1----------------------------------------------------------------------------- 2This file contains a concatenation of the PCRE2 man pages, converted to plain 3text format for ease of searching with a text editor, or for use on systems 4that do not have a man page processor. The small individual files that give 5synopses of each function in the library have not been included. Neither has 6the pcre2demo program. There are separate text files for the pcre2grep and 7pcre2test commands. 8----------------------------------------------------------------------------- 9 10 11PCRE2(3) Library Functions Manual PCRE2(3) 12 13 14 15NAME 16 PCRE2 - Perl-compatible regular expressions (revised API) 17 18INTRODUCTION 19 20 PCRE2 is the name used for a revised API for the PCRE library, which is 21 a set of functions, written in C, that implement regular expression 22 pattern matching using the same syntax and semantics as Perl, with just 23 a few differences. After nearly two decades, the limitations of the 24 original API were making development increasingly difficult. The new 25 API is more extensible, and it was simplified by abolishing the sepa- 26 rate "study" optimizing function; in PCRE2, patterns are automatically 27 optimized where possible. Since forking from PCRE1, the code has been 28 extensively refactored and new features introduced. The old library is 29 now obsolete and is no longer maintained. 30 31 As well as Perl-style regular expression patterns, some features that 32 appeared in Python and the original PCRE before they appeared in Perl 33 are available using the Python syntax. There is also some support for 34 one or two .NET and Oniguruma syntax items, and there are options for 35 requesting some minor changes that give better ECMAScript (aka Java- 36 Script) compatibility. 37 38 The source code for PCRE2 can be compiled to support strings of 8-bit, 39 16-bit, or 32-bit code units, which means that up to three separate li- 40 braries may be installed, one for each code unit size. The size of code 41 unit is not related to the bit size of the underlying hardware. In a 42 64-bit environment that also supports 32-bit applications, versions of 43 PCRE2 that are compiled in both 64-bit and 32-bit modes may be needed. 44 45 The original work to extend PCRE to 16-bit and 32-bit code units was 46 done by Zoltan Herczeg and Christian Persch, respectively. In all three 47 cases, strings can be interpreted either as one character per code 48 unit, or as UTF-encoded Unicode, with support for Unicode general cate- 49 gory properties. Unicode support is optional at build time (but is the 50 default). However, processing strings as UTF code units must be enabled 51 explicitly at run time. The version of Unicode in use can be discovered 52 by running 53 54 pcre2test -C 55 56 The three libraries contain identical sets of functions, with names 57 ending in _8, _16, or _32, respectively (for example, pcre2_com- 58 pile_8()). However, by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or 59 32, a program that uses just one code unit width can be written using 60 generic names such as pcre2_compile(), and the documentation is written 61 assuming that this is the case. 62 63 In addition to the Perl-compatible matching function, PCRE2 contains an 64 alternative function that matches the same compiled patterns in a dif- 65 ferent way. In certain circumstances, the alternative function has some 66 advantages. For a discussion of the two matching algorithms, see the 67 pcre2matching page. 68 69 Details of exactly which Perl regular expression features are and are 70 not supported by PCRE2 are given in separate documents. See the 71 pcre2pattern and pcre2compat pages. There is a syntax summary in the 72 pcre2syntax page. 73 74 Some features of PCRE2 can be included, excluded, or changed when the 75 library is built. The pcre2_config() function makes it possible for a 76 client to discover which features are available. The features them- 77 selves are described in the pcre2build page. Documentation about build- 78 ing PCRE2 for various operating systems can be found in the README and 79 NON-AUTOTOOLS_BUILD files in the source distribution. 80 81 The libraries contains a number of undocumented internal functions and 82 data tables that are used by more than one of the exported external 83 functions, but which are not intended for use by external callers. 84 Their names all begin with "_pcre2", which hopefully will not provoke 85 any name clashes. In some environments, it is possible to control which 86 external symbols are exported when a shared library is built, and in 87 these cases the undocumented symbols are not exported. 88 89 90SECURITY CONSIDERATIONS 91 92 If you are using PCRE2 in a non-UTF application that permits users to 93 supply arbitrary patterns for compilation, you should be aware of a 94 feature that allows users to turn on UTF support from within a pattern. 95 For example, an 8-bit pattern that begins with "(*UTF)" turns on UTF-8 96 mode, which interprets patterns and subjects as strings of UTF-8 code 97 units instead of individual 8-bit characters. This causes both the pat- 98 tern and any data against which it is matched to be checked for UTF-8 99 validity. If the data string is very long, such a check might use suf- 100 ficiently many resources as to cause your application to lose perfor- 101 mance. 102 103 One way of guarding against this possibility is to use the pcre2_pat- 104 tern_info() function to check the compiled pattern's options for 105 PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF option when 106 calling pcre2_compile(). This causes a compile time error if the pat- 107 tern contains a UTF-setting sequence. 108 109 The use of Unicode properties for character types such as \d can also 110 be enabled from within the pattern, by specifying "(*UCP)". This fea- 111 ture can be disallowed by setting the PCRE2_NEVER_UCP option. 112 113 If your application is one that supports UTF, be aware that validity 114 checking can take time. If the same data string is to be matched many 115 times, you can use the PCRE2_NO_UTF_CHECK option for the second and 116 subsequent matches to avoid running redundant checks. 117 118 The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead 119 to problems, because it may leave the current matching point in the 120 middle of a multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C op- 121 tion can be used by an application to lock out the use of \C, causing a 122 compile-time error if it is encountered. It is also possible to build 123 PCRE2 with the use of \C permanently disabled. 124 125 Another way that performance can be hit is by running a pattern that 126 has a very large search tree against a string that will never match. 127 Nested unlimited repeats in a pattern are a common example. PCRE2 pro- 128 vides some protection against this: see the pcre2_set_match_limit() 129 function in the pcre2api page. There is a similar function called 130 pcre2_set_depth_limit() that can be used to restrict the amount of mem- 131 ory that is used. 132 133 134USER DOCUMENTATION 135 136 The user documentation for PCRE2 comprises a number of different sec- 137 tions. In the "man" format, each of these is a separate "man page". In 138 the HTML format, each is a separate page, linked from the index page. 139 In the plain text format, the descriptions of the pcre2grep and 140 pcre2test programs are in files called pcre2grep.txt and pcre2test.txt, 141 respectively. The remaining sections, except for the pcre2demo section 142 (which is a program listing), and the short pages for individual func- 143 tions, are concatenated in pcre2.txt, for ease of searching. The sec- 144 tions are as follows: 145 146 pcre2 this document 147 pcre2-config show PCRE2 installation configuration information 148 pcre2api details of PCRE2's native C API 149 pcre2build building PCRE2 150 pcre2callout details of the pattern callout feature 151 pcre2compat discussion of Perl compatibility 152 pcre2convert details of pattern conversion functions 153 pcre2demo a demonstration C program that uses PCRE2 154 pcre2grep description of the pcre2grep command (8-bit only) 155 pcre2jit discussion of just-in-time optimization support 156 pcre2limits details of size and other limits 157 pcre2matching discussion of the two matching algorithms 158 pcre2partial details of the partial matching facility 159 pcre2pattern syntax and semantics of supported regular 160 expression patterns 161 pcre2perform discussion of performance issues 162 pcre2posix the POSIX-compatible C API for the 8-bit library 163 pcre2sample discussion of the pcre2demo program 164 pcre2serialize details of pattern serialization 165 pcre2syntax quick syntax reference 166 pcre2test description of the pcre2test command 167 pcre2unicode discussion of Unicode and UTF support 168 169 In the "man" and HTML formats, there is also a short page for each C 170 library function, listing its arguments and results. 171 172 173AUTHOR 174 175 Philip Hazel 176 Retired from University Computing Service 177 Cambridge, England. 178 179 Putting an actual email address here is a spam magnet. If you want to 180 email me, use my two names separated by a dot at gmail.com. 181 182 183REVISION 184 185 Last updated: 27 August 2021 186 Copyright (c) 1997-2021 University of Cambridge. 187------------------------------------------------------------------------------ 188 189 190PCRE2API(3) Library Functions Manual PCRE2API(3) 191 192 193 194NAME 195 PCRE2 - Perl-compatible regular expressions (revised API) 196 197 #include <pcre2.h> 198 199 PCRE2 is a new API for PCRE, starting at release 10.0. This document 200 contains a description of all its native functions. See the pcre2 docu- 201 ment for an overview of all the PCRE2 documentation. 202 203 204PCRE2 NATIVE API BASIC FUNCTIONS 205 206 pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length, 207 uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset, 208 pcre2_compile_context *ccontext); 209 210 void pcre2_code_free(pcre2_code *code); 211 212 pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize, 213 pcre2_general_context *gcontext); 214 215 pcre2_match_data *pcre2_match_data_create_from_pattern( 216 const pcre2_code *code, pcre2_general_context *gcontext); 217 218 int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject, 219 PCRE2_SIZE length, PCRE2_SIZE startoffset, 220 uint32_t options, pcre2_match_data *match_data, 221 pcre2_match_context *mcontext); 222 223 int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject, 224 PCRE2_SIZE length, PCRE2_SIZE startoffset, 225 uint32_t options, pcre2_match_data *match_data, 226 pcre2_match_context *mcontext, 227 int *workspace, PCRE2_SIZE wscount); 228 229 void pcre2_match_data_free(pcre2_match_data *match_data); 230 231 232PCRE2 NATIVE API AUXILIARY MATCH FUNCTIONS 233 234 PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data); 235 236 uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data); 237 238 PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data); 239 240 PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data); 241 242 243PCRE2 NATIVE API GENERAL CONTEXT FUNCTIONS 244 245 pcre2_general_context *pcre2_general_context_create( 246 void *(*private_malloc)(PCRE2_SIZE, void *), 247 void (*private_free)(void *, void *), void *memory_data); 248 249 pcre2_general_context *pcre2_general_context_copy( 250 pcre2_general_context *gcontext); 251 252 void pcre2_general_context_free(pcre2_general_context *gcontext); 253 254 255PCRE2 NATIVE API COMPILE CONTEXT FUNCTIONS 256 257 pcre2_compile_context *pcre2_compile_context_create( 258 pcre2_general_context *gcontext); 259 260 pcre2_compile_context *pcre2_compile_context_copy( 261 pcre2_compile_context *ccontext); 262 263 void pcre2_compile_context_free(pcre2_compile_context *ccontext); 264 265 int pcre2_set_bsr(pcre2_compile_context *ccontext, 266 uint32_t value); 267 268 int pcre2_set_character_tables(pcre2_compile_context *ccontext, 269 const uint8_t *tables); 270 271 int pcre2_set_compile_extra_options(pcre2_compile_context *ccontext, 272 uint32_t extra_options); 273 274 int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext, 275 PCRE2_SIZE value); 276 277 int pcre2_set_newline(pcre2_compile_context *ccontext, 278 uint32_t value); 279 280 int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext, 281 uint32_t value); 282 283 int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext, 284 int (*guard_function)(uint32_t, void *), void *user_data); 285 286 287PCRE2 NATIVE API MATCH CONTEXT FUNCTIONS 288 289 pcre2_match_context *pcre2_match_context_create( 290 pcre2_general_context *gcontext); 291 292 pcre2_match_context *pcre2_match_context_copy( 293 pcre2_match_context *mcontext); 294 295 void pcre2_match_context_free(pcre2_match_context *mcontext); 296 297 int pcre2_set_callout(pcre2_match_context *mcontext, 298 int (*callout_function)(pcre2_callout_block *, void *), 299 void *callout_data); 300 301 int pcre2_set_substitute_callout(pcre2_match_context *mcontext, 302 int (*callout_function)(pcre2_substitute_callout_block *, void *), 303 void *callout_data); 304 305 int pcre2_set_offset_limit(pcre2_match_context *mcontext, 306 PCRE2_SIZE value); 307 308 int pcre2_set_heap_limit(pcre2_match_context *mcontext, 309 uint32_t value); 310 311 int pcre2_set_match_limit(pcre2_match_context *mcontext, 312 uint32_t value); 313 314 int pcre2_set_depth_limit(pcre2_match_context *mcontext, 315 uint32_t value); 316 317 318PCRE2 NATIVE API STRING EXTRACTION FUNCTIONS 319 320 int pcre2_substring_copy_byname(pcre2_match_data *match_data, 321 PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen); 322 323 int pcre2_substring_copy_bynumber(pcre2_match_data *match_data, 324 uint32_t number, PCRE2_UCHAR *buffer, 325 PCRE2_SIZE *bufflen); 326 327 void pcre2_substring_free(PCRE2_UCHAR *buffer); 328 329 int pcre2_substring_get_byname(pcre2_match_data *match_data, 330 PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen); 331 332 int pcre2_substring_get_bynumber(pcre2_match_data *match_data, 333 uint32_t number, PCRE2_UCHAR **bufferptr, 334 PCRE2_SIZE *bufflen); 335 336 int pcre2_substring_length_byname(pcre2_match_data *match_data, 337 PCRE2_SPTR name, PCRE2_SIZE *length); 338 339 int pcre2_substring_length_bynumber(pcre2_match_data *match_data, 340 uint32_t number, PCRE2_SIZE *length); 341 342 int pcre2_substring_nametable_scan(const pcre2_code *code, 343 PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last); 344 345 int pcre2_substring_number_from_name(const pcre2_code *code, 346 PCRE2_SPTR name); 347 348 void pcre2_substring_list_free(PCRE2_SPTR *list); 349 350 int pcre2_substring_list_get(pcre2_match_data *match_data, 351 PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr); 352 353 354PCRE2 NATIVE API STRING SUBSTITUTION FUNCTION 355 356 int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject, 357 PCRE2_SIZE length, PCRE2_SIZE startoffset, 358 uint32_t options, pcre2_match_data *match_data, 359 pcre2_match_context *mcontext, PCRE2_SPTR replacementz, 360 PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer, 361 PCRE2_SIZE *outlengthptr); 362 363 364PCRE2 NATIVE API JIT FUNCTIONS 365 366 int pcre2_jit_compile(pcre2_code *code, uint32_t options); 367 368 int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject, 369 PCRE2_SIZE length, PCRE2_SIZE startoffset, 370 uint32_t options, pcre2_match_data *match_data, 371 pcre2_match_context *mcontext); 372 373 void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext); 374 375 pcre2_jit_stack *pcre2_jit_stack_create(PCRE2_SIZE startsize, 376 PCRE2_SIZE maxsize, pcre2_general_context *gcontext); 377 378 void pcre2_jit_stack_assign(pcre2_match_context *mcontext, 379 pcre2_jit_callback callback_function, void *callback_data); 380 381 void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack); 382 383 384PCRE2 NATIVE API SERIALIZATION FUNCTIONS 385 386 int32_t pcre2_serialize_decode(pcre2_code **codes, 387 int32_t number_of_codes, const uint8_t *bytes, 388 pcre2_general_context *gcontext); 389 390 int32_t pcre2_serialize_encode(const pcre2_code **codes, 391 int32_t number_of_codes, uint8_t **serialized_bytes, 392 PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext); 393 394 void pcre2_serialize_free(uint8_t *bytes); 395 396 int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes); 397 398 399PCRE2 NATIVE API AUXILIARY FUNCTIONS 400 401 pcre2_code *pcre2_code_copy(const pcre2_code *code); 402 403 pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code); 404 405 int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer, 406 PCRE2_SIZE bufflen); 407 408 const uint8_t *pcre2_maketables(pcre2_general_context *gcontext); 409 410 void pcre2_maketables_free(pcre2_general_context *gcontext, 411 const uint8_t *tables); 412 413 int pcre2_pattern_info(const pcre2_code *code, uint32_t what, 414 void *where); 415 416 int pcre2_callout_enumerate(const pcre2_code *code, 417 int (*callback)(pcre2_callout_enumerate_block *, void *), 418 void *user_data); 419 420 int pcre2_config(uint32_t what, void *where); 421 422 423PCRE2 NATIVE API OBSOLETE FUNCTIONS 424 425 int pcre2_set_recursion_limit(pcre2_match_context *mcontext, 426 uint32_t value); 427 428 int pcre2_set_recursion_memory_management( 429 pcre2_match_context *mcontext, 430 void *(*private_malloc)(PCRE2_SIZE, void *), 431 void (*private_free)(void *, void *), void *memory_data); 432 433 These functions became obsolete at release 10.30 and are retained only 434 for backward compatibility. They should not be used in new code. The 435 first is replaced by pcre2_set_depth_limit(); the second is no longer 436 needed and has no effect (it always returns zero). 437 438 439PCRE2 EXPERIMENTAL PATTERN CONVERSION FUNCTIONS 440 441 pcre2_convert_context *pcre2_convert_context_create( 442 pcre2_general_context *gcontext); 443 444 pcre2_convert_context *pcre2_convert_context_copy( 445 pcre2_convert_context *cvcontext); 446 447 void pcre2_convert_context_free(pcre2_convert_context *cvcontext); 448 449 int pcre2_set_glob_escape(pcre2_convert_context *cvcontext, 450 uint32_t escape_char); 451 452 int pcre2_set_glob_separator(pcre2_convert_context *cvcontext, 453 uint32_t separator_char); 454 455 int pcre2_pattern_convert(PCRE2_SPTR pattern, PCRE2_SIZE length, 456 uint32_t options, PCRE2_UCHAR **buffer, 457 PCRE2_SIZE *blength, pcre2_convert_context *cvcontext); 458 459 void pcre2_converted_pattern_free(PCRE2_UCHAR *converted_pattern); 460 461 These functions provide a way of converting non-PCRE2 patterns into 462 patterns that can be processed by pcre2_compile(). This facility is ex- 463 perimental and may be changed in future releases. At present, "globs" 464 and POSIX basic and extended patterns can be converted. Details are 465 given in the pcre2convert documentation. 466 467 468PCRE2 8-BIT, 16-BIT, AND 32-BIT LIBRARIES 469 470 There are three PCRE2 libraries, supporting 8-bit, 16-bit, and 32-bit 471 code units, respectively. However, there is just one header file, 472 pcre2.h. This contains the function prototypes and other definitions 473 for all three libraries. One, two, or all three can be installed simul- 474 taneously. On Unix-like systems the libraries are called libpcre2-8, 475 libpcre2-16, and libpcre2-32, and they can also co-exist with the orig- 476 inal PCRE libraries. 477 478 Character strings are passed to and from a PCRE2 library as a sequence 479 of unsigned integers in code units of the appropriate width. Every 480 PCRE2 function comes in three different forms, one for each library, 481 for example: 482 483 pcre2_compile_8() 484 pcre2_compile_16() 485 pcre2_compile_32() 486 487 There are also three different sets of data types: 488 489 PCRE2_UCHAR8, PCRE2_UCHAR16, PCRE2_UCHAR32 490 PCRE2_SPTR8, PCRE2_SPTR16, PCRE2_SPTR32 491 492 The UCHAR types define unsigned code units of the appropriate widths. 493 For example, PCRE2_UCHAR16 is usually defined as `uint16_t'. The SPTR 494 types are constant pointers to the equivalent UCHAR types, that is, 495 they are pointers to vectors of unsigned code units. 496 497 Many applications use only one code unit width. For their convenience, 498 macros are defined whose names are the generic forms such as pcre2_com- 499 pile() and PCRE2_SPTR. These macros use the value of the macro 500 PCRE2_CODE_UNIT_WIDTH to generate the appropriate width-specific func- 501 tion and macro names. PCRE2_CODE_UNIT_WIDTH is not defined by default. 502 An application must define it to be 8, 16, or 32 before including 503 pcre2.h in order to make use of the generic names. 504 505 Applications that use more than one code unit width can be linked with 506 more than one PCRE2 library, but must define PCRE2_CODE_UNIT_WIDTH to 507 be 0 before including pcre2.h, and then use the real function names. 508 Any code that is to be included in an environment where the value of 509 PCRE2_CODE_UNIT_WIDTH is unknown should also use the real function 510 names. (Unfortunately, it is not possible in C code to save and restore 511 the value of a macro.) 512 513 If PCRE2_CODE_UNIT_WIDTH is not defined before including pcre2.h, a 514 compiler error occurs. 515 516 When using multiple libraries in an application, you must take care 517 when processing any particular pattern to use only functions from a 518 single library. For example, if you want to run a match using a pat- 519 tern that was compiled with pcre2_compile_16(), you must do so with 520 pcre2_match_16(), not pcre2_match_8() or pcre2_match_32(). 521 522 In the function summaries above, and in the rest of this document and 523 other PCRE2 documents, functions and data types are described using 524 their generic names, without the _8, _16, or _32 suffix. 525 526 527PCRE2 API OVERVIEW 528 529 PCRE2 has its own native API, which is described in this document. 530 There are also some wrapper functions for the 8-bit library that corre- 531 spond to the POSIX regular expression API, but they do not give access 532 to all the functionality of PCRE2. They are described in the pcre2posix 533 documentation. Both these APIs define a set of C function calls. 534 535 The native API C data types, function prototypes, option values, and 536 error codes are defined in the header file pcre2.h, which also contains 537 definitions of PCRE2_MAJOR and PCRE2_MINOR, the major and minor release 538 numbers for the library. Applications can use these to include support 539 for different releases of PCRE2. 540 541 In a Windows environment, if you want to statically link an application 542 program against a non-dll PCRE2 library, you must define PCRE2_STATIC 543 before including pcre2.h. 544 545 The functions pcre2_compile() and pcre2_match() are used for compiling 546 and matching regular expressions in a Perl-compatible manner. A sample 547 program that demonstrates the simplest way of using them is provided in 548 the file called pcre2demo.c in the PCRE2 source distribution. A listing 549 of this program is given in the pcre2demo documentation, and the 550 pcre2sample documentation describes how to compile and run it. 551 552 The compiling and matching functions recognize various options that are 553 passed as bits in an options argument. There are also some more compli- 554 cated parameters such as custom memory management functions and re- 555 source limits that are passed in "contexts" (which are just memory 556 blocks, described below). Simple applications do not need to make use 557 of contexts. 558 559 Just-in-time (JIT) compiler support is an optional feature of PCRE2 560 that can be built in appropriate hardware environments. It greatly 561 speeds up the matching performance of many patterns. Programs can re- 562 quest that it be used if available by calling pcre2_jit_compile() after 563 a pattern has been successfully compiled by pcre2_compile(). This does 564 nothing if JIT support is not available. 565 566 More complicated programs might need to make use of the specialist 567 functions pcre2_jit_stack_create(), pcre2_jit_stack_free(), and 568 pcre2_jit_stack_assign() in order to control the JIT code's memory us- 569 age. 570 571 JIT matching is automatically used by pcre2_match() if it is available, 572 unless the PCRE2_NO_JIT option is set. There is also a direct interface 573 for JIT matching, which gives improved performance at the expense of 574 less sanity checking. The JIT-specific functions are discussed in the 575 pcre2jit documentation. 576 577 A second matching function, pcre2_dfa_match(), which is not Perl-com- 578 patible, is also provided. This uses a different algorithm for the 579 matching. The alternative algorithm finds all possible matches (at a 580 given point in the subject), and scans the subject just once (unless 581 there are lookaround assertions). However, this algorithm does not re- 582 turn captured substrings. A description of the two matching algorithms 583 and their advantages and disadvantages is given in the pcre2matching 584 documentation. There is no JIT support for pcre2_dfa_match(). 585 586 In addition to the main compiling and matching functions, there are 587 convenience functions for extracting captured substrings from a subject 588 string that has been matched by pcre2_match(). They are: 589 590 pcre2_substring_copy_byname() 591 pcre2_substring_copy_bynumber() 592 pcre2_substring_get_byname() 593 pcre2_substring_get_bynumber() 594 pcre2_substring_list_get() 595 pcre2_substring_length_byname() 596 pcre2_substring_length_bynumber() 597 pcre2_substring_nametable_scan() 598 pcre2_substring_number_from_name() 599 600 pcre2_substring_free() and pcre2_substring_list_free() are also pro- 601 vided, to free memory used for extracted strings. If either of these 602 functions is called with a NULL argument, the function returns immedi- 603 ately without doing anything. 604 605 The function pcre2_substitute() can be called to match a pattern and 606 return a copy of the subject string with substitutions for parts that 607 were matched. 608 609 Functions whose names begin with pcre2_serialize_ are used for saving 610 compiled patterns on disc or elsewhere, and reloading them later. 611 612 Finally, there are functions for finding out information about a com- 613 piled pattern (pcre2_pattern_info()) and about the configuration with 614 which PCRE2 was built (pcre2_config()). 615 616 Functions with names ending with _free() are used for freeing memory 617 blocks of various sorts. In all cases, if one of these functions is 618 called with a NULL argument, it does nothing. 619 620 621STRING LENGTHS AND OFFSETS 622 623 The PCRE2 API uses string lengths and offsets into strings of code 624 units in several places. These values are always of type PCRE2_SIZE, 625 which is an unsigned integer type, currently always defined as size_t. 626 The largest value that can be stored in such a type (that is 627 ~(PCRE2_SIZE)0) is reserved as a special indicator for zero-terminated 628 strings and unset offsets. Therefore, the longest string that can be 629 handled is one less than this maximum. 630 631 632NEWLINES 633 634 PCRE2 supports five different conventions for indicating line breaks in 635 strings: a single CR (carriage return) character, a single LF (line- 636 feed) character, the two-character sequence CRLF, any of the three pre- 637 ceding, or any Unicode newline sequence. The Unicode newline sequences 638 are the three just mentioned, plus the single characters VT (vertical 639 tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line 640 separator, U+2028), and PS (paragraph separator, U+2029). 641 642 Each of the first three conventions is used by at least one operating 643 system as its standard newline sequence. When PCRE2 is built, a default 644 can be specified. If it is not, the default is set to LF, which is the 645 Unix standard. However, the newline convention can be changed by an ap- 646 plication when calling pcre2_compile(), or it can be specified by spe- 647 cial text at the start of the pattern itself; this overrides any other 648 settings. See the pcre2pattern page for details of the special charac- 649 ter sequences. 650 651 In the PCRE2 documentation the word "newline" is used to mean "the 652 character or pair of characters that indicate a line break". The choice 653 of newline convention affects the handling of the dot, circumflex, and 654 dollar metacharacters, the handling of #-comments in /x mode, and, when 655 CRLF is a recognized line ending sequence, the match position advance- 656 ment for a non-anchored pattern. There is more detail about this in the 657 section on pcre2_match() options below. 658 659 The choice of newline convention does not affect the interpretation of 660 the \n or \r escape sequences, nor does it affect what \R matches; this 661 has its own separate convention. 662 663 664MULTITHREADING 665 666 In a multithreaded application it is important to keep thread-specific 667 data separate from data that can be shared between threads. The PCRE2 668 library code itself is thread-safe: it contains no static or global 669 variables. The API is designed to be fairly simple for non-threaded ap- 670 plications while at the same time ensuring that multithreaded applica- 671 tions can use it. 672 673 There are several different blocks of data that are used to pass infor- 674 mation between the application and the PCRE2 libraries. 675 676 The compiled pattern 677 678 A pointer to the compiled form of a pattern is returned to the user 679 when pcre2_compile() is successful. The data in the compiled pattern is 680 fixed, and does not change when the pattern is matched. Therefore, it 681 is thread-safe, that is, the same compiled pattern can be used by more 682 than one thread simultaneously. For example, an application can compile 683 all its patterns at the start, before forking off multiple threads that 684 use them. However, if the just-in-time (JIT) optimization feature is 685 being used, it needs separate memory stack areas for each thread. See 686 the pcre2jit documentation for more details. 687 688 In a more complicated situation, where patterns are compiled only when 689 they are first needed, but are still shared between threads, pointers 690 to compiled patterns must be protected from simultaneous writing by 691 multiple threads. This is somewhat tricky to do correctly. If you know 692 that writing to a pointer is atomic in your environment, you can use 693 logic like this: 694 695 Get a read-only (shared) lock (mutex) for pointer 696 if (pointer == NULL) 697 { 698 Get a write (unique) lock for pointer 699 if (pointer == NULL) pointer = pcre2_compile(... 700 } 701 Release the lock 702 Use pointer in pcre2_match() 703 704 Of course, testing for compilation errors should also be included in 705 the code. 706 707 The reason for checking the pointer a second time is as follows: Sev- 708 eral threads may have acquired the shared lock and tested the pointer 709 for being NULL, but only one of them will be given the write lock, with 710 the rest kept waiting. The winning thread will compile the pattern and 711 store the result. After this thread releases the write lock, another 712 thread will get it, and if it does not retest pointer for being NULL, 713 will recompile the pattern and overwrite the pointer, creating a memory 714 leak and possibly causing other issues. 715 716 In an environment where writing to a pointer may not be atomic, the 717 above logic is not sufficient. The thread that is doing the compiling 718 may be descheduled after writing only part of the pointer, which could 719 cause other threads to use an invalid value. Instead of checking the 720 pointer itself, a separate "pointer is valid" flag (that can be updated 721 atomically) must be used: 722 723 Get a read-only (shared) lock (mutex) for pointer 724 if (!pointer_is_valid) 725 { 726 Get a write (unique) lock for pointer 727 if (!pointer_is_valid) 728 { 729 pointer = pcre2_compile(... 730 pointer_is_valid = TRUE 731 } 732 } 733 Release the lock 734 Use pointer in pcre2_match() 735 736 If JIT is being used, but the JIT compilation is not being done immedi- 737 ately (perhaps waiting to see if the pattern is used often enough), 738 similar logic is required. JIT compilation updates a value within the 739 compiled code block, so a thread must gain unique write access to the 740 pointer before calling pcre2_jit_compile(). Alternatively, 741 pcre2_code_copy() or pcre2_code_copy_with_tables() can be used to ob- 742 tain a private copy of the compiled code before calling the JIT com- 743 piler. 744 745 Context blocks 746 747 The next main section below introduces the idea of "contexts" in which 748 PCRE2 functions are called. A context is nothing more than a collection 749 of parameters that control the way PCRE2 operates. Grouping a number of 750 parameters together in a context is a convenient way of passing them to 751 a PCRE2 function without using lots of arguments. The parameters that 752 are stored in contexts are in some sense "advanced features" of the 753 API. Many straightforward applications will not need to use contexts. 754 755 In a multithreaded application, if the parameters in a context are val- 756 ues that are never changed, the same context can be used by all the 757 threads. However, if any thread needs to change any value in a context, 758 it must make its own thread-specific copy. 759 760 Match blocks 761 762 The matching functions need a block of memory for storing the results 763 of a match. This includes details of what was matched, as well as addi- 764 tional information such as the name of a (*MARK) setting. Each thread 765 must provide its own copy of this memory. 766 767 768PCRE2 CONTEXTS 769 770 Some PCRE2 functions have a lot of parameters, many of which are used 771 only by specialist applications, for example, those that use custom 772 memory management or non-standard character tables. To keep function 773 argument lists at a reasonable size, and at the same time to keep the 774 API extensible, "uncommon" parameters are passed to certain functions 775 in a context instead of directly. A context is just a block of memory 776 that holds the parameter values. Applications that do not need to ad- 777 just any of the context parameters can pass NULL when a context pointer 778 is required. 779 780 There are three different types of context: a general context that is 781 relevant for several PCRE2 operations, a compile-time context, and a 782 match-time context. 783 784 The general context 785 786 At present, this context just contains pointers to (and data for) ex- 787 ternal memory management functions that are called from several places 788 in the PCRE2 library. The context is named `general' rather than 789 specifically `memory' because in future other fields may be added. If 790 you do not want to supply your own custom memory management functions, 791 you do not need to bother with a general context. A general context is 792 created by: 793 794 pcre2_general_context *pcre2_general_context_create( 795 void *(*private_malloc)(PCRE2_SIZE, void *), 796 void (*private_free)(void *, void *), void *memory_data); 797 798 The two function pointers specify custom memory management functions, 799 whose prototypes are: 800 801 void *private_malloc(PCRE2_SIZE, void *); 802 void private_free(void *, void *); 803 804 Whenever code in PCRE2 calls these functions, the final argument is the 805 value of memory_data. Either of the first two arguments of the creation 806 function may be NULL, in which case the system memory management func- 807 tions malloc() and free() are used. (This is not currently useful, as 808 there are no other fields in a general context, but in future there 809 might be.) The private_malloc() function is used (if supplied) to ob- 810 tain memory for storing the context, and all three values are saved as 811 part of the context. 812 813 Whenever PCRE2 creates a data block of any kind, the block contains a 814 pointer to the free() function that matches the malloc() function that 815 was used. When the time comes to free the block, this function is 816 called. 817 818 A general context can be copied by calling: 819 820 pcre2_general_context *pcre2_general_context_copy( 821 pcre2_general_context *gcontext); 822 823 The memory used for a general context should be freed by calling: 824 825 void pcre2_general_context_free(pcre2_general_context *gcontext); 826 827 If this function is passed a NULL argument, it returns immediately 828 without doing anything. 829 830 The compile context 831 832 A compile context is required if you want to provide an external func- 833 tion for stack checking during compilation or to change the default 834 values of any of the following compile-time parameters: 835 836 What \R matches (Unicode newlines or CR, LF, CRLF only) 837 PCRE2's character tables 838 The newline character sequence 839 The compile time nested parentheses limit 840 The maximum length of the pattern string 841 The extra options bits (none set by default) 842 843 A compile context is also required if you are using custom memory man- 844 agement. If none of these apply, just pass NULL as the context argu- 845 ment of pcre2_compile(). 846 847 A compile context is created, copied, and freed by the following func- 848 tions: 849 850 pcre2_compile_context *pcre2_compile_context_create( 851 pcre2_general_context *gcontext); 852 853 pcre2_compile_context *pcre2_compile_context_copy( 854 pcre2_compile_context *ccontext); 855 856 void pcre2_compile_context_free(pcre2_compile_context *ccontext); 857 858 A compile context is created with default values for its parameters. 859 These can be changed by calling the following functions, which return 0 860 on success, or PCRE2_ERROR_BADDATA if invalid data is detected. 861 862 int pcre2_set_bsr(pcre2_compile_context *ccontext, 863 uint32_t value); 864 865 The value must be PCRE2_BSR_ANYCRLF, to specify that \R matches only 866 CR, LF, or CRLF, or PCRE2_BSR_UNICODE, to specify that \R matches any 867 Unicode line ending sequence. The value is used by the JIT compiler and 868 by the two interpreted matching functions, pcre2_match() and 869 pcre2_dfa_match(). 870 871 int pcre2_set_character_tables(pcre2_compile_context *ccontext, 872 const uint8_t *tables); 873 874 The value must be the result of a call to pcre2_maketables(), whose 875 only argument is a general context. This function builds a set of char- 876 acter tables in the current locale. 877 878 int pcre2_set_compile_extra_options(pcre2_compile_context *ccontext, 879 uint32_t extra_options); 880 881 As PCRE2 has developed, almost all the 32 option bits that are avail- 882 able in the options argument of pcre2_compile() have been used up. To 883 avoid running out, the compile context contains a set of extra option 884 bits which are used for some newer, assumed rarer, options. This func- 885 tion sets those bits. It always sets all the bits (either on or off). 886 It does not modify any existing setting. The available options are de- 887 fined in the section entitled "Extra compile options" below. 888 889 int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext, 890 PCRE2_SIZE value); 891 892 This sets a maximum length, in code units, for any pattern string that 893 is compiled with this context. If the pattern is longer, an error is 894 generated. This facility is provided so that applications that accept 895 patterns from external sources can limit their size. The default is the 896 largest number that a PCRE2_SIZE variable can hold, which is effec- 897 tively unlimited. 898 899 int pcre2_set_newline(pcre2_compile_context *ccontext, 900 uint32_t value); 901 902 This specifies which characters or character sequences are to be recog- 903 nized as newlines. The value must be one of PCRE2_NEWLINE_CR (carriage 904 return only), PCRE2_NEWLINE_LF (linefeed only), PCRE2_NEWLINE_CRLF (the 905 two-character sequence CR followed by LF), PCRE2_NEWLINE_ANYCRLF (any 906 of the above), PCRE2_NEWLINE_ANY (any Unicode newline sequence), or 907 PCRE2_NEWLINE_NUL (the NUL character, that is a binary zero). 908 909 A pattern can override the value set in the compile context by starting 910 with a sequence such as (*CRLF). See the pcre2pattern page for details. 911 912 When a pattern is compiled with the PCRE2_EXTENDED or PCRE2_EX- 913 TENDED_MORE option, the newline convention affects the recognition of 914 the end of internal comments starting with #. The value is saved with 915 the compiled pattern for subsequent use by the JIT compiler and by the 916 two interpreted matching functions, pcre2_match() and 917 pcre2_dfa_match(). 918 919 int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext, 920 uint32_t value); 921 922 This parameter adjusts the limit, set when PCRE2 is built (default 923 250), on the depth of parenthesis nesting in a pattern. This limit 924 stops rogue patterns using up too much system stack when being com- 925 piled. The limit applies to parentheses of all kinds, not just captur- 926 ing parentheses. 927 928 int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext, 929 int (*guard_function)(uint32_t, void *), void *user_data); 930 931 There is at least one application that runs PCRE2 in threads with very 932 limited system stack, where running out of stack is to be avoided at 933 all costs. The parenthesis limit above cannot take account of how much 934 stack is actually available during compilation. For a finer control, 935 you can supply a function that is called whenever pcre2_compile() 936 starts to compile a parenthesized part of a pattern. This function can 937 check the actual stack size (or anything else that it wants to, of 938 course). 939 940 The first argument to the callout function gives the current depth of 941 nesting, and the second is user data that is set up by the last argu- 942 ment of pcre2_set_compile_recursion_guard(). The callout function 943 should return zero if all is well, or non-zero to force an error. 944 945 The match context 946 947 A match context is required if you want to: 948 949 Set up a callout function 950 Set an offset limit for matching an unanchored pattern 951 Change the limit on the amount of heap used when matching 952 Change the backtracking match limit 953 Change the backtracking depth limit 954 Set custom memory management specifically for the match 955 956 If none of these apply, just pass NULL as the context argument of 957 pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match(). 958 959 A match context is created, copied, and freed by the following func- 960 tions: 961 962 pcre2_match_context *pcre2_match_context_create( 963 pcre2_general_context *gcontext); 964 965 pcre2_match_context *pcre2_match_context_copy( 966 pcre2_match_context *mcontext); 967 968 void pcre2_match_context_free(pcre2_match_context *mcontext); 969 970 A match context is created with default values for its parameters. 971 These can be changed by calling the following functions, which return 0 972 on success, or PCRE2_ERROR_BADDATA if invalid data is detected. 973 974 int pcre2_set_callout(pcre2_match_context *mcontext, 975 int (*callout_function)(pcre2_callout_block *, void *), 976 void *callout_data); 977 978 This sets up a callout function for PCRE2 to call at specified points 979 during a matching operation. Details are given in the pcre2callout doc- 980 umentation. 981 982 int pcre2_set_substitute_callout(pcre2_match_context *mcontext, 983 int (*callout_function)(pcre2_substitute_callout_block *, void *), 984 void *callout_data); 985 986 This sets up a callout function for PCRE2 to call after each substitu- 987 tion made by pcre2_substitute(). Details are given in the section enti- 988 tled "Creating a new string with substitutions" below. 989 990 int pcre2_set_offset_limit(pcre2_match_context *mcontext, 991 PCRE2_SIZE value); 992 993 The offset_limit parameter limits how far an unanchored search can ad- 994 vance in the subject string. The default value is PCRE2_UNSET. The 995 pcre2_match() and pcre2_dfa_match() functions return PCRE2_ERROR_NO- 996 MATCH if a match with a starting point before or at the given offset is 997 not found. The pcre2_substitute() function makes no more substitutions. 998 999 For example, if the pattern /abc/ is matched against "123abc" with an 1000 offset limit less than 3, the result is PCRE2_ERROR_NOMATCH. A match 1001 can never be found if the startoffset argument of pcre2_match(), 1002 pcre2_dfa_match(), or pcre2_substitute() is greater than the offset 1003 limit set in the match context. 1004 1005 When using this facility, you must set the PCRE2_USE_OFFSET_LIMIT op- 1006 tion when calling pcre2_compile() so that when JIT is in use, different 1007 code can be compiled. If a match is started with a non-default match 1008 limit when PCRE2_USE_OFFSET_LIMIT is not set, an error is generated. 1009 1010 The offset limit facility can be used to track progress when searching 1011 large subject strings or to limit the extent of global substitutions. 1012 See also the PCRE2_FIRSTLINE option, which requires a match to start 1013 before or at the first newline that follows the start of matching in 1014 the subject. If this is set with an offset limit, a match must occur in 1015 the first line and also within the offset limit. In other words, which- 1016 ever limit comes first is used. 1017 1018 int pcre2_set_heap_limit(pcre2_match_context *mcontext, 1019 uint32_t value); 1020 1021 The heap_limit parameter specifies, in units of kibibytes (1024 bytes), 1022 the maximum amount of heap memory that pcre2_match() may use to hold 1023 backtracking information when running an interpretive match. This limit 1024 also applies to pcre2_dfa_match(), which may use the heap when process- 1025 ing patterns with a lot of nested pattern recursion or lookarounds or 1026 atomic groups. This limit does not apply to matching with the JIT opti- 1027 mization, which has its own memory control arrangements (see the 1028 pcre2jit documentation for more details). If the limit is reached, the 1029 negative error code PCRE2_ERROR_HEAPLIMIT is returned. The default 1030 limit can be set when PCRE2 is built; if it is not, the default is set 1031 very large and is essentially unlimited. 1032 1033 A value for the heap limit may also be supplied by an item at the start 1034 of a pattern of the form 1035 1036 (*LIMIT_HEAP=ddd) 1037 1038 where ddd is a decimal number. However, such a setting is ignored un- 1039 less ddd is less than the limit set by the caller of pcre2_match() or, 1040 if no such limit is set, less than the default. 1041 1042 The pcre2_match() function always needs some heap memory, so setting a 1043 value of zero guarantees a "heap limit exceeded" error. Details of how 1044 pcre2_match() uses the heap are given in the pcre2perform documenta- 1045 tion. 1046 1047 For pcre2_dfa_match(), a vector on the system stack is used when pro- 1048 cessing pattern recursions, lookarounds, or atomic groups, and only if 1049 this is not big enough is heap memory used. In this case, setting a 1050 value of zero disables the use of the heap. 1051 1052 int pcre2_set_match_limit(pcre2_match_context *mcontext, 1053 uint32_t value); 1054 1055 The match_limit parameter provides a means of preventing PCRE2 from us- 1056 ing up too many computing resources when processing patterns that are 1057 not going to match, but which have a very large number of possibilities 1058 in their search trees. The classic example is a pattern that uses 1059 nested unlimited repeats. 1060 1061 There is an internal counter in pcre2_match() that is incremented each 1062 time round its main matching loop. If this value reaches the match 1063 limit, pcre2_match() returns the negative value PCRE2_ERROR_MATCHLIMIT. 1064 This has the effect of limiting the amount of backtracking that can 1065 take place. For patterns that are not anchored, the count restarts from 1066 zero for each position in the subject string. This limit also applies 1067 to pcre2_dfa_match(), though the counting is done in a different way. 1068 1069 When pcre2_match() is called with a pattern that was successfully pro- 1070 cessed by pcre2_jit_compile(), the way in which matching is executed is 1071 entirely different. However, there is still the possibility of runaway 1072 matching that goes on for a very long time, and so the match_limit 1073 value is also used in this case (but in a different way) to limit how 1074 long the matching can continue. 1075 1076 The default value for the limit can be set when PCRE2 is built; the de- 1077 fault default is 10 million, which handles all but the most extreme 1078 cases. A value for the match limit may also be supplied by an item at 1079 the start of a pattern of the form 1080 1081 (*LIMIT_MATCH=ddd) 1082 1083 where ddd is a decimal number. However, such a setting is ignored un- 1084 less ddd is less than the limit set by the caller of pcre2_match() or 1085 pcre2_dfa_match() or, if no such limit is set, less than the default. 1086 1087 int pcre2_set_depth_limit(pcre2_match_context *mcontext, 1088 uint32_t value); 1089 1090 This parameter limits the depth of nested backtracking in 1091 pcre2_match(). Each time a nested backtracking point is passed, a new 1092 memory frame is used to remember the state of matching at that point. 1093 Thus, this parameter indirectly limits the amount of memory that is 1094 used in a match. However, because the size of each memory frame depends 1095 on the number of capturing parentheses, the actual memory limit varies 1096 from pattern to pattern. This limit was more useful in versions before 1097 10.30, where function recursion was used for backtracking. 1098 1099 The depth limit is not relevant, and is ignored, when matching is done 1100 using JIT compiled code. However, it is supported by pcre2_dfa_match(), 1101 which uses it to limit the depth of nested internal recursive function 1102 calls that implement atomic groups, lookaround assertions, and pattern 1103 recursions. This limits, indirectly, the amount of system stack that is 1104 used. It was more useful in versions before 10.32, when stack memory 1105 was used for local workspace vectors for recursive function calls. From 1106 version 10.32, only local variables are allocated on the stack and as 1107 each call uses only a few hundred bytes, even a small stack can support 1108 quite a lot of recursion. 1109 1110 If the depth of internal recursive function calls is great enough, lo- 1111 cal workspace vectors are allocated on the heap from version 10.32 on- 1112 wards, so the depth limit also indirectly limits the amount of heap 1113 memory that is used. A recursive pattern such as /(.(?2))((?1)|)/, when 1114 matched to a very long string using pcre2_dfa_match(), can use a great 1115 deal of memory. However, it is probably better to limit heap usage di- 1116 rectly by calling pcre2_set_heap_limit(). 1117 1118 The default value for the depth limit can be set when PCRE2 is built; 1119 if it is not, the default is set to the same value as the default for 1120 the match limit. If the limit is exceeded, pcre2_match() or 1121 pcre2_dfa_match() returns PCRE2_ERROR_DEPTHLIMIT. A value for the depth 1122 limit may also be supplied by an item at the start of a pattern of the 1123 form 1124 1125 (*LIMIT_DEPTH=ddd) 1126 1127 where ddd is a decimal number. However, such a setting is ignored un- 1128 less ddd is less than the limit set by the caller of pcre2_match() or 1129 pcre2_dfa_match() or, if no such limit is set, less than the default. 1130 1131 1132CHECKING BUILD-TIME OPTIONS 1133 1134 int pcre2_config(uint32_t what, void *where); 1135 1136 The function pcre2_config() makes it possible for a PCRE2 client to 1137 find the value of certain configuration parameters and to discover 1138 which optional features have been compiled into the PCRE2 library. The 1139 pcre2build documentation has more details about these features. 1140 1141 The first argument for pcre2_config() specifies which information is 1142 required. The second argument is a pointer to memory into which the in- 1143 formation is placed. If NULL is passed, the function returns the amount 1144 of memory that is needed for the requested information. For calls that 1145 return numerical values, the value is in bytes; when requesting these 1146 values, where should point to appropriately aligned memory. For calls 1147 that return strings, the required length is given in code units, not 1148 counting the terminating zero. 1149 1150 When requesting information, the returned value from pcre2_config() is 1151 non-negative on success, or the negative error code PCRE2_ERROR_BADOP- 1152 TION if the value in the first argument is not recognized. The follow- 1153 ing information is available: 1154 1155 PCRE2_CONFIG_BSR 1156 1157 The output is a uint32_t integer whose value indicates what character 1158 sequences the \R escape sequence matches by default. A value of 1159 PCRE2_BSR_UNICODE means that \R matches any Unicode line ending se- 1160 quence; a value of PCRE2_BSR_ANYCRLF means that \R matches only CR, LF, 1161 or CRLF. The default can be overridden when a pattern is compiled. 1162 1163 PCRE2_CONFIG_COMPILED_WIDTHS 1164 1165 The output is a uint32_t integer whose lower bits indicate which code 1166 unit widths were selected when PCRE2 was built. The 1-bit indicates 1167 8-bit support, and the 2-bit and 4-bit indicate 16-bit and 32-bit sup- 1168 port, respectively. 1169 1170 PCRE2_CONFIG_DEPTHLIMIT 1171 1172 The output is a uint32_t integer that gives the default limit for the 1173 depth of nested backtracking in pcre2_match() or the depth of nested 1174 recursions, lookarounds, and atomic groups in pcre2_dfa_match(). Fur- 1175 ther details are given with pcre2_set_depth_limit() above. 1176 1177 PCRE2_CONFIG_HEAPLIMIT 1178 1179 The output is a uint32_t integer that gives, in kibibytes, the default 1180 limit for the amount of heap memory used by pcre2_match() or 1181 pcre2_dfa_match(). Further details are given with 1182 pcre2_set_heap_limit() above. 1183 1184 PCRE2_CONFIG_JIT 1185 1186 The output is a uint32_t integer that is set to one if support for 1187 just-in-time compiling is available; otherwise it is set to zero. 1188 1189 PCRE2_CONFIG_JITTARGET 1190 1191 The where argument should point to a buffer that is at least 48 code 1192 units long. (The exact length required can be found by calling 1193 pcre2_config() with where set to NULL.) The buffer is filled with a 1194 string that contains the name of the architecture for which the JIT 1195 compiler is configured, for example "x86 32bit (little endian + un- 1196 aligned)". If JIT support is not available, PCRE2_ERROR_BADOPTION is 1197 returned, otherwise the number of code units used is returned. This is 1198 the length of the string, plus one unit for the terminating zero. 1199 1200 PCRE2_CONFIG_LINKSIZE 1201 1202 The output is a uint32_t integer that contains the number of bytes used 1203 for internal linkage in compiled regular expressions. When PCRE2 is 1204 configured, the value can be set to 2, 3, or 4, with the default being 1205 2. This is the value that is returned by pcre2_config(). However, when 1206 the 16-bit library is compiled, a value of 3 is rounded up to 4, and 1207 when the 32-bit library is compiled, internal linkages always use 4 1208 bytes, so the configured value is not relevant. 1209 1210 The default value of 2 for the 8-bit and 16-bit libraries is sufficient 1211 for all but the most massive patterns, since it allows the size of the 1212 compiled pattern to be up to 65535 code units. Larger values allow 1213 larger regular expressions to be compiled by those two libraries, but 1214 at the expense of slower matching. 1215 1216 PCRE2_CONFIG_MATCHLIMIT 1217 1218 The output is a uint32_t integer that gives the default match limit for 1219 pcre2_match(). Further details are given with pcre2_set_match_limit() 1220 above. 1221 1222 PCRE2_CONFIG_NEWLINE 1223 1224 The output is a uint32_t integer whose value specifies the default 1225 character sequence that is recognized as meaning "newline". The values 1226 are: 1227 1228 PCRE2_NEWLINE_CR Carriage return (CR) 1229 PCRE2_NEWLINE_LF Linefeed (LF) 1230 PCRE2_NEWLINE_CRLF Carriage return, linefeed (CRLF) 1231 PCRE2_NEWLINE_ANY Any Unicode line ending 1232 PCRE2_NEWLINE_ANYCRLF Any of CR, LF, or CRLF 1233 PCRE2_NEWLINE_NUL The NUL character (binary zero) 1234 1235 The default should normally correspond to the standard sequence for 1236 your operating system. 1237 1238 PCRE2_CONFIG_NEVER_BACKSLASH_C 1239 1240 The output is a uint32_t integer that is set to one if the use of \C 1241 was permanently disabled when PCRE2 was built; otherwise it is set to 1242 zero. 1243 1244 PCRE2_CONFIG_PARENSLIMIT 1245 1246 The output is a uint32_t integer that gives the maximum depth of nest- 1247 ing of parentheses (of any kind) in a pattern. This limit is imposed to 1248 cap the amount of system stack used when a pattern is compiled. It is 1249 specified when PCRE2 is built; the default is 250. This limit does not 1250 take into account the stack that may already be used by the calling ap- 1251 plication. For finer control over compilation stack usage, see 1252 pcre2_set_compile_recursion_guard(). 1253 1254 PCRE2_CONFIG_STACKRECURSE 1255 1256 This parameter is obsolete and should not be used in new code. The out- 1257 put is a uint32_t integer that is always set to zero. 1258 1259 PCRE2_CONFIG_TABLES_LENGTH 1260 1261 The output is a uint32_t integer that gives the length of PCRE2's char- 1262 acter processing tables in bytes. For details of these tables see the 1263 section on locale support below. 1264 1265 PCRE2_CONFIG_UNICODE_VERSION 1266 1267 The where argument should point to a buffer that is at least 24 code 1268 units long. (The exact length required can be found by calling 1269 pcre2_config() with where set to NULL.) If PCRE2 has been compiled 1270 without Unicode support, the buffer is filled with the text "Unicode 1271 not supported". Otherwise, the Unicode version string (for example, 1272 "8.0.0") is inserted. The number of code units used is returned. This 1273 is the length of the string plus one unit for the terminating zero. 1274 1275 PCRE2_CONFIG_UNICODE 1276 1277 The output is a uint32_t integer that is set to one if Unicode support 1278 is available; otherwise it is set to zero. Unicode support implies UTF 1279 support. 1280 1281 PCRE2_CONFIG_VERSION 1282 1283 The where argument should point to a buffer that is at least 24 code 1284 units long. (The exact length required can be found by calling 1285 pcre2_config() with where set to NULL.) The buffer is filled with the 1286 PCRE2 version string, zero-terminated. The number of code units used is 1287 returned. This is the length of the string plus one unit for the termi- 1288 nating zero. 1289 1290 1291COMPILING A PATTERN 1292 1293 pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length, 1294 uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset, 1295 pcre2_compile_context *ccontext); 1296 1297 void pcre2_code_free(pcre2_code *code); 1298 1299 pcre2_code *pcre2_code_copy(const pcre2_code *code); 1300 1301 pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code); 1302 1303 The pcre2_compile() function compiles a pattern into an internal form. 1304 The pattern is defined by a pointer to a string of code units and a 1305 length (in code units). If the pattern is zero-terminated, the length 1306 can be specified as PCRE2_ZERO_TERMINATED. The function returns a 1307 pointer to a block of memory that contains the compiled pattern and re- 1308 lated data, or NULL if an error occurred. 1309 1310 If the compile context argument ccontext is NULL, memory for the com- 1311 piled pattern is obtained by calling malloc(). Otherwise, it is ob- 1312 tained from the same memory function that was used for the compile con- 1313 text. The caller must free the memory by calling pcre2_code_free() when 1314 it is no longer needed. If pcre2_code_free() is called with a NULL ar- 1315 gument, it returns immediately, without doing anything. 1316 1317 The function pcre2_code_copy() makes a copy of the compiled code in new 1318 memory, using the same memory allocator as was used for the original. 1319 However, if the code has been processed by the JIT compiler (see be- 1320 low), the JIT information cannot be copied (because it is position-de- 1321 pendent). The new copy can initially be used only for non-JIT match- 1322 ing, though it can be passed to pcre2_jit_compile() if required. If 1323 pcre2_code_copy() is called with a NULL argument, it returns NULL. 1324 1325 The pcre2_code_copy() function provides a way for individual threads in 1326 a multithreaded application to acquire a private copy of shared com- 1327 piled code. However, it does not make a copy of the character tables 1328 used by the compiled pattern; the new pattern code points to the same 1329 tables as the original code. (See "Locale Support" below for details 1330 of these character tables.) In many applications the same tables are 1331 used throughout, so this behaviour is appropriate. Nevertheless, there 1332 are occasions when a copy of a compiled pattern and the relevant tables 1333 are needed. The pcre2_code_copy_with_tables() provides this facility. 1334 Copies of both the code and the tables are made, with the new code 1335 pointing to the new tables. The memory for the new tables is automati- 1336 cally freed when pcre2_code_free() is called for the new copy of the 1337 compiled code. If pcre2_code_copy_with_tables() is called with a NULL 1338 argument, it returns NULL. 1339 1340 NOTE: When one of the matching functions is called, pointers to the 1341 compiled pattern and the subject string are set in the match data block 1342 so that they can be referenced by the substring extraction functions 1343 after a successful match. After running a match, you must not free a 1344 compiled pattern or a subject string until after all operations on the 1345 match data block have taken place, unless, in the case of the subject 1346 string, you have used the PCRE2_COPY_MATCHED_SUBJECT option, which is 1347 described in the section entitled "Option bits for pcre2_match()" be- 1348 low. 1349 1350 The options argument for pcre2_compile() contains various bit settings 1351 that affect the compilation. It should be zero if none of them are re- 1352 quired. The available options are described below. Some of them (in 1353 particular, those that are compatible with Perl, but some others as 1354 well) can also be set and unset from within the pattern (see the de- 1355 tailed description in the pcre2pattern documentation). 1356 1357 For those options that can be different in different parts of the pat- 1358 tern, the contents of the options argument specifies their settings at 1359 the start of compilation. The PCRE2_ANCHORED, PCRE2_ENDANCHORED, and 1360 PCRE2_NO_UTF_CHECK options can be set at the time of matching as well 1361 as at compile time. 1362 1363 Some additional options and less frequently required compile-time pa- 1364 rameters (for example, the newline setting) can be provided in a com- 1365 pile context (as described above). 1366 1367 If errorcode or erroroffset is NULL, pcre2_compile() returns NULL imme- 1368 diately. Otherwise, the variables to which these point are set to an 1369 error code and an offset (number of code units) within the pattern, re- 1370 spectively, when pcre2_compile() returns NULL because a compilation er- 1371 ror has occurred. 1372 1373 There are nearly 100 positive error codes that pcre2_compile() may re- 1374 turn if it finds an error in the pattern. There are also some negative 1375 error codes that are used for invalid UTF strings when validity check- 1376 ing is in force. These are the same as given by pcre2_match() and 1377 pcre2_dfa_match(), and are described in the pcre2unicode documentation. 1378 There is no separate documentation for the positive error codes, be- 1379 cause the textual error messages that are obtained by calling the 1380 pcre2_get_error_message() function (see "Obtaining a textual error mes- 1381 sage" below) should be self-explanatory. Macro names starting with 1382 PCRE2_ERROR_ are defined for both positive and negative error codes in 1383 pcre2.h. When compilation is successful errorcode is set to a value 1384 that returns the message "no error" if passed to pcre2_get_error_mes- 1385 sage(). 1386 1387 The value returned in erroroffset is an indication of where in the pat- 1388 tern an error occurred. When there is no error, zero is returned. A 1389 non-zero value is not necessarily the furthest point in the pattern 1390 that was read. For example, after the error "lookbehind assertion is 1391 not fixed length", the error offset points to the start of the failing 1392 assertion. For an invalid UTF-8 or UTF-16 string, the offset is that of 1393 the first code unit of the failing character. 1394 1395 Some errors are not detected until the whole pattern has been scanned; 1396 in these cases, the offset passed back is the length of the pattern. 1397 Note that the offset is in code units, not characters, even in a UTF 1398 mode. It may sometimes point into the middle of a UTF-8 or UTF-16 char- 1399 acter. 1400 1401 This code fragment shows a typical straightforward call to pcre2_com- 1402 pile(): 1403 1404 pcre2_code *re; 1405 PCRE2_SIZE erroffset; 1406 int errorcode; 1407 re = pcre2_compile( 1408 "^A.*Z", /* the pattern */ 1409 PCRE2_ZERO_TERMINATED, /* the pattern is zero-terminated */ 1410 0, /* default options */ 1411 &errorcode, /* for error code */ 1412 &erroffset, /* for error offset */ 1413 NULL); /* no compile context */ 1414 1415 1416 Main compile options 1417 1418 The following names for option bits are defined in the pcre2.h header 1419 file: 1420 1421 PCRE2_ANCHORED 1422 1423 If this bit is set, the pattern is forced to be "anchored", that is, it 1424 is constrained to match only at the first matching point in the string 1425 that is being searched (the "subject string"). This effect can also be 1426 achieved by appropriate constructs in the pattern itself, which is the 1427 only way to do it in Perl. 1428 1429 PCRE2_ALLOW_EMPTY_CLASS 1430 1431 By default, for compatibility with Perl, a closing square bracket that 1432 immediately follows an opening one is treated as a data character for 1433 the class. When PCRE2_ALLOW_EMPTY_CLASS is set, it terminates the 1434 class, which therefore contains no characters and so can never match. 1435 1436 PCRE2_ALT_BSUX 1437 1438 This option request alternative handling of three escape sequences, 1439 which makes PCRE2's behaviour more like ECMAscript (aka JavaScript). 1440 When it is set: 1441 1442 (1) \U matches an upper case "U" character; by default \U causes a com- 1443 pile time error (Perl uses \U to upper case subsequent characters). 1444 1445 (2) \u matches a lower case "u" character unless it is followed by four 1446 hexadecimal digits, in which case the hexadecimal number defines the 1447 code point to match. By default, \u causes a compile time error (Perl 1448 uses it to upper case the following character). 1449 1450 (3) \x matches a lower case "x" character unless it is followed by two 1451 hexadecimal digits, in which case the hexadecimal number defines the 1452 code point to match. By default, as in Perl, a hexadecimal number is 1453 always expected after \x, but it may have zero, one, or two digits (so, 1454 for example, \xz matches a binary zero character followed by z). 1455 1456 ECMAscript 6 added additional functionality to \u. This can be accessed 1457 using the PCRE2_EXTRA_ALT_BSUX extra option (see "Extra compile op- 1458 tions" below). Note that this alternative escape handling applies only 1459 to patterns. Neither of these options affects the processing of re- 1460 placement strings passed to pcre2_substitute(). 1461 1462 PCRE2_ALT_CIRCUMFLEX 1463 1464 In multiline mode (when PCRE2_MULTILINE is set), the circumflex 1465 metacharacter matches at the start of the subject (unless PCRE2_NOTBOL 1466 is set), and also after any internal newline. However, it does not 1467 match after a newline at the end of the subject, for compatibility with 1468 Perl. If you want a multiline circumflex also to match after a termi- 1469 nating newline, you must set PCRE2_ALT_CIRCUMFLEX. 1470 1471 PCRE2_ALT_VERBNAMES 1472 1473 By default, for compatibility with Perl, the name in any verb sequence 1474 such as (*MARK:NAME) is any sequence of characters that does not in- 1475 clude a closing parenthesis. The name is not processed in any way, and 1476 it is not possible to include a closing parenthesis in the name. How- 1477 ever, if the PCRE2_ALT_VERBNAMES option is set, normal backslash pro- 1478 cessing is applied to verb names and only an unescaped closing paren- 1479 thesis terminates the name. A closing parenthesis can be included in a 1480 name either as \) or between \Q and \E. If the PCRE2_EXTENDED or 1481 PCRE2_EXTENDED_MORE option is set with PCRE2_ALT_VERBNAMES, unescaped 1482 whitespace in verb names is skipped and #-comments are recognized, ex- 1483 actly as in the rest of the pattern. 