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1 -----------------------------------------------------------------------------
2 This file contains a concatenation of the PCRE man pages, converted to plain
3 text format for ease of searching with a text editor, or for use on systems
4 that do not have a man page processor. The small individual files that give
5 synopses of each function in the library have not been included. There are
6 separate text files for the pcregrep and pcretest commands.
7 -----------------------------------------------------------------------------
8
9
10 PCRE(3) PCRE(3)
11
12
13 NAME
14 PCRE - Perl-compatible regular expressions
15
16
17 INTRODUCTION
18
19 The PCRE library is a set of functions that implement regular expres-
20 sion pattern matching using the same syntax and semantics as Perl, with
21 just a few differences. (Certain features that appeared in Python and
22 PCRE before they appeared in Perl are also available using the Python
23 syntax.)
24
25 The current implementation of PCRE (release 7.x) corresponds approxi-
26 mately with Perl 5.10, including support for UTF-8 encoded strings and
27 Unicode general category properties. However, UTF-8 and Unicode support
28 has to be explicitly enabled; it is not the default. The Unicode tables
29 correspond to Unicode release 5.0.0.
30
31 In addition to the Perl-compatible matching function, PCRE contains an
32 alternative matching function that matches the same compiled patterns
33 in a different way. In certain circumstances, the alternative function
34 has some advantages. For a discussion of the two matching algorithms,
35 see the pcrematching page.
36
37 PCRE is written in C and released as a C library. A number of people
38 have written wrappers and interfaces of various kinds. In particular,
39 Google Inc. have provided a comprehensive C++ wrapper. This is now
40 included as part of the PCRE distribution. The pcrecpp page has details
41 of this interface. Other people's contributions can be found in the
42 Contrib directory at the primary FTP site, which is:
43
44 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
45
46 Details of exactly which Perl regular expression features are and are
47 not supported by PCRE are given in separate documents. See the pcrepat-
48 tern and pcrecompat pages. There is a syntax summary in the pcresyntax
49 page.
50
51 Some features of PCRE can be included, excluded, or changed when the
52 library is built. The pcre_config() function makes it possible for a
53 client to discover which features are available. The features them-
54 selves are described in the pcrebuild page. Documentation about build-
55 ing PCRE for various operating systems can be found in the README file
56 in the source distribution.
57
58 The library contains a number of undocumented internal functions and
59 data tables that are used by more than one of the exported external
60 functions, but which are not intended for use by external callers.
61 Their names all begin with "_pcre_", which hopefully will not provoke
62 any name clashes. In some environments, it is possible to control which
63 external symbols are exported when a shared library is built, and in
64 these cases the undocumented symbols are not exported.
65
66
67 USER DOCUMENTATION
68
69 The user documentation for PCRE comprises a number of different sec-
70 tions. In the "man" format, each of these is a separate "man page". In
71 the HTML format, each is a separate page, linked from the index page.
72 In the plain text format, all the sections are concatenated, for ease
73 of searching. The sections are as follows:
74
75 pcre this document
76 pcre-config show PCRE installation configuration information
77 pcreapi details of PCRE's native C API
78 pcrebuild options for building PCRE
79 pcrecallout details of the callout feature
80 pcrecompat discussion of Perl compatibility
81 pcrecpp details of the C++ wrapper
82 pcregrep description of the pcregrep command
83 pcrematching discussion of the two matching algorithms
84 pcrepartial details of the partial matching facility
85 pcrepattern syntax and semantics of supported
86 regular expressions
87 pcresyntax quick syntax reference
88 pcreperform discussion of performance issues
89 pcreposix the POSIX-compatible C API
90 pcreprecompile details of saving and re-using precompiled patterns
91 pcresample discussion of the sample program
92 pcrestack discussion of stack usage
93 pcretest description of the pcretest testing command
94
95 In addition, in the "man" and HTML formats, there is a short page for
96 each C library function, listing its arguments and results.
97
98
99 LIMITATIONS
100
101 There are some size limitations in PCRE but it is hoped that they will
102 never in practice be relevant.
103
104 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
105 is compiled with the default internal linkage size of 2. If you want to
106 process regular expressions that are truly enormous, you can compile
107 PCRE with an internal linkage size of 3 or 4 (see the README file in
108 the source distribution and the pcrebuild documentation for details).
109 In these cases the limit is substantially larger. However, the speed
110 of execution is slower.
111
112 All values in repeating quantifiers must be less than 65536.
113
114 There is no limit to the number of parenthesized subpatterns, but there
115 can be no more than 65535 capturing subpatterns.
116
117 The maximum length of name for a named subpattern is 32 characters, and
118 the maximum number of named subpatterns is 10000.
119
120 The maximum length of a subject string is the largest positive number
121 that an integer variable can hold. However, when using the traditional
122 matching function, PCRE uses recursion to handle subpatterns and indef-
123 inite repetition. This means that the available stack space may limit
124 the size of a subject string that can be processed by certain patterns.
125 For a discussion of stack issues, see the pcrestack documentation.
126
127
128 UTF-8 AND UNICODE PROPERTY SUPPORT
129
130 From release 3.3, PCRE has had some support for character strings
131 encoded in the UTF-8 format. For release 4.0 this was greatly extended
132 to cover most common requirements, and in release 5.0 additional sup-
133 port for Unicode general category properties was added.
134
135 In order process UTF-8 strings, you must build PCRE to include UTF-8
136 support in the code, and, in addition, you must call pcre_compile()
137 with the PCRE_UTF8 option flag. When you do this, both the pattern and
138 any subject strings that are matched against it are treated as UTF-8
139 strings instead of just strings of bytes.
140
141 If you compile PCRE with UTF-8 support, but do not use it at run time,
142 the library will be a bit bigger, but the additional run time overhead
143 is limited to testing the PCRE_UTF8 flag occasionally, so should not be
144 very big.
145
146 If PCRE is built with Unicode character property support (which implies
147 UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
148 ported. The available properties that can be tested are limited to the
149 general category properties such as Lu for an upper case letter or Nd
150 for a decimal number, the Unicode script names such as Arabic or Han,
151 and the derived properties Any and L&. A full list is given in the
152 pcrepattern documentation. Only the short names for properties are sup-
153 ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
154 ter}, is not supported. Furthermore, in Perl, many properties may
155 optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE
156 does not support this.
157
158 Validity of UTF-8 strings
159
160 When you set the PCRE_UTF8 flag, the strings passed as patterns and
161 subjects are (by default) checked for validity on entry to the relevant
162 functions. From release 7.3 of PCRE, the check is according the rules
163 of RFC 3629, which are themselves derived from the Unicode specifica-
164 tion. Earlier releases of PCRE followed the rules of RFC 2279, which
165 allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current
166 check allows only values in the range U+0 to U+10FFFF, excluding U+D800
167 to U+DFFF.
168
169 The excluded code points are the "Low Surrogate Area" of Unicode, of
170 which the Unicode Standard says this: "The Low Surrogate Area does not
171 contain any character assignments, consequently no character code
172 charts or namelists are provided for this area. Surrogates are reserved
173 for use with UTF-16 and then must be used in pairs." The code points
174 that are encoded by UTF-16 pairs are available as independent code
175 points in the UTF-8 encoding. (In other words, the whole surrogate
176 thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
177
178 If an invalid UTF-8 string is passed to PCRE, an error return
179 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
180 that your strings are valid, and therefore want to skip these checks in
181 order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
182 compile time or at run time, PCRE assumes that the pattern or subject
183 it is given (respectively) contains only valid UTF-8 codes. In this
184 case, it does not diagnose an invalid UTF-8 string.
185
186 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
187 what happens depends on why the string is invalid. If the string con-
188 forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
189 string of characters in the range 0 to 0x7FFFFFFF. In other words,
190 apart from the initial validity test, PCRE (when in UTF-8 mode) handles
191 strings according to the more liberal rules of RFC 2279. However, if
192 the string does not even conform to RFC 2279, the result is undefined.
193 Your program may crash.
194
195 If you want to process strings of values in the full range 0 to
196 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
197 set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
198 this situation, you will have to apply your own validity check.
199
200 General comments about UTF-8 mode
201
202 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a
203 two-byte UTF-8 character if the value is greater than 127.
204
205 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
206 characters for values greater than \177.
207
208 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
209 vidual bytes, for example: \x{100}{3}.
210
211 4. The dot metacharacter matches one UTF-8 character instead of a sin-
212 gle byte.
213
214 5. The escape sequence \C can be used to match a single byte in UTF-8
215 mode, but its use can lead to some strange effects. This facility is
216 not available in the alternative matching function, pcre_dfa_exec().
217
218 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
219 test characters of any code value, but the characters that PCRE recog-
220 nizes as digits, spaces, or word characters remain the same set as
221 before, all with values less than 256. This remains true even when PCRE
222 includes Unicode property support, because to do otherwise would slow
223 down PCRE in many common cases. If you really want to test for a wider
224 sense of, say, "digit", you must use Unicode property tests such as
225 \p{Nd}.
226
227 7. Similarly, characters that match the POSIX named character classes
228 are all low-valued characters.
229
230 8. However, the Perl 5.10 horizontal and vertical whitespace matching
231 escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
232 acters.
233
234 9. Case-insensitive matching applies only to characters whose values
235 are less than 128, unless PCRE is built with Unicode property support.
236 Even when Unicode property support is available, PCRE still uses its
237 own character tables when checking the case of low-valued characters,
238 so as not to degrade performance. The Unicode property information is
239 used only for characters with higher values. Even when Unicode property
240 support is available, PCRE supports case-insensitive matching only when
241 there is a one-to-one mapping between a letter's cases. There are a
242 small number of many-to-one mappings in Unicode; these are not sup-
243 ported by PCRE.
244
245
246 AUTHOR
247
248 Philip Hazel
249 University Computing Service
250 Cambridge CB2 3QH, England.
251
252 Putting an actual email address here seems to have been a spam magnet,
253 so I've taken it away. If you want to email me, use my two initials,
254 followed by the two digits 10, at the domain cam.ac.uk.
255
256
257 REVISION
258
259 Last updated: 09 August 2007
260 Copyright (c) 1997-2007 University of Cambridge.
261 ------------------------------------------------------------------------------
262
263
264 PCREBUILD(3) PCREBUILD(3)
265
266
267 NAME
268 PCRE - Perl-compatible regular expressions
269
270
271 PCRE BUILD-TIME OPTIONS
272
273 This document describes the optional features of PCRE that can be
274 selected when the library is compiled. They are all selected, or dese-
275 lected, by providing options to the configure script that is run before
276 the make command. The complete list of options for configure (which
277 includes the standard ones such as the selection of the installation
278 directory) can be obtained by running
279
280 ./configure --help
281
282 The following sections include descriptions of options whose names
283 begin with --enable or --disable. These settings specify changes to the
284 defaults for the configure command. Because of the way that configure
285 works, --enable and --disable always come in pairs, so the complemen-
286 tary option always exists as well, but as it specifies the default, it
287 is not described.
