• Home
  • Line#
  • Scopes#
  • Navigate#
  • Raw
  • Download
1Technical Notes about PCRE2
2---------------------------
3
4These are very rough technical notes that record potentially useful information
5about PCRE2 internals. PCRE2 is a library based on the original PCRE library,
6but with a revised (and incompatible) API. To avoid confusion, the original
7library is referred to as PCRE1 below. For information about testing PCRE2, see
8the pcre2test documentation and the comment at the head of the RunTest file.
9
10PCRE1 releases were up to 8.3x when PCRE2 was developed. The 8.xx series will
11continue for bugfixes if necessary. PCRE2 releases started at 10.00 to avoid
12confusion with PCRE1.
13
14
15Historical note 1
16-----------------
17
18Many years ago I implemented some regular expression functions to an algorithm
19suggested by Martin Richards. These were not Unix-like in form, and were quite
20restricted in what they could do by comparison with Perl. The interesting part
21about the algorithm was that the amount of space required to hold the compiled
22form of an expression was known in advance. The code to apply an expression did
23not operate by backtracking, as the original Henry Spencer code and current
24PCRE2 and Perl code does, but instead checked all possibilities simultaneously
25by keeping a list of current states and checking all of them as it advanced
26through the subject string. In the terminology of Jeffrey Friedl's book, it was
27a "DFA algorithm", though it was not a traditional Finite State Machine (FSM).
28When the pattern was all used up, all remaining states were possible matches,
29and the one matching the longest subset of the subject string was chosen. This
30did not necessarily maximize the individual wild portions of the pattern, as is
31expected in Unix and Perl-style regular expressions.
32
33
34Historical note 2
35-----------------
36
37By contrast, the code originally written by Henry Spencer (which was
38subsequently heavily modified for Perl) compiles the expression twice: once in
39a dummy mode in order to find out how much store will be needed, and then for
40real. (The Perl version probably doesn't do this any more; I'm talking about
41the original library.) The execution function operates by backtracking and
42maximizing (or, optionally, minimizing, in Perl) the amount of the subject that
43matches individual wild portions of the pattern. This is an "NFA algorithm" in
44Friedl's terminology.
45
46
47OK, here's the real stuff
48-------------------------
49
50For the set of functions that formed the original PCRE1 library (which are
51unrelated to those mentioned above), I tried at first to invent an algorithm
52that used an amount of store bounded by a multiple of the number of characters
53in the pattern, to save on compiling time. However, because of the greater
54complexity in Perl regular expressions, I couldn't do this. In any case, a
55first pass through the pattern is helpful for other reasons.
56
57
58Support for 16-bit and 32-bit data strings
59-------------------------------------------
60
61The library can be compiled in any combination of 8-bit, 16-bit or 32-bit
62modes, creating up to three different libraries. In the description that
63follows, the word "short" is used for a 16-bit data quantity, and the phrase
64"code unit" is used for a quantity that is a byte in 8-bit mode, a short in
6516-bit mode and a 32-bit word in 32-bit mode. The names of PCRE2 functions are
66given in generic form, without the _8, _16, or _32 suffix.
67
68
69Computing the memory requirement: how it was
70--------------------------------------------
71
72Up to and including release 6.7, PCRE1 worked by running a very degenerate
73first pass to calculate a maximum memory requirement, and then a second pass to
74do the real compile - which might use a bit less than the predicted amount of
75memory. The idea was that this would turn out faster than the Henry Spencer
76code because the first pass is degenerate and the second pass can just store
77stuff straight into memory, which it knows is big enough.
78
79
80Computing the memory requirement: how it is
81-------------------------------------------
82
83By the time I was working on a potential 6.8 release, the degenerate first pass
84had become very complicated and hard to maintain. Indeed one of the early
85things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
86I had a flash of inspiration as to how I could run the real compile function in
87a "fake" mode that enables it to compute how much memory it would need, while
88actually only ever using a few hundred bytes of working memory, and without too
89many tests of the mode that might slow it down. So I refactored the compiling
90functions to work this way. This got rid of about 600 lines of source. It
91should make future maintenance and development easier. As this was such a major
92change, I never released 6.8, instead upping the number to 7.0 (other quite
93major changes were also present in the 7.0 release).
94
95A side effect of this work was that the previous limit of 200 on the nesting
96depth of parentheses was removed. However, there was a downside: compiling ran
97more slowly than before (30% or more, depending on the pattern) because it now
98did a full analysis of the pattern. My hope was that this would not be a big
99issue, and in the event, nobody has commented on it.