1484 1485 PCRE2_AUTO_CALLOUT 1486 1487 If this bit is set, pcre2_compile() automatically inserts callout 1488 items, all with number 255, before each pattern item, except immedi- 1489 ately before or after an explicit callout in the pattern. For discus- 1490 sion of the callout facility, see the pcre2callout documentation. 1491 1492 PCRE2_CASELESS 1493 1494 If this bit is set, letters in the pattern match both upper and lower 1495 case letters in the subject. It is equivalent to Perl's /i option, and 1496 it can be changed within a pattern by a (?i) option setting. If either 1497 PCRE2_UTF or PCRE2_UCP is set, Unicode properties are used for all 1498 characters with more than one other case, and for all characters whose 1499 code points are greater than U+007F. Note that there are two ASCII 1500 characters, K and S, that, in addition to their lower case ASCII equiv- 1501 alents, are case-equivalent with U+212A (Kelvin sign) and U+017F (long 1502 S) respectively. For lower valued characters with only one other case, 1503 a lookup table is used for speed. When neither PCRE2_UTF nor PCRE2_UCP 1504 is set, a lookup table is used for all code points less than 256, and 1505 higher code points (available only in 16-bit or 32-bit mode) are 1506 treated as not having another case. 1507 1508 PCRE2_DOLLAR_ENDONLY 1509 1510 If this bit is set, a dollar metacharacter in the pattern matches only 1511 at the end of the subject string. Without this option, a dollar also 1512 matches immediately before a newline at the end of the string (but not 1513 before any other newlines). The PCRE2_DOLLAR_ENDONLY option is ignored 1514 if PCRE2_MULTILINE is set. There is no equivalent to this option in 1515 Perl, and no way to set it within a pattern. 1516 1517 PCRE2_DOTALL 1518 1519 If this bit is set, a dot metacharacter in the pattern matches any 1520 character, including one that indicates a newline. However, it only 1521 ever matches one character, even if newlines are coded as CRLF. Without 1522 this option, a dot does not match when the current position in the sub- 1523 ject is at a newline. This option is equivalent to Perl's /s option, 1524 and it can be changed within a pattern by a (?s) option setting. A neg- 1525 ative class such as [^a] always matches newline characters, and the \N 1526 escape sequence always matches a non-newline character, independent of 1527 the setting of PCRE2_DOTALL. 1528 1529 PCRE2_DUPNAMES 1530 1531 If this bit is set, names used to identify capture groups need not be 1532 unique. This can be helpful for certain types of pattern when it is 1533 known that only one instance of the named group can ever be matched. 1534 There are more details of named capture groups below; see also the 1535 pcre2pattern documentation. 1536 1537 PCRE2_ENDANCHORED 1538 1539 If this bit is set, the end of any pattern match must be right at the 1540 end of the string being searched (the "subject string"). If the pattern 1541 match succeeds by reaching (*ACCEPT), but does not reach the end of the 1542 subject, the match fails at the current starting point. For unanchored 1543 patterns, a new match is then tried at the next starting point. How- 1544 ever, if the match succeeds by reaching the end of the pattern, but not 1545 the end of the subject, backtracking occurs and an alternative match 1546 may be found. Consider these two patterns: 1547 1548 .(*ACCEPT)|.. 1549 .|.. 1550 1551 If matched against "abc" with PCRE2_ENDANCHORED set, the first matches 1552 "c" whereas the second matches "bc". The effect of PCRE2_ENDANCHORED 1553 can also be achieved by appropriate constructs in the pattern itself, 1554 which is the only way to do it in Perl. 1555 1556 For DFA matching with pcre2_dfa_match(), PCRE2_ENDANCHORED applies only 1557 to the first (that is, the longest) matched string. Other parallel 1558 matches, which are necessarily substrings of the first one, must obvi- 1559 ously end before the end of the subject. 1560 1561 PCRE2_EXTENDED 1562 1563 If this bit is set, most white space characters in the pattern are to- 1564 tally ignored except when escaped or inside a character class. However, 1565 white space is not allowed within sequences such as (?> that introduce 1566 various parenthesized groups, nor within numerical quantifiers such as 1567 {1,3}. Ignorable white space is permitted between an item and a follow- 1568 ing quantifier and between a quantifier and a following + that indi- 1569 cates possessiveness. PCRE2_EXTENDED is equivalent to Perl's /x option, 1570 and it can be changed within a pattern by a (?x) option setting. 1571 1572 When PCRE2 is compiled without Unicode support, PCRE2_EXTENDED recog- 1573 nizes as white space only those characters with code points less than 1574 256 that are flagged as white space in its low-character table. The ta- 1575 ble is normally created by pcre2_maketables(), which uses the isspace() 1576 function to identify space characters. In most ASCII environments, the 1577 relevant characters are those with code points 0x0009 (tab), 0x000A 1578 (linefeed), 0x000B (vertical tab), 0x000C (formfeed), 0x000D (carriage 1579 return), and 0x0020 (space). 1580 1581 When PCRE2 is compiled with Unicode support, in addition to these char- 1582 acters, five more Unicode "Pattern White Space" characters are recog- 1583 nized by PCRE2_EXTENDED. These are U+0085 (next line), U+200E (left-to- 1584 right mark), U+200F (right-to-left mark), U+2028 (line separator), and 1585 U+2029 (paragraph separator). This set of characters is the same as 1586 recognized by Perl's /x option. Note that the horizontal and vertical 1587 space characters that are matched by the \h and \v escapes in patterns 1588 are a much bigger set. 1589 1590 As well as ignoring most white space, PCRE2_EXTENDED also causes char- 1591 acters between an unescaped # outside a character class and the next 1592 newline, inclusive, to be ignored, which makes it possible to include 1593 comments inside complicated patterns. Note that the end of this type of 1594 comment is a literal newline sequence in the pattern; escape sequences 1595 that happen to represent a newline do not count. 1596 1597 Which characters are interpreted as newlines can be specified by a set- 1598 ting in the compile context that is passed to pcre2_compile() or by a 1599 special sequence at the start of the pattern, as described in the sec- 1600 tion entitled "Newline conventions" in the pcre2pattern documentation. 1601 A default is defined when PCRE2 is built. 1602 1603 PCRE2_EXTENDED_MORE 1604 1605 This option has the effect of PCRE2_EXTENDED, but, in addition, un- 1606 escaped space and horizontal tab characters are ignored inside a char- 1607 acter class. Note: only these two characters are ignored, not the full 1608 set of pattern white space characters that are ignored outside a char- 1609 acter class. PCRE2_EXTENDED_MORE is equivalent to Perl's /xx option, 1610 and it can be changed within a pattern by a (?xx) option setting. 1611 1612 PCRE2_FIRSTLINE 1613 1614 If this option is set, the start of an unanchored pattern match must be 1615 before or at the first newline in the subject string following the 1616 start of matching, though the matched text may continue over the new- 1617 line. If startoffset is non-zero, the limiting newline is not necessar- 1618 ily the first newline in the subject. For example, if the subject 1619 string is "abc\nxyz" (where \n represents a single-character newline) a 1620 pattern match for "yz" succeeds with PCRE2_FIRSTLINE if startoffset is 1621 greater than 3. See also PCRE2_USE_OFFSET_LIMIT, which provides a more 1622 general limiting facility. If PCRE2_FIRSTLINE is set with an offset 1623 limit, a match must occur in the first line and also within the offset 1624 limit. In other words, whichever limit comes first is used. 1625 1626 PCRE2_LITERAL 1627 1628 If this option is set, all meta-characters in the pattern are disabled, 1629 and it is treated as a literal string. Matching literal strings with a 1630 regular expression engine is not the most efficient way of doing it. If 1631 you are doing a lot of literal matching and are worried about effi- 1632 ciency, you should consider using other approaches. The only other main 1633 options that are allowed with PCRE2_LITERAL are: PCRE2_ANCHORED, 1634 PCRE2_ENDANCHORED, PCRE2_AUTO_CALLOUT, PCRE2_CASELESS, PCRE2_FIRSTLINE, 1635 PCRE2_MATCH_INVALID_UTF, PCRE2_NO_START_OPTIMIZE, PCRE2_NO_UTF_CHECK, 1636 PCRE2_UTF, and PCRE2_USE_OFFSET_LIMIT. The extra options PCRE2_EX- 1637 TRA_MATCH_LINE and PCRE2_EXTRA_MATCH_WORD are also supported. Any other 1638 options cause an error. 1639 1640 PCRE2_MATCH_INVALID_UTF 1641 1642 This option forces PCRE2_UTF (see below) and also enables support for 1643 matching by pcre2_match() in subject strings that contain invalid UTF 1644 sequences. This facility is not supported for DFA matching. For de- 1645 tails, see the pcre2unicode documentation. 1646 1647 PCRE2_MATCH_UNSET_BACKREF 1648 1649 If this option is set, a backreference to an unset capture group 1650 matches an empty string (by default this causes the current matching 1651 alternative to fail). A pattern such as (\1)(a) succeeds when this op- 1652 tion is set (assuming it can find an "a" in the subject), whereas it 1653 fails by default, for Perl compatibility. Setting this option makes 1654 PCRE2 behave more like ECMAscript (aka JavaScript). 1655 1656 PCRE2_MULTILINE 1657 1658 By default, for the purposes of matching "start of line" and "end of 1659 line", PCRE2 treats the subject string as consisting of a single line 1660 of characters, even if it actually contains newlines. The "start of 1661 line" metacharacter (^) matches only at the start of the string, and 1662 the "end of line" metacharacter ($) matches only at the end of the 1663 string, or before a terminating newline (except when PCRE2_DOLLAR_EN- 1664 DONLY is set). Note, however, that unless PCRE2_DOTALL is set, the "any 1665 character" metacharacter (.) does not match at a newline. This behav- 1666 iour (for ^, $, and dot) is the same as Perl. 1667 1668 When PCRE2_MULTILINE it is set, the "start of line" and "end of line" 1669 constructs match immediately following or immediately before internal 1670 newlines in the subject string, respectively, as well as at the very 1671 start and end. This is equivalent to Perl's /m option, and it can be 1672 changed within a pattern by a (?m) option setting. Note that the "start 1673 of line" metacharacter does not match after a newline at the end of the 1674 subject, for compatibility with Perl. However, you can change this by 1675 setting the PCRE2_ALT_CIRCUMFLEX option. If there are no newlines in a 1676 subject string, or no occurrences of ^ or $ in a pattern, setting 1677 PCRE2_MULTILINE has no effect. 1678 1679 PCRE2_NEVER_BACKSLASH_C 1680 1681 This option locks out the use of \C in the pattern that is being com- 1682 piled. This escape can cause unpredictable behaviour in UTF-8 or 1683 UTF-16 modes, because it may leave the current matching point in the 1684 middle of a multi-code-unit character. This option may be useful in ap- 1685 plications that process patterns from external sources. Note that there 1686 is also a build-time option that permanently locks out the use of \C. 1687 1688 PCRE2_NEVER_UCP 1689 1690 This option locks out the use of Unicode properties for handling \B, 1691 \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes, as 1692 described for the PCRE2_UCP option below. In particular, it prevents 1693 the creator of the pattern from enabling this facility by starting the 1694 pattern with (*UCP). This option may be useful in applications that 1695 process patterns from external sources. The option combination PCRE_UCP 1696 and PCRE_NEVER_UCP causes an error. 1697 1698 PCRE2_NEVER_UTF 1699 1700 This option locks out interpretation of the pattern as UTF-8, UTF-16, 1701 or UTF-32, depending on which library is in use. In particular, it pre- 1702 vents the creator of the pattern from switching to UTF interpretation 1703 by starting the pattern with (*UTF). This option may be useful in ap- 1704 plications that process patterns from external sources. The combination 1705 of PCRE2_UTF and PCRE2_NEVER_UTF causes an error. 1706 1707 PCRE2_NO_AUTO_CAPTURE 1708 1709 If this option is set, it disables the use of numbered capturing paren- 1710 theses in the pattern. Any opening parenthesis that is not followed by 1711 ? behaves as if it were followed by ?: but named parentheses can still 1712 be used for capturing (and they acquire numbers in the usual way). This 1713 is the same as Perl's /n option. Note that, when this option is set, 1714 references to capture groups (backreferences or recursion/subroutine 1715 calls) may only refer to named groups, though the reference can be by 1716 name or by number. 1717 1718 PCRE2_NO_AUTO_POSSESS 1719 1720 If this option is set, it disables "auto-possessification", which is an 1721 optimization that, for example, turns a+b into a++b in order to avoid 1722 backtracks into a+ that can never be successful. However, if callouts 1723 are in use, auto-possessification means that some callouts are never 1724 taken. You can set this option if you want the matching functions to do 1725 a full unoptimized search and run all the callouts, but it is mainly 1726 provided for testing purposes. 1727 1728 PCRE2_NO_DOTSTAR_ANCHOR 1729 1730 If this option is set, it disables an optimization that is applied when 1731 .* is the first significant item in a top-level branch of a pattern, 1732 and all the other branches also start with .* or with \A or \G or ^. 1733 The optimization is automatically disabled for .* if it is inside an 1734 atomic group or a capture group that is the subject of a backreference, 1735 or if the pattern contains (*PRUNE) or (*SKIP). When the optimization 1736 is not disabled, such a pattern is automatically anchored if 1737 PCRE2_DOTALL is set for all the .* items and PCRE2_MULTILINE is not set 1738 for any ^ items. Otherwise, the fact that any match must start either 1739 at the start of the subject or following a newline is remembered. Like 1740 other optimizations, this can cause callouts to be skipped. 1741 1742 PCRE2_NO_START_OPTIMIZE 1743 1744 This is an option whose main effect is at matching time. It does not 1745 change what pcre2_compile() generates, but it does affect the output of 1746 the JIT compiler. 1747 1748 There are a number of optimizations that may occur at the start of a 1749 match, in order to speed up the process. For example, if it is known 1750 that an unanchored match must start with a specific code unit value, 1751 the matching code searches the subject for that value, and fails imme- 1752 diately if it cannot find it, without actually running the main match- 1753 ing function. This means that a special item such as (*COMMIT) at the 1754 start of a pattern is not considered until after a suitable starting 1755 point for the match has been found. Also, when callouts or (*MARK) 1756 items are in use, these "start-up" optimizations can cause them to be 1757 skipped if the pattern is never actually used. The start-up optimiza- 1758 tions are in effect a pre-scan of the subject that takes place before 1759 the pattern is run. 1760 1761 The PCRE2_NO_START_OPTIMIZE option disables the start-up optimizations, 1762 possibly causing performance to suffer, but ensuring that in cases 1763 where the result is "no match", the callouts do occur, and that items 1764 such as (*COMMIT) and (*MARK) are considered at every possible starting 1765 position in the subject string. 1766 1767 Setting PCRE2_NO_START_OPTIMIZE may change the outcome of a matching 1768 operation. Consider the pattern 1769 1770 (*COMMIT)ABC 1771 1772 When this is compiled, PCRE2 records the fact that a match must start 1773 with the character "A". Suppose the subject string is "DEFABC". The 1774 start-up optimization scans along the subject, finds "A" and runs the 1775 first match attempt from there. The (*COMMIT) item means that the pat- 1776 tern must match the current starting position, which in this case, it 1777 does. However, if the same match is run with PCRE2_NO_START_OPTIMIZE 1778 set, the initial scan along the subject string does not happen. The 1779 first match attempt is run starting from "D" and when this fails, 1780 (*COMMIT) prevents any further matches being tried, so the overall re- 1781 sult is "no match". 1782 1783 As another start-up optimization makes use of a minimum length for a 1784 matching subject, which is recorded when possible. Consider the pattern 1785 1786 (*MARK:1)B(*MARK:2)(X|Y) 1787 1788 The minimum length for a match is two characters. If the subject is 1789 "XXBB", the "starting character" optimization skips "XX", then tries to 1790 match "BB", which is long enough. In the process, (*MARK:2) is encoun- 1791 tered and remembered. When the match attempt fails, the next "B" is 1792 found, but there is only one character left, so there are no more at- 1793 tempts, and "no match" is returned with the "last mark seen" set to 1794 "2". If NO_START_OPTIMIZE is set, however, matches are tried at every 1795 possible starting position, including at the end of the subject, where 1796 (*MARK:1) is encountered, but there is no "B", so the "last mark seen" 1797 that is returned is "1". In this case, the optimizations do not affect 1798 the overall match result, which is still "no match", but they do affect 1799 the auxiliary information that is returned. 1800 1801 PCRE2_NO_UTF_CHECK 1802 1803 When PCRE2_UTF is set, the validity of the pattern as a UTF string is 1804 automatically checked. There are discussions about the validity of 1805 UTF-8 strings, UTF-16 strings, and UTF-32 strings in the pcre2unicode 1806 document. If an invalid UTF sequence is found, pcre2_compile() returns 1807 a negative error code. 1808 1809 If you know that your pattern is a valid UTF string, and you want to 1810 skip this check for performance reasons, you can set the 1811 PCRE2_NO_UTF_CHECK option. When it is set, the effect of passing an in- 1812 valid UTF string as a pattern is undefined. It may cause your program 1813 to crash or loop. 1814 1815 Note that this option can also be passed to pcre2_match() and 1816 pcre2_dfa_match(), to suppress UTF validity checking of the subject 1817 string. 1818 1819 Note also that setting PCRE2_NO_UTF_CHECK at compile time does not dis- 1820 able the error that is given if an escape sequence for an invalid Uni- 1821 code code point is encountered in the pattern. In particular, the so- 1822 called "surrogate" code points (0xd800 to 0xdfff) are invalid. If you 1823 want to allow escape sequences such as \x{d800} you can set the 1824 PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES extra option, as described in the 1825 section entitled "Extra compile options" below. However, this is pos- 1826 sible only in UTF-8 and UTF-32 modes, because these values are not rep- 1827 resentable in UTF-16. 1828 1829 PCRE2_UCP 1830 1831 This option has two effects. Firstly, it change the way PCRE2 processes 1832 \B, \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character 1833 classes. By default, only ASCII characters are recognized, but if 1834 PCRE2_UCP is set, Unicode properties are used instead to classify char- 1835 acters. More details are given in the section on generic character 1836 types in the pcre2pattern page. If you set PCRE2_UCP, matching one of 1837 the items it affects takes much longer. 1838 1839 The second effect of PCRE2_UCP is to force the use of Unicode proper- 1840 ties for upper/lower casing operations on characters with code points 1841 greater than 127, even when PCRE2_UTF is not set. This makes it possi- 1842 ble, for example, to process strings in the 16-bit UCS-2 code. This op- 1843 tion is available only if PCRE2 has been compiled with Unicode support 1844 (which is the default). 1845 1846 PCRE2_UNGREEDY 1847 1848 This option inverts the "greediness" of the quantifiers so that they 1849 are not greedy by default, but become greedy if followed by "?". It is 1850 not compatible with Perl. It can also be set by a (?U) option setting 1851 within the pattern. 1852 1853 PCRE2_USE_OFFSET_LIMIT 1854 1855 This option must be set for pcre2_compile() if pcre2_set_offset_limit() 1856 is going to be used to set a non-default offset limit in a match con- 1857 text for matches that use this pattern. An error is generated if an 1858 offset limit is set without this option. For more details, see the de- 1859 scription of pcre2_set_offset_limit() in the section that describes 1860 match contexts. See also the PCRE2_FIRSTLINE option above. 1861 1862 PCRE2_UTF 1863 1864 This option causes PCRE2 to regard both the pattern and the subject 1865 strings that are subsequently processed as strings of UTF characters 1866 instead of single-code-unit strings. It is available when PCRE2 is 1867 built to include Unicode support (which is the default). If Unicode 1868 support is not available, the use of this option provokes an error. De- 1869 tails of how PCRE2_UTF changes the behaviour of PCRE2 are given in the 1870 pcre2unicode page. In particular, note that it changes the way 1871 PCRE2_CASELESS handles characters with code points greater than 127. 1872 1873 Extra compile options 1874 1875 The option bits that can be set in a compile context by calling the 1876 pcre2_set_compile_extra_options() function are as follows: 1877 1878 PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK 1879 1880 Since release 10.38 PCRE2 has forbidden the use of \K within lookaround 1881 assertions, following Perl's lead. This option is provided to re-enable 1882 the previous behaviour (act in positive lookarounds, ignore in negative 1883 ones) in case anybody is relying on it. 1884 1885 PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES 1886 1887 This option applies when compiling a pattern in UTF-8 or UTF-32 mode. 1888 It is forbidden in UTF-16 mode, and ignored in non-UTF modes. Unicode 1889 "surrogate" code points in the range 0xd800 to 0xdfff are used in pairs 1890 in UTF-16 to encode code points with values in the range 0x10000 to 1891 0x10ffff. The surrogates cannot therefore be represented in UTF-16. 1892 They can be represented in UTF-8 and UTF-32, but are defined as invalid 1893 code points, and cause errors if encountered in a UTF-8 or UTF-32 1894 string that is being checked for validity by PCRE2. 1895 1896 These values also cause errors if encountered in escape sequences such 1897 as \x{d912} within a pattern. However, it seems that some applications, 1898 when using PCRE2 to check for unwanted characters in UTF-8 strings, ex- 1899 plicitly test for the surrogates using escape sequences. The 1900 PCRE2_NO_UTF_CHECK option does not disable the error that occurs, be- 1901 cause it applies only to the testing of input strings for UTF validity. 1902 1903 If the extra option PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES is set, surro- 1904 gate code point values in UTF-8 and UTF-32 patterns no longer provoke 1905 errors and are incorporated in the compiled pattern. However, they can 1906 only match subject characters if the matching function is called with 1907 PCRE2_NO_UTF_CHECK set. 1908 1909 PCRE2_EXTRA_ALT_BSUX 1910 1911 The original option PCRE2_ALT_BSUX causes PCRE2 to process \U, \u, and 1912 \x in the way that ECMAscript (aka JavaScript) does. Additional func- 1913 tionality was defined by ECMAscript 6; setting PCRE2_EXTRA_ALT_BSUX has 1914 the effect of PCRE2_ALT_BSUX, but in addition it recognizes \u{hhh..} 1915 as a hexadecimal character code, where hhh.. is any number of hexadeci- 1916 mal digits. 1917 1918 PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL 1919 1920 This is a dangerous option. Use with care. By default, an unrecognized 1921 escape such as \j or a malformed one such as \x{2z} causes a compile- 1922 time error when detected by pcre2_compile(). Perl is somewhat inconsis- 1923 tent in handling such items: for example, \j is treated as a literal 1924 "j", and non-hexadecimal digits in \x{} are just ignored, though warn- 1925 ings are given in both cases if Perl's warning switch is enabled. How- 1926 ever, a malformed octal number after \o{ always causes an error in 1927 Perl. 1928 1929 If the PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL extra option is passed to 1930 pcre2_compile(), all unrecognized or malformed escape sequences are 1931 treated as single-character escapes. For example, \j is a literal "j" 1932 and \x{2z} is treated as the literal string "x{2z}". Setting this op- 1933 tion means that typos in patterns may go undetected and have unexpected 1934 results. Also note that a sequence such as [\N{] is interpreted as a 1935 malformed attempt at [\N{...}] and so is treated as [N{] whereas [\N] 1936 gives an error because an unqualified \N is a valid escape sequence but 1937 is not supported in a character class. To reiterate: this is a danger- 1938 ous option. Use with great care. 1939 1940 PCRE2_EXTRA_ESCAPED_CR_IS_LF 1941 1942 There are some legacy applications where the escape sequence \r in a 1943 pattern is expected to match a newline. If this option is set, \r in a 1944 pattern is converted to \n so that it matches a LF (linefeed) instead 1945 of a CR (carriage return) character. The option does not affect a lit- 1946 eral CR in the pattern, nor does it affect CR specified as an explicit 1947 code point such as \x{0D}. 1948 1949 PCRE2_EXTRA_MATCH_LINE 1950 1951 This option is provided for use by the -x option of pcre2grep. It 1952 causes the pattern only to match complete lines. This is achieved by 1953 automatically inserting the code for "^(?:" at the start of the com- 1954 piled pattern and ")$" at the end. Thus, when PCRE2_MULTILINE is set, 1955 the matched line may be in the middle of the subject string. This op- 1956 tion can be used with PCRE2_LITERAL. 1957 1958 PCRE2_EXTRA_MATCH_WORD 1959 1960 This option is provided for use by the -w option of pcre2grep. It 1961 causes the pattern only to match strings that have a word boundary at 1962 the start and the end. This is achieved by automatically inserting the 1963 code for "\b(?:" at the start of the compiled pattern and ")\b" at the 1964 end. The option may be used with PCRE2_LITERAL. However, it is ignored 1965 if PCRE2_EXTRA_MATCH_LINE is also set. 1966 1967 1968JUST-IN-TIME (JIT) COMPILATION 1969 1970 int pcre2_jit_compile(pcre2_code *code, uint32_t options); 1971 1972 int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject, 1973 PCRE2_SIZE length, PCRE2_SIZE startoffset, 1974 uint32_t options, pcre2_match_data *match_data, 1975 pcre2_match_context *mcontext); 1976 1977 void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext); 1978 1979 pcre2_jit_stack *pcre2_jit_stack_create(PCRE2_SIZE startsize, 1980 PCRE2_SIZE maxsize, pcre2_general_context *gcontext); 1981 1982 void pcre2_jit_stack_assign(pcre2_match_context *mcontext, 1983 pcre2_jit_callback callback_function, void *callback_data); 1984 1985 void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack); 1986 1987 These functions provide support for JIT compilation, which, if the 1988 just-in-time compiler is available, further processes a compiled pat- 1989 tern into machine code that executes much faster than the pcre2_match() 1990 interpretive matching function. Full details are given in the pcre2jit 1991 documentation. 1992 1993 JIT compilation is a heavyweight optimization. It can take some time 1994 for patterns to be analyzed, and for one-off matches and simple pat- 1995 terns the benefit of faster execution might be offset by a much slower 1996 compilation time. Most (but not all) patterns can be optimized by the 1997 JIT compiler. 1998 1999 2000LOCALE SUPPORT 2001 2002 const uint8_t *pcre2_maketables(pcre2_general_context *gcontext); 2003 2004 void pcre2_maketables_free(pcre2_general_context *gcontext, 2005 const uint8_t *tables); 2006 2007 PCRE2 handles caseless matching, and determines whether characters are 2008 letters, digits, or whatever, by reference to a set of tables, indexed 2009 by character code point. However, this applies only to characters whose 2010 code points are less than 256. By default, higher-valued code points 2011 never match escapes such as \w or \d. 2012 2013 When PCRE2 is built with Unicode support (the default), certain Unicode 2014 character properties can be tested with \p and \P, or, alternatively, 2015 the PCRE2_UCP option can be set when a pattern is compiled; this causes 2016 \w and friends to use Unicode property support instead of the built-in 2017 tables. PCRE2_UCP also causes upper/lower casing operations on charac- 2018 ters with code points greater than 127 to use Unicode properties. These 2019 effects apply even when PCRE2_UTF is not set. 2020 2021 The use of locales with Unicode is discouraged. If you are handling 2022 characters with code points greater than 127, you should either use 2023 Unicode support, or use locales, but not try to mix the two. 2024 2025 PCRE2 contains a built-in set of character tables that are used by de- 2026 fault. These are sufficient for many applications. Normally, the in- 2027 ternal tables recognize only ASCII characters. However, when PCRE2 is 2028 built, it is possible to cause the internal tables to be rebuilt in the 2029 default "C" locale of the local system, which may cause them to be dif- 2030 ferent. 2031 2032 The built-in tables can be overridden by tables supplied by the appli- 2033 cation that calls PCRE2. These may be created in a different locale 2034 from the default. As more and more applications change to using Uni- 2035 code, the need for this locale support is expected to die away. 2036 2037 External tables are built by calling the pcre2_maketables() function, 2038 in the relevant locale. The only argument to this function is a general 2039 context, which can be used to pass a custom memory allocator. If the 2040 argument is NULL, the system malloc() is used. The result can be passed 2041 to pcre2_compile() as often as necessary, by creating a compile context 2042 and calling pcre2_set_character_tables() to set the tables pointer 2043 therein. 2044 2045 For example, to build and use tables that are appropriate for the 2046 French locale (where accented characters with values greater than 127 2047 are treated as letters), the following code could be used: 2048 2049 setlocale(LC_CTYPE, "fr_FR"); 2050 tables = pcre2_maketables(NULL); 2051 ccontext = pcre2_compile_context_create(NULL); 2052 pcre2_set_character_tables(ccontext, tables); 2053 re = pcre2_compile(..., ccontext); 2054 2055 The locale name "fr_FR" is used on Linux and other Unix-like systems; 2056 if you are using Windows, the name for the French locale is "french". 2057 2058 The pointer that is passed (via the compile context) to pcre2_compile() 2059 is saved with the compiled pattern, and the same tables are used by the 2060 matching functions. Thus, for any single pattern, compilation and 2061 matching both happen in the same locale, but different patterns can be 2062 processed in different locales. 2063 2064 It is the caller's responsibility to ensure that the memory containing 2065 the tables remains available while they are still in use. When they are 2066 no longer needed, you can discard them using pcre2_maketables_free(), 2067 which should pass as its first parameter the same global context that 2068 was used to create the tables. 2069 2070 Saving locale tables 2071 2072 The tables described above are just a sequence of binary bytes, which 2073 makes them independent of hardware characteristics such as endianness 2074 or whether the processor is 32-bit or 64-bit. A copy of the result of 2075 pcre2_maketables() can therefore be saved in a file or elsewhere and 2076 re-used later, even in a different program or on another computer. The 2077 size of the tables (number of bytes) must be obtained by calling 2078 pcre2_config() with the PCRE2_CONFIG_TABLES_LENGTH option because 2079 pcre2_maketables() does not return this value. Note that the 2080 pcre2_dftables program, which is part of the PCRE2 build system, can be 2081 used stand-alone to create a file that contains a set of binary tables. 2082 See the pcre2build documentation for details. 2083 2084 2085INFORMATION ABOUT A COMPILED PATTERN 2086 2087 int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where); 2088 2089 The pcre2_pattern_info() function returns general information about a 2090 compiled pattern. For information about callouts, see the next section. 2091 The first argument for pcre2_pattern_info() is a pointer to the com- 2092 piled pattern. The second argument specifies which piece of information 2093 is required, and the third argument is a pointer to a variable to re- 2094 ceive the data. If the third argument is NULL, the first argument is 2095 ignored, and the function returns the size in bytes of the variable 2096 that is required for the information requested. Otherwise, the yield of 2097 the function is zero for success, or one of the following negative num- 2098 bers: 2099 2100 PCRE2_ERROR_NULL the argument code was NULL 2101 PCRE2_ERROR_BADMAGIC the "magic number" was not found 2102 PCRE2_ERROR_BADOPTION the value of what was invalid 2103 PCRE2_ERROR_UNSET the requested field is not set 2104 2105 The "magic number" is placed at the start of each compiled pattern as a 2106 simple check against passing an arbitrary memory pointer. Here is a 2107 typical call of pcre2_pattern_info(), to obtain the length of the com- 2108 piled pattern: 2109 2110 int rc; 2111 size_t length; 2112 rc = pcre2_pattern_info( 2113 re, /* result of pcre2_compile() */ 2114 PCRE2_INFO_SIZE, /* what is required */ 2115 &length); /* where to put the data */ 2116 2117 The possible values for the second argument are defined in pcre2.h, and 2118 are as follows: 2119 2120 PCRE2_INFO_ALLOPTIONS 2121 PCRE2_INFO_ARGOPTIONS 2122 PCRE2_INFO_EXTRAOPTIONS 2123 2124 Return copies of the pattern's options. The third argument should point 2125 to a uint32_t variable. PCRE2_INFO_ARGOPTIONS returns exactly the op- 2126 tions that were passed to pcre2_compile(), whereas PCRE2_INFO_ALLOP- 2127 TIONS returns the compile options as modified by any top-level (*XXX) 2128 option settings such as (*UTF) at the start of the pattern itself. 2129 PCRE2_INFO_EXTRAOPTIONS returns the extra options that were set in the 2130 compile context by calling the pcre2_set_compile_extra_options() func- 2131 tion. 2132 2133 For example, if the pattern /(*UTF)abc/ is compiled with the PCRE2_EX- 2134 TENDED option, the result for PCRE2_INFO_ALLOPTIONS is PCRE2_EXTENDED 2135 and PCRE2_UTF. Option settings such as (?i) that can change within a 2136 pattern do not affect the result of PCRE2_INFO_ALLOPTIONS, even if they 2137 appear right at the start of the pattern. (This was different in some 2138 earlier releases.) 2139 2140 A pattern compiled without PCRE2_ANCHORED is automatically anchored by 2141 PCRE2 if the first significant item in every top-level branch is one of 2142 the following: 2143 2144 ^ unless PCRE2_MULTILINE is set 2145 \A always 2146 \G always 2147 .* sometimes - see below 2148 2149 When .* is the first significant item, anchoring is possible only when 2150 all the following are true: 2151 2152 .* is not in an atomic group 2153 .* is not in a capture group that is the subject 2154 of a backreference 2155 PCRE2_DOTALL is in force for .* 2156 Neither (*PRUNE) nor (*SKIP) appears in the pattern 2157 PCRE2_NO_DOTSTAR_ANCHOR is not set 2158 2159 For patterns that are auto-anchored, the PCRE2_ANCHORED bit is set in 2160 the options returned for PCRE2_INFO_ALLOPTIONS. 2161 2162 PCRE2_INFO_BACKREFMAX 2163 2164 Return the number of the highest backreference in the pattern. The 2165 third argument should point to a uint32_t variable. Named capture 2166 groups acquire numbers as well as names, and these count towards the 2167 highest backreference. Backreferences such as \4 or \g{12} match the 2168 captured characters of the given group, but in addition, the check that 2169 a capture group is set in a conditional group such as (?(3)a|b) is also 2170 a backreference. Zero is returned if there are no backreferences. 2171 2172 PCRE2_INFO_BSR 2173 2174 The output is a uint32_t integer whose value indicates what character 2175 sequences the \R escape sequence matches. A value of PCRE2_BSR_UNICODE 2176 means that \R matches any Unicode line ending sequence; a value of 2177 PCRE2_BSR_ANYCRLF means that \R matches only CR, LF, or CRLF. 2178 2179 PCRE2_INFO_CAPTURECOUNT 2180 2181 Return the highest capture group number in the pattern. In patterns 2182 where (?| is not used, this is also the total number of capture groups. 2183 The third argument should point to a uint32_t variable. 2184 2185 PCRE2_INFO_DEPTHLIMIT 2186 2187 If the pattern set a backtracking depth limit by including an item of 2188 the form (*LIMIT_DEPTH=nnnn) at the start, the value is returned. The 2189 third argument should point to a uint32_t integer. If no such value has 2190 been set, the call to pcre2_pattern_info() returns the error PCRE2_ER- 2191 ROR_UNSET. Note that this limit will only be used during matching if it 2192 is less than the limit set or defaulted by the caller of the match 2193 function. 2194 2195 PCRE2_INFO_FIRSTBITMAP 2196 2197 In the absence of a single first code unit for a non-anchored pattern, 2198 pcre2_compile() may construct a 256-bit table that defines a fixed set 2199 of values for the first code unit in any match. For example, a pattern 2200 that starts with [abc] results in a table with three bits set. When 2201 code unit values greater than 255 are supported, the flag bit for 255 2202 means "any code unit of value 255 or above". If such a table was con- 2203 structed, a pointer to it is returned. Otherwise NULL is returned. The 2204 third argument should point to a const uint8_t * variable. 2205 2206 PCRE2_INFO_FIRSTCODETYPE 2207 2208 Return information about the first code unit of any matched string, for 2209 a non-anchored pattern. The third argument should point to a uint32_t 2210 variable. If there is a fixed first value, for example, the letter "c" 2211 from a pattern such as (cat|cow|coyote), 1 is returned, and the value 2212 can be retrieved using PCRE2_INFO_FIRSTCODEUNIT. If there is no fixed 2213 first value, but it is known that a match can occur only at the start 2214 of the subject or following a newline in the subject, 2 is returned. 2215 Otherwise, and for anchored patterns, 0 is returned. 2216 2217 PCRE2_INFO_FIRSTCODEUNIT 2218 2219 Return the value of the first code unit of any matched string for a 2220 pattern where PCRE2_INFO_FIRSTCODETYPE returns 1; otherwise return 0. 2221 The third argument should point to a uint32_t variable. In the 8-bit 2222 library, the value is always less than 256. In the 16-bit library the 2223 value can be up to 0xffff. In the 32-bit library in UTF-32 mode the 2224 value can be up to 0x10ffff, and up to 0xffffffff when not using UTF-32 2225 mode. 2226 2227 PCRE2_INFO_FRAMESIZE 2228 2229 Return the size (in bytes) of the data frames that are used to remember 2230 backtracking positions when the pattern is processed by pcre2_match() 2231 without the use of JIT. The third argument should point to a size_t 2232 variable. The frame size depends on the number of capturing parentheses 2233 in the pattern. Each additional capture group adds two PCRE2_SIZE vari- 2234 ables. 2235 2236 PCRE2_INFO_HASBACKSLASHC 2237 2238 Return 1 if the pattern contains any instances of \C, otherwise 0. The 2239 third argument should point to a uint32_t variable. 2240 2241 PCRE2_INFO_HASCRORLF 2242 2243 Return 1 if the pattern contains any explicit matches for CR or LF 2244 characters, otherwise 0. The third argument should point to a uint32_t 2245 variable. An explicit match is either a literal CR or LF character, or 2246 \r or \n or one of the equivalent hexadecimal or octal escape se- 2247 quences. 2248 2249 PCRE2_INFO_HEAPLIMIT 2250 2251 If the pattern set a heap memory limit by including an item of the form 2252 (*LIMIT_HEAP=nnnn) at the start, the value is returned. The third argu- 2253 ment should point to a uint32_t integer. If no such value has been set, 2254 the call to pcre2_pattern_info() returns the error PCRE2_ERROR_UNSET. 2255 Note that this limit will only be used during matching if it is less 2256 than the limit set or defaulted by the caller of the match function. 2257 2258 PCRE2_INFO_JCHANGED 2259 2260 Return 1 if the (?J) or (?-J) option setting is used in the pattern, 2261 otherwise 0. The third argument should point to a uint32_t variable. 2262 (?J) and (?-J) set and unset the local PCRE2_DUPNAMES option, respec- 2263 tively. 2264 2265 PCRE2_INFO_JITSIZE 2266 2267 If the compiled pattern was successfully processed by pcre2_jit_com- 2268 pile(), return the size of the JIT compiled code, otherwise return 2269 zero. The third argument should point to a size_t variable. 2270 2271 PCRE2_INFO_LASTCODETYPE 2272 2273 Returns 1 if there is a rightmost literal code unit that must exist in 2274 any matched string, other than at its start. The third argument should 2275 point to a uint32_t variable. If there is no such value, 0 is returned. 2276 When 1 is returned, the code unit value itself can be retrieved using 2277 PCRE2_INFO_LASTCODEUNIT. For anchored patterns, a last literal value is 2278 recorded only if it follows something of variable length. For example, 2279 for the pattern /^a\d+z\d+/ the returned value is 1 (with "z" returned 2280 from PCRE2_INFO_LASTCODEUNIT), but for /^a\dz\d/ the returned value is 2281 0. 2282 2283 PCRE2_INFO_LASTCODEUNIT 2284 2285 Return the value of the rightmost literal code unit that must exist in 2286 any matched string, other than at its start, for a pattern where 2287 PCRE2_INFO_LASTCODETYPE returns 1. Otherwise, return 0. The third argu- 2288 ment should point to a uint32_t variable. 2289 2290 PCRE2_INFO_MATCHEMPTY 2291 2292 Return 1 if the pattern might match an empty string, otherwise 0. The 2293 third argument should point to a uint32_t variable. When a pattern con- 2294 tains recursive subroutine calls it is not always possible to determine 2295 whether or not it can match an empty string. PCRE2 takes a cautious ap- 2296 proach and returns 1 in such cases. 2297 2298 PCRE2_INFO_MATCHLIMIT 2299 2300 If the pattern set a match limit by including an item of the form 2301 (*LIMIT_MATCH=nnnn) at the start, the value is returned. The third ar- 2302 gument should point to a uint32_t integer. If no such value has been 2303 set, the call to pcre2_pattern_info() returns the error PCRE2_ERROR_UN- 2304 SET. Note that this limit will only be used during matching if it is 2305 less than the limit set or defaulted by the caller of the match func- 2306 tion. 2307 2308 PCRE2_INFO_MAXLOOKBEHIND 2309 2310 A lookbehind assertion moves back a certain number of characters (not 2311 code units) when it starts to process each of its branches. This re- 2312 quest returns the largest of these backward moves. The third argument 2313 should point to a uint32_t integer. The simple assertions \b and \B re- 2314 quire a one-character lookbehind and cause PCRE2_INFO_MAXLOOKBEHIND to 2315 return 1 in the absence of anything longer. \A also registers a one- 2316 character lookbehind, though it does not actually inspect the previous 2317 character. 2318 2319 Note that this information is useful for multi-segment matching only if 2320 the pattern contains no nested lookbehinds. For example, the pattern 2321 (?<=a(?<=ba)c) returns a maximum lookbehind of 2, but when it is pro- 2322 cessed, the first lookbehind moves back by two characters, matches one 2323 character, then the nested lookbehind also moves back by two charac- 2324 ters. This puts the matching point three characters earlier than it was 2325 at the start. PCRE2_INFO_MAXLOOKBEHIND is really only useful as a de- 2326 bugging tool. See the pcre2partial documentation for a discussion of 2327 multi-segment matching. 2328 2329 PCRE2_INFO_MINLENGTH 2330 2331 If a minimum length for matching subject strings was computed, its 2332 value is returned. Otherwise the returned value is 0. This value is not 2333 computed when PCRE2_NO_START_OPTIMIZE is set. The value is a number of 2334 characters, which in UTF mode may be different from the number of code 2335 units. The third argument should point to a uint32_t variable. The 2336 value is a lower bound to the length of any matching string. There may 2337 not be any strings of that length that do actually match, but every 2338 string that does match is at least that long. 2339 2340 PCRE2_INFO_NAMECOUNT 2341 PCRE2_INFO_NAMEENTRYSIZE 2342 PCRE2_INFO_NAMETABLE 2343 2344 PCRE2 supports the use of named as well as numbered capturing parenthe- 2345 ses. The names are just an additional way of identifying the parenthe- 2346 ses, which still acquire numbers. Several convenience functions such as 2347 pcre2_substring_get_byname() are provided for extracting captured sub- 2348 strings by name. It is also possible to extract the data directly, by 2349 first converting the name to a number in order to access the correct 2350 pointers in the output vector (described with pcre2_match() below). To 2351 do the conversion, you need to use the name-to-number map, which is de- 2352 scribed by these three values. 2353 2354 The map consists of a number of fixed-size entries. PCRE2_INFO_NAME- 2355 COUNT gives the number of entries, and PCRE2_INFO_NAMEENTRYSIZE gives 2356 the size of each entry in code units; both of these return a uint32_t 2357 value. The entry size depends on the length of the longest name. 2358 2359 PCRE2_INFO_NAMETABLE returns a pointer to the first entry of the table. 2360 This is a PCRE2_SPTR pointer to a block of code units. In the 8-bit li- 2361 brary, the first two bytes of each entry are the number of the captur- 2362 ing parenthesis, most significant byte first. In the 16-bit library, 2363 the pointer points to 16-bit code units, the first of which contains 2364 the parenthesis number. In the 32-bit library, the pointer points to 2365 32-bit code units, the first of which contains the parenthesis number. 2366 The rest of the entry is the corresponding name, zero terminated. 2367 2368 The names are in alphabetical order. If (?| is used to create multiple 2369 capture groups with the same number, as described in the section on du- 2370 plicate group numbers in the pcre2pattern page, the groups may be given 2371 the same name, but there is only one entry in the table. Different 2372 names for groups of the same number are not permitted. 2373 2374 Duplicate names for capture groups with different numbers are permit- 2375 ted, but only if PCRE2_DUPNAMES is set. They appear in the table in the 2376 order in which they were found in the pattern. In the absence of (?| 2377 this is the order of increasing number; when (?| is used this is not 2378 necessarily the case because later capture groups may have lower num- 2379 bers. 2380 2381 As a simple example of the name/number table, consider the following 2382 pattern after compilation by the 8-bit library (assume PCRE2_EXTENDED 2383 is set, so white space - including newlines - is ignored): 2384 2385 (?<date> (?<year>(\d\d)?\d\d) - 2386 (?<month>\d\d) - (?<day>\d\d) ) 2387 2388 There are four named capture groups, so the table has four entries, and 2389 each entry in the table is eight bytes long. The table is as follows, 2390 with non-printing bytes shows in hexadecimal, and undefined bytes shown 2391 as ??: 2392 2393 00 01 d a t e 00 ?? 2394 00 05 d a y 00 ?? ?? 2395 00 04 m o n t h 00 2396 00 02 y e a r 00 ?? 2397 2398 When writing code to extract data from named capture groups using the 2399 name-to-number map, remember that the length of the entries is likely 2400 to be different for each compiled pattern. 2401 2402 PCRE2_INFO_NEWLINE 2403 2404 The output is one of the following uint32_t values: 2405 2406 PCRE2_NEWLINE_CR Carriage return (CR) 2407 PCRE2_NEWLINE_LF Linefeed (LF) 2408 PCRE2_NEWLINE_CRLF Carriage return, linefeed (CRLF) 2409 PCRE2_NEWLINE_ANY Any Unicode line ending 2410 PCRE2_NEWLINE_ANYCRLF Any of CR, LF, or CRLF 2411 PCRE2_NEWLINE_NUL The NUL character (binary zero) 2412 2413 This identifies the character sequence that will be recognized as mean- 2414 ing "newline" while matching. 2415 2416 PCRE2_INFO_SIZE 2417 2418 Return the size of the compiled pattern in bytes (for all three li- 2419 braries). The third argument should point to a size_t variable. This 2420 value includes the size of the general data block that precedes the 2421 code units of the compiled pattern itself. The value that is used when 2422 pcre2_compile() is getting memory in which to place the compiled pat- 2423 tern may be slightly larger than the value returned by this option, be- 2424 cause there are cases where the code that calculates the size has to 2425 over-estimate. Processing a pattern with the JIT compiler does not al- 2426 ter the value returned by this option. 2427 2428 2429INFORMATION ABOUT A PATTERN'S CALLOUTS 2430 2431 int pcre2_callout_enumerate(const pcre2_code *code, 2432 int (*callback)(pcre2_callout_enumerate_block *, void *), 2433 void *user_data); 2434 2435 A script language that supports the use of string arguments in callouts 2436 might like to scan all the callouts in a pattern before running the 2437 match. This can be done by calling pcre2_callout_enumerate(). The first 2438 argument is a pointer to a compiled pattern, the second points to a 2439 callback function, and the third is arbitrary user data. The callback 2440 function is called for every callout in the pattern in the order in 2441 which they appear. Its first argument is a pointer to a callout enumer- 2442 ation block, and its second argument is the user_data value that was 2443 passed to pcre2_callout_enumerate(). The contents of the callout enu- 2444 meration block are described in the pcre2callout documentation, which 2445 also gives further details about callouts. 2446 2447 2448SERIALIZATION AND PRECOMPILING 2449 2450 It is possible to save compiled patterns on disc or elsewhere, and 2451 reload them later, subject to a number of restrictions. The host on 2452 which the patterns are reloaded must be running the same version of 2453 PCRE2, with the same code unit width, and must also have the same endi- 2454 anness, pointer width, and PCRE2_SIZE type. Before compiled patterns 2455 can be saved, they must be converted to a "serialized" form, which in 2456 the case of PCRE2 is really just a bytecode dump. The functions whose 2457 names begin with pcre2_serialize_ are used for converting to and from 2458 the serialized form. They are described in the pcre2serialize documen- 2459 tation. Note that PCRE2 serialization does not convert compiled pat- 2460 terns to an abstract format like Java or .NET serialization. 2461 2462 2463THE MATCH DATA BLOCK 2464 2465 pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize, 2466 pcre2_general_context *gcontext); 2467 2468 pcre2_match_data *pcre2_match_data_create_from_pattern( 2469 const pcre2_code *code, pcre2_general_context *gcontext); 2470 2471 void pcre2_match_data_free(pcre2_match_data *match_data); 2472 2473 Information about a successful or unsuccessful match is placed in a 2474 match data block, which is an opaque structure that is accessed by 2475 function calls. In particular, the match data block contains a vector 2476 of offsets into the subject string that define the matched parts of the 2477 subject. This is known as the ovector. 2478 2479 Before calling pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match() 2480 you must create a match data block by calling one of the creation func- 2481 tions above. For pcre2_match_data_create(), the first argument is the 2482 number of pairs of offsets in the ovector. 2483 2484 When using pcre2_match(), one pair of offsets is required to identify 2485 the string that matched the whole pattern, with an additional pair for 2486 each captured substring. For example, a value of 4 creates enough space 2487 to record the matched portion of the subject plus three captured sub- 2488 strings. 2489 2490 When using pcre2_dfa_match() there may be multiple matched substrings 2491 of different lengths at the same point in the subject. The ovector 2492 should be made large enough to hold as many as are expected. 2493 2494 A minimum of at least 1 pair is imposed by pcre2_match_data_create(), 2495 so it is always possible to return the overall matched string in the 2496 case of pcre2_match() or the longest match in the case of 2497 pcre2_dfa_match(). The maximum number of pairs is 65535; if the the 2498 first argument of pcre2_match_data_create() is greater than this, 65535 2499 is used. 2500 2501 The second argument of pcre2_match_data_create() is a pointer to a gen- 2502 eral context, which can specify custom memory management for obtaining 2503 the memory for the match data block. If you are not using custom memory 2504 management, pass NULL, which causes malloc() to be used. 2505 2506 For pcre2_match_data_create_from_pattern(), the first argument is a 2507 pointer to a compiled pattern. The ovector is created to be exactly the 2508 right size to hold all the substrings a pattern might capture when 2509 matched using pcre2_match(). You should not use this call when matching 2510 with pcre2_dfa_match(). The second argument is again a pointer to a 2511 general context, but in this case if NULL is passed, the memory is ob- 2512 tained using the same allocator that was used for the compiled pattern 2513 (custom or default). 2514 2515 A match data block can be used many times, with the same or different 2516 compiled patterns. You can extract information from a match data block 2517 after a match operation has finished, using functions that are de- 2518 scribed in the sections on matched strings and other match data below. 2519 2520 When a call of pcre2_match() fails, valid data is available in the 2521 match block only when the error is PCRE2_ERROR_NOMATCH, PCRE2_ER- 2522 ROR_PARTIAL, or one of the error codes for an invalid UTF string. Ex- 2523 actly what is available depends on the error, and is detailed below. 2524 2525 When one of the matching functions is called, pointers to the compiled 2526 pattern and the subject string are set in the match data block so that 2527 they can be referenced by the extraction functions after a successful 2528 match. After running a match, you must not free a compiled pattern or a 2529 subject string until after all operations on the match data block (for 2530 that match) have taken place, unless, in the case of the subject 2531 string, you have used the PCRE2_COPY_MATCHED_SUBJECT option, which is 2532 described in the section entitled "Option bits for pcre2_match()" be- 2533 low. 2534 2535 When a match data block itself is no longer needed, it should be freed 2536 by calling pcre2_match_data_free(). If this function is called with a 2537 NULL argument, it returns immediately, without doing anything. 2538 2539 2540MATCHING A PATTERN: THE TRADITIONAL FUNCTION 2541 2542 int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject, 2543 PCRE2_SIZE length, PCRE2_SIZE startoffset, 2544 uint32_t options, pcre2_match_data *match_data, 2545 pcre2_match_context *mcontext); 2546 2547 The function pcre2_match() is called to match a subject string against 2548 a compiled pattern, which is passed in the code argument. You can call 2549 pcre2_match() with the same code argument as many times as you like, in 2550 order to find multiple matches in the subject string or to match dif- 2551 ferent subject strings with the same pattern. 2552 2553 This function is the main matching facility of the library, and it op- 2554 erates in a Perl-like manner. For specialist use there is also an al- 2555 ternative matching function, which is described below in the section 2556 about the pcre2_dfa_match() function. 2557 2558 Here is an example of a simple call to pcre2_match(): 2559 2560 pcre2_match_data *md = pcre2_match_data_create(4, NULL); 2561 int rc = pcre2_match( 2562 re, /* result of pcre2_compile() */ 2563 "some string", /* the subject string */ 2564 11, /* the length of the subject string */ 2565 0, /* start at offset 0 in the subject */ 2566 0, /* default options */ 2567 md, /* the match data block */ 2568 NULL); /* a match context; NULL means use defaults */ 2569 2570 If the subject string is zero-terminated, the length can be given as 2571 PCRE2_ZERO_TERMINATED. A match context must be provided if certain less 2572 common matching parameters are to be changed. For details, see the sec- 2573 tion on the match context above. 2574 2575 The string to be matched by pcre2_match() 2576 2577 The subject string is passed to pcre2_match() as a pointer in subject, 2578 a length in length, and a starting offset in startoffset. The length 2579 and offset are in code units, not characters. That is, they are in 2580 bytes for the 8-bit library, 16-bit code units for the 16-bit library, 2581 and 32-bit code units for the 32-bit library, whether or not UTF pro- 2582 cessing is enabled. As a special case, if subject is NULL and length is 2583 zero, the subject is assumed to be an empty string. If length is non- 2584 zero, an error occurs if subject is NULL. 2585 2586 If startoffset is greater than the length of the subject, pcre2_match() 2587 returns PCRE2_ERROR_BADOFFSET. When the starting offset is zero, the 2588 search for a match starts at the beginning of the subject, and this is 2589 by far the most common case. In UTF-8 or UTF-16 mode, the starting off- 2590 set must point to the start of a character, or to the end of the sub- 2591 ject (in UTF-32 mode, one code unit equals one character, so all off- 2592 sets are valid). Like the pattern string, the subject may contain bi- 2593 nary zeros. 2594 2595 A non-zero starting offset is useful when searching for another match 2596 in the same subject by calling pcre2_match() again after a previous 2597 success. Setting startoffset differs from passing over a shortened 2598 string and setting PCRE2_NOTBOL in the case of a pattern that begins 2599 with any kind of lookbehind. For example, consider the pattern 2600 2601 \Biss\B 2602 2603 which finds occurrences of "iss" in the middle of words. (\B matches 2604 only if the current position in the subject is not a word boundary.) 2605 When applied to the string "Mississippi" the first call to 2606 pcre2_match() finds the first occurrence. If pcre2_match() is called 2607 again with just the remainder of the subject, namely "issippi", it does 2608 not match, because \B is always false at the start of the subject, 2609 which is deemed to be a word boundary. However, if pcre2_match() is 2610 passed the entire string again, but with startoffset set to 4, it finds 2611 the second occurrence of "iss" because it is able to look behind the 2612 starting point to discover that it is preceded by a letter. 2613 2614 Finding all the matches in a subject is tricky when the pattern can 2615 match an empty string. It is possible to emulate Perl's /g behaviour by 2616 first trying the match again at the same offset, with the 2617 PCRE2_NOTEMPTY_ATSTART and PCRE2_ANCHORED options, and then if that 2618 fails, advancing the starting offset and trying an ordinary match 2619 again. There is some code that demonstrates how to do this in the 2620 pcre2demo sample program. In the most general case, you have to check 2621 to see if the newline convention recognizes CRLF as a newline, and if 2622 so, and the current character is CR followed by LF, advance the start- 2623 ing offset by two characters instead of one. 2624 2625 If a non-zero starting offset is passed when the pattern is anchored, a 2626 single attempt to match at the given offset is made. This can only suc- 2627 ceed if the pattern does not require the match to be at the start of 2628 the subject. In other words, the anchoring must be the result of set- 2629 ting the PCRE2_ANCHORED option or the use of .* with PCRE2_DOTALL, not 2630 by starting the pattern with ^ or \A. 2631 2632 Option bits for pcre2_match() 2633 2634 The unused bits of the options argument for pcre2_match() must be zero. 2635 The only bits that may be set are PCRE2_ANCHORED, 2636 PCRE2_COPY_MATCHED_SUBJECT, PCRE2_ENDANCHORED, PCRE2_NOTBOL, PCRE2_NO- 2637 TEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_JIT, 2638 PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT. Their 2639 action is described below. 2640 2641 Setting PCRE2_ANCHORED or PCRE2_ENDANCHORED at match time is not sup- 2642 ported by the just-in-time (JIT) compiler. If it is set, JIT matching 2643 is disabled and the interpretive code in pcre2_match() is run. Apart 2644 from PCRE2_NO_JIT (obviously), the remaining options are supported for 2645 JIT matching. 2646 2647 PCRE2_ANCHORED 2648 2649 The PCRE2_ANCHORED option limits pcre2_match() to matching at the first 2650 matching position. If a pattern was compiled with PCRE2_ANCHORED, or 2651 turned out to be anchored by virtue of its contents, it cannot be made 2652 unachored at matching time. Note that setting the option at match time 2653 disables JIT matching. 2654 2655 PCRE2_COPY_MATCHED_SUBJECT 2656 2657 By default, a pointer to the subject is remembered in the match data 2658 block so that, after a successful match, it can be referenced by the 2659 substring extraction functions. This means that the subject's memory 2660 must not be freed until all such operations are complete. For some ap- 2661 plications where the lifetime of the subject string is not guaranteed, 2662 it may be necessary to make a copy of the subject string, but it is 2663 wasteful to do this unless the match is successful. After a successful 2664 match, if PCRE2_COPY_MATCHED_SUBJECT is set, the subject is copied and 2665 the new pointer is remembered in the match data block instead of the 2666 original subject pointer. The memory allocator that was used for the 2667 match block itself is used. The copy is automatically freed when 2668 pcre2_match_data_free() is called to free the match data block. It is 2669 also automatically freed if the match data block is re-used for another 2670 match operation. 2671 2672 PCRE2_ENDANCHORED 2673 2674 If the PCRE2_ENDANCHORED option is set, any string that pcre2_match() 2675 matches must be right at the end of the subject string. Note that set- 2676 ting the option at match time disables JIT matching. 2677 2678 PCRE2_NOTBOL 2679 2680 This option specifies that first character of the subject string is not 2681 the beginning of a line, so the circumflex metacharacter should not 2682 match before it. Setting this without having set PCRE2_MULTILINE at 2683 compile time causes circumflex never to match. This option affects only 2684 the behaviour of the circumflex metacharacter. It does not affect \A. 2685 2686 PCRE2_NOTEOL 2687 2688 This option specifies that the end of the subject string is not the end 2689 of a line, so the dollar metacharacter should not match it nor (except 2690 in multiline mode) a newline immediately before it. Setting this with- 2691 out having set PCRE2_MULTILINE at compile time causes dollar never to 2692 match. This option affects only the behaviour of the dollar metacharac- 2693 ter. It does not affect \Z or \z. 2694 2695 PCRE2_NOTEMPTY 2696 2697 An empty string is not considered to be a valid match if this option is 2698 set. If there are alternatives in the pattern, they are tried. If all 2699 the alternatives match the empty string, the entire match fails. For 2700 example, if the pattern 2701 2702 a?b? 2703 2704 is applied to a string not beginning with "a" or "b", it matches an 2705 empty string at the start of the subject. With PCRE2_NOTEMPTY set, this 2706 match is not valid, so pcre2_match() searches further into the string 2707 for occurrences of "a" or "b". 2708 2709 PCRE2_NOTEMPTY_ATSTART 2710 2711 This is like PCRE2_NOTEMPTY, except that it locks out an empty string 2712 match only at the first matching position, that is, at the start of the 2713 subject plus the starting offset. An empty string match later in the 2714 subject is permitted. If the pattern is anchored, such a match can oc- 2715 cur only if the pattern contains \K. 2716 2717 PCRE2_NO_JIT 2718 2719 By default, if a pattern has been successfully processed by 2720 pcre2_jit_compile(), JIT is automatically used when pcre2_match() is 2721 called with options that JIT supports. Setting PCRE2_NO_JIT disables 2722 the use of JIT; it forces matching to be done by the interpreter. 2723 2724 PCRE2_NO_UTF_CHECK 2725 2726 When PCRE2_UTF is set at compile time, the validity of the subject as a 2727 UTF string is checked unless PCRE2_NO_UTF_CHECK is passed to 2728 pcre2_match() or PCRE2_MATCH_INVALID_UTF was passed to pcre2_compile(). 2729 The latter special case is discussed in detail in the pcre2unicode doc- 2730 umentation. 2731 2732 In the default case, if a non-zero starting offset is given, the check 2733 is applied only to that part of the subject that could be inspected 2734 during matching, and there is a check that the starting offset points 2735 to the first code unit of a character or to the end of the subject. If 2736 there are no lookbehind assertions in the pattern, the check starts at 2737 the starting offset. Otherwise, it starts at the length of the longest 2738 lookbehind before the starting offset, or at the start of the subject 2739 if there are not that many characters before the starting offset. Note 2740 that the sequences \b and \B are one-character lookbehinds. 2741 2742 The check is carried out before any other processing takes place, and a 2743 negative error code is returned if the check fails. There are several 2744 UTF error codes for each code unit width, corresponding to different 2745 problems with the code unit sequence. There are discussions about the 2746 validity of UTF-8 strings, UTF-16 strings, and UTF-32 strings in the 2747 pcre2unicode documentation. 2748 2749 If you know that your subject is valid, and you want to skip this check 2750 for performance reasons, you can set the PCRE2_NO_UTF_CHECK option when 2751 calling pcre2_match(). You might want to do this for the second and 2752 subsequent calls to pcre2_match() if you are making repeated calls to 2753 find multiple matches in the same subject string. 2754 2755 Warning: Unless PCRE2_MATCH_INVALID_UTF was set at compile time, when 2756 PCRE2_NO_UTF_CHECK is set at match time the effect of passing an in- 2757 valid string as a subject, or an invalid value of startoffset, is unde- 2758 fined. Your program may crash or loop indefinitely or give wrong re- 2759 sults. 2760 2761 PCRE2_PARTIAL_HARD 2762 PCRE2_PARTIAL_SOFT 2763 2764 These options turn on the partial matching feature. A partial match oc- 2765 curs if the end of the subject string is reached successfully, but 2766 there are not enough subject characters to complete the match. In addi- 2767 tion, either at least one character must have been inspected or the 2768 pattern must contain a lookbehind, or the pattern must be one that 2769 could match an empty string. 2770 2771 If this situation arises when PCRE2_PARTIAL_SOFT (but not PCRE2_PAR- 2772 TIAL_HARD) is set, matching continues by testing any remaining alterna- 2773 tives. Only if no complete match can be found is PCRE2_ERROR_PARTIAL 2774 returned instead of PCRE2_ERROR_NOMATCH. In other words, PCRE2_PAR- 2775 TIAL_SOFT specifies that the caller is prepared to handle a partial 2776 match, but only if no complete match can be found. 2777 2778 If PCRE2_PARTIAL_HARD is set, it overrides PCRE2_PARTIAL_SOFT. In this 2779 case, if a partial match is found, pcre2_match() immediately returns 2780 PCRE2_ERROR_PARTIAL, without considering any other alternatives. In 2781 other words, when PCRE2_PARTIAL_HARD is set, a partial match is consid- 2782 ered to be more important that an alternative complete match. 2783 2784 There is a more detailed discussion of partial and multi-segment match- 2785 ing, with examples, in the pcre2partial documentation. 2786 2787 2788NEWLINE HANDLING WHEN MATCHING 2789 2790 When PCRE2 is built, a default newline convention is set; this is usu- 2791 ally the standard convention for the operating system. The default can 2792 be overridden in a compile context by calling pcre2_set_newline(). It 2793 can also be overridden by starting a pattern string with, for example, 2794 (*CRLF), as described in the section on newline conventions in the 2795 pcre2pattern page. During matching, the newline choice affects the be- 2796 haviour of the dot, circumflex, and dollar metacharacters. It may also 2797 alter the way the match starting position is advanced after a match 2798 failure for an unanchored pattern. 