288
289
290 C++ SUPPORT
291
292 By default, the configure script will search for a C++ compiler and C++
293 header files. If it finds them, it automatically builds the C++ wrapper
294 library for PCRE. You can disable this by adding
295
296 --disable-cpp
297
298 to the configure command.
299
300
301 UTF-8 SUPPORT
302
303 To build PCRE with support for UTF-8 character strings, add
304
305 --enable-utf8
306
307 to the configure command. Of itself, this does not make PCRE treat
308 strings as UTF-8. As well as compiling PCRE with this option, you also
309 have have to set the PCRE_UTF8 option when you call the pcre_compile()
310 function.
311
312
313 UNICODE CHARACTER PROPERTY SUPPORT
314
315 UTF-8 support allows PCRE to process character values greater than 255
316 in the strings that it handles. On its own, however, it does not pro-
317 vide any facilities for accessing the properties of such characters. If
318 you want to be able to use the pattern escapes \P, \p, and \X, which
319 refer to Unicode character properties, you must add
320
321 --enable-unicode-properties
322
323 to the configure command. This implies UTF-8 support, even if you have
324 not explicitly requested it.
325
326 Including Unicode property support adds around 30K of tables to the
327 PCRE library. Only the general category properties such as Lu and Nd
328 are supported. Details are given in the pcrepattern documentation.
329
330
331 CODE VALUE OF NEWLINE
332
333 By default, PCRE interprets character 10 (linefeed, LF) as indicating
334 the end of a line. This is the normal newline character on Unix-like
335 systems. You can compile PCRE to use character 13 (carriage return, CR)
336 instead, by adding
337
338 --enable-newline-is-cr
339
340 to the configure command. There is also a --enable-newline-is-lf
341 option, which explicitly specifies linefeed as the newline character.
342
343 Alternatively, you can specify that line endings are to be indicated by
344 the two character sequence CRLF. If you want this, add
345
346 --enable-newline-is-crlf
347
348 to the configure command. There is a fourth option, specified by
349
350 --enable-newline-is-anycrlf
351
352 which causes PCRE to recognize any of the three sequences CR, LF, or
353 CRLF as indicating a line ending. Finally, a fifth option, specified by
354
355 --enable-newline-is-any
356
357 causes PCRE to recognize any Unicode newline sequence.
358
359 Whatever line ending convention is selected when PCRE is built can be
360 overridden when the library functions are called. At build time it is
361 conventional to use the standard for your operating system.
362
363
364 WHAT \R MATCHES
365
366 By default, the sequence \R in a pattern matches any Unicode newline
367 sequence, whatever has been selected as the line ending sequence. If
368 you specify
369
370 --enable-bsr-anycrlf
371
372 the default is changed so that \R matches only CR, LF, or CRLF. What-
373 ever is selected when PCRE is built can be overridden when the library
374 functions are called.
375
376
377 BUILDING SHARED AND STATIC LIBRARIES
378
379 The PCRE building process uses libtool to build both shared and static
380 Unix libraries by default. You can suppress one of these by adding one
381 of
382
383 --disable-shared
384 --disable-static
385
386 to the configure command, as required.
387
388
389 POSIX MALLOC USAGE
390
391 When PCRE is called through the POSIX interface (see the pcreposix doc-
392 umentation), additional working storage is required for holding the
393 pointers to capturing substrings, because PCRE requires three integers
394 per substring, whereas the POSIX interface provides only two. If the
395 number of expected substrings is small, the wrapper function uses space
396 on the stack, because this is faster than using malloc() for each call.
397 The default threshold above which the stack is no longer used is 10; it
398 can be changed by adding a setting such as
399
400 --with-posix-malloc-threshold=20
401
402 to the configure command.
403
404
405 HANDLING VERY LARGE PATTERNS
406
407 Within a compiled pattern, offset values are used to point from one
408 part to another (for example, from an opening parenthesis to an alter-
409 nation metacharacter). By default, two-byte values are used for these
410 offsets, leading to a maximum size for a compiled pattern of around
411 64K. This is sufficient to handle all but the most gigantic patterns.
412 Nevertheless, some people do want to process enormous patterns, so it
413 is possible to compile PCRE to use three-byte or four-byte offsets by
414 adding a setting such as
415
416 --with-link-size=3
417
418 to the configure command. The value given must be 2, 3, or 4. Using
419 longer offsets slows down the operation of PCRE because it has to load
420 additional bytes when handling them.
421
422
423 AVOIDING EXCESSIVE STACK USAGE
424
425 When matching with the pcre_exec() function, PCRE implements backtrack-
426 ing by making recursive calls to an internal function called match().
427 In environments where the size of the stack is limited, this can se-
428 verely limit PCRE's operation. (The Unix environment does not usually
429 suffer from this problem, but it may sometimes be necessary to increase
430 the maximum stack size. There is a discussion in the pcrestack docu-
431 mentation.) An alternative approach to recursion that uses memory from
432 the heap to remember data, instead of using recursive function calls,
433 has been implemented to work round the problem of limited stack size.
434 If you want to build a version of PCRE that works this way, add
435
436 --disable-stack-for-recursion
437
438 to the configure command. With this configuration, PCRE will use the
439 pcre_stack_malloc and pcre_stack_free variables to call memory manage-
440 ment functions. By default these point to malloc() and free(), but you
441 can replace the pointers so that your own functions are used.
442
443 Separate functions are provided rather than using pcre_malloc and
444 pcre_free because the usage is very predictable: the block sizes
445 requested are always the same, and the blocks are always freed in
446 reverse order. A calling program might be able to implement optimized
447 functions that perform better than malloc() and free(). PCRE runs
448 noticeably more slowly when built in this way. This option affects only
449 the pcre_exec() function; it is not relevant for the the
450 pcre_dfa_exec() function.
451
452
453 LIMITING PCRE RESOURCE USAGE
454
455 Internally, PCRE has a function called match(), which it calls repeat-
456 edly (sometimes recursively) when matching a pattern with the
457 pcre_exec() function. By controlling the maximum number of times this
458 function may be called during a single matching operation, a limit can
459 be placed on the resources used by a single call to pcre_exec(). The
460 limit can be changed at run time, as described in the pcreapi documen-
461 tation. The default is 10 million, but this can be changed by adding a
462 setting such as
463
464 --with-match-limit=500000
465
466 to the configure command. This setting has no effect on the
467 pcre_dfa_exec() matching function.
468
469 In some environments it is desirable to limit the depth of recursive
470 calls of match() more strictly than the total number of calls, in order
471 to restrict the maximum amount of stack (or heap, if --disable-stack-
472 for-recursion is specified) that is used. A second limit controls this;
473 it defaults to the value that is set for --with-match-limit, which
474 imposes no additional constraints. However, you can set a lower limit
475 by adding, for example,
476
477 --with-match-limit-recursion=10000
478
479 to the configure command. This value can also be overridden at run
480 time.
481
482
483 CREATING CHARACTER TABLES AT BUILD TIME
484
485 PCRE uses fixed tables for processing characters whose code values are
486 less than 256. By default, PCRE is built with a set of tables that are
487 distributed in the file pcre_chartables.c.dist. These tables are for
488 ASCII codes only. If you add
489
490 --enable-rebuild-chartables
491
492 to the configure command, the distributed tables are no longer used.
493 Instead, a program called dftables is compiled and run. This outputs
494 the source for new set of tables, created in the default locale of your
495 C runtime system. (This method of replacing the tables does not work if
496 you are cross compiling, because dftables is run on the local host. If
497 you need to create alternative tables when cross compiling, you will
498 have to do so "by hand".)
499
500
501 USING EBCDIC CODE
502
503 PCRE assumes by default that it will run in an environment where the
504 character code is ASCII (or Unicode, which is a superset of ASCII).
505 This is the case for most computer operating systems. PCRE can, how-
506 ever, be compiled to run in an EBCDIC environment by adding
507
508 --enable-ebcdic
509
510 to the configure command. This setting implies --enable-rebuild-charta-
511 bles. You should only use it if you know that you are in an EBCDIC
512 environment (for example, an IBM mainframe operating system).
513
514
515 SEE ALSO
516
517 pcreapi(3), pcre_config(3).
518
519
520 AUTHOR
521
522 Philip Hazel
523 University Computing Service
524 Cambridge CB2 3QH, England.
525
526
527 REVISION
528
529 Last updated: 11 September 2007
530 Copyright (c) 1997-2007 University of Cambridge.
531 ------------------------------------------------------------------------------
532
533
534 PCREMATCHING(3) PCREMATCHING(3)
535
536
537 NAME
538 PCRE - Perl-compatible regular expressions
539
540
541 PCRE MATCHING ALGORITHMS
542
543 This document describes the two different algorithms that are available
544 in PCRE for matching a compiled regular expression against a given sub-
545 ject string. The "standard" algorithm is the one provided by the
546 pcre_exec() function. This works in the same was as Perl's matching
547 function, and provides a Perl-compatible matching operation.
548
549 An alternative algorithm is provided by the pcre_dfa_exec() function;
550 this operates in a different way, and is not Perl-compatible. It has
551 advantages and disadvantages compared with the standard algorithm, and
552 these are described below.
553
554 When there is only one possible way in which a given subject string can
555 match a pattern, the two algorithms give the same answer. A difference
556 arises, however, when there are multiple possibilities. For example, if
557 the pattern
558
559 ^<.*>
560
561 is matched against the string
562
563 <something> <something else> <something further>
564
565 there are three possible answers. The standard algorithm finds only one
566 of them, whereas the alternative algorithm finds all three.
567
568
569 REGULAR EXPRESSIONS AS TREES
570
571 The set of strings that are matched by a regular expression can be rep-
572 resented as a tree structure. An unlimited repetition in the pattern
573 makes the tree of infinite size, but it is still a tree. Matching the
574 pattern to a given subject string (from a given starting point) can be
575 thought of as a search of the tree. There are two ways to search a
576 tree: depth-first and breadth-first, and these correspond to the two
577 matching algorithms provided by PCRE.
578
579
580 THE STANDARD MATCHING ALGORITHM
581
582 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
583 sions", the standard algorithm is an "NFA algorithm". It conducts a
584 depth-first search of the pattern tree. That is, it proceeds along a
585 single path through the tree, checking that the subject matches what is
586 required. When there is a mismatch, the algorithm tries any alterna-
587 tives at the current point, and if they all fail, it backs up to the
588 previous branch point in the tree, and tries the next alternative
589 branch at that level. This often involves backing up (moving to the
590 left) in the subject string as well. The order in which repetition
591 branches are tried is controlled by the greedy or ungreedy nature of
592 the quantifier.
593
594 If a leaf node is reached, a matching string has been found, and at
595 that point the algorithm stops. Thus, if there is more than one possi-
596 ble match, this algorithm returns the first one that it finds. Whether
597 this is the shortest, the longest, or some intermediate length depends
598 on the way the greedy and ungreedy repetition quantifiers are specified
599 in the pattern.
