1<html> 2<head> 3<title>pcrepattern specification</title> 4</head> 5<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"> 6<h1>pcrepattern man page</h1> 7<p> 8Return to the <a href="index.html">PCRE index page</a>. 9</p> 10<p> 11This page is part of the PCRE HTML documentation. It was generated automatically 12from the original man page. If there is any nonsense in it, please consult the 13man page, in case the conversion went wrong. 14<br> 15<ul> 16<li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a> 17<li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a> 18<li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a> 19<li><a name="TOC4" href="#SEC4">BACKSLASH</a> 20<li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a> 21<li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT) AND \N</a> 22<li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a> 23<li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a> 24<li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a> 25<li><a name="TOC10" href="#SEC10">VERTICAL BAR</a> 26<li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a> 27<li><a name="TOC12" href="#SEC12">SUBPATTERNS</a> 28<li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a> 29<li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a> 30<li><a name="TOC15" href="#SEC15">REPETITION</a> 31<li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a> 32<li><a name="TOC17" href="#SEC17">BACK REFERENCES</a> 33<li><a name="TOC18" href="#SEC18">ASSERTIONS</a> 34<li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a> 35<li><a name="TOC20" href="#SEC20">COMMENTS</a> 36<li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a> 37<li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a> 38<li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a> 39<li><a name="TOC24" href="#SEC24">CALLOUTS</a> 40<li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a> 41<li><a name="TOC26" href="#SEC26">SEE ALSO</a> 42<li><a name="TOC27" href="#SEC27">AUTHOR</a> 43<li><a name="TOC28" href="#SEC28">REVISION</a> 44</ul> 45<br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br> 46<P> 47The syntax and semantics of the regular expressions that are supported by PCRE 48are described in detail below. There is a quick-reference syntax summary in the 49<a href="pcresyntax.html"><b>pcresyntax</b></a> 50page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE 51also supports some alternative regular expression syntax (which does not 52conflict with the Perl syntax) in order to provide some compatibility with 53regular expressions in Python, .NET, and Oniguruma. 54</P> 55<P> 56Perl's regular expressions are described in its own documentation, and 57regular expressions in general are covered in a number of books, some of which 58have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", 59published by O'Reilly, covers regular expressions in great detail. This 60description of PCRE's regular expressions is intended as reference material. 61</P> 62<P> 63The original operation of PCRE was on strings of one-byte characters. However, 64there is now also support for UTF-8 character strings. To use this, 65PCRE must be built to include UTF-8 support, and you must call 66<b>pcre_compile()</b> or <b>pcre_compile2()</b> with the PCRE_UTF8 option. There 67is also a special sequence that can be given at the start of a pattern: 68<pre> 69 (*UTF8) 70</pre> 71Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8 72option. This feature is not Perl-compatible. How setting UTF-8 mode affects 73pattern matching is mentioned in several places below. There is also a summary 74of UTF-8 features in the 75<a href="pcre.html#utf8support">section on UTF-8 support</a> 76in the main 77<a href="pcre.html"><b>pcre</b></a> 78page. 79</P> 80<P> 81Another special sequence that may appear at the start of a pattern or in 82combination with (*UTF8) is: 83<pre> 84 (*UCP) 85</pre> 86This has the same effect as setting the PCRE_UCP option: it causes sequences 87such as \d and \w to use Unicode properties to determine character types, 88instead of recognizing only characters with codes less than 128 via a lookup 89table. 90</P> 91<P> 92If a pattern starts with (*NO_START_OPT), it has the same effect as setting the 93PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are 94also some more of these special sequences that are concerned with the handling 95of newlines; they are described below. 96</P> 97<P> 98The remainder of this document discusses the patterns that are supported by 99PCRE when its main matching function, <b>pcre_exec()</b>, is used. 100From release 6.0, PCRE offers a second matching function, 101<b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not 102Perl-compatible. Some of the features discussed below are not available when 103<b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the 104alternative function, and how it differs from the normal function, are 105discussed in the 106<a href="pcrematching.html"><b>pcrematching</b></a> 107page. 108<a name="newlines"></a></P> 109<br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br> 110<P> 111PCRE supports five different conventions for indicating line breaks in 112strings: a single CR (carriage return) character, a single LF (linefeed) 113character, the two-character sequence CRLF, any of the three preceding, or any 114Unicode newline sequence. The 115<a href="pcreapi.html"><b>pcreapi</b></a> 116page has 117<a href="pcreapi.html#newlines">further discussion</a> 118about newlines, and shows how to set the newline convention in the 119<i>options</i> arguments for the compiling and matching functions. 120</P> 121<P> 122It is also possible to specify a newline convention by starting a pattern 123string with one of the following five sequences: 124<pre> 125 (*CR) carriage return 126 (*LF) linefeed 127 (*CRLF) carriage return, followed by linefeed 128 (*ANYCRLF) any of the three above 129 (*ANY) all Unicode newline sequences 130</pre> 131These override the default and the options given to <b>pcre_compile()</b> or 132<b>pcre_compile2()</b>. For example, on a Unix system where LF is the default 133newline sequence, the pattern 134<pre> 135 (*CR)a.b 136</pre> 137changes the convention to CR. That pattern matches "a\nb" because LF is no 138longer a newline. Note that these special settings, which are not 139Perl-compatible, are recognized only at the very start of a pattern, and that 140they must be in upper case. If more than one of them is present, the last one 141is used. 142</P> 143<P> 144The newline convention affects the interpretation of the dot metacharacter when 145PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not 146affect what the \R escape sequence matches. By default, this is any Unicode 147newline sequence, for Perl compatibility. However, this can be changed; see the 148description of \R in the section entitled 149<a href="#newlineseq">"Newline sequences"</a> 150below. A change of \R setting can be combined with a change of newline 151convention. 152</P> 153<br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br> 154<P> 155A regular expression is a pattern that is matched against a subject string from 156left to right. Most characters stand for themselves in a pattern, and match the 157corresponding characters in the subject. As a trivial example, the pattern 158<pre> 159 The quick brown fox 160</pre> 161matches a portion of a subject string that is identical to itself. When 162caseless matching is specified (the PCRE_CASELESS option), letters are matched 163independently of case. In UTF-8 mode, PCRE always understands the concept of 164case for characters whose values are less than 128, so caseless matching is 165always possible. For characters with higher values, the concept of case is 166supported if PCRE is compiled with Unicode property support, but not otherwise. 167If you want to use caseless matching for characters 128 and above, you must 168ensure that PCRE is compiled with Unicode property support as well as with 169UTF-8 support. 170</P> 171<P> 172The power of regular expressions comes from the ability to include alternatives 173and repetitions in the pattern. These are encoded in the pattern by the use of 174<i>metacharacters</i>, which do not stand for themselves but instead are 175interpreted in some special way. 176</P> 177<P> 178There are two different sets of metacharacters: those that are recognized 179anywhere in the pattern except within square brackets, and those that are 180recognized within square brackets. Outside square brackets, the metacharacters 181are as follows: 182<pre> 183 \ general escape character with several uses 184 ^ assert start of string (or line, in multiline mode) 185 $ assert end of string (or line, in multiline mode) 186 . match any character except newline (by default) 187 [ start character class definition 188 | start of alternative branch 189 ( start subpattern 190 ) end subpattern 191 ? extends the meaning of ( 192 also 0 or 1 quantifier 193 also quantifier minimizer 194 * 0 or more quantifier 195 + 1 or more quantifier 196 also "possessive quantifier" 197 { start min/max quantifier 198</pre> 199Part of a pattern that is in square brackets is called a "character class". In 200a character class the only metacharacters are: 201<pre> 202 \ general escape character 203 ^ negate the class, but only if the first character 204 - indicates character range 205 [ POSIX character class (only if followed by POSIX syntax) 206 ] terminates the character class 207</pre> 208The following sections describe the use of each of the metacharacters. 209</P> 210<br><a name="SEC4" href="#TOC1">BACKSLASH</a><br> 211<P> 212The backslash character has several uses. Firstly, if it is followed by a 213character that is not a number or a letter, it takes away any special meaning 214that character may have. This use of backslash as an escape character applies 215both inside and outside character classes. 216</P> 217<P> 218For example, if you want to match a * character, you write \* in the pattern. 219This escaping action applies whether or not the following character would 220otherwise be interpreted as a metacharacter, so it is always safe to precede a 221non-alphanumeric with backslash to specify that it stands for itself. In 222particular, if you want to match a backslash, you write \\. 223</P> 224<P> 225In UTF-8 mode, only ASCII numbers and letters have any special meaning after a 226backslash. All other characters (in particular, those whose codepoints are 227greater than 127) are treated as literals. 228</P> 229<P> 230If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the 231pattern (other than in a character class) and characters between a # outside 232a character class and the next newline are ignored. An escaping backslash can 233be used to include a whitespace or # character as part of the pattern. 234</P> 235<P> 236If you want to remove the special meaning from a sequence of characters, you 237can do so by putting them between \Q and \E. This is different from Perl in 238that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in 239Perl, $ and @ cause variable interpolation. Note the following examples: 240<pre> 241 Pattern PCRE matches Perl matches 242 243 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz 244 \Qabc\$xyz\E abc\$xyz abc\$xyz 245 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 246</pre> 247The \Q...\E sequence is recognized both inside and outside character classes. 248An isolated \E that is not preceded by \Q is ignored. 249<a name="digitsafterbackslash"></a></P> 250<br><b> 251Non-printing characters 252</b><br> 253<P> 254A second use of backslash provides a way of encoding non-printing characters 255in patterns in a visible manner. There is no restriction on the appearance of 256non-printing characters, apart from the binary zero that terminates a pattern, 257but when a pattern is being prepared by text editing, it is often easier to use 258one of the following escape sequences than the binary character it represents: 259<pre> 260 \a alarm, that is, the BEL character (hex 07) 261 \cx "control-x", where x is any ASCII character 262 \e escape (hex 1B) 263 \f formfeed (hex 0C) 264 \n linefeed (hex 0A) 265 \r carriage return (hex 0D) 266 \t tab (hex 09) 267 \ddd character with octal code ddd, or back reference 268 \xhh character with hex code hh 269 \x{hhh..} character with hex code hhh.. 270</pre> 271The precise effect of \cx is as follows: if x is a lower case letter, it 272is converted to upper case. Then bit 6 of the character (hex 40) is inverted. 273Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while 274\c; becomes hex 7B (; is 3B). If the byte following \c has a value greater 275than 127, a compile-time error occurs. This locks out non-ASCII characters in 276both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte 277values are valid. A lower case letter is converted to upper case, and then the 2780xc0 bits are flipped.) 279</P> 280<P> 281After \x, from zero to two hexadecimal digits are read (letters can be in 282upper or lower case). Any number of hexadecimal digits may appear between \x{ 283and }, but the value of the character code must be less than 256 in non-UTF-8 284mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in 285hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code 286point, which is 10FFFF. 287</P> 288<P> 289If characters other than hexadecimal digits appear between \x{ and }, or if 290there is no terminating }, this form of escape is not recognized. Instead, the 291initial \x will be interpreted as a basic hexadecimal escape, with no 292following digits, giving a character whose value is zero. 293</P> 294<P> 295Characters whose value is less than 256 can be defined by either of the two 296syntaxes for \x. There is no difference in the way they are handled. For 297example, \xdc is exactly the same as \x{dc}. 298</P> 299<P> 300After \0 up to two further octal digits are read. If there are fewer than two 301digits, just those that are present are used. Thus the sequence \0\x\07 302specifies two binary zeros followed by a BEL character (code value 7). Make 303sure you supply two digits after the initial zero if the pattern character that 304follows is itself an octal digit. 305</P> 306<P> 307The handling of a backslash followed by a digit other than 0 is complicated. 308Outside a character class, PCRE reads it and any following digits as a decimal 309number. If the number is less than 10, or if there have been at least that many 310previous capturing left parentheses in the expression, the entire sequence is 311taken as a <i>back reference</i>. A description of how this works is given 312<a href="#backreferences">later,</a> 313following the discussion of 314<a href="#subpattern">parenthesized subpatterns.