2799 2800 When PCRE2_NEWLINE_CRLF, PCRE2_NEWLINE_ANYCRLF, or PCRE2_NEWLINE_ANY is 2801 set as the newline convention, and a match attempt for an unanchored 2802 pattern fails when the current starting position is at a CRLF sequence, 2803 and the pattern contains no explicit matches for CR or LF characters, 2804 the match position is advanced by two characters instead of one, in 2805 other words, to after the CRLF. 2806 2807 The above rule is a compromise that makes the most common cases work as 2808 expected. For example, if the pattern is .+A (and the PCRE2_DOTALL op- 2809 tion is not set), it does not match the string "\r\nA" because, after 2810 failing at the start, it skips both the CR and the LF before retrying. 2811 However, the pattern [\r\n]A does match that string, because it con- 2812 tains an explicit CR or LF reference, and so advances only by one char- 2813 acter after the first failure. 2814 2815 An explicit match for CR of LF is either a literal appearance of one of 2816 those characters in the pattern, or one of the \r or \n or equivalent 2817 octal or hexadecimal escape sequences. Implicit matches such as [^X] do 2818 not count, nor does \s, even though it includes CR and LF in the char- 2819 acters that it matches. 2820 2821 Notwithstanding the above, anomalous effects may still occur when CRLF 2822 is a valid newline sequence and explicit \r or \n escapes appear in the 2823 pattern. 2824 2825 2826HOW PCRE2_MATCH() RETURNS A STRING AND CAPTURED SUBSTRINGS 2827 2828 uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data); 2829 2830 PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data); 2831 2832 In general, a pattern matches a certain portion of the subject, and in 2833 addition, further substrings from the subject may be picked out by 2834 parenthesized parts of the pattern. Following the usage in Jeffrey 2835 Friedl's book, this is called "capturing" in what follows, and the 2836 phrase "capture group" (Perl terminology) is used for a fragment of a 2837 pattern that picks out a substring. PCRE2 supports several other kinds 2838 of parenthesized group that do not cause substrings to be captured. The 2839 pcre2_pattern_info() function can be used to find out how many capture 2840 groups there are in a compiled pattern. 2841 2842 You can use auxiliary functions for accessing captured substrings by 2843 number or by name, as described in sections below. 2844 2845 Alternatively, you can make direct use of the vector of PCRE2_SIZE val- 2846 ues, called the ovector, which contains the offsets of captured 2847 strings. It is part of the match data block. The function 2848 pcre2_get_ovector_pointer() returns the address of the ovector, and 2849 pcre2_get_ovector_count() returns the number of pairs of values it con- 2850 tains. 2851 2852 Within the ovector, the first in each pair of values is set to the off- 2853 set of the first code unit of a substring, and the second is set to the 2854 offset of the first code unit after the end of a substring. These val- 2855 ues are always code unit offsets, not character offsets. That is, they 2856 are byte offsets in the 8-bit library, 16-bit offsets in the 16-bit li- 2857 brary, and 32-bit offsets in the 32-bit library. 2858 2859 After a partial match (error return PCRE2_ERROR_PARTIAL), only the 2860 first pair of offsets (that is, ovector[0] and ovector[1]) are set. 2861 They identify the part of the subject that was partially matched. See 2862 the pcre2partial documentation for details of partial matching. 2863 2864 After a fully successful match, the first pair of offsets identifies 2865 the portion of the subject string that was matched by the entire pat- 2866 tern. The next pair is used for the first captured substring, and so 2867 on. The value returned by pcre2_match() is one more than the highest 2868 numbered pair that has been set. For example, if two substrings have 2869 been captured, the returned value is 3. If there are no captured sub- 2870 strings, the return value from a successful match is 1, indicating that 2871 just the first pair of offsets has been set. 2872 2873 If a pattern uses the \K escape sequence within a positive assertion, 2874 the reported start of a successful match can be greater than the end of 2875 the match. For example, if the pattern (?=ab\K) is matched against 2876 "ab", the start and end offset values for the match are 2 and 0. 2877 2878 If a capture group is matched repeatedly within a single match opera- 2879 tion, it is the last portion of the subject that it matched that is re- 2880 turned. 2881 2882 If the ovector is too small to hold all the captured substring offsets, 2883 as much as possible is filled in, and the function returns a value of 2884 zero. If captured substrings are not of interest, pcre2_match() may be 2885 called with a match data block whose ovector is of minimum length (that 2886 is, one pair). 2887 2888 It is possible for capture group number n+1 to match some part of the 2889 subject when group n has not been used at all. For example, if the 2890 string "abc" is matched against the pattern (a|(z))(bc) the return from 2891 the function is 4, and groups 1 and 3 are matched, but 2 is not. When 2892 this happens, both values in the offset pairs corresponding to unused 2893 groups are set to PCRE2_UNSET. 2894 2895 Offset values that correspond to unused groups at the end of the ex- 2896 pression are also set to PCRE2_UNSET. For example, if the string "abc" 2897 is matched against the pattern (abc)(x(yz)?)? groups 2 and 3 are not 2898 matched. The return from the function is 2, because the highest used 2899 capture group number is 1. The offsets for for the second and third 2900 capture groupss (assuming the vector is large enough, of course) are 2901 set to PCRE2_UNSET. 2902 2903 Elements in the ovector that do not correspond to capturing parentheses 2904 in the pattern are never changed. That is, if a pattern contains n cap- 2905 turing parentheses, no more than ovector[0] to ovector[2n+1] are set by 2906 pcre2_match(). The other elements retain whatever values they previ- 2907 ously had. After a failed match attempt, the contents of the ovector 2908 are unchanged. 2909 2910 2911OTHER INFORMATION ABOUT A MATCH 2912 2913 PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data); 2914 2915 PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data); 2916 2917 As well as the offsets in the ovector, other information about a match 2918 is retained in the match data block and can be retrieved by the above 2919 functions in appropriate circumstances. If they are called at other 2920 times, the result is undefined. 2921 2922 After a successful match, a partial match (PCRE2_ERROR_PARTIAL), or a 2923 failure to match (PCRE2_ERROR_NOMATCH), a mark name may be available. 2924 The function pcre2_get_mark() can be called to access this name, which 2925 can be specified in the pattern by any of the backtracking control 2926 verbs, not just (*MARK). The same function applies to all the verbs. It 2927 returns a pointer to the zero-terminated name, which is within the com- 2928 piled pattern. If no name is available, NULL is returned. The length of 2929 the name (excluding the terminating zero) is stored in the code unit 2930 that precedes the name. You should use this length instead of relying 2931 on the terminating zero if the name might contain a binary zero. 2932 2933 After a successful match, the name that is returned is the last mark 2934 name encountered on the matching path through the pattern. Instances of 2935 backtracking verbs without names do not count. Thus, for example, if 2936 the matching path contains (*MARK:A)(*PRUNE), the name "A" is returned. 2937 After a "no match" or a partial match, the last encountered name is re- 2938 turned. For example, consider this pattern: 2939 2940 ^(*MARK:A)((*MARK:B)a|b)c 2941 2942 When it matches "bc", the returned name is A. The B mark is "seen" in 2943 the first branch of the group, but it is not on the matching path. On 2944 the other hand, when this pattern fails to match "bx", the returned 2945 name is B. 2946 2947 Warning: By default, certain start-of-match optimizations are used to 2948 give a fast "no match" result in some situations. For example, if the 2949 anchoring is removed from the pattern above, there is an initial check 2950 for the presence of "c" in the subject before running the matching en- 2951 gine. This check fails for "bx", causing a match failure without seeing 2952 any marks. You can disable the start-of-match optimizations by setting 2953 the PCRE2_NO_START_OPTIMIZE option for pcre2_compile() or by starting 2954 the pattern with (*NO_START_OPT). 2955 2956 After a successful match, a partial match, or one of the invalid UTF 2957 errors (for example, PCRE2_ERROR_UTF8_ERR5), pcre2_get_startchar() can 2958 be called. After a successful or partial match it returns the code unit 2959 offset of the character at which the match started. For a non-partial 2960 match, this can be different to the value of ovector[0] if the pattern 2961 contains the \K escape sequence. After a partial match, however, this 2962 value is always the same as ovector[0] because \K does not affect the 2963 result of a partial match. 2964 2965 After a UTF check failure, pcre2_get_startchar() can be used to obtain 2966 the code unit offset of the invalid UTF character. Details are given in 2967 the pcre2unicode page. 2968 2969 2970ERROR RETURNS FROM pcre2_match() 2971 2972 If pcre2_match() fails, it returns a negative number. This can be con- 2973 verted to a text string by calling the pcre2_get_error_message() func- 2974 tion (see "Obtaining a textual error message" below). Negative error 2975 codes are also returned by other functions, and are documented with 2976 them. The codes are given names in the header file. If UTF checking is 2977 in force and an invalid UTF subject string is detected, one of a number 2978 of UTF-specific negative error codes is returned. Details are given in 2979 the pcre2unicode page. The following are the other errors that may be 2980 returned by pcre2_match(): 2981 2982 PCRE2_ERROR_NOMATCH 2983 2984 The subject string did not match the pattern. 2985 2986 PCRE2_ERROR_PARTIAL 2987 2988 The subject string did not match, but it did match partially. See the 2989 pcre2partial documentation for details of partial matching. 2990 2991 PCRE2_ERROR_BADMAGIC 2992 2993 PCRE2 stores a 4-byte "magic number" at the start of the compiled code, 2994 to catch the case when it is passed a junk pointer. This is the error 2995 that is returned when the magic number is not present. 2996 2997 PCRE2_ERROR_BADMODE 2998 2999 This error is given when a compiled pattern is passed to a function in 3000 a library of a different code unit width, for example, a pattern com- 3001 piled by the 8-bit library is passed to a 16-bit or 32-bit library 3002 function. 3003 3004 PCRE2_ERROR_BADOFFSET 3005 3006 The value of startoffset was greater than the length of the subject. 3007 3008 PCRE2_ERROR_BADOPTION 3009 3010 An unrecognized bit was set in the options argument. 3011 3012 PCRE2_ERROR_BADUTFOFFSET 3013 3014 The UTF code unit sequence that was passed as a subject was checked and 3015 found to be valid (the PCRE2_NO_UTF_CHECK option was not set), but the 3016 value of startoffset did not point to the beginning of a UTF character 3017 or the end of the subject. 3018 3019 PCRE2_ERROR_CALLOUT 3020 3021 This error is never generated by pcre2_match() itself. It is provided 3022 for use by callout functions that want to cause pcre2_match() or 3023 pcre2_callout_enumerate() to return a distinctive error code. See the 3024 pcre2callout documentation for details. 3025 3026 PCRE2_ERROR_DEPTHLIMIT 3027 3028 The nested backtracking depth limit was reached. 3029 3030 PCRE2_ERROR_HEAPLIMIT 3031 3032 The heap limit was reached. 3033 3034 PCRE2_ERROR_INTERNAL 3035 3036 An unexpected internal error has occurred. This error could be caused 3037 by a bug in PCRE2 or by overwriting of the compiled pattern. 3038 3039 PCRE2_ERROR_JIT_STACKLIMIT 3040 3041 This error is returned when a pattern that was successfully studied us- 3042 ing JIT is being matched, but the memory available for the just-in-time 3043 processing stack is not large enough. See the pcre2jit documentation 3044 for more details. 3045 3046 PCRE2_ERROR_MATCHLIMIT 3047 3048 The backtracking match limit was reached. 3049 3050 PCRE2_ERROR_NOMEMORY 3051 3052 Heap memory is used to remember backgracking points. This error is 3053 given when the memory allocation function (default or custom) fails. 3054 Note that a different error, PCRE2_ERROR_HEAPLIMIT, is given if the 3055 amount of memory needed exceeds the heap limit. PCRE2_ERROR_NOMEMORY is 3056 also returned if PCRE2_COPY_MATCHED_SUBJECT is set and memory alloca- 3057 tion fails. 3058 3059 PCRE2_ERROR_NULL 3060 3061 Either the code, subject, or match_data argument was passed as NULL. 3062 3063 PCRE2_ERROR_RECURSELOOP 3064 3065 This error is returned when pcre2_match() detects a recursion loop 3066 within the pattern. Specifically, it means that either the whole pat- 3067 tern or a capture group has been called recursively for the second time 3068 at the same position in the subject string. Some simple patterns that 3069 might do this are detected and faulted at compile time, but more com- 3070 plicated cases, in particular mutual recursions between two different 3071 groups, cannot be detected until matching is attempted. 3072 3073 3074OBTAINING A TEXTUAL ERROR MESSAGE 3075 3076 int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer, 3077 PCRE2_SIZE bufflen); 3078 3079 A text message for an error code from any PCRE2 function (compile, 3080 match, or auxiliary) can be obtained by calling pcre2_get_error_mes- 3081 sage(). The code is passed as the first argument, with the remaining 3082 two arguments specifying a code unit buffer and its length in code 3083 units, into which the text message is placed. The message is returned 3084 in code units of the appropriate width for the library that is being 3085 used. 3086 3087 The returned message is terminated with a trailing zero, and the func- 3088 tion returns the number of code units used, excluding the trailing 3089 zero. If the error number is unknown, the negative error code PCRE2_ER- 3090 ROR_BADDATA is returned. If the buffer is too small, the message is 3091 truncated (but still with a trailing zero), and the negative error code 3092 PCRE2_ERROR_NOMEMORY is returned. None of the messages are very long; 3093 a buffer size of 120 code units is ample. 3094 3095 3096EXTRACTING CAPTURED SUBSTRINGS BY NUMBER 3097 3098 int pcre2_substring_length_bynumber(pcre2_match_data *match_data, 3099 uint32_t number, PCRE2_SIZE *length); 3100 3101 int pcre2_substring_copy_bynumber(pcre2_match_data *match_data, 3102 uint32_t number, PCRE2_UCHAR *buffer, 3103 PCRE2_SIZE *bufflen); 3104 3105 int pcre2_substring_get_bynumber(pcre2_match_data *match_data, 3106 uint32_t number, PCRE2_UCHAR **bufferptr, 3107 PCRE2_SIZE *bufflen); 3108 3109 void pcre2_substring_free(PCRE2_UCHAR *buffer); 3110 3111 Captured substrings can be accessed directly by using the ovector as 3112 described above. For convenience, auxiliary functions are provided for 3113 extracting captured substrings as new, separate, zero-terminated 3114 strings. A substring that contains a binary zero is correctly extracted 3115 and has a further zero added on the end, but the result is not, of 3116 course, a C string. 3117 3118 The functions in this section identify substrings by number. The number 3119 zero refers to the entire matched substring, with higher numbers refer- 3120 ring to substrings captured by parenthesized groups. After a partial 3121 match, only substring zero is available. An attempt to extract any 3122 other substring gives the error PCRE2_ERROR_PARTIAL. The next section 3123 describes similar functions for extracting captured substrings by name. 3124 3125 If a pattern uses the \K escape sequence within a positive assertion, 3126 the reported start of a successful match can be greater than the end of 3127 the match. For example, if the pattern (?=ab\K) is matched against 3128 "ab", the start and end offset values for the match are 2 and 0. In 3129 this situation, calling these functions with a zero substring number 3130 extracts a zero-length empty string. 3131 3132 You can find the length in code units of a captured substring without 3133 extracting it by calling pcre2_substring_length_bynumber(). The first 3134 argument is a pointer to the match data block, the second is the group 3135 number, and the third is a pointer to a variable into which the length 3136 is placed. If you just want to know whether or not the substring has 3137 been captured, you can pass the third argument as NULL. 3138 3139 The pcre2_substring_copy_bynumber() function copies a captured sub- 3140 string into a supplied buffer, whereas pcre2_substring_get_bynumber() 3141 copies it into new memory, obtained using the same memory allocation 3142 function that was used for the match data block. The first two argu- 3143 ments of these functions are a pointer to the match data block and a 3144 capture group number. 3145 3146 The final arguments of pcre2_substring_copy_bynumber() are a pointer to 3147 the buffer and a pointer to a variable that contains its length in code 3148 units. This is updated to contain the actual number of code units used 3149 for the extracted substring, excluding the terminating zero. 3150 3151 For pcre2_substring_get_bynumber() the third and fourth arguments point 3152 to variables that are updated with a pointer to the new memory and the 3153 number of code units that comprise the substring, again excluding the 3154 terminating zero. When the substring is no longer needed, the memory 3155 should be freed by calling pcre2_substring_free(). 3156 3157 The return value from all these functions is zero for success, or a 3158 negative error code. If the pattern match failed, the match failure 3159 code is returned. If a substring number greater than zero is used af- 3160 ter a partial match, PCRE2_ERROR_PARTIAL is returned. Other possible 3161 error codes are: 3162 3163 PCRE2_ERROR_NOMEMORY 3164 3165 The buffer was too small for pcre2_substring_copy_bynumber(), or the 3166 attempt to get memory failed for pcre2_substring_get_bynumber(). 3167 3168 PCRE2_ERROR_NOSUBSTRING 3169 3170 There is no substring with that number in the pattern, that is, the 3171 number is greater than the number of capturing parentheses. 3172 3173 PCRE2_ERROR_UNAVAILABLE 3174 3175 The substring number, though not greater than the number of captures in 3176 the pattern, is greater than the number of slots in the ovector, so the 3177 substring could not be captured. 3178 3179 PCRE2_ERROR_UNSET 3180 3181 The substring did not participate in the match. For example, if the 3182 pattern is (abc)|(def) and the subject is "def", and the ovector con- 3183 tains at least two capturing slots, substring number 1 is unset. 3184 3185 3186EXTRACTING A LIST OF ALL CAPTURED SUBSTRINGS 3187 3188 int pcre2_substring_list_get(pcre2_match_data *match_data, 3189 PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr); 3190 3191 void pcre2_substring_list_free(PCRE2_SPTR *list); 3192 3193 The pcre2_substring_list_get() function extracts all available sub- 3194 strings and builds a list of pointers to them. It also (optionally) 3195 builds a second list that contains their lengths (in code units), ex- 3196 cluding a terminating zero that is added to each of them. All this is 3197 done in a single block of memory that is obtained using the same memory 3198 allocation function that was used to get the match data block. 3199 3200 This function must be called only after a successful match. If called 3201 after a partial match, the error code PCRE2_ERROR_PARTIAL is returned. 3202 3203 The address of the memory block is returned via listptr, which is also 3204 the start of the list of string pointers. The end of the list is marked 3205 by a NULL pointer. The address of the list of lengths is returned via 3206 lengthsptr. If your strings do not contain binary zeros and you do not 3207 therefore need the lengths, you may supply NULL as the lengthsptr argu- 3208 ment to disable the creation of a list of lengths. The yield of the 3209 function is zero if all went well, or PCRE2_ERROR_NOMEMORY if the mem- 3210 ory block could not be obtained. When the list is no longer needed, it 3211 should be freed by calling pcre2_substring_list_free(). 3212 3213 If this function encounters a substring that is unset, which can happen 3214 when capture group number n+1 matches some part of the subject, but 3215 group n has not been used at all, it returns an empty string. This can 3216 be distinguished from a genuine zero-length substring by inspecting the 3217 appropriate offset in the ovector, which contain PCRE2_UNSET for unset 3218 substrings, or by calling pcre2_substring_length_bynumber(). 3219 3220 3221EXTRACTING CAPTURED SUBSTRINGS BY NAME 3222 3223 int pcre2_substring_number_from_name(const pcre2_code *code, 3224 PCRE2_SPTR name); 3225 3226 int pcre2_substring_length_byname(pcre2_match_data *match_data, 3227 PCRE2_SPTR name, PCRE2_SIZE *length); 3228 3229 int pcre2_substring_copy_byname(pcre2_match_data *match_data, 3230 PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen); 3231 3232 int pcre2_substring_get_byname(pcre2_match_data *match_data, 3233 PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen); 3234 3235 void pcre2_substring_free(PCRE2_UCHAR *buffer); 3236 3237 To extract a substring by name, you first have to find associated num- 3238 ber. For example, for this pattern: 3239 3240 (a+)b(?<xxx>\d+)... 3241 3242 the number of the capture group called "xxx" is 2. If the name is known 3243 to be unique (PCRE2_DUPNAMES was not set), you can find the number from 3244 the name by calling pcre2_substring_number_from_name(). The first argu- 3245 ment is the compiled pattern, and the second is the name. The yield of 3246 the function is the group number, PCRE2_ERROR_NOSUBSTRING if there is 3247 no group with that name, or PCRE2_ERROR_NOUNIQUESUBSTRING if there is 3248 more than one group with that name. Given the number, you can extract 3249 the substring directly from the ovector, or use one of the "bynumber" 3250 functions described above. 3251 3252 For convenience, there are also "byname" functions that correspond to 3253 the "bynumber" functions, the only difference being that the second ar- 3254 gument is a name instead of a number. If PCRE2_DUPNAMES is set and 3255 there are duplicate names, these functions scan all the groups with the 3256 given name, and return the captured substring from the first named 3257 group that is set. 3258 3259 If there are no groups with the given name, PCRE2_ERROR_NOSUBSTRING is 3260 returned. If all groups with the name have numbers that are greater 3261 than the number of slots in the ovector, PCRE2_ERROR_UNAVAILABLE is re- 3262 turned. If there is at least one group with a slot in the ovector, but 3263 no group is found to be set, PCRE2_ERROR_UNSET is returned. 3264 3265 Warning: If the pattern uses the (?| feature to set up multiple capture 3266 groups with the same number, as described in the section on duplicate 3267 group numbers in the pcre2pattern page, you cannot use names to distin- 3268 guish the different capture groups, because names are not included in 3269 the compiled code. The matching process uses only numbers. For this 3270 reason, the use of different names for groups with the same number 3271 causes an error at compile time. 3272 3273 3274CREATING A NEW STRING WITH SUBSTITUTIONS 3275 3276 int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject, 3277 PCRE2_SIZE length, PCRE2_SIZE startoffset, 3278 uint32_t options, pcre2_match_data *match_data, 3279 pcre2_match_context *mcontext, PCRE2_SPTR replacement, 3280 PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer, 3281 PCRE2_SIZE *outlengthptr); 3282 3283 This function optionally calls pcre2_match() and then makes a copy of 3284 the subject string in outputbuffer, replacing parts that were matched 3285 with the replacement string, whose length is supplied in rlength, which 3286 can be given as PCRE2_ZERO_TERMINATED for a zero-terminated string. As 3287 a special case, if replacement is NULL and rlength is zero, the re- 3288 placement is assumed to be an empty string. If rlength is non-zero, an 3289 error occurs if replacement is NULL. 3290 3291 There is an option (see PCRE2_SUBSTITUTE_REPLACEMENT_ONLY below) to re- 3292 turn just the replacement string(s). The default action is to perform 3293 just one replacement if the pattern matches, but there is an option 3294 that requests multiple replacements (see PCRE2_SUBSTITUTE_GLOBAL be- 3295 low). 3296 3297 If successful, pcre2_substitute() returns the number of substitutions 3298 that were carried out. This may be zero if no match was found, and is 3299 never greater than one unless PCRE2_SUBSTITUTE_GLOBAL is set. A nega- 3300 tive value is returned if an error is detected. 3301 3302 Matches in which a \K item in a lookahead in the pattern causes the 3303 match to end before it starts are not supported, and give rise to an 3304 error return. For global replacements, matches in which \K in a lookbe- 3305 hind causes the match to start earlier than the point that was reached 3306 in the previous iteration are also not supported. 3307 3308 The first seven arguments of pcre2_substitute() are the same as for 3309 pcre2_match(), except that the partial matching options are not permit- 3310 ted, and match_data may be passed as NULL, in which case a match data 3311 block is obtained and freed within this function, using memory manage- 3312 ment functions from the match context, if provided, or else those that 3313 were used to allocate memory for the compiled code. 3314 3315 If match_data is not NULL and PCRE2_SUBSTITUTE_MATCHED is not set, the 3316 provided block is used for all calls to pcre2_match(), and its contents 3317 afterwards are the result of the final call. For global changes, this 3318 will always be a no-match error. The contents of the ovector within the 3319 match data block may or may not have been changed. 3320 3321 As well as the usual options for pcre2_match(), a number of additional 3322 options can be set in the options argument of pcre2_substitute(). One 3323 such option is PCRE2_SUBSTITUTE_MATCHED. When this is set, an external 3324 match_data block must be provided, and it must have already been used 3325 for an external call to pcre2_match() with the same pattern and subject 3326 arguments. The data in the match_data block (return code, offset vec- 3327 tor) is then used for the first substitution instead of calling 3328 pcre2_match() from within pcre2_substitute(). This allows an applica- 3329 tion to check for a match before choosing to substitute, without having 3330 to repeat the match. 3331 3332 The contents of the externally supplied match data block are not 3333 changed when PCRE2_SUBSTITUTE_MATCHED is set. If PCRE2_SUBSTI- 3334 TUTE_GLOBAL is also set, pcre2_match() is called after the first sub- 3335 stitution to check for further matches, but this is done using an in- 3336 ternally obtained match data block, thus always leaving the external 3337 block unchanged. 3338 3339 The code argument is not used for matching before the first substitu- 3340 tion when PCRE2_SUBSTITUTE_MATCHED is set, but it must be provided, 3341 even when PCRE2_SUBSTITUTE_GLOBAL is not set, because it contains in- 3342 formation such as the UTF setting and the number of capturing parenthe- 3343 ses in the pattern. 3344 3345 The default action of pcre2_substitute() is to return a copy of the 3346 subject string with matched substrings replaced. However, if PCRE2_SUB- 3347 STITUTE_REPLACEMENT_ONLY is set, only the replacement substrings are 3348 returned. In the global case, multiple replacements are concatenated in 3349 the output buffer. Substitution callouts (see below) can be used to 3350 separate them if necessary. 3351 3352 The outlengthptr argument of pcre2_substitute() must point to a vari- 3353 able that contains the length, in code units, of the output buffer. If 3354 the function is successful, the value is updated to contain the length 3355 in code units of the new string, excluding the trailing zero that is 3356 automatically added. 3357 3358 If the function is not successful, the value set via outlengthptr de- 3359 pends on the type of error. For syntax errors in the replacement 3360 string, the value is the offset in the replacement string where the er- 3361 ror was detected. For other errors, the value is PCRE2_UNSET by de- 3362 fault. This includes the case of the output buffer being too small, un- 3363 less PCRE2_SUBSTITUTE_OVERFLOW_LENGTH is set. 3364 3365 PCRE2_SUBSTITUTE_OVERFLOW_LENGTH changes what happens when the output 3366 buffer is too small. The default action is to return PCRE2_ERROR_NOMEM- 3367 ORY immediately. If this option is set, however, pcre2_substitute() 3368 continues to go through the motions of matching and substituting (with- 3369 out, of course, writing anything) in order to compute the size of buf- 3370 fer that is needed. This value is passed back via the outlengthptr 3371 variable, with the result of the function still being PCRE2_ER- 3372 ROR_NOMEMORY. 3373 3374 Passing a buffer size of zero is a permitted way of finding out how 3375 much memory is needed for given substitution. However, this does mean 3376 that the entire operation is carried out twice. Depending on the appli- 3377 cation, it may be more efficient to allocate a large buffer and free 3378 the excess afterwards, instead of using PCRE2_SUBSTITUTE_OVER- 3379 FLOW_LENGTH. 3380 3381 The replacement string, which is interpreted as a UTF string in UTF 3382 mode, is checked for UTF validity unless PCRE2_NO_UTF_CHECK is set. An 3383 invalid UTF replacement string causes an immediate return with the rel- 3384 evant UTF error code. 3385 3386 If PCRE2_SUBSTITUTE_LITERAL is set, the replacement string is not in- 3387 terpreted in any way. By default, however, a dollar character is an es- 3388 cape character that can specify the insertion of characters from cap- 3389 ture groups and names from (*MARK) or other control verbs in the pat- 3390 tern. The following forms are always recognized: 3391 3392 $$ insert a dollar character 3393 $<n> or ${<n>} insert the contents of group <n> 3394 $*MARK or ${*MARK} insert a control verb name 3395 3396 Either a group number or a group name can be given for <n>. Curly 3397 brackets are required only if the following character would be inter- 3398 preted as part of the number or name. The number may be zero to include 3399 the entire matched string. For example, if the pattern a(b)c is 3400 matched with "=abc=" and the replacement string "+$1$0$1+", the result 3401 is "=+babcb+=". 3402 3403 $*MARK inserts the name from the last encountered backtracking control 3404 verb on the matching path that has a name. (*MARK) must always include 3405 a name, but the other verbs need not. For example, in the case of 3406 (*MARK:A)(*PRUNE) the name inserted is "A", but for (*MARK:A)(*PRUNE:B) 3407 the relevant name is "B". This facility can be used to perform simple 3408 simultaneous substitutions, as this pcre2test example shows: 3409 3410 /(*MARK:pear)apple|(*MARK:orange)lemon/g,replace=${*MARK} 3411 apple lemon 3412 2: pear orange 3413 3414 PCRE2_SUBSTITUTE_GLOBAL causes the function to iterate over the subject 3415 string, replacing every matching substring. If this option is not set, 3416 only the first matching substring is replaced. The search for matches 3417 takes place in the original subject string (that is, previous replace- 3418 ments do not affect it). Iteration is implemented by advancing the 3419 startoffset value for each search, which is always passed the entire 3420 subject string. If an offset limit is set in the match context, search- 3421 ing stops when that limit is reached. 3422 3423 You can restrict the effect of a global substitution to a portion of 3424 the subject string by setting either or both of startoffset and an off- 3425 set limit. Here is a pcre2test example: 3426 3427 /B/g,replace=!,use_offset_limit 3428 ABC ABC ABC ABC\=offset=3,offset_limit=12 3429 2: ABC A!C A!C ABC 3430 3431 When continuing with global substitutions after matching a substring 3432 with zero length, an attempt to find a non-empty match at the same off- 3433 set is performed. If this is not successful, the offset is advanced by 3434 one character except when CRLF is a valid newline sequence and the next 3435 two characters are CR, LF. In this case, the offset is advanced by two 3436 characters. 3437 3438 PCRE2_SUBSTITUTE_UNKNOWN_UNSET causes references to capture groups that 3439 do not appear in the pattern to be treated as unset groups. This option 3440 should be used with care, because it means that a typo in a group name 3441 or number no longer causes the PCRE2_ERROR_NOSUBSTRING error. 3442 3443 PCRE2_SUBSTITUTE_UNSET_EMPTY causes unset capture groups (including un- 3444 known groups when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set) to be treated 3445 as empty strings when inserted as described above. If this option is 3446 not set, an attempt to insert an unset group causes the PCRE2_ERROR_UN- 3447 SET error. This option does not influence the extended substitution 3448 syntax described below. 3449 3450 PCRE2_SUBSTITUTE_EXTENDED causes extra processing to be applied to the 3451 replacement string. Without this option, only the dollar character is 3452 special, and only the group insertion forms listed above are valid. 3453 When PCRE2_SUBSTITUTE_EXTENDED is set, two things change: 3454 3455 Firstly, backslash in a replacement string is interpreted as an escape 3456 character. The usual forms such as \n or \x{ddd} can be used to specify 3457 particular character codes, and backslash followed by any non-alphanu- 3458 meric character quotes that character. Extended quoting can be coded 3459 using \Q...\E, exactly as in pattern strings. 3460 3461 There are also four escape sequences for forcing the case of inserted 3462 letters. The insertion mechanism has three states: no case forcing, 3463 force upper case, and force lower case. The escape sequences change the 3464 current state: \U and \L change to upper or lower case forcing, respec- 3465 tively, and \E (when not terminating a \Q quoted sequence) reverts to 3466 no case forcing. The sequences \u and \l force the next character (if 3467 it is a letter) to upper or lower case, respectively, and then the 3468 state automatically reverts to no case forcing. Case forcing applies to 3469 all inserted characters, including those from capture groups and let- 3470 ters within \Q...\E quoted sequences. If either PCRE2_UTF or PCRE2_UCP 3471 was set when the pattern was compiled, Unicode properties are used for 3472 case forcing characters whose code points are greater than 127. 3473 3474 Note that case forcing sequences such as \U...\E do not nest. For exam- 3475 ple, the result of processing "\Uaa\LBB\Ecc\E" is "AAbbcc"; the final 3476 \E has no effect. Note also that the PCRE2_ALT_BSUX and PCRE2_EX- 3477 TRA_ALT_BSUX options do not apply to replacement strings. 3478 3479 The second effect of setting PCRE2_SUBSTITUTE_EXTENDED is to add more 3480 flexibility to capture group substitution. The syntax is similar to 3481 that used by Bash: 3482 3483 ${<n>:-<string>} 3484 ${<n>:+<string1>:<string2>} 3485 3486 As before, <n> may be a group number or a name. The first form speci- 3487 fies a default value. If group <n> is set, its value is inserted; if 3488 not, <string> is expanded and the result inserted. The second form 3489 specifies strings that are expanded and inserted when group <n> is set 3490 or unset, respectively. The first form is just a convenient shorthand 3491 for 3492 3493 ${<n>:+${<n>}:<string>} 3494 3495 Backslash can be used to escape colons and closing curly brackets in 3496 the replacement strings. A change of the case forcing state within a 3497 replacement string remains in force afterwards, as shown in this 3498 pcre2test example: 3499 3500 /(some)?(body)/substitute_extended,replace=${1:+\U:\L}HeLLo 3501 body 3502 1: hello 3503 somebody 3504 1: HELLO 3505 3506 The PCRE2_SUBSTITUTE_UNSET_EMPTY option does not affect these extended 3507 substitutions. However, PCRE2_SUBSTITUTE_UNKNOWN_UNSET does cause un- 3508 known groups in the extended syntax forms to be treated as unset. 3509 3510 If PCRE2_SUBSTITUTE_LITERAL is set, PCRE2_SUBSTITUTE_UNKNOWN_UNSET, 3511 PCRE2_SUBSTITUTE_UNSET_EMPTY, and PCRE2_SUBSTITUTE_EXTENDED are irrele- 3512 vant and are ignored. 3513 3514 Substitution errors 3515 3516 In the event of an error, pcre2_substitute() returns a negative error 3517 code. Except for PCRE2_ERROR_NOMATCH (which is never returned), errors 3518 from pcre2_match() are passed straight back. 3519 3520 PCRE2_ERROR_NOSUBSTRING is returned for a non-existent substring inser- 3521 tion, unless PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set. 3522 3523 PCRE2_ERROR_UNSET is returned for an unset substring insertion (includ- 3524 ing an unknown substring when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set) 3525 when the simple (non-extended) syntax is used and PCRE2_SUBSTITUTE_UN- 3526 SET_EMPTY is not set. 3527 3528 PCRE2_ERROR_NOMEMORY is returned if the output buffer is not big 3529 enough. If the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option is set, the size 3530 of buffer that is needed is returned via outlengthptr. Note that this 3531 does not happen by default. 3532 3533 PCRE2_ERROR_NULL is returned if PCRE2_SUBSTITUTE_MATCHED is set but the 3534 match_data argument is NULL or if the subject or replacement arguments 3535 are NULL. For backward compatibility reasons an exception is made for 3536 the replacement argument if the rlength argument is also 0. 3537 3538 PCRE2_ERROR_BADREPLACEMENT is used for miscellaneous syntax errors in 3539 the replacement string, with more particular errors being PCRE2_ER- 3540 ROR_BADREPESCAPE (invalid escape sequence), PCRE2_ERROR_REPMISSINGBRACE 3541 (closing curly bracket not found), PCRE2_ERROR_BADSUBSTITUTION (syntax 3542 error in extended group substitution), and PCRE2_ERROR_BADSUBSPATTERN 3543 (the pattern match ended before it started or the match started earlier 3544 than the current position in the subject, which can happen if \K is 3545 used in an assertion). 3546 3547 As for all PCRE2 errors, a text message that describes the error can be 3548 obtained by calling the pcre2_get_error_message() function (see "Ob- 3549 taining a textual error message" above). 3550 3551 Substitution callouts 3552 3553 int pcre2_set_substitute_callout(pcre2_match_context *mcontext, 3554 int (*callout_function)(pcre2_substitute_callout_block *, void *), 3555 void *callout_data); 3556 3557 The pcre2_set_substitution_callout() function can be used to specify a 3558 callout function for pcre2_substitute(). This information is passed in 3559 a match context. The callout function is called after each substitution 3560 has been processed, but it can cause the replacement not to happen. The 3561 callout function is not called for simulated substitutions that happen 3562 as a result of the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option. 3563 3564 The first argument of the callout function is a pointer to a substitute 3565 callout block structure, which contains the following fields, not nec- 3566 essarily in this order: 3567 3568 uint32_t version; 3569 uint32_t subscount; 3570 PCRE2_SPTR input; 3571 PCRE2_SPTR output; 3572 PCRE2_SIZE *ovector; 3573 uint32_t oveccount; 3574 PCRE2_SIZE output_offsets[2]; 3575 3576 The version field contains the version number of the block format. The 3577 current version is 0. The version number will increase in future if 3578 more fields are added, but the intention is never to remove any of the 3579 existing fields. 3580 3581 The subscount field is the number of the current match. It is 1 for the 3582 first callout, 2 for the second, and so on. The input and output point- 3583 ers are copies of the values passed to pcre2_substitute(). 3584 3585 The ovector field points to the ovector, which contains the result of 3586 the most recent match. The oveccount field contains the number of pairs 3587 that are set in the ovector, and is always greater than zero. 3588 3589 The output_offsets vector contains the offsets of the replacement in 3590 the output string. This has already been processed for dollar and (if 3591 requested) backslash substitutions as described above. 3592 3593 The second argument of the callout function is the value passed as 3594 callout_data when the function was registered. The value returned by 3595 the callout function is interpreted as follows: 3596 3597 If the value is zero, the replacement is accepted, and, if PCRE2_SUB- 3598 STITUTE_GLOBAL is set, processing continues with a search for the next 3599 match. If the value is not zero, the current replacement is not ac- 3600 cepted. If the value is greater than zero, processing continues when 3601 PCRE2_SUBSTITUTE_GLOBAL is set. Otherwise (the value is less than zero 3602 or PCRE2_SUBSTITUTE_GLOBAL is not set), the the rest of the input is 3603 copied to the output and the call to pcre2_substitute() exits, return- 3604 ing the number of matches so far. 3605 3606 3607DUPLICATE CAPTURE GROUP NAMES 3608 3609 int pcre2_substring_nametable_scan(const pcre2_code *code, 3610 PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last); 3611 3612 When a pattern is compiled with the PCRE2_DUPNAMES option, names for 3613 capture groups are not required to be unique. Duplicate names are al- 3614 ways allowed for groups with the same number, created by using the (?| 3615 feature. Indeed, if such groups are named, they are required to use the 3616 same names. 3617 3618 Normally, patterns that use duplicate names are such that in any one 3619 match, only one of each set of identically-named groups participates. 3620 An example is shown in the pcre2pattern documentation. 3621 3622 When duplicates are present, pcre2_substring_copy_byname() and 3623 pcre2_substring_get_byname() return the first substring corresponding 3624 to the given name that is set. Only if none are set is PCRE2_ERROR_UN- 3625 SET is returned. The pcre2_substring_number_from_name() function re- 3626 turns the error PCRE2_ERROR_NOUNIQUESUBSTRING when there are duplicate 3627 names. 3628 3629 If you want to get full details of all captured substrings for a given 3630 name, you must use the pcre2_substring_nametable_scan() function. The 3631 first argument is the compiled pattern, and the second is the name. If 3632 the third and fourth arguments are NULL, the function returns a group 3633 number for a unique name, or PCRE2_ERROR_NOUNIQUESUBSTRING otherwise. 3634 3635 When the third and fourth arguments are not NULL, they must be pointers 3636 to variables that are updated by the function. After it has run, they 3637 point to the first and last entries in the name-to-number table for the 3638 given name, and the function returns the length of each entry in code 3639 units. In both cases, PCRE2_ERROR_NOSUBSTRING is returned if there are 3640 no entries for the given name. 3641 3642 The format of the name table is described above in the section entitled 3643 Information about a pattern. Given all the relevant entries for the 3644 name, you can extract each of their numbers, and hence the captured 3645 data. 3646 3647 3648FINDING ALL POSSIBLE MATCHES AT ONE POSITION 3649 3650 The traditional matching function uses a similar algorithm to Perl, 3651 which stops when it finds the first match at a given point in the sub- 3652 ject. If you want to find all possible matches, or the longest possible 3653 match at a given position, consider using the alternative matching 3654 function (see below) instead. If you cannot use the alternative func- 3655 tion, you can kludge it up by making use of the callout facility, which 3656 is described in the pcre2callout documentation. 3657 3658 What you have to do is to insert a callout right at the end of the pat- 3659 tern. When your callout function is called, extract and save the cur- 3660 rent matched substring. Then return 1, which forces pcre2_match() to 3661 backtrack and try other alternatives. Ultimately, when it runs out of 3662 matches, pcre2_match() will yield PCRE2_ERROR_NOMATCH. 3663 3664 3665MATCHING A PATTERN: THE ALTERNATIVE FUNCTION 3666 3667 int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject, 3668 PCRE2_SIZE length, PCRE2_SIZE startoffset, 3669 uint32_t options, pcre2_match_data *match_data, 3670 pcre2_match_context *mcontext, 3671 int *workspace, PCRE2_SIZE wscount); 3672 3673 The function pcre2_dfa_match() is called to match a subject string 3674 against a compiled pattern, using a matching algorithm that scans the 3675 subject string just once (not counting lookaround assertions), and does 3676 not backtrack (except when processing lookaround assertions). This has 3677 different characteristics to the normal algorithm, and is not compati- 3678 ble with Perl. Some of the features of PCRE2 patterns are not sup- 3679 ported. Nevertheless, there are times when this kind of matching can be 3680 useful. For a discussion of the two matching algorithms, and a list of 3681 features that pcre2_dfa_match() does not support, see the pcre2matching 3682 documentation. 3683 3684 The arguments for the pcre2_dfa_match() function are the same as for 3685 pcre2_match(), plus two extras. The ovector within the match data block 3686 is used in a different way, and this is described below. The other com- 3687 mon arguments are used in the same way as for pcre2_match(), so their 3688 description is not repeated here. 3689 3690 The two additional arguments provide workspace for the function. The 3691 workspace vector should contain at least 20 elements. It is used for 3692 keeping track of multiple paths through the pattern tree. More 3693 workspace is needed for patterns and subjects where there are a lot of 3694 potential matches. 3695 3696 Here is an example of a simple call to pcre2_dfa_match(): 3697 3698 int wspace[20]; 3699 pcre2_match_data *md = pcre2_match_data_create(4, NULL); 3700 int rc = pcre2_dfa_match( 3701 re, /* result of pcre2_compile() */ 3702 "some string", /* the subject string */ 3703 11, /* the length of the subject string */ 3704 0, /* start at offset 0 in the subject */ 3705 0, /* default options */ 3706 md, /* the match data block */ 3707 NULL, /* a match context; NULL means use defaults */ 3708 wspace, /* working space vector */ 3709 20); /* number of elements (NOT size in bytes) */ 3710 3711 Option bits for pcre2_dfa_match() 3712 3713 The unused bits of the options argument for pcre2_dfa_match() must be 3714 zero. The only bits that may be set are PCRE2_ANCHORED, 3715 PCRE2_COPY_MATCHED_SUBJECT, PCRE2_ENDANCHORED, PCRE2_NOTBOL, PCRE2_NO- 3716 TEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_UTF_CHECK, 3717 PCRE2_PARTIAL_HARD, PCRE2_PARTIAL_SOFT, PCRE2_DFA_SHORTEST, and 3718 PCRE2_DFA_RESTART. All but the last four of these are exactly the same 3719 as for pcre2_match(), so their description is not repeated here. 3720 3721 PCRE2_PARTIAL_HARD 3722 PCRE2_PARTIAL_SOFT 3723 3724 These have the same general effect as they do for pcre2_match(), but 3725 the details are slightly different. When PCRE2_PARTIAL_HARD is set for 3726 pcre2_dfa_match(), it returns PCRE2_ERROR_PARTIAL if the end of the 3727 subject is reached and there is still at least one matching possibility 3728 that requires additional characters. This happens even if some complete 3729 matches have already been found. When PCRE2_PARTIAL_SOFT is set, the 3730 return code PCRE2_ERROR_NOMATCH is converted into PCRE2_ERROR_PARTIAL 3731 if the end of the subject is reached, there have been no complete 3732 matches, but there is still at least one matching possibility. The por- 3733 tion of the string that was inspected when the longest partial match 3734 was found is set as the first matching string in both cases. There is a 3735 more detailed discussion of partial and multi-segment matching, with 3736 examples, in the pcre2partial documentation. 3737 3738 PCRE2_DFA_SHORTEST 3739 3740 Setting the PCRE2_DFA_SHORTEST option causes the matching algorithm to 3741 stop as soon as it has found one match. Because of the way the alterna- 3742 tive algorithm works, this is necessarily the shortest possible match 3743 at the first possible matching point in the subject string. 3744 3745 PCRE2_DFA_RESTART 3746 3747 When pcre2_dfa_match() returns a partial match, it is possible to call 3748 it again, with additional subject characters, and have it continue with 3749 the same match. The PCRE2_DFA_RESTART option requests this action; when 3750 it is set, the workspace and wscount options must reference the same 3751 vector as before because data about the match so far is left in them 3752 after a partial match. There is more discussion of this facility in the 3753 pcre2partial documentation. 3754 3755 Successful returns from pcre2_dfa_match() 3756 3757 When pcre2_dfa_match() succeeds, it may have matched more than one sub- 3758 string in the subject. Note, however, that all the matches from one run 3759 of the function start at the same point in the subject. The shorter 3760 matches are all initial substrings of the longer matches. For example, 3761 if the pattern 3762 3763 <.*> 3764 3765 is matched against the string 3766 3767 This is <something> <something else> <something further> no more 3768 3769 the three matched strings are 3770 3771 <something> <something else> <something further> 3772 <something> <something else> 3773 <something> 3774 3775 On success, the yield of the function is a number greater than zero, 3776 which is the number of matched substrings. The offsets of the sub- 3777 strings are returned in the ovector, and can be extracted by number in 3778 the same way as for pcre2_match(), but the numbers bear no relation to 3779 any capture groups that may exist in the pattern, because DFA matching 3780 does not support capturing. 3781 3782 Calls to the convenience functions that extract substrings by name re- 3783 turn the error PCRE2_ERROR_DFA_UFUNC (unsupported function) if used af- 3784 ter a DFA match. The convenience functions that extract substrings by 3785 number never return PCRE2_ERROR_NOSUBSTRING. 3786 3787 The matched strings are stored in the ovector in reverse order of 3788 length; that is, the longest matching string is first. If there were 3789 too many matches to fit into the ovector, the yield of the function is 3790 zero, and the vector is filled with the longest matches. 3791 3792 NOTE: PCRE2's "auto-possessification" optimization usually applies to 3793 character repeats at the end of a pattern (as well as internally). For 3794 example, the pattern "a\d+" is compiled as if it were "a\d++". For DFA 3795 matching, this means that only one possible match is found. If you re- 3796 ally do want multiple matches in such cases, either use an ungreedy re- 3797 peat such as "a\d+?" or set the PCRE2_NO_AUTO_POSSESS option when com- 3798 piling. 3799 3800 Error returns from pcre2_dfa_match() 3801 3802 The pcre2_dfa_match() function returns a negative number when it fails. 3803 Many of the errors are the same as for pcre2_match(), as described 3804 above. There are in addition the following errors that are specific to 3805 pcre2_dfa_match(): 3806 3807 PCRE2_ERROR_DFA_UITEM 3808 3809 This return is given if pcre2_dfa_match() encounters an item in the 3810 pattern that it does not support, for instance, the use of \C in a UTF 3811 mode or a backreference. 3812 3813 PCRE2_ERROR_DFA_UCOND 3814 3815 This return is given if pcre2_dfa_match() encounters a condition item 3816 that uses a backreference for the condition, or a test for recursion in 3817 a specific capture group. These are not supported. 3818 3819 PCRE2_ERROR_DFA_UINVALID_UTF 3820 3821 This return is given if pcre2_dfa_match() is called for a pattern that 3822 was compiled with PCRE2_MATCH_INVALID_UTF. This is not supported for 3823 DFA matching. 3824 3825 PCRE2_ERROR_DFA_WSSIZE 3826 3827 This return is given if pcre2_dfa_match() runs out of space in the 3828 workspace vector. 3829 3830 PCRE2_ERROR_DFA_RECURSE 3831 3832 When a recursion or subroutine call is processed, the matching function 3833 calls itself recursively, using private memory for the ovector and 3834 workspace. This error is given if the internal ovector is not large 3835 enough. This should be extremely rare, as a vector of size 1000 is 3836 used. 3837 3838 PCRE2_ERROR_DFA_BADRESTART 3839 3840 When pcre2_dfa_match() is called with the PCRE2_DFA_RESTART option, 3841 some plausibility checks are made on the contents of the workspace, 3842 which should contain data about the previous partial match. If any of 3843 these checks fail, this error is given. 3844 3845 3846SEE ALSO 3847 3848 pcre2build(3), pcre2callout(3), pcre2demo(3), pcre2matching(3), 3849 pcre2partial(3), pcre2posix(3), pcre2sample(3), pcre2unicode(3). 3850 3851 3852AUTHOR 3853 3854 Philip Hazel 3855 Retired from University Computing Service 3856 Cambridge, England. 3857 3858 3859REVISION 3860 3861 Last updated: 27 July 2022 3862 Copyright (c) 1997-2022 University of Cambridge. 3863------------------------------------------------------------------------------ 3864 3865 3866PCRE2BUILD(3) Library Functions Manual PCRE2BUILD(3) 3867 3868 3869 3870NAME 3871 PCRE2 - Perl-compatible regular expressions (revised API) 3872 3873BUILDING PCRE2 3874 3875 PCRE2 is distributed with a configure script that can be used to build 3876 the library in Unix-like environments using the applications known as 3877 Autotools. Also in the distribution are files to support building using 3878 CMake instead of configure. The text file README contains general in- 3879 formation about building with Autotools (some of which is repeated be- 3880 low), and also has some comments about building on various operating 3881 systems. There is a lot more information about building PCRE2 without 3882 using Autotools (including information about using CMake and building 3883 "by hand") in the text file called NON-AUTOTOOLS-BUILD. You should 3884 consult this file as well as the README file if you are building in a 3885 non-Unix-like environment. 3886 3887 3888PCRE2 BUILD-TIME OPTIONS 3889 3890 The rest of this document describes the optional features of PCRE2 that 3891 can be selected when the library is compiled. It assumes use of the 3892 configure script, where the optional features are selected or dese- 3893 lected by providing options to configure before running the make com- 3894 mand. However, the same options can be selected in both Unix-like and 3895 non-Unix-like environments if you are using CMake instead of configure 3896 to build PCRE2. 3897 3898 If you are not using Autotools or CMake, option selection can be done 3899 by editing the config.h file, or by passing parameter settings to the 3900 compiler, as described in NON-AUTOTOOLS-BUILD. 3901 3902 The complete list of options for configure (which includes the standard 3903 ones such as the selection of the installation directory) can be ob- 3904 tained by running 3905 3906 ./configure --help 3907 3908 The following sections include descriptions of "on/off" options whose 3909 names begin with --enable or --disable. Because of the way that config- 3910 ure works, --enable and --disable always come in pairs, so the comple- 3911 mentary option always exists as well, but as it specifies the default, 3912 it is not described. Options that specify values have names that start 3913 with --with. At the end of a configure run, a summary of the configura- 3914 tion is output. 3915 3916 3917BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES 3918 3919 By default, a library called libpcre2-8 is built, containing functions 3920 that take string arguments contained in arrays of bytes, interpreted 3921 either as single-byte characters, or UTF-8 strings. You can also build 3922 two other libraries, called libpcre2-16 and libpcre2-32, which process 3923 strings that are contained in arrays of 16-bit and 32-bit code units, 3924 respectively. These can be interpreted either as single-unit characters 3925 or UTF-16/UTF-32 strings. To build these additional libraries, add one 3926 or both of the following to the configure command: 3927 3928 --enable-pcre2-16 3929 --enable-pcre2-32 3930 3931 If you do not want the 8-bit library, add 3932 3933 --disable-pcre2-8 3934 3935 as well. At least one of the three libraries must be built. Note that 3936 the POSIX wrapper is for the 8-bit library only, and that pcre2grep is 3937 an 8-bit program. Neither of these are built if you select only the 3938 16-bit or 32-bit libraries. 3939 3940 3941BUILDING SHARED AND STATIC LIBRARIES 3942 3943 The Autotools PCRE2 building process uses libtool to build both shared 3944 and static libraries by default. You can suppress an unwanted library 3945 by adding one of 3946 3947 --disable-shared 3948 --disable-static 3949 3950 to the configure command. 3951 3952 3953UNICODE AND UTF SUPPORT 3954 3955 By default, PCRE2 is built with support for Unicode and UTF character 3956 strings. To build it without Unicode support, add 3957 3958 --disable-unicode 3959 3960 to the configure command. This setting applies to all three libraries. 3961 It is not possible to build one library with Unicode support and an- 3962 other without in the same configuration. 3963 3964 Of itself, Unicode support does not make PCRE2 treat strings as UTF-8, 3965 UTF-16 or UTF-32. To do that, applications that use the library can set 3966 the PCRE2_UTF option when they call pcre2_compile() to compile a pat- 3967 tern. Alternatively, patterns may be started with (*UTF) unless the 3968 application has locked this out by setting PCRE2_NEVER_UTF. 3969 3970 UTF support allows the libraries to process character code points up to 3971 0x10ffff in the strings that they handle. Unicode support also gives 3972 access to the Unicode properties of characters, using pattern escapes 3973 such as \P, \p, and \X. Only the general category properties such as Lu 3974 and Nd, script names, and some bi-directional properties are supported. 3975 Details are given in the pcre2pattern documentation. 3976 3977 Pattern escapes such as \d and \w do not by default make use of Unicode 3978 properties. The application can request that they do by setting the 3979 PCRE2_UCP option. Unless the application has set PCRE2_NEVER_UCP, a 3980 pattern may also request this by starting with (*UCP). 3981 3982 3983DISABLING THE USE OF \C 3984 3985 The \C escape sequence, which matches a single code unit, even in a UTF 3986 mode, can cause unpredictable behaviour because it may leave the cur- 3987 rent matching point in the middle of a multi-code-unit character. The 3988 application can lock it out by setting the PCRE2_NEVER_BACKSLASH_C op- 3989 tion when calling pcre2_compile(). There is also a build-time option 3990 3991 --enable-never-backslash-C 3992 3993 (note the upper case C) which locks out the use of \C entirely. 3994 3995 3996JUST-IN-TIME COMPILER SUPPORT 3997 3998 Just-in-time (JIT) compiler support is included in the build by speci- 3999 fying 4000 4001 --enable-jit 4002 4003 This support is available only for certain hardware architectures. If 4004 this option is set for an unsupported architecture, a building error 4005 occurs. If in doubt, use 4006 4007 --enable-jit=auto 4008 4009 which enables JIT only if the current hardware is supported. You can 4010 check if JIT is enabled in the configuration summary that is output at 4011 the end of a configure run. If you are enabling JIT under SELinux you 4012 may also want to add 4013 4014 --enable-jit-sealloc 4015 4016 which enables the use of an execmem allocator in JIT that is compatible 4017 with SELinux. This has no effect if JIT is not enabled. See the 4018 pcre2jit documentation for a discussion of JIT usage. When JIT support 4019 is enabled, pcre2grep automatically makes use of it, unless you add 4020 4021 --disable-pcre2grep-jit 4022 4023 to the configure command. 4024 4025 4026NEWLINE RECOGNITION 4027 4028 By default, PCRE2 interprets the linefeed (LF) character as indicating 4029 the end of a line. This is the normal newline character on Unix-like 4030 systems. You can compile PCRE2 to use carriage return (CR) instead, by 4031 adding 4032 4033 --enable-newline-is-cr 4034 4035 to the configure command. There is also an --enable-newline-is-lf op- 4036 tion, which explicitly specifies linefeed as the newline character. 4037 4038 Alternatively, you can specify that line endings are to be indicated by 4039 the two-character sequence CRLF (CR immediately followed by LF). If you 4040 want this, add 4041 4042 --enable-newline-is-crlf 4043 4044 to the configure command. There is a fourth option, specified by 4045 4046 --enable-newline-is-anycrlf 4047 4048 which causes PCRE2 to recognize any of the three sequences CR, LF, or 4049 CRLF as indicating a line ending. A fifth option, specified by 4050 4051 --enable-newline-is-any 4052 4053 causes PCRE2 to recognize any Unicode newline sequence. The Unicode 4054 newline sequences are the three just mentioned, plus the single charac- 4055 ters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line, 4056 U+0085), LS (line separator, U+2028), and PS (paragraph separator, 4057 U+2029). The final option is 4058 4059 --enable-newline-is-nul 4060 4061 which causes NUL (binary zero) to be set as the default line-ending 4062 character. 4063 4064 Whatever default line ending convention is selected when PCRE2 is built 4065 can be overridden by applications that use the library. At build time 4066 it is recommended to use the standard for your operating system. 4067 4068 4069WHAT \R MATCHES 4070 4071 By default, the sequence \R in a pattern matches any Unicode newline 4072 sequence, independently of what has been selected as the line ending 4073 sequence. If you specify 4074 4075 --enable-bsr-anycrlf 4076 4077 the default is changed so that \R matches only CR, LF, or CRLF. What- 4078 ever is selected when PCRE2 is built can be overridden by applications 4079 that use the library. 4080 4081 4082HANDLING VERY LARGE PATTERNS 4083 4084 Within a compiled pattern, offset values are used to point from one 4085 part to another (for example, from an opening parenthesis to an alter- 4086 nation metacharacter). By default, in the 8-bit and 16-bit libraries, 4087 two-byte values are used for these offsets, leading to a maximum size 4088 for a compiled pattern of around 64 thousand code units. This is suffi- 4089 cient to handle all but the most gigantic patterns. Nevertheless, some 4090 people do want to process truly enormous patterns, so it is possible to 4091 compile PCRE2 to use three-byte or four-byte offsets by adding a set- 4092 ting such as 4093 4094 --with-link-size=3 4095 4096 to the configure command. The value given must be 2, 3, or 4. For the 4097 16-bit library, a value of 3 is rounded up to 4. In these libraries, 4098 using longer offsets slows down the operation of PCRE2 because it has 4099 to load additional data when handling them. For the 32-bit library the 4100 value is always 4 and cannot be overridden; the value of --with-link- 4101 size is ignored. 4102 4103 4104LIMITING PCRE2 RESOURCE USAGE 4105 4106 The pcre2_match() function increments a counter each time it goes round 4107 its main loop. Putting a limit on this counter controls the amount of 4108 computing resource used by a single call to pcre2_match(). The limit 4109 can be changed at run time, as described in the pcre2api documentation. 4110 The default is 10 million, but this can be changed by adding a setting 4111 such as 4112 4113 --with-match-limit=500000 4114 4115 to the configure command. This setting also applies to the 4116 pcre2_dfa_match() matching function, and to JIT matching (though the 4117 counting is done differently). 4118 4119 The pcre2_match() function uses heap memory to record backtracking 4120 points. The more nested backtracking points there are (that is, the 4121 deeper the search tree), the more memory is needed. There is an upper 4122 limit, specified in kibibytes (units of 1024 bytes). This limit can be 4123 changed at run time, as described in the pcre2api documentation. The 4124 default limit (in effect unlimited) is 20 million. You can change this 4125 by a setting such as 4126 4127 --with-heap-limit=500 4128 4129 which limits the amount of heap to 500 KiB. This limit applies only to 4130 interpretive matching in pcre2_match() and pcre2_dfa_match(), which may 4131 also use the heap for internal workspace when processing complicated 4132 patterns. This limit does not apply when JIT (which has its own memory 4133 arrangements) is used. 4134 4135 You can also explicitly limit the depth of nested backtracking in the 4136 pcre2_match() interpreter. This limit defaults to the value that is set 4137 for --with-match-limit. You can set a lower default limit by adding, 4138 for example, 4139 4140 --with-match-limit-depth=10000 4141 4142 to the configure command. This value can be overridden at run time. 4143 This depth limit indirectly limits the amount of heap memory that is 4144 used, but because the size of each backtracking "frame" depends on the 4145 number of capturing parentheses in a pattern, the amount of heap that 4146 is used before the limit is reached varies from pattern to pattern. 4147 This limit was more useful in versions before 10.30, where function re- 4148 cursion was used for backtracking. 4149 4150 As well as applying to pcre2_match(), the depth limit also controls the 4151 depth of recursive function calls in pcre2_dfa_match(). These are used 4152 for lookaround assertions, atomic groups, and recursion within pat- 4153 terns. The limit does not apply to JIT matching. 4154 4155 4156CREATING CHARACTER TABLES AT BUILD TIME 4157 4158 PCRE2 uses fixed tables for processing characters whose code points are 4159 less than 256. By default, PCRE2 is built with a set of tables that are 4160 distributed in the file src/pcre2_chartables.c.dist. These tables are 4161 for ASCII codes only. If you add 4162 4163 --enable-rebuild-chartables 4164 4165 to the configure command, the distributed tables are no longer used. 4166 Instead, a program called pcre2_dftables is compiled and run. This out- 4167 puts the source for new set of tables, created in the default locale of 4168 your C run-time system. This method of replacing the tables does not 4169 work if you are cross compiling, because pcre2_dftables needs to be run 4170 on the local host and therefore not compiled with the cross compiler. 4171 4172 If you need to create alternative tables when cross compiling, you will 4173 have to do so "by hand". There may also be other reasons for creating 4174 tables manually. To cause pcre2_dftables to be built on the local 4175 host, run a normal compiling command, and then run the program with the 4176 output file as its argument, for example: 4177 4178 cc src/pcre2_dftables.c -o pcre2_dftables 4179 ./pcre2_dftables src/pcre2_chartables.c 4180 4181 This builds the tables in the default locale of the local host. If you 4182 want to specify a locale, you must use the -L option: 4183 4184 LC_ALL=fr_FR ./pcre2_dftables -L src/pcre2_chartables.c 4185 4186 You can also specify -b (with or without -L). This causes the tables to 4187 be written in binary instead of as source code. A set of binary tables 4188 can be loaded into memory by an application and passed to pcre2_com- 4189 pile() in the same way as tables created by calling pcre2_maketables(). 4190 The tables are just a string of bytes, independent of hardware charac- 4191 teristics such as endianness. This means they can be bundled with an 4192 application that runs in different environments, to ensure consistent 4193 behaviour. 4194 4195 4196USING EBCDIC CODE 4197 4198 PCRE2 assumes by default that it will run in an environment where the 4199 character code is ASCII or Unicode, which is a superset of ASCII. This 4200 is the case for most computer operating systems. PCRE2 can, however, be 4201 compiled to run in an 8-bit EBCDIC environment by adding 4202 4203 --enable-ebcdic --disable-unicode 4204 4205 to the configure command. This setting implies --enable-rebuild-charta- 4206 bles. You should only use it if you know that you are in an EBCDIC en- 4207 vironment (for example, an IBM mainframe operating system). 