600
601 Because it ends up with a single path through the tree, it is rela-
602 tively straightforward for this algorithm to keep track of the sub-
603 strings that are matched by portions of the pattern in parentheses.
604 This provides support for capturing parentheses and back references.
605
606
607 THE ALTERNATIVE MATCHING ALGORITHM
608
609 This algorithm conducts a breadth-first search of the tree. Starting
610 from the first matching point in the subject, it scans the subject
611 string from left to right, once, character by character, and as it does
612 this, it remembers all the paths through the tree that represent valid
613 matches. In Friedl's terminology, this is a kind of "DFA algorithm",
614 though it is not implemented as a traditional finite state machine (it
615 keeps multiple states active simultaneously).
616
617 The scan continues until either the end of the subject is reached, or
618 there are no more unterminated paths. At this point, terminated paths
619 represent the different matching possibilities (if there are none, the
620 match has failed). Thus, if there is more than one possible match,
621 this algorithm finds all of them, and in particular, it finds the long-
622 est. In PCRE, there is an option to stop the algorithm after the first
623 match (which is necessarily the shortest) has been found.
624
625 Note that all the matches that are found start at the same point in the
626 subject. If the pattern
627
628 cat(er(pillar)?)
629
630 is matched against the string "the caterpillar catchment", the result
631 will be the three strings "cat", "cater", and "caterpillar" that start
632 at the fourth character of the subject. The algorithm does not automat-
633 ically move on to find matches that start at later positions.
634
635 There are a number of features of PCRE regular expressions that are not
636 supported by the alternative matching algorithm. They are as follows:
637
638 1. Because the algorithm finds all possible matches, the greedy or
639 ungreedy nature of repetition quantifiers is not relevant. Greedy and
640 ungreedy quantifiers are treated in exactly the same way. However, pos-
641 sessive quantifiers can make a difference when what follows could also
642 match what is quantified, for example in a pattern like this:
643
644 ^a++\w!
645
646 This pattern matches "aaab!" but not "aaa!", which would be matched by
647 a non-possessive quantifier. Similarly, if an atomic group is present,
648 it is matched as if it were a standalone pattern at the current point,
649 and the longest match is then "locked in" for the rest of the overall
650 pattern.
651
652 2. When dealing with multiple paths through the tree simultaneously, it
653 is not straightforward to keep track of captured substrings for the
654 different matching possibilities, and PCRE's implementation of this
655 algorithm does not attempt to do this. This means that no captured sub-
656 strings are available.
657
658 3. Because no substrings are captured, back references within the pat-
659 tern are not supported, and cause errors if encountered.
660
661 4. For the same reason, conditional expressions that use a backrefer-
662 ence as the condition or test for a specific group recursion are not
663 supported.
664
665 5. Because many paths through the tree may be active, the \K escape
666 sequence, which resets the start of the match when encountered (but may
667 be on some paths and not on others), is not supported. It causes an
668 error if encountered.
669
670 6. Callouts are supported, but the value of the capture_top field is
671 always 1, and the value of the capture_last field is always -1.
672
673 7. The \C escape sequence, which (in the standard algorithm) matches a
674 single byte, even in UTF-8 mode, is not supported because the alterna-
675 tive algorithm moves through the subject string one character at a
676 time, for all active paths through the tree.
677
678 8. None of the backtracking control verbs such as (*PRUNE) are sup-
679 ported.
680
681
682 ADVANTAGES OF THE ALTERNATIVE ALGORITHM
683
684 Using the alternative matching algorithm provides the following advan-
685 tages:
686
687 1. All possible matches (at a single point in the subject) are automat-
688 ically found, and in particular, the longest match is found. To find
689 more than one match using the standard algorithm, you have to do kludgy
690 things with callouts.
691
692 2. There is much better support for partial matching. The restrictions
693 on the content of the pattern that apply when using the standard algo-
694 rithm for partial matching do not apply to the alternative algorithm.
695 For non-anchored patterns, the starting position of a partial match is
696 available.
697
698 3. Because the alternative algorithm scans the subject string just
699 once, and never needs to backtrack, it is possible to pass very long
700 subject strings to the matching function in several pieces, checking
701 for partial matching each time.
702
703
704 DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
705
706 The alternative algorithm suffers from a number of disadvantages:
707
708 1. It is substantially slower than the standard algorithm. This is
709 partly because it has to search for all possible matches, but is also
710 because it is less susceptible to optimization.
711
712 2. Capturing parentheses and back references are not supported.
713
714 3. Although atomic groups are supported, their use does not provide the
715 performance advantage that it does for the standard algorithm.
716
717
718 AUTHOR
719
720 Philip Hazel
721 University Computing Service
722 Cambridge CB2 3QH, England.
723
724
725 REVISION
726
727 Last updated: 08 August 2007
728 Copyright (c) 1997-2007 University of Cambridge.
729 ------------------------------------------------------------------------------
730
731
732 PCREAPI(3) PCREAPI(3)
733
734
735 NAME
736 PCRE - Perl-compatible regular expressions
737
738
739 PCRE NATIVE API
740
741 #include <pcre.h>
742
743 pcre *pcre_compile(const char *pattern, int options,
744 const char **errptr, int *erroffset,
745 const unsigned char *tableptr);
746
747 pcre *pcre_compile2(const char *pattern, int options,
748 int *errorcodeptr,
749 const char **errptr, int *erroffset,
750 const unsigned char *tableptr);
751
752 pcre_extra *pcre_study(const pcre *code, int options,
753 const char **errptr);
754
755 int pcre_exec(const pcre *code, const pcre_extra *extra,
756 const char *subject, int length, int startoffset,
757 int options, int *ovector, int ovecsize);
758
759 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
760 const char *subject, int length, int startoffset,
761 int options, int *ovector, int ovecsize,
762 int *workspace, int wscount);
763
764 int pcre_copy_named_substring(const pcre *code,
765 const char *subject, int *ovector,
766 int stringcount, const char *stringname,
767 char *buffer, int buffersize);
768
769 int pcre_copy_substring(const char *subject, int *ovector,
770 int stringcount, int stringnumber, char *buffer,
771 int buffersize);
772
773 int pcre_get_named_substring(const pcre *code,
774 const char *subject, int *ovector,
775 int stringcount, const char *stringname,
776 const char **stringptr);
777
778 int pcre_get_stringnumber(const pcre *code,
779 const char *name);
780
781 int pcre_get_stringtable_entries(const pcre *code,
782 const char *name, char **first, char **last);
783
784 int pcre_get_substring(const char *subject, int *ovector,
785 int stringcount, int stringnumber,
786 const char **stringptr);
787
788 int pcre_get_substring_list(const char *subject,
789 int *ovector, int stringcount, const char ***listptr);
790
791 void pcre_free_substring(const char *stringptr);
792
793 void pcre_free_substring_list(const char **stringptr);
794
795 const unsigned char *pcre_maketables(void);
796
797 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
798 int what, void *where);
799
800 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
801
802 int pcre_refcount(pcre *code, int adjust);
803
804 int pcre_config(int what, void *where);
805
806 char *pcre_version(void);
807
808 void *(*pcre_malloc)(size_t);
809
810 void (*pcre_free)(void *);
811
812 void *(*pcre_stack_malloc)(size_t);
813
814 void (*pcre_stack_free)(void *);
815
816 int (*pcre_callout)(pcre_callout_block *);
817
818
819 PCRE API OVERVIEW
820
821 PCRE has its own native API, which is described in this document. There
822 are also some wrapper functions that correspond to the POSIX regular
823 expression API. These are described in the pcreposix documentation.
824 Both of these APIs define a set of C function calls. A C++ wrapper is
825 distributed with PCRE. It is documented in the pcrecpp page.
826
827 The native API C function prototypes are defined in the header file
828 pcre.h, and on Unix systems the library itself is called libpcre. It
829 can normally be accessed by adding -lpcre to the command for linking an
830 application that uses PCRE. The header file defines the macros
831 PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
832 bers for the library. Applications can use these to include support
833 for different releases of PCRE.
834
835 The functions pcre_compile(), pcre_compile2(), pcre_study(), and
836 pcre_exec() are used for compiling and matching regular expressions in
837 a Perl-compatible manner. A sample program that demonstrates the sim-
838 plest way of using them is provided in the file called pcredemo.c in
839 the source distribution. The pcresample documentation describes how to
840 run it.
841
842 A second matching function, pcre_dfa_exec(), which is not Perl-compati-
843 ble, is also provided. This uses a different algorithm for the match-
844 ing. The alternative algorithm finds all possible matches (at a given
845 point in the subject), and scans the subject just once. However, this
846 algorithm does not return captured substrings. A description of the two
847 matching algorithms and their advantages and disadvantages is given in
848 the pcrematching documentation.
849
850 In addition to the main compiling and matching functions, there are
851 convenience functions for extracting captured substrings from a subject
852 string that is matched by pcre_exec(). They are:
853
854 pcre_copy_substring()
855 pcre_copy_named_substring()
856 pcre_get_substring()
857 pcre_get_named_substring()
858 pcre_get_substring_list()
859 pcre_get_stringnumber()
860 pcre_get_stringtable_entries()
861
862 pcre_free_substring() and pcre_free_substring_list() are also provided,
863 to free the memory used for extracted strings.
864
865 The function pcre_maketables() is used to build a set of character
866 tables in the current locale for passing to pcre_compile(),
867 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
868 provided for specialist use. Most commonly, no special tables are
869 passed, in which case internal tables that are generated when PCRE is
870 built are used.
871
872 The function pcre_fullinfo() is used to find out information about a
873 compiled pattern; pcre_info() is an obsolete version that returns only
874 some of the available information, but is retained for backwards com-
875 patibility. The function pcre_version() returns a pointer to a string
876 containing the version of PCRE and its date of release.
877
878 The function pcre_refcount() maintains a reference count in a data
879 block containing a compiled pattern. This is provided for the benefit
880 of object-oriented applications.
881
882 The global variables pcre_malloc and pcre_free initially contain the
883 entry points of the standard malloc() and free() functions, respec-
884 tively. PCRE calls the memory management functions via these variables,
885 so a calling program can replace them if it wishes to intercept the
886 calls. This should be done before calling any PCRE functions.
887
888 The global variables pcre_stack_malloc and pcre_stack_free are also
889 indirections to memory management functions. These special functions
890 are used only when PCRE is compiled to use the heap for remembering
891 data, instead of recursive function calls, when running the pcre_exec()
892 function. See the pcrebuild documentation for details of how to do
893 this. It is a non-standard way of building PCRE, for use in environ-
894 ments that have limited stacks. Because of the greater use of memory
895 management, it runs more slowly. Separate functions are provided so
896 that special-purpose external code can be used for this case. When
897 used, these functions are always called in a stack-like manner (last
898 obtained, first freed), and always for memory blocks of the same size.
899 There is a discussion about PCRE's stack usage in the pcrestack docu-
900 mentation.
901
902 The global variable pcre_callout initially contains NULL. It can be set
903 by the caller to a "callout" function, which PCRE will then call at
904 specified points during a matching operation. Details are given in the
905 pcrecallout documentation.