</a> 315</P> 316<P> 317Inside a character class, or if the decimal number is greater than 9 and there 318have not been that many capturing subpatterns, PCRE re-reads up to three octal 319digits following the backslash, and uses them to generate a data character. Any 320subsequent digits stand for themselves. In non-UTF-8 mode, the value of a 321character specified in octal must be less than \400. In UTF-8 mode, values up 322to \777 are permitted. For example: 323<pre> 324 \040 is another way of writing a space 325 \40 is the same, provided there are fewer than 40 previous capturing subpatterns 326 \7 is always a back reference 327 \11 might be a back reference, or another way of writing a tab 328 \011 is always a tab 329 \0113 is a tab followed by the character "3" 330 \113 might be a back reference, otherwise the character with octal code 113 331 \377 might be a back reference, otherwise the byte consisting entirely of 1 bits 332 \81 is either a back reference, or a binary zero followed by the two characters "8" and "1" 333</pre> 334Note that octal values of 100 or greater must not be introduced by a leading 335zero, because no more than three octal digits are ever read. 336</P> 337<P> 338All the sequences that define a single character value can be used both inside 339and outside character classes. In addition, inside a character class, the 340sequence \b is interpreted as the backspace character (hex 08). The sequences 341\B, \N, \R, and \X are not special inside a character class. Like any other 342unrecognized escape sequences, they are treated as the literal characters "B", 343"N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is 344set. Outside a character class, these sequences have different meanings. 345</P> 346<br><b> 347Absolute and relative back references 348</b><br> 349<P> 350The sequence \g followed by an unsigned or a negative number, optionally 351enclosed in braces, is an absolute or relative back reference. A named back 352reference can be coded as \g{name}. Back references are discussed 353<a href="#backreferences">later,</a> 354following the discussion of 355<a href="#subpattern">parenthesized subpatterns.</a> 356</P> 357<br><b> 358Absolute and relative subroutine calls 359</b><br> 360<P> 361For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or 362a number enclosed either in angle brackets or single quotes, is an alternative 363syntax for referencing a subpattern as a "subroutine". Details are discussed 364<a href="#onigurumasubroutines">later.</a> 365Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i> 366synonymous. The former is a back reference; the latter is a 367<a href="#subpatternsassubroutines">subroutine</a> 368call. 369<a name="genericchartypes"></a></P> 370<br><b> 371Generic character types 372</b><br> 373<P> 374Another use of backslash is for specifying generic character types: 375<pre> 376 \d any decimal digit 377 \D any character that is not a decimal digit 378 \h any horizontal whitespace character 379 \H any character that is not a horizontal whitespace character 380 \s any whitespace character 381 \S any character that is not a whitespace character 382 \v any vertical whitespace character 383 \V any character that is not a vertical whitespace character 384 \w any "word" character 385 \W any "non-word" character 386</pre> 387There is also the single sequence \N, which matches a non-newline character. 388This is the same as 389<a href="#fullstopdot">the "." metacharacter</a> 390when PCRE_DOTALL is not set. 391</P> 392<P> 393Each pair of lower and upper case escape sequences partitions the complete set 394of characters into two disjoint sets. Any given character matches one, and only 395one, of each pair. The sequences can appear both inside and outside character 396classes. They each match one character of the appropriate type. If the current 397matching point is at the end of the subject string, all of them fail, because 398there is no character to match. 399</P> 400<P> 401For compatibility with Perl, \s does not match the VT character (code 11). 402This makes it different from the the POSIX "space" class. The \s characters 403are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is 404included in a Perl script, \s may match the VT character. In PCRE, it never 405does. 406</P> 407<P> 408A "word" character is an underscore or any character that is a letter or digit. 409By default, the definition of letters and digits is controlled by PCRE's 410low-valued character tables, and may vary if locale-specific matching is taking 411place (see 412<a href="pcreapi.html#localesupport">"Locale support"</a> 413in the 414<a href="pcreapi.html"><b>pcreapi</b></a> 415page). For example, in a French locale such as "fr_FR" in Unix-like systems, 416or "french" in Windows, some character codes greater than 128 are used for 417accented letters, and these are then matched by \w. The use of locales with 418Unicode is discouraged. 419</P> 420<P> 421By default, in UTF-8 mode, characters with values greater than 128 never match 422\d, \s, or \w, and always match \D, \S, and \W. These sequences retain 423their original meanings from before UTF-8 support was available, mainly for 424efficiency reasons. However, if PCRE is compiled with Unicode property support, 425and the PCRE_UCP option is set, the behaviour is changed so that Unicode 426properties are used to determine character types, as follows: 427<pre> 428 \d any character that \p{Nd} matches (decimal digit) 429 \s any character that \p{Z} matches, plus HT, LF, FF, CR 430 \w any character that \p{L} or \p{N} matches, plus underscore 431</pre> 432The upper case escapes match the inverse sets of characters. Note that \d 433matches only decimal digits, whereas \w matches any Unicode digit, as well as 434any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and 435\B because they are defined in terms of \w and \W. Matching these sequences 436is noticeably slower when PCRE_UCP is set. 437</P> 438<P> 439The sequences \h, \H, \v, and \V are features that were added to Perl at 440release 5.10. In contrast to the other sequences, which match only ASCII 441characters by default, these always match certain high-valued codepoints in 442UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters 443are: 444<pre> 445 U+0009 Horizontal tab 446 U+0020 Space 447 U+00A0 Non-break space 448 U+1680 Ogham space mark 449 U+180E Mongolian vowel separator 450 U+2000 En quad 451 U+2001 Em quad 452 U+2002 En space 453 U+2003 Em space 454 U+2004 Three-per-em space 455 U+2005 Four-per-em space 456 U+2006 Six-per-em space 457 U+2007 Figure space 458 U+2008 Punctuation space 459 U+2009 Thin space 460 U+200A Hair space 461 U+202F Narrow no-break space 462 U+205F Medium mathematical space 463 U+3000 Ideographic space 464</pre> 465The vertical space characters are: 466<pre> 467 U+000A Linefeed 468 U+000B Vertical tab 469 U+000C Formfeed 470 U+000D Carriage return 471 U+0085 Next line 472 U+2028 Line separator 473 U+2029 Paragraph separator 474<a name="newlineseq"></a></PRE> 475</P> 476<br><b> 477Newline sequences 478</b><br> 479<P> 480Outside a character class, by default, the escape sequence \R matches any 481Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the following: 482<pre> 483 (?>\r\n|\n|\x0b|\f|\r|\x85) 484</pre> 485This is an example of an "atomic group", details of which are given 486<a href="#atomicgroup">below.</a> 487This particular group matches either the two-character sequence CR followed by 488LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, 489U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next 490line, U+0085). The two-character sequence is treated as a single unit that 491cannot be split. 492</P> 493<P> 494In UTF-8 mode, two additional characters whose codepoints are greater than 255 495are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). 496Unicode character property support is not needed for these characters to be 497recognized. 498</P> 499<P> 500It is possible to restrict \R to match only CR, LF, or CRLF (instead of the 501complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF 502either at compile time or when the pattern is matched. (BSR is an abbrevation 503for "backslash R".) This can be made the default when PCRE is built; if this is 504the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option. 505It is also possible to specify these settings by starting a pattern string with 506one of the following sequences: 507<pre> 508 (*BSR_ANYCRLF) CR, LF, or CRLF only 509 (*BSR_UNICODE) any Unicode newline sequence 510</pre> 511These override the default and the options given to <b>pcre_compile()</b> or 512<b>pcre_compile2()</b>, but they can be overridden by options given to 513<b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. Note that these special settings, 514which are not Perl-compatible, are recognized only at the very start of a 515pattern, and that they must be in upper case. If more than one of them is 516present, the last one is used. They can be combined with a change of newline 517convention; for example, a pattern can start with: 518<pre> 519 (*ANY)(*BSR_ANYCRLF) 520</pre> 521They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside 522a character class, \R is treated as an unrecognized escape sequence, and so 523matches the letter "R" by default, but causes an error if PCRE_EXTRA is set. 524<a name="uniextseq"></a></P> 525<br><b> 526Unicode character properties 527</b><br> 528<P> 529When PCRE is built with Unicode character property support, three additional 530escape sequences that match characters with specific properties are available. 531When not in UTF-8 mode, these sequences are of course limited to testing 532characters whose codepoints are less than 256, but they do work in this mode. 533The extra escape sequences are: 534<pre> 535 \p{<i>xx</i>} a character with the <i>xx</i> property 536 \P{<i>xx</i>} a character without the <i>xx</i> property 537 \X an extended Unicode sequence 538</pre> 539The property names represented by <i>xx</i> above are limited to the Unicode 540script names, the general category properties, "Any", which matches any 541character (including newline), and some special PCRE properties (described 542in the 543<a href="#extraprops">next section).</a> 544Other Perl properties such as "InMusicalSymbols" are not currently supported by 545PCRE. Note that \P{Any} does not match any characters, so always causes a 546match failure. 547</P> 548<P> 549Sets of Unicode characters are defined as belonging to certain scripts. A 550character from one of these sets can be matched using a script name. For 551example: 552<pre> 553 \p{Greek} 554 \P{Han} 555</pre> 556Those that are not part of an identified script are lumped together as 557"Common". The current list of scripts is: 558</P> 559<P> 560Arabic, 561Armenian, 562Avestan, 563Balinese, 564Bamum, 565Bengali, 566Bopomofo, 567Braille, 568Buginese, 569Buhid, 570Canadian_Aboriginal, 571Carian, 572Cham, 573Cherokee, 574Common, 575Coptic, 576Cuneiform, 577Cypriot, 578Cyrillic, 579Deseret, 580Devanagari, 581Egyptian_Hieroglyphs, 582Ethiopic, 583Georgian, 584Glagolitic, 585Gothic, 586Greek, 587Gujarati, 588Gurmukhi, 589Han, 590Hangul, 591Hanunoo, 592Hebrew, 593Hiragana, 594Imperial_Aramaic, 595Inherited, 596Inscriptional_Pahlavi, 597Inscriptional_Parthian, 598Javanese, 599Kaithi, 600Kannada, 601Katakana, 602Kayah_Li, 603Kharoshthi, 604Khmer, 605Lao, 606Latin, 607Lepcha, 608Limbu, 609Linear_B, 610Lisu, 611Lycian, 612Lydian, 613Malayalam, 614Meetei_Mayek, 615Mongolian, 616Myanmar, 617New_Tai_Lue, 618Nko, 619Ogham, 620Old_Italic, 621Old_Persian, 622Old_South_Arabian, 623Old_Turkic, 624Ol_Chiki, 625Oriya, 626Osmanya, 627Phags_Pa, 628Phoenician, 629Rejang, 630Runic, 631Samaritan, 632Saurashtra, 633Shavian, 634Sinhala, 635Sundanese, 636Syloti_Nagri, 637Syriac, 638Tagalog, 639Tagbanwa, 640Tai_Le, 641Tai_Tham, 642Tai_Viet, 643Tamil, 644Telugu, 645Thaana, 646Thai, 647Tibetan, 648Tifinagh, 649Ugaritic, 650Vai, 651Yi. 652</P> 653<P> 654Each character has exactly one Unicode general category property, specified by 655a two-letter abbreviation. For compatibility with Perl, negation can be 656specified by including a circumflex between the opening brace and the property 657name. For example, \p{^Lu} is the same as \P{Lu}. 658</P> 659<P> 660If only one letter is specified with \p or \P, it includes all the general 661category properties that start with that letter. In this case, in the absence 662of negation, the curly brackets in the escape sequence are optional; these two 663examples have the same effect: 664<pre> 665 \p{L} 666 \pL 667</pre> 668The following general category property codes are supported: 669<pre> 670 C Other 671 Cc Control 672 Cf Format 673 Cn Unassigned 674 Co Private use 675 Cs Surrogate 676 677 L Letter 678 Ll Lower case letter 679 Lm Modifier letter 680 Lo Other letter 681 Lt Title case letter 682 Lu Upper case letter 683 684 M Mark 685 Mc Spacing mark 686 Me Enclosing mark 687 Mn Non-spacing mark 688 689 N Number 690 Nd Decimal number 691 Nl Letter number 692 No Other number 693 694 P Punctuation 695 Pc Connector punctuation 696 Pd Dash punctuation 697 Pe Close punctuation 698 Pf Final punctuation 699 Pi Initial punctuation 700 Po Other punctuation 701 Ps Open punctuation 702 703 S Symbol 704 Sc Currency symbol 705 Sk Modifier symbol 706 Sm Mathematical symbol 707 So Other symbol 708 709 Z Separator 710 Zl Line separator 711 Zp Paragraph separator 712 Zs Space separator 713</pre> 714The special property L& is also supported: it matches a character that has 715the Lu, Ll, or Lt property, in other words, a letter that is not classified as 716a modifier or "other". 717</P> 718<P> 719The Cs (Surrogate) property applies only to characters in the range U+D800 to 720U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so 721cannot be tested by PCRE, unless UTF-8 validity checking has been turned off 722(see the discussion of PCRE_NO_UTF8_CHECK in the 723<a href="pcreapi.html"><b>pcreapi</b></a> 724page). Perl does not support the Cs property. 725</P> 726<P> 727The long synonyms for property names that Perl supports (such as \p{Letter}) 728are not supported by PCRE, nor is it permitted to prefix any of these 729properties with "Is". 730</P> 731<P> 732No character that is in the Unicode table has the Cn (unassigned) property. 733Instead, this property is assumed for any code point that is not in the 734Unicode table. 735</P> 736<P> 737Specifying caseless matching does not affect these escape sequences. For 738example, \p{Lu} always matches only upper case letters. 739</P> 740<P> 741The \X escape matches any number of Unicode characters that form an extended 742Unicode sequence. \X is equivalent to 743<pre> 744 (?>\PM\pM*) 745</pre> 746That is, it matches a character without the "mark" property, followed by zero 747or more characters with the "mark" property, and treats the sequence as an 748atomic group 749<a href="#atomicgroup">(see below).</a> 750Characters with the "mark" property are typically accents that affect the 751preceding character. None of them have codepoints less than 256, so in 752non-UTF-8 mode \X matches any one character. 753</P> 754<P> 755Matching characters by Unicode property is not fast, because PCRE has to search 756a structure that contains data for over fifteen thousand characters. That is 757why the traditional escape sequences such as \d and \w do not use Unicode 758properties in PCRE by default, though you can make them do so by setting the 759PCRE_UCP option for <b>pcre_compile()</b> or by starting the pattern with 760(*UCP). 761<a name="extraprops"></a></P> 762<br><b> 763PCRE's additional properties 764</b><br> 765<P> 766As well as the standard Unicode properties described in the previous 767section, PCRE supports four more that make it possible to convert traditional 768escape sequences such as \w and \s and POSIX character classes to use Unicode 769properties. PCRE uses these non-standard, non-Perl properties internally when 770PCRE_UCP is set. They are: 771<pre> 772 Xan Any alphanumeric character 773 Xps Any POSIX space character 774 Xsp Any Perl space character 775 Xwd Any Perl "word" character 776</pre> 777Xan matches characters that have either the L (letter) or the N (number) 778property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or 779carriage return, and any other character that has the Z (separator) property. 780Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the 781same characters as Xan, plus underscore. 782<a name="resetmatchstart"></a></P> 783<br><b> 784Resetting the match start 785</b><br> 786<P> 787The escape sequence \K causes any previously matched characters not to be 788included in the final matched sequence. For example, the pattern: 789<pre> 790 foo\Kbar 791</pre> 792matches "foobar", but reports that it has matched "bar". This feature is 793similar to a lookbehind assertion 794<a href="#lookbehind">(described below).</a> 795However, in this case, the part of the subject before the real match does not 796have to be of fixed length, as lookbehind assertions do. The use of \K does 797not interfere with the setting of 798<a href="#subpattern">captured substrings.</a> 799For example, when the pattern 800<pre> 801 (foo)\Kbar 802</pre> 803matches "foobar", the first substring is still set to "foo". 804</P> 805<P> 806Perl documents that the use of \K within assertions is "not well defined". In 807PCRE, \K is acted upon when it occurs inside positive assertions, but is 808ignored in negative assertions. 809<a name="smallassertions"></a></P> 810<br><b> 811Simple assertions 812</b><br> 813<P> 814The final use of backslash is for certain simple assertions. An assertion 815specifies a condition that has to be met at a particular point in a match, 816without consuming any characters from the subject string. The use of 817subpatterns for more complicated assertions is described 818<a href="#bigassertions">below.</a> 819The backslashed assertions are: 820<pre> 821 \b matches at a word boundary 822 \B matches when not at a word boundary 823 \A matches at the start of the subject 824 \Z matches at the end of the subject 825 also matches before a newline at the end of the subject 826 \z matches only at the end of the subject 827 \G matches at the first matching position in the subject 828</pre> 829Inside a character class, \b has a different meaning; it matches the backspace 830character. If any other of these assertions appears in a character class, by 831default it matches the corresponding literal character (for example, \B 832matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid 833escape sequence" error is generated instead. 834</P> 835<P> 836A word boundary is a position in the subject string where the current character 837and the previous character do not both match \w or \W (i.e. one matches 838\w and the other matches \W), or the start or end of the string if the 839first or last character matches \w, respectively. In UTF-8 mode, the meanings 840of \w and \W can be changed by setting the PCRE_UCP option. When this is 841done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start 842of word" or "end of word" metasequence. However, whatever follows \b normally 843determines which it is. For example, the fragment \ba matches "a" at the start 844of a word. 845</P> 846<P> 847The \A, \Z, and \z assertions differ from the traditional circumflex and 848dollar (described in the next section) in that they only ever match at the very 849start and end of the subject string, whatever options are set. Thus, they are 850independent of multiline mode. These three assertions are not affected by the 851PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the 852circumflex and dollar metacharacters. However, if the <i>startoffset</i> 853argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start 854at a point other than the beginning of the subject, \A can never match. The 855difference between \Z and \z is that \Z matches before a newline at the end 856of the string as well as at the very end, whereas \z matches only at the end. 857</P> 858<P> 859The \G assertion is true only when the current matching position is at the 860start point of the match, as specified by the <i>startoffset</i> argument of 861<b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is 862non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate 863arguments, you can mimic Perl's /g option, and it is in this kind of 864implementation where \G can be useful. 865</P> 866<P> 867Note, however, that PCRE's interpretation of \G, as the start of the current 868match, is subtly different from Perl's, which defines it as the end of the 869previous match. In Perl, these can be different when the previously matched 870string was empty. Because PCRE does just one match at a time, it cannot 871reproduce this behaviour. 872</P> 873<P> 874If all the alternatives of a pattern begin with \G, the expression is anchored 875to the starting match position, and the "anchored" flag is set in the compiled 876regular expression. 877</P> 878<br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br> 879<P> 880Outside a character class, in the default matching mode, the circumflex 881character is an assertion that is true only if the current matching point is 882at the start of the subject string. If the <i>startoffset</i> argument of 883<b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE 884option is unset. Inside a character class, circumflex has an entirely different 885meaning 886<a href="#characterclass">(see below).</a> 887</P> 888<P> 889Circumflex need not be the first character of the pattern if a number of 890alternatives are involved, but it should be the first thing in each alternative 891in which it appears if the pattern is ever to match that branch. If all 892possible alternatives start with a circumflex, that is, if the pattern is 893constrained to match only at the start of the subject, it is said to be an 894"anchored" pattern. (There are also other constructs that can cause a pattern 895to be anchored.) 896</P> 897<P> 898A dollar character is an assertion that is true only if the current matching 899point is at the end of the subject string, or immediately before a newline 900at the end of the string (by default). Dollar need not be the last character of 901the pattern if a number of alternatives are involved, but it should be the last 902item in any branch in which it appears. Dollar has no special meaning in a 903character class. 904</P> 905<P> 906The meaning of dollar can be changed so that it matches only at the very end of 907the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This 908does not affect the \Z assertion. 909</P> 910<P> 911The meanings of the circumflex and dollar characters are changed if the 912PCRE_MULTILINE option is set. When this is the case, a circumflex matches 913immediately after internal newlines as well as at the start of the subject 914string. It does not match after a newline that ends the string. A dollar 915matches before any newlines in the string, as well as at the very end, when 916PCRE_MULTILINE is set. When newline is specified as the two-character 917sequence CRLF, isolated CR and LF characters do not indicate newlines. 918</P> 919<P> 920For example, the pattern /^abc$/ matches the subject string "def\nabc" (where 921\n represents a newline) in multiline mode, but not otherwise. Consequently, 922patterns that are anchored in single line mode because all branches start with 923^ are not anchored in multiline mode, and a match for circumflex is possible 924when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The 925PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. 926</P> 927<P> 928Note that the sequences \A, \Z, and \z can be used to match the start and 929end of the subject in both modes, and if all branches of a pattern start with 930\A it is always anchored, whether or not PCRE_MULTILINE is set. 931<a name="fullstopdot"></a></P> 932<br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br> 933<P> 934Outside a character class, a dot in the pattern matches any one character in 935the subject string except (by default) a character that signifies the end of a 936line. In UTF-8 mode, the matched character may be more than one byte long. 937</P> 938<P> 939When a line ending is defined as a single character, dot never matches that 940character; when the two-character sequence CRLF is used, dot does not match CR 941if it is immediately followed by LF, but otherwise it matches all characters 942(including isolated CRs and LFs). When any Unicode line endings are being 943recognized, dot does not match CR or LF or any of the other line ending 944characters. 945</P> 946<P> 947The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL 948option is set, a dot matches any one character, without exception. If the 949two-character sequence CRLF is present in the subject string, it takes two dots 950to match it. 951</P> 952<P> 953The handling of dot is entirely independent of the handling of circumflex and 954dollar, the only relationship being that they both involve newlines. Dot has no 955special meaning in a character class. 956</P> 957<P> 958The escape sequence \N behaves like a dot, except that it is not affected by 959the PCRE_DOTALL option. In other words, it matches any character except one 960that signifies the end of a line. 961</P> 962<br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br> 963<P> 964Outside a character class, the escape sequence \C matches any one byte, both 965in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending 966characters. The feature is provided in Perl in order to match individual bytes 967in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the 968rest of the string may start with a malformed UTF-8 character. For this reason, 969the \C escape sequence is best avoided. 970</P> 971<P> 972PCRE does not allow \C to appear in lookbehind assertions 973<a href="#lookbehind">(described below),</a> 974because in UTF-8 mode this would make it impossible to calculate the length of 975the lookbehind. 976<a name="characterclass"></a></P> 977<br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br> 978<P> 979An opening square bracket introduces a character class, terminated by a closing 980square bracket. A closing square bracket on its own is not special by default. 981However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square 982bracket causes a compile-time error. If a closing square bracket is required as 983a member of the class, it should be the first data character in the class 984(after an initial circumflex, if present) or escaped with a backslash. 985</P> 986<P> 987A character class matches a single character in the subject. In UTF-8 mode, the 988character may be more than one byte long. A matched character must be in the 989set of characters defined by the class, unless the first character in the class 990definition is a circumflex, in which case the subject character must not be in 991the set defined by the class. If a circumflex is actually required as a member 992of the class, ensure it is not the first character, or escape it with a 993backslash. 994</P> 995<P> 996For example, the character class [aeiou] matches any lower case vowel, while 997[^aeiou] matches any character that is not a lower case vowel. Note that a 998circumflex is just a convenient notation for specifying the characters that 999are in the class by enumerating those that are not. A class that starts with a 1000circumflex is not an assertion; it still consumes a character from the subject 1001string, and therefore it fails if the current pointer is at the end of the 1002string. 1003</P> 1004<P> 1005In UTF-8 mode, characters with values greater than 255 can be included in a 1006class as a literal string of bytes, or by using the \x{ escaping mechanism. 1007</P> 1008<P> 1009When caseless matching is set, any letters in a class represent both their 1010upper case and lower case versions, so for example, a caseless [aeiou] matches 1011"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a 1012caseful version would. In UTF-8 mode, PCRE always understands the concept of 1013case for characters whose values are less than 128, so caseless matching is 1014always possible. For characters with higher values, the concept of case is 1015supported if PCRE is compiled with Unicode property support, but not otherwise. 1016If you want to use caseless matching in UTF8-mode for characters 128 and above, 1017you must ensure that PCRE is compiled with Unicode property support as well as 1018with UTF-8 support. 1019</P> 1020<P> 1021Characters that might indicate line breaks are never treated in any special way 1022when matching character classes, whatever line-ending sequence is in use, and 1023whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class 1024such as [^a] always matches one of these characters. 1025</P> 1026<P> 1027The minus (hyphen) character can be used to specify a range of characters in a 1028character class. For example, [d-m] matches any letter between d and m, 1029inclusive. If a minus character is required in a class, it must be escaped with 1030a backslash or appear in a position where it cannot be interpreted as 1031indicating a range, typically as the first or last character in the class. 1032</P> 1033<P> 1034It is not possible to have the literal character "]" as the end character of a 1035range. A pattern such as [W-]46] is interpreted as a class of two characters 1036("W" and "-") followed by a literal string "46]", so it would match "W46]" or 1037"-46]". However, if the "]" is escaped with a backslash it is interpreted as 1038the end of range, so [W-\]46] is interpreted as a class containing a range 1039followed by two other characters. The octal or hexadecimal representation of 1040"]" can also be used to end a range. 1041</P> 1042<P> 1043Ranges operate in the collating sequence of character values. They can also be 1044used for characters specified numerically, for example [\000-\037]. In UTF-8 1045mode, ranges can include characters whose values are greater than 255, for 1046example [\x{100}-\x{2ff}]. 1047</P> 1048<P> 1049If a range that includes letters is used when caseless matching is set, it 1050matches the letters in either case. For example, [W-c] is equivalent to 1051[][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character 1052tables for a French locale are in use, [\xc8-\xcb] matches accented E 1053characters in both cases. In UTF-8 mode, PCRE supports the concept of case for 1054characters with values greater than 128 only when it is compiled with Unicode 1055property support. 1056</P> 1057<P> 1058The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, 1059\V, \w, and \W may appear in a character class, and add the characters that 1060they match to the class. For example, [\dABCDEF] matches any hexadecimal 1061digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \d, \s, \w 1062and their upper case partners, just as it does when they appear outside a 1063character class, as described in the section entitled 1064<a href="#genericchartypes">"Generic character types"</a> 1065above. The escape sequence \b has a different meaning inside a character 1066class; it matches the backspace character. The sequences \B, \N, \R, and \X 1067are not special inside a character class. Like any other unrecognized escape 1068sequences, they are treated as the literal characters "B", "N", "R", and "X" by 1069default, but cause an error if the PCRE_EXTRA option is set. 1070</P> 1071<P> 1072A circumflex can conveniently be used with the upper case character types to 1073specify a more restricted set of characters than the matching lower case type. 1074For example, the class [^\W_] matches any letter or digit, but not underscore, 1075whereas [\w] includes underscore. A positive character class should be read as 1076"something OR something OR ..." and a negative class as "NOT something AND NOT 1077something AND NOT ...". 