4208 4209 It is not possible to support both EBCDIC and UTF-8 codes in the same 4210 version of the library. Consequently, --enable-unicode and --enable- 4211 ebcdic are mutually exclusive. 4212 4213 The EBCDIC character that corresponds to an ASCII LF is assumed to have 4214 the value 0x15 by default. However, in some EBCDIC environments, 0x25 4215 is used. In such an environment you should use 4216 4217 --enable-ebcdic-nl25 4218 4219 as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR 4220 has the same value as in ASCII, namely, 0x0d. Whichever of 0x15 and 4221 0x25 is not chosen as LF is made to correspond to the Unicode NEL char- 4222 acter (which, in Unicode, is 0x85). 4223 4224 The options that select newline behaviour, such as --enable-newline-is- 4225 cr, and equivalent run-time options, refer to these character values in 4226 an EBCDIC environment. 4227 4228 4229PCRE2GREP SUPPORT FOR EXTERNAL SCRIPTS 4230 4231 By default pcre2grep supports the use of callouts with string arguments 4232 within the patterns it is matching. There are two kinds: one that gen- 4233 erates output using local code, and another that calls an external pro- 4234 gram or script. If --disable-pcre2grep-callout-fork is added to the 4235 configure command, only the first kind of callout is supported; if 4236 --disable-pcre2grep-callout is used, all callouts are completely ig- 4237 nored. For more details of pcre2grep callouts, see the pcre2grep docu- 4238 mentation. 4239 4240 4241PCRE2GREP OPTIONS FOR COMPRESSED FILE SUPPORT 4242 4243 By default, pcre2grep reads all files as plain text. You can build it 4244 so that it recognizes files whose names end in .gz or .bz2, and reads 4245 them with libz or libbz2, respectively, by adding one or both of 4246 4247 --enable-pcre2grep-libz 4248 --enable-pcre2grep-libbz2 4249 4250 to the configure command. These options naturally require that the rel- 4251 evant libraries are installed on your system. Configuration will fail 4252 if they are not. 4253 4254 4255PCRE2GREP BUFFER SIZE 4256 4257 pcre2grep uses an internal buffer to hold a "window" on the file it is 4258 scanning, in order to be able to output "before" and "after" lines when 4259 it finds a match. The default starting size of the buffer is 20KiB. The 4260 buffer itself is three times this size, but because of the way it is 4261 used for holding "before" lines, the longest line that is guaranteed to 4262 be processable is the notional buffer size. If a longer line is encoun- 4263 tered, pcre2grep automatically expands the buffer, up to a specified 4264 maximum size, whose default is 1MiB or the starting size, whichever is 4265 the larger. You can change the default parameter values by adding, for 4266 example, 4267 4268 --with-pcre2grep-bufsize=51200 4269 --with-pcre2grep-max-bufsize=2097152 4270 4271 to the configure command. The caller of pcre2grep can override these 4272 values by using --buffer-size and --max-buffer-size on the command 4273 line. 4274 4275 4276PCRE2TEST OPTION FOR LIBREADLINE SUPPORT 4277 4278 If you add one of 4279 4280 --enable-pcre2test-libreadline 4281 --enable-pcre2test-libedit 4282 4283 to the configure command, pcre2test is linked with the libreadline or- 4284 libedit library, respectively, and when its input is from a terminal, 4285 it reads it using the readline() function. This provides line-editing 4286 and history facilities. Note that libreadline is GPL-licensed, so if 4287 you distribute a binary of pcre2test linked in this way, there may be 4288 licensing issues. These can be avoided by linking instead with libedit, 4289 which has a BSD licence. 4290 4291 Setting --enable-pcre2test-libreadline causes the -lreadline option to 4292 be added to the pcre2test build. In many operating environments with a 4293 sytem-installed readline library this is sufficient. However, in some 4294 environments (e.g. if an unmodified distribution version of readline is 4295 in use), some extra configuration may be necessary. The INSTALL file 4296 for libreadline says this: 4297 4298 "Readline uses the termcap functions, but does not link with 4299 the termcap or curses library itself, allowing applications 4300 which link with readline the to choose an appropriate library." 4301 4302 If your environment has not been set up so that an appropriate library 4303 is automatically included, you may need to add something like 4304 4305 LIBS="-ncurses" 4306 4307 immediately before the configure command. 4308 4309 4310INCLUDING DEBUGGING CODE 4311 4312 If you add 4313 4314 --enable-debug 4315 4316 to the configure command, additional debugging code is included in the 4317 build. This feature is intended for use by the PCRE2 maintainers. 4318 4319 4320DEBUGGING WITH VALGRIND SUPPORT 4321 4322 If you add 4323 4324 --enable-valgrind 4325 4326 to the configure command, PCRE2 will use valgrind annotations to mark 4327 certain memory regions as unaddressable. This allows it to detect in- 4328 valid memory accesses, and is mostly useful for debugging PCRE2 itself. 4329 4330 4331CODE COVERAGE REPORTING 4332 4333 If your C compiler is gcc, you can build a version of PCRE2 that can 4334 generate a code coverage report for its test suite. To enable this, you 4335 must install lcov version 1.6 or above. Then specify 4336 4337 --enable-coverage 4338 4339 to the configure command and build PCRE2 in the usual way. 4340 4341 Note that using ccache (a caching C compiler) is incompatible with code 4342 coverage reporting. If you have configured ccache to run automatically 4343 on your system, you must set the environment variable 4344 4345 CCACHE_DISABLE=1 4346 4347 before running make to build PCRE2, so that ccache is not used. 4348 4349 When --enable-coverage is used, the following addition targets are 4350 added to the Makefile: 4351 4352 make coverage 4353 4354 This creates a fresh coverage report for the PCRE2 test suite. It is 4355 equivalent to running "make coverage-reset", "make coverage-baseline", 4356 "make check", and then "make coverage-report". 4357 4358 make coverage-reset 4359 4360 This zeroes the coverage counters, but does nothing else. 4361 4362 make coverage-baseline 4363 4364 This captures baseline coverage information. 4365 4366 make coverage-report 4367 4368 This creates the coverage report. 4369 4370 make coverage-clean-report 4371 4372 This removes the generated coverage report without cleaning the cover- 4373 age data itself. 4374 4375 make coverage-clean-data 4376 4377 This removes the captured coverage data without removing the coverage 4378 files created at compile time (*.gcno). 4379 4380 make coverage-clean 4381 4382 This cleans all coverage data including the generated coverage report. 4383 For more information about code coverage, see the gcov and lcov docu- 4384 mentation. 4385 4386 4387DISABLING THE Z AND T FORMATTING MODIFIERS 4388 4389 The C99 standard defines formatting modifiers z and t for size_t and 4390 ptrdiff_t values, respectively. By default, PCRE2 uses these modifiers 4391 in environments other than old versions of Microsoft Visual Studio when 4392 __STDC_VERSION__ is defined and has a value greater than or equal to 4393 199901L (indicating support for C99). However, there is at least one 4394 environment that claims to be C99 but does not support these modifiers. 4395 If 4396 4397 --disable-percent-zt 4398 4399 is specified, no use is made of the z or t modifiers. Instead of %td or 4400 %zu, a suitable format is used depending in the size of long for the 4401 platform. 4402 4403 4404SUPPORT FOR FUZZERS 4405 4406 There is a special option for use by people who want to run fuzzing 4407 tests on PCRE2: 4408 4409 --enable-fuzz-support 4410 4411 At present this applies only to the 8-bit library. If set, it causes an 4412 extra library called libpcre2-fuzzsupport.a to be built, but not in- 4413 stalled. This contains a single function called LLVMFuzzerTestOneIn- 4414 put() whose arguments are a pointer to a string and the length of the 4415 string. When called, this function tries to compile the string as a 4416 pattern, and if that succeeds, to match it. This is done both with no 4417 options and with some random options bits that are generated from the 4418 string. 4419 4420 Setting --enable-fuzz-support also causes a binary called pcre2fuz- 4421 zcheck to be created. This is normally run under valgrind or used when 4422 PCRE2 is compiled with address sanitizing enabled. It calls the fuzzing 4423 function and outputs information about what it is doing. The input 4424 strings are specified by arguments: if an argument starts with "=" the 4425 rest of it is a literal input string. Otherwise, it is assumed to be a 4426 file name, and the contents of the file are the test string. 4427 4428 4429OBSOLETE OPTION 4430 4431 In versions of PCRE2 prior to 10.30, there were two ways of handling 4432 backtracking in the pcre2_match() function. The default was to use the 4433 system stack, but if 4434 4435 --disable-stack-for-recursion 4436 4437 was set, memory on the heap was used. From release 10.30 onwards this 4438 has changed (the stack is no longer used) and this option now does 4439 nothing except give a warning. 4440 4441 4442SEE ALSO 4443 4444 pcre2api(3), pcre2-config(3). 4445 4446 4447AUTHOR 4448 4449 Philip Hazel 4450 Retired from University Computing Service 4451 Cambridge, England. 4452 4453 4454REVISION 4455 4456 Last updated: 27 July 2022 4457 Copyright (c) 1997-2022 University of Cambridge. 4458------------------------------------------------------------------------------ 4459 4460 4461PCRE2CALLOUT(3) Library Functions Manual PCRE2CALLOUT(3) 4462 4463 4464 4465NAME 4466 PCRE2 - Perl-compatible regular expressions (revised API) 4467 4468SYNOPSIS 4469 4470 #include <pcre2.h> 4471 4472 int (*pcre2_callout)(pcre2_callout_block *, void *); 4473 4474 int pcre2_callout_enumerate(const pcre2_code *code, 4475 int (*callback)(pcre2_callout_enumerate_block *, void *), 4476 void *user_data); 4477 4478 4479DESCRIPTION 4480 4481 PCRE2 provides a feature called "callout", which is a means of tempo- 4482 rarily passing control to the caller of PCRE2 in the middle of pattern 4483 matching. The caller of PCRE2 provides an external function by putting 4484 its entry point in a match context (see pcre2_set_callout() in the 4485 pcre2api documentation). 4486 4487 When using the pcre2_substitute() function, an additional callout fea- 4488 ture is available. This does a callout after each change to the subject 4489 string and is described in the pcre2api documentation; the rest of this 4490 document is concerned with callouts during pattern matching. 4491 4492 Within a regular expression, (?C<arg>) indicates a point at which the 4493 external function is to be called. Different callout points can be 4494 identified by putting a number less than 256 after the letter C. The 4495 default value is zero. Alternatively, the argument may be a delimited 4496 string. The starting delimiter must be one of ` ' " ^ % # $ { and the 4497 ending delimiter is the same as the start, except for {, where the end- 4498 ing delimiter is }. If the ending delimiter is needed within the 4499 string, it must be doubled. For example, this pattern has two callout 4500 points: 4501 4502 (?C1)abc(?C"some ""arbitrary"" text")def 4503 4504 If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled, 4505 PCRE2 automatically inserts callouts, all with number 255, before each 4506 item in the pattern except for immediately before or after an explicit 4507 callout. For example, if PCRE2_AUTO_CALLOUT is used with the pattern 4508 4509 A(?C3)B 4510 4511 it is processed as if it were 4512 4513 (?C255)A(?C3)B(?C255) 4514 4515 Here is a more complicated example: 4516 4517 A(\d{2}|--) 4518 4519 With PCRE2_AUTO_CALLOUT, this pattern is processed as if it were 4520 4521 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) 4522 4523 Notice that there is a callout before and after each parenthesis and 4524 alternation bar. If the pattern contains a conditional group whose con- 4525 dition is an assertion, an automatic callout is inserted immediately 4526 before the condition. Such a callout may also be inserted explicitly, 4527 for example: 4528 4529 (?(?C9)(?=a)ab|de) (?(?C%text%)(?!=d)ab|de) 4530 4531 This applies only to assertion conditions (because they are themselves 4532 independent groups). 4533 4534 Callouts can be useful for tracking the progress of pattern matching. 4535 The pcre2test program has a pattern qualifier (/auto_callout) that sets 4536 automatic callouts. When any callouts are present, the output from 4537 pcre2test indicates how the pattern is being matched. This is useful 4538 information when you are trying to optimize the performance of a par- 4539 ticular pattern. 4540 4541 4542MISSING CALLOUTS 4543 4544 You should be aware that, because of optimizations in the way PCRE2 4545 compiles and matches patterns, callouts sometimes do not happen exactly 4546 as you might expect. 4547 4548 Auto-possessification 4549 4550 At compile time, PCRE2 "auto-possessifies" repeated items when it knows 4551 that what follows cannot be part of the repeat. For example, a+[bc] is 4552 compiled as if it were a++[bc]. The pcre2test output when this pattern 4553 is compiled with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied 4554 to the string "aaaa" is: 4555 4556 --->aaaa 4557 +0 ^ a+ 4558 +2 ^ ^ [bc] 4559 No match 4560 4561 This indicates that when matching [bc] fails, there is no backtracking 4562 into a+ (because it is being treated as a++) and therefore the callouts 4563 that would be taken for the backtracks do not occur. You can disable 4564 the auto-possessify feature by passing PCRE2_NO_AUTO_POSSESS to 4565 pcre2_compile(), or starting the pattern with (*NO_AUTO_POSSESS). In 4566 this case, the output changes to this: 4567 4568 --->aaaa 4569 +0 ^ a+ 4570 +2 ^ ^ [bc] 4571 +2 ^ ^ [bc] 4572 +2 ^ ^ [bc] 4573 +2 ^^ [bc] 4574 No match 4575 4576 This time, when matching [bc] fails, the matcher backtracks into a+ and 4577 tries again, repeatedly, until a+ itself fails. 4578 4579 Automatic .* anchoring 4580 4581 By default, an optimization is applied when .* is the first significant 4582 item in a pattern. If PCRE2_DOTALL is set, so that the dot can match 4583 any character, the pattern is automatically anchored. If PCRE2_DOTALL 4584 is not set, a match can start only after an internal newline or at the 4585 beginning of the subject, and pcre2_compile() remembers this. If a pat- 4586 tern has more than one top-level branch, automatic anchoring occurs if 4587 all branches are anchorable. 4588 4589 This optimization is disabled, however, if .* is in an atomic group or 4590 if there is a backreference to the capture group in which it appears. 4591 It is also disabled if the pattern contains (*PRUNE) or (*SKIP). How- 4592 ever, the presence of callouts does not affect it. 4593 4594 For example, if the pattern .*\d is compiled with PCRE2_AUTO_CALLOUT 4595 and applied to the string "aa", the pcre2test output is: 4596 4597 --->aa 4598 +0 ^ .* 4599 +2 ^ ^ \d 4600 +2 ^^ \d 4601 +2 ^ \d 4602 No match 4603 4604 This shows that all match attempts start at the beginning of the sub- 4605 ject. In other words, the pattern is anchored. You can disable this op- 4606 timization by passing PCRE2_NO_DOTSTAR_ANCHOR to pcre2_compile(), or 4607 starting the pattern with (*NO_DOTSTAR_ANCHOR). In this case, the out- 4608 put changes to: 4609 4610 --->aa 4611 +0 ^ .* 4612 +2 ^ ^ \d 4613 +2 ^^ \d 4614 +2 ^ \d 4615 +0 ^ .* 4616 +2 ^^ \d 4617 +2 ^ \d 4618 No match 4619 4620 This shows more match attempts, starting at the second subject charac- 4621 ter. Another optimization, described in the next section, means that 4622 there is no subsequent attempt to match with an empty subject. 4623 4624 Other optimizations 4625 4626 Other optimizations that provide fast "no match" results also affect 4627 callouts. For example, if the pattern is 4628 4629 ab(?C4)cd 4630 4631 PCRE2 knows that any matching string must contain the letter "d". If 4632 the subject string is "abyz", the lack of "d" means that matching 4633 doesn't ever start, and the callout is never reached. However, with 4634 "abyd", though the result is still no match, the callout is obeyed. 4635 4636 For most patterns PCRE2 also knows the minimum length of a matching 4637 string, and will immediately give a "no match" return without actually 4638 running a match if the subject is not long enough, or, for unanchored 4639 patterns, if it has been scanned far enough. 4640 4641 You can disable these optimizations by passing the PCRE2_NO_START_OPTI- 4642 MIZE option to pcre2_compile(), or by starting the pattern with 4643 (*NO_START_OPT). This slows down the matching process, but does ensure 4644 that callouts such as the example above are obeyed. 4645 4646 4647THE CALLOUT INTERFACE 4648 4649 During matching, when PCRE2 reaches a callout point, if an external 4650 function is provided in the match context, it is called. This applies 4651 to both normal, DFA, and JIT matching. The first argument to the call- 4652 out function is a pointer to a pcre2_callout block. The second argument 4653 is the void * callout data that was supplied when the callout was set 4654 up by calling pcre2_set_callout() (see the pcre2api documentation). The 4655 callout block structure contains the following fields, not necessarily 4656 in this order: 4657 4658 uint32_t version; 4659 uint32_t callout_number; 4660 uint32_t capture_top; 4661 uint32_t capture_last; 4662 uint32_t callout_flags; 4663 PCRE2_SIZE *offset_vector; 4664 PCRE2_SPTR mark; 4665 PCRE2_SPTR subject; 4666 PCRE2_SIZE subject_length; 4667 PCRE2_SIZE start_match; 4668 PCRE2_SIZE current_position; 4669 PCRE2_SIZE pattern_position; 4670 PCRE2_SIZE next_item_length; 4671 PCRE2_SIZE callout_string_offset; 4672 PCRE2_SIZE callout_string_length; 4673 PCRE2_SPTR callout_string; 4674 4675 The version field contains the version number of the block format. The 4676 current version is 2; the three callout string fields were added for 4677 version 1, and the callout_flags field for version 2. If you are writ- 4678 ing an application that might use an earlier release of PCRE2, you 4679 should check the version number before accessing any of these fields. 4680 The version number will increase in future if more fields are added, 4681 but the intention is never to remove any of the existing fields. 4682 4683 Fields for numerical callouts 4684 4685 For a numerical callout, callout_string is NULL, and callout_number 4686 contains the number of the callout, in the range 0-255. This is the 4687 number that follows (?C for callouts that part of the pattern; it is 4688 255 for automatically generated callouts. 4689 4690 Fields for string callouts 4691 4692 For callouts with string arguments, callout_number is always zero, and 4693 callout_string points to the string that is contained within the com- 4694 piled pattern. Its length is given by callout_string_length. Duplicated 4695 ending delimiters that were present in the original pattern string have 4696 been turned into single characters, but there is no other processing of 4697 the callout string argument. An additional code unit containing binary 4698 zero is present after the string, but is not included in the length. 4699 The delimiter that was used to start the string is also stored within 4700 the pattern, immediately before the string itself. You can access this 4701 delimiter as callout_string[-1] if you need it. 4702 4703 The callout_string_offset field is the code unit offset to the start of 4704 the callout argument string within the original pattern string. This is 4705 provided for the benefit of applications such as script languages that 4706 might need to report errors in the callout string within the pattern. 4707 4708 Fields for all callouts 4709 4710 The remaining fields in the callout block are the same for both kinds 4711 of callout. 4712 4713 The offset_vector field is a pointer to a vector of capturing offsets 4714 (the "ovector"). You may read the elements in this vector, but you must 4715 not change any of them. 4716 4717 For calls to pcre2_match(), the offset_vector field is not (since re- 4718 lease 10.30) a pointer to the actual ovector that was passed to the 4719 matching function in the match data block. Instead it points to an in- 4720 ternal ovector of a size large enough to hold all possible captured 4721 substrings in the pattern. Note that whenever a recursion or subroutine 4722 call within a pattern completes, the capturing state is reset to what 4723 it was before. 4724 4725 The capture_last field contains the number of the most recently cap- 4726 tured substring, and the capture_top field contains one more than the 4727 number of the highest numbered captured substring so far. If no sub- 4728 strings have yet been captured, the value of capture_last is 0 and the 4729 value of capture_top is 1. The values of these fields do not always 4730 differ by one; for example, when the callout in the pattern 4731 ((a)(b))(?C2) is taken, capture_last is 1 but capture_top is 4. 4732 4733 The contents of ovector[2] to ovector[<capture_top>*2-1] can be in- 4734 spected in order to extract substrings that have been matched so far, 4735 in the same way as extracting substrings after a match has completed. 4736 The values in ovector[0] and ovector[1] are always PCRE2_UNSET because 4737 the match is by definition not complete. Substrings that have not been 4738 captured but whose numbers are less than capture_top also have both of 4739 their ovector slots set to PCRE2_UNSET. 4740 4741 For DFA matching, the offset_vector field points to the ovector that 4742 was passed to the matching function in the match data block for call- 4743 outs at the top level, but to an internal ovector during the processing 4744 of pattern recursions, lookarounds, and atomic groups. However, these 4745 ovectors hold no useful information because pcre2_dfa_match() does not 4746 support substring capturing. The value of capture_top is always 1 and 4747 the value of capture_last is always 0 for DFA matching. 4748 4749 The subject and subject_length fields contain copies of the values that 4750 were passed to the matching function. 4751 4752 The start_match field normally contains the offset within the subject 4753 at which the current match attempt started. However, if the escape se- 4754 quence \K has been encountered, this value is changed to reflect the 4755 modified starting point. If the pattern is not anchored, the callout 4756 function may be called several times from the same point in the pattern 4757 for different starting points in the subject. 4758 4759 The current_position field contains the offset within the subject of 4760 the current match pointer. 4761 4762 The pattern_position field contains the offset in the pattern string to 4763 the next item to be matched. 4764 4765 The next_item_length field contains the length of the next item to be 4766 processed in the pattern string. When the callout is at the end of the 4767 pattern, the length is zero. When the callout precedes an opening 4768 parenthesis, the length includes meta characters that follow the paren- 4769 thesis. For example, in a callout before an assertion such as (?=ab) 4770 the length is 3. For an an alternation bar or a closing parenthesis, 4771 the length is one, unless a closing parenthesis is followed by a quan- 4772 tifier, in which case its length is included. (This changed in release 4773 10.23. In earlier releases, before an opening parenthesis the length 4774 was that of the entire group, and before an alternation bar or a clos- 4775 ing parenthesis the length was zero.) 4776 4777 The pattern_position and next_item_length fields are intended to help 4778 in distinguishing between different automatic callouts, which all have 4779 the same callout number. However, they are set for all callouts, and 4780 are used by pcre2test to show the next item to be matched when display- 4781 ing callout information. 4782 4783 In callouts from pcre2_match() the mark field contains a pointer to the 4784 zero-terminated name of the most recently passed (*MARK), (*PRUNE), or 4785 (*THEN) item in the match, or NULL if no such items have been passed. 4786 Instances of (*PRUNE) or (*THEN) without a name do not obliterate a 4787 previous (*MARK). In callouts from the DFA matching function this field 4788 always contains NULL. 4789 4790 The callout_flags field is always zero in callouts from 4791 pcre2_dfa_match() or when JIT is being used. When pcre2_match() without 4792 JIT is used, the following bits may be set: 4793 4794 PCRE2_CALLOUT_STARTMATCH 4795 4796 This is set for the first callout after the start of matching for each 4797 new starting position in the subject. 4798 4799 PCRE2_CALLOUT_BACKTRACK 4800 4801 This is set if there has been a matching backtrack since the previous 4802 callout, or since the start of matching if this is the first callout 4803 from a pcre2_match() run. 4804 4805 Both bits are set when a backtrack has caused a "bumpalong" to a new 4806 starting position in the subject. Output from pcre2test does not indi- 4807 cate the presence of these bits unless the callout_extra modifier is 4808 set. 4809 4810 The information in the callout_flags field is provided so that applica- 4811 tions can track and tell their users how matching with backtracking is 4812 done. This can be useful when trying to optimize patterns, or just to 4813 understand how PCRE2 works. There is no support in pcre2_dfa_match() 4814 because there is no backtracking in DFA matching, and there is no sup- 4815 port in JIT because JIT is all about maximimizing matching performance. 4816 In both these cases the callout_flags field is always zero. 4817 4818 4819RETURN VALUES FROM CALLOUTS 4820 4821 The external callout function returns an integer to PCRE2. If the value 4822 is zero, matching proceeds as normal. If the value is greater than 4823 zero, matching fails at the current point, but the testing of other 4824 matching possibilities goes ahead, just as if a lookahead assertion had 4825 failed. If the value is less than zero, the match is abandoned, and the 4826 matching function returns the negative value. 4827 4828 Negative values should normally be chosen from the set of PCRE2_ER- 4829 ROR_xxx values. In particular, PCRE2_ERROR_NOMATCH forces a standard 4830 "no match" failure. The error number PCRE2_ERROR_CALLOUT is reserved 4831 for use by callout functions; it will never be used by PCRE2 itself. 4832 4833 4834CALLOUT ENUMERATION 4835 4836 int pcre2_callout_enumerate(const pcre2_code *code, 4837 int (*callback)(pcre2_callout_enumerate_block *, void *), 4838 void *user_data); 4839 4840 A script language that supports the use of string arguments in callouts 4841 might like to scan all the callouts in a pattern before running the 4842 match. This can be done by calling pcre2_callout_enumerate(). The first 4843 argument is a pointer to a compiled pattern, the second points to a 4844 callback function, and the third is arbitrary user data. The callback 4845 function is called for every callout in the pattern in the order in 4846 which they appear. Its first argument is a pointer to a callout enumer- 4847 ation block, and its second argument is the user_data value that was 4848 passed to pcre2_callout_enumerate(). The data block contains the fol- 4849 lowing fields: 4850 4851 version Block version number 4852 pattern_position Offset to next item in pattern 4853 next_item_length Length of next item in pattern 4854 callout_number Number for numbered callouts 4855 callout_string_offset Offset to string within pattern 4856 callout_string_length Length of callout string 4857 callout_string Points to callout string or is NULL 4858 4859 The version number is currently 0. It will increase if new fields are 4860 ever added to the block. The remaining fields are the same as their 4861 namesakes in the pcre2_callout block that is used for callouts during 4862 matching, as described above. 4863 4864 Note that the value of pattern_position is unique for each callout. 4865 However, if a callout occurs inside a group that is quantified with a 4866 non-zero minimum or a fixed maximum, the group is replicated inside the 4867 compiled pattern. For example, a pattern such as /(a){2}/ is compiled 4868 as if it were /(a)(a)/. This means that the callout will be enumerated 4869 more than once, but with the same value for pattern_position in each 4870 case. 4871 4872 The callback function should normally return zero. If it returns a non- 4873 zero value, scanning the pattern stops, and that value is returned from 4874 pcre2_callout_enumerate(). 4875 4876 4877AUTHOR 4878 4879 Philip Hazel 4880 University Computing Service 4881 Cambridge, England. 4882 4883 4884REVISION 4885 4886 Last updated: 03 February 2019 4887 Copyright (c) 1997-2019 University of Cambridge. 4888------------------------------------------------------------------------------ 4889 4890 4891PCRE2COMPAT(3) Library Functions Manual PCRE2COMPAT(3) 4892 4893 4894 4895NAME 4896 PCRE2 - Perl-compatible regular expressions (revised API) 4897 4898DIFFERENCES BETWEEN PCRE2 AND PERL 4899 4900 This document describes some of the differences in the ways that PCRE2 4901 and Perl handle regular expressions. The differences described here are 4902 with respect to Perl version 5.34.0, but as both Perl and PCRE2 are 4903 continually changing, the information may at times be out of date. 4904 4905 1. When PCRE2_DOTALL (equivalent to Perl's /s qualifier) is not set, 4906 the behaviour of the '.' metacharacter differs from Perl. In PCRE2, '.' 4907 matches the next character unless it is the start of a newline se- 4908 quence. This means that, if the newline setting is CR, CRLF, or NUL, 4909 '.' will match the code point LF (0x0A) in ASCII/Unicode environments, 4910 and NL (either 0x15 or 0x25) when using EBCDIC. In Perl, '.' appears 4911 never to match LF, even when 0x0A is not a newline indicator. 4912 4913 2. PCRE2 has only a subset of Perl's Unicode support. Details of what 4914 it does have are given in the pcre2unicode page. 4915 4916 3. Like Perl, PCRE2 allows repeat quantifiers on parenthesized asser- 4917 tions, but they do not mean what you might think. For example, (?!a){3} 4918 does not assert that the next three characters are not "a". It just as- 4919 serts that the next character is not "a" three times (in principle; 4920 PCRE2 optimizes this to run the assertion just once). Perl allows some 4921 repeat quantifiers on other assertions, for example, \b* , but these do 4922 not seem to have any use. PCRE2 does not allow any kind of quantifier 4923 on non-lookaround assertions. 4924 4925 4. Capture groups that occur inside negative lookaround assertions are 4926 counted, but their entries in the offsets vector are set only when a 4927 negative assertion is a condition that has a matching branch (that is, 4928 the condition is false). Perl may set such capture groups in other 4929 circumstances. 4930 4931 5. The following Perl escape sequences are not supported: \F, \l, \L, 4932 \u, \U, and \N when followed by a character name. \N on its own, match- 4933 ing a non-newline character, and \N{U+dd..}, matching a Unicode code 4934 point, are supported. The escapes that modify the case of following 4935 letters are implemented by Perl's general string-handling and are not 4936 part of its pattern matching engine. If any of these are encountered by 4937 PCRE2, an error is generated by default. However, if either of the 4938 PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX options is set, \U and \u are 4939 interpreted as ECMAScript interprets them. 4940 4941 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE2 4942 is built with Unicode support (the default). The properties that can be 4943 tested with \p and \P are limited to the general category properties 4944 such as Lu and Nd, script names such as Greek or Han, Bidi_Class, 4945 Bidi_Control, and the derived properties Any and LC (synonym L&). Both 4946 PCRE2 and Perl support the Cs (surrogate) property, but in PCRE2 its 4947 use is limited. See the pcre2pattern documentation for details. The 4948 long synonyms for property names that Perl supports (such as \p{Let- 4949 ter}) are not supported by PCRE2, nor is it permitted to prefix any of 4950 these properties with "Is". 4951 4952 7. PCRE2 supports the \Q...\E escape for quoting substrings. Characters 4953 in between are treated as literals. However, this is slightly different 4954 from Perl in that $ and @ are also handled as literals inside the 4955 quotes. In Perl, they cause variable interpolation (PCRE2 does not have 4956 variables). Also, Perl does "double-quotish backslash interpolation" on 4957 any backslashes between \Q and \E which, its documentation says, "may 4958 lead to confusing results". PCRE2 treats a backslash between \Q and \E 4959 just like any other character. Note the following examples: 4960 4961 Pattern PCRE2 matches Perl matches 4962 4963 \Qabc$xyz\E abc$xyz abc followed by the 4964 contents of $xyz 4965 \Qabc\$xyz\E abc\$xyz abc\$xyz 4966 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 4967 \QA\B\E A\B A\B 4968 \Q\\E \ \\E 4969 4970 The \Q...\E sequence is recognized both inside and outside character 4971 classes by both PCRE2 and Perl. 4972 4973 8. Fairly obviously, PCRE2 does not support the (?{code}) and 4974 (??{code}) constructions. However, PCRE2 does have a "callout" feature, 4975 which allows an external function to be called during pattern matching. 4976 See the pcre2callout documentation for details. 4977 4978 9. Subroutine calls (whether recursive or not) were treated as atomic 4979 groups up to PCRE2 release 10.23, but from release 10.30 this changed, 4980 and backtracking into subroutine calls is now supported, as in Perl. 4981 4982 10. In PCRE2, if any of the backtracking control verbs are used in a 4983 group that is called as a subroutine (whether or not recursively), 4984 their effect is confined to that group; it does not extend to the sur- 4985 rounding pattern. This is not always the case in Perl. In particular, 4986 if (*THEN) is present in a group that is called as a subroutine, its 4987 action is limited to that group, even if the group does not contain any 4988 | characters. Note that such groups are processed as anchored at the 4989 point where they are tested. 4990 4991 11. If a pattern contains more than one backtracking control verb, the 4992 first one that is backtracked onto acts. For example, in the pattern 4993 A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure 4994 in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases 4995 it is the same as PCRE2, but there are cases where it differs. 4996 4997 12. There are some differences that are concerned with the settings of 4998 captured strings when part of a pattern is repeated. For example, 4999 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 un- 5000 set, but in PCRE2 it is set to "b". 5001 5002 13. PCRE2's handling of duplicate capture group numbers and names is 5003 not as general as Perl's. This is a consequence of the fact the PCRE2 5004 works internally just with numbers, using an external table to trans- 5005 late between numbers and names. In particular, a pattern such as 5006 (?|(?<a>A)|(?<b>B)), where the two capture groups have the same number 5007 but different names, is not supported, and causes an error at compile 5008 time. If it were allowed, it would not be possible to distinguish which 5009 group matched, because both names map to capture group number 1. To 5010 avoid this confusing situation, an error is given at compile time. 5011 5012 14. Perl used to recognize comments in some places that PCRE2 does not, 5013 for example, between the ( and ? at the start of a group. If the /x 5014 modifier is set, Perl allowed white space between ( and ? though the 5015 latest Perls give an error (for a while it was just deprecated). There 5016 may still be some cases where Perl behaves differently. 5017 5018 15. Perl, when in warning mode, gives warnings for character classes 5019 such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter- 5020 als. PCRE2 has no warning features, so it gives an error in these cases 5021 because they are almost certainly user mistakes. 5022 5023 16. In PCRE2, the upper/lower case character properties Lu and Ll are 5024 not affected when case-independent matching is specified. For example, 5025 \p{Lu} always matches an upper case letter. I think Perl has changed in 5026 this respect; in the release at the time of writing (5.34), \p{Lu} and 5027 \p{Ll} match all letters, regardless of case, when case independence is 5028 specified. 5029 5030 17. From release 5.32.0, Perl locks out the use of \K in lookaround as- 5031 sertions. From release 10.38 PCRE2 does the same by default. However, 5032 there is an option for re-enabling the previous behaviour. When this 5033 option is set, \K is acted on when it occurs in positive assertions, 5034 but is ignored in negative assertions. 5035 5036 18. PCRE2 provides some extensions to the Perl regular expression fa- 5037 cilities. Perl 5.10 included new features that were not in earlier 5038 versions of Perl, some of which (such as named parentheses) were in 5039 PCRE2 for some time before. This list is with respect to Perl 5.34: 5040 5041 (a) Although lookbehind assertions in PCRE2 must match fixed length 5042 strings, each alternative toplevel branch of a lookbehind assertion can 5043 match a different length of string. Perl used to require them all to 5044 have the same length, but the latest version has some variable length 5045 support. 5046 5047 (b) From PCRE2 10.23, backreferences to groups of fixed length are sup- 5048 ported in lookbehinds, provided that there is no possibility of refer- 5049 encing a non-unique number or name. Perl does not support backrefer- 5050 ences in lookbehinds. 5051 5052 (c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the 5053 $ meta-character matches only at the very end of the string. 5054 5055 (d) A backslash followed by a letter with no special meaning is 5056 faulted. (Perl can be made to issue a warning.) 5057 5058 (e) If PCRE2_UNGREEDY is set, the greediness of the repetition quanti- 5059 fiers is inverted, that is, by default they are not greedy, but if fol- 5060 lowed by a question mark they are. 5061 5062 (f) PCRE2_ANCHORED can be used at matching time to force a pattern to 5063 be tried only at the first matching position in the subject string. 5064 5065 (g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and 5066 PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents. 5067 5068 (h) The \R escape sequence can be restricted to match only CR, LF, or 5069 CRLF by the PCRE2_BSR_ANYCRLF option. 5070 5071 (i) The callout facility is PCRE2-specific. Perl supports codeblocks 5072 and variable interpolation, but not general hooks on every match. 5073 5074 (j) The partial matching facility is PCRE2-specific. 5075 5076 (k) The alternative matching function (pcre2_dfa_match() matches in a 5077 different way and is not Perl-compatible. 5078 5079 (l) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT) 5080 at the start of a pattern. These set overall options that cannot be 5081 changed within the pattern. 5082 5083 (m) PCRE2 supports non-atomic positive lookaround assertions. This is 5084 an extension to the lookaround facilities. The default, Perl-compatible 5085 lookarounds are atomic. 5086 5087 19. The Perl /a modifier restricts /d numbers to pure ascii, and the 5088 /aa modifier restricts /i case-insensitive matching to pure ascii, ig- 5089 noring Unicode rules. This separation cannot be represented with 5090 PCRE2_UCP. 5091 5092 20. Perl has different limits than PCRE2. See the pcre2limit documenta- 5093 tion for details. Perl went with 5.10 from recursion to iteration keep- 5094 ing the intermediate matches on the heap, which is ~10% slower but does 5095 not fall into any stack-overflow limit. PCRE2 made a similar change at 5096 release 10.30, and also has many build-time and run-time customizable 5097 limits. 5098 5099 5100AUTHOR 5101 5102 Philip Hazel 5103 Retired from University Computing Service 5104 Cambridge, England. 5105 5106 5107REVISION 5108 5109 Last updated: 08 December 2021 5110 Copyright (c) 1997-2021 University of Cambridge. 5111------------------------------------------------------------------------------ 5112 5113 5114PCRE2JIT(3) Library Functions Manual PCRE2JIT(3) 5115 5116 5117 5118NAME 5119 PCRE2 - Perl-compatible regular expressions (revised API) 5120 5121PCRE2 JUST-IN-TIME COMPILER SUPPORT 5122 5123 Just-in-time compiling is a heavyweight optimization that can greatly 5124 speed up pattern matching. However, it comes at the cost of extra pro- 5125 cessing before the match is performed, so it is of most benefit when 5126 the same pattern is going to be matched many times. This does not nec- 5127 essarily mean many calls of a matching function; if the pattern is not 5128 anchored, matching attempts may take place many times at various posi- 5129 tions in the subject, even for a single call. Therefore, if the subject 5130 string is very long, it may still pay to use JIT even for one-off 5131 matches. JIT support is available for all of the 8-bit, 16-bit and 5132 32-bit PCRE2 libraries. 5133 5134 JIT support applies only to the traditional Perl-compatible matching 5135 function. It does not apply when the DFA matching function is being 5136 used. The code for this support was written by Zoltan Herczeg. 5137 5138 5139AVAILABILITY OF JIT SUPPORT 5140 5141 JIT support is an optional feature of PCRE2. The "configure" option 5142 --enable-jit (or equivalent CMake option) must be set when PCRE2 is 5143 built if you want to use JIT. The support is limited to the following 5144 hardware platforms: 5145 5146 ARM 32-bit (v5, v7, and Thumb2) 5147 ARM 64-bit 5148 IBM s390x 64 bit 5149 Intel x86 32-bit and 64-bit 5150 MIPS 32-bit and 64-bit 5151 Power PC 32-bit and 64-bit 5152 SPARC 32-bit 5153 5154 If --enable-jit is set on an unsupported platform, compilation fails. 5155 5156 A program can tell if JIT support is available by calling pcre2_con- 5157 fig() with the PCRE2_CONFIG_JIT option. The result is 1 when JIT is 5158 available, and 0 otherwise. However, a simple program does not need to 5159 check this in order to use JIT. The API is implemented in a way that 5160 falls back to the interpretive code if JIT is not available. For pro- 5161 grams that need the best possible performance, there is also a "fast 5162 path" API that is JIT-specific. 5163 5164 5165SIMPLE USE OF JIT 5166 5167 To make use of the JIT support in the simplest way, all you have to do 5168 is to call pcre2_jit_compile() after successfully compiling a pattern 5169 with pcre2_compile(). This function has two arguments: the first is the 5170 compiled pattern pointer that was returned by pcre2_compile(), and the 5171 second is zero or more of the following option bits: PCRE2_JIT_COM- 5172 PLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT. 5173 5174 If JIT support is not available, a call to pcre2_jit_compile() does 5175 nothing and returns PCRE2_ERROR_JIT_BADOPTION. Otherwise, the compiled 5176 pattern is passed to the JIT compiler, which turns it into machine code 5177 that executes much faster than the normal interpretive code, but yields 5178 exactly the same results. The returned value from pcre2_jit_compile() 5179 is zero on success, or a negative error code. 5180 5181 There is a limit to the size of pattern that JIT supports, imposed by 5182 the size of machine stack that it uses. The exact rules are not docu- 5183 mented because they may change at any time, in particular, when new op- 5184 timizations are introduced. If a pattern is too big, a call to 5185 pcre2_jit_compile() returns PCRE2_ERROR_NOMEMORY. 5186 5187 PCRE2_JIT_COMPLETE requests the JIT compiler to generate code for com- 5188 plete matches. If you want to run partial matches using the PCRE2_PAR- 5189 TIAL_HARD or PCRE2_PARTIAL_SOFT options of pcre2_match(), you should 5190 set one or both of the other options as well as, or instead of 5191 PCRE2_JIT_COMPLETE. The JIT compiler generates different optimized code 5192 for each of the three modes (normal, soft partial, hard partial). When 5193 pcre2_match() is called, the appropriate code is run if it is avail- 5194 able. Otherwise, the pattern is matched using interpretive code. 5195 5196 You can call pcre2_jit_compile() multiple times for the same compiled 5197 pattern. It does nothing if it has previously compiled code for any of 5198 the option bits. For example, you can call it once with PCRE2_JIT_COM- 5199 PLETE and (perhaps later, when you find you need partial matching) 5200 again with PCRE2_JIT_COMPLETE and PCRE2_JIT_PARTIAL_HARD. This time it 5201 will ignore PCRE2_JIT_COMPLETE and just compile code for partial match- 5202 ing. If pcre2_jit_compile() is called with no option bits set, it imme- 5203 diately returns zero. This is an alternative way of testing whether JIT 5204 is available. 5205 5206 At present, it is not possible to free JIT compiled code except when 5207 the entire compiled pattern is freed by calling pcre2_code_free(). 5208 5209 In some circumstances you may need to call additional functions. These 5210 are described in the section entitled "Controlling the JIT stack" be- 5211 low. 5212 5213 There are some pcre2_match() options that are not supported by JIT, and 5214 there are also some pattern items that JIT cannot handle. Details are 5215 given below. In both cases, matching automatically falls back to the 5216 interpretive code. If you want to know whether JIT was actually used 5217 for a particular match, you should arrange for a JIT callback function 5218 to be set up as described in the section entitled "Controlling the JIT 5219 stack" below, even if you do not need to supply a non-default JIT 5220 stack. Such a callback function is called whenever JIT code is about to 5221 be obeyed. If the match-time options are not right for JIT execution, 5222 the callback function is not obeyed. 5223 5224 If the JIT compiler finds an unsupported item, no JIT data is gener- 5225 ated. You can find out if JIT matching is available after compiling a 5226 pattern by calling pcre2_pattern_info() with the PCRE2_INFO_JITSIZE op- 5227 tion. A non-zero result means that JIT compilation was successful. A 5228 result of 0 means that JIT support is not available, or the pattern was 5229 not processed by pcre2_jit_compile(), or the JIT compiler was not able 5230 to handle the pattern. 5231 5232 5233MATCHING SUBJECTS CONTAINING INVALID UTF 5234 5235 When a pattern is compiled with the PCRE2_UTF option, subject strings 5236 are normally expected to be a valid sequence of UTF code units. By de- 5237 fault, this is checked at the start of matching and an error is gener- 5238 ated if invalid UTF is detected. The PCRE2_NO_UTF_CHECK option can be 5239 passed to pcre2_match() to skip the check (for improved performance) if 5240 you are sure that a subject string is valid. If this option is used 5241 with an invalid string, the result is undefined. 5242 5243 However, a way of running matches on strings that may contain invalid 5244 UTF sequences is available. Calling pcre2_compile() with the 5245 PCRE2_MATCH_INVALID_UTF option has two effects: it tells the inter- 5246 preter in pcre2_match() to support invalid UTF, and, if pcre2_jit_com- 5247 pile() is called, the compiled JIT code also supports invalid UTF. De- 5248 tails of how this support works, in both the JIT and the interpretive 5249 cases, is given in the pcre2unicode documentation. 5250 5251 There is also an obsolete option for pcre2_jit_compile() called 5252 PCRE2_JIT_INVALID_UTF, which currently exists only for backward compat- 5253 ibility. It is superseded by the pcre2_compile() option 5254 PCRE2_MATCH_INVALID_UTF and should no longer be used. It may be removed 5255 in future. 5256 5257 5258UNSUPPORTED OPTIONS AND PATTERN ITEMS 5259 5260 The pcre2_match() options that are supported for JIT matching are 5261 PCRE2_COPY_MATCHED_SUBJECT, PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, 5262 PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and 5263 PCRE2_PARTIAL_SOFT. The PCRE2_ANCHORED and PCRE2_ENDANCHORED options 5264 are not supported at match time. 5265 5266 If the PCRE2_NO_JIT option is passed to pcre2_match() it disables the 5267 use of JIT, forcing matching by the interpreter code. 5268 5269 The only unsupported pattern items are \C (match a single data unit) 5270 when running in a UTF mode, and a callout immediately before an asser- 5271 tion condition in a conditional group. 5272 5273 5274RETURN VALUES FROM JIT MATCHING 5275 5276 When a pattern is matched using JIT matching, the return values are the 5277 same as those given by the interpretive pcre2_match() code, with the 5278 addition of one new error code: PCRE2_ERROR_JIT_STACKLIMIT. This means 5279 that the memory used for the JIT stack was insufficient. See "Control- 5280 ling the JIT stack" below for a discussion of JIT stack usage. 5281 5282 The error code PCRE2_ERROR_MATCHLIMIT is returned by the JIT code if 5283 searching a very large pattern tree goes on for too long, as it is in 5284 the same circumstance when JIT is not used, but the details of exactly 5285 what is counted are not the same. The PCRE2_ERROR_DEPTHLIMIT error code 5286 is never returned when JIT matching is used. 5287 5288 5289CONTROLLING THE JIT STACK 5290 5291 When the compiled JIT code runs, it needs a block of memory to use as a 5292 stack. By default, it uses 32KiB on the machine stack. However, some 5293 large or complicated patterns need more than this. The error PCRE2_ER- 5294 ROR_JIT_STACKLIMIT is given when there is not enough stack. Three func- 5295 tions are provided for managing blocks of memory for use as JIT stacks. 5296 There is further discussion about the use of JIT stacks in the section 5297 entitled "JIT stack FAQ" below. 5298 5299 The pcre2_jit_stack_create() function creates a JIT stack. Its argu- 5300 ments are a starting size, a maximum size, and a general context (for 5301 memory allocation functions, or NULL for standard memory allocation). 5302 It returns a pointer to an opaque structure of type pcre2_jit_stack, or 5303 NULL if there is an error. The pcre2_jit_stack_free() function is used 5304 to free a stack that is no longer needed. If its argument is NULL, this 5305 function returns immediately, without doing anything. (For the techni- 5306 cally minded: the address space is allocated by mmap or VirtualAlloc.) 5307 A maximum stack size of 512KiB to 1MiB should be more than enough for 5308 any pattern. 5309 5310 The pcre2_jit_stack_assign() function specifies which stack JIT code 5311 should use. Its arguments are as follows: 5312 5313 pcre2_match_context *mcontext 5314 pcre2_jit_callback callback 5315 void *data 5316 5317 The first argument is a pointer to a match context. When this is subse- 5318 quently passed to a matching function, its information determines which 5319 JIT stack is used. If this argument is NULL, the function returns imme- 5320 diately, without doing anything. There are three cases for the values 5321 of the other two options: 5322 5323 (1) If callback is NULL and data is NULL, an internal 32KiB block 5324 on the machine stack is used. This is the default when a match 5325 context is created. 5326 5327 (2) If callback is NULL and data is not NULL, data must be 5328 a pointer to a valid JIT stack, the result of calling 5329 pcre2_jit_stack_create(). 5330 5331 (3) If callback is not NULL, it must point to a function that is 5332 called with data as an argument at the start of matching, in 5333 order to set up a JIT stack. If the return from the callback 5334 function is NULL, the internal 32KiB stack is used; otherwise the 5335 return value must be a valid JIT stack, the result of calling 5336 pcre2_jit_stack_create(). 5337 5338 A callback function is obeyed whenever JIT code is about to be run; it 5339 is not obeyed when pcre2_match() is called with options that are incom- 5340 patible for JIT matching. A callback function can therefore be used to 5341 determine whether a match operation was executed by JIT or by the in- 5342 terpreter. 5343 5344 You may safely use the same JIT stack for more than one pattern (either 5345 by assigning directly or by callback), as long as the patterns are 5346 matched sequentially in the same thread. Currently, the only way to set 5347 up non-sequential matches in one thread is to use callouts: if a call- 5348 out function starts another match, that match must use a different JIT 5349 stack to the one used for currently suspended match(es). 5350 5351 In a multithread application, if you do not specify a JIT stack, or if 5352 you assign or pass back NULL from a callback, that is thread-safe, be- 5353 cause each thread has its own machine stack. However, if you assign or 5354 pass back a non-NULL JIT stack, this must be a different stack for each 5355 thread so that the application is thread-safe. 5356 5357 Strictly speaking, even more is allowed. You can assign the same non- 5358 NULL stack to a match context that is used by any number of patterns, 5359 as long as they are not used for matching by multiple threads at the 5360 same time. For example, you could use the same stack in all compiled 5361 patterns, with a global mutex in the callback to wait until the stack 5362 is available for use. However, this is an inefficient solution, and not 5363 recommended. 5364 5365 This is a suggestion for how a multithreaded program that needs to set 5366 up non-default JIT stacks might operate: 5367 5368 During thread initialization 5369 thread_local_var = pcre2_jit_stack_create(...) 5370 5371 During thread exit 5372 pcre2_jit_stack_free(thread_local_var) 5373 5374 Use a one-line callback function 5375 return thread_local_var 5376 5377 All the functions described in this section do nothing if JIT is not 5378 available. 5379 5380 5381JIT STACK FAQ 5382 5383 (1) Why do we need JIT stacks? 5384 5385 PCRE2 (and JIT) is a recursive, depth-first engine, so it needs a stack 5386 where the local data of the current node is pushed before checking its 5387 child nodes. Allocating real machine stack on some platforms is diffi- 5388 cult. For example, the stack chain needs to be updated every time if we 5389 extend the stack on PowerPC. Although it is possible, its updating 5390 time overhead decreases performance. So we do the recursion in memory. 5391 5392 (2) Why don't we simply allocate blocks of memory with malloc()? 5393 5394 Modern operating systems have a nice feature: they can reserve an ad- 5395 dress space instead of allocating memory. We can safely allocate memory 5396 pages inside this address space, so the stack could grow without moving 5397 memory data (this is important because of pointers). Thus we can allo- 5398 cate 1MiB address space, and use only a single memory page (usually 5399 4KiB) if that is enough. However, we can still grow up to 1MiB anytime 5400 if needed. 5401 5402 (3) Who "owns" a JIT stack? 5403 5404 The owner of the stack is the user program, not the JIT studied pattern 5405 or anything else. The user program must ensure that if a stack is being 5406 used by pcre2_match(), (that is, it is assigned to a match context that 5407 is passed to the pattern currently running), that stack must not be 5408 used by any other threads (to avoid overwriting the same memory area). 5409 The best practice for multithreaded programs is to allocate a stack for 5410 each thread, and return this stack through the JIT callback function. 5411 5412 (4) When should a JIT stack be freed? 5413 5414 You can free a JIT stack at any time, as long as it will not be used by 5415 pcre2_match() again. When you assign the stack to a match context, only 5416 a pointer is set. There is no reference counting or any other magic. 5417 You can free compiled patterns, contexts, and stacks in any order, any- 5418 time. Just do not call pcre2_match() with a match context pointing to 5419 an already freed stack, as that will cause SEGFAULT. (Also, do not free 5420 a stack currently used by pcre2_match() in another thread). You can 5421 also replace the stack in a context at any time when it is not in use. 5422 You should free the previous stack before assigning a replacement. 5423 5424 (5) Should I allocate/free a stack every time before/after calling 5425 pcre2_match()? 5426 5427 No, because this is too costly in terms of resources. However, you 5428 could implement some clever idea which release the stack if it is not 5429 used in let's say two minutes. The JIT callback can help to achieve 5430 this without keeping a list of patterns. 5431 5432 (6) OK, the stack is for long term memory allocation. But what happens 5433 if a pattern causes stack overflow with a stack of 1MiB? Is that 1MiB 5434 kept until the stack is freed? 5435 5436 Especially on embedded sytems, it might be a good idea to release mem- 5437 ory sometimes without freeing the stack. There is no API for this at 5438 the moment. Probably a function call which returns with the currently 5439 allocated memory for any stack and another which allows releasing mem- 5440 ory (shrinking the stack) would be a good idea if someone needs this. 5441 5442 (7) This is too much of a headache. Isn't there any better solution for 5443 JIT stack handling? 5444 5445 No, thanks to Windows. If POSIX threads were used everywhere, we could 5446 throw out this complicated API. 5447 5448 5449FREEING JIT SPECULATIVE MEMORY 5450 5451 void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext); 5452 5453 The JIT executable allocator does not free all memory when it is possi- 5454 ble. It expects new allocations, and keeps some free memory around to 5455 improve allocation speed. However, in low memory conditions, it might 5456 be better to free all possible memory. You can cause this to happen by 5457 calling pcre2_jit_free_unused_memory(). Its argument is a general con- 5458 text, for custom memory management, or NULL for standard memory manage- 5459 ment. 5460 5461 5462EXAMPLE CODE 5463 5464 This is a single-threaded example that specifies a JIT stack without 5465 using a callback. A real program should include error checking after 5466 all the function calls. 5467 5468 int rc; 5469 pcre2_code *re; 5470 pcre2_match_data *match_data; 5471 pcre2_match_context *mcontext; 5472 pcre2_jit_stack *jit_stack; 5473 5474 re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0, 5475 &errornumber, &erroffset, NULL); 5476 rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE); 5477 mcontext = pcre2_match_context_create(NULL); 5478 jit_stack = pcre2_jit_stack_create(32*1024, 512*1024, NULL); 5479 pcre2_jit_stack_assign(mcontext, NULL, jit_stack); 5480 match_data = pcre2_match_data_create(re, 10); 5481 rc = pcre2_match(re, subject, length, 0, 0, match_data, mcontext); 5482 /* Process result */ 5483 5484 pcre2_code_free(re); 5485 pcre2_match_data_free(match_data); 5486 pcre2_match_context_free(mcontext); 5487 pcre2_jit_stack_free(jit_stack); 5488 5489 5490JIT FAST PATH API 5491 5492 Because the API described above falls back to interpreted matching when 5493 JIT is not available, it is convenient for programs that are written 5494 for general use in many environments. However, calling JIT via 5495 pcre2_match() does have a performance impact. Programs that are written 5496 for use where JIT is known to be available, and which need the best 5497 possible performance, can instead use a "fast path" API to call JIT 5498 matching directly instead of calling pcre2_match() (obviously only for 5499 patterns that have been successfully processed by pcre2_jit_compile()). 5500 5501 The fast path function is called pcre2_jit_match(), and it takes ex- 5502 actly the same arguments as pcre2_match(). However, the subject string 5503 must be specified with a length; PCRE2_ZERO_TERMINATED is not sup- 5504 ported. Unsupported option bits (for example, PCRE2_ANCHORED, PCRE2_EN- 5505 DANCHORED and PCRE2_COPY_MATCHED_SUBJECT) are ignored, as is the 5506 PCRE2_NO_JIT option. The return values are also the same as for 5507 pcre2_match(), plus PCRE2_ERROR_JIT_BADOPTION if a matching mode (par- 5508 tial or complete) is requested that was not compiled. 5509 5510 When you call pcre2_match(), as well as testing for invalid options, a 5511 number of other sanity checks are performed on the arguments. For exam- 5512 ple, if the subject pointer is NULL but the length is non-zero, an im- 5513 mediate error is given. Also, unless PCRE2_NO_UTF_CHECK is set, a UTF 5514 subject string is tested for validity. In the interests of speed, these 5515 checks do not happen on the JIT fast path, and if invalid data is 5516 passed, the result is undefined. 5517 5518 Bypassing the sanity checks and the pcre2_match() wrapping can give 5519 speedups of more than 10%. 5520 5521 5522SEE ALSO 5523 5524 pcre2api(3) 5525 5526 5527AUTHOR 5528 5529 Philip Hazel (FAQ by Zoltan Herczeg) 5530 University Computing Service 5531 Cambridge, England. 5532 5533 5534REVISION 5535 5536 Last updated: 30 November 2021 5537 Copyright (c) 1997-2021 University of Cambridge. 5538------------------------------------------------------------------------------ 5539 5540 5541PCRE2LIMITS(3) Library Functions Manual PCRE2LIMITS(3) 5542 5543 5544 5545NAME 5546 PCRE2 - Perl-compatible regular expressions (revised API) 5547 5548SIZE AND OTHER LIMITATIONS 5549 5550 There are some size limitations in PCRE2 but it is hoped that they will 5551 never in practice be relevant. 5552 5553 The maximum size of a compiled pattern is approximately 64 thousand 5554 code units for the 8-bit and 16-bit libraries if PCRE2 is compiled with 5555 the default internal linkage size, which is 2 bytes for these li- 5556 braries. If you want to process regular expressions that are truly 5557 enormous, you can compile PCRE2 with an internal linkage size of 3 or 4 5558 (when building the 16-bit library, 3 is rounded up to 4). See the 5559 README file in the source distribution and the pcre2build documentation 5560 for details. In these cases the limit is substantially larger. How- 5561 ever, the speed of execution is slower. In the 32-bit library, the in- 5562 ternal linkage size is always 4. 5563 5564 The maximum length of a source pattern string is essentially unlimited; 5565 it is the largest number a PCRE2_SIZE variable can hold. However, the 5566 program that calls pcre2_compile() can specify a smaller limit. 5567 5568 The maximum length (in code units) of a subject string is one less than 5569 the largest number a PCRE2_SIZE variable can hold. PCRE2_SIZE is an un- 5570 signed integer type, usually defined as size_t. Its maximum value (that 5571 is ~(PCRE2_SIZE)0) is reserved as a special indicator for zero-termi- 5572 nated strings and unset offsets. 5573 5574 All values in repeating quantifiers must be less than 65536. 5575 5576 The maximum length of a lookbehind assertion is 65535 characters. 5577 5578 There is no limit to the number of parenthesized groups, but there can 5579 be no more than 65535 capture groups, and there is a limit to the depth 5580 of nesting of parenthesized subpatterns of all kinds. This is imposed 5581 in order to limit the amount of system stack used at compile time. The 5582 default limit can be specified when PCRE2 is built; if not, the default 5583 is set to 250. An application can change this limit by calling 5584 pcre2_set_parens_nest_limit() to set the limit in a compile context. 5585 5586 The maximum length of name for a named capture group is 32 code units, 5587 and the maximum number of such groups is 10000. 5588 5589 The maximum length of a name in a (*MARK), (*PRUNE), (*SKIP), or 5590 (*THEN) verb is 255 code units for the 8-bit library and 65535 code 5591 units for the 16-bit and 32-bit libraries. 5592 5593 The maximum length of a string argument to a callout is the largest 5594 number a 32-bit unsigned integer can hold. 5595 5596 The maximum amount of heap memory used for matching is controlled by 5597 the heap limit, which can be set in a pattern or in a match context. 5598 The default is a very large number, effectively unlimited. 5599 5600 5601AUTHOR 5602 5603 Philip Hazel 5604 Retired from University Computing Service 5605 Cambridge, England. 5606 5607 5608REVISION 5609 5610 Last updated: 26 July 2022 5611 Copyright (c) 1997-2022 University of Cambridge. 5612------------------------------------------------------------------------------ 5613 5614 5615PCRE2MATCHING(3) Library Functions Manual PCRE2MATCHING(3) 5616 5617 5618 5619NAME 5620 PCRE2 - Perl-compatible regular expressions (revised API) 5621 5622PCRE2 MATCHING ALGORITHMS 5623 5624 This document describes the two different algorithms that are available 5625 in PCRE2 for matching a compiled regular expression against a given 5626 subject string. The "standard" algorithm is the one provided by the 5627 pcre2_match() function. This works in the same as as Perl's matching 5628 function, and provide a Perl-compatible matching operation. The just- 5629 in-time (JIT) optimization that is described in the pcre2jit documenta- 5630 tion is compatible with this function. 5631 5632 An alternative algorithm is provided by the pcre2_dfa_match() function; 5633 it operates in a different way, and is not Perl-compatible. This alter- 5634 native has advantages and disadvantages compared with the standard al- 5635 gorithm, and these are described below. 5636 5637 When there is only one possible way in which a given subject string can 5638 match a pattern, the two algorithms give the same answer. A difference 5639 arises, however, when there are multiple possibilities. For example, if 5640 the pattern 5641 5642 ^<.*> 5643 5644 is matched against the string 5645 5646 <something> <something else> <something further> 5647 5648 there are three possible answers. The standard algorithm finds only one 5649 of them, whereas the alternative algorithm finds all three. 5650 5651 5652REGULAR EXPRESSIONS AS TREES 5653 5654 The set of strings that are matched by a regular expression can be rep- 5655 resented as a tree structure. An unlimited repetition in the pattern 5656 makes the tree of infinite size, but it is still a tree. Matching the 5657 pattern to a given subject string (from a given starting point) can be 5658 thought of as a search of the tree. There are two ways to search a 5659 tree: depth-first and breadth-first, and these correspond to the two 5660 matching algorithms provided by PCRE2. 5661 5662 5663THE STANDARD MATCHING ALGORITHM 5664 5665 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres- 5666 sions", the standard algorithm is an "NFA algorithm". It conducts a 5667 depth-first search of the pattern tree. That is, it proceeds along a 5668 single path through the tree, checking that the subject matches what is 5669 required. When there is a mismatch, the algorithm tries any alterna- 5670 tives at the current point, and if they all fail, it backs up to the 5671 previous branch point in the tree, and tries the next alternative 5672 branch at that level. This often involves backing up (moving to the 5673 left) in the subject string as well. The order in which repetition 5674 branches are tried is controlled by the greedy or ungreedy nature of 5675 the quantifier. 5676 5677 If a leaf node is reached, a matching string has been found, and at 5678 that point the algorithm stops. Thus, if there is more than one possi- 5679 ble match, this algorithm returns the first one that it finds. Whether 5680 this is the shortest, the longest, or some intermediate length depends 5681 on the way the alternations and the greedy or ungreedy repetition quan- 5682 tifiers are specified in the pattern. 5683 5684 Because it ends up with a single path through the tree, it is rela- 5685 tively straightforward for this algorithm to keep track of the sub- 5686 strings that are matched by portions of the pattern in parentheses. 5687 This provides support for capturing parentheses and backreferences. 5688 5689 5690THE ALTERNATIVE MATCHING ALGORITHM 5691 5692 This algorithm conducts a breadth-first search of the tree. Starting 5693 from the first matching point in the subject, it scans the subject 5694 string from left to right, once, character by character, and as it does 5695 this, it remembers all the paths through the tree that represent valid 5696 matches. In Friedl's terminology, this is a kind of "DFA algorithm", 5697 though it is not implemented as a traditional finite state machine (it 5698 keeps multiple states active simultaneously). 5699 5700 Although the general principle of this matching algorithm is that it 5701 scans the subject string only once, without backtracking, there is one 5702 exception: when a lookaround assertion is encountered, the characters 5703 following or preceding the current point have to be independently in- 5704 spected. 