100
101At release 8.34, a limit on the nesting depth of parentheses was re-introduced
102(default 250, settable at build time) so as to put a limit on the amount of
103system stack used by the compile function, which uses recursive function calls
104for nested parenthesized groups. This is a safety feature for environments with
105small stacks where the patterns are provided by users.
106
107History repeated itself for release 10.20. A number of bugs relating to named
108subpatterns had been discovered by fuzzers. Most of these were related to the
109handling of forward references when it was not known if the named pattern was
110unique. (References to non-unique names use a different opcode and more
111memory.) The use of duplicate group numbers (the (?| facility) also caused
112issues.
113
114To get around these problems I adopted a new approach by adding a third pass,
115really a "pre-pass", over the pattern, which does nothing other than identify
116all the named subpatterns and their corresponding group numbers. This means
117that the actual compile (both pre-pass and real compile) have full knowledge of
118group names and numbers throughout. Several dozen lines of messy code were
119eliminated, though the new pre-pass is not short (skipping over [] classes is
120complicated).
121
122
123Traditional matching function
124-----------------------------
125
126The "traditional", and original, matching function is called pcre2_match(), and
127it implements an NFA algorithm, similar to the original Henry Spencer algorithm
128and the way that Perl works. This is not surprising, since it is intended to be
129as compatible with Perl as possible. This is the function most users of PCRE2
130will use most of the time. If PCRE2 is compiled with just-in-time (JIT)
131support, and studying a compiled pattern with JIT is successful, the JIT code
132is run instead of the normal pcre2_match() code, but the result is the same.
133
134
135Supplementary matching function
136-------------------------------
137
138There is also a supplementary matching function called pcre2_dfa_match(). This
139implements a DFA matching algorithm that searches simultaneously for all
140possible matches that start at one point in the subject string. (Going back to
141my roots: see Historical Note 1 above.) This function intreprets the same
142compiled pattern data as pcre2_match(); however, not all the facilities are
143available, and those that are do not always work in quite the same way. See the
144user documentation for details.
145
146The algorithm that is used for pcre2_dfa_match() is not a traditional FSM,
147because it may have a number of states active at one time. More work would be
148needed at compile time to produce a traditional FSM where only one state is
149ever active at once. I believe some other regex matchers work this way. JIT
150support is not available for this kind of matching.
151
152
153Changeable options
154------------------
155
156The /i, /m, or /s options (PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL, and
157some others) may change in the middle of patterns. Their processing is handled
158entirely at compile time by generating different opcodes for the different
159settings. The runtime functions do not need to keep track of an options state.
160
161
162Format of compiled patterns
163---------------------------
164
165The compiled form of a pattern is a vector of unsigned code units (bytes in
1668-bit mode, shorts in 16-bit mode, 32-bit words in 32-bit mode), containing
167items of variable length. The first code unit in an item contains an opcode,
168and the length of the item is either implicit in the opcode or contained in the
169data that follows it.
170
171In many cases listed below, LINK_SIZE data values are specified for offsets
172within the compiled pattern. LINK_SIZE always specifies a number of bytes. The
173default value for LINK_SIZE is 2, except for the 32-bit library, where it can
174only be 4. The 8-bit library can be compiled to used 3-byte or 4-byte values,
175and the 16-bit library can be compiled to use 4-byte values, though this
176impairs performance. Specifing a LINK_SIZE larger than 2 for these libraries is
177necessary only when patterns whose compiled length is greater than 64K code
178units are going to be processed. When a LINK_SIZE value uses more than one code
179unit, the most significant unit is first.
180
181In this description, we assume the "normal" compilation options. Data values
182that are counts (e.g. quantifiers) are always two bytes long in 8-bit mode
183(most significant byte first), or one code unit in 16-bit and 32-bit modes.