1078</P> 1079<P> 1080The only metacharacters that are recognized in character classes are backslash, 1081hyphen (only where it can be interpreted as specifying a range), circumflex 1082(only at the start), opening square bracket (only when it can be interpreted as 1083introducing a POSIX class name - see the next section), and the terminating 1084closing square bracket. However, escaping other non-alphanumeric characters 1085does no harm. 1086</P> 1087<br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br> 1088<P> 1089Perl supports the POSIX notation for character classes. This uses names 1090enclosed by [: and :] within the enclosing square brackets. PCRE also supports 1091this notation. For example, 1092<pre> 1093 [01[:alpha:]%] 1094</pre> 1095matches "0", "1", any alphabetic character, or "%". The supported class names 1096are: 1097<pre> 1098 alnum letters and digits 1099 alpha letters 1100 ascii character codes 0 - 127 1101 blank space or tab only 1102 cntrl control characters 1103 digit decimal digits (same as \d) 1104 graph printing characters, excluding space 1105 lower lower case letters 1106 print printing characters, including space 1107 punct printing characters, excluding letters and digits and space 1108 space white space (not quite the same as \s) 1109 upper upper case letters 1110 word "word" characters (same as \w) 1111 xdigit hexadecimal digits 1112</pre> 1113The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and 1114space (32). Notice that this list includes the VT character (code 11). This 1115makes "space" different to \s, which does not include VT (for Perl 1116compatibility). 1117</P> 1118<P> 1119The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 11205.8. Another Perl extension is negation, which is indicated by a ^ character 1121after the colon. For example, 1122<pre> 1123 [12[:^digit:]] 1124</pre> 1125matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX 1126syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not 1127supported, and an error is given if they are encountered. 1128</P> 1129<P> 1130By default, in UTF-8 mode, characters with values greater than 128 do not match 1131any of the POSIX character classes. However, if the PCRE_UCP option is passed 1132to <b>pcre_compile()</b>, some of the classes are changed so that Unicode 1133character properties are used. This is achieved by replacing the POSIX classes 1134by other sequences, as follows: 1135<pre> 1136 [:alnum:] becomes \p{Xan} 1137 [:alpha:] becomes \p{L} 1138 [:blank:] becomes \h 1139 [:digit:] becomes \p{Nd} 1140 [:lower:] becomes \p{Ll} 1141 [:space:] becomes \p{Xps} 1142 [:upper:] becomes \p{Lu} 1143 [:word:] becomes \p{Xwd} 1144</pre> 1145Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX 1146classes are unchanged, and match only characters with code points less than 1147128. 1148</P> 1149<br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br> 1150<P> 1151Vertical bar characters are used to separate alternative patterns. For example, 1152the pattern 1153<pre> 1154 gilbert|sullivan 1155</pre> 1156matches either "gilbert" or "sullivan". Any number of alternatives may appear, 1157and an empty alternative is permitted (matching the empty string). The matching 1158process tries each alternative in turn, from left to right, and the first one 1159that succeeds is used. If the alternatives are within a subpattern 1160<a href="#subpattern">(defined below),</a> 1161"succeeds" means matching the rest of the main pattern as well as the 1162alternative in the subpattern. 1163</P> 1164<br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br> 1165<P> 1166The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and 1167PCRE_EXTENDED options (which are Perl-compatible) can be changed from within 1168the pattern by a sequence of Perl option letters enclosed between "(?" and ")". 1169The option letters are 1170<pre> 1171 i for PCRE_CASELESS 1172 m for PCRE_MULTILINE 1173 s for PCRE_DOTALL 1174 x for PCRE_EXTENDED 1175</pre> 1176For example, (?im) sets caseless, multiline matching. It is also possible to 1177unset these options by preceding the letter with a hyphen, and a combined 1178setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and 1179PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also 1180permitted. If a letter appears both before and after the hyphen, the option is 1181unset. 1182</P> 1183<P> 1184The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be 1185changed in the same way as the Perl-compatible options by using the characters 1186J, U and X respectively. 1187</P> 1188<P> 1189When one of these option changes occurs at top level (that is, not inside 1190subpattern parentheses), the change applies to the remainder of the pattern 1191that follows. If the change is placed right at the start of a pattern, PCRE 1192extracts it into the global options (and it will therefore show up in data 1193extracted by the <b>pcre_fullinfo()</b> function). 1194</P> 1195<P> 1196An option change within a subpattern (see below for a description of 1197subpatterns) affects only that part of the subpattern that follows it, so 1198<pre> 1199 (a(?i)b)c 1200</pre> 1201matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). 1202By this means, options can be made to have different settings in different 1203parts of the pattern. Any changes made in one alternative do carry on 1204into subsequent branches within the same subpattern. For example, 1205<pre> 1206 (a(?i)b|c) 1207</pre> 1208matches "ab", "aB", "c", and "C", even though when matching "C" the first 1209branch is abandoned before the option setting. This is because the effects of 1210option settings happen at compile time. There would be some very weird 1211behaviour otherwise. 1212</P> 1213<P> 1214<b>Note:</b> There are other PCRE-specific options that can be set by the 1215application when the compile or match functions are called. In some cases the 1216pattern can contain special leading sequences such as (*CRLF) to override what 1217the application has set or what has been defaulted. Details are given in the 1218section entitled 1219<a href="#newlineseq">"Newline sequences"</a> 1220above. There are also the (*UTF8) and (*UCP) leading sequences that can be used 1221to set UTF-8 and Unicode property modes; they are equivalent to setting the 1222PCRE_UTF8 and the PCRE_UCP options, respectively. 1223<a name="subpattern"></a></P> 1224<br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br> 1225<P> 1226Subpatterns are delimited by parentheses (round brackets), which can be nested. 1227Turning part of a pattern into a subpattern does two things: 1228<br> 1229<br> 12301. It localizes a set of alternatives. For example, the pattern 1231<pre> 1232 cat(aract|erpillar|) 1233</pre> 1234matches "cataract", "caterpillar", or "cat". Without the parentheses, it would 1235match "cataract", "erpillar" or an empty string. 1236<br> 1237<br> 12382. It sets up the subpattern as a capturing subpattern. This means that, when 1239the whole pattern matches, that portion of the subject string that matched the 1240subpattern is passed back to the caller via the <i>ovector</i> argument of 1241<b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting 1242from 1) to obtain numbers for the capturing subpatterns. For example, if the 1243string "the red king" is matched against the pattern 1244<pre> 1245 the ((red|white) (king|queen)) 1246</pre> 1247the captured substrings are "red king", "red", and "king", and are numbered 1, 12482, and 3, respectively. 1249</P> 1250<P> 1251The fact that plain parentheses fulfil two functions is not always helpful. 1252There are often times when a grouping subpattern is required without a 1253capturing requirement. If an opening parenthesis is followed by a question mark 1254and a colon, the subpattern does not do any capturing, and is not counted when 1255computing the number of any subsequent capturing subpatterns. For example, if 1256the string "the white queen" is matched against the pattern 1257<pre> 1258 the ((?:red|white) (king|queen)) 1259</pre> 1260the captured substrings are "white queen" and "queen", and are numbered 1 and 12612. The maximum number of capturing subpatterns is 65535. 1262</P> 1263<P> 1264As a convenient shorthand, if any option settings are required at the start of 1265a non-capturing subpattern, the option letters may appear between the "?" and 1266the ":". Thus the two patterns 1267<pre> 1268 (?i:saturday|sunday) 1269 (?:(?i)saturday|sunday) 1270</pre> 1271match exactly the same set of strings. Because alternative branches are tried 1272from left to right, and options are not reset until the end of the subpattern 1273is reached, an option setting in one branch does affect subsequent branches, so 1274the above patterns match "SUNDAY" as well as "Saturday". 1275<a name="dupsubpatternnumber"></a></P> 1276<br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br> 1277<P> 1278Perl 5.10 introduced a feature whereby each alternative in a subpattern uses 1279the same numbers for its capturing parentheses. Such a subpattern starts with 1280(?| and is itself a non-capturing subpattern. For example, consider this 1281pattern: 1282<pre> 1283 (?|(Sat)ur|(Sun))day 1284</pre> 1285Because the two alternatives are inside a (?| group, both sets of capturing 1286parentheses are numbered one. Thus, when the pattern matches, you can look 1287at captured substring number one, whichever alternative matched. This construct 1288is useful when you want to capture part, but not all, of one of a number of 1289alternatives. Inside a (?| group, parentheses are numbered as usual, but the 1290number is reset at the start of each branch. The numbers of any capturing 1291parentheses that follow the subpattern start after the highest number used in 1292any branch. The following example is taken from the Perl documentation. The 1293numbers underneath show in which buffer the captured content will be stored. 1294<pre> 1295 # before ---------------branch-reset----------- after 1296 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x 1297 # 1 2 2 3 2 3 4 1298</pre> 1299A back reference to a numbered subpattern uses the most recent value that is 1300set for that number by any subpattern. The following pattern matches "abcabc" 1301or "defdef": 1302<pre> 1303 /(?|(abc)|(def))\1/ 1304</pre> 1305In contrast, a recursive or "subroutine" call to a numbered subpattern always 1306refers to the first one in the pattern with the given number. The following 1307pattern matches "abcabc" or "defabc": 1308<pre> 1309 /(?|(abc)|(def))(?1)/ 1310</pre> 1311If a 1312<a href="#conditions">condition test</a> 1313for a subpattern's having matched refers to a non-unique number, the test is 1314true if any of the subpatterns of that number have matched. 1315</P> 1316<P> 1317An alternative approach to using this "branch reset" feature is to use 1318duplicate named subpatterns, as described in the next section. 1319</P> 1320<br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br> 1321<P> 1322Identifying capturing parentheses by number is simple, but it can be very hard 1323to keep track of the numbers in complicated regular expressions. Furthermore, 1324if an expression is modified, the numbers may change. To help with this 1325difficulty, PCRE supports the naming of subpatterns. This feature was not 1326added to Perl until release 5.10. Python had the feature earlier, and PCRE 1327introduced it at release 4.0, using the Python syntax. PCRE now supports both 1328the Perl and the Python syntax. Perl allows identically numbered subpatterns to 1329have different names, but PCRE does not. 1330</P> 1331<P> 1332In PCRE, a subpattern can be named in one of three ways: (?<name>...) or 1333(?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing 1334parentheses from other parts of the pattern, such as 1335<a href="#backreferences">back references,</a> 1336<a href="#recursion">recursion,</a> 1337and 1338<a href="#conditions">conditions,</a> 1339can be made by name as well as by number. 1340</P> 1341<P> 1342Names consist of up to 32 alphanumeric characters and underscores. Named 1343capturing parentheses are still allocated numbers as well as names, exactly as 1344if the names were not present. The PCRE API provides function calls for 1345extracting the name-to-number translation table from a compiled pattern. There 1346is also a convenience function for extracting a captured substring by name. 1347</P> 1348<P> 1349By default, a name must be unique within a pattern, but it is possible to relax 1350this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate 1351names are also always permitted for subpatterns with the same number, set up as 1352described in the previous section.) Duplicate names can be useful for patterns 1353where only one instance of the named parentheses can match. Suppose you want to 1354match the name of a weekday, either as a 3-letter abbreviation or as the full 1355name, and in both cases you want to extract the abbreviation. This pattern 1356(ignoring the line breaks) does the job: 1357<pre> 1358 (?<DN>Mon|Fri|Sun)(?:day)?| 1359 (?<DN>Tue)(?:sday)?| 1360 (?<DN>Wed)(?:nesday)?| 1361 (?<DN>Thu)(?:rsday)?| 1362 (?<DN>Sat)(?:urday)? 1363</pre> 1364There are five capturing substrings, but only one is ever set after a match. 1365(An alternative way of solving this problem is to use a "branch reset" 1366subpattern, as described in the previous section.) 1367</P> 1368<P> 1369The convenience function for extracting the data by name returns the substring 1370for the first (and in this example, the only) subpattern of that name that 1371matched. This saves searching to find which numbered subpattern it was. 1372</P> 1373<P> 1374If you make a back reference to a non-unique named subpattern from elsewhere in 1375the pattern, the one that corresponds to the first occurrence of the name is 1376used. In the absence of duplicate numbers (see the previous section) this is 1377the one with the lowest number. If you use a named reference in a condition 1378test (see the 1379<a href="#conditions">section about conditions</a> 1380below), either to check whether a subpattern has matched, or to check for 1381recursion, all subpatterns with the same name are tested. If the condition is 1382true for any one of them, the overall condition is true. This is the same 1383behaviour as testing by number. For further details of the interfaces for 1384handling named subpatterns, see the 1385<a href="pcreapi.html"><b>pcreapi</b></a> 1386documentation. 