5705 5706 The scan continues until either the end of the subject is reached, or 5707 there are no more unterminated paths. At this point, terminated paths 5708 represent the different matching possibilities (if there are none, the 5709 match has failed). Thus, if there is more than one possible match, 5710 this algorithm finds all of them, and in particular, it finds the long- 5711 est. The matches are returned in the output vector in decreasing order 5712 of length. There is an option to stop the algorithm after the first 5713 match (which is necessarily the shortest) is found. 5714 5715 Note that the size of vector needed to contain all the results depends 5716 on the number of simultaneous matches, not on the number of parentheses 5717 in the pattern. Using pcre2_match_data_create_from_pattern() to create 5718 the match data block is therefore not advisable when doing DFA match- 5719 ing. 5720 5721 Note also that all the matches that are found start at the same point 5722 in the subject. If the pattern 5723 5724 cat(er(pillar)?)? 5725 5726 is matched against the string "the caterpillar catchment", the result 5727 is the three strings "caterpillar", "cater", and "cat" that start at 5728 the fifth character of the subject. The algorithm does not automati- 5729 cally move on to find matches that start at later positions. 5730 5731 PCRE2's "auto-possessification" optimization usually applies to charac- 5732 ter repeats at the end of a pattern (as well as internally). For exam- 5733 ple, the pattern "a\d+" is compiled as if it were "a\d++" because there 5734 is no point even considering the possibility of backtracking into the 5735 repeated digits. For DFA matching, this means that only one possible 5736 match is found. If you really do want multiple matches in such cases, 5737 either use an ungreedy repeat ("a\d+?") or set the PCRE2_NO_AUTO_POS- 5738 SESS option when compiling. 5739 5740 There are a number of features of PCRE2 regular expressions that are 5741 not supported or behave differently in the alternative matching func- 5742 tion. Those that are not supported cause an error if encountered. 5743 5744 1. Because the algorithm finds all possible matches, the greedy or un- 5745 greedy nature of repetition quantifiers is not relevant (though it may 5746 affect auto-possessification, as just described). During matching, 5747 greedy and ungreedy quantifiers are treated in exactly the same way. 5748 However, possessive quantifiers can make a difference when what follows 5749 could also match what is quantified, for example in a pattern like 5750 this: 5751 5752 ^a++\w! 5753 5754 This pattern matches "aaab!" but not "aaa!", which would be matched by 5755 a non-possessive quantifier. Similarly, if an atomic group is present, 5756 it is matched as if it were a standalone pattern at the current point, 5757 and the longest match is then "locked in" for the rest of the overall 5758 pattern. 5759 5760 2. When dealing with multiple paths through the tree simultaneously, it 5761 is not straightforward to keep track of captured substrings for the 5762 different matching possibilities, and PCRE2's implementation of this 5763 algorithm does not attempt to do this. This means that no captured sub- 5764 strings are available. 5765 5766 3. Because no substrings are captured, backreferences within the pat- 5767 tern are not supported. 5768 5769 4. For the same reason, conditional expressions that use a backrefer- 5770 ence as the condition or test for a specific group recursion are not 5771 supported. 5772 5773 5. Again for the same reason, script runs are not supported. 5774 5775 6. Because many paths through the tree may be active, the \K escape se- 5776 quence, which resets the start of the match when encountered (but may 5777 be on some paths and not on others), is not supported. 5778 5779 7. Callouts are supported, but the value of the capture_top field is 5780 always 1, and the value of the capture_last field is always 0. 5781 5782 8. The \C escape sequence, which (in the standard algorithm) always 5783 matches a single code unit, even in a UTF mode, is not supported in 5784 these modes, because the alternative algorithm moves through the sub- 5785 ject string one character (not code unit) at a time, for all active 5786 paths through the tree. 5787 5788 9. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) 5789 are not supported. (*FAIL) is supported, and behaves like a failing 5790 negative assertion. 5791 5792 10. The PCRE2_MATCH_INVALID_UTF option for pcre2_compile() is not sup- 5793 ported by pcre2_dfa_match(). 5794 5795 5796ADVANTAGES OF THE ALTERNATIVE ALGORITHM 5797 5798 The main advantage of the alternative algorithm is that all possible 5799 matches (at a single point in the subject) are automatically found, and 5800 in particular, the longest match is found. To find more than one match 5801 at the same point using the standard algorithm, you have to do kludgy 5802 things with callouts. 5803 5804 Partial matching is possible with this algorithm, though it has some 5805 limitations. The pcre2partial documentation gives details of partial 5806 matching and discusses multi-segment matching. 5807 5808 5809DISADVANTAGES OF THE ALTERNATIVE ALGORITHM 5810 5811 The alternative algorithm suffers from a number of disadvantages: 5812 5813 1. It is substantially slower than the standard algorithm. This is 5814 partly because it has to search for all possible matches, but is also 5815 because it is less susceptible to optimization. 5816 5817 2. Capturing parentheses, backreferences, script runs, and matching 5818 within invalid UTF string are not supported. 5819 5820 3. Although atomic groups are supported, their use does not provide the 5821 performance advantage that it does for the standard algorithm. 5822 5823 4. JIT optimization is not supported. 5824 5825 5826AUTHOR 5827 5828 Philip Hazel 5829 Retired from University Computing Service 5830 Cambridge, England. 5831 5832 5833REVISION 5834 5835 Last updated: 28 August 2021 5836 Copyright (c) 1997-2021 University of Cambridge. 5837------------------------------------------------------------------------------ 5838 5839 5840PCRE2PARTIAL(3) Library Functions Manual PCRE2PARTIAL(3) 5841 5842 5843 5844NAME 5845 PCRE2 - Perl-compatible regular expressions 5846 5847PARTIAL MATCHING IN PCRE2 5848 5849 In normal use of PCRE2, if there is a match up to the end of a subject 5850 string, but more characters are needed to match the entire pattern, 5851 PCRE2_ERROR_NOMATCH is returned, just like any other failing match. 5852 There are circumstances where it might be helpful to distinguish this 5853 "partial match" case. 5854 5855 One example is an application where the subject string is very long, 5856 and not all available at once. The requirement here is to be able to do 5857 the matching segment by segment, but special action is needed when a 5858 matched substring spans the boundary between two segments. 5859 5860 Another example is checking a user input string as it is typed, to en- 5861 sure that it conforms to a required format. Invalid characters can be 5862 immediately diagnosed and rejected, giving instant feedback. 5863 5864 Partial matching is a PCRE2-specific feature; it is not Perl-compati- 5865 ble. It is requested by setting one of the PCRE2_PARTIAL_HARD or 5866 PCRE2_PARTIAL_SOFT options when calling a matching function. The dif- 5867 ference between the two options is whether or not a partial match is 5868 preferred to an alternative complete match, though the details differ 5869 between the two types of matching function. If both options are set, 5870 PCRE2_PARTIAL_HARD takes precedence. 5871 5872 If you want to use partial matching with just-in-time optimized code, 5873 as well as setting a partial match option for the matching function, 5874 you must also call pcre2_jit_compile() with one or both of these op- 5875 tions: 5876 5877 PCRE2_JIT_PARTIAL_HARD 5878 PCRE2_JIT_PARTIAL_SOFT 5879 5880 PCRE2_JIT_COMPLETE should also be set if you are going to run non-par- 5881 tial matches on the same pattern. Separate code is compiled for each 5882 mode. If the appropriate JIT mode has not been compiled, interpretive 5883 matching code is used. 5884 5885 Setting a partial matching option disables two of PCRE2's standard op- 5886 timization hints. PCRE2 remembers the last literal code unit in a pat- 5887 tern, and abandons matching immediately if it is not present in the 5888 subject string. This optimization cannot be used for a subject string 5889 that might match only partially. PCRE2 also remembers a minimum length 5890 of a matching string, and does not bother to run the matching function 5891 on shorter strings. This optimization is also disabled for partial 5892 matching. 5893 5894 5895REQUIREMENTS FOR A PARTIAL MATCH 5896 5897 A possible partial match occurs during matching when the end of the 5898 subject string is reached successfully, but either more characters are 5899 needed to complete the match, or the addition of more characters might 5900 change what is matched. 5901 5902 Example 1: if the pattern is /abc/ and the subject is "ab", more char- 5903 acters are definitely needed to complete a match. In this case both 5904 hard and soft matching options yield a partial match. 5905 5906 Example 2: if the pattern is /ab+/ and the subject is "ab", a complete 5907 match can be found, but the addition of more characters might change 5908 what is matched. In this case, only PCRE2_PARTIAL_HARD returns a par- 5909 tial match; PCRE2_PARTIAL_SOFT returns the complete match. 5910 5911 On reaching the end of the subject, when PCRE2_PARTIAL_HARD is set, if 5912 the next pattern item is \z, \Z, \b, \B, or $ there is always a partial 5913 match. Otherwise, for both options, the next pattern item must be one 5914 that inspects a character, and at least one of the following must be 5915 true: 5916 5917 (1) At least one character has already been inspected. An inspected 5918 character need not form part of the final matched string; lookbehind 5919 assertions and the \K escape sequence provide ways of inspecting char- 5920 acters before the start of a matched string. 5921 5922 (2) The pattern contains one or more lookbehind assertions. This condi- 5923 tion exists in case there is a lookbehind that inspects characters be- 5924 fore the start of the match. 5925 5926 (3) There is a special case when the whole pattern can match an empty 5927 string. When the starting point is at the end of the subject, the 5928 empty string match is a possibility, and if PCRE2_PARTIAL_SOFT is set 5929 and neither of the above conditions is true, it is returned. However, 5930 because adding more characters might result in a non-empty match, 5931 PCRE2_PARTIAL_HARD returns a partial match, which in this case means 5932 "there is going to be a match at this point, but until some more char- 5933 acters are added, we do not know if it will be an empty string or some- 5934 thing longer". 5935 5936 5937PARTIAL MATCHING USING pcre2_match() 5938 5939 When a partial matching option is set, the result of calling 5940 pcre2_match() can be one of the following: 5941 5942 A successful match 5943 A complete match has been found, starting and ending within this sub- 5944 ject. 5945 5946 PCRE2_ERROR_NOMATCH 5947 No match can start anywhere in this subject. 5948 5949 PCRE2_ERROR_PARTIAL 5950 Adding more characters may result in a complete match that uses one 5951 or more characters from the end of this subject. 5952 5953 When a partial match is returned, the first two elements in the ovector 5954 point to the portion of the subject that was matched, but the values in 5955 the rest of the ovector are undefined. The appearance of \K in the pat- 5956 tern has no effect for a partial match. Consider this pattern: 5957 5958 /abc\K123/ 5959 5960 If it is matched against "456abc123xyz" the result is a complete match, 5961 and the ovector defines the matched string as "123", because \K resets 5962 the "start of match" point. However, if a partial match is requested 5963 and the subject string is "456abc12", a partial match is found for the 5964 string "abc12", because all these characters are needed for a subse- 5965 quent re-match with additional characters. 5966 5967 If there is more than one partial match, the first one that was found 5968 provides the data that is returned. Consider this pattern: 5969 5970 /123\w+X|dogY/ 5971 5972 If this is matched against the subject string "abc123dog", both alter- 5973 natives fail to match, but the end of the subject is reached during 5974 matching, so PCRE2_ERROR_PARTIAL is returned. The offsets are set to 3 5975 and 9, identifying "123dog" as the first partial match. (In this exam- 5976 ple, there are two partial matches, because "dog" on its own partially 5977 matches the second alternative.) 5978 5979 How a partial match is processed by pcre2_match() 5980 5981 What happens when a partial match is identified depends on which of the 5982 two partial matching options is set. 5983 5984 If PCRE2_PARTIAL_HARD is set, PCRE2_ERROR_PARTIAL is returned as soon 5985 as a partial match is found, without continuing to search for possible 5986 complete matches. This option is "hard" because it prefers an earlier 5987 partial match over a later complete match. For this reason, the assump- 5988 tion is made that the end of the supplied subject string is not the 5989 true end of the available data, which is why \z, \Z, \b, \B, and $ al- 5990 ways give a partial match. 5991 5992 If PCRE2_PARTIAL_SOFT is set, the partial match is remembered, but 5993 matching continues as normal, and other alternatives in the pattern are 5994 tried. If no complete match can be found, PCRE2_ERROR_PARTIAL is re- 5995 turned instead of PCRE2_ERROR_NOMATCH. This option is "soft" because it 5996 prefers a complete match over a partial match. All the various matching 5997 items in a pattern behave as if the subject string is potentially com- 5998 plete; \z, \Z, and $ match at the end of the subject, as normal, and 5999 for \b and \B the end of the subject is treated as a non-alphanumeric. 6000 6001 The difference between the two partial matching options can be illus- 6002 trated by a pattern such as: 6003 6004 /dog(sbody)?/ 6005 6006 This matches either "dog" or "dogsbody", greedily (that is, it prefers 6007 the longer string if possible). If it is matched against the string 6008 "dog" with PCRE2_PARTIAL_SOFT, it yields a complete match for "dog". 6009 However, if PCRE2_PARTIAL_HARD is set, the result is PCRE2_ERROR_PAR- 6010 TIAL. On the other hand, if the pattern is made ungreedy the result is 6011 different: 6012 6013 /dog(sbody)??/ 6014 6015 In this case the result is always a complete match because that is 6016 found first, and matching never continues after finding a complete 6017 match. It might be easier to follow this explanation by thinking of the 6018 two patterns like this: 6019 6020 /dog(sbody)?/ is the same as /dogsbody|dog/ 6021 /dog(sbody)??/ is the same as /dog|dogsbody/ 6022 6023 The second pattern will never match "dogsbody", because it will always 6024 find the shorter match first. 6025 6026 Example of partial matching using pcre2test 6027 6028 The pcre2test data modifiers partial_hard (or ph) and partial_soft (or 6029 ps) set PCRE2_PARTIAL_HARD and PCRE2_PARTIAL_SOFT, respectively, when 6030 calling pcre2_match(). Here is a run of pcre2test using a pattern that 6031 matches the whole subject in the form of a date: 6032 6033 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ 6034 data> 25dec3\=ph 6035 Partial match: 23dec3 6036 data> 3ju\=ph 6037 Partial match: 3ju 6038 data> 3juj\=ph 6039 No match 6040 6041 This example gives the same results for both hard and soft partial 6042 matching options. Here is an example where there is a difference: 6043 6044 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ 6045 data> 25jun04\=ps 6046 0: 25jun04 6047 1: jun 6048 data> 25jun04\=ph 6049 Partial match: 25jun04 6050 6051 With PCRE2_PARTIAL_SOFT, the subject is matched completely. For 6052 PCRE2_PARTIAL_HARD, however, the subject is assumed not to be complete, 6053 so there is only a partial match. 6054 6055 6056MULTI-SEGMENT MATCHING WITH pcre2_match() 6057 6058 PCRE was not originally designed with multi-segment matching in mind. 6059 However, over time, features (including partial matching) that make 6060 multi-segment matching possible have been added. A very long string can 6061 be searched segment by segment by calling pcre2_match() repeatedly, 6062 with the aim of achieving the same results that would happen if the en- 6063 tire string was available for searching all the time. Normally, the 6064 strings that are being sought are much shorter than each individual 6065 segment, and are in the middle of very long strings, so the pattern is 6066 normally not anchored. 6067 6068 Special logic must be implemented to handle a matched substring that 6069 spans a segment boundary. PCRE2_PARTIAL_HARD should be used, because it 6070 returns a partial match at the end of a segment whenever there is the 6071 possibility of changing the match by adding more characters. The 6072 PCRE2_NOTBOL option should also be set for all but the first segment. 6073 6074 When a partial match occurs, the next segment must be added to the cur- 6075 rent subject and the match re-run, using the startoffset argument of 6076 pcre2_match() to begin at the point where the partial match started. 6077 For example: 6078 6079 re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ 6080 data> ...the date is 23ja\=ph 6081 Partial match: 23ja 6082 data> ...the date is 23jan19 and on that day...\=offset=15 6083 0: 23jan19 6084 1: jan 6085 6086 Note the use of the offset modifier to start the new match where the 6087 partial match was found. In this example, the next segment was added to 6088 the one in which the partial match was found. This is the most 6089 straightforward approach, typically using a memory buffer that is twice 6090 the size of each segment. After a partial match, the first half of the 6091 buffer is discarded, the second half is moved to the start of the buf- 6092 fer, and a new segment is added before repeating the match as in the 6093 example above. After a no match, the entire buffer can be discarded. 6094 6095 If there are memory constraints, you may want to discard text that pre- 6096 cedes a partial match before adding the next segment. Unfortunately, 6097 this is not at present straightforward. In cases such as the above, 6098 where the pattern does not contain any lookbehinds, it is sufficient to 6099 retain only the partially matched substring. However, if the pattern 6100 contains a lookbehind assertion, characters that precede the start of 6101 the partial match may have been inspected during the matching process. 6102 When pcre2test displays a partial match, it indicates these characters 6103 with '<' if the allusedtext modifier is set: 6104 6105 re> "(?<=123)abc" 6106 data> xx123ab\=ph,allusedtext 6107 Partial match: 123ab 6108 <<< 6109 6110 However, the allusedtext modifier is not available for JIT matching, 6111 because JIT matching does not record the first (or last) consulted 6112 characters. For this reason, this information is not available via the 6113 API. It is therefore not possible in general to obtain the exact number 6114 of characters that must be retained in order to get the right match re- 6115 sult. If you cannot retain the entire segment, you must find some 6116 heuristic way of choosing. 6117 6118 If you know the approximate length of the matching substrings, you can 6119 use that to decide how much text to retain. The only lookbehind infor- 6120 mation that is currently available via the API is the length of the 6121 longest individual lookbehind in a pattern, but this can be misleading 6122 if there are nested lookbehinds. The value returned by calling 6123 pcre2_pattern_info() with the PCRE2_INFO_MAXLOOKBEHIND option is the 6124 maximum number of characters (not code units) that any individual look- 6125 behind moves back when it is processed. A pattern such as 6126 "(?<=(?<!b)a)" has a maximum lookbehind value of one, but inspects two 6127 characters before its starting point. 6128 6129 In a non-UTF or a 32-bit case, moving back is just a subtraction, but 6130 in UTF-8 or UTF-16 you have to count characters while moving back 6131 through the code units. 6132 6133 6134PARTIAL MATCHING USING pcre2_dfa_match() 6135 6136 The DFA function moves along the subject string character by character, 6137 without backtracking, searching for all possible matches simultane- 6138 ously. If the end of the subject is reached before the end of the pat- 6139 tern, there is the possibility of a partial match. 6140 6141 When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned only if 6142 there have been no complete matches. Otherwise, the complete matches 6143 are returned. If PCRE2_PARTIAL_HARD is set, a partial match takes 6144 precedence over any complete matches. The portion of the string that 6145 was matched when the longest partial match was found is set as the 6146 first matching string. 6147 6148 Because the DFA function always searches for all possible matches, and 6149 there is no difference between greedy and ungreedy repetition, its be- 6150 haviour is different from the pcre2_match(). Consider the string "dog" 6151 matched against this ungreedy pattern: 6152 6153 /dog(sbody)??/ 6154 6155 Whereas the standard function stops as soon as it finds the complete 6156 match for "dog", the DFA function also finds the partial match for 6157 "dogsbody", and so returns that when PCRE2_PARTIAL_HARD is set. 6158 6159 6160MULTI-SEGMENT MATCHING WITH pcre2_dfa_match() 6161 6162 When a partial match has been found using the DFA matching function, it 6163 is possible to continue the match by providing additional subject data 6164 and calling the function again with the same compiled regular expres- 6165 sion, this time setting the PCRE2_DFA_RESTART option. You must pass the 6166 same working space as before, because this is where details of the pre- 6167 vious partial match are stored. You can set the PCRE2_PARTIAL_SOFT or 6168 PCRE2_PARTIAL_HARD options with PCRE2_DFA_RESTART to continue partial 6169 matching over multiple segments. Here is an example using pcre2test: 6170 6171 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ 6172 data> 23ja\=dfa,ps 6173 Partial match: 23ja 6174 data> n05\=dfa,dfa_restart 6175 0: n05 6176 6177 The first call has "23ja" as the subject, and requests partial match- 6178 ing; the second call has "n05" as the subject for the continued 6179 (restarted) match. Notice that when the match is complete, only the 6180 last part is shown; PCRE2 does not retain the previously partially- 6181 matched string. It is up to the calling program to do that if it needs 6182 to. This means that, for an unanchored pattern, if a continued match 6183 fails, it is not possible to try again at a new starting point. All 6184 this facility is capable of doing is continuing with the previous match 6185 attempt. For example, consider this pattern: 6186 6187 1234|3789 6188 6189 If the first part of the subject is "ABC123", a partial match of the 6190 first alternative is found at offset 3. There is no partial match for 6191 the second alternative, because such a match does not start at the same 6192 point in the subject string. Attempting to continue with the string 6193 "7890" does not yield a match because only those alternatives that 6194 match at one point in the subject are remembered. Depending on the ap- 6195 plication, this may or may not be what you want. 6196 6197 If you do want to allow for starting again at the next character, one 6198 way of doing it is to retain some or all of the segment and try a new 6199 complete match, as described for pcre2_match() above. Another possibil- 6200 ity is to work with two buffers. If a partial match at offset n in the 6201 first buffer is followed by "no match" when PCRE2_DFA_RESTART is used 6202 on the second buffer, you can then try a new match starting at offset 6203 n+1 in the first buffer. 6204 6205 6206AUTHOR 6207 6208 Philip Hazel 6209 University Computing Service 6210 Cambridge, England. 6211 6212 6213REVISION 6214 6215 Last updated: 04 September 2019 6216 Copyright (c) 1997-2019 University of Cambridge. 6217------------------------------------------------------------------------------ 6218 6219 6220PCRE2PATTERN(3) Library Functions Manual PCRE2PATTERN(3) 6221 6222 6223 6224NAME 6225 PCRE2 - Perl-compatible regular expressions (revised API) 6226 6227PCRE2 REGULAR EXPRESSION DETAILS 6228 6229 The syntax and semantics of the regular expressions that are supported 6230 by PCRE2 are described in detail below. There is a quick-reference syn- 6231 tax summary in the pcre2syntax page. PCRE2 tries to match Perl syntax 6232 and semantics as closely as it can. PCRE2 also supports some alterna- 6233 tive regular expression syntax (which does not conflict with the Perl 6234 syntax) in order to provide some compatibility with regular expressions 6235 in Python, .NET, and Oniguruma. 6236 6237 Perl's regular expressions are described in its own documentation, and 6238 regular expressions in general are covered in a number of books, some 6239 of which have copious examples. Jeffrey Friedl's "Mastering Regular Ex- 6240 pressions", published by O'Reilly, covers regular expressions in great 6241 detail. This description of PCRE2's regular expressions is intended as 6242 reference material. 6243 6244 This document discusses the regular expression patterns that are sup- 6245 ported by PCRE2 when its main matching function, pcre2_match(), is 6246 used. PCRE2 also has an alternative matching function, 6247 pcre2_dfa_match(), which matches using a different algorithm that is 6248 not Perl-compatible. Some of the features discussed below are not 6249 available when DFA matching is used. The advantages and disadvantages 6250 of the alternative function, and how it differs from the normal func- 6251 tion, are discussed in the pcre2matching page. 6252 6253 6254SPECIAL START-OF-PATTERN ITEMS 6255 6256 A number of options that can be passed to pcre2_compile() can also be 6257 set by special items at the start of a pattern. These are not Perl-com- 6258 patible, but are provided to make these options accessible to pattern 6259 writers who are not able to change the program that processes the pat- 6260 tern. Any number of these items may appear, but they must all be to- 6261 gether right at the start of the pattern string, and the letters must 6262 be in upper case. 6263 6264 UTF support 6265 6266 In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either 6267 as single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32 6268 can be specified for the 32-bit library, in which case it constrains 6269 the character values to valid Unicode code points. To process UTF 6270 strings, PCRE2 must be built to include Unicode support (which is the 6271 default). When using UTF strings you must either call the compiling 6272 function with one or both of the PCRE2_UTF or PCRE2_MATCH_INVALID_UTF 6273 options, or the pattern must start with the special sequence (*UTF), 6274 which is equivalent to setting the relevant PCRE2_UTF. How setting a 6275 UTF mode affects pattern matching is mentioned in several places below. 6276 There is also a summary of features in the pcre2unicode page. 6277 6278 Some applications that allow their users to supply patterns may wish to 6279 restrict them to non-UTF data for security reasons. If the 6280 PCRE2_NEVER_UTF option is passed to pcre2_compile(), (*UTF) is not al- 6281 lowed, and its appearance in a pattern causes an error. 6282 6283 Unicode property support 6284 6285 Another special sequence that may appear at the start of a pattern is 6286 (*UCP). This has the same effect as setting the PCRE2_UCP option: it 6287 causes sequences such as \d and \w to use Unicode properties to deter- 6288 mine character types, instead of recognizing only characters with codes 6289 less than 256 via a lookup table. If also causes upper/lower casing op- 6290 erations to use Unicode properties for characters with code points 6291 greater than 127, even when UTF is not set. 6292 6293 Some applications that allow their users to supply patterns may wish to 6294 restrict them for security reasons. If the PCRE2_NEVER_UCP option is 6295 passed to pcre2_compile(), (*UCP) is not allowed, and its appearance in 6296 a pattern causes an error. 6297 6298 Locking out empty string matching 6299 6300 Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same 6301 effect as passing the PCRE2_NOTEMPTY or PCRE2_NOTEMPTY_ATSTART option 6302 to whichever matching function is subsequently called to match the pat- 6303 tern. These options lock out the matching of empty strings, either en- 6304 tirely, or only at the start of the subject. 6305 6306 Disabling auto-possessification 6307 6308 If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as 6309 setting the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from making 6310 quantifiers possessive when what follows cannot match the repeated 6311 item. For example, by default a+b is treated as a++b. For more details, 6312 see the pcre2api documentation. 6313 6314 Disabling start-up optimizations 6315 6316 If a pattern starts with (*NO_START_OPT), it has the same effect as 6317 setting the PCRE2_NO_START_OPTIMIZE option. This disables several opti- 6318 mizations for quickly reaching "no match" results. For more details, 6319 see the pcre2api documentation. 6320 6321 Disabling automatic anchoring 6322 6323 If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the same effect 6324 as setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables optimiza- 6325 tions that apply to patterns whose top-level branches all start with .* 6326 (match any number of arbitrary characters). For more details, see the 6327 pcre2api documentation. 6328 6329 Disabling JIT compilation 6330 6331 If a pattern that starts with (*NO_JIT) is successfully compiled, an 6332 attempt by the application to apply the JIT optimization by calling 6333 pcre2_jit_compile() is ignored. 6334 6335 Setting match resource limits 6336 6337 The pcre2_match() function contains a counter that is incremented every 6338 time it goes round its main loop. The caller of pcre2_match() can set a 6339 limit on this counter, which therefore limits the amount of computing 6340 resource used for a match. The maximum depth of nested backtracking can 6341 also be limited; this indirectly restricts the amount of heap memory 6342 that is used, but there is also an explicit memory limit that can be 6343 set. 6344 6345 These facilities are provided to catch runaway matches that are pro- 6346 voked by patterns with huge matching trees. A common example is a pat- 6347 tern with nested unlimited repeats applied to a long string that does 6348 not match. When one of these limits is reached, pcre2_match() gives an 6349 error return. The limits can also be set by items at the start of the 6350 pattern of the form 6351 6352 (*LIMIT_HEAP=d) 6353 (*LIMIT_MATCH=d) 6354 (*LIMIT_DEPTH=d) 6355 6356 where d is any number of decimal digits. However, the value of the set- 6357 ting must be less than the value set (or defaulted) by the caller of 6358 pcre2_match() for it to have any effect. In other words, the pattern 6359 writer can lower the limits set by the programmer, but not raise them. 6360 If there is more than one setting of one of these limits, the lower 6361 value is used. The heap limit is specified in kibibytes (units of 1024 6362 bytes). 6363 6364 Prior to release 10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This 6365 name is still recognized for backwards compatibility. 6366 6367 The heap limit applies only when the pcre2_match() or pcre2_dfa_match() 6368 interpreters are used for matching. It does not apply to JIT. The match 6369 limit is used (but in a different way) when JIT is being used, or when 6370 pcre2_dfa_match() is called, to limit computing resource usage by those 6371 matching functions. The depth limit is ignored by JIT but is relevant 6372 for DFA matching, which uses function recursion for recursions within 6373 the pattern and for lookaround assertions and atomic groups. In this 6374 case, the depth limit controls the depth of such recursion. 6375 6376 Newline conventions 6377 6378 PCRE2 supports six different conventions for indicating line breaks in 6379 strings: a single CR (carriage return) character, a single LF (line- 6380 feed) character, the two-character sequence CRLF, any of the three pre- 6381 ceding, any Unicode newline sequence, or the NUL character (binary 6382 zero). The pcre2api page has further discussion about newlines, and 6383 shows how to set the newline convention when calling pcre2_compile(). 6384 6385 It is also possible to specify a newline convention by starting a pat- 6386 tern string with one of the following sequences: 6387 6388 (*CR) carriage return 6389 (*LF) linefeed 6390 (*CRLF) carriage return, followed by linefeed 6391 (*ANYCRLF) any of the three above 6392 (*ANY) all Unicode newline sequences 6393 (*NUL) the NUL character (binary zero) 6394 6395 These override the default and the options given to the compiling func- 6396 tion. For example, on a Unix system where LF is the default newline se- 6397 quence, the pattern 6398 6399 (*CR)a.b 6400 6401 changes the convention to CR. That pattern matches "a\nb" because LF is 6402 no longer a newline. If more than one of these settings is present, the 6403 last one is used. 6404 6405 The newline convention affects where the circumflex and dollar asser- 6406 tions are true. It also affects the interpretation of the dot metachar- 6407 acter when PCRE2_DOTALL is not set, and the behaviour of \N when not 6408 followed by an opening brace. However, it does not affect what the \R 6409 escape sequence matches. By default, this is any Unicode newline se- 6410 quence, for Perl compatibility. However, this can be changed; see the 6411 next section and the description of \R in the section entitled "Newline 6412 sequences" below. A change of \R setting can be combined with a change 6413 of newline convention. 6414 6415 Specifying what \R matches 6416 6417 It is possible to restrict \R to match only CR, LF, or CRLF (instead of 6418 the complete set of Unicode line endings) by setting the option 6419 PCRE2_BSR_ANYCRLF at compile time. This effect can also be achieved by 6420 starting a pattern with (*BSR_ANYCRLF). For completeness, (*BSR_UNI- 6421 CODE) is also recognized, corresponding to PCRE2_BSR_UNICODE. 6422 6423 6424EBCDIC CHARACTER CODES 6425 6426 PCRE2 can be compiled to run in an environment that uses EBCDIC as its 6427 character code instead of ASCII or Unicode (typically a mainframe sys- 6428 tem). In the sections below, character code values are ASCII or Uni- 6429 code; in an EBCDIC environment these characters may have different code 6430 values, and there are no code points greater than 255. 6431 6432 6433CHARACTERS AND METACHARACTERS 6434 6435 A regular expression is a pattern that is matched against a subject 6436 string from left to right. Most characters stand for themselves in a 6437 pattern, and match the corresponding characters in the subject. As a 6438 trivial example, the pattern 6439 6440 The quick brown fox 6441 6442 matches a portion of a subject string that is identical to itself. When 6443 caseless matching is specified (the PCRE2_CASELESS option or (?i) 6444 within the pattern), letters are matched independently of case. Note 6445 that there are two ASCII characters, K and S, that, in addition to 6446 their lower case ASCII equivalents, are case-equivalent with Unicode 6447 U+212A (Kelvin sign) and U+017F (long S) respectively when either 6448 PCRE2_UTF or PCRE2_UCP is set. 6449 6450 The power of regular expressions comes from the ability to include wild 6451 cards, character classes, alternatives, and repetitions in the pattern. 6452 These are encoded in the pattern by the use of metacharacters, which do 6453 not stand for themselves but instead are interpreted in some special 6454 way. 6455 6456 There are two different sets of metacharacters: those that are recog- 6457 nized anywhere in the pattern except within square brackets, and those 6458 that are recognized within square brackets. Outside square brackets, 6459 the metacharacters are as follows: 6460 6461 \ general escape character with several uses 6462 ^ assert start of string (or line, in multiline mode) 6463 $ assert end of string (or line, in multiline mode) 6464 . match any character except newline (by default) 6465 [ start character class definition 6466 | start of alternative branch 6467 ( start group or control verb 6468 ) end group or control verb 6469 * 0 or more quantifier 6470 + 1 or more quantifier; also "possessive quantifier" 6471 ? 0 or 1 quantifier; also quantifier minimizer 6472 { start min/max quantifier 6473 6474 Part of a pattern that is in square brackets is called a "character 6475 class". In a character class the only metacharacters are: 6476 6477 \ general escape character 6478 ^ negate the class, but only if the first character 6479 - indicates character range 6480 [ POSIX character class (if followed by POSIX syntax) 6481 ] terminates the character class 6482 6483 If a pattern is compiled with the PCRE2_EXTENDED option, most white 6484 space in the pattern, other than in a character class, and characters 6485 between a # outside a character class and the next newline, inclusive, 6486 are ignored. An escaping backslash can be used to include a white space 6487 or a # character as part of the pattern. If the PCRE2_EXTENDED_MORE op- 6488 tion is set, the same applies, but in addition unescaped space and hor- 6489 izontal tab characters are ignored inside a character class. Note: only 6490 these two characters are ignored, not the full set of pattern white 6491 space characters that are ignored outside a character class. Option 6492 settings can be changed within a pattern; see the section entitled "In- 6493 ternal Option Setting" below. 6494 6495 The following sections describe the use of each of the metacharacters. 6496 6497 6498BACKSLASH 6499 6500 The backslash character has several uses. Firstly, if it is followed by 6501 a character that is not a digit or a letter, it takes away any special 6502 meaning that character may have. This use of backslash as an escape 6503 character applies both inside and outside character classes. 6504 6505 For example, if you want to match a * character, you must write \* in 6506 the pattern. This escaping action applies whether or not the following 6507 character would otherwise be interpreted as a metacharacter, so it is 6508 always safe to precede a non-alphanumeric with backslash to specify 6509 that it stands for itself. In particular, if you want to match a back- 6510 slash, you write \\. 6511 6512 Only ASCII digits and letters have any special meaning after a back- 6513 slash. All other characters (in particular, those whose code points are 6514 greater than 127) are treated as literals. 6515 6516 If you want to treat all characters in a sequence as literals, you can 6517 do so by putting them between \Q and \E. This is different from Perl in 6518 that $ and @ are handled as literals in \Q...\E sequences in PCRE2, 6519 whereas in Perl, $ and @ cause variable interpolation. Also, Perl does 6520 "double-quotish backslash interpolation" on any backslashes between \Q 6521 and \E which, its documentation says, "may lead to confusing results". 6522 PCRE2 treats a backslash between \Q and \E just like any other charac- 6523 ter. Note the following examples: 6524 6525 Pattern PCRE2 matches Perl matches 6526 6527 \Qabc$xyz\E abc$xyz abc followed by the 6528 contents of $xyz 6529 \Qabc\$xyz\E abc\$xyz abc\$xyz 6530 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 6531 \QA\B\E A\B A\B 6532 \Q\\E \ \\E 6533 6534 The \Q...\E sequence is recognized both inside and outside character 6535 classes. An isolated \E that is not preceded by \Q is ignored. If \Q 6536 is not followed by \E later in the pattern, the literal interpretation 6537 continues to the end of the pattern (that is, \E is assumed at the 6538 end). If the isolated \Q is inside a character class, this causes an 6539 error, because the character class is not terminated by a closing 6540 square bracket. 6541 6542 Non-printing characters 6543 6544 A second use of backslash provides a way of encoding non-printing char- 6545 acters in patterns in a visible manner. There is no restriction on the 6546 appearance of non-printing characters in a pattern, but when a pattern 6547 is being prepared by text editing, it is often easier to use one of the 6548 following escape sequences instead of the binary character it repre- 6549 sents. In an ASCII or Unicode environment, these escapes are as fol- 6550 lows: 6551 6552 \a alarm, that is, the BEL character (hex 07) 6553 \cx "control-x", where x is any printable ASCII character 6554 \e escape (hex 1B) 6555 \f form feed (hex 0C) 6556 \n linefeed (hex 0A) 6557 \r carriage return (hex 0D) (but see below) 6558 \t tab (hex 09) 6559 \0dd character with octal code 0dd 6560 \ddd character with octal code ddd, or backreference 6561 \o{ddd..} character with octal code ddd.. 6562 \xhh character with hex code hh 6563 \x{hhh..} character with hex code hhh.. 6564 \N{U+hhh..} character with Unicode hex code point hhh.. 6565 6566 By default, after \x that is not followed by {, from zero to two hexa- 6567 decimal digits are read (letters can be in upper or lower case). Any 6568 number of hexadecimal digits may appear between \x{ and }. If a charac- 6569 ter other than a hexadecimal digit appears between \x{ and }, or if 6570 there is no terminating }, an error occurs. 6571 6572 Characters whose code points are less than 256 can be defined by either 6573 of the two syntaxes for \x or by an octal sequence. There is no differ- 6574 ence in the way they are handled. For example, \xdc is exactly the same 6575 as \x{dc} or \334. However, using the braced versions does make such 6576 sequences easier to read. 6577 6578 Support is available for some ECMAScript (aka JavaScript) escape se- 6579 quences via two compile-time options. If PCRE2_ALT_BSUX is set, the se- 6580 quence \x followed by { is not recognized. Only if \x is followed by 6581 two hexadecimal digits is it recognized as a character escape. Other- 6582 wise it is interpreted as a literal "x" character. In this mode, sup- 6583 port for code points greater than 256 is provided by \u, which must be 6584 followed by four hexadecimal digits; otherwise it is interpreted as a 6585 literal "u" character. 6586 6587 PCRE2_EXTRA_ALT_BSUX has the same effect as PCRE2_ALT_BSUX and, in ad- 6588 dition, \u{hhh..} is recognized as the character specified by hexadeci- 6589 mal code point. There may be any number of hexadecimal digits. This 6590 syntax is from ECMAScript 6. 6591 6592 The \N{U+hhh..} escape sequence is recognized only when PCRE2 is oper- 6593 ating in UTF mode. Perl also uses \N{name} to specify characters by 6594 Unicode name; PCRE2 does not support this. Note that when \N is not 6595 followed by an opening brace (curly bracket) it has an entirely differ- 6596 ent meaning, matching any character that is not a newline. 6597 6598 There are some legacy applications where the escape sequence \r is ex- 6599 pected to match a newline. If the PCRE2_EXTRA_ESCAPED_CR_IS_LF option 6600 is set, \r in a pattern is converted to \n so that it matches a LF 6601 (linefeed) instead of a CR (carriage return) character. 6602 6603 The precise effect of \cx on ASCII characters is as follows: if x is a 6604 lower case letter, it is converted to upper case. Then bit 6 of the 6605 character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A 6606 (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes 6607 hex 7B (; is 3B). If the code unit following \c has a value less than 6608 32 or greater than 126, a compile-time error occurs. 6609 6610 When PCRE2 is compiled in EBCDIC mode, \N{U+hhh..} is not supported. 6611 \a, \e, \f, \n, \r, and \t generate the appropriate EBCDIC code values. 6612 The \c escape is processed as specified for Perl in the perlebcdic doc- 6613 ument. The only characters that are allowed after \c are A-Z, a-z, or 6614 one of @, [, \, ], ^, _, or ?. Any other character provokes a compile- 6615 time error. The sequence \c@ encodes character code 0; after \c the 6616 letters (in either case) encode characters 1-26 (hex 01 to hex 1A); [, 6617 \, ], ^, and _ encode characters 27-31 (hex 1B to hex 1F), and \c? be- 6618 comes either 255 (hex FF) or 95 (hex 5F). 6619 6620 Thus, apart from \c?, these escapes generate the same character code 6621 values as they do in an ASCII environment, though the meanings of the 6622 values mostly differ. For example, \cG always generates code value 7, 6623 which is BEL in ASCII but DEL in EBCDIC. 6624 6625 The sequence \c? generates DEL (127, hex 7F) in an ASCII environment, 6626 but because 127 is not a control character in EBCDIC, Perl makes it 6627 generate the APC character. Unfortunately, there are several variants 6628 of EBCDIC. In most of them the APC character has the value 255 (hex 6629 FF), but in the one Perl calls POSIX-BC its value is 95 (hex 5F). If 6630 certain other characters have POSIX-BC values, PCRE2 makes \c? generate 6631 95; otherwise it generates 255. 6632 6633 After \0 up to two further octal digits are read. If there are fewer 6634 than two digits, just those that are present are used. Thus the se- 6635 quence \0\x\015 specifies two binary zeros followed by a CR character 6636 (code value 13). Make sure you supply two digits after the initial zero 6637 if the pattern character that follows is itself an octal digit. 6638 6639 The escape \o must be followed by a sequence of octal digits, enclosed 6640 in braces. An error occurs if this is not the case. This escape is a 6641 recent addition to Perl; it provides way of specifying character code 6642 points as octal numbers greater than 0777, and it also allows octal 6643 numbers and backreferences to be unambiguously specified. 6644 6645 For greater clarity and unambiguity, it is best to avoid following \ by 6646 a digit greater than zero. Instead, use \o{} or \x{} to specify numeri- 6647 cal character code points, and \g{} to specify backreferences. The fol- 6648 lowing paragraphs describe the old, ambiguous syntax. 6649 6650 The handling of a backslash followed by a digit other than 0 is compli- 6651 cated, and Perl has changed over time, causing PCRE2 also to change. 6652 6653 Outside a character class, PCRE2 reads the digit and any following dig- 6654 its as a decimal number. If the number is less than 10, begins with the 6655 digit 8 or 9, or if there are at least that many previous capture 6656 groups in the expression, the entire sequence is taken as a backrefer- 6657 ence. A description of how this works is given later, following the 6658 discussion of parenthesized groups. Otherwise, up to three octal dig- 6659 its are read to form a character code. 6660 6661 Inside a character class, PCRE2 handles \8 and \9 as the literal char- 6662 acters "8" and "9", and otherwise reads up to three octal digits fol- 6663 lowing the backslash, using them to generate a data character. Any sub- 6664 sequent digits stand for themselves. For example, outside a character 6665 class: 6666 6667 \040 is another way of writing an ASCII space 6668 \40 is the same, provided there are fewer than 40 6669 previous capture groups 6670 \7 is always a backreference 6671 \11 might be a backreference, or another way of 6672 writing a tab 6673 \011 is always a tab 6674 \0113 is a tab followed by the character "3" 6675 \113 might be a backreference, otherwise the 6676 character with octal code 113 6677 \377 might be a backreference, otherwise 6678 the value 255 (decimal) 6679 \81 is always a backreference 6680 6681 Note that octal values of 100 or greater that are specified using this 6682 syntax must not be introduced by a leading zero, because no more than 6683 three octal digits are ever read. 6684 6685 Constraints on character values 6686 6687 Characters that are specified using octal or hexadecimal numbers are 6688 limited to certain values, as follows: 6689 6690 8-bit non-UTF mode no greater than 0xff 6691 16-bit non-UTF mode no greater than 0xffff 6692 32-bit non-UTF mode no greater than 0xffffffff 6693 All UTF modes no greater than 0x10ffff and a valid code point 6694 6695 Invalid Unicode code points are all those in the range 0xd800 to 0xdfff 6696 (the so-called "surrogate" code points). The check for these can be 6697 disabled by the caller of pcre2_compile() by setting the option 6698 PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES. However, this is possible only in 6699 UTF-8 and UTF-32 modes, because these values are not representable in 6700 UTF-16. 6701 6702 Escape sequences in character classes 6703 6704 All the sequences that define a single character value can be used both 6705 inside and outside character classes. In addition, inside a character 6706 class, \b is interpreted as the backspace character (hex 08). 6707 6708 When not followed by an opening brace, \N is not allowed in a character 6709 class. \B, \R, and \X are not special inside a character class. Like 6710 other unrecognized alphabetic escape sequences, they cause an error. 6711 Outside a character class, these sequences have different meanings. 6712 6713 Unsupported escape sequences 6714 6715 In Perl, the sequences \F, \l, \L, \u, and \U are recognized by its 6716 string handler and used to modify the case of following characters. By 6717 default, PCRE2 does not support these escape sequences in patterns. 6718 However, if either of the PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX op- 6719 tions is set, \U matches a "U" character, and \u can be used to define 6720 a character by code point, as described above. 6721 6722 Absolute and relative backreferences 6723 6724 The sequence \g followed by a signed or unsigned number, optionally en- 6725 closed in braces, is an absolute or relative backreference. A named 6726 backreference can be coded as \g{name}. Backreferences are discussed 6727 later, following the discussion of parenthesized groups. 6728 6729 Absolute and relative subroutine calls 6730 6731 For compatibility with Oniguruma, the non-Perl syntax \g followed by a 6732 name or a number enclosed either in angle brackets or single quotes, is 6733 an alternative syntax for referencing a capture group as a subroutine. 6734 Details are discussed later. Note that \g{...} (Perl syntax) and 6735 \g<...> (Oniguruma syntax) are not synonymous. The former is a backref- 6736 erence; the latter is a subroutine call. 6737 6738 Generic character types 6739 6740 Another use of backslash is for specifying generic character types: 6741 6742 \d any decimal digit 6743 \D any character that is not a decimal digit 6744 \h any horizontal white space character 6745 \H any character that is not a horizontal white space character 6746 \N any character that is not a newline 6747 \s any white space character 6748 \S any character that is not a white space character 6749 \v any vertical white space character 6750 \V any character that is not a vertical white space character 6751 \w any "word" character 6752 \W any "non-word" character 6753 6754 The \N escape sequence has the same meaning as the "." metacharacter 6755 when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not change 6756 the meaning of \N. Note that when \N is followed by an opening brace it 6757 has a different meaning. See the section entitled "Non-printing charac- 6758 ters" above for details. Perl also uses \N{name} to specify characters 6759 by Unicode name; PCRE2 does not support this. 6760 6761 Each pair of lower and upper case escape sequences partitions the com- 6762 plete set of characters into two disjoint sets. Any given character 6763 matches one, and only one, of each pair. The sequences can appear both 6764 inside and outside character classes. They each match one character of 6765 the appropriate type. If the current matching point is at the end of 6766 the subject string, all of them fail, because there is no character to 6767 match. 6768 6769 The default \s characters are HT (9), LF (10), VT (11), FF (12), CR 6770 (13), and space (32), which are defined as white space in the "C" lo- 6771 cale. This list may vary if locale-specific matching is taking place. 6772 For example, in some locales the "non-breaking space" character (\xA0) 6773 is recognized as white space, and in others the VT character is not. 6774 6775 A "word" character is an underscore or any character that is a letter 6776 or digit. By default, the definition of letters and digits is con- 6777 trolled by PCRE2's low-valued character tables, and may vary if locale- 6778 specific matching is taking place (see "Locale support" in the pcre2api 6779 page). For example, in a French locale such as "fr_FR" in Unix-like 6780 systems, or "french" in Windows, some character codes greater than 127 6781 are used for accented letters, and these are then matched by \w. The 6782 use of locales with Unicode is discouraged. 6783 6784 By default, characters whose code points are greater than 127 never 6785 match \d, \s, or \w, and always match \D, \S, and \W, although this may 6786 be different for characters in the range 128-255 when locale-specific 6787 matching is happening. These escape sequences retain their original 6788 meanings from before Unicode support was available, mainly for effi- 6789 ciency reasons. If the PCRE2_UCP option is set, the behaviour is 6790 changed so that Unicode properties are used to determine character 6791 types, as follows: 6792 6793 \d any character that matches \p{Nd} (decimal digit) 6794 \s any character that matches \p{Z} or \h or \v 6795 \w any character that matches \p{L} or \p{N}, plus underscore 6796 6797 The upper case escapes match the inverse sets of characters. Note that 6798 \d matches only decimal digits, whereas \w matches any Unicode digit, 6799 as well as any Unicode letter, and underscore. Note also that PCRE2_UCP 6800 affects \b, and \B because they are defined in terms of \w and \W. 6801 Matching these sequences is noticeably slower when PCRE2_UCP is set. 6802 6803 The sequences \h, \H, \v, and \V, in contrast to the other sequences, 6804 which match only ASCII characters by default, always match a specific 6805 list of code points, whether or not PCRE2_UCP is set. The horizontal 6806 space characters are: 6807 6808 U+0009 Horizontal tab (HT) 6809 U+0020 Space 6810 U+00A0 Non-break space 6811 U+1680 Ogham space mark 6812 U+180E Mongolian vowel separator 6813 U+2000 En quad 6814 U+2001 Em quad 6815 U+2002 En space 6816 U+2003 Em space 6817 U+2004 Three-per-em space 6818 U+2005 Four-per-em space 6819 U+2006 Six-per-em space 6820 U+2007 Figure space 6821 U+2008 Punctuation space 6822 U+2009 Thin space 6823 U+200A Hair space 6824 U+202F Narrow no-break space 6825 U+205F Medium mathematical space 6826 U+3000 Ideographic space 6827 6828 The vertical space characters are: 6829 6830 U+000A Linefeed (LF) 6831 U+000B Vertical tab (VT) 6832 U+000C Form feed (FF) 6833 U+000D Carriage return (CR) 6834 U+0085 Next line (NEL) 6835 U+2028 Line separator 6836 U+2029 Paragraph separator 6837 6838 In 8-bit, non-UTF-8 mode, only the characters with code points less 6839 than 256 are relevant. 6840 6841 Newline sequences 6842 6843 Outside a character class, by default, the escape sequence \R matches 6844 any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent 6845 to the following: 6846 6847 (?>\r\n|\n|\x0b|\f|\r|\x85) 6848 6849 This is an example of an "atomic group", details of which are given be- 6850 low. This particular group matches either the two-character sequence 6851 CR followed by LF, or one of the single characters LF (linefeed, 6852 U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car- 6853 riage return, U+000D), or NEL (next line, U+0085). Because this is an 6854 atomic group, the two-character sequence is treated as a single unit 6855 that cannot be split. 6856 6857 In other modes, two additional characters whose code points are greater 6858 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa- 6859 rator, U+2029). Unicode support is not needed for these characters to 6860 be recognized. 6861 6862 It is possible to restrict \R to match only CR, LF, or CRLF (instead of 6863 the complete set of Unicode line endings) by setting the option 6864 PCRE2_BSR_ANYCRLF at compile time. (BSR is an abbreviation for "back- 6865 slash R".) This can be made the default when PCRE2 is built; if this is 6866 the case, the other behaviour can be requested via the PCRE2_BSR_UNI- 6867 CODE option. It is also possible to specify these settings by starting 6868 a pattern string with one of the following sequences: 6869 6870 (*BSR_ANYCRLF) CR, LF, or CRLF only 6871 (*BSR_UNICODE) any Unicode newline sequence 6872 6873 These override the default and the options given to the compiling func- 6874 tion. Note that these special settings, which are not Perl-compatible, 6875 are recognized only at the very start of a pattern, and that they must 6876 be in upper case. If more than one of them is present, the last one is 6877 used. They can be combined with a change of newline convention; for ex- 6878 ample, a pattern can start with: 6879 6880 (*ANY)(*BSR_ANYCRLF) 6881 6882 They can also be combined with the (*UTF) or (*UCP) special sequences. 6883 Inside a character class, \R is treated as an unrecognized escape se- 6884 quence, and causes an error. 6885 6886 Unicode character properties 6887 6888 When PCRE2 is built with Unicode support (the default), three addi- 6889 tional escape sequences that match characters with specific properties 6890 are available. They can be used in any mode, though in 8-bit and 16-bit 6891 non-UTF modes these sequences are of course limited to testing charac- 6892 ters whose code points are less than U+0100 and U+10000, respectively. 6893 In 32-bit non-UTF mode, code points greater than 0x10ffff (the Unicode 6894 limit) may be encountered. These are all treated as being in the Un- 6895 known script and with an unassigned type. 6896 6897 Matching characters by Unicode property is not fast, because PCRE2 has 6898 to do a multistage table lookup in order to find a character's prop- 6899 erty. That is why the traditional escape sequences such as \d and \w do 6900 not use Unicode properties in PCRE2 by default, though you can make 6901 them do so by setting the PCRE2_UCP option or by starting the pattern 6902 with (*UCP). 6903 6904 The extra escape sequences that provide property support are: 6905 6906 \p{xx} a character with the xx property 6907 \P{xx} a character without the xx property 6908 \X a Unicode extended grapheme cluster 6909 6910 The property names represented by xx above are not case-sensitive, and 6911 in accordance with Unicode's "loose matching" rules, spaces, hyphens, 6912 and underscores are ignored. There is support for Unicode script names, 6913 Unicode general category properties, "Any", which matches any character 6914 (including newline), Bidi_Class, a number of binary (yes/no) proper- 6915 ties, and some special PCRE2 properties (described below). Certain 6916 other Perl properties such as "InMusicalSymbols" are not supported by 6917 PCRE2. Note that \P{Any} does not match any characters, so always 6918 causes a match failure. 6919 6920 Script properties for \p and \P 6921 6922 There are three different syntax forms for matching a script. Each Uni- 6923 code character has a basic script and, optionally, a list of other 6924 scripts ("Script Extensions") with which it is commonly used. Using the 6925 Adlam script as an example, \p{sc:Adlam} matches characters whose basic 6926 script is Adlam, whereas \p{scx:Adlam} matches, in addition, characters 6927 that have Adlam in their extensions list. The full names "script" and 6928 "script extensions" for the property types are recognized, and a equals 6929 sign is an alternative to the colon. If a script name is given without 6930 a property type, for example, \p{Adlam}, it is treated as \p{scx:Ad- 6931 lam}. Perl changed to this interpretation at release 5.26 and PCRE2 6932 changed at release 10.40. 6933 6934 Unassigned characters (and in non-UTF 32-bit mode, characters with code 6935 points greater than 0x10FFFF) are assigned the "Unknown" script. Others 6936 that are not part of an identified script are lumped together as "Com- 6937 mon". The current list of recognized script names and their 4-character 6938 abbreviations can be obtained by running this command: 6939 6940 pcre2test -LS 6941 6942 6943 The general category property for \p and \P 6944 6945 Each character has exactly one Unicode general category property, spec- 6946 ified by a two-letter abbreviation. For compatibility with Perl, nega- 6947 tion can be specified by including a circumflex between the opening 6948 brace and the property name. For example, \p{^Lu} is the same as 6949 \P{Lu}. 6950 6951 If only one letter is specified with \p or \P, it includes all the gen- 6952 eral category properties that start with that letter. In this case, in 6953 the absence of negation, the curly brackets in the escape sequence are 6954 optional; these two examples have the same effect: 6955 6956 \p{L} 6957 \pL 6958 6959 The following general category property codes are supported: 6960 6961 C Other 6962 Cc Control 6963 Cf Format 6964 Cn Unassigned 6965 Co Private use 6966 Cs Surrogate 6967 6968 L Letter 6969 Ll Lower case letter 6970 Lm Modifier letter 6971 Lo Other letter 6972 Lt Title case letter 6973 Lu Upper case letter 6974 6975 M Mark 6976 Mc Spacing mark 6977 Me Enclosing mark 6978 Mn Non-spacing mark 6979 6980 N Number 6981 Nd Decimal number 6982 Nl Letter number 6983 No Other number 6984 6985 P Punctuation 6986 Pc Connector punctuation 6987 Pd Dash punctuation 6988 Pe Close punctuation 6989 Pf Final punctuation 6990 Pi Initial punctuation 6991 Po Other punctuation 6992 Ps Open punctuation 6993 6994 S Symbol 6995 Sc Currency symbol 6996 Sk Modifier symbol 6997 Sm Mathematical symbol 6998 So Other symbol 6999 7000 Z Separator 7001 Zl Line separator 7002 Zp Paragraph separator 7003 Zs Space separator 7004 7005 The special property LC, which has the synonym L&, is also supported: 7006 it matches a character that has the Lu, Ll, or Lt property, in other 7007 words, a letter that is not classified as a modifier or "other". 7008 7009 The Cs (Surrogate) property applies only to characters whose code 7010 points are in the range U+D800 to U+DFFF. These characters are no dif- 7011 ferent to any other character when PCRE2 is not in UTF mode (using the 7012 16-bit or 32-bit library). However, they are not valid in Unicode 7013 strings and so cannot be tested by PCRE2 in UTF mode, unless UTF valid- 7014 ity checking has been turned off (see the discussion of 7015 PCRE2_NO_UTF_CHECK in the pcre2api page). 7016 7017 The long synonyms for property names that Perl supports (such as 7018 \p{Letter}) are not supported by PCRE2, nor is it permitted to prefix 7019 any of these properties with "Is". 7020 7021 No character that is in the Unicode table has the Cn (unassigned) prop- 7022 erty. Instead, this property is assumed for any code point that is not 7023 in the Unicode table. 7024 7025 Specifying caseless matching does not affect these escape sequences. 7026 For example, \p{Lu} always matches only upper case letters. This is 7027 different from the behaviour of current versions of Perl. 7028 7029 Binary (yes/no) properties for \p and \P 7030 7031 Unicode defines a number of binary properties, that is, properties 7032 whose only values are true or false. You can obtain a list of those 7033 that are recognized by \p and \P, along with their abbreviations, by 7034 running this command: 7035 7036 pcre2test -LP 7037 7038 7039 The Bidi_Class property for \p and \P 7040 7041 \p{Bidi_Class:<class>} matches a character with the given class 7042 \p{BC:<class>} matches a character with the given class 7043 7044 The recognized classes are: 7045 7046 AL Arabic letter 7047 AN Arabic number 7048 B paragraph separator 7049 BN boundary neutral 7050 CS common separator 7051 EN European number 7052 ES European separator 7053 ET European terminator 7054 FSI first strong isolate 7055 L left-to-right 7056 LRE left-to-right embedding 7057 LRI left-to-right isolate 7058 LRO left-to-right override 7059 NSM non-spacing mark 7060 ON other neutral 7061 PDF pop directional format 7062 PDI pop directional isolate 7063 R right-to-left 7064 RLE right-to-left embedding 7065 RLI right-to-left isolate 7066 RLO right-to-left override 7067 S segment separator 7068 WS which space 7069 7070 An equals sign may be used instead of a colon. The class names are 7071 case-insensitive; only the short names listed above are recognized. 7072 7073 Extended grapheme clusters 7074 7075 The \X escape matches any number of Unicode characters that form an 7076 "extended grapheme cluster", and treats the sequence as an atomic group 7077 (see below). Unicode supports various kinds of composite character by 7078 giving each character a grapheme breaking property, and having rules 7079 that use these properties to define the boundaries of extended grapheme 7080 clusters. The rules are defined in Unicode Standard Annex 29, "Unicode 7081 Text Segmentation". Unicode 11.0.0 abandoned the use of some previous 7082 properties that had been used for emojis. Instead it introduced vari- 7083 ous emoji-specific properties. PCRE2 uses only the Extended Picto- 7084 graphic property. 7085 7086 \X always matches at least one character. Then it decides whether to 7087 add additional characters according to the following rules for ending a 7088 cluster: 7089 7090 1. End at the end of the subject string. 7091 7092 2. Do not end between CR and LF; otherwise end after any control char- 7093 acter. 7094 7095 3. Do not break Hangul (a Korean script) syllable sequences. Hangul 7096 characters are of five types: L, V, T, LV, and LVT. An L character may 7097 be followed by an L, V, LV, or LVT character; an LV or V character may 7098 be followed by a V or T character; an LVT or T character may be fol- 7099 lowed only by a T character. 7100 7101 4. Do not end before extending characters or spacing marks or the 7102 "zero-width joiner" character. Characters with the "mark" property al- 7103 ways have the "extend" grapheme breaking property. 7104 7105 5. Do not end after prepend characters. 7106 7107 6. Do not break within emoji modifier sequences or emoji zwj sequences. 7108 That is, do not break between characters with the Extended_Pictographic 7109 property. Extend and ZWJ characters are allowed between the charac- 7110 ters. 7111 7112 7. Do not break within emoji flag sequences. That is, do not break be- 7113 tween regional indicator (RI) characters if there are an odd number of 7114 RI characters before the break point. 7115 7116 8. Otherwise, end the cluster. 7117 7118 PCRE2's additional properties 7119 7120 As well as the standard Unicode properties described above, PCRE2 sup- 7121 ports four more that make it possible to convert traditional escape se- 7122 quences such as \w and \s to use Unicode properties. PCRE2 uses these 7123 non-standard, non-Perl properties internally when PCRE2_UCP is set. 7124 However, they may also be used explicitly. These properties are: 7125 7126 Xan Any alphanumeric character 7127 Xps Any POSIX space character 7128 Xsp Any Perl space character 7129 Xwd Any Perl "word" character 7130 7131 Xan matches characters that have either the L (letter) or the N (num- 7132 ber) property. Xps matches the characters tab, linefeed, vertical tab, 7133 form feed, or carriage return, and any other character that has the Z 7134 (separator) property. Xsp is the same as Xps; in PCRE1 it used to ex- 7135 clude vertical tab, for Perl compatibility, but Perl changed. Xwd 7136 matches the same characters as Xan, plus underscore. 7137 7138 There is another non-standard property, Xuc, which matches any charac- 7139 ter that can be represented by a Universal Character Name in C++ and 7140 other programming languages. These are the characters $, @, ` (grave 7141 accent), and all characters with Unicode code points greater than or 7142 equal to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that 7143 most base (ASCII) characters are excluded. (Universal Character Names 7144 are of the form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit. 7145 Note that the Xuc property does not match these sequences but the char- 7146 acters that they represent.) 