184
185
186Opcodes with no following data
187------------------------------
188
189These items are all just one unit long
190
191  OP_END                 end of pattern
192  OP_ANY                 match any one character other than newline
193  OP_ALLANY              match any one character, including newline
194  OP_ANYBYTE             match any single code unit, even in UTF-8/16 mode
195  OP_SOD                 match start of data: \A
196  OP_SOM,                start of match (subject + offset): \G
197  OP_SET_SOM,            set start of match (\K)
198  OP_CIRC                ^ (start of data)
199  OP_CIRCM               ^ multiline mode (start of data or after newline)
200  OP_NOT_WORD_BOUNDARY   \W
201  OP_WORD_BOUNDARY       \w
202  OP_NOT_DIGIT           \D
203  OP_DIGIT               \d
204  OP_NOT_HSPACE          \H
205  OP_HSPACE              \h
206  OP_NOT_WHITESPACE      \S
207  OP_WHITESPACE          \s
208  OP_NOT_VSPACE          \V
209  OP_VSPACE              \v
210  OP_NOT_WORDCHAR        \W
211  OP_WORDCHAR            \w
212  OP_EODN                match end of data or newline at end: \Z
213  OP_EOD                 match end of data: \z
214  OP_DOLL                $ (end of data, or before final newline)
215  OP_DOLLM               $ multiline mode (end of data or before newline)
216  OP_EXTUNI              match an extended Unicode grapheme cluster
217  OP_ANYNL               match any Unicode newline sequence
218
219  OP_ASSERT_ACCEPT       )
220  OP_ACCEPT              ) These are Perl 5.10's "backtracking control
221  OP_COMMIT              ) verbs". If OP_ACCEPT is inside capturing
222  OP_FAIL                ) parentheses, it may be preceded by one or more
223  OP_PRUNE               ) OP_CLOSE, each followed by a count that
224  OP_SKIP                ) indicates which parentheses must be closed.
225  OP_THEN                )
226
227OP_ASSERT_ACCEPT is used when (*ACCEPT) is encountered within an assertion.
228This ends the assertion, not the entire pattern match. The assertion (?!) is
229always optimized to OP_FAIL.
230
231OP_ALLANY is used for '.' when PCRE2_DOTALL is set. It is also used for \C in
232non-UTF modes and in UTF-32 mode (since one code unit still equals one
233character). Another use is for [^] when empty classes are permitted
234(PCRE2_ALLOW_EMPTY_CLASS is set).
235
236
237Backtracking control verbs with optional data
238---------------------------------------------
239
240(*THEN) without an argument generates the opcode OP_THEN and no following data.
241OP_MARK is followed by the mark name, preceded by a length in one code unit,
242and followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with
243arguments, the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used,
244with the name following in the same format as OP_MARK.
245
246
247Matching literal characters
248---------------------------
249
250The OP_CHAR opcode is followed by a single character that is to be matched
251casefully. For caseless matching, OP_CHARI is used. In UTF-8 or UTF-16 modes,
252the character may be more than one code unit long. In UTF-32 mode, characters
253are always exactly one code unit long.
254
255If there is only one character in a character class, OP_CHAR or OP_CHARI is
256used for a positive class, and OP_NOT or OP_NOTI for a negative one (that is,
257for something like [^a]).
258
259
260Repeating single characters
261---------------------------
262
263The common repeats (*, +, ?), when applied to a single character, use the
264following opcodes, which come in caseful and caseless versions:
265
266  Caseful         Caseless
267  OP_STAR         OP_STARI
268  OP_MINSTAR      OP_MINSTARI
269  OP_POSSTAR      OP_POSSTARI
270  OP_PLUS         OP_PLUSI
271  OP_MINPLUS      OP_MINPLUSI
272  OP_POSPLUS      OP_POSPLUSI
273  OP_QUERY        OP_QUERYI
274  OP_MINQUERY     OP_MINQUERYI
275  OP_POSQUERY     OP_POSQUERYI
276
277Each opcode is followed by the character that is to be repeated. In ASCII or
278UTF-32 modes, these are two-code-unit items; in UTF-8 or UTF-16 modes, the
279length is variable. Those with "MIN" in their names are the minimizing
280versions. Those with "POS" in their names are possessive versions. Other kinds
281of repeat make use of these opcodes:
282
283  Caseful         Caseless
284  OP_UPTO         OP_UPTOI
285  OP_MINUPTO      OP_MINUPTOI
286  OP_POSUPTO      OP_POSUPTOI
287  OP_EXACT        OP_EXACTI
288
289Each of these is followed by a count and then the repeated character. The count
290is two bytes long in 8-bit mode (most significant byte first), or one code unit
291in 16-bit and 32-bit modes.
292
293OP_UPTO matches from 0 to the given number. A repeat with a non-zero minimum
294and a fixed maximum is coded as an OP_EXACT followed by an OP_UPTO (or
295OP_MINUPTO or OPT_POSUPTO).