906
907
908 NEWLINES
909
910 PCRE supports five different conventions for indicating line breaks in
911 strings: a single CR (carriage return) character, a single LF (line-
912 feed) character, the two-character sequence CRLF, any of the three pre-
913 ceding, or any Unicode newline sequence. The Unicode newline sequences
914 are the three just mentioned, plus the single characters VT (vertical
915 tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
916 separator, U+2028), and PS (paragraph separator, U+2029).
917
918 Each of the first three conventions is used by at least one operating
919 system as its standard newline sequence. When PCRE is built, a default
920 can be specified. The default default is LF, which is the Unix stan-
921 dard. When PCRE is run, the default can be overridden, either when a
922 pattern is compiled, or when it is matched.
923
924 At compile time, the newline convention can be specified by the options
925 argument of pcre_compile(), or it can be specified by special text at
926 the start of the pattern itself; this overrides any other settings. See
927 the pcrepattern page for details of the special character sequences.
928
929 In the PCRE documentation the word "newline" is used to mean "the char-
930 acter or pair of characters that indicate a line break". The choice of
931 newline convention affects the handling of the dot, circumflex, and
932 dollar metacharacters, the handling of #-comments in /x mode, and, when
933 CRLF is a recognized line ending sequence, the match position advance-
934 ment for a non-anchored pattern. There is more detail about this in the
935 section on pcre_exec() options below.
936
937 The choice of newline convention does not affect the interpretation of
938 the \n or \r escape sequences, nor does it affect what \R matches,
939 which is controlled in a similar way, but by separate options.
940
941
942 MULTITHREADING
943
944 The PCRE functions can be used in multi-threading applications, with
945 the proviso that the memory management functions pointed to by
946 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
947 callout function pointed to by pcre_callout, are shared by all threads.
948
949 The compiled form of a regular expression is not altered during match-
950 ing, so the same compiled pattern can safely be used by several threads
951 at once.
952
953
954 SAVING PRECOMPILED PATTERNS FOR LATER USE
955
956 The compiled form of a regular expression can be saved and re-used at a
957 later time, possibly by a different program, and even on a host other
958 than the one on which it was compiled. Details are given in the
959 pcreprecompile documentation. However, compiling a regular expression
960 with one version of PCRE for use with a different version is not guar-
961 anteed to work and may cause crashes.
962
963
964 CHECKING BUILD-TIME OPTIONS
965
966 int pcre_config(int what, void *where);
967
968 The function pcre_config() makes it possible for a PCRE client to dis-
969 cover which optional features have been compiled into the PCRE library.
970 The pcrebuild documentation has more details about these optional fea-
971 tures.
972
973 The first argument for pcre_config() is an integer, specifying which
974 information is required; the second argument is a pointer to a variable
975 into which the information is placed. The following information is
976 available:
977
978 PCRE_CONFIG_UTF8
979
980 The output is an integer that is set to one if UTF-8 support is avail-
981 able; otherwise it is set to zero.
982
983 PCRE_CONFIG_UNICODE_PROPERTIES
984
985 The output is an integer that is set to one if support for Unicode
986 character properties is available; otherwise it is set to zero.
987
988 PCRE_CONFIG_NEWLINE
989
990 The output is an integer whose value specifies the default character
991 sequence that is recognized as meaning "newline". The four values that
992 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
993 and -1 for ANY. The default should normally be the standard sequence
994 for your operating system.
995
996 PCRE_CONFIG_BSR
997
998 The output is an integer whose value indicates what character sequences
999 the \R escape sequence matches by default. A value of 0 means that \R
1000 matches any Unicode line ending sequence; a value of 1 means that \R
1001 matches only CR, LF, or CRLF. The default can be overridden when a pat-
1002 tern is compiled or matched.
1003
1004 PCRE_CONFIG_LINK_SIZE
1005
1006 The output is an integer that contains the number of bytes used for
1007 internal linkage in compiled regular expressions. The value is 2, 3, or
1008 4. Larger values allow larger regular expressions to be compiled, at
1009 the expense of slower matching. The default value of 2 is sufficient
1010 for all but the most massive patterns, since it allows the compiled
1011 pattern to be up to 64K in size.
1012
1013 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
1014
1015 The output is an integer that contains the threshold above which the
1016 POSIX interface uses malloc() for output vectors. Further details are
1017 given in the pcreposix documentation.
1018
1019 PCRE_CONFIG_MATCH_LIMIT
1020
1021 The output is an integer that gives the default limit for the number of
1022 internal matching function calls in a pcre_exec() execution. Further
1023 details are given with pcre_exec() below.
1024
1025 PCRE_CONFIG_MATCH_LIMIT_RECURSION
1026
1027 The output is an integer that gives the default limit for the depth of
1028 recursion when calling the internal matching function in a pcre_exec()
1029 execution. Further details are given with pcre_exec() below.
1030
1031 PCRE_CONFIG_STACKRECURSE
1032
1033 The output is an integer that is set to one if internal recursion when
1034 running pcre_exec() is implemented by recursive function calls that use
1035 the stack to remember their state. This is the usual way that PCRE is
1036 compiled. The output is zero if PCRE was compiled to use blocks of data
1037 on the heap instead of recursive function calls. In this case,
1038 pcre_stack_malloc and pcre_stack_free are called to manage memory
1039 blocks on the heap, thus avoiding the use of the stack.
1040
1041
1042 COMPILING A PATTERN
1043
1044 pcre *pcre_compile(const char *pattern, int options,
1045 const char **errptr, int *erroffset,
1046 const unsigned char *tableptr);
1047
1048 pcre *pcre_compile2(const char *pattern, int options,
1049 int *errorcodeptr,
1050 const char **errptr, int *erroffset,
1051 const unsigned char *tableptr);
1052
1053 Either of the functions pcre_compile() or pcre_compile2() can be called
1054 to compile a pattern into an internal form. The only difference between
1055 the two interfaces is that pcre_compile2() has an additional argument,
1056 errorcodeptr, via which a numerical error code can be returned.
1057
1058 The pattern is a C string terminated by a binary zero, and is passed in
1059 the pattern argument. A pointer to a single block of memory that is
1060 obtained via pcre_malloc is returned. This contains the compiled code
1061 and related data. The pcre type is defined for the returned block; this
1062 is a typedef for a structure whose contents are not externally defined.
1063 It is up to the caller to free the memory (via pcre_free) when it is no
1064 longer required.
1065
1066 Although the compiled code of a PCRE regex is relocatable, that is, it
1067 does not depend on memory location, the complete pcre data block is not
1068 fully relocatable, because it may contain a copy of the tableptr argu-
1069 ment, which is an address (see below).
1070
1071 The options argument contains various bit settings that affect the com-
1072 pilation. It should be zero if no options are required. The available
1073 options are described below. Some of them, in particular, those that
1074 are compatible with Perl, can also be set and unset from within the
1075 pattern (see the detailed description in the pcrepattern documenta-
1076 tion). For these options, the contents of the options argument speci-
1077 fies their initial settings at the start of compilation and execution.
1078 The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at the time
1079 of matching as well as at compile time.
1080
1081 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1082 if compilation of a pattern fails, pcre_compile() returns NULL, and
1083 sets the variable pointed to by errptr to point to a textual error mes-
1084 sage. This is a static string that is part of the library. You must not
1085 try to free it. The offset from the start of the pattern to the charac-
1086 ter where the error was discovered is placed in the variable pointed to
1087 by erroffset, which must not be NULL. If it is, an immediate error is
1088 given.
1089
1090 If pcre_compile2() is used instead of pcre_compile(), and the error-
1091 codeptr argument is not NULL, a non-zero error code number is returned
1092 via this argument in the event of an error. This is in addition to the
1093 textual error message. Error codes and messages are listed below.
1094
1095 If the final argument, tableptr, is NULL, PCRE uses a default set of
1096 character tables that are built when PCRE is compiled, using the
1097 default C locale. Otherwise, tableptr must be an address that is the
1098 result of a call to pcre_maketables(). This value is stored with the
1099 compiled pattern, and used again by pcre_exec(), unless another table
1100 pointer is passed to it. For more discussion, see the section on locale
1101 support below.
1102
1103 This code fragment shows a typical straightforward call to pcre_com-
1104 pile():
1105
1106 pcre *re;
1107 const char *error;
1108 int erroffset;
1109 re = pcre_compile(
1110 "^A.*Z", /* the pattern */
1111 0, /* default options */
1112 &error, /* for error message */
1113 &erroffset, /* for error offset */
1114 NULL); /* use default character tables */
1115
1116 The following names for option bits are defined in the pcre.h header
1117 file:
1118
1119 PCRE_ANCHORED
1120
1121 If this bit is set, the pattern is forced to be "anchored", that is, it
1122 is constrained to match only at the first matching point in the string
1123 that is being searched (the "subject string"). This effect can also be
1124 achieved by appropriate constructs in the pattern itself, which is the
1125 only way to do it in Perl.
1126
1127 PCRE_AUTO_CALLOUT
1128
1129 If this bit is set, pcre_compile() automatically inserts callout items,
1130 all with number 255, before each pattern item. For discussion of the
1131 callout facility, see the pcrecallout documentation.
1132
1133 PCRE_BSR_ANYCRLF
1134 PCRE_BSR_UNICODE
1135
1136 These options (which are mutually exclusive) control what the \R escape
1137 sequence matches. The choice is either to match only CR, LF, or CRLF,
1138 or to match any Unicode newline sequence. The default is specified when
1139 PCRE is built. It can be overridden from within the pattern, or by set-
1140 ting an option when a compiled pattern is matched.
1141
1142 PCRE_CASELESS
1143
1144 If this bit is set, letters in the pattern match both upper and lower
1145 case letters. It is equivalent to Perl's /i option, and it can be
1146 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1147 always understands the concept of case for characters whose values are
1148 less than 128, so caseless matching is always possible. For characters
1149 with higher values, the concept of case is supported if PCRE is com-
1150 piled with Unicode property support, but not otherwise. If you want to
1151 use caseless matching for characters 128 and above, you must ensure
1152 that PCRE is compiled with Unicode property support as well as with
1153 UTF-8 support.
1154
1155 PCRE_DOLLAR_ENDONLY
1156
1157 If this bit is set, a dollar metacharacter in the pattern matches only
1158 at the end of the subject string. Without this option, a dollar also
1159 matches immediately before a newline at the end of the string (but not
1160 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1161 if PCRE_MULTILINE is set. There is no equivalent to this option in
1162 Perl, and no way to set it within a pattern.
1163
1164 PCRE_DOTALL
1165
1166 If this bit is set, a dot metacharater in the pattern matches all char-
1167 acters, including those that indicate newline. Without it, a dot does
1168 not match when the current position is at a newline. This option is
1169 equivalent to Perl's /s option, and it can be changed within a pattern
1170 by a (?s) option setting. A negative class such as [^a] always matches
1171 newline characters, independent of the setting of this option.