7147 7148 Resetting the match start 7149 7150 In normal use, the escape sequence \K causes any previously matched 7151 characters not to be included in the final matched sequence that is re- 7152 turned. For example, the pattern: 7153 7154 foo\Kbar 7155 7156 matches "foobar", but reports that it has matched "bar". \K does not 7157 interact with anchoring in any way. The pattern: 7158 7159 ^foo\Kbar 7160 7161 matches only when the subject begins with "foobar" (in single line 7162 mode), though it again reports the matched string as "bar". This fea- 7163 ture is similar to a lookbehind assertion (described below). However, 7164 in this case, the part of the subject before the real match does not 7165 have to be of fixed length, as lookbehind assertions do. The use of \K 7166 does not interfere with the setting of captured substrings. For exam- 7167 ple, when the pattern 7168 7169 (foo)\Kbar 7170 7171 matches "foobar", the first substring is still set to "foo". 7172 7173 From version 5.32.0 Perl forbids the use of \K in lookaround asser- 7174 tions. From release 10.38 PCRE2 also forbids this by default. However, 7175 the PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK option can be used when calling 7176 pcre2_compile() to re-enable the previous behaviour. When this option 7177 is set, \K is acted upon when it occurs inside positive assertions, but 7178 is ignored in negative assertions. Note that when a pattern such as 7179 (?=ab\K) matches, the reported start of the match can be greater than 7180 the end of the match. Using \K in a lookbehind assertion at the start 7181 of a pattern can also lead to odd effects. For example, consider this 7182 pattern: 7183 7184 (?<=\Kfoo)bar 7185 7186 If the subject is "foobar", a call to pcre2_match() with a starting 7187 offset of 3 succeeds and reports the matching string as "foobar", that 7188 is, the start of the reported match is earlier than where the match 7189 started. 7190 7191 Simple assertions 7192 7193 The final use of backslash is for certain simple assertions. An asser- 7194 tion specifies a condition that has to be met at a particular point in 7195 a match, without consuming any characters from the subject string. The 7196 use of groups for more complicated assertions is described below. The 7197 backslashed assertions are: 7198 7199 \b matches at a word boundary 7200 \B matches when not at a word boundary 7201 \A matches at the start of the subject 7202 \Z matches at the end of the subject 7203 also matches before a newline at the end of the subject 7204 \z matches only at the end of the subject 7205 \G matches at the first matching position in the subject 7206 7207 Inside a character class, \b has a different meaning; it matches the 7208 backspace character. If any other of these assertions appears in a 7209 character class, an "invalid escape sequence" error is generated. 7210 7211 A word boundary is a position in the subject string where the current 7212 character and the previous character do not both match \w or \W (i.e. 7213 one matches \w and the other matches \W), or the start or end of the 7214 string if the first or last character matches \w, respectively. When 7215 PCRE2 is built with Unicode support, the meanings of \w and \W can be 7216 changed by setting the PCRE2_UCP option. When this is done, it also af- 7217 fects \b and \B. Neither PCRE2 nor Perl has a separate "start of word" 7218 or "end of word" metasequence. However, whatever follows \b normally 7219 determines which it is. For example, the fragment \ba matches "a" at 7220 the start of a word. 7221 7222 The \A, \Z, and \z assertions differ from the traditional circumflex 7223 and dollar (described in the next section) in that they only ever match 7224 at the very start and end of the subject string, whatever options are 7225 set. Thus, they are independent of multiline mode. These three asser- 7226 tions are not affected by the PCRE2_NOTBOL or PCRE2_NOTEOL options, 7227 which affect only the behaviour of the circumflex and dollar metachar- 7228 acters. However, if the startoffset argument of pcre2_match() is non- 7229 zero, indicating that matching is to start at a point other than the 7230 beginning of the subject, \A can never match. The difference between 7231 \Z and \z is that \Z matches before a newline at the end of the string 7232 as well as at the very end, whereas \z matches only at the end. 7233 7234 The \G assertion is true only when the current matching position is at 7235 the start point of the matching process, as specified by the startoff- 7236 set argument of pcre2_match(). It differs from \A when the value of 7237 startoffset is non-zero. By calling pcre2_match() multiple times with 7238 appropriate arguments, you can mimic Perl's /g option, and it is in 7239 this kind of implementation where \G can be useful. 7240 7241 Note, however, that PCRE2's implementation of \G, being true at the 7242 starting character of the matching process, is subtly different from 7243 Perl's, which defines it as true at the end of the previous match. In 7244 Perl, these can be different when the previously matched string was 7245 empty. Because PCRE2 does just one match at a time, it cannot reproduce 7246 this behaviour. 7247 7248 If all the alternatives of a pattern begin with \G, the expression is 7249 anchored to the starting match position, and the "anchored" flag is set 7250 in the compiled regular expression. 7251 7252 7253CIRCUMFLEX AND DOLLAR 7254 7255 The circumflex and dollar metacharacters are zero-width assertions. 7256 That is, they test for a particular condition being true without con- 7257 suming any characters from the subject string. These two metacharacters 7258 are concerned with matching the starts and ends of lines. If the new- 7259 line convention is set so that only the two-character sequence CRLF is 7260 recognized as a newline, isolated CR and LF characters are treated as 7261 ordinary data characters, and are not recognized as newlines. 7262 7263 Outside a character class, in the default matching mode, the circumflex 7264 character is an assertion that is true only if the current matching 7265 point is at the start of the subject string. If the startoffset argu- 7266 ment of pcre2_match() is non-zero, or if PCRE2_NOTBOL is set, circum- 7267 flex can never match if the PCRE2_MULTILINE option is unset. Inside a 7268 character class, circumflex has an entirely different meaning (see be- 7269 low). 7270 7271 Circumflex need not be the first character of the pattern if a number 7272 of alternatives are involved, but it should be the first thing in each 7273 alternative in which it appears if the pattern is ever to match that 7274 branch. If all possible alternatives start with a circumflex, that is, 7275 if the pattern is constrained to match only at the start of the sub- 7276 ject, it is said to be an "anchored" pattern. (There are also other 7277 constructs that can cause a pattern to be anchored.) 7278 7279 The dollar character is an assertion that is true only if the current 7280 matching point is at the end of the subject string, or immediately be- 7281 fore a newline at the end of the string (by default), unless PCRE2_NO- 7282 TEOL is set. Note, however, that it does not actually match the new- 7283 line. Dollar need not be the last character of the pattern if a number 7284 of alternatives are involved, but it should be the last item in any 7285 branch in which it appears. Dollar has no special meaning in a charac- 7286 ter class. 7287 7288 The meaning of dollar can be changed so that it matches only at the 7289 very end of the string, by setting the PCRE2_DOLLAR_ENDONLY option at 7290 compile time. This does not affect the \Z assertion. 7291 7292 The meanings of the circumflex and dollar metacharacters are changed if 7293 the PCRE2_MULTILINE option is set. When this is the case, a dollar 7294 character matches before any newlines in the string, as well as at the 7295 very end, and a circumflex matches immediately after internal newlines 7296 as well as at the start of the subject string. It does not match after 7297 a newline that ends the string, for compatibility with Perl. However, 7298 this can be changed by setting the PCRE2_ALT_CIRCUMFLEX option. 7299 7300 For example, the pattern /^abc$/ matches the subject string "def\nabc" 7301 (where \n represents a newline) in multiline mode, but not otherwise. 7302 Consequently, patterns that are anchored in single line mode because 7303 all branches start with ^ are not anchored in multiline mode, and a 7304 match for circumflex is possible when the startoffset argument of 7305 pcre2_match() is non-zero. The PCRE2_DOLLAR_ENDONLY option is ignored 7306 if PCRE2_MULTILINE is set. 7307 7308 When the newline convention (see "Newline conventions" below) recog- 7309 nizes the two-character sequence CRLF as a newline, this is preferred, 7310 even if the single characters CR and LF are also recognized as new- 7311 lines. For example, if the newline convention is "any", a multiline 7312 mode circumflex matches before "xyz" in the string "abc\r\nxyz" rather 7313 than after CR, even though CR on its own is a valid newline. (It also 7314 matches at the very start of the string, of course.) 7315 7316 Note that the sequences \A, \Z, and \z can be used to match the start 7317 and end of the subject in both modes, and if all branches of a pattern 7318 start with \A it is always anchored, whether or not PCRE2_MULTILINE is 7319 set. 7320 7321 7322FULL STOP (PERIOD, DOT) AND \N 7323 7324 Outside a character class, a dot in the pattern matches any one charac- 7325 ter in the subject string except (by default) a character that signi- 7326 fies the end of a line. One or more characters may be specified as line 7327 terminators (see "Newline conventions" above). 7328 7329 Dot never matches a single line-ending character. When the two-charac- 7330 ter sequence CRLF is the only line ending, dot does not match CR if it 7331 is immediately followed by LF, but otherwise it matches all characters 7332 (including isolated CRs and LFs). When ANYCRLF is selected for line 7333 endings, no occurences of CR of LF match dot. When all Unicode line 7334 endings are being recognized, dot does not match CR or LF or any of the 7335 other line ending characters. 7336 7337 The behaviour of dot with regard to newlines can be changed. If the 7338 PCRE2_DOTALL option is set, a dot matches any one character, without 7339 exception. If the two-character sequence CRLF is present in the sub- 7340 ject string, it takes two dots to match it. 7341 7342 The handling of dot is entirely independent of the handling of circum- 7343 flex and dollar, the only relationship being that they both involve 7344 newlines. Dot has no special meaning in a character class. 7345 7346 The escape sequence \N when not followed by an opening brace behaves 7347 like a dot, except that it is not affected by the PCRE2_DOTALL option. 7348 In other words, it matches any character except one that signifies the 7349 end of a line. 7350 7351 When \N is followed by an opening brace it has a different meaning. See 7352 the section entitled "Non-printing characters" above for details. Perl 7353 also uses \N{name} to specify characters by Unicode name; PCRE2 does 7354 not support this. 7355 7356 7357MATCHING A SINGLE CODE UNIT 7358 7359 Outside a character class, the escape sequence \C matches any one code 7360 unit, whether or not a UTF mode is set. In the 8-bit library, one code 7361 unit is one byte; in the 16-bit library it is a 16-bit unit; in the 7362 32-bit library it is a 32-bit unit. Unlike a dot, \C always matches 7363 line-ending characters. The feature is provided in Perl in order to 7364 match individual bytes in UTF-8 mode, but it is unclear how it can use- 7365 fully be used. 7366 7367 Because \C breaks up characters into individual code units, matching 7368 one unit with \C in UTF-8 or UTF-16 mode means that the rest of the 7369 string may start with a malformed UTF character. This has undefined re- 7370 sults, because PCRE2 assumes that it is matching character by character 7371 in a valid UTF string (by default it checks the subject string's valid- 7372 ity at the start of processing unless the PCRE2_NO_UTF_CHECK or 7373 PCRE2_MATCH_INVALID_UTF option is used). 7374 7375 An application can lock out the use of \C by setting the 7376 PCRE2_NEVER_BACKSLASH_C option when compiling a pattern. It is also 7377 possible to build PCRE2 with the use of \C permanently disabled. 7378 7379 PCRE2 does not allow \C to appear in lookbehind assertions (described 7380 below) in UTF-8 or UTF-16 modes, because this would make it impossible 7381 to calculate the length of the lookbehind. Neither the alternative 7382 matching function pcre2_dfa_match() nor the JIT optimizer support \C in 7383 these UTF modes. The former gives a match-time error; the latter fails 7384 to optimize and so the match is always run using the interpreter. 7385 7386 In the 32-bit library, however, \C is always supported (when not ex- 7387 plicitly locked out) because it always matches a single code unit, 7388 whether or not UTF-32 is specified. 7389 7390 In general, the \C escape sequence is best avoided. However, one way of 7391 using it that avoids the problem of malformed UTF-8 or UTF-16 charac- 7392 ters is to use a lookahead to check the length of the next character, 7393 as in this pattern, which could be used with a UTF-8 string (ignore 7394 white space and line breaks): 7395 7396 (?| (?=[\x00-\x7f])(\C) | 7397 (?=[\x80-\x{7ff}])(\C)(\C) | 7398 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | 7399 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) 7400 7401 In this example, a group that starts with (?| resets the capturing 7402 parentheses numbers in each alternative (see "Duplicate Group Numbers" 7403 below). The assertions at the start of each branch check the next UTF-8 7404 character for values whose encoding uses 1, 2, 3, or 4 bytes, respec- 7405 tively. The character's individual bytes are then captured by the ap- 7406 propriate number of \C groups. 7407 7408 7409SQUARE BRACKETS AND CHARACTER CLASSES 7410 7411 An opening square bracket introduces a character class, terminated by a 7412 closing square bracket. A closing square bracket on its own is not spe- 7413 cial by default. If a closing square bracket is required as a member 7414 of the class, it should be the first data character in the class (after 7415 an initial circumflex, if present) or escaped with a backslash. This 7416 means that, by default, an empty class cannot be defined. However, if 7417 the PCRE2_ALLOW_EMPTY_CLASS option is set, a closing square bracket at 7418 the start does end the (empty) class. 7419 7420 A character class matches a single character in the subject. A matched 7421 character must be in the set of characters defined by the class, unless 7422 the first character in the class definition is a circumflex, in which 7423 case the subject character must not be in the set defined by the class. 7424 If a circumflex is actually required as a member of the class, ensure 7425 it is not the first character, or escape it with a backslash. 7426 7427 For example, the character class [aeiou] matches any lower case vowel, 7428 while [^aeiou] matches any character that is not a lower case vowel. 7429 Note that a circumflex is just a convenient notation for specifying the 7430 characters that are in the class by enumerating those that are not. A 7431 class that starts with a circumflex is not an assertion; it still con- 7432 sumes a character from the subject string, and therefore it fails if 7433 the current pointer is at the end of the string. 7434 7435 Characters in a class may be specified by their code points using \o, 7436 \x, or \N{U+hh..} in the usual way. When caseless matching is set, any 7437 letters in a class represent both their upper case and lower case ver- 7438 sions, so for example, a caseless [aeiou] matches "A" as well as "a", 7439 and a caseless [^aeiou] does not match "A", whereas a caseful version 7440 would. Note that there are two ASCII characters, K and S, that, in ad- 7441 dition to their lower case ASCII equivalents, are case-equivalent with 7442 Unicode U+212A (Kelvin sign) and U+017F (long S) respectively when ei- 7443 ther PCRE2_UTF or PCRE2_UCP is set. 7444 7445 Characters that might indicate line breaks are never treated in any 7446 special way when matching character classes, whatever line-ending se- 7447 quence is in use, and whatever setting of the PCRE2_DOTALL and 7448 PCRE2_MULTILINE options is used. A class such as [^a] always matches 7449 one of these characters. 7450 7451 The generic character type escape sequences \d, \D, \h, \H, \p, \P, \s, 7452 \S, \v, \V, \w, and \W may appear in a character class, and add the 7453 characters that they match to the class. For example, [\dABCDEF] 7454 matches any hexadecimal digit. In UTF modes, the PCRE2_UCP option af- 7455 fects the meanings of \d, \s, \w and their upper case partners, just as 7456 it does when they appear outside a character class, as described in the 7457 section entitled "Generic character types" above. The escape sequence 7458 \b has a different meaning inside a character class; it matches the 7459 backspace character. The sequences \B, \R, and \X are not special in- 7460 side a character class. Like any other unrecognized escape sequences, 7461 they cause an error. The same is true for \N when not followed by an 7462 opening brace. 7463 7464 The minus (hyphen) character can be used to specify a range of charac- 7465 ters in a character class. For example, [d-m] matches any letter be- 7466 tween d and m, inclusive. If a minus character is required in a class, 7467 it must be escaped with a backslash or appear in a position where it 7468 cannot be interpreted as indicating a range, typically as the first or 7469 last character in the class, or immediately after a range. For example, 7470 [b-d-z] matches letters in the range b to d, a hyphen character, or z. 7471 7472 Perl treats a hyphen as a literal if it appears before or after a POSIX 7473 class (see below) or before or after a character type escape such as as 7474 \d or \H. However, unless the hyphen is the last character in the 7475 class, Perl outputs a warning in its warning mode, as this is most 7476 likely a user error. As PCRE2 has no facility for warning, an error is 7477 given in these cases. 7478 7479 It is not possible to have the literal character "]" as the end charac- 7480 ter of a range. A pattern such as [W-]46] is interpreted as a class of 7481 two characters ("W" and "-") followed by a literal string "46]", so it 7482 would match "W46]" or "-46]". However, if the "]" is escaped with a 7483 backslash it is interpreted as the end of range, so [W-\]46] is inter- 7484 preted as a class containing a range followed by two other characters. 7485 The octal or hexadecimal representation of "]" can also be used to end 7486 a range. 7487 7488 Ranges normally include all code points between the start and end char- 7489 acters, inclusive. They can also be used for code points specified nu- 7490 merically, for example [\000-\037]. Ranges can include any characters 7491 that are valid for the current mode. In any UTF mode, the so-called 7492 "surrogate" characters (those whose code points lie between 0xd800 and 7493 0xdfff inclusive) may not be specified explicitly by default (the 7494 PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES option disables this check). How- 7495 ever, ranges such as [\x{d7ff}-\x{e000}], which include the surrogates, 7496 are always permitted. 7497 7498 There is a special case in EBCDIC environments for ranges whose end 7499 points are both specified as literal letters in the same case. For com- 7500 patibility with Perl, EBCDIC code points within the range that are not 7501 letters are omitted. For example, [h-k] matches only four characters, 7502 even though the codes for h and k are 0x88 and 0x92, a range of 11 code 7503 points. However, if the range is specified numerically, for example, 7504 [\x88-\x92] or [h-\x92], all code points are included. 7505 7506 If a range that includes letters is used when caseless matching is set, 7507 it matches the letters in either case. For example, [W-c] is equivalent 7508 to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if 7509 character tables for a French locale are in use, [\xc8-\xcb] matches 7510 accented E characters in both cases. 7511 7512 A circumflex can conveniently be used with the upper case character 7513 types to specify a more restricted set of characters than the matching 7514 lower case type. For example, the class [^\W_] matches any letter or 7515 digit, but not underscore, whereas [\w] includes underscore. A positive 7516 character class should be read as "something OR something OR ..." and a 7517 negative class as "NOT something AND NOT something AND NOT ...". 7518 7519 The only metacharacters that are recognized in character classes are 7520 backslash, hyphen (only where it can be interpreted as specifying a 7521 range), circumflex (only at the start), opening square bracket (only 7522 when it can be interpreted as introducing a POSIX class name, or for a 7523 special compatibility feature - see the next two sections), and the 7524 terminating closing square bracket. However, escaping other non-al- 7525 phanumeric characters does no harm. 7526 7527 7528POSIX CHARACTER CLASSES 7529 7530 Perl supports the POSIX notation for character classes. This uses names 7531 enclosed by [: and :] within the enclosing square brackets. PCRE2 also 7532 supports this notation. For example, 7533 7534 [01[:alpha:]%] 7535 7536 matches "0", "1", any alphabetic character, or "%". The supported class 7537 names are: 7538 7539 alnum letters and digits 7540 alpha letters 7541 ascii character codes 0 - 127 7542 blank space or tab only 7543 cntrl control characters 7544 digit decimal digits (same as \d) 7545 graph printing characters, excluding space 7546 lower lower case letters 7547 print printing characters, including space 7548 punct printing characters, excluding letters and digits and space 7549 space white space (the same as \s from PCRE2 8.34) 7550 upper upper case letters 7551 word "word" characters (same as \w) 7552 xdigit hexadecimal digits 7553 7554 The default "space" characters are HT (9), LF (10), VT (11), FF (12), 7555 CR (13), and space (32). If locale-specific matching is taking place, 7556 the list of space characters may be different; there may be fewer or 7557 more of them. "Space" and \s match the same set of characters. 7558 7559 The name "word" is a Perl extension, and "blank" is a GNU extension 7560 from Perl 5.8. Another Perl extension is negation, which is indicated 7561 by a ^ character after the colon. For example, 7562 7563 [12[:^digit:]] 7564 7565 matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the 7566 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but 7567 these are not supported, and an error is given if they are encountered. 7568 7569 By default, characters with values greater than 127 do not match any of 7570 the POSIX character classes, although this may be different for charac- 7571 ters in the range 128-255 when locale-specific matching is happening. 7572 However, if the PCRE2_UCP option is passed to pcre2_compile(), some of 7573 the classes are changed so that Unicode character properties are used. 7574 This is achieved by replacing certain POSIX classes with other se- 7575 quences, as follows: 7576 7577 [:alnum:] becomes \p{Xan} 7578 [:alpha:] becomes \p{L} 7579 [:blank:] becomes \h 7580 [:cntrl:] becomes \p{Cc} 7581 [:digit:] becomes \p{Nd} 7582 [:lower:] becomes \p{Ll} 7583 [:space:] becomes \p{Xps} 7584 [:upper:] becomes \p{Lu} 7585 [:word:] becomes \p{Xwd} 7586 7587 Negated versions, such as [:^alpha:] use \P instead of \p. Three other 7588 POSIX classes are handled specially in UCP mode: 7589 7590 [:graph:] This matches characters that have glyphs that mark the page 7591 when printed. In Unicode property terms, it matches all char- 7592 acters with the L, M, N, P, S, or Cf properties, except for: 7593 7594 U+061C Arabic Letter Mark 7595 U+180E Mongolian Vowel Separator 7596 U+2066 - U+2069 Various "isolate"s 7597 7598 7599 [:print:] This matches the same characters as [:graph:] plus space 7600 characters that are not controls, that is, characters with 7601 the Zs property. 7602 7603 [:punct:] This matches all characters that have the Unicode P (punctua- 7604 tion) property, plus those characters with code points less 7605 than 256 that have the S (Symbol) property. 7606 7607 The other POSIX classes are unchanged, and match only characters with 7608 code points less than 256. 7609 7610 7611COMPATIBILITY FEATURE FOR WORD BOUNDARIES 7612 7613 In the POSIX.2 compliant library that was included in 4.4BSD Unix, the 7614 ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word" 7615 and "end of word". PCRE2 treats these items as follows: 7616 7617 [[:<:]] is converted to \b(?=\w) 7618 [[:>:]] is converted to \b(?<=\w) 7619 7620 Only these exact character sequences are recognized. A sequence such as 7621 [a[:<:]b] provokes error for an unrecognized POSIX class name. This 7622 support is not compatible with Perl. It is provided to help migrations 7623 from other environments, and is best not used in any new patterns. Note 7624 that \b matches at the start and the end of a word (see "Simple asser- 7625 tions" above), and in a Perl-style pattern the preceding or following 7626 character normally shows which is wanted, without the need for the as- 7627 sertions that are used above in order to give exactly the POSIX behav- 7628 iour. 7629 7630 7631VERTICAL BAR 7632 7633 Vertical bar characters are used to separate alternative patterns. For 7634 example, the pattern 7635 7636 gilbert|sullivan 7637 7638 matches either "gilbert" or "sullivan". Any number of alternatives may 7639 appear, and an empty alternative is permitted (matching the empty 7640 string). The matching process tries each alternative in turn, from left 7641 to right, and the first one that succeeds is used. If the alternatives 7642 are within a group (defined below), "succeeds" means matching the rest 7643 of the main pattern as well as the alternative in the group. 7644 7645 7646INTERNAL OPTION SETTING 7647 7648 The settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL, 7649 PCRE2_EXTENDED, PCRE2_EXTENDED_MORE, and PCRE2_NO_AUTO_CAPTURE options 7650 can be changed from within the pattern by a sequence of letters en- 7651 closed between "(?" and ")". These options are Perl-compatible, and 7652 are described in detail in the pcre2api documentation. The option let- 7653 ters are: 7654 7655 i for PCRE2_CASELESS 7656 m for PCRE2_MULTILINE 7657 n for PCRE2_NO_AUTO_CAPTURE 7658 s for PCRE2_DOTALL 7659 x for PCRE2_EXTENDED 7660 xx for PCRE2_EXTENDED_MORE 7661 7662 For example, (?im) sets caseless, multiline matching. It is also possi- 7663 ble to unset these options by preceding the relevant letters with a hy- 7664 phen, for example (?-im). The two "extended" options are not indepen- 7665 dent; unsetting either one cancels the effects of both of them. 7666 7667 A combined setting and unsetting such as (?im-sx), which sets 7668 PCRE2_CASELESS and PCRE2_MULTILINE while unsetting PCRE2_DOTALL and 7669 PCRE2_EXTENDED, is also permitted. Only one hyphen may appear in the 7670 options string. If a letter appears both before and after the hyphen, 7671 the option is unset. An empty options setting "(?)" is allowed. Need- 7672 less to say, it has no effect. 7673 7674 If the first character following (? is a circumflex, it causes all of 7675 the above options to be unset. Thus, (?^) is equivalent to (?-imnsx). 7676 Letters may follow the circumflex to cause some options to be re-in- 7677 stated, but a hyphen may not appear. 7678 7679 The PCRE2-specific options PCRE2_DUPNAMES and PCRE2_UNGREEDY can be 7680 changed in the same way as the Perl-compatible options by using the 7681 characters J and U respectively. However, these are not unset by (?^). 7682 7683 When one of these option changes occurs at top level (that is, not in- 7684 side group parentheses), the change applies to the remainder of the 7685 pattern that follows. An option change within a group (see below for a 7686 description of groups) affects only that part of the group that follows 7687 it, so 7688 7689 (a(?i)b)c 7690 7691 matches abc and aBc and no other strings (assuming PCRE2_CASELESS is 7692 not used). By this means, options can be made to have different set- 7693 tings in different parts of the pattern. Any changes made in one alter- 7694 native do carry on into subsequent branches within the same group. For 7695 example, 7696 7697 (a(?i)b|c) 7698 7699 matches "ab", "aB", "c", and "C", even though when matching "C" the 7700 first branch is abandoned before the option setting. This is because 7701 the effects of option settings happen at compile time. There would be 7702 some very weird behaviour otherwise. 7703 7704 As a convenient shorthand, if any option settings are required at the 7705 start of a non-capturing group (see the next section), the option let- 7706 ters may appear between the "?" and the ":". Thus the two patterns 7707 7708 (?i:saturday|sunday) 7709 (?:(?i)saturday|sunday) 7710 7711 match exactly the same set of strings. 7712 7713 Note: There are other PCRE2-specific options, applying to the whole 7714 pattern, which can be set by the application when the compiling func- 7715 tion is called. In addition, the pattern can contain special leading 7716 sequences such as (*CRLF) to override what the application has set or 7717 what has been defaulted. Details are given in the section entitled 7718 "Newline sequences" above. There are also the (*UTF) and (*UCP) leading 7719 sequences that can be used to set UTF and Unicode property modes; they 7720 are equivalent to setting the PCRE2_UTF and PCRE2_UCP options, respec- 7721 tively. However, the application can set the PCRE2_NEVER_UTF and 7722 PCRE2_NEVER_UCP options, which lock out the use of the (*UTF) and 7723 (*UCP) sequences. 7724 7725 7726GROUPS 7727 7728 Groups are delimited by parentheses (round brackets), which can be 7729 nested. Turning part of a pattern into a group does two things: 7730 7731 1. It localizes a set of alternatives. For example, the pattern 7732 7733 cat(aract|erpillar|) 7734 7735 matches "cataract", "caterpillar", or "cat". Without the parentheses, 7736 it would match "cataract", "erpillar" or an empty string. 7737 7738 2. It creates a "capture group". This means that, when the whole pat- 7739 tern matches, the portion of the subject string that matched the group 7740 is passed back to the caller, separately from the portion that matched 7741 the whole pattern. (This applies only to the traditional matching 7742 function; the DFA matching function does not support capturing.) 7743 7744 Opening parentheses are counted from left to right (starting from 1) to 7745 obtain numbers for capture groups. For example, if the string "the red 7746 king" is matched against the pattern 7747 7748 the ((red|white) (king|queen)) 7749 7750 the captured substrings are "red king", "red", and "king", and are num- 7751 bered 1, 2, and 3, respectively. 7752 7753 The fact that plain parentheses fulfil two functions is not always 7754 helpful. There are often times when grouping is required without cap- 7755 turing. If an opening parenthesis is followed by a question mark and a 7756 colon, the group does not do any capturing, and is not counted when 7757 computing the number of any subsequent capture groups. For example, if 7758 the string "the white queen" is matched against the pattern 7759 7760 the ((?:red|white) (king|queen)) 7761 7762 the captured substrings are "white queen" and "queen", and are numbered 7763 1 and 2. The maximum number of capture groups is 65535. 7764 7765 As a convenient shorthand, if any option settings are required at the 7766 start of a non-capturing group, the option letters may appear between 7767 the "?" and the ":". Thus the two patterns 7768 7769 (?i:saturday|sunday) 7770 (?:(?i)saturday|sunday) 7771 7772 match exactly the same set of strings. Because alternative branches are 7773 tried from left to right, and options are not reset until the end of 7774 the group is reached, an option setting in one branch does affect sub- 7775 sequent branches, so the above patterns match "SUNDAY" as well as "Sat- 7776 urday". 7777 7778 7779DUPLICATE GROUP NUMBERS 7780 7781 Perl 5.10 introduced a feature whereby each alternative in a group uses 7782 the same numbers for its capturing parentheses. Such a group starts 7783 with (?| and is itself a non-capturing group. For example, consider 7784 this pattern: 7785 7786 (?|(Sat)ur|(Sun))day 7787 7788 Because the two alternatives are inside a (?| group, both sets of cap- 7789 turing parentheses are numbered one. Thus, when the pattern matches, 7790 you can look at captured substring number one, whichever alternative 7791 matched. This construct is useful when you want to capture part, but 7792 not all, of one of a number of alternatives. Inside a (?| group, paren- 7793 theses are numbered as usual, but the number is reset at the start of 7794 each branch. The numbers of any capturing parentheses that follow the 7795 whole group start after the highest number used in any branch. The fol- 7796 lowing example is taken from the Perl documentation. The numbers under- 7797 neath show in which buffer the captured content will be stored. 7798 7799 # before ---------------branch-reset----------- after 7800 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x 7801 # 1 2 2 3 2 3 4 7802 7803 A backreference to a capture group uses the most recent value that is 7804 set for the group. The following pattern matches "abcabc" or "defdef": 7805 7806 /(?|(abc)|(def))\1/ 7807 7808 In contrast, a subroutine call to a capture group always refers to the 7809 first one in the pattern with the given number. The following pattern 7810 matches "abcabc" or "defabc": 7811 7812 /(?|(abc)|(def))(?1)/ 7813 7814 A relative reference such as (?-1) is no different: it is just a conve- 7815 nient way of computing an absolute group number. 7816 7817 If a condition test for a group's having matched refers to a non-unique 7818 number, the test is true if any group with that number has matched. 7819 7820 An alternative approach to using this "branch reset" feature is to use 7821 duplicate named groups, as described in the next section. 7822 7823 7824NAMED CAPTURE GROUPS 7825 7826 Identifying capture groups by number is simple, but it can be very hard 7827 to keep track of the numbers in complicated patterns. Furthermore, if 7828 an expression is modified, the numbers may change. To help with this 7829 difficulty, PCRE2 supports the naming of capture groups. This feature 7830 was not added to Perl until release 5.10. Python had the feature ear- 7831 lier, and PCRE1 introduced it at release 4.0, using the Python syntax. 7832 PCRE2 supports both the Perl and the Python syntax. 7833 7834 In PCRE2, a capture group can be named in one of three ways: 7835 (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python. 7836 Names may be up to 32 code units long. When PCRE2_UTF is not set, they 7837 may contain only ASCII alphanumeric characters and underscores, but 7838 must start with a non-digit. When PCRE2_UTF is set, the syntax of group 7839 names is extended to allow any Unicode letter or Unicode decimal digit. 7840 In other words, group names must match one of these patterns: 7841 7842 ^[_A-Za-z][_A-Za-z0-9]*\z when PCRE2_UTF is not set 7843 ^[_\p{L}][_\p{L}\p{Nd}]*\z when PCRE2_UTF is set 7844 7845 References to capture groups from other parts of the pattern, such as 7846 backreferences, recursion, and conditions, can all be made by name as 7847 well as by number. 7848 7849 Named capture groups are allocated numbers as well as names, exactly as 7850 if the names were not present. In both PCRE2 and Perl, capture groups 7851 are primarily identified by numbers; any names are just aliases for 7852 these numbers. The PCRE2 API provides function calls for extracting the 7853 complete name-to-number translation table from a compiled pattern, as 7854 well as convenience functions for extracting captured substrings by 7855 name. 7856 7857 Warning: When more than one capture group has the same number, as de- 7858 scribed in the previous section, a name given to one of them applies to 7859 all of them. Perl allows identically numbered groups to have different 7860 names. Consider this pattern, where there are two capture groups, both 7861 numbered 1: 7862 7863 (?|(?<AA>aa)|(?<BB>bb)) 7864 7865 Perl allows this, with both names AA and BB as aliases of group 1. 7866 Thus, after a successful match, both names yield the same value (either 7867 "aa" or "bb"). 7868 7869 In an attempt to reduce confusion, PCRE2 does not allow the same group 7870 number to be associated with more than one name. The example above pro- 7871 vokes a compile-time error. However, there is still scope for confu- 7872 sion. Consider this pattern: 7873 7874 (?|(?<AA>aa)|(bb)) 7875 7876 Although the second group number 1 is not explicitly named, the name AA 7877 is still an alias for any group 1. Whether the pattern matches "aa" or 7878 "bb", a reference by name to group AA yields the matched string. 7879 7880 By default, a name must be unique within a pattern, except that dupli- 7881 cate names are permitted for groups with the same number, for example: 7882 7883 (?|(?<AA>aa)|(?<AA>bb)) 7884 7885 The duplicate name constraint can be disabled by setting the PCRE2_DUP- 7886 NAMES option at compile time, or by the use of (?J) within the pattern, 7887 as described in the section entitled "Internal Option Setting" above. 7888 7889 Duplicate names can be useful for patterns where only one instance of 7890 the named capture group can match. Suppose you want to match the name 7891 of a weekday, either as a 3-letter abbreviation or as the full name, 7892 and in both cases you want to extract the abbreviation. This pattern 7893 (ignoring the line breaks) does the job: 7894 7895 (?J) 7896 (?<DN>Mon|Fri|Sun)(?:day)?| 7897 (?<DN>Tue)(?:sday)?| 7898 (?<DN>Wed)(?:nesday)?| 7899 (?<DN>Thu)(?:rsday)?| 7900 (?<DN>Sat)(?:urday)? 7901 7902 There are five capture groups, but only one is ever set after a match. 7903 The convenience functions for extracting the data by name returns the 7904 substring for the first (and in this example, the only) group of that 7905 name that matched. This saves searching to find which numbered group it 7906 was. (An alternative way of solving this problem is to use a "branch 7907 reset" group, as described in the previous section.) 7908 7909 If you make a backreference to a non-unique named group from elsewhere 7910 in the pattern, the groups to which the name refers are checked in the 7911 order in which they appear in the overall pattern. The first one that 7912 is set is used for the reference. For example, this pattern matches 7913 both "foofoo" and "barbar" but not "foobar" or "barfoo": 7914 7915 (?J)(?:(?<n>foo)|(?<n>bar))\k<n> 7916 7917 7918 If you make a subroutine call to a non-unique named group, the one that 7919 corresponds to the first occurrence of the name is used. In the absence 7920 of duplicate numbers this is the one with the lowest number. 7921 7922 If you use a named reference in a condition test (see the section about 7923 conditions below), either to check whether a capture group has matched, 7924 or to check for recursion, all groups with the same name are tested. If 7925 the condition is true for any one of them, the overall condition is 7926 true. This is the same behaviour as testing by number. For further de- 7927 tails of the interfaces for handling named capture groups, see the 7928 pcre2api documentation. 7929 7930 7931REPETITION 7932 7933 Repetition is specified by quantifiers, which can follow any of the 7934 following items: 7935 7936 a literal data character 7937 the dot metacharacter 7938 the \C escape sequence 7939 the \R escape sequence 7940 the \X escape sequence 7941 an escape such as \d or \pL that matches a single character 7942 a character class 7943 a backreference 7944 a parenthesized group (including lookaround assertions) 7945 a subroutine call (recursive or otherwise) 7946 7947 The general repetition quantifier specifies a minimum and maximum num- 7948 ber of permitted matches, by giving the two numbers in curly brackets 7949 (braces), separated by a comma. The numbers must be less than 65536, 7950 and the first must be less than or equal to the second. For example, 7951 7952 z{2,4} 7953 7954 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a 7955 special character. If the second number is omitted, but the comma is 7956 present, there is no upper limit; if the second number and the comma 7957 are both omitted, the quantifier specifies an exact number of required 7958 matches. Thus 7959 7960 [aeiou]{3,} 7961 7962 matches at least 3 successive vowels, but may match many more, whereas 7963 7964 \d{8} 7965 7966 matches exactly 8 digits. An opening curly bracket that appears in a 7967 position where a quantifier is not allowed, or one that does not match 7968 the syntax of a quantifier, is taken as a literal character. For exam- 7969 ple, {,6} is not a quantifier, but a literal string of four characters. 7970 7971 In UTF modes, quantifiers apply to characters rather than to individual 7972 code units. Thus, for example, \x{100}{2} matches two characters, each 7973 of which is represented by a two-byte sequence in a UTF-8 string. Simi- 7974 larly, \X{3} matches three Unicode extended grapheme clusters, each of 7975 which may be several code units long (and they may be of different 7976 lengths). 7977 7978 The quantifier {0} is permitted, causing the expression to behave as if 7979 the previous item and the quantifier were not present. This may be use- 7980 ful for capture groups that are referenced as subroutines from else- 7981 where in the pattern (but see also the section entitled "Defining cap- 7982 ture groups for use by reference only" below). Except for parenthesized 7983 groups, items that have a {0} quantifier are omitted from the compiled 7984 pattern. 7985 7986 For convenience, the three most common quantifiers have single-charac- 7987 ter abbreviations: 7988 7989 * is equivalent to {0,} 7990 + is equivalent to {1,} 7991 ? is equivalent to {0,1} 7992 7993 It is possible to construct infinite loops by following a group that 7994 can match no characters with a quantifier that has no upper limit, for 7995 example: 7996 7997 (a?)* 7998 7999 Earlier versions of Perl and PCRE1 used to give an error at compile 8000 time for such patterns. However, because there are cases where this can 8001 be useful, such patterns are now accepted, but whenever an iteration of 8002 such a group matches no characters, matching moves on to the next item 8003 in the pattern instead of repeatedly matching an empty string. This 8004 does not prevent backtracking into any of the iterations if a subse- 8005 quent item fails to match. 8006 8007 By default, quantifiers are "greedy", that is, they match as much as 8008 possible (up to the maximum number of permitted times), without causing 8009 the rest of the pattern to fail. The classic example of where this 8010 gives problems is in trying to match comments in C programs. These ap- 8011 pear between /* and */ and within the comment, individual * and / char- 8012 acters may appear. An attempt to match C comments by applying the pat- 8013 tern 8014 8015 /\*.*\*/ 8016 8017 to the string 8018 8019 /* first comment */ not comment /* second comment */ 8020 8021 fails, because it matches the entire string owing to the greediness of 8022 the .* item. However, if a quantifier is followed by a question mark, 8023 it ceases to be greedy, and instead matches the minimum number of times 8024 possible, so the pattern 8025 8026 /\*.*?\*/ 8027 8028 does the right thing with the C comments. The meaning of the various 8029 quantifiers is not otherwise changed, just the preferred number of 8030 matches. Do not confuse this use of question mark with its use as a 8031 quantifier in its own right. Because it has two uses, it can sometimes 8032 appear doubled, as in 8033 8034 \d??\d 8035 8036 which matches one digit by preference, but can match two if that is the 8037 only way the rest of the pattern matches. 8038 8039 If the PCRE2_UNGREEDY option is set (an option that is not available in 8040 Perl), the quantifiers are not greedy by default, but individual ones 8041 can be made greedy by following them with a question mark. In other 8042 words, it inverts the default behaviour. 8043 8044 When a parenthesized group is quantified with a minimum repeat count 8045 that is greater than 1 or with a limited maximum, more memory is re- 8046 quired for the compiled pattern, in proportion to the size of the mini- 8047 mum or maximum. 8048 8049 If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option 8050 (equivalent to Perl's /s) is set, thus allowing the dot to match new- 8051 lines, the pattern is implicitly anchored, because whatever follows 8052 will be tried against every character position in the subject string, 8053 so there is no point in retrying the overall match at any position af- 8054 ter the first. PCRE2 normally treats such a pattern as though it were 8055 preceded by \A. 8056 8057 In cases where it is known that the subject string contains no new- 8058 lines, it is worth setting PCRE2_DOTALL in order to obtain this opti- 8059 mization, or alternatively, using ^ to indicate anchoring explicitly. 8060 8061 However, there are some cases where the optimization cannot be used. 8062 When .* is inside capturing parentheses that are the subject of a 8063 backreference elsewhere in the pattern, a match at the start may fail 8064 where a later one succeeds. Consider, for example: 8065 8066 (.*)abc\1 8067 8068 If the subject is "xyz123abc123" the match point is the fourth charac- 8069 ter. For this reason, such a pattern is not implicitly anchored. 8070 8071 Another case where implicit anchoring is not applied is when the lead- 8072 ing .* is inside an atomic group. Once again, a match at the start may 8073 fail where a later one succeeds. Consider this pattern: 8074 8075 (?>.*?a)b 8076 8077 It matches "ab" in the subject "aab". The use of the backtracking con- 8078 trol verbs (*PRUNE) and (*SKIP) also disable this optimization, and 8079 there is an option, PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly. 8080 8081 When a capture group is repeated, the value captured is the substring 8082 that matched the final iteration. For example, after 8083 8084 (tweedle[dume]{3}\s*)+ 8085 8086 has matched "tweedledum tweedledee" the value of the captured substring 8087 is "tweedledee". However, if there are nested capture groups, the cor- 8088 responding captured values may have been set in previous iterations. 8089 For example, after 8090 8091 (a|(b))+ 8092 8093 matches "aba" the value of the second captured substring is "b". 8094 8095 8096ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS 8097 8098 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 8099 repetition, failure of what follows normally causes the repeated item 8100 to be re-evaluated to see if a different number of repeats allows the 8101 rest of the pattern to match. Sometimes it is useful to prevent this, 8102 either to change the nature of the match, or to cause it fail earlier 8103 than it otherwise might, when the author of the pattern knows there is 8104 no point in carrying on. 8105 8106 Consider, for example, the pattern \d+foo when applied to the subject 8107 line 8108 8109 123456bar 8110 8111 After matching all 6 digits and then failing to match "foo", the normal 8112 action of the matcher is to try again with only 5 digits matching the 8113 \d+ item, and then with 4, and so on, before ultimately failing. 8114 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides 8115 the means for specifying that once a group has matched, it is not to be 8116 re-evaluated in this way. 8117 8118 If we use atomic grouping for the previous example, the matcher gives 8119 up immediately on failing to match "foo" the first time. The notation 8120 is a kind of special parenthesis, starting with (?> as in this example: 8121 8122 (?>\d+)foo 8123 8124 Perl 5.28 introduced an experimental alphabetic form starting with (* 8125 which may be easier to remember: 8126 8127 (*atomic:\d+)foo 8128 8129 This kind of parenthesized group "locks up" the part of the pattern it 8130 contains once it has matched, and a failure further into the pattern is 8131 prevented from backtracking into it. Backtracking past it to previous 8132 items, however, works as normal. 8133 8134 An alternative description is that a group of this type matches exactly 8135 the string of characters that an identical standalone pattern would 8136 match, if anchored at the current point in the subject string. 8137 8138 Atomic groups are not capture groups. Simple cases such as the above 8139 example can be thought of as a maximizing repeat that must swallow ev- 8140 erything it can. So, while both \d+ and \d+? are prepared to adjust 8141 the number of digits they match in order to make the rest of the pat- 8142 tern match, (?>\d+) can only match an entire sequence of digits. 8143 8144 Atomic groups in general can of course contain arbitrarily complicated 8145 expressions, and can be nested. However, when the contents of an atomic 8146 group is just a single repeated item, as in the example above, a sim- 8147 pler notation, called a "possessive quantifier" can be used. This con- 8148 sists of an additional + character following a quantifier. Using this 8149 notation, the previous example can be rewritten as 8150 8151 \d++foo 8152 8153 Note that a possessive quantifier can be used with an entire group, for 8154 example: 8155 8156 (abc|xyz){2,3}+ 8157 8158 Possessive quantifiers are always greedy; the setting of the PCRE2_UN- 8159 GREEDY option is ignored. They are a convenient notation for the sim- 8160 pler forms of atomic group. However, there is no difference in the 8161 meaning of a possessive quantifier and the equivalent atomic group, 8162 though there may be a performance difference; possessive quantifiers 8163 should be slightly faster. 8164 8165 The possessive quantifier syntax is an extension to the Perl 5.8 syn- 8166 tax. Jeffrey Friedl originated the idea (and the name) in the first 8167 edition of his book. Mike McCloskey liked it, so implemented it when he 8168 built Sun's Java package, and PCRE1 copied it from there. It found its 8169 way into Perl at release 5.10. 8170 8171 PCRE2 has an optimization that automatically "possessifies" certain 8172 simple pattern constructs. For example, the sequence A+B is treated as 8173 A++B because there is no point in backtracking into a sequence of A's 8174 when B must follow. This feature can be disabled by the PCRE2_NO_AUTO- 8175 POSSESS option, or starting the pattern with (*NO_AUTO_POSSESS). 8176 8177 When a pattern contains an unlimited repeat inside a group that can it- 8178 self be repeated an unlimited number of times, the use of an atomic 8179 group is the only way to avoid some failing matches taking a very long 8180 time indeed. The pattern 8181 8182 (\D+|<\d+>)*[!?] 8183 8184 matches an unlimited number of substrings that either consist of non- 8185 digits, or digits enclosed in <>, followed by either ! or ?. When it 8186 matches, it runs quickly. However, if it is applied to 8187 8188 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 8189 8190 it takes a long time before reporting failure. This is because the 8191 string can be divided between the internal \D+ repeat and the external 8192 * repeat in a large number of ways, and all have to be tried. (The ex- 8193 ample uses [!?] rather than a single character at the end, because both 8194 PCRE2 and Perl have an optimization that allows for fast failure when a 8195 single character is used. They remember the last single character that 8196 is required for a match, and fail early if it is not present in the 8197 string.) If the pattern is changed so that it uses an atomic group, 8198 like this: 8199 8200 ((?>\D+)|<\d+>)*[!?] 8201 8202 sequences of non-digits cannot be broken, and failure happens quickly. 8203 8204 8205BACKREFERENCES 8206 8207 Outside a character class, a backslash followed by a digit greater than 8208 0 (and possibly further digits) is a backreference to a capture group 8209 earlier (that is, to its left) in the pattern, provided there have been 8210 that many previous capture groups. 8211 8212 However, if the decimal number following the backslash is less than 8, 8213 it is always taken as a backreference, and causes an error only if 8214 there are not that many capture groups in the entire pattern. In other 8215 words, the group that is referenced need not be to the left of the ref- 8216 erence for numbers less than 8. A "forward backreference" of this type 8217 can make sense when a repetition is involved and the group to the right 8218 has participated in an earlier iteration. 8219 8220 It is not possible to have a numerical "forward backreference" to a 8221 group whose number is 8 or more using this syntax because a sequence 8222 such as \50 is interpreted as a character defined in octal. See the 8223 subsection entitled "Non-printing characters" above for further details 8224 of the handling of digits following a backslash. Other forms of back- 8225 referencing do not suffer from this restriction. In particular, there 8226 is no problem when named capture groups are used (see below). 8227 8228 Another way of avoiding the ambiguity inherent in the use of digits 8229 following a backslash is to use the \g escape sequence. This escape 8230 must be followed by a signed or unsigned number, optionally enclosed in 8231 braces. These examples are all identical: 8232 8233 (ring), \1 8234 (ring), \g1 8235 (ring), \g{1} 8236 8237 An unsigned number specifies an absolute reference without the ambigu- 8238 ity that is present in the older syntax. It is also useful when literal 8239 digits follow the reference. A signed number is a relative reference. 8240 Consider this example: 8241 8242 (abc(def)ghi)\g{-1} 8243 8244 The sequence \g{-1} is a reference to the most recently started capture 8245 group before \g, that is, is it equivalent to \2 in this example. Simi- 8246 larly, \g{-2} would be equivalent to \1. The use of relative references 8247 can be helpful in long patterns, and also in patterns that are created 8248 by joining together fragments that contain references within them- 8249 selves. 8250 8251 The sequence \g{+1} is a reference to the next capture group. This kind 8252 of forward reference can be useful in patterns that repeat. Perl does 8253 not support the use of + in this way. 8254 8255 A backreference matches whatever actually most recently matched the 8256 capture group in the current subject string, rather than anything at 8257 all that matches the group (see "Groups as subroutines" below for a way 8258 of doing that). So the pattern 8259 8260 (sens|respons)e and \1ibility 8261 8262 matches "sense and sensibility" and "response and responsibility", but 8263 not "sense and responsibility". If caseful matching is in force at the 8264 time of the backreference, the case of letters is relevant. For exam- 8265 ple, 8266 8267 ((?i)rah)\s+\1 8268 8269 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the 8270 original capture group is matched caselessly. 8271 8272 There are several different ways of writing backreferences to named 8273 capture groups. The .NET syntax \k{name} and the Perl syntax \k<name> 8274 or \k'name' are supported, as is the Python syntax (?P=name). Perl 8275 5.10's unified backreference syntax, in which \g can be used for both 8276 numeric and named references, is also supported. We could rewrite the 8277 above example in any of the following ways: 8278 8279 (?<p1>(?i)rah)\s+\k<p1> 8280 (?'p1'(?i)rah)\s+\k{p1} 8281 (?P<p1>(?i)rah)\s+(?P=p1) 8282 (?<p1>(?i)rah)\s+\g{p1} 8283 8284 A capture group that is referenced by name may appear in the pattern 8285 before or after the reference. 8286 8287 There may be more than one backreference to the same group. If a group 8288 has not actually been used in a particular match, backreferences to it 8289 always fail by default. For example, the pattern 8290 8291 (a|(bc))\2 8292 8293 always fails if it starts to match "a" rather than "bc". However, if 8294 the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backref- 8295 erence to an unset value matches an empty string. 8296 8297 Because there may be many capture groups in a pattern, all digits fol- 8298 lowing a backslash are taken as part of a potential backreference num- 8299 ber. If the pattern continues with a digit character, some delimiter 8300 must be used to terminate the backreference. If the PCRE2_EXTENDED or 8301 PCRE2_EXTENDED_MORE option is set, this can be white space. Otherwise, 8302 the \g{} syntax or an empty comment (see "Comments" below) can be used. 8303 8304 Recursive backreferences 8305 8306 A backreference that occurs inside the group to which it refers fails 8307 when the group is first used, so, for example, (a\1) never matches. 8308 However, such references can be useful inside repeated groups. For ex- 8309 ample, the pattern 8310 8311 (a|b\1)+ 8312 8313 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- 8314 ation of the group, the backreference matches the character string cor- 8315 responding to the previous iteration. In order for this to work, the 8316 pattern must be such that the first iteration does not need to match 8317 the backreference. This can be done using alternation, as in the exam- 8318 ple above, or by a quantifier with a minimum of zero. 8319 8320 For versions of PCRE2 less than 10.25, backreferences of this type used 8321 to cause the group that they reference to be treated as an atomic 8322 group. This restriction no longer applies, and backtracking into such 8323 groups can occur as normal. 8324 8325 8326ASSERTIONS 8327 8328 An assertion is a test on the characters following or preceding the 8329 current matching point that does not consume any characters. The simple 8330 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described 8331 above. 8332 8333 More complicated assertions are coded as parenthesized groups. There 8334 are two kinds: those that look ahead of the current position in the 8335 subject string, and those that look behind it, and in each case an as- 8336 sertion may be positive (must match for the assertion to be true) or 8337 negative (must not match for the assertion to be true). An assertion 8338 group is matched in the normal way, and if it is true, matching contin- 8339 ues after it, but with the matching position in the subject string re- 8340 set to what it was before the assertion was processed. 8341 8342 The Perl-compatible lookaround assertions are atomic. If an assertion 8343 is true, but there is a subsequent matching failure, there is no back- 8344 tracking into the assertion. However, there are some cases where non- 8345 atomic assertions can be useful. PCRE2 has some support for these, de- 8346 scribed in the section entitled "Non-atomic assertions" below, but they 8347 are not Perl-compatible. 8348 8349 A lookaround assertion may appear as the condition in a conditional 8350 group (see below). In this case, the result of matching the assertion 8351 determines which branch of the condition is followed. 8352 8353 Assertion groups are not capture groups. If an assertion contains cap- 8354 ture groups within it, these are counted for the purposes of numbering 8355 the capture groups in the whole pattern. Within each branch of an as- 8356 sertion, locally captured substrings may be referenced in the usual 8357 way. For example, a sequence such as (.)\g{-1} can be used to check 8358 that two adjacent characters are the same. 8359 8360 When a branch within an assertion fails to match, any substrings that 8361 were captured are discarded (as happens with any pattern branch that 8362 fails to match). A negative assertion is true only when all its 8363 branches fail to match; this means that no captured substrings are ever 8364 retained after a successful negative assertion. When an assertion con- 8365 tains a matching branch, what happens depends on the type of assertion. 8366 8367 For a positive assertion, internally captured substrings in the suc- 8368 cessful branch are retained, and matching continues with the next pat- 8369 tern item after the assertion. For a negative assertion, a matching 8370 branch means that the assertion is not true. If such an assertion is 8371 being used as a condition in a conditional group (see below), captured 8372 substrings are retained, because matching continues with the "no" 8373 branch of the condition. For other failing negative assertions, control 8374 passes to the previous backtracking point, thus discarding any captured 8375 strings within the assertion. 8376 8377 Most assertion groups may be repeated; though it makes no sense to as- 8378 sert the same thing several times, the side effect of capturing in pos- 8379 itive assertions may occasionally be useful. However, an assertion that 8380 forms the condition for a conditional group may not be quantified. 8381 PCRE2 used to restrict the repetition of assertions, but from release 8382 10.35 the only restriction is that an unlimited maximum repetition is 8383 changed to be one more than the minimum. For example, {3,} is treated 8384 as {3,4}. 8385 8386 Alphabetic assertion names 8387 8388 Traditionally, symbolic sequences such as (?= and (?<= have been used 8389 to specify lookaround assertions. Perl 5.28 introduced some experimen- 8390 tal alphabetic alternatives which might be easier to remember. They all 8391 start with (* instead of (? and must be written using lower case let- 8392 ters. PCRE2 supports the following synonyms: 8393 8394 (*positive_lookahead: or (*pla: is the same as (?= 8395 (*negative_lookahead: or (*nla: is the same as (?! 8396 (*positive_lookbehind: or (*plb: is the same as (?<= 8397 (*negative_lookbehind: or (*nlb: is the same as (?<! 8398 8399 For example, (*pla:foo) is the same assertion as (?=foo). In the fol- 8400 lowing sections, the various assertions are described using the origi- 8401 nal symbolic forms. 8402 8403 Lookahead assertions 8404 8405 Lookahead assertions start with (?= for positive assertions and (?! for 8406 negative assertions. For example, 8407 8408 \w+(?=;) 8409 8410 matches a word followed by a semicolon, but does not include the semi- 8411 colon in the match, and 8412 8413 foo(?!bar) 8414 8415 matches any occurrence of "foo" that is not followed by "bar". Note 8416 that the apparently similar pattern 8417 8418 (?!foo)bar 8419 8420 does not find an occurrence of "bar" that is preceded by something 8421 other than "foo"; it finds any occurrence of "bar" whatsoever, because 8422 the assertion (?!foo) is always true when the next three characters are 8423 "bar". A lookbehind assertion is needed to achieve the other effect. 8424 8425 If you want to force a matching failure at some point in a pattern, the 8426 most convenient way to do it is with (?!) because an empty string al- 8427 ways matches, so an assertion that requires there not to be an empty 8428 string must always fail. The backtracking control verb (*FAIL) or (*F) 8429 is a synonym for (?!). 8430 8431 Lookbehind assertions 8432 8433 Lookbehind assertions start with (?<= for positive assertions and (?<! 8434 for negative assertions. For example, 8435 8436 (?<!foo)bar 8437 8438 does find an occurrence of "bar" that is not preceded by "foo". The 8439 contents of a lookbehind assertion are restricted such that all the 8440 strings it matches must have a fixed length. However, if there are sev- 8441 eral top-level alternatives, they do not all have to have the same 8442 fixed length. Thus 8443 8444 (?<=bullock|donkey) 8445 8446 is permitted, but 8447 8448 (?<!dogs?|cats?) 8449 8450 causes an error at compile time. Branches that match different length 8451 strings are permitted only at the top level of a lookbehind assertion. 8452 This is an extension compared with Perl, which requires all branches to 8453 match the same length of string. An assertion such as 8454 8455 (?<=ab(c|de)) 8456 8457 is not permitted, because its single top-level branch can match two 8458 different lengths, but it is acceptable to PCRE2 if rewritten to use 8459 two top-level branches: 8460 8461 (?<=abc|abde) 8462 8463 In some cases, the escape sequence \K (see above) can be used instead 8464 of a lookbehind assertion to get round the fixed-length restriction. 8465 8466 The implementation of lookbehind assertions is, for each alternative, 8467 to temporarily move the current position back by the fixed length and 8468 then try to match. If there are insufficient characters before the cur- 8469 rent position, the assertion fails. 8470 8471 In UTF-8 and UTF-16 modes, PCRE2 does not allow the \C escape (which 8472 matches a single code unit even in a UTF mode) to appear in lookbehind 8473 assertions, because it makes it impossible to calculate the length of 8474 the lookbehind. The \X and \R escapes, which can match different num- 8475 bers of code units, are never permitted in lookbehinds. 8476 8477 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in 8478 lookbehinds, as long as the called capture group matches a fixed-length 8479 string. However, recursion, that is, a "subroutine" call into a group 8480 that is already active, is not supported. 8481 8482 Perl does not support backreferences in lookbehinds. PCRE2 does support 8483 them, but only if certain conditions are met. The PCRE2_MATCH_UN- 8484 SET_BACKREF option must not be set, there must be no use of (?| in the 8485 pattern (it creates duplicate group numbers), and if the backreference 8486 is by name, the name must be unique. Of course, the referenced group 8487 must itself match a fixed length substring. The following pattern 8488 matches words containing at least two characters that begin and end 8489 with the same character: 8490 8491 \b(\w)\w++(?<=\1) 8492 8493 Possessive quantifiers can be used in conjunction with lookbehind as- 8494 sertions to specify efficient matching of fixed-length strings at the 8495 end of subject strings. Consider a simple pattern such as 8496 8497 abcd$ 8498 8499 when applied to a long string that does not match. Because matching 8500 proceeds from left to right, PCRE2 will look for each "a" in the sub- 8501 ject and then see if what follows matches the rest of the pattern. If 8502 the pattern is specified as 8503 8504 ^.*abcd$ 8505 8506 the initial .* matches the entire string at first, but when this fails 8507 (because there is no following "a"), it backtracks to match all but the 8508 last character, then all but the last two characters, and so on. Once 8509 again the search for "a" covers the entire string, from right to left, 8510 so we are no better off. However, if the pattern is written as 8511 8512 ^.*+(?<=abcd) 8513 8514 there can be no backtracking for the .*+ item because of the possessive 8515 quantifier; it can match only the entire string. The subsequent lookbe- 8516 hind assertion does a single test on the last four characters. If it 8517 fails, the match fails immediately. For long strings, this approach 8518 makes a significant difference to the processing time. 8519 8520 Using multiple assertions 8521 8522 Several assertions (of any sort) may occur in succession. For example, 8523 8524 (?<=\d{3})(?<!999)foo 8525 8526 matches "foo" preceded by three digits that are not "999". Notice that 8527 each of the assertions is applied independently at the same point in 8528 the subject string. First there is a check that the previous three 8529 characters are all digits, and then there is a check that the same 8530 three characters are not "999". This pattern does not match "foo" pre- 8531 ceded by six characters, the first of which are digits and the last 8532 three of which are not "999". For example, it doesn't match "123abc- 8533 foo". A pattern to do that is 8534 8535 (?<=\d{3}...)(?<!999)foo 8536 8537 This time the first assertion looks at the preceding six characters, 8538 checking that the first three are digits, and then the second assertion 8539 checks that the preceding three characters are not "999". 8540 8541 Assertions can be nested in any combination. For example, 8542 8543 (?<=(?<!foo)bar)baz 8544 8545 matches an occurrence of "baz" that is preceded by "bar" which in turn 8546 is not preceded by "foo", while 8547 8548 (?<=\d{3}(?!999)...)foo 8549 8550 is another pattern that matches "foo" preceded by three digits and any 8551 three characters that are not "999". 