296
297Another set of matching repeating opcodes (called OP_NOTSTAR, OP_NOTSTARI,
298etc.) are used for repeated, negated, single-character classes such as [^a]*.
299The normal single-character opcodes (OP_STAR, etc.) are used for repeated
300positive single-character classes.
301
302
303Repeating character types
304-------------------------
305
306Repeats of things like \d are done exactly as for single characters, except
307that instead of a character, the opcode for the type (e.g. OP_DIGIT) is stored
308in the next code unit. The opcodes are:
309
310  OP_TYPESTAR
311  OP_TYPEMINSTAR
312  OP_TYPEPOSSTAR
313  OP_TYPEPLUS
314  OP_TYPEMINPLUS
315  OP_TYPEPOSPLUS
316  OP_TYPEQUERY
317  OP_TYPEMINQUERY
318  OP_TYPEPOSQUERY
319  OP_TYPEUPTO
320  OP_TYPEMINUPTO
321  OP_TYPEPOSUPTO
322  OP_TYPEEXACT
323
324
325Match by Unicode property
326-------------------------
327
328OP_PROP and OP_NOTPROP are used for positive and negative matches of a
329character by testing its Unicode property (the \p and \P escape sequences).
330Each is followed by two code units that encode the desired property as a type
331and a value. The types are a set of #defines of the form PT_xxx, and the values
332are enumerations of the form ucp_xx, defined in the pcre2_ucp.h source file.
333The value is relevant only for PT_GC (General Category), PT_PC (Particular
334Category), and PT_SC (Script).
335
336Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
337three code units: OP_PROP or OP_NOTPROP, and then the desired property type and
338value.
339
340
341Character classes
342-----------------
343
344If there is only one character in a class, OP_CHAR or OP_CHARI is used for a
345positive class, and OP_NOT or OP_NOTI for a negative one (that is, for
346something like [^a]).
347
348A set of repeating opcodes (called OP_NOTSTAR etc.) are used for repeated,
349negated, single-character classes. The normal single-character opcodes
350(OP_STAR, etc.) are used for repeated positive single-character classes.
351
352When there is more than one character in a class, and all the code points are
353less than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a
354negative one. In either case, the opcode is followed by a 32-byte (16-short,
3558-word) bit map containing a 1 bit for every character that is acceptable. The
356bits are counted from the least significant end of each unit. In caseless mode,
357bits for both cases are set.
358
359The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 and
36016-bit and 32-bit modes, subject characters with values greater than 255 can be
361handled correctly. For OP_CLASS they do not match, whereas for OP_NCLASS they
362do.
363
364For classes containing characters with values greater than 255 or that contain
365\p or \P, OP_XCLASS is used. It optionally uses a bit map if any acceptable
366code points are less than 256, followed by a list of pairs (for a range) and/or
367single characters and/or properties. In caseless mode, both cases are
368explicitly listed.
369
370OP_XCLASS is followed by a LINK_SIZE value containing the total length of the
371opcode and its data. This is followed by a code unit containing flag bits:
372XCL_NOT indicates that this is a negative class, and XCL_MAP indicates that a
373bit map is present. There follows the bit map, if XCL_MAP is set, and then a
374sequence of items coded as follows:
375
376  XCL_END      marks the end of the list
377  XCL_SINGLE   one character follows
378  XCL_RANGE    two characters follow
379  XCL_PROP     a Unicode property (type, value) follows
380  XCL_NOTPROP  a Unicode property (type, value) follows
381
382If a range starts with a code point less than 256 and ends with one greater
383than 255, it is split into two ranges, with characters less than 256 being
384indicated in the bit map, and the rest with XCL_RANGE.
385
386When XCL_NOT is set, the bit map, if present, contains bits for characters that
387are allowed (exactly as for OP_NCLASS), but the list of items that follow it
388specifies characters and properties that are not allowed.
389
390
391Back references
392---------------
393
394OP_REF (caseful) or OP_REFI (caseless) is followed by a count containing the
395reference number when the reference is to a unique capturing group (either by
396number or by name). When named groups are used, there may be more than one
397group with the same name. In this case, a reference to such a group by name
398generates OP_DNREF or OP_DNREFI. These are followed by two counts: the index
399(not the byte offset) in the group name table of the first entry for the
400required name, followed by the number of groups with the same name. The
401matching code can then search for the first one that is set.