1172
1173 PCRE_DUPNAMES
1174
1175 If this bit is set, names used to identify capturing subpatterns need
1176 not be unique. This can be helpful for certain types of pattern when it
1177 is known that only one instance of the named subpattern can ever be
1178 matched. There are more details of named subpatterns below; see also
1179 the pcrepattern documentation.
1180
1181 PCRE_EXTENDED
1182
1183 If this bit is set, whitespace data characters in the pattern are
1184 totally ignored except when escaped or inside a character class. White-
1185 space does not include the VT character (code 11). In addition, charac-
1186 ters between an unescaped # outside a character class and the next new-
1187 line, inclusive, are also ignored. This is equivalent to Perl's /x
1188 option, and it can be changed within a pattern by a (?x) option set-
1189 ting.
1190
1191 This option makes it possible to include comments inside complicated
1192 patterns. Note, however, that this applies only to data characters.
1193 Whitespace characters may never appear within special character
1194 sequences in a pattern, for example within the sequence (?( which
1195 introduces a conditional subpattern.
1196
1197 PCRE_EXTRA
1198
1199 This option was invented in order to turn on additional functionality
1200 of PCRE that is incompatible with Perl, but it is currently of very
1201 little use. When set, any backslash in a pattern that is followed by a
1202 letter that has no special meaning causes an error, thus reserving
1203 these combinations for future expansion. By default, as in Perl, a
1204 backslash followed by a letter with no special meaning is treated as a
1205 literal. (Perl can, however, be persuaded to give a warning for this.)
1206 There are at present no other features controlled by this option. It
1207 can also be set by a (?X) option setting within a pattern.
1208
1209 PCRE_FIRSTLINE
1210
1211 If this option is set, an unanchored pattern is required to match
1212 before or at the first newline in the subject string, though the
1213 matched text may continue over the newline.
1214
1215 PCRE_MULTILINE
1216
1217 By default, PCRE treats the subject string as consisting of a single
1218 line of characters (even if it actually contains newlines). The "start
1219 of line" metacharacter (^) matches only at the start of the string,
1220 while the "end of line" metacharacter ($) matches only at the end of
1221 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1222 is set). This is the same as Perl.
1223
1224 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1225 constructs match immediately following or immediately before internal
1226 newlines in the subject string, respectively, as well as at the very
1227 start and end. This is equivalent to Perl's /m option, and it can be
1228 changed within a pattern by a (?m) option setting. If there are no new-
1229 lines in a subject string, or no occurrences of ^ or $ in a pattern,
1230 setting PCRE_MULTILINE has no effect.
1231
1232 PCRE_NEWLINE_CR
1233 PCRE_NEWLINE_LF
1234 PCRE_NEWLINE_CRLF
1235 PCRE_NEWLINE_ANYCRLF
1236 PCRE_NEWLINE_ANY
1237
1238 These options override the default newline definition that was chosen
1239 when PCRE was built. Setting the first or the second specifies that a
1240 newline is indicated by a single character (CR or LF, respectively).
1241 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1242 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1243 that any of the three preceding sequences should be recognized. Setting
1244 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1245 recognized. The Unicode newline sequences are the three just mentioned,
1246 plus the single characters VT (vertical tab, U+000B), FF (formfeed,
1247 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1248 (paragraph separator, U+2029). The last two are recognized only in
1249 UTF-8 mode.
1250
1251 The newline setting in the options word uses three bits that are
1252 treated as a number, giving eight possibilities. Currently only six are
1253 used (default plus the five values above). This means that if you set
1254 more than one newline option, the combination may or may not be sensi-
1255 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1256 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1257 cause an error.
1258
1259 The only time that a line break is specially recognized when compiling
1260 a pattern is if PCRE_EXTENDED is set, and an unescaped # outside a
1261 character class is encountered. This indicates a comment that lasts
1262 until after the next line break sequence. In other circumstances, line
1263 break sequences are treated as literal data, except that in
1264 PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1265 and are therefore ignored.
1266
1267 The newline option that is set at compile time becomes the default that
1268 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1269
1270 PCRE_NO_AUTO_CAPTURE
1271
1272 If this option is set, it disables the use of numbered capturing paren-
1273 theses in the pattern. Any opening parenthesis that is not followed by
1274 ? behaves as if it were followed by ?: but named parentheses can still
1275 be used for capturing (and they acquire numbers in the usual way).
1276 There is no equivalent of this option in Perl.
1277
1278 PCRE_UNGREEDY
1279
1280 This option inverts the "greediness" of the quantifiers so that they
1281 are not greedy by default, but become greedy if followed by "?". It is
1282 not compatible with Perl. It can also be set by a (?U) option setting
1283 within the pattern.
1284
1285 PCRE_UTF8
1286
1287 This option causes PCRE to regard both the pattern and the subject as
1288 strings of UTF-8 characters instead of single-byte character strings.
1289 However, it is available only when PCRE is built to include UTF-8 sup-
1290 port. If not, the use of this option provokes an error. Details of how
1291 this option changes the behaviour of PCRE are given in the section on
1292 UTF-8 support in the main pcre page.
1293
1294 PCRE_NO_UTF8_CHECK
1295
1296 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1297 automatically checked. There is a discussion about the validity of
1298 UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of
1299 bytes is found, pcre_compile() returns an error. If you already know
1300 that your pattern is valid, and you want to skip this check for perfor-
1301 mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is
1302 set, the effect of passing an invalid UTF-8 string as a pattern is
1303 undefined. It may cause your program to crash. Note that this option
1304 can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
1305 UTF-8 validity checking of subject strings.
1306
1307
1308 COMPILATION ERROR CODES
1309
1310 The following table lists the error codes than may be returned by
1311 pcre_compile2(), along with the error messages that may be returned by
1312 both compiling functions. As PCRE has developed, some error codes have
1313 fallen out of use. To avoid confusion, they have not been re-used.
1314
1315 0 no error
1316 1 \ at end of pattern
1317 2 \c at end of pattern
1318 3 unrecognized character follows \
1319 4 numbers out of order in {} quantifier
1320 5 number too big in {} quantifier
1321 6 missing terminating ] for character class
1322 7 invalid escape sequence in character class
1323 8 range out of order in character class
1324 9 nothing to repeat
1325 10 [this code is not in use]
1326 11 internal error: unexpected repeat
1327 12 unrecognized character after (?
1328 13 POSIX named classes are supported only within a class
1329 14 missing )
1330 15 reference to non-existent subpattern
1331 16 erroffset passed as NULL
1332 17 unknown option bit(s) set
1333 18 missing ) after comment
1334 19 [this code is not in use]
1335 20 regular expression too large
1336 21 failed to get memory
1337 22 unmatched parentheses
1338 23 internal error: code overflow
1339 24 unrecognized character after (?<
1340 25 lookbehind assertion is not fixed length
1341 26 malformed number or name after (?(
1342 27 conditional group contains more than two branches
1343 28 assertion expected after (?(
1344 29 (?R or (?[+-]digits must be followed by )
1345 30 unknown POSIX class name
1346 31 POSIX collating elements are not supported
1347 32 this version of PCRE is not compiled with PCRE_UTF8 support
1348 33 [this code is not in use]
1349 34 character value in \x{...} sequence is too large
1350 35 invalid condition (?(0)
1351 36 \C not allowed in lookbehind assertion
1352 37 PCRE does not support \L, \l, \N, \U, or \u
1353 38 number after (?C is > 255
1354 39 closing ) for (?C expected
1355 40 recursive call could loop indefinitely
1356 41 unrecognized character after (?P
1357 42 syntax error in subpattern name (missing terminator)
1358 43 two named subpatterns have the same name
1359 44 invalid UTF-8 string
1360 45 support for \P, \p, and \X has not been compiled
1361 46 malformed \P or \p sequence
1362 47 unknown property name after \P or \p
1363 48 subpattern name is too long (maximum 32 characters)
1364 49 too many named subpatterns (maximum 10,000)
1365 50 [this code is not in use]
1366 51 octal value is greater than \377 (not in UTF-8 mode)
1367 52 internal error: overran compiling workspace
1368 53 internal error: previously-checked referenced subpattern not
1369 found
1370 54 DEFINE group contains more than one branch
1371 55 repeating a DEFINE group is not allowed
1372 56 inconsistent NEWLINE options
1373 57 \g is not followed by a braced name or an optionally braced
1374 non-zero number
1375 58 (?+ or (?- or (?(+ or (?(- must be followed by a non-zero number
1376
1377
1378 STUDYING A PATTERN
1379
1380 pcre_extra *pcre_study(const pcre *code, int options
1381 const char **errptr);
1382
1383 If a compiled pattern is going to be used several times, it is worth
1384 spending more time analyzing it in order to speed up the time taken for
1385 matching. The function pcre_study() takes a pointer to a compiled pat-
1386 tern as its first argument. If studying the pattern produces additional
1387 information that will help speed up matching, pcre_study() returns a
1388 pointer to a pcre_extra block, in which the study_data field points to
1389 the results of the study.
1390
1391 The returned value from pcre_study() can be passed directly to
1392 pcre_exec(). However, a pcre_extra block also contains other fields
1393 that can be set by the caller before the block is passed; these are
1394 described below in the section on matching a pattern.
1395
1396 If studying the pattern does not produce any additional information
1397 pcre_study() returns NULL. In that circumstance, if the calling program
1398 wants to pass any of the other fields to pcre_exec(), it must set up
1399 its own pcre_extra block.
1400
1401 The second argument of pcre_study() contains option bits. At present,
1402 no options are defined, and this argument should always be zero.
1403
1404 The third argument for pcre_study() is a pointer for an error message.
1405 If studying succeeds (even if no data is returned), the variable it
1406 points to is set to NULL. Otherwise it is set to point to a textual
1407 error message. This is a static string that is part of the library. You
1408 must not try to free it. You should test the error pointer for NULL
1409 after calling pcre_study(), to be sure that it has run successfully.
1410
1411 This is a typical call to pcre_study():
1412
1413 pcre_extra *pe;
1414 pe = pcre_study(
1415 re, /* result of pcre_compile() */
1416 0, /* no options exist */
1417 &error); /* set to NULL or points to a message */
1418
1419 At present, studying a pattern is useful only for non-anchored patterns
1420 that do not have a single fixed starting character. A bitmap of possi-
1421 ble starting bytes is created.
1422
1423
1424 LOCALE SUPPORT
1425
1426 PCRE handles caseless matching, and determines whether characters are
1427 letters, digits, or whatever, by reference to a set of tables, indexed
1428 by character value. When running in UTF-8 mode, this applies only to
1429 characters with codes less than 128. Higher-valued codes never match
1430 escapes such as \w or \d, but can be tested with \p if PCRE is built
1431 with Unicode character property support. The use of locales with Uni-
1432 code is discouraged. If you are handling characters with codes greater
1433 than 128, you should either use UTF-8 and Unicode, or use locales, but
1434 not try to mix the two.