8552 8553 8554NON-ATOMIC ASSERTIONS 8555 8556 The traditional Perl-compatible lookaround assertions are atomic. That 8557 is, if an assertion is true, but there is a subsequent matching fail- 8558 ure, there is no backtracking into the assertion. However, there are 8559 some cases where non-atomic positive assertions can be useful. PCRE2 8560 provides these using the following syntax: 8561 8562 (*non_atomic_positive_lookahead: or (*napla: or (?* 8563 (*non_atomic_positive_lookbehind: or (*naplb: or (?<* 8564 8565 Consider the problem of finding the right-most word in a string that 8566 also appears earlier in the string, that is, it must appear at least 8567 twice in total. This pattern returns the required result as captured 8568 substring 1: 8569 8570 ^(?x)(*napla: .* \b(\w++)) (?> .*? \b\1\b ){2} 8571 8572 For a subject such as "word1 word2 word3 word2 word3 word4" the result 8573 is "word3". How does it work? At the start, ^(?x) anchors the pattern 8574 and sets the "x" option, which causes white space (introduced for read- 8575 ability) to be ignored. Inside the assertion, the greedy .* at first 8576 consumes the entire string, but then has to backtrack until the rest of 8577 the assertion can match a word, which is captured by group 1. In other 8578 words, when the assertion first succeeds, it captures the right-most 8579 word in the string. 8580 8581 The current matching point is then reset to the start of the subject, 8582 and the rest of the pattern match checks for two occurrences of the 8583 captured word, using an ungreedy .*? to scan from the left. If this 8584 succeeds, we are done, but if the last word in the string does not oc- 8585 cur twice, this part of the pattern fails. If a traditional atomic 8586 lookhead (?= or (*pla: had been used, the assertion could not be re-en- 8587 tered, and the whole match would fail. The pattern would succeed only 8588 if the very last word in the subject was found twice. 8589 8590 Using a non-atomic lookahead, however, means that when the last word 8591 does not occur twice in the string, the lookahead can backtrack and 8592 find the second-last word, and so on, until either the match succeeds, 8593 or all words have been tested. 8594 8595 Two conditions must be met for a non-atomic assertion to be useful: the 8596 contents of one or more capturing groups must change after a backtrack 8597 into the assertion, and there must be a backreference to a changed 8598 group later in the pattern. If this is not the case, the rest of the 8599 pattern match fails exactly as before because nothing has changed, so 8600 using a non-atomic assertion just wastes resources. 8601 8602 There is one exception to backtracking into a non-atomic assertion. If 8603 an (*ACCEPT) control verb is triggered, the assertion succeeds atomi- 8604 cally. That is, a subsequent match failure cannot backtrack into the 8605 assertion. 8606 8607 Non-atomic assertions are not supported by the alternative matching 8608 function pcre2_dfa_match(). They are supported by JIT, but only if they 8609 do not contain any control verbs such as (*ACCEPT). (This may change in 8610 future). Note that assertions that appear as conditions for conditional 8611 groups (see below) must be atomic. 8612 8613 8614SCRIPT RUNS 8615 8616 In concept, a script run is a sequence of characters that are all from 8617 the same Unicode script such as Latin or Greek. However, because some 8618 scripts are commonly used together, and because some diacritical and 8619 other marks are used with multiple scripts, it is not that simple. 8620 There is a full description of the rules that PCRE2 uses in the section 8621 entitled "Script Runs" in the pcre2unicode documentation. 8622 8623 If part of a pattern is enclosed between (*script_run: or (*sr: and a 8624 closing parenthesis, it fails if the sequence of characters that it 8625 matches are not a script run. After a failure, normal backtracking oc- 8626 curs. Script runs can be used to detect spoofing attacks using charac- 8627 ters that look the same, but are from different scripts. The string 8628 "paypal.com" is an infamous example, where the letters could be a mix- 8629 ture of Latin and Cyrillic. This pattern ensures that the matched char- 8630 acters in a sequence of non-spaces that follow white space are a script 8631 run: 8632 8633 \s+(*sr:\S+) 8634 8635 To be sure that they are all from the Latin script (for example), a 8636 lookahead can be used: 8637 8638 \s+(?=\p{Latin})(*sr:\S+) 8639 8640 This works as long as the first character is expected to be a character 8641 in that script, and not (for example) punctuation, which is allowed 8642 with any script. If this is not the case, a more creative lookahead is 8643 needed. For example, if digits, underscore, and dots are permitted at 8644 the start: 8645 8646 \s+(?=[0-9_.]*\p{Latin})(*sr:\S+) 8647 8648 8649 In many cases, backtracking into a script run pattern fragment is not 8650 desirable. The script run can employ an atomic group to prevent this. 8651 Because this is a common requirement, a shorthand notation is provided 8652 by (*atomic_script_run: or (*asr: 8653 8654 (*asr:...) is the same as (*sr:(?>...)) 8655 8656 Note that the atomic group is inside the script run. Putting it outside 8657 would not prevent backtracking into the script run pattern. 8658 8659 Support for script runs is not available if PCRE2 is compiled without 8660 Unicode support. A compile-time error is given if any of the above con- 8661 structs is encountered. Script runs are not supported by the alternate 8662 matching function, pcre2_dfa_match() because they use the same mecha- 8663 nism as capturing parentheses. 8664 8665 Warning: The (*ACCEPT) control verb (see below) should not be used 8666 within a script run group, because it causes an immediate exit from the 8667 group, bypassing the script run checking. 8668 8669 8670CONDITIONAL GROUPS 8671 8672 It is possible to cause the matching process to obey a pattern fragment 8673 conditionally or to choose between two alternative fragments, depending 8674 on the result of an assertion, or whether a specific capture group has 8675 already been matched. The two possible forms of conditional group are: 8676 8677 (?(condition)yes-pattern) 8678 (?(condition)yes-pattern|no-pattern) 8679 8680 If the condition is satisfied, the yes-pattern is used; otherwise the 8681 no-pattern (if present) is used. An absent no-pattern is equivalent to 8682 an empty string (it always matches). If there are more than two alter- 8683 natives in the group, a compile-time error occurs. Each of the two al- 8684 ternatives may itself contain nested groups of any form, including con- 8685 ditional groups; the restriction to two alternatives applies only at 8686 the level of the condition itself. This pattern fragment is an example 8687 where the alternatives are complex: 8688 8689 (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) 8690 8691 8692 There are five kinds of condition: references to capture groups, refer- 8693 ences to recursion, two pseudo-conditions called DEFINE and VERSION, 8694 and assertions. 8695 8696 Checking for a used capture group by number 8697 8698 If the text between the parentheses consists of a sequence of digits, 8699 the condition is true if a capture group of that number has previously 8700 matched. If there is more than one capture group with the same number 8701 (see the earlier section about duplicate group numbers), the condition 8702 is true if any of them have matched. An alternative notation is to pre- 8703 cede the digits with a plus or minus sign. In this case, the group num- 8704 ber is relative rather than absolute. The most recently opened capture 8705 group can be referenced by (?(-1), the next most recent by (?(-2), and 8706 so on. Inside loops it can also make sense to refer to subsequent 8707 groups. The next capture group can be referenced as (?(+1), and so on. 8708 (The value zero in any of these forms is not used; it provokes a com- 8709 pile-time error.) 8710 8711 Consider the following pattern, which contains non-significant white 8712 space to make it more readable (assume the PCRE2_EXTENDED option) and 8713 to divide it into three parts for ease of discussion: 8714 8715 ( \( )? [^()]+ (?(1) \) ) 8716 8717 The first part matches an optional opening parenthesis, and if that 8718 character is present, sets it as the first captured substring. The sec- 8719 ond part matches one or more characters that are not parentheses. The 8720 third part is a conditional group that tests whether or not the first 8721 capture group matched. If it did, that is, if subject started with an 8722 opening parenthesis, the condition is true, and so the yes-pattern is 8723 executed and a closing parenthesis is required. Otherwise, since no- 8724 pattern is not present, the conditional group matches nothing. In other 8725 words, this pattern matches a sequence of non-parentheses, optionally 8726 enclosed in parentheses. 8727 8728 If you were embedding this pattern in a larger one, you could use a 8729 relative reference: 8730 8731 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... 8732 8733 This makes the fragment independent of the parentheses in the larger 8734 pattern. 8735 8736 Checking for a used capture group by name 8737 8738 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a 8739 used capture group by name. For compatibility with earlier versions of 8740 PCRE1, which had this facility before Perl, the syntax (?(name)...) is 8741 also recognized. Note, however, that undelimited names consisting of 8742 the letter R followed by digits are ambiguous (see the following sec- 8743 tion). Rewriting the above example to use a named group gives this: 8744 8745 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) 8746 8747 If the name used in a condition of this kind is a duplicate, the test 8748 is applied to all groups of the same name, and is true if any one of 8749 them has matched. 8750 8751 Checking for pattern recursion 8752 8753 "Recursion" in this sense refers to any subroutine-like call from one 8754 part of the pattern to another, whether or not it is actually recur- 8755 sive. See the sections entitled "Recursive patterns" and "Groups as 8756 subroutines" below for details of recursion and subroutine calls. 8757 8758 If a condition is the string (R), and there is no capture group with 8759 the name R, the condition is true if matching is currently in a recur- 8760 sion or subroutine call to the whole pattern or any capture group. If 8761 digits follow the letter R, and there is no group with that name, the 8762 condition is true if the most recent call is into a group with the 8763 given number, which must exist somewhere in the overall pattern. This 8764 is a contrived example that is equivalent to a+b: 8765 8766 ((?(R1)a+|(?1)b)) 8767 8768 However, in both cases, if there is a capture group with a matching 8769 name, the condition tests for its being set, as described in the sec- 8770 tion above, instead of testing for recursion. For example, creating a 8771 group with the name R1 by adding (?<R1>) to the above pattern com- 8772 pletely changes its meaning. 8773 8774 If a name preceded by ampersand follows the letter R, for example: 8775 8776 (?(R&name)...) 8777 8778 the condition is true if the most recent recursion is into a group of 8779 that name (which must exist within the pattern). 8780 8781 This condition does not check the entire recursion stack. It tests only 8782 the current level. If the name used in a condition of this kind is a 8783 duplicate, the test is applied to all groups of the same name, and is 8784 true if any one of them is the most recent recursion. 8785 8786 At "top level", all these recursion test conditions are false. 8787 8788 Defining capture groups for use by reference only 8789 8790 If the condition is the string (DEFINE), the condition is always false, 8791 even if there is a group with the name DEFINE. In this case, there may 8792 be only one alternative in the rest of the conditional group. It is al- 8793 ways skipped if control reaches this point in the pattern; the idea of 8794 DEFINE is that it can be used to define subroutines that can be refer- 8795 enced from elsewhere. (The use of subroutines is described below.) For 8796 example, a pattern to match an IPv4 address such as "192.168.23.245" 8797 could be written like this (ignore white space and line breaks): 8798 8799 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) 8800 \b (?&byte) (\.(?&byte)){3} \b 8801 8802 The first part of the pattern is a DEFINE group inside which another 8803 group named "byte" is defined. This matches an individual component of 8804 an IPv4 address (a number less than 256). When matching takes place, 8805 this part of the pattern is skipped because DEFINE acts like a false 8806 condition. The rest of the pattern uses references to the named group 8807 to match the four dot-separated components of an IPv4 address, insist- 8808 ing on a word boundary at each end. 8809 8810 Checking the PCRE2 version 8811 8812 Programs that link with a PCRE2 library can check the version by call- 8813 ing pcre2_config() with appropriate arguments. Users of applications 8814 that do not have access to the underlying code cannot do this. A spe- 8815 cial "condition" called VERSION exists to allow such users to discover 8816 which version of PCRE2 they are dealing with by using this condition to 8817 match a string such as "yesno". VERSION must be followed either by "=" 8818 or ">=" and a version number. For example: 8819 8820 (?(VERSION>=10.4)yes|no) 8821 8822 This pattern matches "yes" if the PCRE2 version is greater or equal to 8823 10.4, or "no" otherwise. The fractional part of the version number may 8824 not contain more than two digits. 8825 8826 Assertion conditions 8827 8828 If the condition is not in any of the above formats, it must be a 8829 parenthesized assertion. This may be a positive or negative lookahead 8830 or lookbehind assertion. However, it must be a traditional atomic as- 8831 sertion, not one of the PCRE2-specific non-atomic assertions. 8832 8833 Consider this pattern, again containing non-significant white space, 8834 and with the two alternatives on the second line: 8835 8836 (?(?=[^a-z]*[a-z]) 8837 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) 8838 8839 The condition is a positive lookahead assertion that matches an op- 8840 tional sequence of non-letters followed by a letter. In other words, it 8841 tests for the presence of at least one letter in the subject. If a let- 8842 ter is found, the subject is matched against the first alternative; 8843 otherwise it is matched against the second. This pattern matches 8844 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are 8845 letters and dd are digits. 8846 8847 When an assertion that is a condition contains capture groups, any cap- 8848 turing that occurs in a matching branch is retained afterwards, for 8849 both positive and negative assertions, because matching always contin- 8850 ues after the assertion, whether it succeeds or fails. (Compare non- 8851 conditional assertions, for which captures are retained only for posi- 8852 tive assertions that succeed.) 8853 8854 8855COMMENTS 8856 8857 There are two ways of including comments in patterns that are processed 8858 by PCRE2. In both cases, the start of the comment must not be in a 8859 character class, nor in the middle of any other sequence of related 8860 characters such as (?: or a group name or number. The characters that 8861 make up a comment play no part in the pattern matching. 8862 8863 The sequence (?# marks the start of a comment that continues up to the 8864 next closing parenthesis. Nested parentheses are not permitted. If the 8865 PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, an unescaped # 8866 character also introduces a comment, which in this case continues to 8867 immediately after the next newline character or character sequence in 8868 the pattern. Which characters are interpreted as newlines is controlled 8869 by an option passed to the compiling function or by a special sequence 8870 at the start of the pattern, as described in the section entitled "New- 8871 line conventions" above. Note that the end of this type of comment is a 8872 literal newline sequence in the pattern; escape sequences that happen 8873 to represent a newline do not count. For example, consider this pattern 8874 when PCRE2_EXTENDED is set, and the default newline convention (a sin- 8875 gle linefeed character) is in force: 8876 8877 abc #comment \n still comment 8878 8879 On encountering the # character, pcre2_compile() skips along, looking 8880 for a newline in the pattern. The sequence \n is still literal at this 8881 stage, so it does not terminate the comment. Only an actual character 8882 with the code value 0x0a (the default newline) does so. 8883 8884 8885RECURSIVE PATTERNS 8886 8887 Consider the problem of matching a string in parentheses, allowing for 8888 unlimited nested parentheses. Without the use of recursion, the best 8889 that can be done is to use a pattern that matches up to some fixed 8890 depth of nesting. It is not possible to handle an arbitrary nesting 8891 depth. 8892 8893 For some time, Perl has provided a facility that allows regular expres- 8894 sions to recurse (amongst other things). It does this by interpolating 8895 Perl code in the expression at run time, and the code can refer to the 8896 expression itself. A Perl pattern using code interpolation to solve the 8897 parentheses problem can be created like this: 8898 8899 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; 8900 8901 The (?p{...}) item interpolates Perl code at run time, and in this case 8902 refers recursively to the pattern in which it appears. 8903 8904 Obviously, PCRE2 cannot support the interpolation of Perl code. In- 8905 stead, it supports special syntax for recursion of the entire pattern, 8906 and also for individual capture group recursion. After its introduction 8907 in PCRE1 and Python, this kind of recursion was subsequently introduced 8908 into Perl at release 5.10. 8909 8910 A special item that consists of (? followed by a number greater than 8911 zero and a closing parenthesis is a recursive subroutine call of the 8912 capture group of the given number, provided that it occurs inside that 8913 group. (If not, it is a non-recursive subroutine call, which is de- 8914 scribed in the next section.) The special item (?R) or (?0) is a recur- 8915 sive call of the entire regular expression. 8916 8917 This PCRE2 pattern solves the nested parentheses problem (assume the 8918 PCRE2_EXTENDED option is set so that white space is ignored): 8919 8920 \( ( [^()]++ | (?R) )* \) 8921 8922 First it matches an opening parenthesis. Then it matches any number of 8923 substrings which can either be a sequence of non-parentheses, or a re- 8924 cursive match of the pattern itself (that is, a correctly parenthesized 8925 substring). Finally there is a closing parenthesis. Note the use of a 8926 possessive quantifier to avoid backtracking into sequences of non- 8927 parentheses. 8928 8929 If this were part of a larger pattern, you would not want to recurse 8930 the entire pattern, so instead you could use this: 8931 8932 ( \( ( [^()]++ | (?1) )* \) ) 8933 8934 We have put the pattern into parentheses, and caused the recursion to 8935 refer to them instead of the whole pattern. 8936 8937 In a larger pattern, keeping track of parenthesis numbers can be 8938 tricky. This is made easier by the use of relative references. Instead 8939 of (?1) in the pattern above you can write (?-2) to refer to the second 8940 most recently opened parentheses preceding the recursion. In other 8941 words, a negative number counts capturing parentheses leftwards from 8942 the point at which it is encountered. 8943 8944 Be aware however, that if duplicate capture group numbers are in use, 8945 relative references refer to the earliest group with the appropriate 8946 number. Consider, for example: 8947 8948 (?|(a)|(b)) (c) (?-2) 8949 8950 The first two capture groups (a) and (b) are both numbered 1, and group 8951 (c) is number 2. When the reference (?-2) is encountered, the second 8952 most recently opened parentheses has the number 1, but it is the first 8953 such group (the (a) group) to which the recursion refers. This would be 8954 the same if an absolute reference (?1) was used. In other words, rela- 8955 tive references are just a shorthand for computing a group number. 8956 8957 It is also possible to refer to subsequent capture groups, by writing 8958 references such as (?+2). However, these cannot be recursive because 8959 the reference is not inside the parentheses that are referenced. They 8960 are always non-recursive subroutine calls, as described in the next 8961 section. 8962 8963 An alternative approach is to use named parentheses. The Perl syntax 8964 for this is (?&name); PCRE1's earlier syntax (?P>name) is also sup- 8965 ported. We could rewrite the above example as follows: 8966 8967 (?<pn> \( ( [^()]++ | (?&pn) )* \) ) 8968 8969 If there is more than one group with the same name, the earliest one is 8970 used. 8971 8972 The example pattern that we have been looking at contains nested unlim- 8973 ited repeats, and so the use of a possessive quantifier for matching 8974 strings of non-parentheses is important when applying the pattern to 8975 strings that do not match. For example, when this pattern is applied to 8976 8977 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 8978 8979 it yields "no match" quickly. However, if a possessive quantifier is 8980 not used, the match runs for a very long time indeed because there are 8981 so many different ways the + and * repeats can carve up the subject, 8982 and all have to be tested before failure can be reported. 8983 8984 At the end of a match, the values of capturing parentheses are those 8985 from the outermost level. If you want to obtain intermediate values, a 8986 callout function can be used (see below and the pcre2callout documenta- 8987 tion). If the pattern above is matched against 8988 8989 (ab(cd)ef) 8990 8991 the value for the inner capturing parentheses (numbered 2) is "ef", 8992 which is the last value taken on at the top level. If a capture group 8993 is not matched at the top level, its final captured value is unset, 8994 even if it was (temporarily) set at a deeper level during the matching 8995 process. 8996 8997 Do not confuse the (?R) item with the condition (R), which tests for 8998 recursion. Consider this pattern, which matches text in angle brack- 8999 ets, allowing for arbitrary nesting. Only digits are allowed in nested 9000 brackets (that is, when recursing), whereas any characters are permit- 9001 ted at the outer level. 9002 9003 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > 9004 9005 In this pattern, (?(R) is the start of a conditional group, with two 9006 different alternatives for the recursive and non-recursive cases. The 9007 (?R) item is the actual recursive call. 9008 9009 Differences in recursion processing between PCRE2 and Perl 9010 9011 Some former differences between PCRE2 and Perl no longer exist. 9012 9013 Before release 10.30, recursion processing in PCRE2 differed from Perl 9014 in that a recursive subroutine call was always treated as an atomic 9015 group. That is, once it had matched some of the subject string, it was 9016 never re-entered, even if it contained untried alternatives and there 9017 was a subsequent matching failure. (Historical note: PCRE implemented 9018 recursion before Perl did.) 9019 9020 Starting with release 10.30, recursive subroutine calls are no longer 9021 treated as atomic. That is, they can be re-entered to try unused alter- 9022 natives if there is a matching failure later in the pattern. This is 9023 now compatible with the way Perl works. If you want a subroutine call 9024 to be atomic, you must explicitly enclose it in an atomic group. 9025 9026 Supporting backtracking into recursions simplifies certain types of re- 9027 cursive pattern. For example, this pattern matches palindromic strings: 9028 9029 ^((.)(?1)\2|.?)$ 9030 9031 The second branch in the group matches a single central character in 9032 the palindrome when there are an odd number of characters, or nothing 9033 when there are an even number of characters, but in order to work it 9034 has to be able to try the second case when the rest of the pattern 9035 match fails. If you want to match typical palindromic phrases, the pat- 9036 tern has to ignore all non-word characters, which can be done like 9037 this: 9038 9039 ^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$ 9040 9041 If run with the PCRE2_CASELESS option, this pattern matches phrases 9042 such as "A man, a plan, a canal: Panama!". Note the use of the posses- 9043 sive quantifier *+ to avoid backtracking into sequences of non-word 9044 characters. Without this, PCRE2 takes a great deal longer (ten times or 9045 more) to match typical phrases, and Perl takes so long that you think 9046 it has gone into a loop. 9047 9048 Another way in which PCRE2 and Perl used to differ in their recursion 9049 processing is in the handling of captured values. Formerly in Perl, 9050 when a group was called recursively or as a subroutine (see the next 9051 section), it had no access to any values that were captured outside the 9052 recursion, whereas in PCRE2 these values can be referenced. Consider 9053 this pattern: 9054 9055 ^(.)(\1|a(?2)) 9056 9057 This pattern matches "bab". The first capturing parentheses match "b", 9058 then in the second group, when the backreference \1 fails to match "b", 9059 the second alternative matches "a" and then recurses. In the recursion, 9060 \1 does now match "b" and so the whole match succeeds. This match used 9061 to fail in Perl, but in later versions (I tried 5.024) it now works. 9062 9063 9064GROUPS AS SUBROUTINES 9065 9066 If the syntax for a recursive group call (either by number or by name) 9067 is used outside the parentheses to which it refers, it operates a bit 9068 like a subroutine in a programming language. More accurately, PCRE2 9069 treats the referenced group as an independent subpattern which it tries 9070 to match at the current matching position. The called group may be de- 9071 fined before or after the reference. A numbered reference can be abso- 9072 lute or relative, as in these examples: 9073 9074 (...(absolute)...)...(?2)... 9075 (...(relative)...)...(?-1)... 9076 (...(?+1)...(relative)... 9077 9078 An earlier example pointed out that the pattern 9079 9080 (sens|respons)e and \1ibility 9081 9082 matches "sense and sensibility" and "response and responsibility", but 9083 not "sense and responsibility". If instead the pattern 9084 9085 (sens|respons)e and (?1)ibility 9086 9087 is used, it does match "sense and responsibility" as well as the other 9088 two strings. Another example is given in the discussion of DEFINE 9089 above. 9090 9091 Like recursions, subroutine calls used to be treated as atomic, but 9092 this changed at PCRE2 release 10.30, so backtracking into subroutine 9093 calls can now occur. However, any capturing parentheses that are set 9094 during the subroutine call revert to their previous values afterwards. 9095 9096 Processing options such as case-independence are fixed when a group is 9097 defined, so if it is used as a subroutine, such options cannot be 9098 changed for different calls. For example, consider this pattern: 9099 9100 (abc)(?i:(?-1)) 9101 9102 It matches "abcabc". It does not match "abcABC" because the change of 9103 processing option does not affect the called group. 9104 9105 The behaviour of backtracking control verbs in groups when called as 9106 subroutines is described in the section entitled "Backtracking verbs in 9107 subroutines" below. 9108 9109 9110ONIGURUMA SUBROUTINE SYNTAX 9111 9112 For compatibility with Oniguruma, the non-Perl syntax \g followed by a 9113 name or a number enclosed either in angle brackets or single quotes, is 9114 an alternative syntax for calling a group as a subroutine, possibly re- 9115 cursively. Here are two of the examples used above, rewritten using 9116 this syntax: 9117 9118 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) 9119 (sens|respons)e and \g'1'ibility 9120 9121 PCRE2 supports an extension to Oniguruma: if a number is preceded by a 9122 plus or a minus sign it is taken as a relative reference. For example: 9123 9124 (abc)(?i:\g<-1>) 9125 9126 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not 9127 synonymous. The former is a backreference; the latter is a subroutine 9128 call. 9129 9130 9131CALLOUTS 9132 9133 Perl has a feature whereby using the sequence (?{...}) causes arbitrary 9134 Perl code to be obeyed in the middle of matching a regular expression. 9135 This makes it possible, amongst other things, to extract different sub- 9136 strings that match the same pair of parentheses when there is a repeti- 9137 tion. 9138 9139 PCRE2 provides a similar feature, but of course it cannot obey arbi- 9140 trary Perl code. The feature is called "callout". The caller of PCRE2 9141 provides an external function by putting its entry point in a match 9142 context using the function pcre2_set_callout(), and then passing that 9143 context to pcre2_match() or pcre2_dfa_match(). If no match context is 9144 passed, or if the callout entry point is set to NULL, callouts are dis- 9145 abled. 9146 9147 Within a regular expression, (?C<arg>) indicates a point at which the 9148 external function is to be called. There are two kinds of callout: 9149 those with a numerical argument and those with a string argument. (?C) 9150 on its own with no argument is treated as (?C0). A numerical argument 9151 allows the application to distinguish between different callouts. 9152 String arguments were added for release 10.20 to make it possible for 9153 script languages that use PCRE2 to embed short scripts within patterns 9154 in a similar way to Perl. 9155 9156 During matching, when PCRE2 reaches a callout point, the external func- 9157 tion is called. It is provided with the number or string argument of 9158 the callout, the position in the pattern, and one item of data that is 9159 also set in the match block. The callout function may cause matching to 9160 proceed, to backtrack, or to fail. 9161 9162 By default, PCRE2 implements a number of optimizations at matching 9163 time, and one side-effect is that sometimes callouts are skipped. If 9164 you need all possible callouts to happen, you need to set options that 9165 disable the relevant optimizations. More details, including a complete 9166 description of the programming interface to the callout function, are 9167 given in the pcre2callout documentation. 9168 9169 Callouts with numerical arguments 9170 9171 If you just want to have a means of identifying different callout 9172 points, put a number less than 256 after the letter C. For example, 9173 this pattern has two callout points: 9174 9175 (?C1)abc(?C2)def 9176 9177 If the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical 9178 callouts are automatically installed before each item in the pattern. 9179 They are all numbered 255. If there is a conditional group in the pat- 9180 tern whose condition is an assertion, an additional callout is inserted 9181 just before the condition. An explicit callout may also be set at this 9182 position, as in this example: 9183 9184 (?(?C9)(?=a)abc|def) 9185 9186 Note that this applies only to assertion conditions, not to other types 9187 of condition. 9188 9189 Callouts with string arguments 9190 9191 A delimited string may be used instead of a number as a callout argu- 9192 ment. The starting delimiter must be one of ` ' " ^ % # $ { and the 9193 ending delimiter is the same as the start, except for {, where the end- 9194 ing delimiter is }. If the ending delimiter is needed within the 9195 string, it must be doubled. For example: 9196 9197 (?C'ab ''c'' d')xyz(?C{any text})pqr 9198 9199 The doubling is removed before the string is passed to the callout 9200 function. 9201 9202 9203BACKTRACKING CONTROL 9204 9205 There are a number of special "Backtracking Control Verbs" (to use 9206 Perl's terminology) that modify the behaviour of backtracking during 9207 matching. They are generally of the form (*VERB) or (*VERB:NAME). Some 9208 verbs take either form, and may behave differently depending on whether 9209 or not a name argument is present. The names are not required to be 9210 unique within the pattern. 9211 9212 By default, for compatibility with Perl, a name is any sequence of 9213 characters that does not include a closing parenthesis. The name is not 9214 processed in any way, and it is not possible to include a closing 9215 parenthesis in the name. This can be changed by setting the 9216 PCRE2_ALT_VERBNAMES option, but the result is no longer Perl-compati- 9217 ble. 9218 9219 When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to 9220 verb names and only an unescaped closing parenthesis terminates the 9221 name. However, the only backslash items that are permitted are \Q, \E, 9222 and sequences such as \x{100} that define character code points. Char- 9223 acter type escapes such as \d are faulted. 9224 9225 A closing parenthesis can be included in a name either as \) or between 9226 \Q and \E. In addition to backslash processing, if the PCRE2_EXTENDED 9227 or PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb 9228 names is skipped, and #-comments are recognized, exactly as in the rest 9229 of the pattern. PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect 9230 verb names unless PCRE2_ALT_VERBNAMES is also set. 9231 9232 The maximum length of a name is 255 in the 8-bit library and 65535 in 9233 the 16-bit and 32-bit libraries. If the name is empty, that is, if the 9234 closing parenthesis immediately follows the colon, the effect is as if 9235 the colon were not there. Any number of these verbs may occur in a pat- 9236 tern. Except for (*ACCEPT), they may not be quantified. 9237 9238 Since these verbs are specifically related to backtracking, most of 9239 them can be used only when the pattern is to be matched using the tra- 9240 ditional matching function, because that uses a backtracking algorithm. 9241 With the exception of (*FAIL), which behaves like a failing negative 9242 assertion, the backtracking control verbs cause an error if encountered 9243 by the DFA matching function. 9244 9245 The behaviour of these verbs in repeated groups, assertions, and in 9246 capture groups called as subroutines (whether or not recursively) is 9247 documented below. 9248 9249 Optimizations that affect backtracking verbs 9250 9251 PCRE2 contains some optimizations that are used to speed up matching by 9252 running some checks at the start of each match attempt. For example, it 9253 may know the minimum length of matching subject, or that a particular 9254 character must be present. When one of these optimizations bypasses the 9255 running of a match, any included backtracking verbs will not, of 9256 course, be processed. You can suppress the start-of-match optimizations 9257 by setting the PCRE2_NO_START_OPTIMIZE option when calling pcre2_com- 9258 pile(), or by starting the pattern with (*NO_START_OPT). There is more 9259 discussion of this option in the section entitled "Compiling a pattern" 9260 in the pcre2api documentation. 9261 9262 Experiments with Perl suggest that it too has similar optimizations, 9263 and like PCRE2, turning them off can change the result of a match. 9264 9265 Verbs that act immediately 9266 9267 The following verbs act as soon as they are encountered. 9268 9269 (*ACCEPT) or (*ACCEPT:NAME) 9270 9271 This verb causes the match to end successfully, skipping the remainder 9272 of the pattern. However, when it is inside a capture group that is 9273 called as a subroutine, only that group is ended successfully. Matching 9274 then continues at the outer level. If (*ACCEPT) in triggered in a posi- 9275 tive assertion, the assertion succeeds; in a negative assertion, the 9276 assertion fails. 9277 9278 If (*ACCEPT) is inside capturing parentheses, the data so far is cap- 9279 tured. For example: 9280 9281 A((?:A|B(*ACCEPT)|C)D) 9282 9283 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- 9284 tured by the outer parentheses. 9285 9286 (*ACCEPT) is the only backtracking verb that is allowed to be quanti- 9287 fied because an ungreedy quantification with a minimum of zero acts 9288 only when a backtrack happens. Consider, for example, 9289 9290 (A(*ACCEPT)??B)C 9291 9292 where A, B, and C may be complex expressions. After matching "A", the 9293 matcher processes "BC"; if that fails, causing a backtrack, (*ACCEPT) 9294 is triggered and the match succeeds. In both cases, all but C is cap- 9295 tured. Whereas (*COMMIT) (see below) means "fail on backtrack", a re- 9296 peated (*ACCEPT) of this type means "succeed on backtrack". 9297 9298 Warning: (*ACCEPT) should not be used within a script run group, be- 9299 cause it causes an immediate exit from the group, bypassing the script 9300 run checking. 9301 9302 (*FAIL) or (*FAIL:NAME) 9303 9304 This verb causes a matching failure, forcing backtracking to occur. It 9305 may be abbreviated to (*F). It is equivalent to (?!) but easier to 9306 read. The Perl documentation notes that it is probably useful only when 9307 combined with (?{}) or (??{}). Those are, of course, Perl features that 9308 are not present in PCRE2. The nearest equivalent is the callout fea- 9309 ture, as for example in this pattern: 9310 9311 a+(?C)(*FAIL) 9312 9313 A match with the string "aaaa" always fails, but the callout is taken 9314 before each backtrack happens (in this example, 10 times). 9315 9316 (*ACCEPT:NAME) and (*FAIL:NAME) behave the same as (*MARK:NAME)(*AC- 9317 CEPT) and (*MARK:NAME)(*FAIL), respectively, that is, a (*MARK) is 9318 recorded just before the verb acts. 9319 9320 Recording which path was taken 9321 9322 There is one verb whose main purpose is to track how a match was ar- 9323 rived at, though it also has a secondary use in conjunction with ad- 9324 vancing the match starting point (see (*SKIP) below). 9325 9326 (*MARK:NAME) or (*:NAME) 9327 9328 A name is always required with this verb. For all the other backtrack- 9329 ing control verbs, a NAME argument is optional. 9330 9331 When a match succeeds, the name of the last-encountered mark name on 9332 the matching path is passed back to the caller as described in the sec- 9333 tion entitled "Other information about the match" in the pcre2api docu- 9334 mentation. This applies to all instances of (*MARK) and other verbs, 9335 including those inside assertions and atomic groups. However, there are 9336 differences in those cases when (*MARK) is used in conjunction with 9337 (*SKIP) as described below. 9338 9339 The mark name that was last encountered on the matching path is passed 9340 back. A verb without a NAME argument is ignored for this purpose. Here 9341 is an example of pcre2test output, where the "mark" modifier requests 9342 the retrieval and outputting of (*MARK) data: 9343 9344 re> /X(*MARK:A)Y|X(*MARK:B)Z/mark 9345 data> XY 9346 0: XY 9347 MK: A 9348 XZ 9349 0: XZ 9350 MK: B 9351 9352 The (*MARK) name is tagged with "MK:" in this output, and in this exam- 9353 ple it indicates which of the two alternatives matched. This is a more 9354 efficient way of obtaining this information than putting each alterna- 9355 tive in its own capturing parentheses. 9356 9357 If a verb with a name is encountered in a positive assertion that is 9358 true, the name is recorded and passed back if it is the last-encoun- 9359 tered. This does not happen for negative assertions or failing positive 9360 assertions. 9361 9362 After a partial match or a failed match, the last encountered name in 9363 the entire match process is returned. For example: 9364 9365 re> /X(*MARK:A)Y|X(*MARK:B)Z/mark 9366 data> XP 9367 No match, mark = B 9368 9369 Note that in this unanchored example the mark is retained from the 9370 match attempt that started at the letter "X" in the subject. Subsequent 9371 match attempts starting at "P" and then with an empty string do not get 9372 as far as the (*MARK) item, but nevertheless do not reset it. 9373 9374 If you are interested in (*MARK) values after failed matches, you 9375 should probably set the PCRE2_NO_START_OPTIMIZE option (see above) to 9376 ensure that the match is always attempted. 9377 9378 Verbs that act after backtracking 9379 9380 The following verbs do nothing when they are encountered. Matching con- 9381 tinues with what follows, but if there is a subsequent match failure, 9382 causing a backtrack to the verb, a failure is forced. That is, back- 9383 tracking cannot pass to the left of the verb. However, when one of 9384 these verbs appears inside an atomic group or in a lookaround assertion 9385 that is true, its effect is confined to that group, because once the 9386 group has been matched, there is never any backtracking into it. Back- 9387 tracking from beyond an assertion or an atomic group ignores the entire 9388 group, and seeks a preceding backtracking point. 9389 9390 These verbs differ in exactly what kind of failure occurs when back- 9391 tracking reaches them. The behaviour described below is what happens 9392 when the verb is not in a subroutine or an assertion. Subsequent sec- 9393 tions cover these special cases. 9394 9395 (*COMMIT) or (*COMMIT:NAME) 9396 9397 This verb causes the whole match to fail outright if there is a later 9398 matching failure that causes backtracking to reach it. Even if the pat- 9399 tern is unanchored, no further attempts to find a match by advancing 9400 the starting point take place. If (*COMMIT) is the only backtracking 9401 verb that is encountered, once it has been passed pcre2_match() is com- 9402 mitted to finding a match at the current starting point, or not at all. 9403 For example: 9404 9405 a+(*COMMIT)b 9406 9407 This matches "xxaab" but not "aacaab". It can be thought of as a kind 9408 of dynamic anchor, or "I've started, so I must finish." 9409 9410 The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COM- 9411 MIT). It is like (*MARK:NAME) in that the name is remembered for pass- 9412 ing back to the caller. However, (*SKIP:NAME) searches only for names 9413 that are set with (*MARK), ignoring those set by any of the other back- 9414 tracking verbs. 9415 9416 If there is more than one backtracking verb in a pattern, a different 9417 one that follows (*COMMIT) may be triggered first, so merely passing 9418 (*COMMIT) during a match does not always guarantee that a match must be 9419 at this starting point. 9420 9421 Note that (*COMMIT) at the start of a pattern is not the same as an an- 9422 chor, unless PCRE2's start-of-match optimizations are turned off, as 9423 shown in this output from pcre2test: 9424 9425 re> /(*COMMIT)abc/ 9426 data> xyzabc 9427 0: abc 9428 data> 9429 re> /(*COMMIT)abc/no_start_optimize 9430 data> xyzabc 9431 No match 9432 9433 For the first pattern, PCRE2 knows that any match must start with "a", 9434 so the optimization skips along the subject to "a" before applying the 9435 pattern to the first set of data. The match attempt then succeeds. The 9436 second pattern disables the optimization that skips along to the first 9437 character. The pattern is now applied starting at "x", and so the 9438 (*COMMIT) causes the match to fail without trying any other starting 9439 points. 9440 9441 (*PRUNE) or (*PRUNE:NAME) 9442 9443 This verb causes the match to fail at the current starting position in 9444 the subject if there is a later matching failure that causes backtrack- 9445 ing to reach it. If the pattern is unanchored, the normal "bumpalong" 9446 advance to the next starting character then happens. Backtracking can 9447 occur as usual to the left of (*PRUNE), before it is reached, or when 9448 matching to the right of (*PRUNE), but if there is no match to the 9449 right, backtracking cannot cross (*PRUNE). In simple cases, the use of 9450 (*PRUNE) is just an alternative to an atomic group or possessive quan- 9451 tifier, but there are some uses of (*PRUNE) that cannot be expressed in 9452 any other way. In an anchored pattern (*PRUNE) has the same effect as 9453 (*COMMIT). 9454 9455 The behaviour of (*PRUNE:NAME) is not the same as (*MARK:NAME)(*PRUNE). 9456 It is like (*MARK:NAME) in that the name is remembered for passing back 9457 to the caller. However, (*SKIP:NAME) searches only for names set with 9458 (*MARK), ignoring those set by other backtracking verbs. 9459 9460 (*SKIP) 9461 9462 This verb, when given without a name, is like (*PRUNE), except that if 9463 the pattern is unanchored, the "bumpalong" advance is not to the next 9464 character, but to the position in the subject where (*SKIP) was encoun- 9465 tered. (*SKIP) signifies that whatever text was matched leading up to 9466 it cannot be part of a successful match if there is a later mismatch. 9467 Consider: 9468 9469 a+(*SKIP)b 9470 9471 If the subject is "aaaac...", after the first match attempt fails 9472 (starting at the first character in the string), the starting point 9473 skips on to start the next attempt at "c". Note that a possessive quan- 9474 tifier does not have the same effect as this example; although it would 9475 suppress backtracking during the first match attempt, the second at- 9476 tempt would start at the second character instead of skipping on to 9477 "c". 9478 9479 If (*SKIP) is used to specify a new starting position that is the same 9480 as the starting position of the current match, or (by being inside a 9481 lookbehind) earlier, the position specified by (*SKIP) is ignored, and 9482 instead the normal "bumpalong" occurs. 9483 9484 (*SKIP:NAME) 9485 9486 When (*SKIP) has an associated name, its behaviour is modified. When 9487 such a (*SKIP) is triggered, the previous path through the pattern is 9488 searched for the most recent (*MARK) that has the same name. If one is 9489 found, the "bumpalong" advance is to the subject position that corre- 9490 sponds to that (*MARK) instead of to where (*SKIP) was encountered. If 9491 no (*MARK) with a matching name is found, the (*SKIP) is ignored. 9492 9493 The search for a (*MARK) name uses the normal backtracking mechanism, 9494 which means that it does not see (*MARK) settings that are inside 9495 atomic groups or assertions, because they are never re-entered by back- 9496 tracking. Compare the following pcre2test examples: 9497 9498 re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/ 9499 data: abc 9500 0: a 9501 1: a 9502 data: 9503 re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/ 9504 data: abc 9505 0: b 9506 1: b 9507 9508 In the first example, the (*MARK) setting is in an atomic group, so it 9509 is not seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored. 9510 This allows the second branch of the pattern to be tried at the first 9511 character position. In the second example, the (*MARK) setting is not 9512 in an atomic group. This allows (*SKIP:X) to find the (*MARK) when it 9513 backtracks, and this causes a new matching attempt to start at the sec- 9514 ond character. This time, the (*MARK) is never seen because "a" does 9515 not match "b", so the matcher immediately jumps to the second branch of 9516 the pattern. 9517 9518 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It 9519 ignores names that are set by other backtracking verbs. 9520 9521 (*THEN) or (*THEN:NAME) 9522 9523 This verb causes a skip to the next innermost alternative when back- 9524 tracking reaches it. That is, it cancels any further backtracking 9525 within the current alternative. Its name comes from the observation 9526 that it can be used for a pattern-based if-then-else block: 9527 9528 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... 9529 9530 If the COND1 pattern matches, FOO is tried (and possibly further items 9531 after the end of the group if FOO succeeds); on failure, the matcher 9532 skips to the second alternative and tries COND2, without backtracking 9533 into COND1. If that succeeds and BAR fails, COND3 is tried. If subse- 9534 quently BAZ fails, there are no more alternatives, so there is a back- 9535 track to whatever came before the entire group. If (*THEN) is not in- 9536 side an alternation, it acts like (*PRUNE). 9537 9538 The behaviour of (*THEN:NAME) is not the same as (*MARK:NAME)(*THEN). 9539 It is like (*MARK:NAME) in that the name is remembered for passing back 9540 to the caller. However, (*SKIP:NAME) searches only for names set with 9541 (*MARK), ignoring those set by other backtracking verbs. 9542 9543 A group that does not contain a | character is just a part of the en- 9544 closing alternative; it is not a nested alternation with only one al- 9545 ternative. The effect of (*THEN) extends beyond such a group to the en- 9546 closing alternative. Consider this pattern, where A, B, etc. are com- 9547 plex pattern fragments that do not contain any | characters at this 9548 level: 9549 9550 A (B(*THEN)C) | D 9551 9552 If A and B are matched, but there is a failure in C, matching does not 9553 backtrack into A; instead it moves to the next alternative, that is, D. 9554 However, if the group containing (*THEN) is given an alternative, it 9555 behaves differently: 9556 9557 A (B(*THEN)C | (*FAIL)) | D 9558 9559 The effect of (*THEN) is now confined to the inner group. After a fail- 9560 ure in C, matching moves to (*FAIL), which causes the whole group to 9561 fail because there are no more alternatives to try. In this case, 9562 matching does backtrack into A. 9563 9564 Note that a conditional group is not considered as having two alterna- 9565 tives, because only one is ever used. In other words, the | character 9566 in a conditional group has a different meaning. Ignoring white space, 9567 consider: 9568 9569 ^.*? (?(?=a) a | b(*THEN)c ) 9570 9571 If the subject is "ba", this pattern does not match. Because .*? is un- 9572 greedy, it initially matches zero characters. The condition (?=a) then 9573 fails, the character "b" is matched, but "c" is not. At this point, 9574 matching does not backtrack to .*? as might perhaps be expected from 9575 the presence of the | character. The conditional group is part of the 9576 single alternative that comprises the whole pattern, and so the match 9577 fails. (If there was a backtrack into .*?, allowing it to match "b", 9578 the match would succeed.) 9579 9580 The verbs just described provide four different "strengths" of control 9581 when subsequent matching fails. (*THEN) is the weakest, carrying on the 9582 match at the next alternative. (*PRUNE) comes next, failing the match 9583 at the current starting position, but allowing an advance to the next 9584 character (for an unanchored pattern). (*SKIP) is similar, except that 9585 the advance may be more than one character. (*COMMIT) is the strongest, 9586 causing the entire match to fail. 9587 9588 More than one backtracking verb 9589 9590 If more than one backtracking verb is present in a pattern, the one 9591 that is backtracked onto first acts. For example, consider this pat- 9592 tern, where A, B, etc. are complex pattern fragments: 9593 9594 (A(*COMMIT)B(*THEN)C|ABD) 9595 9596 If A matches but B fails, the backtrack to (*COMMIT) causes the entire 9597 match to fail. However, if A and B match, but C fails, the backtrack to 9598 (*THEN) causes the next alternative (ABD) to be tried. This behaviour 9599 is consistent, but is not always the same as Perl's. It means that if 9600 two or more backtracking verbs appear in succession, all the the last 9601 of them has no effect. Consider this example: 9602 9603 ...(*COMMIT)(*PRUNE)... 9604 9605 If there is a matching failure to the right, backtracking onto (*PRUNE) 9606 causes it to be triggered, and its action is taken. There can never be 9607 a backtrack onto (*COMMIT). 9608 9609 Backtracking verbs in repeated groups 9610 9611 PCRE2 sometimes differs from Perl in its handling of backtracking verbs 9612 in repeated groups. For example, consider: 9613 9614 /(a(*COMMIT)b)+ac/ 9615 9616 If the subject is "abac", Perl matches unless its optimizations are 9617 disabled, but PCRE2 always fails because the (*COMMIT) in the second 9618 repeat of the group acts. 9619 9620 Backtracking verbs in assertions 9621 9622 (*FAIL) in any assertion has its normal effect: it forces an immediate 9623 backtrack. The behaviour of the other backtracking verbs depends on 9624 whether or not the assertion is standalone or acting as the condition 9625 in a conditional group. 9626 9627 (*ACCEPT) in a standalone positive assertion causes the assertion to 9628 succeed without any further processing; captured strings and a mark 9629 name (if set) are retained. In a standalone negative assertion, (*AC- 9630 CEPT) causes the assertion to fail without any further processing; cap- 9631 tured substrings and any mark name are discarded. 9632 9633 If the assertion is a condition, (*ACCEPT) causes the condition to be 9634 true for a positive assertion and false for a negative one; captured 9635 substrings are retained in both cases. 9636 9637 The remaining verbs act only when a later failure causes a backtrack to 9638 reach them. This means that, for the Perl-compatible assertions, their 9639 effect is confined to the assertion, because Perl lookaround assertions 9640 are atomic. A backtrack that occurs after such an assertion is complete 9641 does not jump back into the assertion. Note in particular that a 9642 (*MARK) name that is set in an assertion is not "seen" by an instance 9643 of (*SKIP:NAME) later in the pattern. 9644 9645 PCRE2 now supports non-atomic positive assertions, as described in the 9646 section entitled "Non-atomic assertions" above. These assertions must 9647 be standalone (not used as conditions). They are not Perl-compatible. 9648 For these assertions, a later backtrack does jump back into the asser- 9649 tion, and therefore verbs such as (*COMMIT) can be triggered by back- 9650 tracks from later in the pattern. 9651 9652 The effect of (*THEN) is not allowed to escape beyond an assertion. If 9653 there are no more branches to try, (*THEN) causes a positive assertion 9654 to be false, and a negative assertion to be true. 9655 9656 The other backtracking verbs are not treated specially if they appear 9657 in a standalone positive assertion. In a conditional positive asser- 9658 tion, backtracking (from within the assertion) into (*COMMIT), (*SKIP), 9659 or (*PRUNE) causes the condition to be false. However, for both stand- 9660 alone and conditional negative assertions, backtracking into (*COMMIT), 9661 (*SKIP), or (*PRUNE) causes the assertion to be true, without consider- 9662 ing any further alternative branches. 9663 9664 Backtracking verbs in subroutines 9665 9666 These behaviours occur whether or not the group is called recursively. 9667 9668 (*ACCEPT) in a group called as a subroutine causes the subroutine match 9669 to succeed without any further processing. Matching then continues af- 9670 ter the subroutine call. Perl documents this behaviour. Perl's treat- 9671 ment of the other verbs in subroutines is different in some cases. 9672 9673 (*FAIL) in a group called as a subroutine has its normal effect: it 9674 forces an immediate backtrack. 9675 9676 (*COMMIT), (*SKIP), and (*PRUNE) cause the subroutine match to fail 9677 when triggered by being backtracked to in a group called as a subrou- 9678 tine. There is then a backtrack at the outer level. 9679 9680 (*THEN), when triggered, skips to the next alternative in the innermost 9681 enclosing group that has alternatives (its normal behaviour). However, 9682 if there is no such group within the subroutine's group, the subroutine 9683 match fails and there is a backtrack at the outer level. 9684 9685 9686SEE ALSO 9687 9688 pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2syntax(3), 9689 pcre2(3). 9690 9691 9692AUTHOR 9693 9694 Philip Hazel 9695 Retired from University Computing Service 9696 Cambridge, England. 9697 9698 9699REVISION 9700 9701 Last updated: 12 January 2022 9702 Copyright (c) 1997-2022 University of Cambridge. 9703------------------------------------------------------------------------------ 9704 9705 9706PCRE2PERFORM(3) Library Functions Manual PCRE2PERFORM(3) 9707 9708 9709 9710NAME 9711 PCRE2 - Perl-compatible regular expressions (revised API) 9712 9713PCRE2 PERFORMANCE 9714 9715 Two aspects of performance are discussed below: memory usage and pro- 9716 cessing time. The way you express your pattern as a regular expression 9717 can affect both of them. 9718 9719 9720COMPILED PATTERN MEMORY USAGE 9721 9722 Patterns are compiled by PCRE2 into a reasonably efficient interpretive 9723 code, so that most simple patterns do not use much memory for storing 9724 the compiled version. However, there is one case where the memory usage 9725 of a compiled pattern can be unexpectedly large. If a parenthesized 9726 group has a quantifier with a minimum greater than 1 and/or a limited 9727 maximum, the whole group is repeated in the compiled code. For example, 9728 the pattern 9729 9730 (abc|def){2,4} 9731 9732 is compiled as if it were 9733 9734 (abc|def)(abc|def)((abc|def)(abc|def)?)? 9735 9736 (Technical aside: It is done this way so that backtrack points within 9737 each of the repetitions can be independently maintained.) 9738 9739 For regular expressions whose quantifiers use only small numbers, this 9740 is not usually a problem. However, if the numbers are large, and par- 9741 ticularly if such repetitions are nested, the memory usage can become 9742 an embarrassment. For example, the very simple pattern 9743 9744 ((ab){1,1000}c){1,3} 9745 9746 uses over 50KiB when compiled using the 8-bit library. When PCRE2 is 9747 compiled with its default internal pointer size of two bytes, the size 9748 limit on a compiled pattern is 65535 code units in the 8-bit and 16-bit 9749 libraries, and this is reached with the above pattern if the outer rep- 9750 etition is increased from 3 to 4. PCRE2 can be compiled to use larger 9751 internal pointers and thus handle larger compiled patterns, but it is 9752 better to try to rewrite your pattern to use less memory if you can. 9753 9754 One way of reducing the memory usage for such patterns is to make use 9755 of PCRE2's "subroutine" facility. Re-writing the above pattern as 9756 9757 ((ab)(?2){0,999}c)(?1){0,2} 9758 9759 reduces the memory requirements to around 16KiB, and indeed it remains 9760 under 20KiB even with the outer repetition increased to 100. However, 9761 this kind of pattern is not always exactly equivalent, because any cap- 9762 tures within subroutine calls are lost when the subroutine completes. 9763 If this is not a problem, this kind of rewriting will allow you to 9764 process patterns that PCRE2 cannot otherwise handle. The matching per- 9765 formance of the two different versions of the pattern are roughly the 9766 same. (This applies from release 10.30 - things were different in ear- 9767 lier releases.) 9768 9769 9770STACK AND HEAP USAGE AT RUN TIME 9771 9772 From release 10.30, the interpretive (non-JIT) version of pcre2_match() 9773 uses very little system stack at run time. In earlier releases recur- 9774 sive function calls could use a great deal of stack, and this could 9775 cause problems, but this usage has been eliminated. Backtracking posi- 9776 tions are now explicitly remembered in memory frames controlled by the 9777 code. 9778 9779 The size of each frame depends on the size of pointer variables and the 9780 number of capturing parenthesized groups in the pattern being matched. 9781 On a 64-bit system the frame size for a pattern with no captures is 128 9782 bytes. For each capturing group the size increases by 16 bytes. 9783 9784 Until release 10.41, an initial 20KiB frames vector was allocated on 9785 the system stack, but this still caused some issues for multi-thread 9786 applications where each thread has a very small stack. From release 9787 10.41 backtracking memory frames are always held in heap memory. An 9788 initial heap allocation is obtained the first time any match data block 9789 is passed to pcre2_match(). This is remembered with the match data 9790 block and re-used if that block is used for another match. It is freed 9791 when the match data block itself is freed. 9792 9793 The size of the initial block is the larger of 20KiB or ten times the 9794 pattern's frame size, unless the heap limit is less than this, in which 9795 case the heap limit is used. If the initial block proves to be too 9796 small during matching, it is replaced by a larger block, subject to the 9797 heap limit. The heap limit is checked only when a new block is to be 9798 allocated. Reducing the heap limit between calls to pcre2_match() with 9799 the same match data block does not affect the saved block. 9800 9801 In contrast to pcre2_match(), pcre2_dfa_match() does use recursive 9802 function calls, but only for processing atomic groups, lookaround as- 9803 sertions, and recursion within the pattern. The original version of the 9804 code used to allocate quite large internal workspace vectors on the 9805 stack, which caused some problems for some patterns in environments 9806 with small stacks. From release 10.32 the code for pcre2_dfa_match() 9807 has been re-factored to use heap memory when necessary for internal 9808 workspace when recursing, though recursive function calls are still 9809 used. 9810 9811 The "match depth" parameter can be used to limit the depth of function 9812 recursion, and the "match heap" parameter to limit heap memory in 9813 pcre2_dfa_match(). 9814 9815 9816PROCESSING TIME 9817 9818 Certain items in regular expression patterns are processed more effi- 9819 ciently than others. It is more efficient to use a character class like 9820 [aeiou] than a set of single-character alternatives such as 9821 (a|e|i|o|u). In general, the simplest construction that provides the 9822 required behaviour is usually the most efficient. Jeffrey Friedl's book 9823 contains a lot of useful general discussion about optimizing regular 9824 expressions for efficient performance. This document contains a few ob- 9825 servations about PCRE2. 9826 9827 Using Unicode character properties (the \p, \P, and \X escapes) is 9828 slow, because PCRE2 has to use a multi-stage table lookup whenever it 9829 needs a character's property. If you can find an alternative pattern 9830 that does not use character properties, it will probably be faster. 9831 9832 By default, the escape sequences \b, \d, \s, and \w, and the POSIX 9833 character classes such as [:alpha:] do not use Unicode properties, 9834 partly for backwards compatibility, and partly for performance reasons. 9835 However, you can set the PCRE2_UCP option or start the pattern with 9836 (*UCP) if you want Unicode character properties to be used. This can 9837 double the matching time for items such as \d, when matched with 9838 pcre2_match(); the performance loss is less with a DFA matching func- 9839 tion, and in both cases there is not much difference for \b. 9840 9841 When a pattern begins with .* not in atomic parentheses, nor in paren- 9842 theses that are the subject of a backreference, and the PCRE2_DOTALL 9843 option is set, the pattern is implicitly anchored by PCRE2, since it 9844 can match only at the start of a subject string. If the pattern has 9845 multiple top-level branches, they must all be anchorable. The optimiza- 9846 tion can be disabled by the PCRE2_NO_DOTSTAR_ANCHOR option, and is au- 9847 tomatically disabled if the pattern contains (*PRUNE) or (*SKIP). 9848 9849 If PCRE2_DOTALL is not set, PCRE2 cannot make this optimization, be- 9850 cause the dot metacharacter does not then match a newline, and if the 9851 subject string contains newlines, the pattern may match from the char- 9852 acter immediately following one of them instead of from the very start. 9853 For example, the pattern 9854 9855 .*second 9856 9857 matches the subject "first\nand second" (where \n stands for a newline 9858 character), with the match starting at the seventh character. In order 9859 to do this, PCRE2 has to retry the match starting after every newline 9860 in the subject. 9861 9862 If you are using such a pattern with subject strings that do not con- 9863 tain newlines, the best performance is obtained by setting 9864 PCRE2_DOTALL, or starting the pattern with ^.* or ^.*? to indicate ex- 9865 plicit anchoring. That saves PCRE2 from having to scan along the sub- 9866 ject looking for a newline to restart at. 9867 9868 Beware of patterns that contain nested indefinite repeats. These can 9869 take a long time to run when applied to a string that does not match. 9870 Consider the pattern fragment 9871 9872 ^(a+)* 9873 9874 This can match "aaaa" in 16 different ways, and this number increases 9875 very rapidly as the string gets longer. (The * repeat can match 0, 1, 9876 2, 3, or 4 times, and for each of those cases other than 0 or 4, the + 9877 repeats can match different numbers of times.) When the remainder of 9878 the pattern is such that the entire match is going to fail, PCRE2 has 9879 in principle to try every possible variation, and this can take an ex- 9880 tremely long time, even for relatively short strings. 9881 9882 An optimization catches some of the more simple cases such as 9883 9884 (a+)*b 9885 9886 where a literal character follows. Before embarking on the standard 9887 matching procedure, PCRE2 checks that there is a "b" later in the sub- 9888 ject string, and if there is not, it fails the match immediately. How- 9889 ever, when there is no following literal this optimization cannot be 9890 used. You can see the difference by comparing the behaviour of 9891 9892 (a+)*\d 9893 9894 with the pattern above. The former gives a failure almost instantly 9895 when applied to a whole line of "a" characters, whereas the latter 9896 takes an appreciable time with strings longer than about 20 characters. 9897 9898 In many cases, the solution to this kind of performance issue is to use 9899 an atomic group or a possessive quantifier. This can often reduce mem- 9900 ory requirements as well. As another example, consider this pattern: 9901 9902 ([^<]|<(?!inet))+ 9903 9904 It matches from wherever it starts until it encounters "<inet" or the 9905 end of the data, and is the kind of pattern that might be used when 9906 processing an XML file. Each iteration of the outer parentheses matches 9907 either one character that is not "<" or a "<" that is not followed by 9908 "inet". However, each time a parenthesis is processed, a backtracking 9909 position is passed, so this formulation uses a memory frame for each 9910 matched character. For a long string, a lot of memory is required. Con- 9911 sider now this rewritten pattern, which matches exactly the same 9912 strings: 9913 9914 ([^<]++|<(?!inet))+ 9915 9916 This runs much faster, because sequences of characters that do not con- 9917 tain "<" are "swallowed" in one item inside the parentheses, and a pos- 9918 sessive quantifier is used to stop any backtracking into the runs of 9919 non-"<" characters. This version also uses a lot less memory because 9920 entry to a new set of parentheses happens only when a "<" character 9921 that is not followed by "inet" is encountered (and we assume this is 9922 relatively rare). 9923 9924 This example shows that one way of optimizing performance when matching 9925 long subject strings is to write repeated parenthesized subpatterns to 9926 match more than one character whenever possible. 9927 9928 SETTING RESOURCE LIMITS 9929 9930 You can set limits on the amount of processing that takes place when 9931 matching, and on the amount of heap memory that is used. The default 9932 values of the limits are very large, and unlikely ever to operate. They 9933 can be changed when PCRE2 is built, and they can also be set when 9934 pcre2_match() or pcre2_dfa_match() is called. For details of these in- 9935 terfaces, see the pcre2build documentation and the section entitled 9936 "The match context" in the pcre2api documentation. 9937 9938 The pcre2test test program has a modifier called "find_limits" which, 9939 if applied to a subject line, causes it to find the smallest limits 9940 that allow a pattern to match. This is done by repeatedly matching with 9941 different limits. 9942 9943 9944AUTHOR 9945 9946 Philip Hazel 9947 Retired from University Computing Service 9948 Cambridge, England. 9949 9950 9951REVISION 9952 9953 Last updated: 27 July 2022 9954 Copyright (c) 1997-2022 University of Cambridge. 