402
403
404Repeating character classes and back references
405-----------------------------------------------
406
407Single-character classes are handled specially (see above). This section
408applies to other classes and also to back references. In both cases, the repeat
409information follows the base item. The matching code looks at the following
410opcode to see if it is one of these:
411
412  OP_CRSTAR
413  OP_CRMINSTAR
414  OP_CRPOSSTAR
415  OP_CRPLUS
416  OP_CRMINPLUS
417  OP_CRPOSPLUS
418  OP_CRQUERY
419  OP_CRMINQUERY
420  OP_CRPOSQUERY
421  OP_CRRANGE
422  OP_CRMINRANGE
423  OP_CRPOSRANGE
424
425All but the last three are single-code-unit items, with no data. The others are
426followed by the minimum and maximum repeat counts.
427
428
429Brackets and alternation
430------------------------
431
432A pair of non-capturing round brackets is wrapped round each expression at
433compile time, so alternation always happens in the context of brackets.
434
435[Note for North Americans: "bracket" to some English speakers, including
436myself, can be round, square, curly, or pointy. Hence this usage rather than
437"parentheses".]
438
439Non-capturing brackets use the opcode OP_BRA, capturing brackets use OP_CBRA. A
440bracket opcode is followed by a LINK_SIZE value which gives the offset to the
441next alternative OP_ALT or, if there aren't any branches, to the matching
442OP_KET opcode. Each OP_ALT is followed by a LINK_SIZE value giving the offset
443to the next one, or to the OP_KET opcode. For capturing brackets, the bracket
444number is a count that immediately follows the offset.
445
446OP_KET is used for subpatterns that do not repeat indefinitely, and OP_KETRMIN
447and OP_KETRMAX are used for indefinite repetitions, minimally or maximally
448respectively (see below for possessive repetitions). All three are followed by
449a LINK_SIZE value giving (as a positive number) the offset back to the matching
450bracket opcode.
451
452If a subpattern is quantified such that it is permitted to match zero times, it
453is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are
454single-unit opcodes that tell the matcher that skipping the following
455subpattern entirely is a valid match. In the case of the first two, not
456skipping the pattern is also valid (greedy and non-greedy). The third is used
457when a pattern has the quantifier {0,0}. It cannot be entirely discarded,
458because it may be called as a subroutine from elsewhere in the pattern.
459
460A subpattern with an indefinite maximum repetition is replicated in the
461compiled data its minimum number of times (or once with OP_BRAZERO if the
462minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
463as appropriate.
464
465A subpattern with a bounded maximum repetition is replicated in a nested
466fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
467before each replication after the minimum, so that, for example, (abc){2,5} is
468compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
469has the same number.
470
471When a repeated subpattern has an unbounded upper limit, it is checked to see
472whether it could match an empty string. If this is the case, the opcode in the
473final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
474that it needs to check for matching an empty string when it hits OP_KETRMIN or
475OP_KETRMAX, and if so, to break the loop.
476
477
478Possessive brackets
479-------------------
480
481When a repeated group (capturing or non-capturing) is marked as possessive by
482the "+" notation, e.g. (abc)++, different opcodes are used. Their names all
483have POS on the end, e.g. OP_BRAPOS instead of OP_BRA and OP_SCBRAPOS instead
484of OP_SCBRA. The end of such a group is marked by OP_KETRPOS. If the minimum
485repetition is zero, the group is preceded by OP_BRAPOSZERO.
486
487
488Once-only (atomic) groups
489-------------------------
490
491These are just like other subpatterns, but they start with the opcode
492OP_ONCE or OP_ONCE_NC. The former is used when there are no capturing brackets
493within the atomic group; the latter when there are. The distinction is needed
494for when there is a backtrack to before the group - any captures within the
495group must be reset, so it is necessary to retain backtracking points inside
496the group, even after it is complete, in order to do this. When there are no
497captures in an atomic group, all the backtracking can be discarded when it is
498complete. This is more efficient, and also uses less stack.
499
500The check for matching an empty string in an unbounded repeat is handled
501entirely at runtime, so there are just these two opcodes for atomic groups.
502
503
504Assertions
505----------
506
507Forward assertions are also just like other subpatterns, but starting with one
508of the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
509OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
510is OP_REVERSE, followed by a count of the number of characters to move back the
511pointer in the subject string. In ASCII or UTF-32 mode, the count is also the
512number of code units, but in UTF-8/16 mode each character may occupy more than
513one code unit. A separate count is present in each alternative of a lookbehind
514assertion, allowing them to have different (but fixed) lengths.