1435
1436 PCRE contains an internal set of tables that are used when the final
1437 argument of pcre_compile() is NULL. These are sufficient for many
1438 applications. Normally, the internal tables recognize only ASCII char-
1439 acters. However, when PCRE is built, it is possible to cause the inter-
1440 nal tables to be rebuilt in the default "C" locale of the local system,
1441 which may cause them to be different.
1442
1443 The internal tables can always be overridden by tables supplied by the
1444 application that calls PCRE. These may be created in a different locale
1445 from the default. As more and more applications change to using Uni-
1446 code, the need for this locale support is expected to die away.
1447
1448 External tables are built by calling the pcre_maketables() function,
1449 which has no arguments, in the relevant locale. The result can then be
1450 passed to pcre_compile() or pcre_exec() as often as necessary. For
1451 example, to build and use tables that are appropriate for the French
1452 locale (where accented characters with values greater than 128 are
1453 treated as letters), the following code could be used:
1454
1455 setlocale(LC_CTYPE, "fr_FR");
1456 tables = pcre_maketables();
1457 re = pcre_compile(..., tables);
1458
1459 The locale name "fr_FR" is used on Linux and other Unix-like systems;
1460 if you are using Windows, the name for the French locale is "french".
1461
1462 When pcre_maketables() runs, the tables are built in memory that is
1463 obtained via pcre_malloc. It is the caller's responsibility to ensure
1464 that the memory containing the tables remains available for as long as
1465 it is needed.
1466
1467 The pointer that is passed to pcre_compile() is saved with the compiled
1468 pattern, and the same tables are used via this pointer by pcre_study()
1469 and normally also by pcre_exec(). Thus, by default, for any single pat-
1470 tern, compilation, studying and matching all happen in the same locale,
1471 but different patterns can be compiled in different locales.
1472
1473 It is possible to pass a table pointer or NULL (indicating the use of
1474 the internal tables) to pcre_exec(). Although not intended for this
1475 purpose, this facility could be used to match a pattern in a different
1476 locale from the one in which it was compiled. Passing table pointers at
1477 run time is discussed below in the section on matching a pattern.
1478
1479
1480 INFORMATION ABOUT A PATTERN
1481
1482 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1483 int what, void *where);
1484
1485 The pcre_fullinfo() function returns information about a compiled pat-
1486 tern. It replaces the obsolete pcre_info() function, which is neverthe-
1487 less retained for backwards compability (and is documented below).
1488
1489 The first argument for pcre_fullinfo() is a pointer to the compiled
1490 pattern. The second argument is the result of pcre_study(), or NULL if
1491 the pattern was not studied. The third argument specifies which piece
1492 of information is required, and the fourth argument is a pointer to a
1493 variable to receive the data. The yield of the function is zero for
1494 success, or one of the following negative numbers:
1495
1496 PCRE_ERROR_NULL the argument code was NULL
1497 the argument where was NULL
1498 PCRE_ERROR_BADMAGIC the "magic number" was not found
1499 PCRE_ERROR_BADOPTION the value of what was invalid
1500
1501 The "magic number" is placed at the start of each compiled pattern as
1502 an simple check against passing an arbitrary memory pointer. Here is a
1503 typical call of pcre_fullinfo(), to obtain the length of the compiled
1504 pattern:
1505
1506 int rc;
1507 size_t length;
1508 rc = pcre_fullinfo(
1509 re, /* result of pcre_compile() */
1510 pe, /* result of pcre_study(), or NULL */
1511 PCRE_INFO_SIZE, /* what is required */
1512 &length); /* where to put the data */
1513
1514 The possible values for the third argument are defined in pcre.h, and
1515 are as follows:
1516
1517 PCRE_INFO_BACKREFMAX
1518
1519 Return the number of the highest back reference in the pattern. The
1520 fourth argument should point to an int variable. Zero is returned if
1521 there are no back references.
1522
1523 PCRE_INFO_CAPTURECOUNT
1524
1525 Return the number of capturing subpatterns in the pattern. The fourth
1526 argument should point to an int variable.
1527
1528 PCRE_INFO_DEFAULT_TABLES
1529
1530 Return a pointer to the internal default character tables within PCRE.
1531 The fourth argument should point to an unsigned char * variable. This
1532 information call is provided for internal use by the pcre_study() func-
1533 tion. External callers can cause PCRE to use its internal tables by
1534 passing a NULL table pointer.
1535
1536 PCRE_INFO_FIRSTBYTE
1537
1538 Return information about the first byte of any matched string, for a
1539 non-anchored pattern. The fourth argument should point to an int vari-
1540 able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1541 is still recognized for backwards compatibility.)
1542
1543 If there is a fixed first byte, for example, from a pattern such as
1544 (cat|cow|coyote), its value is returned. Otherwise, if either
1545
1546 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
1547 branch starts with "^", or
1548
1549 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1550 set (if it were set, the pattern would be anchored),
1551
1552 -1 is returned, indicating that the pattern matches only at the start
1553 of a subject string or after any newline within the string. Otherwise
1554 -2 is returned. For anchored patterns, -2 is returned.
1555
1556 PCRE_INFO_FIRSTTABLE
1557
1558 If the pattern was studied, and this resulted in the construction of a
1559 256-bit table indicating a fixed set of bytes for the first byte in any
1560 matching string, a pointer to the table is returned. Otherwise NULL is
1561 returned. The fourth argument should point to an unsigned char * vari-
1562 able.
1563
1564 PCRE_INFO_HASCRORLF
1565
1566 Return 1 if the pattern contains any explicit matches for CR or LF
1567 characters, otherwise 0. The fourth argument should point to an int
1568 variable.
1569
1570 PCRE_INFO_JCHANGED
1571
1572 Return 1 if the (?J) option setting is used in the pattern, otherwise
1573 0. The fourth argument should point to an int variable. The (?J) inter-
1574 nal option setting changes the local PCRE_DUPNAMES option.
1575
1576 PCRE_INFO_LASTLITERAL
1577
1578 Return the value of the rightmost literal byte that must exist in any
1579 matched string, other than at its start, if such a byte has been
1580 recorded. The fourth argument should point to an int variable. If there
1581 is no such byte, -1 is returned. For anchored patterns, a last literal
1582 byte is recorded only if it follows something of variable length. For
1583 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1584 /^a\dz\d/ the returned value is -1.
1585
1586 PCRE_INFO_NAMECOUNT
1587 PCRE_INFO_NAMEENTRYSIZE
1588 PCRE_INFO_NAMETABLE
1589
1590 PCRE supports the use of named as well as numbered capturing parenthe-
1591 ses. The names are just an additional way of identifying the parenthe-
1592 ses, which still acquire numbers. Several convenience functions such as
1593 pcre_get_named_substring() are provided for extracting captured sub-
1594 strings by name. It is also possible to extract the data directly, by
1595 first converting the name to a number in order to access the correct
1596 pointers in the output vector (described with pcre_exec() below). To do
1597 the conversion, you need to use the name-to-number map, which is
1598 described by these three values.
1599
1600 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1601 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1602 of each entry; both of these return an int value. The entry size
1603 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
1604 a pointer to the first entry of the table (a pointer to char). The
1605 first two bytes of each entry are the number of the capturing parenthe-
1606 sis, most significant byte first. The rest of the entry is the corre-
1607 sponding name, zero terminated. The names are in alphabetical order.
1608 When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1609 theses numbers. For example, consider the following pattern (assume
1610 PCRE_EXTENDED is set, so white space - including newlines - is
1611 ignored):
1612
1613 (?<date> (?<year>(\d\d)?\d\d) -
1614 (?<month>\d\d) - (?<day>\d\d) )
1615
1616 There are four named subpatterns, so the table has four entries, and
1617 each entry in the table is eight bytes long. The table is as follows,
1618 with non-printing bytes shows in hexadecimal, and undefined bytes shown
1619 as ??:
1620
1621 00 01 d a t e 00 ??
1622 00 05 d a y 00 ?? ??
1623 00 04 m o n t h 00
1624 00 02 y e a r 00 ??
1625
1626 When writing code to extract data from named subpatterns using the
1627 name-to-number map, remember that the length of the entries is likely
1628 to be different for each compiled pattern.
1629
1630 PCRE_INFO_OKPARTIAL
1631
1632 Return 1 if the pattern can be used for partial matching, otherwise 0.
1633 The fourth argument should point to an int variable. The pcrepartial
1634 documentation lists the restrictions that apply to patterns when par-
1635 tial matching is used.
1636
1637 PCRE_INFO_OPTIONS
1638
1639 Return a copy of the options with which the pattern was compiled. The
1640 fourth argument should point to an unsigned long int variable. These
1641 option bits are those specified in the call to pcre_compile(), modified
1642 by any top-level option settings at the start of the pattern itself. In
1643 other words, they are the options that will be in force when matching
1644 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
1645 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
1646 and PCRE_EXTENDED.
1647
1648 A pattern is automatically anchored by PCRE if all of its top-level
1649 alternatives begin with one of the following:
1650
1651 ^ unless PCRE_MULTILINE is set
1652 \A always
1653 \G always
1654 .* if PCRE_DOTALL is set and there are no back
1655 references to the subpattern in which .* appears
1656
1657 For such patterns, the PCRE_ANCHORED bit is set in the options returned
1658 by pcre_fullinfo().
1659
1660 PCRE_INFO_SIZE
1661
1662 Return the size of the compiled pattern, that is, the value that was
1663 passed as the argument to pcre_malloc() when PCRE was getting memory in
1664 which to place the compiled data. The fourth argument should point to a
1665 size_t variable.
1666
1667 PCRE_INFO_STUDYSIZE
1668
1669 Return the size of the data block pointed to by the study_data field in
1670 a pcre_extra block. That is, it is the value that was passed to
1671 pcre_malloc() when PCRE was getting memory into which to place the data
1672 created by pcre_study(). The fourth argument should point to a size_t
1673 variable.
1674
1675
1676 OBSOLETE INFO FUNCTION
1677
1678 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1679
1680 The pcre_info() function is now obsolete because its interface is too
1681 restrictive to return all the available data about a compiled pattern.
1682 New programs should use pcre_fullinfo() instead. The yield of
1683 pcre_info() is the number of capturing subpatterns, or one of the fol-
1684 lowing negative numbers:
1685
1686 PCRE_ERROR_NULL the argument code was NULL
1687 PCRE_ERROR_BADMAGIC the "magic number" was not found
1688
1689 If the optptr argument is not NULL, a copy of the options with which
1690 the pattern was compiled is placed in the integer it points to (see
1691 PCRE_INFO_OPTIONS above).
1692
1693 If the pattern is not anchored and the firstcharptr argument is not
1694 NULL, it is used to pass back information about the first character of
1695 any matched string (see PCRE_INFO_FIRSTBYTE above).
1696
1697
1698 REFERENCE COUNTS
1699
1700 int pcre_refcount(pcre *code, int adjust);
1701
1702 The pcre_refcount() function is used to maintain a reference count in
1703 the data block that contains a compiled pattern. It is provided for the
1704 benefit of applications that operate in an object-oriented manner,
1705 where different parts of the application may be using the same compiled
1706 pattern, but you want to free the block when they are all done.