1387</P> 1388<P> 1389<b>Warning:</b> You cannot use different names to distinguish between two 1390subpatterns with the same number because PCRE uses only the numbers when 1391matching. For this reason, an error is given at compile time if different names 1392are given to subpatterns with the same number. However, you can give the same 1393name to subpatterns with the same number, even when PCRE_DUPNAMES is not set. 1394</P> 1395<br><a name="SEC15" href="#TOC1">REPETITION</a><br> 1396<P> 1397Repetition is specified by quantifiers, which can follow any of the following 1398items: 1399<pre> 1400 a literal data character 1401 the dot metacharacter 1402 the \C escape sequence 1403 the \X escape sequence (in UTF-8 mode with Unicode properties) 1404 the \R escape sequence 1405 an escape such as \d or \pL that matches a single character 1406 a character class 1407 a back reference (see next section) 1408 a parenthesized subpattern (unless it is an assertion) 1409 a recursive or "subroutine" call to a subpattern 1410</pre> 1411The general repetition quantifier specifies a minimum and maximum number of 1412permitted matches, by giving the two numbers in curly brackets (braces), 1413separated by a comma. The numbers must be less than 65536, and the first must 1414be less than or equal to the second. For example: 1415<pre> 1416 z{2,4} 1417</pre> 1418matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special 1419character. If the second number is omitted, but the comma is present, there is 1420no upper limit; if the second number and the comma are both omitted, the 1421quantifier specifies an exact number of required matches. Thus 1422<pre> 1423 [aeiou]{3,} 1424</pre> 1425matches at least 3 successive vowels, but may match many more, while 1426<pre> 1427 \d{8} 1428</pre> 1429matches exactly 8 digits. An opening curly bracket that appears in a position 1430where a quantifier is not allowed, or one that does not match the syntax of a 1431quantifier, is taken as a literal character. For example, {,6} is not a 1432quantifier, but a literal string of four characters. 1433</P> 1434<P> 1435In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual 1436bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of 1437which is represented by a two-byte sequence. Similarly, when Unicode property 1438support is available, \X{3} matches three Unicode extended sequences, each of 1439which may be several bytes long (and they may be of different lengths). 1440</P> 1441<P> 1442The quantifier {0} is permitted, causing the expression to behave as if the 1443previous item and the quantifier were not present. This may be useful for 1444subpatterns that are referenced as 1445<a href="#subpatternsassubroutines">subroutines</a> 1446from elsewhere in the pattern (but see also the section entitled 1447<a href="#subdefine">"Defining subpatterns for use by reference only"</a> 1448below). Items other than subpatterns that have a {0} quantifier are omitted 1449from the compiled pattern. 1450</P> 1451<P> 1452For convenience, the three most common quantifiers have single-character 1453abbreviations: 1454<pre> 1455 * is equivalent to {0,} 1456 + is equivalent to {1,} 1457 ? is equivalent to {0,1} 1458</pre> 1459It is possible to construct infinite loops by following a subpattern that can 1460match no characters with a quantifier that has no upper limit, for example: 1461<pre> 1462 (a?)* 1463</pre> 1464Earlier versions of Perl and PCRE used to give an error at compile time for 1465such patterns. However, because there are cases where this can be useful, such 1466patterns are now accepted, but if any repetition of the subpattern does in fact 1467match no characters, the loop is forcibly broken. 1468</P> 1469<P> 1470By default, the quantifiers are "greedy", that is, they match as much as 1471possible (up to the maximum number of permitted times), without causing the 1472rest of the pattern to fail. The classic example of where this gives problems 1473is in trying to match comments in C programs. These appear between /* and */ 1474and within the comment, individual * and / characters may appear. An attempt to 1475match C comments by applying the pattern 1476<pre> 1477 /\*.*\*/ 1478</pre> 1479to the string 1480<pre> 1481 /* first comment */ not comment /* second comment */ 1482</pre> 1483fails, because it matches the entire string owing to the greediness of the .* 1484item. 1485</P> 1486<P> 1487However, if a quantifier is followed by a question mark, it ceases to be 1488greedy, and instead matches the minimum number of times possible, so the 1489pattern 1490<pre> 1491 /\*.*?\*/ 1492</pre> 1493does the right thing with the C comments. The meaning of the various 1494quantifiers is not otherwise changed, just the preferred number of matches. 1495Do not confuse this use of question mark with its use as a quantifier in its 1496own right. Because it has two uses, it can sometimes appear doubled, as in 1497<pre> 1498 \d??\d 1499</pre> 1500which matches one digit by preference, but can match two if that is the only 1501way the rest of the pattern matches. 1502</P> 1503<P> 1504If the PCRE_UNGREEDY option is set (an option that is not available in Perl), 1505the quantifiers are not greedy by default, but individual ones can be made 1506greedy by following them with a question mark. In other words, it inverts the 1507default behaviour. 1508</P> 1509<P> 1510When a parenthesized subpattern is quantified with a minimum repeat count that 1511is greater than 1 or with a limited maximum, more memory is required for the 1512compiled pattern, in proportion to the size of the minimum or maximum. 1513</P> 1514<P> 1515If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent 1516to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is 1517implicitly anchored, because whatever follows will be tried against every 1518character position in the subject string, so there is no point in retrying the 1519overall match at any position after the first. PCRE normally treats such a 1520pattern as though it were preceded by \A. 1521</P> 1522<P> 1523In cases where it is known that the subject string contains no newlines, it is 1524worth setting PCRE_DOTALL in order to obtain this optimization, or 1525alternatively using ^ to indicate anchoring explicitly. 1526</P> 1527<P> 1528However, there is one situation where the optimization cannot be used. When .* 1529is inside capturing parentheses that are the subject of a back reference 1530elsewhere in the pattern, a match at the start may fail where a later one 1531succeeds. Consider, for example: 1532<pre> 1533 (.*)abc\1 1534</pre> 1535If the subject is "xyz123abc123" the match point is the fourth character. For 1536this reason, such a pattern is not implicitly anchored. 1537</P> 1538<P> 1539When a capturing subpattern is repeated, the value captured is the substring 1540that matched the final iteration. For example, after 1541<pre> 1542 (tweedle[dume]{3}\s*)+ 1543</pre> 1544has matched "tweedledum tweedledee" the value of the captured substring is 1545"tweedledee". However, if there are nested capturing subpatterns, the 1546corresponding captured values may have been set in previous iterations. For 1547example, after 1548<pre> 1549 /(a|(b))+/ 1550</pre> 1551matches "aba" the value of the second captured substring is "b". 1552<a name="atomicgroup"></a></P> 1553<br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br> 1554<P> 1555With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 1556repetition, failure of what follows normally causes the repeated item to be 1557re-evaluated to see if a different number of repeats allows the rest of the 1558pattern to match. Sometimes it is useful to prevent this, either to change the 1559nature of the match, or to cause it fail earlier than it otherwise might, when 1560the author of the pattern knows there is no point in carrying on. 1561</P> 1562<P> 1563Consider, for example, the pattern \d+foo when applied to the subject line 1564<pre> 1565 123456bar 1566</pre> 1567After matching all 6 digits and then failing to match "foo", the normal 1568action of the matcher is to try again with only 5 digits matching the \d+ 1569item, and then with 4, and so on, before ultimately failing. "Atomic grouping" 1570(a term taken from Jeffrey Friedl's book) provides the means for specifying 1571that once a subpattern has matched, it is not to be re-evaluated in this way. 1572</P> 1573<P> 1574If we use atomic grouping for the previous example, the matcher gives up 1575immediately on failing to match "foo" the first time. The notation is a kind of 1576special parenthesis, starting with (?> as in this example: 1577<pre> 1578 (?>\d+)foo 1579</pre> 1580This kind of parenthesis "locks up" the part of the pattern it contains once 1581it has matched, and a failure further into the pattern is prevented from 1582backtracking into it. Backtracking past it to previous items, however, works as 1583normal. 1584</P> 1585<P> 1586An alternative description is that a subpattern of this type matches the string 1587of characters that an identical standalone pattern would match, if anchored at 1588the current point in the subject string. 1589</P> 1590<P> 1591Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as 1592the above example can be thought of as a maximizing repeat that must swallow 1593everything it can. So, while both \d+ and \d+? are prepared to adjust the 1594number of digits they match in order to make the rest of the pattern match, 1595(?>\d+) can only match an entire sequence of digits. 1596</P> 1597<P> 1598Atomic groups in general can of course contain arbitrarily complicated 1599subpatterns, and can be nested. However, when the subpattern for an atomic 1600group is just a single repeated item, as in the example above, a simpler 1601notation, called a "possessive quantifier" can be used. This consists of an 1602additional + character following a quantifier. Using this notation, the 1603previous example can be rewritten as 1604<pre> 1605 \d++foo 1606</pre> 1607Note that a possessive quantifier can be used with an entire group, for 1608example: 1609<pre> 1610 (abc|xyz){2,3}+ 1611</pre> 1612Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY 1613option is ignored. They are a convenient notation for the simpler forms of 1614atomic group. However, there is no difference in the meaning of a possessive 1615quantifier and the equivalent atomic group, though there may be a performance 1616difference; possessive quantifiers should be slightly faster. 1617</P> 1618<P> 1619The possessive quantifier syntax is an extension to the Perl 5.8 syntax. 1620Jeffrey Friedl originated the idea (and the name) in the first edition of his 1621book. Mike McCloskey liked it, so implemented it when he built Sun's Java 1622package, and PCRE copied it from there. It ultimately found its way into Perl 1623at release 5.10. 1624</P> 1625<P> 1626PCRE has an optimization that automatically "possessifies" certain simple 1627pattern constructs. For example, the sequence A+B is treated as A++B because 1628there is no point in backtracking into a sequence of A's when B must follow. 1629</P> 1630<P> 1631When a pattern contains an unlimited repeat inside a subpattern that can itself 1632be repeated an unlimited number of times, the use of an atomic group is the 1633only way to avoid some failing matches taking a very long time indeed. The 1634pattern 1635<pre> 1636 (\D+|<\d+>)*[!?] 1637</pre> 1638matches an unlimited number of substrings that either consist of non-digits, or 1639digits enclosed in <>, followed by either ! or ?. When it matches, it runs 1640quickly. However, if it is applied to 1641<pre> 1642 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 1643</pre> 1644it takes a long time before reporting failure. This is because the string can 1645be divided between the internal \D+ repeat and the external * repeat in a 1646large number of ways, and all have to be tried. (The example uses [!?] rather 1647than a single character at the end, because both PCRE and Perl have an 1648optimization that allows for fast failure when a single character is used. They 1649remember the last single character that is required for a match, and fail early 1650if it is not present in the string.) If the pattern is changed so that it uses 1651an atomic group, like this: 1652<pre> 1653 ((?>\D+)|<\d+>)*[!?] 1654</pre> 1655sequences of non-digits cannot be broken, and failure happens quickly. 1656<a name="backreferences"></a></P> 1657<br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br> 1658<P> 1659Outside a character class, a backslash followed by a digit greater than 0 (and 1660possibly further digits) is a back reference to a capturing subpattern earlier 1661(that is, to its left) in the pattern, provided there have been that many 1662previous capturing left parentheses. 1663</P> 1664<P> 1665However, if the decimal number following the backslash is less than 10, it is 1666always taken as a back reference, and causes an error only if there are not 1667that many capturing left parentheses in the entire pattern. In other words, the 1668parentheses that are referenced need not be to the left of the reference for 1669numbers less than 10. A "forward back reference" of this type can make sense 1670when a repetition is involved and the subpattern to the right has participated 1671in an earlier iteration. 1672</P> 1673<P> 1674It is not possible to have a numerical "forward back reference" to a subpattern 1675whose number is 10 or more using this syntax because a sequence such as \50 is 1676interpreted as a character defined in octal. See the subsection entitled 1677"Non-printing characters" 1678<a href="#digitsafterbackslash">above</a> 1679for further details of the handling of digits following a backslash. There is 1680no such problem when named parentheses are used. A back reference to any 1681subpattern is possible using named parentheses (see below). 1682</P> 1683<P> 1684Another way of avoiding the ambiguity inherent in the use of digits following a 1685backslash is to use the \g escape sequence. This escape must be followed by an 1686unsigned number or a negative number, optionally enclosed in braces. These 1687examples are all identical: 1688<pre> 1689 (ring), \1 1690 (ring), \g1 1691 (ring), \g{1} 1692</pre> 1693An unsigned number specifies an absolute reference without the ambiguity that 1694is present in the older syntax. It is also useful when literal digits follow 1695the reference. A negative number is a relative reference. Consider this 1696example: 1697<pre> 1698 (abc(def)ghi)\g{-1} 1699</pre> 1700The sequence \g{-1} is a reference to the most recently started capturing 1701subpattern before \g, that is, is it equivalent to \2 in this example. 1702Similarly, \g{-2} would be equivalent to \1. The use of relative references 1703can be helpful in long patterns, and also in patterns that are created by 1704joining together fragments that contain references within themselves. 1705</P> 1706<P> 1707A back reference matches whatever actually matched the capturing subpattern in 1708the current subject string, rather than anything matching the subpattern 1709itself (see 1710<a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a> 1711below for a way of doing that). So the pattern 1712<pre> 1713 (sens|respons)e and \1ibility 1714</pre> 1715matches "sense and sensibility" and "response and responsibility", but not 1716"sense and responsibility". If caseful matching is in force at the time of the 1717back reference, the case of letters is relevant. For example, 1718<pre> 1719 ((?i)rah)\s+\1 1720</pre> 1721matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original 1722capturing subpattern is matched caselessly. 1723</P> 1724<P> 1725There are several different ways of writing back references to named 1726subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or 1727\k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified 1728back reference syntax, in which \g can be used for both numeric and named 1729references, is also supported. We could rewrite the above example in any of 1730the following ways: 1731<pre> 1732 (?