515
516
517Conditional subpatterns
518-----------------------
519
520These are like other subpatterns, but they start with the opcode OP_COND, or
521OP_SCOND for one that might match an empty string in an unbounded repeat.
522
523If the condition is a back reference, this is stored at the start of the
524subpattern using the opcode OP_CREF followed by a count containing the
525reference number, provided that the reference is to a unique capturing group.
526If the reference was by name and there is more than one group with that name,
527OP_DNCREF is used instead. It is followed by two counts: the index in the group
528names table, and the number of groups with the same name. The allows the
529matcher to check if any group with the given name is set.
530
531If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
532group x" (coded as "(?(Rx)"), the group number is stored at the start of the
533subpattern using the opcode OP_RREF (with a value of RREF_ANY (0xffff) for "the
534whole pattern") or OP_DNRREF (with data as for OP_DNCREF).
535
536For a DEFINE condition, OP_FALSE is used (with no associated data). During
537compilation, however, a DEFINE condition is coded as OP_DEFINE so that, when
538the conditional group is complete, there can be a check to ensure that it
539contains only one top-level branch. Once this has happened, the opcode is
540changed to OP_FALSE, so the matcher never sees OP_DEFINE.
541
542There is a special PCRE2-specific condition of the form (VERSION[>]=x.y), which
543tests the PCRE2 version number. This compiles into one of the opcodes OP_TRUE
544or OP_FALSE.
545
546If a condition is not a back reference, recursion test, DEFINE, or VERSION, it
547must start with an assertion, whose opcode normally immediately follows OP_COND
548or OP_SCOND. However, if automatic callouts are enabled, a callout is inserted
549immediately before the assertion. It is also possible to insert a manual
550callout at this point. Only assertion conditions may have callouts preceding
551the condition.
552
553A condition that is the negative assertion (?!) is optimized to OP_FAIL in all
554parts of the pattern, so this is another opcode that may appear as a condition.
555It is treated the same as OP_FALSE.
556
557
558Recursion
559---------
560
561Recursion either matches the current pattern, or some subexpression. The opcode
562OP_RECURSE is followed by a LINK_SIZE value that is the offset to the starting
563bracket from the start of the whole pattern. OP_RECURSE is also used for
564"subroutine" calls, even though they are not strictly a recursion. Repeated
565recursions are automatically wrapped inside OP_ONCE brackets, because otherwise
566some patterns broke them. A non-repeated recursion is not wrapped in OP_ONCE
567brackets, but it is nevertheless still treated as an atomic group.
568
569
570Callout
571-------
572
573A callout can nowadays have either a numerical argument or a string argument.
574These use OP_CALLOUT or OP_CALLOUT_STR, respectively. In each case these are
575followed by two LINK_SIZE values giving the offset in the pattern string to the
576start of the following item, and another count giving the length of this item.
577These values make it possible for pcre2test to output useful tracing
578information using callouts.
579
580In the case of a numeric callout, after these two values there is a single code
581unit containing the callout number, in the range 0-255, with 255 being used for
582callouts that are automatically inserted as a result of the PCRE2_AUTO_CALLOUT
583option. Thus, this opcode item is of fixed length:
584
585  [OP_CALLOUT] [PATTERN_OFFSET] [PATTERN_LENGTH] [NUMBER]
586
587For callouts with string arguments, OP_CALLOUT_STR has three more data items:
588a LINK_SIZE value giving the complete length of the entire opcode item, a
589LINK_SIZE item containing the offset within the pattern string to the start of
590the string argument, and the string itself, preceded by its starting delimiter
591and followed by a binary zero. When a callout function is called, a pointer to
592the actual string is passed, but the delimiter can be accessed as string[-1] if
593the application needs it. In the 8-bit library, the callout in /X(?C'abc')Y/ is
594compiled as the following bytes (decimal numbers represent binary values):
595
596  [OP_CALLOUT]  [0] [10]  [0] [1]  [0] [14]  [0] [5] ['] [a] [b] [c] [0]
597                --------  -------  --------  -------
598                   |         |        |         |
599                   ------- LINK_SIZE items ------
600
601Opcode table checking
602---------------------
603
604The last opcode that is defined in pcre2_internal.h is OP_TABLE_LENGTH. This is
605not a real opcode, but is used to check that tables indexed by opcode are the
606correct length, in order to catch updating errors.
607
608Philip Hazel
609June 2016
610