1707
1708 When a pattern is compiled, the reference count field is initialized to
1709 zero. It is changed only by calling this function, whose action is to
1710 add the adjust value (which may be positive or negative) to it. The
1711 yield of the function is the new value. However, the value of the count
1712 is constrained to lie between 0 and 65535, inclusive. If the new value
1713 is outside these limits, it is forced to the appropriate limit value.
1714
1715 Except when it is zero, the reference count is not correctly preserved
1716 if a pattern is compiled on one host and then transferred to a host
1717 whose byte-order is different. (This seems a highly unlikely scenario.)
1718
1719
1720 MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1721
1722 int pcre_exec(const pcre *code, const pcre_extra *extra,
1723 const char *subject, int length, int startoffset,
1724 int options, int *ovector, int ovecsize);
1725
1726 The function pcre_exec() is called to match a subject string against a
1727 compiled pattern, which is passed in the code argument. If the pattern
1728 has been studied, the result of the study should be passed in the extra
1729 argument. This function is the main matching facility of the library,
1730 and it operates in a Perl-like manner. For specialist use there is also
1731 an alternative matching function, which is described below in the sec-
1732 tion about the pcre_dfa_exec() function.
1733
1734 In most applications, the pattern will have been compiled (and option-
1735 ally studied) in the same process that calls pcre_exec(). However, it
1736 is possible to save compiled patterns and study data, and then use them
1737 later in different processes, possibly even on different hosts. For a
1738 discussion about this, see the pcreprecompile documentation.
1739
1740 Here is an example of a simple call to pcre_exec():
1741
1742 int rc;
1743 int ovector[30];
1744 rc = pcre_exec(
1745 re, /* result of pcre_compile() */
1746 NULL, /* we didn't study the pattern */
1747 "some string", /* the subject string */
1748 11, /* the length of the subject string */
1749 0, /* start at offset 0 in the subject */
1750 0, /* default options */
1751 ovector, /* vector of integers for substring information */
1752 30); /* number of elements (NOT size in bytes) */
1753
1754 Extra data for pcre_exec()
1755
1756 If the extra argument is not NULL, it must point to a pcre_extra data
1757 block. The pcre_study() function returns such a block (when it doesn't
1758 return NULL), but you can also create one for yourself, and pass addi-
1759 tional information in it. The pcre_extra block contains the following
1760 fields (not necessarily in this order):
1761
1762 unsigned long int flags;
1763 void *study_data;
1764 unsigned long int match_limit;
1765 unsigned long int match_limit_recursion;
1766 void *callout_data;
1767 const unsigned char *tables;
1768
1769 The flags field is a bitmap that specifies which of the other fields
1770 are set. The flag bits are:
1771
1772 PCRE_EXTRA_STUDY_DATA
1773 PCRE_EXTRA_MATCH_LIMIT
1774 PCRE_EXTRA_MATCH_LIMIT_RECURSION
1775 PCRE_EXTRA_CALLOUT_DATA
1776 PCRE_EXTRA_TABLES
1777
1778 Other flag bits should be set to zero. The study_data field is set in
1779 the pcre_extra block that is returned by pcre_study(), together with
1780 the appropriate flag bit. You should not set this yourself, but you may
1781 add to the block by setting the other fields and their corresponding
1782 flag bits.
1783
1784 The match_limit field provides a means of preventing PCRE from using up
1785 a vast amount of resources when running patterns that are not going to
1786 match, but which have a very large number of possibilities in their
1787 search trees. The classic example is the use of nested unlimited
1788 repeats.
1789
1790 Internally, PCRE uses a function called match() which it calls repeat-
1791 edly (sometimes recursively). The limit set by match_limit is imposed
1792 on the number of times this function is called during a match, which
1793 has the effect of limiting the amount of backtracking that can take
1794 place. For patterns that are not anchored, the count restarts from zero
1795 for each position in the subject string.
1796
1797 The default value for the limit can be set when PCRE is built; the
1798 default default is 10 million, which handles all but the most extreme
1799 cases. You can override the default by suppling pcre_exec() with a
1800 pcre_extra block in which match_limit is set, and
1801 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
1802 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1803
1804 The match_limit_recursion field is similar to match_limit, but instead
1805 of limiting the total number of times that match() is called, it limits
1806 the depth of recursion. The recursion depth is a smaller number than
1807 the total number of calls, because not all calls to match() are recur-
1808 sive. This limit is of use only if it is set smaller than match_limit.
1809
1810 Limiting the recursion depth limits the amount of stack that can be
1811 used, or, when PCRE has been compiled to use memory on the heap instead
1812 of the stack, the amount of heap memory that can be used.
1813
1814 The default value for match_limit_recursion can be set when PCRE is
1815 built; the default default is the same value as the default for
1816 match_limit. You can override the default by suppling pcre_exec() with
1817 a pcre_extra block in which match_limit_recursion is set, and
1818 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
1819 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1820
1821 The pcre_callout field is used in conjunction with the "callout" fea-
1822 ture, which is described in the pcrecallout documentation.
1823
1824 The tables field is used to pass a character tables pointer to
1825 pcre_exec(); this overrides the value that is stored with the compiled
1826 pattern. A non-NULL value is stored with the compiled pattern only if
1827 custom tables were supplied to pcre_compile() via its tableptr argu-
1828 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
1829 PCRE's internal tables to be used. This facility is helpful when re-
1830 using patterns that have been saved after compiling with an external
1831 set of tables, because the external tables might be at a different
1832 address when pcre_exec() is called. See the pcreprecompile documenta-
1833 tion for a discussion of saving compiled patterns for later use.
1834
1835 Option bits for pcre_exec()
1836
1837 The unused bits of the options argument for pcre_exec() must be zero.
1838 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
1839 PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and
1840 PCRE_PARTIAL.
1841
1842 PCRE_ANCHORED
1843
1844 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
1845 matching position. If a pattern was compiled with PCRE_ANCHORED, or
1846 turned out to be anchored by virtue of its contents, it cannot be made
1847 unachored at matching time.
1848
1849 PCRE_BSR_ANYCRLF
1850 PCRE_BSR_UNICODE
1851
1852 These options (which are mutually exclusive) control what the \R escape
1853 sequence matches. The choice is either to match only CR, LF, or CRLF,
1854 or to match any Unicode newline sequence. These options override the
1855 choice that was made or defaulted when the pattern was compiled.
1856
1857 PCRE_NEWLINE_CR
1858 PCRE_NEWLINE_LF
1859 PCRE_NEWLINE_CRLF
1860 PCRE_NEWLINE_ANYCRLF
1861 PCRE_NEWLINE_ANY
1862
1863 These options override the newline definition that was chosen or
1864 defaulted when the pattern was compiled. For details, see the descrip-
1865 tion of pcre_compile() above. During matching, the newline choice
1866 affects the behaviour of the dot, circumflex, and dollar metacharac-
1867 ters. It may also alter the way the match position is advanced after a
1868 match failure for an unanchored pattern.
1869
1870 When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
1871 set, and a match attempt for an unanchored pattern fails when the cur-
1872 rent position is at a CRLF sequence, and the pattern contains no
1873 explicit matches for CR or LF characters, the match position is
1874 advanced by two characters instead of one, in other words, to after the
1875 CRLF.
1876
1877 The above rule is a compromise that makes the most common cases work as
1878 expected. For example, if the pattern is .+A (and the PCRE_DOTALL
1879 option is not set), it does not match the string "\r\nA" because, after
1880 failing at the start, it skips both the CR and the LF before retrying.
1881 However, the pattern [\r\n]A does match that string, because it con-
1882 tains an explicit CR or LF reference, and so advances only by one char-
1883 acter after the first failure.
1884
1885 An explicit match for CR of LF is either a literal appearance of one of
1886 those characters, or one of the \r or \n escape sequences. Implicit
1887 matches such as [^X] do not count, nor does \s (which includes CR and
1888 LF in the characters that it matches).
1889
1890 Notwithstanding the above, anomalous effects may still occur when CRLF
1891 is a valid newline sequence and explicit \r or \n escapes appear in the
1892 pattern.
1893
1894 PCRE_NOTBOL
1895
1896 This option specifies that first character of the subject string is not
1897 the beginning of a line, so the circumflex metacharacter should not
1898 match before it. Setting this without PCRE_MULTILINE (at compile time)
1899 causes circumflex never to match. This option affects only the behav-
1900 iour of the circumflex metacharacter. It does not affect \A.
1901
1902 PCRE_NOTEOL
1903
1904 This option specifies that the end of the subject string is not the end
1905 of a line, so the dollar metacharacter should not match it nor (except
1906 in multiline mode) a newline immediately before it. Setting this with-
1907 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1908 option affects only the behaviour of the dollar metacharacter. It does
1909 not affect \Z or \z.
1910
1911 PCRE_NOTEMPTY
1912
1913 An empty string is not considered to be a valid match if this option is
1914 set. If there are alternatives in the pattern, they are tried. If all
1915 the alternatives match the empty string, the entire match fails. For
1916 example, if the pattern
1917
1918 a?b?
1919
1920 is applied to a string not beginning with "a" or "b", it matches the
1921 empty string at the start of the subject. With PCRE_NOTEMPTY set, this
1922 match is not valid, so PCRE searches further into the string for occur-
1923 rences of "a" or "b".
1924
1925 Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1926 cial case of a pattern match of the empty string within its split()
1927 function, and when using the /g modifier. It is possible to emulate
1928 Perl's behaviour after matching a null string by first trying the match
1929 again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1930 if that fails by advancing the starting offset (see below) and trying
1931 an ordinary match again. There is some code that demonstrates how to do
1932 this in the pcredemo.c sample program.
1933
1934 PCRE_NO_UTF8_CHECK
1935
1936 When PCRE_UTF8 is set at compile time, the validity of the subject as a
1937 UTF-8 string is automatically checked when pcre_exec() is subsequently
1938 called. The value of startoffset is also checked to ensure that it
1939 points to the start of a UTF-8 character. There is a discussion about
1940 the validity of UTF-8 strings in the section on UTF-8 support in the
1941 main pcre page. If an invalid UTF-8 sequence of bytes is found,
1942 pcre_exec() returns the error PCRE_ERROR_BADUTF8. If startoffset con-
1943 tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
1944
1945 If you already know that your subject is valid, and you want to skip
1946 these checks for performance reasons, you can set the
1947 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
1948 do this for the second and subsequent calls to pcre_exec() if you are
1949 making repeated calls to find all the matches in a single subject
1950 string. However, you should be sure that the value of startoffset
1951 points to the start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
1952 set, the effect of passing an invalid UTF-8 string as a subject, or a
1953 value of startoffset that does not point to the start of a UTF-8 char-
1954 acter, is undefined. Your program may crash.