<p1>(?i)rah)\s+\k<p1> 1733 (?'p1'(?i)rah)\s+\k{p1} 1734 (?P<p1>(?i)rah)\s+(?P=p1) 1735 (?<p1>(?i)rah)\s+\g{p1} 1736</pre> 1737A subpattern that is referenced by name may appear in the pattern before or 1738after the reference. 1739</P> 1740<P> 1741There may be more than one back reference to the same subpattern. If a 1742subpattern has not actually been used in a particular match, any back 1743references to it always fail by default. For example, the pattern 1744<pre> 1745 (a|(bc))\2 1746</pre> 1747always fails if it starts to match "a" rather than "bc". However, if the 1748PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an 1749unset value matches an empty string. 1750</P> 1751<P> 1752Because there may be many capturing parentheses in a pattern, all digits 1753following a backslash are taken as part of a potential back reference number. 1754If the pattern continues with a digit character, some delimiter must be used to 1755terminate the back reference. If the PCRE_EXTENDED option is set, this can be 1756whitespace. Otherwise, the \g{ syntax or an empty comment (see 1757<a href="#comments">"Comments"</a> 1758below) can be used. 1759</P> 1760<br><b> 1761Recursive back references 1762</b><br> 1763<P> 1764A back reference that occurs inside the parentheses to which it refers fails 1765when the subpattern is first used, so, for example, (a\1) never matches. 1766However, such references can be useful inside repeated subpatterns. For 1767example, the pattern 1768<pre> 1769 (a|b\1)+ 1770</pre> 1771matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of 1772the subpattern, the back reference matches the character string corresponding 1773to the previous iteration. In order for this to work, the pattern must be such 1774that the first iteration does not need to match the back reference. This can be 1775done using alternation, as in the example above, or by a quantifier with a 1776minimum of zero. 1777</P> 1778<P> 1779Back references of this type cause the group that they reference to be treated 1780as an 1781<a href="#atomicgroup">atomic group.</a> 1782Once the whole group has been matched, a subsequent matching failure cannot 1783cause backtracking into the middle of the group. 1784<a name="bigassertions"></a></P> 1785<br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br> 1786<P> 1787An assertion is a test on the characters following or preceding the current 1788matching point that does not actually consume any characters. The simple 1789assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described 1790<a href="#smallassertions">above.</a> 1791</P> 1792<P> 1793More complicated assertions are coded as subpatterns. There are two kinds: 1794those that look ahead of the current position in the subject string, and those 1795that look behind it. An assertion subpattern is matched in the normal way, 1796except that it does not cause the current matching position to be changed. 1797</P> 1798<P> 1799Assertion subpatterns are not capturing subpatterns, and may not be repeated, 1800because it makes no sense to assert the same thing several times. If any kind 1801of assertion contains capturing subpatterns within it, these are counted for 1802the purposes of numbering the capturing subpatterns in the whole pattern. 1803However, substring capturing is carried out only for positive assertions, 1804because it does not make sense for negative assertions. 1805</P> 1806<br><b> 1807Lookahead assertions 1808</b><br> 1809<P> 1810Lookahead assertions start with (?= for positive assertions and (?! for 1811negative assertions. For example, 1812<pre> 1813 \w+(?=;) 1814</pre> 1815matches a word followed by a semicolon, but does not include the semicolon in 1816the match, and 1817<pre> 1818 foo(?!bar) 1819</pre> 1820matches any occurrence of "foo" that is not followed by "bar". Note that the 1821apparently similar pattern 1822<pre> 1823 (?!foo)bar 1824</pre> 1825does not find an occurrence of "bar" that is preceded by something other than 1826"foo"; it finds any occurrence of "bar" whatsoever, because the assertion 1827(?!foo) is always true when the next three characters are "bar". A 1828lookbehind assertion is needed to achieve the other effect. 1829</P> 1830<P> 1831If you want to force a matching failure at some point in a pattern, the most 1832convenient way to do it is with (?!) because an empty string always matches, so 1833an assertion that requires there not to be an empty string must always fail. 1834The backtracking control verb (*FAIL) or (*F) is a synonym for (?!). 1835<a name="lookbehind"></a></P> 1836<br><b> 1837Lookbehind assertions 1838</b><br> 1839<P> 1840Lookbehind assertions start with (?<= for positive assertions and (?<! for 1841negative assertions. For example, 1842<pre> 1843 (?<!foo)bar 1844</pre> 1845does find an occurrence of "bar" that is not preceded by "foo". The contents of 1846a lookbehind assertion are restricted such that all the strings it matches must 1847have a fixed length. However, if there are several top-level alternatives, they 1848do not all have to have the same fixed length. Thus 1849<pre> 1850 (?<=bullock|donkey) 1851</pre> 1852is permitted, but 1853<pre> 1854 (?<!dogs?|cats?) 1855</pre> 1856causes an error at compile time. Branches that match different length strings 1857are permitted only at the top level of a lookbehind assertion. This is an 1858extension compared with Perl, which requires all branches to match the same 1859length of string. An assertion such as 1860<pre> 1861 (?<=ab(c|de)) 1862</pre> 1863is not permitted, because its single top-level branch can match two different 1864lengths, but it is acceptable to PCRE if rewritten to use two top-level 1865branches: 1866<pre> 1867 (?<=abc|abde) 1868</pre> 1869In some cases, the escape sequence \K 1870<a href="#resetmatchstart">(see above)</a> 1871can be used instead of a lookbehind assertion to get round the fixed-length 1872restriction. 1873</P> 1874<P> 1875The implementation of lookbehind assertions is, for each alternative, to 1876temporarily move the current position back by the fixed length and then try to 1877match. If there are insufficient characters before the current position, the 1878assertion fails. 1879</P> 1880<P> 1881PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode) 1882to appear in lookbehind assertions, because it makes it impossible to calculate 1883the length of the lookbehind. The \X and \R escapes, which can match 1884different numbers of bytes, are also not permitted. 1885</P> 1886<P> 1887<a href="#subpatternsassubroutines">"Subroutine"</a> 1888calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long 1889as the subpattern matches a fixed-length string. 1890<a href="#recursion">Recursion,</a> 1891however, is not supported. 1892</P> 1893<P> 1894Possessive quantifiers can be used in conjunction with lookbehind assertions to 1895specify efficient matching of fixed-length strings at the end of subject 1896strings. Consider a simple pattern such as 1897<pre> 1898 abcd$ 1899</pre> 1900when applied to a long string that does not match. Because matching proceeds 1901from left to right, PCRE will look for each "a" in the subject and then see if 1902what follows matches the rest of the pattern. If the pattern is specified as 1903<pre> 1904 ^.*abcd$ 1905</pre> 1906the initial .* matches the entire string at first, but when this fails (because 1907there is no following "a"), it backtracks to match all but the last character, 1908then all but the last two characters, and so on. Once again the search for "a" 1909covers the entire string, from right to left, so we are no better off. However, 1910if the pattern is written as 1911<pre> 1912 ^.*+(?<=abcd) 1913</pre> 1914there can be no backtracking for the .*+ item; it can match only the entire 1915string. The subsequent lookbehind assertion does a single test on the last four 1916characters. If it fails, the match fails immediately. For long strings, this 1917approach makes a significant difference to the processing time. 1918</P> 1919<br><b> 1920Using multiple assertions 1921</b><br> 1922<P> 1923Several assertions (of any sort) may occur in succession. For example, 1924<pre> 1925 (?<=\d{3})(?<!999)foo 1926</pre> 1927matches "foo" preceded by three digits that are not "999". Notice that each of 1928the assertions is applied independently at the same point in the subject 1929string. First there is a check that the previous three characters are all 1930digits, and then there is a check that the same three characters are not "999". 1931This pattern does <i>not</i> match "foo" preceded by six characters, the first 1932of which are digits and the last three of which are not "999". For example, it 1933doesn't match "123abcfoo". A pattern to do that is 1934<pre> 1935 (?<=\d{3}...)(?<!999)foo 1936</pre> 1937This time the first assertion looks at the preceding six characters, checking 1938that the first three are digits, and then the second assertion checks that the 1939preceding three characters are not "999". 1940</P> 1941<P> 1942Assertions can be nested in any combination. For example, 1943<pre> 1944 (?<=(?<!foo)bar)baz 1945</pre> 1946matches an occurrence of "baz" that is preceded by "bar" which in turn is not 1947preceded by "foo", while 1948<pre> 1949 (?<=\d{3}(?!999)...)foo 1950</pre> 1951is another pattern that matches "foo" preceded by three digits and any three 1952characters that are not "999". 1953<a name="conditions"></a></P> 1954<br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br> 1955<P> 1956It is possible to cause the matching process to obey a subpattern 1957conditionally or to choose between two alternative subpatterns, depending on 1958the result of an assertion, or whether a specific capturing subpattern has 1959already been matched. The two possible forms of conditional subpattern are: 1960<pre> 1961 (?(condition)yes-pattern) 1962 (?(condition)yes-pattern|no-pattern) 1963</pre> 1964If the condition is satisfied, the yes-pattern is used; otherwise the 1965no-pattern (if present) is used. If there are more than two alternatives in the 1966subpattern, a compile-time error occurs. Each of the two alternatives may 1967itself contain nested subpatterns of any form, including conditional 1968subpatterns; the restriction to two alternatives applies only at the level of 1969the condition. This pattern fragment is an example where the alternatives are 1970complex: 1971<pre> 1972 (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) 1973 1974</PRE> 1975</P> 1976<P> 1977There are four kinds of condition: references to subpatterns, references to 1978recursion, a pseudo-condition called DEFINE, and assertions. 1979</P> 1980<br><b> 1981Checking for a used subpattern by number 1982</b><br> 1983<P> 1984If the text between the parentheses consists of a sequence of digits, the 1985condition is true if a capturing subpattern of that number has previously 1986matched. If there is more than one capturing subpattern with the same number 1987(see the earlier 1988<a href="#recursion">section about duplicate subpattern numbers),</a> 1989the condition is true if any of them have matched. An alternative notation is 1990to precede the digits with a plus or minus sign. In this case, the subpattern 1991number is relative rather than absolute. The most recently opened parentheses 1992can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside 1993loops it can also make sense to refer to subsequent groups. The next 1994parentheses to be opened can be referenced as (?(+1), and so on. (The value 1995zero in any of these forms is not used; it provokes a compile-time error.) 1996</P> 1997<P> 1998Consider the following pattern, which contains non-significant white space to 1999make it more readable (assume the PCRE_EXTENDED option) and to divide it into 2000three parts for ease of discussion: 2001<pre> 2002 ( \( )? [^()]+ (?(1) \) ) 2003</pre> 2004The first part matches an optional opening parenthesis, and if that 2005character is present, sets it as the first captured substring. The second part 2006matches one or more characters that are not parentheses. The third part is a 2007conditional subpattern that tests whether or not the first set of parentheses 2008matched. If they did, that is, if subject started with an opening parenthesis, 2009the condition is true, and so the yes-pattern is executed and a closing 2010parenthesis is required. Otherwise, since no-pattern is not present, the 2011subpattern matches nothing. In other words, this pattern matches a sequence of 2012non-parentheses, optionally enclosed in parentheses. 2013</P> 2014<P> 2015If you were embedding this pattern in a larger one, you could use a relative 2016reference: 2017<pre> 2018 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... 2019</pre> 2020This makes the fragment independent of the parentheses in the larger pattern. 2021</P> 2022<br><b> 2023Checking for a used subpattern by name 2024</b><br> 2025<P> 2026Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used 2027subpattern by name. For compatibility with earlier versions of PCRE, which had 2028this facility before Perl, the syntax (?(name)...) is also recognized. However, 2029there is a possible ambiguity with this syntax, because subpattern names may 2030consist entirely of digits. PCRE looks first for a named subpattern; if it 2031cannot find one and the name consists entirely of digits, PCRE looks for a 2032subpattern of that number, which must be greater than zero. Using subpattern 2033names that consist entirely of digits is not recommended. 2034</P> 2035<P> 2036Rewriting the above example to use a named subpattern gives this: 2037<pre> 2038 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) 2039</pre> 2040If the name used in a condition of this kind is a duplicate, the test is 2041applied to all subpatterns of the same name, and is true if any one of them has 2042matched. 2043</P> 2044<br><b> 2045Checking for pattern recursion 2046</b><br> 2047<P> 2048If the condition is the string (R), and there is no subpattern with the name R, 2049the condition is true if a recursive call to the whole pattern or any 2050subpattern has been made. If digits or a name preceded by ampersand follow the 2051letter R, for example: 2052<pre> 2053 (?(R3)...) or (?(R&name)...) 2054</pre> 2055the condition is true if the most recent recursion is into a subpattern whose 2056number or name is given. This condition does not check the entire recursion 2057stack. If the name used in a condition of this kind is a duplicate, the test is 2058applied to all subpatterns of the same name, and is true if any one of them is 2059the most recent recursion. 2060</P> 2061<P> 2062At "top level", all these recursion test conditions are false. 2063<a href="#recursion">The syntax for recursive patterns</a> 2064is described below. 2065<a name="subdefine"></a></P> 2066<br><b> 2067Defining subpatterns for use by reference only 2068</b><br> 2069<P> 2070If the condition is the string (DEFINE), and there is no subpattern with the 2071name DEFINE, the condition is always false. In this case, there may be only one 2072alternative in the subpattern. It is always skipped if control reaches this 2073point in the pattern; the idea of DEFINE is that it can be used to define 2074"subroutines" that can be referenced from elsewhere. (The use of 2075<a href="#subpatternsassubroutines">"subroutines"</a> 2076is described below.) For example, a pattern to match an IPv4 address such as 2077"192.168.23.245" could be written like this (ignore whitespace and line 2078breaks): 2079<pre> 2080 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) 2081 \b (?