9955------------------------------------------------------------------------------ 9956 9957 9958PCRE2POSIX(3) Library Functions Manual PCRE2POSIX(3) 9959 9960 9961 9962NAME 9963 PCRE2 - Perl-compatible regular expressions (revised API) 9964 9965SYNOPSIS 9966 9967 #include <pcre2posix.h> 9968 9969 int pcre2_regcomp(regex_t *preg, const char *pattern, 9970 int cflags); 9971 9972 int pcre2_regexec(const regex_t *preg, const char *string, 9973 size_t nmatch, regmatch_t pmatch[], int eflags); 9974 9975 size_t pcre2_regerror(int errcode, const regex_t *preg, 9976 char *errbuf, size_t errbuf_size); 9977 9978 void pcre2_regfree(regex_t *preg); 9979 9980 9981DESCRIPTION 9982 9983 This set of functions provides a POSIX-style API for the PCRE2 regular 9984 expression 8-bit library. There are no POSIX-style wrappers for PCRE2's 9985 16-bit and 32-bit libraries. See the pcre2api documentation for a de- 9986 scription of PCRE2's native API, which contains much additional func- 9987 tionality. 9988 9989 The functions described here are wrapper functions that ultimately call 9990 the PCRE2 native API. Their prototypes are defined in the pcre2posix.h 9991 header file, and they all have unique names starting with pcre2_. How- 9992 ever, the pcre2posix.h header also contains macro definitions that con- 9993 vert the standard POSIX names such regcomp() into pcre2_regcomp() etc. 9994 This means that a program can use the usual POSIX names without running 9995 the risk of accidentally linking with POSIX functions from a different 9996 library. 9997 9998 On Unix-like systems the PCRE2 POSIX library is called libpcre2-posix, 9999 so can be accessed by adding -lpcre2-posix to the command for linking 10000 an application. Because the POSIX functions call the native ones, it is 10001 also necessary to add -lpcre2-8. 10002 10003 Although they were not defined as protypes in pcre2posix.h, releases 10004 10.33 to 10.36 of the library contained functions with the POSIX names 10005 regcomp() etc. These simply passed their arguments to the PCRE2 func- 10006 tions. These functions were provided for backwards compatibility with 10007 earlier versions of PCRE2, which had only POSIX names. However, this 10008 has proved troublesome in situations where a program links with several 10009 libraries, some of which use PCRE2's POSIX interface while others use 10010 the real POSIX functions. For this reason, the POSIX names have been 10011 removed since release 10.37. 10012 10013 Calling the header file pcre2posix.h avoids any conflict with other 10014 POSIX libraries. It can, of course, be renamed or aliased as regex.h, 10015 which is the "correct" name, if there is no clash. It provides two 10016 structure types, regex_t for compiled internal forms, and regmatch_t 10017 for returning captured substrings. It also defines some constants whose 10018 names start with "REG_"; these are used for setting options and identi- 10019 fying error codes. 10020 10021 10022USING THE POSIX FUNCTIONS 10023 10024 Those POSIX option bits that can reasonably be mapped to PCRE2 native 10025 options have been implemented. In addition, the option REG_EXTENDED is 10026 defined with the value zero. This has no effect, but since programs 10027 that are written to the POSIX interface often use it, this makes it 10028 easier to slot in PCRE2 as a replacement library. Other POSIX options 10029 are not even defined. 10030 10031 There are also some options that are not defined by POSIX. These have 10032 been added at the request of users who want to make use of certain 10033 PCRE2-specific features via the POSIX calling interface or to add BSD 10034 or GNU functionality. 10035 10036 When PCRE2 is called via these functions, it is only the API that is 10037 POSIX-like in style. The syntax and semantics of the regular expres- 10038 sions themselves are still those of Perl, subject to the setting of 10039 various PCRE2 options, as described below. "POSIX-like in style" means 10040 that the API approximates to the POSIX definition; it is not fully 10041 POSIX-compatible, and in multi-unit encoding domains it is probably 10042 even less compatible. 10043 10044 The descriptions below use the actual names of the functions, but, as 10045 described above, the standard POSIX names (without the pcre2_ prefix) 10046 may also be used. 10047 10048 10049COMPILING A PATTERN 10050 10051 The function pcre2_regcomp() is called to compile a pattern into an in- 10052 ternal form. By default, the pattern is a C string terminated by a bi- 10053 nary zero (but see REG_PEND below). The preg argument is a pointer to a 10054 regex_t structure that is used as a base for storing information about 10055 the compiled regular expression. (It is also used for input when 10056 REG_PEND is set.) 10057 10058 The argument cflags is either zero, or contains one or more of the bits 10059 defined by the following macros: 10060 10061 REG_DOTALL 10062 10063 The PCRE2_DOTALL option is set when the regular expression is passed 10064 for compilation to the native function. Note that REG_DOTALL is not 10065 part of the POSIX standard. 10066 10067 REG_ICASE 10068 10069 The PCRE2_CASELESS option is set when the regular expression is passed 10070 for compilation to the native function. 10071 10072 REG_NEWLINE 10073 10074 The PCRE2_MULTILINE option is set when the regular expression is passed 10075 for compilation to the native function. Note that this does not mimic 10076 the defined POSIX behaviour for REG_NEWLINE (see the following sec- 10077 tion). 10078 10079 REG_NOSPEC 10080 10081 The PCRE2_LITERAL option is set when the regular expression is passed 10082 for compilation to the native function. This disables all meta charac- 10083 ters in the pattern, causing it to be treated as a literal string. The 10084 only other options that are allowed with REG_NOSPEC are REG_ICASE, 10085 REG_NOSUB, REG_PEND, and REG_UTF. Note that REG_NOSPEC is not part of 10086 the POSIX standard. 10087 10088 REG_NOSUB 10089 10090 When a pattern that is compiled with this flag is passed to 10091 pcre2_regexec() for matching, the nmatch and pmatch arguments are ig- 10092 nored, and no captured strings are returned. Versions of the PCRE li- 10093 brary prior to 10.22 used to set the PCRE2_NO_AUTO_CAPTURE compile op- 10094 tion, but this no longer happens because it disables the use of back- 10095 references. 10096 10097 REG_PEND 10098 10099 If this option is set, the reg_endp field in the preg structure (which 10100 has the type const char *) must be set to point to the character beyond 10101 the end of the pattern before calling pcre2_regcomp(). The pattern it- 10102 self may now contain binary zeros, which are treated as data charac- 10103 ters. Without REG_PEND, a binary zero terminates the pattern and the 10104 re_endp field is ignored. This is a GNU extension to the POSIX standard 10105 and should be used with caution in software intended to be portable to 10106 other systems. 10107 10108 REG_UCP 10109 10110 The PCRE2_UCP option is set when the regular expression is passed for 10111 compilation to the native function. This causes PCRE2 to use Unicode 10112 properties when matchine \d, \w, etc., instead of just recognizing 10113 ASCII values. Note that REG_UCP is not part of the POSIX standard. 10114 10115 REG_UNGREEDY 10116 10117 The PCRE2_UNGREEDY option is set when the regular expression is passed 10118 for compilation to the native function. Note that REG_UNGREEDY is not 10119 part of the POSIX standard. 10120 10121 REG_UTF 10122 10123 The PCRE2_UTF option is set when the regular expression is passed for 10124 compilation to the native function. This causes the pattern itself and 10125 all data strings used for matching it to be treated as UTF-8 strings. 10126 Note that REG_UTF is not part of the POSIX standard. 10127 10128 In the absence of these flags, no options are passed to the native 10129 function. This means the the regex is compiled with PCRE2 default se- 10130 mantics. In particular, the way it handles newline characters in the 10131 subject string is the Perl way, not the POSIX way. Note that setting 10132 PCRE2_MULTILINE has only some of the effects specified for REG_NEWLINE. 10133 It does not affect the way newlines are matched by the dot metacharac- 10134 ter (they are not) or by a negative class such as [^a] (they are). 10135 10136 The yield of pcre2_regcomp() is zero on success, and non-zero other- 10137 wise. The preg structure is filled in on success, and one other member 10138 of the structure (as well as re_endp) is public: re_nsub contains the 10139 number of capturing subpatterns in the regular expression. Various er- 10140 ror codes are defined in the header file. 10141 10142 NOTE: If the yield of pcre2_regcomp() is non-zero, you must not attempt 10143 to use the contents of the preg structure. If, for example, you pass it 10144 to pcre2_regexec(), the result is undefined and your program is likely 10145 to crash. 10146 10147 10148MATCHING NEWLINE CHARACTERS 10149 10150 This area is not simple, because POSIX and Perl take different views of 10151 things. It is not possible to get PCRE2 to obey POSIX semantics, but 10152 then PCRE2 was never intended to be a POSIX engine. The following table 10153 lists the different possibilities for matching newline characters in 10154 Perl and PCRE2: 10155 10156 Default Change with 10157 10158 . matches newline no PCRE2_DOTALL 10159 newline matches [^a] yes not changeable 10160 $ matches \n at end yes PCRE2_DOLLAR_ENDONLY 10161 $ matches \n in middle no PCRE2_MULTILINE 10162 ^ matches \n in middle no PCRE2_MULTILINE 10163 10164 This is the equivalent table for a POSIX-compatible pattern matcher: 10165 10166 Default Change with 10167 10168 . matches newline yes REG_NEWLINE 10169 newline matches [^a] yes REG_NEWLINE 10170 $ matches \n at end no REG_NEWLINE 10171 $ matches \n in middle no REG_NEWLINE 10172 ^ matches \n in middle no REG_NEWLINE 10173 10174 This behaviour is not what happens when PCRE2 is called via its POSIX 10175 API. By default, PCRE2's behaviour is the same as Perl's, except that 10176 there is no equivalent for PCRE2_DOLLAR_ENDONLY in Perl. In both PCRE2 10177 and Perl, there is no way to stop newline from matching [^a]. 10178 10179 Default POSIX newline handling can be obtained by setting PCRE2_DOTALL 10180 and PCRE2_DOLLAR_ENDONLY when calling pcre2_compile() directly, but 10181 there is no way to make PCRE2 behave exactly as for the REG_NEWLINE ac- 10182 tion. When using the POSIX API, passing REG_NEWLINE to PCRE2's 10183 pcre2_regcomp() function causes PCRE2_MULTILINE to be passed to 10184 pcre2_compile(), and REG_DOTALL passes PCRE2_DOTALL. There is no way to 10185 pass PCRE2_DOLLAR_ENDONLY. 10186 10187 10188MATCHING A PATTERN 10189 10190 The function pcre2_regexec() is called to match a compiled pattern preg 10191 against a given string, which is by default terminated by a zero byte 10192 (but see REG_STARTEND below), subject to the options in eflags. These 10193 can be: 10194 10195 REG_NOTBOL 10196 10197 The PCRE2_NOTBOL option is set when calling the underlying PCRE2 match- 10198 ing function. 10199 10200 REG_NOTEMPTY 10201 10202 The PCRE2_NOTEMPTY option is set when calling the underlying PCRE2 10203 matching function. Note that REG_NOTEMPTY is not part of the POSIX 10204 standard. However, setting this option can give more POSIX-like behav- 10205 iour in some situations. 10206 10207 REG_NOTEOL 10208 10209 The PCRE2_NOTEOL option is set when calling the underlying PCRE2 match- 10210 ing function. 10211 10212 REG_STARTEND 10213 10214 When this option is set, the subject string starts at string + 10215 pmatch[0].rm_so and ends at string + pmatch[0].rm_eo, which should 10216 point to the first character beyond the string. There may be binary ze- 10217 ros within the subject string, and indeed, using REG_STARTEND is the 10218 only way to pass a subject string that contains a binary zero. 10219 10220 Whatever the value of pmatch[0].rm_so, the offsets of the matched 10221 string and any captured substrings are still given relative to the 10222 start of string itself. (Before PCRE2 release 10.30 these were given 10223 relative to string + pmatch[0].rm_so, but this differs from other im- 10224 plementations.) 10225 10226 This is a BSD extension, compatible with but not specified by IEEE 10227 Standard 1003.2 (POSIX.2), and should be used with caution in software 10228 intended to be portable to other systems. Note that a non-zero rm_so 10229 does not imply REG_NOTBOL; REG_STARTEND affects only the location and 10230 length of the string, not how it is matched. Setting REG_STARTEND and 10231 passing pmatch as NULL are mutually exclusive; the error REG_INVARG is 10232 returned. 10233 10234 If the pattern was compiled with the REG_NOSUB flag, no data about any 10235 matched strings is returned. The nmatch and pmatch arguments of 10236 pcre2_regexec() are ignored (except possibly as input for REG_STAR- 10237 TEND). 10238 10239 The value of nmatch may be zero, and the value pmatch may be NULL (un- 10240 less REG_STARTEND is set); in both these cases no data about any 10241 matched strings is returned. 10242 10243 Otherwise, the portion of the string that was matched, and also any 10244 captured substrings, are returned via the pmatch argument, which points 10245 to an array of nmatch structures of type regmatch_t, containing the 10246 members rm_so and rm_eo. These contain the byte offset to the first 10247 character of each substring and the offset to the first character after 10248 the end of each substring, respectively. The 0th element of the vector 10249 relates to the entire portion of string that was matched; subsequent 10250 elements relate to the capturing subpatterns of the regular expression. 10251 Unused entries in the array have both structure members set to -1. 10252 10253 A successful match yields a zero return; various error codes are de- 10254 fined in the header file, of which REG_NOMATCH is the "expected" fail- 10255 ure code. 10256 10257 10258ERROR MESSAGES 10259 10260 The pcre2_regerror() function maps a non-zero errorcode from either 10261 pcre2_regcomp() or pcre2_regexec() to a printable message. If preg is 10262 not NULL, the error should have arisen from the use of that structure. 10263 A message terminated by a binary zero is placed in errbuf. If the buf- 10264 fer is too short, only the first errbuf_size - 1 characters of the er- 10265 ror message are used. The yield of the function is the size of buffer 10266 needed to hold the whole message, including the terminating zero. This 10267 value is greater than errbuf_size if the message was truncated. 10268 10269 10270MEMORY USAGE 10271 10272 Compiling a regular expression causes memory to be allocated and asso- 10273 ciated with the preg structure. The function pcre2_regfree() frees all 10274 such memory, after which preg may no longer be used as a compiled ex- 10275 pression. 10276 10277 10278AUTHOR 10279 10280 Philip Hazel 10281 University Computing Service 10282 Cambridge, England. 10283 10284 10285REVISION 10286 10287 Last updated: 26 April 2021 10288 Copyright (c) 1997-2021 University of Cambridge. 10289------------------------------------------------------------------------------ 10290 10291 10292PCRE2SAMPLE(3) Library Functions Manual PCRE2SAMPLE(3) 10293 10294 10295 10296NAME 10297 PCRE2 - Perl-compatible regular expressions (revised API) 10298 10299PCRE2 SAMPLE PROGRAM 10300 10301 A simple, complete demonstration program to get you started with using 10302 PCRE2 is supplied in the file pcre2demo.c in the src directory in the 10303 PCRE2 distribution. A listing of this program is given in the pcre2demo 10304 documentation. If you do not have a copy of the PCRE2 distribution, you 10305 can save this listing to re-create the contents of pcre2demo.c. 10306 10307 The demonstration program compiles the regular expression that is its 10308 first argument, and matches it against the subject string in its second 10309 argument. No PCRE2 options are set, and default character tables are 10310 used. If matching succeeds, the program outputs the portion of the sub- 10311 ject that matched, together with the contents of any captured sub- 10312 strings. 10313 10314 If the -g option is given on the command line, the program then goes on 10315 to check for further matches of the same regular expression in the same 10316 subject string. The logic is a little bit tricky because of the possi- 10317 bility of matching an empty string. Comments in the code explain what 10318 is going on. 10319 10320 The code in pcre2demo.c is an 8-bit program that uses the PCRE2 8-bit 10321 library. It handles strings and characters that are stored in 8-bit 10322 code units. By default, one character corresponds to one code unit, 10323 but if the pattern starts with "(*UTF)", both it and the subject are 10324 treated as UTF-8 strings, where characters may occupy multiple code 10325 units. 10326 10327 If PCRE2 is installed in the standard include and library directories 10328 for your operating system, you should be able to compile the demonstra- 10329 tion program using a command like this: 10330 10331 cc -o pcre2demo pcre2demo.c -lpcre2-8 10332 10333 If PCRE2 is installed elsewhere, you may need to add additional options 10334 to the command line. For example, on a Unix-like system that has PCRE2 10335 installed in /usr/local, you can compile the demonstration program us- 10336 ing a command like this: 10337 10338 cc -o pcre2demo -I/usr/local/include pcre2demo.c \ 10339 -L/usr/local/lib -lpcre2-8 10340 10341 Once you have built the demonstration program, you can run simple tests 10342 like this: 10343 10344 ./pcre2demo 'cat|dog' 'the cat sat on the mat' 10345 ./pcre2demo -g 'cat|dog' 'the dog sat on the cat' 10346 10347 Note that there is a much more comprehensive test program, called 10348 pcre2test, which supports many more facilities for testing regular ex- 10349 pressions using all three PCRE2 libraries (8-bit, 16-bit, and 32-bit, 10350 though not all three need be installed). The pcre2demo program is pro- 10351 vided as a relatively simple coding example. 10352 10353 If you try to run pcre2demo when PCRE2 is not installed in the standard 10354 library directory, you may get an error like this on some operating 10355 systems (e.g. Solaris): 10356 10357 ld.so.1: pcre2demo: fatal: libpcre2-8.so.0: open failed: No such file 10358 or directory 10359 10360 This is caused by the way shared library support works on those sys- 10361 tems. You need to add 10362 10363 -R/usr/local/lib 10364 10365 (for example) to the compile command to get round this problem. 10366 10367 10368AUTHOR 10369 10370 Philip Hazel 10371 University Computing Service 10372 Cambridge, England. 10373 10374 10375REVISION 10376 10377 Last updated: 02 February 2016 10378 Copyright (c) 1997-2016 University of Cambridge. 10379------------------------------------------------------------------------------ 10380PCRE2SERIALIZE(3) Library Functions Manual PCRE2SERIALIZE(3) 10381 10382 10383 10384NAME 10385 PCRE2 - Perl-compatible regular expressions (revised API) 10386 10387SAVING AND RE-USING PRECOMPILED PCRE2 PATTERNS 10388 10389 int32_t pcre2_serialize_decode(pcre2_code **codes, 10390 int32_t number_of_codes, const uint8_t *bytes, 10391 pcre2_general_context *gcontext); 10392 10393 int32_t pcre2_serialize_encode(const pcre2_code **codes, 10394 int32_t number_of_codes, uint8_t **serialized_bytes, 10395 PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext); 10396 10397 void pcre2_serialize_free(uint8_t *bytes); 10398 10399 int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes); 10400 10401 If you are running an application that uses a large number of regular 10402 expression patterns, it may be useful to store them in a precompiled 10403 form instead of having to compile them every time the application is 10404 run. However, if you are using the just-in-time optimization feature, 10405 it is not possible to save and reload the JIT data, because it is posi- 10406 tion-dependent. The host on which the patterns are reloaded must be 10407 running the same version of PCRE2, with the same code unit width, and 10408 must also have the same endianness, pointer width and PCRE2_SIZE type. 10409 For example, patterns compiled on a 32-bit system using PCRE2's 16-bit 10410 library cannot be reloaded on a 64-bit system, nor can they be reloaded 10411 using the 8-bit library. 10412 10413 Note that "serialization" in PCRE2 does not convert compiled patterns 10414 to an abstract format like Java or .NET serialization. The serialized 10415 output is really just a bytecode dump, which is why it can only be 10416 reloaded in the same environment as the one that created it. Hence the 10417 restrictions mentioned above. Applications that are not statically 10418 linked with a fixed version of PCRE2 must be prepared to recompile pat- 10419 terns from their sources, in order to be immune to PCRE2 upgrades. 10420 10421 10422SECURITY CONCERNS 10423 10424 The facility for saving and restoring compiled patterns is intended for 10425 use within individual applications. As such, the data supplied to 10426 pcre2_serialize_decode() is expected to be trusted data, not data from 10427 arbitrary external sources. There is only some simple consistency 10428 checking, not complete validation of what is being re-loaded. Corrupted 10429 data may cause undefined results. For example, if the length field of a 10430 pattern in the serialized data is corrupted, the deserializing code may 10431 read beyond the end of the byte stream that is passed to it. 10432 10433 10434SAVING COMPILED PATTERNS 10435 10436 Before compiled patterns can be saved they must be serialized, which in 10437 PCRE2 means converting the pattern to a stream of bytes. A single byte 10438 stream may contain any number of compiled patterns, but they must all 10439 use the same character tables. A single copy of the tables is included 10440 in the byte stream (its size is 1088 bytes). For more details of char- 10441 acter tables, see the section on locale support in the pcre2api docu- 10442 mentation. 10443 10444 The function pcre2_serialize_encode() creates a serialized byte stream 10445 from a list of compiled patterns. Its first two arguments specify the 10446 list, being a pointer to a vector of pointers to compiled patterns, and 10447 the length of the vector. The third and fourth arguments point to vari- 10448 ables which are set to point to the created byte stream and its length, 10449 respectively. The final argument is a pointer to a general context, 10450 which can be used to specify custom memory mangagement functions. If 10451 this argument is NULL, malloc() is used to obtain memory for the byte 10452 stream. The yield of the function is the number of serialized patterns, 10453 or one of the following negative error codes: 10454 10455 PCRE2_ERROR_BADDATA the number of patterns is zero or less 10456 PCRE2_ERROR_BADMAGIC mismatch of id bytes in one of the patterns 10457 PCRE2_ERROR_NOMEMORY memory allocation failed 10458 PCRE2_ERROR_MIXEDTABLES the patterns do not all use the same tables 10459 PCRE2_ERROR_NULL the 1st, 3rd, or 4th argument is NULL 10460 10461 PCRE2_ERROR_BADMAGIC means either that a pattern's code has been cor- 10462 rupted, or that a slot in the vector does not point to a compiled pat- 10463 tern. 10464 10465 Once a set of patterns has been serialized you can save the data in any 10466 appropriate manner. Here is sample code that compiles two patterns and 10467 writes them to a file. It assumes that the variable fd refers to a file 10468 that is open for output. The error checking that should be present in a 10469 real application has been omitted for simplicity. 10470 10471 int errorcode; 10472 uint8_t *bytes; 10473 PCRE2_SIZE erroroffset; 10474 PCRE2_SIZE bytescount; 10475 pcre2_code *list_of_codes[2]; 10476 list_of_codes[0] = pcre2_compile("first pattern", 10477 PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL); 10478 list_of_codes[1] = pcre2_compile("second pattern", 10479 PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL); 10480 errorcode = pcre2_serialize_encode(list_of_codes, 2, &bytes, 10481 &bytescount, NULL); 10482 errorcode = fwrite(bytes, 1, bytescount, fd); 10483 10484 Note that the serialized data is binary data that may contain any of 10485 the 256 possible byte values. On systems that make a distinction be- 10486 tween binary and non-binary data, be sure that the file is opened for 10487 binary output. 10488 10489 Serializing a set of patterns leaves the original data untouched, so 10490 they can still be used for matching. Their memory must eventually be 10491 freed in the usual way by calling pcre2_code_free(). When you have fin- 10492 ished with the byte stream, it too must be freed by calling pcre2_seri- 10493 alize_free(). If this function is called with a NULL argument, it re- 10494 turns immediately without doing anything. 10495 10496 10497RE-USING PRECOMPILED PATTERNS 10498 10499 In order to re-use a set of saved patterns you must first make the se- 10500 rialized byte stream available in main memory (for example, by reading 10501 from a file). The management of this memory block is up to the applica- 10502 tion. You can use the pcre2_serialize_get_number_of_codes() function to 10503 find out how many compiled patterns are in the serialized data without 10504 actually decoding the patterns: 10505 10506 uint8_t *bytes = <serialized data>; 10507 int32_t number_of_codes = pcre2_serialize_get_number_of_codes(bytes); 10508 10509 The pcre2_serialize_decode() function reads a byte stream and recreates 10510 the compiled patterns in new memory blocks, setting pointers to them in 10511 a vector. The first two arguments are a pointer to a suitable vector 10512 and its length, and the third argument points to a byte stream. The fi- 10513 nal argument is a pointer to a general context, which can be used to 10514 specify custom memory mangagement functions for the decoded patterns. 10515 If this argument is NULL, malloc() and free() are used. After deserial- 10516 ization, the byte stream is no longer needed and can be discarded. 10517 10518 pcre2_code *list_of_codes[2]; 10519 uint8_t *bytes = <serialized data>; 10520 int32_t number_of_codes = 10521 pcre2_serialize_decode(list_of_codes, 2, bytes, NULL); 10522 10523 If the vector is not large enough for all the patterns in the byte 10524 stream, it is filled with those that fit, and the remainder are ig- 10525 nored. The yield of the function is the number of decoded patterns, or 10526 one of the following negative error codes: 10527 10528 PCRE2_ERROR_BADDATA second argument is zero or less 10529 PCRE2_ERROR_BADMAGIC mismatch of id bytes in the data 10530 PCRE2_ERROR_BADMODE mismatch of code unit size or PCRE2 version 10531 PCRE2_ERROR_BADSERIALIZEDDATA other sanity check failure 10532 PCRE2_ERROR_MEMORY memory allocation failed 10533 PCRE2_ERROR_NULL first or third argument is NULL 10534 10535 PCRE2_ERROR_BADMAGIC may mean that the data is corrupt, or that it was 10536 compiled on a system with different endianness. 10537 10538 Decoded patterns can be used for matching in the usual way, and must be 10539 freed by calling pcre2_code_free(). However, be aware that there is a 10540 potential race issue if you are using multiple patterns that were de- 10541 coded from a single byte stream in a multithreaded application. A sin- 10542 gle copy of the character tables is used by all the decoded patterns 10543 and a reference count is used to arrange for its memory to be automati- 10544 cally freed when the last pattern is freed, but there is no locking on 10545 this reference count. Therefore, if you want to call pcre2_code_free() 10546 for these patterns in different threads, you must arrange your own 10547 locking, and ensure that pcre2_code_free() cannot be called by two 10548 threads at the same time. 10549 10550 If a pattern was processed by pcre2_jit_compile() before being serial- 10551 ized, the JIT data is discarded and so is no longer available after a 10552 save/restore cycle. You can, however, process a restored pattern with 10553 pcre2_jit_compile() if you wish. 10554 10555 10556AUTHOR 10557 10558 Philip Hazel 10559 University Computing Service 10560 Cambridge, England. 10561 10562 10563REVISION 10564 10565 Last updated: 27 June 2018 10566 Copyright (c) 1997-2018 University of Cambridge. 10567------------------------------------------------------------------------------ 10568 10569 10570PCRE2SYNTAX(3) Library Functions Manual PCRE2SYNTAX(3) 10571 10572 10573 10574NAME 10575 PCRE2 - Perl-compatible regular expressions (revised API) 10576 10577PCRE2 REGULAR EXPRESSION SYNTAX SUMMARY 10578 10579 The full syntax and semantics of the regular expressions that are sup- 10580 ported by PCRE2 are described in the pcre2pattern documentation. This 10581 document contains a quick-reference summary of the syntax. 10582 10583 10584QUOTING 10585 10586 \x where x is non-alphanumeric is a literal x 10587 \Q...\E treat enclosed characters as literal 10588 10589 10590ESCAPED CHARACTERS 10591 10592 This table applies to ASCII and Unicode environments. An unrecognized 10593 escape sequence causes an error. 10594 10595 \a alarm, that is, the BEL character (hex 07) 10596 \cx "control-x", where x is any ASCII printing character 10597 \e escape (hex 1B) 10598 \f form feed (hex 0C) 10599 \n newline (hex 0A) 10600 \r carriage return (hex 0D) 10601 \t tab (hex 09) 10602 \0dd character with octal code 0dd 10603 \ddd character with octal code ddd, or backreference 10604 \o{ddd..} character with octal code ddd.. 10605 \N{U+hh..} character with Unicode code point hh.. (Unicode mode only) 10606 \xhh character with hex code hh 10607 \x{hh..} character with hex code hh.. 10608 10609 If PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX is set ("ALT_BSUX mode"), the 10610 following are also recognized: 10611 10612 \U the character "U" 10613 \uhhhh character with hex code hhhh 10614 \u{hh..} character with hex code hh.. but only for EXTRA_ALT_BSUX 10615 10616 When \x is not followed by {, from zero to two hexadecimal digits are 10617 read, but in ALT_BSUX mode \x must be followed by two hexadecimal dig- 10618 its to be recognized as a hexadecimal escape; otherwise it matches a 10619 literal "x". Likewise, if \u (in ALT_BSUX mode) is not followed by 10620 four hexadecimal digits or (in EXTRA_ALT_BSUX mode) a sequence of hex 10621 digits in curly brackets, it matches a literal "u". 10622 10623 Note that \0dd is always an octal code. The treatment of backslash fol- 10624 lowed by a non-zero digit is complicated; for details see the section 10625 "Non-printing characters" in the pcre2pattern documentation, where de- 10626 tails of escape processing in EBCDIC environments are also given. 10627 \N{U+hh..} is synonymous with \x{hh..} in PCRE2 but is not supported in 10628 EBCDIC environments. Note that \N not followed by an opening curly 10629 bracket has a different meaning (see below). 10630 10631 10632CHARACTER TYPES 10633 10634 . any character except newline; 10635 in dotall mode, any character whatsoever 10636 \C one code unit, even in UTF mode (best avoided) 10637 \d a decimal digit 10638 \D a character that is not a decimal digit 10639 \h a horizontal white space character 10640 \H a character that is not a horizontal white space character 10641 \N a character that is not a newline 10642 \p{xx} a character with the xx property 10643 \P{xx} a character without the xx property 10644 \R a newline sequence 10645 \s a white space character 10646 \S a character that is not a white space character 10647 \v a vertical white space character 10648 \V a character that is not a vertical white space character 10649 \w a "word" character 10650 \W a "non-word" character 10651 \X a Unicode extended grapheme cluster 10652 10653 \C is dangerous because it may leave the current matching point in the 10654 middle of a UTF-8 or UTF-16 character. The application can lock out the 10655 use of \C by setting the PCRE2_NEVER_BACKSLASH_C option. It is also 10656 possible to build PCRE2 with the use of \C permanently disabled. 10657 10658 By default, \d, \s, and \w match only ASCII characters, even in UTF-8 10659 mode or in the 16-bit and 32-bit libraries. However, if locale-specific 10660 matching is happening, \s and \w may also match characters with code 10661 points in the range 128-255. If the PCRE2_UCP option is set, the behav- 10662 iour of these escape sequences is changed to use Unicode properties and 10663 they match many more characters. 10664 10665 Property descriptions in \p and \P are matched caselessly; hyphens, un- 10666 derscores, and white space are ignored, in accordance with Unicode's 10667 "loose matching" rules. 10668 10669 10670GENERAL CATEGORY PROPERTIES FOR \p and \P 10671 10672 C Other 10673 Cc Control 10674 Cf Format 10675 Cn Unassigned 10676 Co Private use 10677 Cs Surrogate 10678 10679 L Letter 10680 Ll Lower case letter 10681 Lm Modifier letter 10682 Lo Other letter 10683 Lt Title case letter 10684 Lu Upper case letter 10685 Lc Ll, Lu, or Lt 10686 L& Ll, Lu, or Lt 10687 10688 M Mark 10689 Mc Spacing mark 10690 Me Enclosing mark 10691 Mn Non-spacing mark 10692 10693 N Number 10694 Nd Decimal number 10695 Nl Letter number 10696 No Other number 10697 10698 P Punctuation 10699 Pc Connector punctuation 10700 Pd Dash punctuation 10701 Pe Close punctuation 10702 Pf Final punctuation 10703 Pi Initial punctuation 10704 Po Other punctuation 10705 Ps Open punctuation 10706 10707 S Symbol 10708 Sc Currency symbol 10709 Sk Modifier symbol 10710 Sm Mathematical symbol 10711 So Other symbol 10712 10713 Z Separator 10714 Zl Line separator 10715 Zp Paragraph separator 10716 Zs Space separator 10717 10718 10719PCRE2 SPECIAL CATEGORY PROPERTIES FOR \p and \P 10720 10721 Xan Alphanumeric: union of properties L and N 10722 Xps POSIX space: property Z or tab, NL, VT, FF, CR 10723 Xsp Perl space: property Z or tab, NL, VT, FF, CR 10724 Xuc Univerally-named character: one that can be 10725 represented by a Universal Character Name 10726 Xwd Perl word: property Xan or underscore 10727 10728 Perl and POSIX space are now the same. Perl added VT to its space char- 10729 acter set at release 5.18. 10730 10731 10732BINARY PROPERTIES FOR \p AND \P 10733 10734 Unicode defines a number of binary properties, that is, properties 10735 whose only values are true or false. You can obtain a list of those 10736 that are recognized by \p and \P, along with their abbreviations, by 10737 running this command: 10738 10739 pcre2test -LP 10740 10741 10742SCRIPT MATCHING WITH \p AND \P 10743 10744 Many script names and their 4-letter abbreviations are recognized in 10745 \p{sc:...} or \p{scx:...} items, or on their own with \p (and also \P 10746 of course). You can obtain a list of these scripts by running this com- 10747 mand: 10748 10749 pcre2test -LS 10750 10751 10752THE BIDI_CLASS PROPERTY FOR \p AND \P 10753 10754 \p{Bidi_Class:<class>} matches a character with the given class 10755 \p{BC:<class>} matches a character with the given class 10756 10757 The recognized classes are: 10758 10759 AL Arabic letter 10760 AN Arabic number 10761 B paragraph separator 10762 BN boundary neutral 10763 CS common separator 10764 EN European number 10765 ES European separator 10766 ET European terminator 10767 FSI first strong isolate 10768 L left-to-right 10769 LRE left-to-right embedding 10770 LRI left-to-right isolate 10771 LRO left-to-right override 10772 NSM non-spacing mark 10773 ON other neutral 10774 PDF pop directional format 10775 PDI pop directional isolate 10776 R right-to-left 10777 RLE right-to-left embedding 10778 RLI right-to-left isolate 10779 RLO right-to-left override 10780 S segment separator 10781 WS which space 10782 10783 10784CHARACTER CLASSES 10785 10786 [...] positive character class 10787 [^...] negative character class 10788 [x-y] range (can be used for hex characters) 10789 [[:xxx:]] positive POSIX named set 10790 [[:^xxx:]] negative POSIX named set 10791 10792 alnum alphanumeric 10793 alpha alphabetic 10794 ascii 0-127 10795 blank space or tab 10796 cntrl control character 10797 digit decimal digit 10798 graph printing, excluding space 10799 lower lower case letter 10800 print printing, including space 10801 punct printing, excluding alphanumeric 10802 space white space 10803 upper upper case letter 10804 word same as \w 10805 xdigit hexadecimal digit 10806 10807 In PCRE2, POSIX character set names recognize only ASCII characters by 10808 default, but some of them use Unicode properties if PCRE2_UCP is set. 10809 You can use \Q...\E inside a character class. 10810 10811 10812QUANTIFIERS 10813 10814 ? 0 or 1, greedy 10815 ?+ 0 or 1, possessive 10816 ?? 0 or 1, lazy 10817 * 0 or more, greedy 10818 *+ 0 or more, possessive 10819 *? 0 or more, lazy 10820 + 1 or more, greedy 10821 ++ 1 or more, possessive 10822 +? 1 or more, lazy 10823 {n} exactly n 10824 {n,m} at least n, no more than m, greedy 10825 {n,m}+ at least n, no more than m, possessive 10826 {n,m}? at least n, no more than m, lazy 10827 {n,} n or more, greedy 10828 {n,}+ n or more, possessive 10829 {n,}? n or more, lazy 10830 10831 10832ANCHORS AND SIMPLE ASSERTIONS 10833 10834 \b word boundary 10835 \B not a word boundary 10836 ^ start of subject 10837 also after an internal newline in multiline mode 10838 (after any newline if PCRE2_ALT_CIRCUMFLEX is set) 10839 \A start of subject 10840 $ end of subject 10841 also before newline at end of subject 10842 also before internal newline in multiline mode 10843 \Z end of subject 10844 also before newline at end of subject 10845 \z end of subject 10846 \G first matching position in subject 10847 10848 10849REPORTED MATCH POINT SETTING 10850 10851 \K set reported start of match 10852 10853 From release 10.38 \K is not permitted by default in lookaround asser- 10854 tions, for compatibility with Perl. However, if the PCRE2_EXTRA_AL- 10855 LOW_LOOKAROUND_BSK option is set, the previous behaviour is re-enabled. 10856 When this option is set, \K is honoured in positive assertions, but ig- 10857 nored in negative ones. 10858 10859 10860ALTERNATION 10861 10862 expr|expr|expr... 10863 10864 10865CAPTURING 10866 10867 (...) capture group 10868 (?<name>...) named capture group (Perl) 10869 (?'name'...) named capture group (Perl) 10870 (?P<name>...) named capture group (Python) 10871 (?:...) non-capture group 10872 (?|...) non-capture group; reset group numbers for 10873 capture groups in each alternative 10874 10875 In non-UTF modes, names may contain underscores and ASCII letters and 10876 digits; in UTF modes, any Unicode letters and Unicode decimal digits 10877 are permitted. In both cases, a name must not start with a digit. 10878 10879 10880ATOMIC GROUPS 10881 10882 (?>...) atomic non-capture group 10883 (*atomic:...) atomic non-capture group 10884 10885 10886COMMENT 10887 10888 (?#....) comment (not nestable) 10889 10890 10891OPTION SETTING 10892 Changes of these options within a group are automatically cancelled at 10893 the end of the group. 10894 10895 (?i) caseless 10896 (?J) allow duplicate named groups 10897 (?m) multiline 10898 (?n) no auto capture 10899 (?s) single line (dotall) 10900 (?U) default ungreedy (lazy) 10901 (?x) extended: ignore white space except in classes 10902 (?xx) as (?x) but also ignore space and tab in classes 10903 (?-...) unset option(s) 10904 (?^) unset imnsx options 10905 10906 Unsetting x or xx unsets both. Several options may be set at once, and 10907 a mixture of setting and unsetting such as (?i-x) is allowed, but there 10908 may be only one hyphen. Setting (but no unsetting) is allowed after (?^ 10909 for example (?^in). An option setting may appear at the start of a non- 10910 capture group, for example (?i:...). 10911 10912 The following are recognized only at the very start of a pattern or af- 10913 ter one of the newline or \R options with similar syntax. More than one 10914 of them may appear. For the first three, d is a decimal number. 10915 10916 (*LIMIT_DEPTH=d) set the backtracking limit to d 10917 (*LIMIT_HEAP=d) set the heap size limit to d * 1024 bytes 10918 (*LIMIT_MATCH=d) set the match limit to d 10919 (*NOTEMPTY) set PCRE2_NOTEMPTY when matching 10920 (*NOTEMPTY_ATSTART) set PCRE2_NOTEMPTY_ATSTART when matching 10921 (*NO_AUTO_POSSESS) no auto-possessification (PCRE2_NO_AUTO_POSSESS) 10922 (*NO_DOTSTAR_ANCHOR) no .* anchoring (PCRE2_NO_DOTSTAR_ANCHOR) 10923 (*NO_JIT) disable JIT optimization 10924 (*NO_START_OPT) no start-match optimization (PCRE2_NO_START_OPTIMIZE) 10925 (*UTF) set appropriate UTF mode for the library in use 10926 (*UCP) set PCRE2_UCP (use Unicode properties for \d etc) 10927 10928 Note that LIMIT_DEPTH, LIMIT_HEAP, and LIMIT_MATCH can only reduce the 10929 value of the limits set by the caller of pcre2_match() or 10930 pcre2_dfa_match(), not increase them. LIMIT_RECURSION is an obsolete 10931 synonym for LIMIT_DEPTH. The application can lock out the use of (*UTF) 10932 and (*UCP) by setting the PCRE2_NEVER_UTF or PCRE2_NEVER_UCP options, 10933 respectively, at compile time. 10934 10935 10936NEWLINE CONVENTION 10937 10938 These are recognized only at the very start of the pattern or after op- 10939 tion settings with a similar syntax. 10940 10941 (*CR) carriage return only 10942 (*LF) linefeed only 10943 (*CRLF) carriage return followed by linefeed 10944 (*ANYCRLF) all three of the above 10945 (*ANY) any Unicode newline sequence 10946 (*NUL) the NUL character (binary zero) 10947 10948 10949WHAT \R MATCHES 10950 10951 These are recognized only at the very start of the pattern or after op- 10952 tion setting with a similar syntax. 10953 10954 (*BSR_ANYCRLF) CR, LF, or CRLF 10955 (*BSR_UNICODE) any Unicode newline sequence 10956 10957 10958LOOKAHEAD AND LOOKBEHIND ASSERTIONS 10959 10960 (?=...) ) 10961 (*pla:...) ) positive lookahead 10962 (*positive_lookahead:...) ) 10963 10964 (?!...) ) 10965 (*nla:...) ) negative lookahead 10966 (*negative_lookahead:...) ) 10967 10968 (?<=...) ) 10969 (*plb:...) ) positive lookbehind 10970 (*positive_lookbehind:...) ) 10971 10972 (?<!...) ) 10973 (*nlb:...) ) negative lookbehind 10974 (*negative_lookbehind:...) ) 10975 10976 Each top-level branch of a lookbehind must be of a fixed length. 10977 10978 10979NON-ATOMIC LOOKAROUND ASSERTIONS 10980 10981 These assertions are specific to PCRE2 and are not Perl-compatible. 10982 10983 (?*...) ) 10984 (*napla:...) ) synonyms 10985 (*non_atomic_positive_lookahead:...) ) 10986 10987 (?<*...) ) 10988 (*naplb:...) ) synonyms 10989 (*non_atomic_positive_lookbehind:...) ) 10990 10991 10992SCRIPT RUNS 10993 10994 (*script_run:...) ) script run, can be backtracked into 10995 (*sr:...) ) 10996 10997 (*atomic_script_run:...) ) atomic script run 10998 (*asr:...) ) 10999 11000 11001BACKREFERENCES 11002 11003 \n reference by number (can be ambiguous) 11004 \gn reference by number 11005 \g{n} reference by number 11006 \g+n relative reference by number (PCRE2 extension) 11007 \g-n relative reference by number 11008 \g{+n} relative reference by number (PCRE2 extension) 11009 \g{-n} relative reference by number 11010 \k<name> reference by name (Perl) 11011 \k'name' reference by name (Perl) 11012 \g{name} reference by name (Perl) 11013 \k{name} reference by name (.NET) 11014 (?P=name) reference by name (Python) 11015 11016 11017SUBROUTINE REFERENCES (POSSIBLY RECURSIVE) 11018 11019 (?R) recurse whole pattern 11020 (?n) call subroutine by absolute number 11021 (?+n) call subroutine by relative number 11022 (?-n) call subroutine by relative number 11023 (?&name) call subroutine by name (Perl) 11024 (?P>name) call subroutine by name (Python) 11025 \g<name> call subroutine by name (Oniguruma) 11026 \g'name' call subroutine by name (Oniguruma) 11027 \g<n> call subroutine by absolute number (Oniguruma) 11028 \g'n' call subroutine by absolute number (Oniguruma) 11029 \g<+n> call subroutine by relative number (PCRE2 extension) 11030 \g'+n' call subroutine by relative number (PCRE2 extension) 11031 \g<-n> call subroutine by relative number (PCRE2 extension) 11032 \g'-n' call subroutine by relative number (PCRE2 extension) 11033 11034 11035CONDITIONAL PATTERNS 11036 11037 (?(condition)yes-pattern) 11038 (?(condition)yes-pattern|no-pattern) 11039 11040 (?(n) absolute reference condition 11041 (?(+n) relative reference condition 11042 (?(-n) relative reference condition 11043 (?(<name>) named reference condition (Perl) 11044 (?('name') named reference condition (Perl) 11045 (?(name) named reference condition (PCRE2, deprecated) 11046 (?(R) overall recursion condition 11047 (?(Rn) specific numbered group recursion condition 11048 (?(R&name) specific named group recursion condition 11049 (?(DEFINE) define groups for reference 11050 (?(VERSION[>]=n.m) test PCRE2 version 11051 (?(assert) assertion condition 11052 11053 Note the ambiguity of (?(R) and (?(Rn) which might be named reference 11054 conditions or recursion tests. Such a condition is interpreted as a 11055 reference condition if the relevant named group exists. 11056 11057 11058BACKTRACKING CONTROL 11059 11060 All backtracking control verbs may be in the form (*VERB:NAME). For 11061 (*MARK) the name is mandatory, for the others it is optional. (*SKIP) 11062 changes its behaviour if :NAME is present. The others just set a name 11063 for passing back to the caller, but this is not a name that (*SKIP) can 11064 see. The following act immediately they are reached: 11065 11066 (*ACCEPT) force successful match 11067 (*FAIL) force backtrack; synonym (*F) 11068 (*MARK:NAME) set name to be passed back; synonym (*:NAME) 11069 11070 The following act only when a subsequent match failure causes a back- 11071 track to reach them. They all force a match failure, but they differ in 11072 what happens afterwards. Those that advance the start-of-match point do 11073 so only if the pattern is not anchored. 11074 11075 (*COMMIT) overall failure, no advance of starting point 11076 (*PRUNE) advance to next starting character 11077 (*SKIP) advance to current matching position 11078 (*SKIP:NAME) advance to position corresponding to an earlier 11079 (*MARK:NAME); if not found, the (*SKIP) is ignored 11080 (*THEN) local failure, backtrack to next alternation 11081 11082 The effect of one of these verbs in a group called as a subroutine is 11083 confined to the subroutine call. 11084 11085 11086CALLOUTS 11087 11088 (?C) callout (assumed number 0) 11089 (?Cn) callout with numerical data n 11090 (?C"text") callout with string data 11091 11092 The allowed string delimiters are ` ' " ^ % # $ (which are the same for 11093 the start and the end), and the starting delimiter { matched with the 11094 ending delimiter }. To encode the ending delimiter within the string, 11095 double it. 11096 11097 11098SEE ALSO 11099 11100 pcre2pattern(3), pcre2api(3), pcre2callout(3), pcre2matching(3), 11101 pcre2(3). 11102 11103 11104AUTHOR 11105 11106 Philip Hazel 11107 Retired from University Computing Service 11108 Cambridge, England. 11109 11110 11111REVISION 11112 11113 Last updated: 12 January 2022 11114 Copyright (c) 1997-2022 University of Cambridge. 11115------------------------------------------------------------------------------ 11116 11117 11118PCRE2UNICODE(3) Library Functions Manual PCRE2UNICODE(3) 11119 11120 11121 11122NAME 11123 PCRE - Perl-compatible regular expressions (revised API) 11124 11125UNICODE AND UTF SUPPORT 11126 11127 PCRE2 is normally built with Unicode support, though if you do not need 11128 it, you can build it without, in which case the library will be 11129 smaller. With Unicode support, PCRE2 has knowledge of Unicode character 11130 properties and can process strings of text in UTF-8, UTF-16, and UTF-32 11131 format (depending on the code unit width), but this is not the default. 11132 Unless specifically requested, PCRE2 treats each code unit in a string 11133 as one character. 11134 11135 There are two ways of telling PCRE2 to switch to UTF mode, where char- 11136 acters may consist of more than one code unit and the range of values 11137 is constrained. The program can call pcre2_compile() with the PCRE2_UTF 11138 option, or the pattern may start with the sequence (*UTF). However, 11139 the latter facility can be locked out by the PCRE2_NEVER_UTF option. 11140 That is, the programmer can prevent the supplier of the pattern from 11141 switching to UTF mode. 11142 11143 Note that the PCRE2_MATCH_INVALID_UTF option (see below) forces 11144 PCRE2_UTF to be set. 11145 11146 In UTF mode, both the pattern and any subject strings that are matched 11147 against it are treated as UTF strings instead of strings of individual 11148 one-code-unit characters. There are also some other changes to the way 11149 characters are handled, as documented below. 11150 11151 11152UNICODE PROPERTY SUPPORT 11153 11154 When PCRE2 is built with Unicode support, the escape sequences \p{..}, 11155 \P{..}, and \X can be used. This is not dependent on the PCRE2_UTF set- 11156 ting. The Unicode properties that can be tested are a subset of those 11157 that Perl supports. Currently they are limited to the general category 11158 properties such as Lu for an upper case letter or Nd for a decimal num- 11159 ber, the Unicode script names such as Arabic or Han, Bidi_Class, 11160 Bidi_Control, and the derived properties Any and LC (synonym L&). Full 11161 lists are given in the pcre2pattern and pcre2syntax documentation. In 11162 general, only the short names for properties are supported. For exam- 11163 ple, \p{L} matches a letter. Its longer synonym, \p{Letter}, is not 11164 supported. Furthermore, in Perl, many properties may optionally be pre- 11165 fixed by "Is", for compatibility with Perl 5.6. PCRE2 does not support 11166 this. 11167 11168 11169WIDE CHARACTERS AND UTF MODES 11170 11171 Code points less than 256 can be specified in patterns by either braced 11172 or unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3). 11173 Larger values have to use braced sequences. Unbraced octal code points 11174 up to \777 are also recognized; larger ones can be coded using \o{...}. 11175 11176 The escape sequence \N{U+<hex digits>} is recognized as another way of 11177 specifying a Unicode character by code point in a UTF mode. It is not 11178 allowed in non-UTF mode. 11179 11180 In UTF mode, repeat quantifiers apply to complete UTF characters, not 11181 to individual code units. 11182 11183 In UTF mode, the dot metacharacter matches one UTF character instead of 11184 a single code unit. 11185 11186 In UTF mode, capture group names are not restricted to ASCII, and may 11187 contain any Unicode letters and decimal digits, as well as underscore. 11188 11189 The escape sequence \C can be used to match a single code unit in UTF 11190 mode, but its use can lead to some strange effects because it breaks up 11191 multi-unit characters (see the description of \C in the pcre2pattern 11192 documentation). For this reason, there is a build-time option that dis- 11193 ables support for \C completely. There is also a less draconian com- 11194 pile-time option for locking out the use of \C when a pattern is com- 11195 piled. 11196 11197 The use of \C is not supported by the alternative matching function 11198 pcre2_dfa_match() when in UTF-8 or UTF-16 mode, that is, when a charac- 11199 ter may consist of more than one code unit. The use of \C in these 11200 modes provokes a match-time error. Also, the JIT optimization does not 11201 support \C in these modes. If JIT optimization is requested for a UTF-8 11202 or UTF-16 pattern that contains \C, it will not succeed, and so when 11203 pcre2_match() is called, the matching will be carried out by the inter- 11204 pretive function. 11205 11206 The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test 11207 characters of any code value, but, by default, the characters that 11208 PCRE2 recognizes as digits, spaces, or word characters remain the same 11209 set as in non-UTF mode, all with code points less than 256. This re- 11210 mains true even when PCRE2 is built to include Unicode support, because 11211 to do otherwise would slow down matching in many common cases. Note 11212 that this also applies to \b and \B, because they are defined in terms 11213 of \w and \W. If you want to test for a wider sense of, say, "digit", 11214 you can use explicit Unicode property tests such as \p{Nd}. Alterna- 11215 tively, if you set the PCRE2_UCP option, the way that the character es- 11216 capes work is changed so that Unicode properties are used to determine 11217 which characters match. There are more details in the section on 11218 generic character types in the pcre2pattern documentation. 11219 11220 Similarly, characters that match the POSIX named character classes are 11221 all low-valued characters, unless the PCRE2_UCP option is set. 11222 11223 However, the special horizontal and vertical white space matching es- 11224 capes (\h, \H, \v, and \V) do match all the appropriate Unicode charac- 11225 ters, whether or not PCRE2_UCP is set. 11226 11227 11228UNICODE CASE-EQUIVALENCE 11229 11230 If either PCRE2_UTF or PCRE2_UCP is set, upper/lower case processing 11231 makes use of Unicode properties except for characters whose code points 11232 are less than 128 and that have at most two case-equivalent values. For 11233 these, a direct table lookup is used for speed. A few Unicode charac- 11234 ters such as Greek sigma have more than two code points that are case- 11235 equivalent, and these are treated specially. Setting PCRE2_UCP without 11236 PCRE2_UTF allows Unicode-style case processing for non-UTF character 11237 encodings such as UCS-2. 11238 11239 11240SCRIPT RUNS 11241 11242 The pattern constructs (*script_run:...) and (*atomic_script_run:...), 11243 with synonyms (*sr:...) and (*asr:...), verify that the string matched 11244 within the parentheses is a script run. In concept, a script run is a 11245 sequence of characters that are all from the same Unicode script. How- 11246 ever, because some scripts are commonly used together, and because some 11247 diacritical and other marks are used with multiple scripts, it is not 11248 that simple. 11249 11250 Every Unicode character has a Script property, mostly with a value cor- 11251 responding to the name of a script, such as Latin, Greek, or Cyrillic. 11252 There are also three special values: 11253 11254 "Unknown" is used for code points that have not been assigned, and also 11255 for the surrogate code points. In the PCRE2 32-bit library, characters 11256 whose code points are greater than the Unicode maximum (U+10FFFF), 11257 which are accessible only in non-UTF mode, are assigned the Unknown 11258 script. 11259 11260 "Common" is used for characters that are used with many scripts. These 11261 include punctuation, emoji, mathematical, musical, and currency sym- 11262 bols, and the ASCII digits 0 to 9. 11263 11264 "Inherited" is used for characters such as diacritical marks that mod- 11265 ify a previous character. These are considered to take on the script of 11266 the character that they modify. 11267 11268 Some Inherited characters are used with many scripts, but many of them 11269 are only normally used with a small number of scripts. For example, 11270 U+102E0 (Coptic Epact thousands mark) is used only with Arabic and Cop- 11271 tic. In order to make it possible to check this, a Unicode property 11272 called Script Extension exists. Its value is a list of scripts that ap- 11273 ply to the character. For the majority of characters, the list contains 11274 just one script, the same one as the Script property. However, for 11275 characters such as U+102E0 more than one Script is listed. There are 11276 also some Common characters that have a single, non-Common script in 11277 their Script Extension list. 11278 11279 The next section describes the basic rules for deciding whether a given 11280 string of characters is a script run. Note, however, that there are 11281 some special cases involving the Chinese Han script, and an additional 11282 constraint for decimal digits. These are covered in subsequent sec- 11283 tions. 11284 11285 Basic script run rules 11286 11287 A string that is less than two characters long is a script run. This is 11288 the only case in which an Unknown character can be part of a script 11289 run. Longer strings are checked using only the Script Extensions prop- 11290 erty, not the basic Script property. 11291 11292 If a character's Script Extension property is the single value "Inher- 11293 ited", it is always accepted as part of a script run. This is also true 11294 for the property "Common", subject to the checking of decimal digits 11295 described below. All the remaining characters in a script run must have 11296 at least one script in common in their Script Extension lists. In set- 11297 theoretic terminology, the intersection of all the sets of scripts must 11298 not be empty. 11299 11300 A simple example is an Internet name such as "google.com". The letters 11301 are all in the Latin script, and the dot is Common, so this string is a 11302 script run. However, the Cyrillic letter "o" looks exactly the same as 11303 the Latin "o"; a string that looks the same, but with Cyrillic "o"s is 11304 not a script run. 11305 11306 More interesting examples involve characters with more than one script 11307 in their Script Extension. Consider the following characters: 11308 11309 U+060C Arabic comma 11310 U+06D4 Arabic full stop 11311 11312 The first has the Script Extension list Arabic, Hanifi Rohingya, Syr- 11313 iac, and Thaana; the second has just Arabic and Hanifi Rohingya. Both 11314 of them could appear in script runs of either Arabic or Hanifi Ro- 11315 hingya. The first could also appear in Syriac or Thaana script runs, 11316 but the second could not. 11317 11318 The Chinese Han script 11319 11320 The Chinese Han script is commonly used in conjunction with other 11321 scripts for writing certain languages. Japanese uses the Hiragana and 11322 Katakana scripts together with Han; Korean uses Hangul and Han; Tai- 11323 wanese Mandarin uses Bopomofo and Han. These three combinations are 11324 treated as special cases when checking script runs and are, in effect, 11325 "virtual scripts". Thus, a script run may contain a mixture of Hira- 11326 gana, Katakana, and Han, or a mixture of Hangul and Han, or a mixture 11327 of Bopomofo and Han, but not, for example, a mixture of Hangul and 11328 Bopomofo and Han. PCRE2 (like Perl) follows Unicode's Technical Stan- 11329 dard 39 ("Unicode Security Mechanisms", http://unicode.org/re- 11330 ports/tr39/) in allowing such mixtures. 11331 11332 Decimal digits 11333 11334 Unicode contains many sets of 10 decimal digits in different scripts, 11335 and some scripts (including the Common script) contain more than one 11336 set. Some of these decimal digits them are visually indistinguishable 11337 from the common ASCII digits. In addition to the script checking de- 11338 scribed above, if a script run contains any decimal digits, they must 11339 all come from the same set of 10 adjacent characters. 11340 11341 11342VALIDITY OF UTF STRINGS 11343 11344 When the PCRE2_UTF option is set, the strings passed as patterns and 11345 subjects are (by default) checked for validity on entry to the relevant 11346 functions. If an invalid UTF string is passed, a negative error code is 11347 returned. The code unit offset to the offending character can be ex- 11348 tracted from the match data block by calling pcre2_get_startchar(), 11349 which is used for this purpose after a UTF error. 11350 11351 In some situations, you may already know that your strings are valid, 11352 and therefore want to skip these checks in order to improve perfor- 11353 mance, for example in the case of a long subject string that is being 11354 scanned repeatedly. If you set the PCRE2_NO_UTF_CHECK option at com- 11355 pile time or at match time, PCRE2 assumes that the pattern or subject 11356 it is given (respectively) contains only valid UTF code unit sequences. 11357 11358 If you pass an invalid UTF string when PCRE2_NO_UTF_CHECK is set, the 11359 result is undefined and your program may crash or loop indefinitely or 11360 give incorrect results. There is, however, one mode of matching that 11361 can handle invalid UTF subject strings. This is enabled by passing 11362 PCRE2_MATCH_INVALID_UTF to pcre2_compile() and is discussed below in 11363 the next section. The rest of this section covers the case when 11364 PCRE2_MATCH_INVALID_UTF is not set. 11365 11366 Passing PCRE2_NO_UTF_CHECK to pcre2_compile() just disables the UTF 11367 check for the pattern; it does not also apply to subject strings. If 11368 you want to disable the check for a subject string you must pass this 11369 same option to pcre2_match() or pcre2_dfa_match(). 11370 11371 UTF-16 and UTF-32 strings can indicate their endianness by special code 11372 knows as a byte-order mark (BOM). The PCRE2 functions do not handle 11373 this, expecting strings to be in host byte order. 11374 11375 Unless PCRE2_NO_UTF_CHECK is set, a UTF string is checked before any 11376 other processing takes place. In the case of pcre2_match() and 11377 pcre2_dfa_match() calls with a non-zero starting offset, the check is 11378 applied only to that part of the subject that could be inspected during 11379 matching, and there is a check that the starting offset points to the 11380 first code unit of a character or to the end of the subject. If there 11381 are no lookbehind assertions in the pattern, the check starts at the 11382 starting offset. Otherwise, it starts at the length of the longest 11383 lookbehind before the starting offset, or at the start of the subject 11384 if there are not that many characters before the starting offset. Note 11385 that the sequences \b and \B are one-character lookbehinds. 11386 11387 In addition to checking the format of the string, there is a check to 11388 ensure that all code points lie in the range U+0 to U+10FFFF, excluding 11389 the surrogate area. The so-called "non-character" code points are not 11390 excluded because Unicode corrigendum #9 makes it clear that they should 11391 not be. 11392 11393 Characters in the "Surrogate Area" of Unicode are reserved for use by 11394 UTF-16, where they are used in pairs to encode code points with values 11395 greater than 0xFFFF. The code points that are encoded by UTF-16 pairs 11396 are available independently in the UTF-8 and UTF-32 encodings. (In 11397 other words, the whole surrogate thing is a fudge for UTF-16 which un- 11398 fortunately messes up UTF-8 and UTF-32.) 11399 11400 Setting PCRE2_NO_UTF_CHECK at compile time does not disable the error 11401 that is given if an escape sequence for an invalid Unicode code point 11402 is encountered in the pattern. If you want to allow escape sequences 11403 such as \x{d800} (a surrogate code point) you can set the PCRE2_EX- 11404 TRA_ALLOW_SURROGATE_ESCAPES extra option. However, this is possible 11405 only in UTF-8 and UTF-32 modes, because these values are not repre- 11406 sentable in UTF-16. 11407 11408 Errors in UTF-8 strings 11409 11410 The following negative error codes are given for invalid UTF-8 strings: 11411 11412 PCRE2_ERROR_UTF8_ERR1 11413 PCRE2_ERROR_UTF8_ERR2 11414 PCRE2_ERROR_UTF8_ERR3 11415 PCRE2_ERROR_UTF8_ERR4 11416 PCRE2_ERROR_UTF8_ERR5 11417 11418 The string ends with a truncated UTF-8 character; the code specifies 11419 how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8 11420 characters to be no longer than 4 bytes, the encoding scheme (origi- 11421 nally defined by RFC 2279) allows for up to 6 bytes, and this is 11422 checked first; hence the possibility of 4 or 5 missing bytes. 11423 11424 PCRE2_ERROR_UTF8_ERR6 11425 PCRE2_ERROR_UTF8_ERR7 11426 PCRE2_ERROR_UTF8_ERR8 11427 PCRE2_ERROR_UTF8_ERR9 11428 PCRE2_ERROR_UTF8_ERR10 11429 11430 The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of 11431 the character do not have the binary value 0b10 (that is, either the 11432 most significant bit is 0, or the next bit is 1). 11433 11434 PCRE2_ERROR_UTF8_ERR11 11435 PCRE2_ERROR_UTF8_ERR12 11436 11437 A character that is valid by the RFC 2279 rules is either 5 or 6 bytes 11438 long; these code points are excluded by RFC 3629. 11439 11440 PCRE2_ERROR_UTF8_ERR13 11441 11442 A 4-byte character has a value greater than 0x10ffff; these code points 11443 are excluded by RFC 3629. 11444 11445 PCRE2_ERROR_UTF8_ERR14 11446 11447 A 3-byte character has a value in the range 0xd800 to 0xdfff; this 11448 range of code points are reserved by RFC 3629 for use with UTF-16, and 11449 so are excluded from UTF-8. 11450 11451 PCRE2_ERROR_UTF8_ERR15 11452 PCRE2_ERROR_UTF8_ERR16 11453 PCRE2_ERROR_UTF8_ERR17 11454 PCRE2_ERROR_UTF8_ERR18 11455 PCRE2_ERROR_UTF8_ERR19 11456 11457 A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes 11458 for a value that can be represented by fewer bytes, which is invalid. 11459 For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor- 11460 rect coding uses just one byte. 11461 11462 PCRE2_ERROR_UTF8_ERR20 11463 11464 The two most significant bits of the first byte of a character have the 11465 binary value 0b10 (that is, the most significant bit is 1 and the sec- 11466 ond is 0). Such a byte can only validly occur as the second or subse- 11467 quent byte of a multi-byte character. 11468 11469 PCRE2_ERROR_UTF8_ERR21 11470 11471 The first byte of a character has the value 0xfe or 0xff. These values 11472 can never occur in a valid UTF-8 string. 11473 11474 Errors in UTF-16 strings 11475 11476 The following negative error codes are given for invalid UTF-16 11477 strings: 11478 11479 PCRE2_ERROR_UTF16_ERR1 Missing low surrogate at end of string 11480 PCRE2_ERROR_UTF16_ERR2 Invalid low surrogate follows high surrogate 11481 PCRE2_ERROR_UTF16_ERR3 Isolated low surrogate 11482 11483 11484 Errors in UTF-32 strings 11485 11486 The following negative error codes are given for invalid UTF-32 11487 strings: 11488 11489 PCRE2_ERROR_UTF32_ERR1 Surrogate character (0xd800 to 0xdfff) 11490 PCRE2_ERROR_UTF32_ERR2 Code point is greater than 0x10ffff 11491 11492 11493MATCHING IN INVALID UTF STRINGS 11494 11495 You can run pattern matches on subject strings that may contain invalid 11496 UTF sequences if you call pcre2_compile() with the PCRE2_MATCH_IN- 11497 VALID_UTF option. This is supported by pcre2_match(), including JIT 11498 matching, but not by pcre2_dfa_match(). When PCRE2_MATCH_INVALID_UTF is 11499 set, it forces PCRE2_UTF to be set as well. Note, however, that the 11500 pattern itself must be a valid UTF string. 11501 11502 Setting PCRE2_MATCH_INVALID_UTF does not affect what pcre2_compile() 11503 generates, but if pcre2_jit_compile() is subsequently called, it does 11504 generate different code. If JIT is not used, the option affects the be- 11505 haviour of the interpretive code in pcre2_match(). When PCRE2_MATCH_IN- 11506 VALID_UTF is set at compile time, PCRE2_NO_UTF_CHECK is ignored at 11507 match time. 11508 11509 In this mode, an invalid code unit sequence in the subject never 11510 matches any pattern item. It does not match dot, it does not match 11511 \p{Any}, it does not even match negative items such as [^X]. A lookbe- 11512 hind assertion fails if it encounters an invalid sequence while moving 11513 the current point backwards. In other words, an invalid UTF code unit 11514 sequence acts as a barrier which no match can cross. 11515 11516 You can also think of this as the subject being split up into fragments 11517 of valid UTF, delimited internally by invalid code unit sequences. The 11518 pattern is matched fragment by fragment. The result of a successful 11519 match, however, is given as code unit offsets in the entire subject 11520 string in the usual way. There are a few points to consider: 11521 11522 The internal boundaries are not interpreted as the beginnings or ends 11523 of lines and so do not match circumflex or dollar characters in the 11524 pattern. 11525 11526 If pcre2_match() is called with an offset that points to an invalid 11527 UTF-sequence, that sequence is skipped, and the match starts at the 11528 next valid UTF character, or the end of the subject. 11529 11530 At internal fragment boundaries, \b and \B behave in the same way as at 11531 the beginning and end of the subject. For example, a sequence such as 11532 \bWORD\b would match an instance of WORD that is surrounded by invalid 11533 UTF code units. 11534 11535 Using PCRE2_MATCH_INVALID_UTF, an application can run matches on arbi- 11536 trary data, knowing that any matched strings that are returned are 11537 valid UTF. This can be useful when searching for UTF text in executable 11538 or other binary files. 11539 11540 11541AUTHOR 11542 11543 Philip Hazel 11544 Retired from University Computing Service 11545 Cambridge, England. 11546 11547 11548REVISION 11549 11550 Last updated: 22 December 2021 11551 Copyright (c) 1997-2021 University of Cambridge. 11552------------------------------------------------------------------------------ 11553 11554 11555