&byte) (\.(?&byte)){3} \b 2082</pre> 2083The first part of the pattern is a DEFINE group inside which a another group 2084named "byte" is defined. This matches an individual component of an IPv4 2085address (a number less than 256). When matching takes place, this part of the 2086pattern is skipped because DEFINE acts like a false condition. The rest of the 2087pattern uses references to the named group to match the four dot-separated 2088components of an IPv4 address, insisting on a word boundary at each end. 2089</P> 2090<br><b> 2091Assertion conditions 2092</b><br> 2093<P> 2094If the condition is not in any of the above formats, it must be an assertion. 2095This may be a positive or negative lookahead or lookbehind assertion. Consider 2096this pattern, again containing non-significant white space, and with the two 2097alternatives on the second line: 2098<pre> 2099 (?(?=[^a-z]*[a-z]) 2100 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) 2101</pre> 2102The condition is a positive lookahead assertion that matches an optional 2103sequence of non-letters followed by a letter. In other words, it tests for the 2104presence of at least one letter in the subject. If a letter is found, the 2105subject is matched against the first alternative; otherwise it is matched 2106against the second. This pattern matches strings in one of the two forms 2107dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. 2108<a name="comments"></a></P> 2109<br><a name="SEC20" href="#TOC1">COMMENTS</a><br> 2110<P> 2111There are two ways of including comments in patterns that are processed by 2112PCRE. In both cases, the start of the comment must not be in a character class, 2113nor in the middle of any other sequence of related characters such as (?: or a 2114subpattern name or number. The characters that make up a comment play no part 2115in the pattern matching. 2116</P> 2117<P> 2118The sequence (?# marks the start of a comment that continues up to the next 2119closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED 2120option is set, an unescaped # character also introduces a comment, which in 2121this case continues to immediately after the next newline character or 2122character sequence in the pattern. Which characters are interpreted as newlines 2123is controlled by the options passed to <b>pcre_compile()</b> or by a special 2124sequence at the start of the pattern, as described in the section entitled 2125<a href="#newlines">"Newline conventions"</a> 2126above. Note that the end of this type of comment is a literal newline sequence 2127in the pattern; escape sequences that happen to represent a newline do not 2128count. For example, consider this pattern when PCRE_EXTENDED is set, and the 2129default newline convention is in force: 2130<pre> 2131 abc #comment \n still comment 2132</pre> 2133On encountering the # character, <b>pcre_compile()</b> skips along, looking for 2134a newline in the pattern. The sequence \n is still literal at this stage, so 2135it does not terminate the comment. Only an actual character with the code value 21360x0a (the default newline) does so. 2137<a name="recursion"></a></P> 2138<br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br> 2139<P> 2140Consider the problem of matching a string in parentheses, allowing for 2141unlimited nested parentheses. Without the use of recursion, the best that can 2142be done is to use a pattern that matches up to some fixed depth of nesting. It 2143is not possible to handle an arbitrary nesting depth. 2144</P> 2145<P> 2146For some time, Perl has provided a facility that allows regular expressions to 2147recurse (amongst other things). It does this by interpolating Perl code in the 2148expression at run time, and the code can refer to the expression itself. A Perl 2149pattern using code interpolation to solve the parentheses problem can be 2150created like this: 2151<pre> 2152 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; 2153</pre> 2154The (?p{...}) item interpolates Perl code at run time, and in this case refers 2155recursively to the pattern in which it appears. 2156</P> 2157<P> 2158Obviously, PCRE cannot support the interpolation of Perl code. Instead, it 2159supports special syntax for recursion of the entire pattern, and also for 2160individual subpattern recursion. After its introduction in PCRE and Python, 2161this kind of recursion was subsequently introduced into Perl at release 5.10. 2162</P> 2163<P> 2164A special item that consists of (? followed by a number greater than zero and a 2165closing parenthesis is a recursive call of the subpattern of the given number, 2166provided that it occurs inside that subpattern. (If not, it is a 2167<a href="#subpatternsassubroutines">"subroutine"</a> 2168call, which is described in the next section.) The special item (?R) or (?0) is 2169a recursive call of the entire regular expression. 2170</P> 2171<P> 2172This PCRE pattern solves the nested parentheses problem (assume the 2173PCRE_EXTENDED option is set so that white space is ignored): 2174<pre> 2175 \( ( [^()]++ | (?R) )* \) 2176</pre> 2177First it matches an opening parenthesis. Then it matches any number of 2178substrings which can either be a sequence of non-parentheses, or a recursive 2179match of the pattern itself (that is, a correctly parenthesized substring). 2180Finally there is a closing parenthesis. Note the use of a possessive quantifier 2181to avoid backtracking into sequences of non-parentheses. 2182</P> 2183<P> 2184If this were part of a larger pattern, you would not want to recurse the entire 2185pattern, so instead you could use this: 2186<pre> 2187 ( \( ( [^()]++ | (?1) )* \) ) 2188</pre> 2189We have put the pattern into parentheses, and caused the recursion to refer to 2190them instead of the whole pattern. 2191</P> 2192<P> 2193In a larger pattern, keeping track of parenthesis numbers can be tricky. This 2194is made easier by the use of relative references. Instead of (?1) in the 2195pattern above you can write (?-2) to refer to the second most recently opened 2196parentheses preceding the recursion. In other words, a negative number counts 2197capturing parentheses leftwards from the point at which it is encountered. 2198</P> 2199<P> 2200It is also possible to refer to subsequently opened parentheses, by writing 2201references such as (?+2). However, these cannot be recursive because the 2202reference is not inside the parentheses that are referenced. They are always 2203<a href="#subpatternsassubroutines">"subroutine"</a> 2204calls, as described in the next section. 2205</P> 2206<P> 2207An alternative approach is to use named parentheses instead. The Perl syntax 2208for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We 2209could rewrite the above example as follows: 2210<pre> 2211 (?<pn> \( ( [^()]++ | (?&pn) )* \) ) 2212</pre> 2213If there is more than one subpattern with the same name, the earliest one is 2214used. 2215</P> 2216<P> 2217This particular example pattern that we have been looking at contains nested 2218unlimited repeats, and so the use of a possessive quantifier for matching 2219strings of non-parentheses is important when applying the pattern to strings 2220that do not match. For example, when this pattern is applied to 2221<pre> 2222 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 2223</pre> 2224it yields "no match" quickly. However, if a possessive quantifier is not used, 2225the match runs for a very long time indeed because there are so many different 2226ways the + and * repeats can carve up the subject, and all have to be tested 2227before failure can be reported. 2228</P> 2229<P> 2230At the end of a match, the values of capturing parentheses are those from 2231the outermost level. If you want to obtain intermediate values, a callout 2232function can be used (see below and the 2233<a href="pcrecallout.html"><b>pcrecallout</b></a> 2234documentation). If the pattern above is matched against 2235<pre> 2236 (ab(cd)ef) 2237</pre> 2238the value for the inner capturing parentheses (numbered 2) is "ef", which is 2239the last value taken on at the top level. If a capturing subpattern is not 2240matched at the top level, its final value is unset, even if it is (temporarily) 2241set at a deeper level. 2242</P> 2243<P> 2244If there are more than 15 capturing parentheses in a pattern, PCRE has to 2245obtain extra memory to store data during a recursion, which it does by using 2246<b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can 2247be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. 2248</P> 2249<P> 2250Do not confuse the (?R) item with the condition (R), which tests for recursion. 2251Consider this pattern, which matches text in angle brackets, allowing for 2252arbitrary nesting. Only digits are allowed in nested brackets (that is, when 2253recursing), whereas any characters are permitted at the outer level. 2254<pre> 2255 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > 2256</pre> 2257In this pattern, (?(R) is the start of a conditional subpattern, with two 2258different alternatives for the recursive and non-recursive cases. The (?R) item 2259is the actual recursive call. 2260<a name="recursiondifference"></a></P> 2261<br><b> 2262Recursion difference from Perl 2263</b><br> 2264<P> 2265In PCRE (like Python, but unlike Perl), a recursive subpattern call is always 2266treated as an atomic group. That is, once it has matched some of the subject 2267string, it is never re-entered, even if it contains untried alternatives and 2268there is a subsequent matching failure. This can be illustrated by the 2269following pattern, which purports to match a palindromic string that contains 2270an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"): 2271<pre> 2272 ^(.|(.)(?1)\2)$ 2273</pre> 2274The idea is that it either matches a single character, or two identical 2275characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE 2276it does not if the pattern is longer than three characters. Consider the 2277subject string "abcba": 2278</P> 2279<P> 2280At the top level, the first character is matched, but as it is not at the end 2281of the string, the first alternative fails; the second alternative is taken 2282and the recursion kicks in. The recursive call to subpattern 1 successfully 2283matches the next character ("b"). (Note that the beginning and end of line 2284tests are not part of the recursion). 2285</P> 2286<P> 2287Back at the top level, the next character ("c") is compared with what 2288subpattern 2 matched, which was "a". This fails. Because the recursion is 2289treated as an atomic group, there are now no backtracking points, and so the 2290entire match fails. (Perl is able, at this point, to re-enter the recursion and 2291try the second alternative.) However, if the pattern is written with the 2292alternatives in the other order, things are different: 2293<pre> 2294 ^((.)(?1)\2|.)$ 2295</pre> 2296This time, the recursing alternative is tried first, and continues to recurse 2297until it runs out of characters, at which point the recursion fails. But this 2298time we do have another alternative to try at the higher level. That is the big 2299difference: in the previous case the remaining alternative is at a deeper 2300recursion level, which PCRE cannot use. 2301</P> 2302<P> 2303To change the pattern so that it matches all palindromic strings, not just 2304those with an odd number of characters, it is tempting to change the pattern to 2305this: 2306<pre> 2307 ^((.)(?1)\2|.?)$ 2308</pre> 2309Again, this works in Perl, but not in PCRE, and for the same reason. When a 2310deeper recursion has matched a single character, it cannot be entered again in 2311order to match an empty string. The solution is to separate the two cases, and 2312write out the odd and even cases as alternatives at the higher level: 2313<pre> 2314 ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) 2315</pre> 2316If you want to match typical palindromic phrases, the pattern has to ignore all 2317non-word characters, which can be done like this: 2318<pre> 2319 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ 2320</pre> 2321If run with the PCRE_CASELESS option, this pattern matches phrases such as "A 2322man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note 2323the use of the possessive quantifier *+ to avoid backtracking into sequences of 2324non-word characters. Without this, PCRE takes a great deal longer (ten times or 2325more) to match typical phrases, and Perl takes so long that you think it has 2326gone into a loop. 2327</P> 2328<P> 2329<b>WARNING</b>: The palindrome-matching patterns above work only if the subject 2330string does not start with a palindrome that is shorter than the entire string. 2331For example, although "abcba" is correctly matched, if the subject is "ababa", 2332PCRE finds the palindrome "aba" at the start, then fails at top level because 2333the end of the string does not follow. Once again, it cannot jump back into the 2334recursion to try other alternatives, so the entire match fails. 2335<a name="subpatternsassubroutines"></a></P> 2336<br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br> 2337<P> 2338If the syntax for a recursive subpattern reference (either by number or by 2339name) is used outside the parentheses to which it refers, it operates like a 2340subroutine in a programming language. The "called" subpattern may be defined 2341before or after the reference. A numbered reference can be absolute or 2342relative, as in these examples: 2343<pre> 2344 (...(absolute)...)...(?2)... 2345 (...(relative)...)...(?-1)... 2346 (...(?+1)...(relative)... 2347</pre> 2348An earlier example pointed out that the pattern 2349<pre> 2350 (sens|respons)e and \1ibility 2351</pre> 2352matches "sense and sensibility" and "response and responsibility", but not 2353"sense and responsibility". If instead the pattern 2354<pre> 2355 (sens|respons)e and (?1)ibility 2356</pre> 2357is used, it does match "sense and responsibility" as well as the other two 2358strings. Another example is given in the discussion of DEFINE above. 2359</P> 2360<P> 2361Like recursive subpatterns, a subroutine call is always treated as an atomic 2362group. That is, once it has matched some of the subject string, it is never 2363re-entered, even if it contains untried alternatives and there is a subsequent 2364matching failure. Any capturing parentheses that are set during the subroutine 2365call revert to their previous values afterwards. 2366</P> 2367<P> 2368When a subpattern is used as a subroutine, processing options such as 2369case-independence are fixed when the subpattern is defined. They cannot be 2370changed for different calls. For example, consider this pattern: 2371<pre> 2372 (abc)(?i:(?-1)) 2373</pre> 2374It matches "abcabc". It does not match "abcABC" because the change of 2375processing option does not affect the called subpattern. 2376<a name="onigurumasubroutines"></a></P> 2377<br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br> 2378<P> 2379For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or 2380a number enclosed either in angle brackets or single quotes, is an alternative 2381syntax for referencing a subpattern as a subroutine, possibly recursively. Here 2382are two of the examples used above, rewritten using this syntax: 2383<pre> 2384 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) 2385 (sens|respons)e and \g'1'ibility 2386</pre> 2387PCRE supports an extension to Oniguruma: if a number is preceded by a 2388plus or a minus sign it is taken as a relative reference. For example: 2389<pre> 2390 (abc)(?i:\g<-1>) 2391</pre> 2392Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i> 2393synonymous. The former is a back reference; the latter is a subroutine call. 2394</P> 2395<br><a name="SEC24" href="#TOC1">CALLOUTS</a><br> 2396<P> 2397Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl 2398code to be obeyed in the middle of matching a regular expression. This makes it 2399possible, amongst other things, to extract different substrings that match the 2400same pair of parentheses when there is a repetition. 2401</P> 2402<P> 2403PCRE provides a similar feature, but of course it cannot obey arbitrary Perl 2404code. The feature is called "callout". The caller of PCRE provides an external 2405function by putting its entry point in the global variable <i>pcre_callout</i>. 2406By default, this variable contains NULL, which disables all calling out. 2407</P> 2408<P> 2409Within a regular expression, (?C) indicates the points at which the external 2410function is to be called. If you want to identify different callout points, you 2411can put a number less than 256 after the letter C. The default value is zero. 2412For example, this pattern has two callout points: 2413<pre> 2414 (?C1)abc(?C2)def 2415</pre> 2416If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are 2417automatically installed before each item in the pattern. They are all numbered 2418255. 2419</P> 2420<P> 2421During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is 2422set), the external function is called. It is provided with the number of the 2423callout, the position in the pattern, and, optionally, one item of data 2424originally supplied by the caller of <b>pcre_exec()</b>. The callout function 2425may cause matching to proceed, to backtrack, or to fail altogether. A complete 2426description of the interface to the callout function is given in the 2427<a href="pcrecallout.html"><b>pcrecallout</b></a> 2428documentation. 2429<a name="backtrackcontrol"></a></P> 2430<br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br> 2431<P> 2432Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which 2433are described in the Perl documentation as "experimental and subject to change 2434or removal in a future version of Perl". It goes on to say: "Their usage in 2435production code should be noted to avoid problems during upgrades." The same 2436remarks apply to the PCRE features described in this section. 2437</P> 2438<P> 2439Since these verbs are specifically related to backtracking, most of them can be 2440used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses 2441a backtracking algorithm. With the exception of (*FAIL), which behaves like a 2442failing negative assertion, they cause an error if encountered by 2443<b>pcre_dfa_exec()</b>. 2444</P> 2445<P> 2446If any of these verbs are used in an assertion or subroutine subpattern 2447(including recursive subpatterns), their effect is confined to that subpattern; 2448it does not extend to the surrounding pattern. Note that such subpatterns are 2449processed as anchored at the point where they are tested. 2450</P> 2451<P> 2452The new verbs make use of what was previously invalid syntax: an opening 2453parenthesis followed by an asterisk. They are generally of the form 2454(*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour, 2455depending on whether or not an argument is present. An name is a sequence of 2456letters, digits, and underscores. If the name is empty, that is, if the closing 2457parenthesis immediately follows the colon, the effect is as if the colon were 2458not there. Any number of these verbs may occur in a pattern. 2459</P> 2460<P> 2461PCRE contains some optimizations that are used to speed up matching by running 2462some checks at the start of each match attempt. For example, it may know the 2463minimum length of matching subject, or that a particular character must be 2464present. When one of these optimizations suppresses the running of a match, any 2465included backtracking verbs will not, of course, be processed. You can suppress 2466the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option 2467when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the 2468pattern with (*NO_START_OPT). 2469</P> 2470<br><b> 2471Verbs that act immediately 2472</b><br> 2473<P> 2474The following verbs act as soon as they are encountered. They may not be 2475followed by a name. 2476<pre> 2477 (*ACCEPT) 2478</pre> 2479This verb causes the match to end successfully, skipping the remainder of the 2480pattern. When inside a recursion, only the innermost pattern is ended 2481immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is 2482captured. (This feature was added to PCRE at release 8.00.) For example: 2483<pre> 2484 A((?:A|B(*ACCEPT)|C)D) 2485</pre> 2486This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by 2487the outer parentheses. 2488<pre> 2489 (*FAIL) or (*F) 2490</pre> 2491This verb causes the match to fail, forcing backtracking to occur. It is 2492equivalent to (?!) but easier to read. The Perl documentation notes that it is 2493probably useful only when combined with (?{}) or (??{}). Those are, of course, 2494Perl features that are not present in PCRE. The nearest equivalent is the 2495callout feature, as for example in this pattern: 2496<pre> 2497 a+(?C)(*FAIL) 2498</pre> 2499A match with the string "aaaa" always fails, but the callout is taken before 2500each backtrack happens (in this example, 10 times). 2501</P> 2502<br><b> 2503Recording which path was taken 2504</b><br> 2505<P> 2506There is one verb whose main purpose is to track how a match was arrived at, 2507though it also has a secondary use in conjunction with advancing the match 2508starting point (see (*SKIP) below). 2509<pre> 2510 (*MARK:NAME) or (*:NAME) 2511</pre> 2512A name is always required with this verb. There may be as many instances of 2513(*MARK) as you like in a pattern, and their names do not have to be unique. 2514</P> 2515<P> 2516When a match succeeds, the name of the last-encountered (*MARK) is passed back 2517to the caller via the <i>pcre_extra</i> data structure, as described in the 2518<a href="pcreapi.html#extradata">section on <i>pcre_extra</i></a> 2519in the 2520<a href="pcreapi.html"><b>pcreapi</b></a> 2521documentation. No data is returned for a partial match. Here is an example of 2522<b>pcretest</b> output, where the /K modifier requests the retrieval and 2523outputting of (*MARK) data: 2524<pre> 2525 /X(*MARK:A)Y|X(*MARK:B)Z/K 2526 XY 2527 0: XY 2528 MK: A 2529 XZ 2530 0: XZ 2531 MK: B 2532</pre> 2533The (*MARK) name is tagged with "MK:" in this output, and in this example it 2534indicates which of the two alternatives matched. This is a more efficient way 2535of obtaining this information than putting each alternative in its own 2536capturing parentheses. 2537</P> 2538<P> 2539A name may also be returned after a failed match if the final path through the 2540pattern involves (*MARK). However, unless (*MARK) used in conjunction with 2541(*COMMIT), this is unlikely to happen for an unanchored pattern because, as the 2542starting point for matching is advanced, the final check is often with an empty 2543string, causing a failure before (*MARK) is reached. For example: 2544<pre> 2545 /X(*MARK:A)Y|X(*MARK:B)Z/K 2546 XP 2547 No match 2548</pre> 2549There are three potential starting points for this match (starting with X, 2550starting with P, and with an empty string). If the pattern is anchored, the 2551result is different: 2552<pre> 2553 /^X(*MARK:A)Y|^X(*MARK:B)Z/K 2554 XP 2555 No match, mark = B 2556</pre> 2557PCRE's start-of-match optimizations can also interfere with this. For example, 2558if, as a result of a call to <b>pcre_study()</b>, it knows the minimum 2559subject length for a match, a shorter subject will not be scanned at all. 2560</P> 2561<P> 2562Note that similar anomalies (though different in detail) exist in Perl, no 2563doubt for the same reasons. The use of (*MARK) data after a failed match of an 2564unanchored pattern is not recommended, unless (*COMMIT) is involved. 2565</P> 2566<br><b> 2567Verbs that act after backtracking 2568</b><br> 2569<P> 2570The following verbs do nothing when they are encountered. Matching continues 2571with what follows, but if there is no subsequent match, causing a backtrack to 2572the verb, a failure is forced. That is, backtracking cannot pass to the left of 2573the verb. However, when one of these verbs appears inside an atomic group, its 2574effect is confined to that group, because once the group has been matched, 2575there is never any backtracking into it. In this situation, backtracking can 2576"jump back" to the left of the entire atomic group. (Remember also, as stated 2577above, that this localization also applies in subroutine calls and assertions.) 2578</P> 2579<P> 2580These verbs differ in exactly what kind of failure occurs when backtracking 2581reaches them. 2582<pre> 2583 (*COMMIT) 2584</pre> 2585This verb, which may not be followed by a name, causes the whole match to fail 2586outright if the rest of the pattern does not match. Even if the pattern is 2587unanchored, no further attempts to find a match by advancing the starting point 2588take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to 2589finding a match at the current starting point, or not at all. For example: 2590<pre> 2591 a+(*COMMIT)b 2592</pre> 2593This matches "xxaab" but not "aacaab". It can be thought of as a kind of 2594dynamic anchor, or "I've started, so I must finish." The name of the most 2595recently passed (*MARK) in the path is passed back when (*COMMIT) forces a 2596match failure. 2597</P> 2598<P> 2599Note that (*COMMIT) at the start of a pattern is not the same as an anchor, 2600unless PCRE's start-of-match optimizations are turned off, as shown in this 2601<b>pcretest</b> example: 2602<pre> 2603 /(*COMMIT)abc/ 2604 xyzabc 2605 0: abc 2606 xyzabc\Y 2607 No match 2608</pre> 2609PCRE knows that any match must start with "a", so the optimization skips along 2610the subject to "a" before running the first match attempt, which succeeds. When 2611the optimization is disabled by the \Y escape in the second subject, the match 2612starts at "x" and so the (*COMMIT) causes it to fail without trying any other 2613starting points. 2614<pre> 2615 (*PRUNE) or (*PRUNE:NAME) 2616</pre> 2617This verb causes the match to fail at the current starting position in the 2618subject if the rest of the pattern does not match. If the pattern is 2619unanchored, the normal "bumpalong" advance to the next starting character then 2620happens. Backtracking can occur as usual to the left of (*PRUNE), before it is 2621reached, or when matching to the right of (*PRUNE), but if there is no match to 2622the right, backtracking cannot cross (*PRUNE). In simple cases, the use of 2623(*PRUNE) is just an alternative to an atomic group or possessive quantifier, 2624but there are some uses of (*PRUNE) that cannot be expressed in any other way. 2625The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the 2626match fails completely; the name is passed back if this is the final attempt. 2627(*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored 2628pattern (*PRUNE) has the same effect as (*COMMIT). 2629<pre> 2630 (*SKIP) 2631</pre> 2632This verb, when given without a name, is like (*PRUNE), except that if the 2633pattern is unanchored, the "bumpalong" advance is not to the next character, 2634but to the position in the subject where (*SKIP) was encountered. (*SKIP) 2635signifies that whatever text was matched leading up to it cannot be part of a 2636successful match. Consider: 2637<pre> 2638 a+(*SKIP)b 2639</pre> 2640If the subject is "aaaac...", after the first match attempt fails (starting at 2641the first character in the string), the starting point skips on to start the 2642next attempt at "c". Note that a possessive quantifer does not have the same 2643effect as this example; although it would suppress backtracking during the 2644first match attempt, the second attempt would start at the second character 2645instead of skipping on to "c". 2646<pre> 2647 (*SKIP:NAME) 2648</pre> 2649When (*SKIP) has an associated name, its behaviour is modified. If the 2650following pattern fails to match, the previous path through the pattern is 2651searched for the most recent (*MARK) that has the same name. If one is found, 2652the "bumpalong" advance is to the subject position that corresponds to that 2653(*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a 2654matching name is found, normal "bumpalong" of one character happens (the 2655(*SKIP) is ignored). 2656<pre> 2657 (*THEN) or (*THEN:NAME) 2658</pre> 2659This verb causes a skip to the next alternation in the innermost enclosing 2660group if the rest of the pattern does not match. That is, it cancels pending 2661backtracking, but only within the current alternation. Its name comes from the 2662observation that it can be used for a pattern-based if-then-else block: 2663<pre> 2664 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... 2665</pre> 2666If the COND1 pattern matches, FOO is tried (and possibly further items after 2667the end of the group if FOO succeeds); on failure the matcher skips to the 2668second alternative and tries COND2, without backtracking into COND1. The 2669behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the 2670overall match fails. If (*THEN) is not directly inside an alternation, it acts 2671like (*PRUNE). 2672</P> 2673<P> 2674The above verbs provide four different "strengths" of control when subsequent 2675matching fails. (*THEN) is the weakest, carrying on the match at the next 2676alternation. (*PRUNE) comes next, failing the match at the current starting 2677position, but allowing an advance to the next character (for an unanchored 2678pattern). (*SKIP) is similar, except that the advance may be more than one 2679character. (*COMMIT) is the strongest, causing the entire match to fail. 2680</P> 2681<P> 2682If more than one is present in a pattern, the "stongest" one wins. For example, 2683consider this pattern, where A, B, etc. are complex pattern fragments: 2684<pre> 2685 (A(*COMMIT)B(*THEN)C|D) 2686</pre> 2687Once A has matched, PCRE is committed to this match, at the current starting 2688position. If subsequently B matches, but C does not, the normal (*THEN) action 2689of trying the next alternation (that is, D) does not happen because (*COMMIT) 2690overrides. 2691</P> 2692<br><a name="SEC26" href="#TOC1">SEE ALSO</a><br> 2693<P> 2694<b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), 2695<b>pcresyntax</b>(3), <b>pcre</b>(3). 2696</P> 2697<br><a name="SEC27" href="#TOC1">AUTHOR</a><br> 2698<P> 2699Philip Hazel 2700<br> 2701University Computing Service 2702<br> 2703Cambridge CB2 3QH, England. 2704<br> 2705</P> 2706<br><a name="SEC28" href="#TOC1">REVISION</a><br> 2707<P> 2708Last updated: 21 November 2010 2709<br> 2710Copyright © 1997-2010 University of Cambridge. 2711<br> 2712<p> 2713Return to the <a href="index.html">PCRE index page</a>. 2714</p> 2715