1955
1956 PCRE_PARTIAL
1957
1958 This option turns on the partial matching feature. If the subject
1959 string fails to match the pattern, but at some point during the match-
1960 ing process the end of the subject was reached (that is, the subject
1961 partially matches the pattern and the failure to match occurred only
1962 because there were not enough subject characters), pcre_exec() returns
1963 PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
1964 used, there are restrictions on what may appear in the pattern. These
1965 are discussed in the pcrepartial documentation.
1966
1967 The string to be matched by pcre_exec()
1968
1969 The subject string is passed to pcre_exec() as a pointer in subject, a
1970 length in length, and a starting byte offset in startoffset. In UTF-8
1971 mode, the byte offset must point to the start of a UTF-8 character.
1972 Unlike the pattern string, the subject may contain binary zero bytes.
1973 When the starting offset is zero, the search for a match starts at the
1974 beginning of the subject, and this is by far the most common case.
1975
1976 A non-zero starting offset is useful when searching for another match
1977 in the same subject by calling pcre_exec() again after a previous suc-
1978 cess. Setting startoffset differs from just passing over a shortened
1979 string and setting PCRE_NOTBOL in the case of a pattern that begins
1980 with any kind of lookbehind. For example, consider the pattern
1981
1982 \Biss\B
1983
1984 which finds occurrences of "iss" in the middle of words. (\B matches
1985 only if the current position in the subject is not a word boundary.)
1986 When applied to the string "Mississipi" the first call to pcre_exec()
1987 finds the first occurrence. If pcre_exec() is called again with just
1988 the remainder of the subject, namely "issipi", it does not match,
1989 because \B is always false at the start of the subject, which is deemed
1990 to be a word boundary. However, if pcre_exec() is passed the entire
1991 string again, but with startoffset set to 4, it finds the second occur-
1992 rence of "iss" because it is able to look behind the starting point to
1993 discover that it is preceded by a letter.
1994
1995 If a non-zero starting offset is passed when the pattern is anchored,
1996 one attempt to match at the given offset is made. This can only succeed
1997 if the pattern does not require the match to be at the start of the
1998 subject.
1999
2000 How pcre_exec() returns captured substrings
2001
2002 In general, a pattern matches a certain portion of the subject, and in
2003 addition, further substrings from the subject may be picked out by
2004 parts of the pattern. Following the usage in Jeffrey Friedl's book,
2005 this is called "capturing" in what follows, and the phrase "capturing
2006 subpattern" is used for a fragment of a pattern that picks out a sub-
2007 string. PCRE supports several other kinds of parenthesized subpattern
2008 that do not cause substrings to be captured.
2009
2010 Captured substrings are returned to the caller via a vector of integer
2011 offsets whose address is passed in ovector. The number of elements in
2012 the vector is passed in ovecsize, which must be a non-negative number.
2013 Note: this argument is NOT the size of ovector in bytes.
2014
2015 The first two-thirds of the vector is used to pass back captured sub-
2016 strings, each substring using a pair of integers. The remaining third
2017 of the vector is used as workspace by pcre_exec() while matching cap-
2018 turing subpatterns, and is not available for passing back information.
2019 The length passed in ovecsize should always be a multiple of three. If
2020 it is not, it is rounded down.
2021
2022 When a match is successful, information about captured substrings is
2023 returned in pairs of integers, starting at the beginning of ovector,
2024 and continuing up to two-thirds of its length at the most. The first
2025 element of a pair is set to the offset of the first character in a sub-
2026 string, and the second is set to the offset of the first character
2027 after the end of a substring. The first pair, ovector[0] and ovec-
2028 tor[1], identify the portion of the subject string matched by the
2029 entire pattern. The next pair is used for the first capturing subpat-
2030 tern, and so on. The value returned by pcre_exec() is one more than the
2031 highest numbered pair that has been set. For example, if two substrings
2032 have been captured, the returned value is 3. If there are no capturing
2033 subpatterns, the return value from a successful match is 1, indicating
2034 that just the first pair of offsets has been set.
2035
2036 If a capturing subpattern is matched repeatedly, it is the last portion
2037 of the string that it matched that is returned.
2038
2039 If the vector is too small to hold all the captured substring offsets,
2040 it is used as far as possible (up to two-thirds of its length), and the
2041 function returns a value of zero. In particular, if the substring off-
2042 sets are not of interest, pcre_exec() may be called with ovector passed
2043 as NULL and ovecsize as zero. However, if the pattern contains back
2044 references and the ovector is not big enough to remember the related
2045 substrings, PCRE has to get additional memory for use during matching.
2046 Thus it is usually advisable to supply an ovector.
2047
2048 The pcre_info() function can be used to find out how many capturing
2049 subpatterns there are in a compiled pattern. The smallest size for
2050 ovector that will allow for n captured substrings, in addition to the
2051 offsets of the substring matched by the whole pattern, is (n+1)*3.
2052
2053 It is possible for capturing subpattern number n+1 to match some part
2054 of the subject when subpattern n has not been used at all. For example,
2055 if the string "abc" is matched against the pattern (a|(z))(bc) the
2056 return from the function is 4, and subpatterns 1 and 3 are matched, but
2057 2 is not. When this happens, both values in the offset pairs corre-
2058 sponding to unused subpatterns are set to -1.
2059
2060 Offset values that correspond to unused subpatterns at the end of the
2061 expression are also set to -1. For example, if the string "abc" is
2062 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2063 matched. The return from the function is 2, because the highest used
2064 capturing subpattern number is 1. However, you can refer to the offsets
2065 for the second and third capturing subpatterns if you wish (assuming
2066 the vector is large enough, of course).
2067
2068 Some convenience functions are provided for extracting the captured
2069 substrings as separate strings. These are described below.
2070
2071 Error return values from pcre_exec()
2072
2073 If pcre_exec() fails, it returns a negative number. The following are
2074 defined in the header file:
2075
2076 PCRE_ERROR_NOMATCH (-1)
2077
2078 The subject string did not match the pattern.
2079
2080 PCRE_ERROR_NULL (-2)
2081
2082 Either code or subject was passed as NULL, or ovector was NULL and
2083 ovecsize was not zero.
2084
2085 PCRE_ERROR_BADOPTION (-3)
2086
2087 An unrecognized bit was set in the options argument.
2088
2089 PCRE_ERROR_BADMAGIC (-4)
2090
2091 PCRE stores a 4-byte "magic number" at the start of the compiled code,
2092 to catch the case when it is passed a junk pointer and to detect when a
2093 pattern that was compiled in an environment of one endianness is run in
2094 an environment with the other endianness. This is the error that PCRE
2095 gives when the magic number is not present.
2096
2097 PCRE_ERROR_UNKNOWN_OPCODE (-5)
2098
2099 While running the pattern match, an unknown item was encountered in the
2100 compiled pattern. This error could be caused by a bug in PCRE or by
2101 overwriting of the compiled pattern.
2102
2103 PCRE_ERROR_NOMEMORY (-6)
2104
2105 If a pattern contains back references, but the ovector that is passed
2106 to pcre_exec() is not big enough to remember the referenced substrings,
2107 PCRE gets a block of memory at the start of matching to use for this
2108 purpose. If the call via pcre_malloc() fails, this error is given. The
2109 memory is automatically freed at the end of matching.
2110
2111 PCRE_ERROR_NOSUBSTRING (-7)
2112
2113 This error is used by the pcre_copy_substring(), pcre_get_substring(),
2114 and pcre_get_substring_list() functions (see below). It is never
2115 returned by pcre_exec().
2116
2117 PCRE_ERROR_MATCHLIMIT (-8)
2118
2119 The backtracking limit, as specified by the match_limit field in a
2120 pcre_extra structure (or defaulted) was reached. See the description
2121 above.
2122
2123 PCRE_ERROR_CALLOUT (-9)
2124
2125 This error is never generated by pcre_exec() itself. It is provided for
2126 use by callout functions that want to yield a distinctive error code.
2127 See the pcrecallout documentation for details.
2128
2129 PCRE_ERROR_BADUTF8 (-10)
2130
2131 A string that contains an invalid UTF-8 byte sequence was passed as a
2132 subject.
2133
2134 PCRE_ERROR_BADUTF8_OFFSET (-11)
2135
2136 The UTF-8 byte sequence that was passed as a subject was valid, but the
2137 value of startoffset did not point to the beginning of a UTF-8 charac-
2138 ter.
2139
2140 PCRE_ERROR_PARTIAL (-12)
2141
2142 The subject string did not match, but it did match partially. See the
2143 pcrepartial documentation for details of partial matching.
2144
2145 PCRE_ERROR_BADPARTIAL (-13)
2146
2147 The PCRE_PARTIAL option was used with a compiled pattern containing
2148 items that are not supported for partial matching. See the pcrepartial
2149 documentation for details of partial matching.
2150
2151 PCRE_ERROR_INTERNAL (-14)
2152
2153 An unexpected internal error has occurred. This error could be caused
2154 by a bug in PCRE or by overwriting of the compiled pattern.
2155
2156 PCRE_ERROR_BADCOUNT (-15)
2157
2158 This error is given if the value of the ovecsize argument is negative.
2159
2160 PCRE_ERROR_RECURSIONLIMIT (-21)
2161
2162 The internal recursion limit, as specified by the match_limit_recursion
2163 field in a pcre_extra structure (or defaulted) was reached. See the
2164 description above.
2165
2166 PCRE_ERROR_BADNEWLINE (-23)
2167
2168 An invalid combination of PCRE_NEWLINE_xxx options was given.
2169
2170 Error numbers -16 to -20 and -22 are not used by pcre_exec().
2171
2172
2173 EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2174
2175 int pcre_copy_substring(const char *subject, int *ovector,
2176 int stringcount, int stringnumber, char *buffer,
2177 int buffersize);
2178
2179 int pcre_get_substring(const char *subject, int *ovector,
2180 int stringcount, int stringnumber,
2181 const char **stringptr);
2182
2183 int pcre_get_substring_list(const char *subject,
2184 int *ovector, int stringcount, const char ***listptr);
2185
2186 Captured substrings can be accessed directly by using the offsets
2187 returned by pcre_exec() in ovector. For convenience, the functions
2188 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
2189 string_list() are provided for extracting captured substrings as new,
2190 separate, zero-terminated strings. These functions identify substrings
2191 by number. The next section describes functions for extracting named
2192 substrings.
2193
2194 A substring that contains a binary zero is correctly extracted and has
2195 a further zero added on the end, but the result is not, of course, a C
2196 string. However, you can process such a string by referring to the
2197 length that is returned by pcre_copy_substring() and pcre_get_sub-
2198 string(). Unfortunately, the interface to pcre_get_substring_list() is
2199 not adequate for handling strings containing binary zeros, because the
2200 end of the final string is not independently indicated.
2201
2202 The first three arguments are the same for all three of these func-
2203 tions: subject is the subject string that has just been successfully
2204 matched, ovector is a pointer to the vector of integer offsets that was
2205 passed to pcre_exec(), and stringcount is the number of substrings that
2206 were captured by the match, including the substring that matched the
2207 entire regular expression. This is the value returned by pcre_exec() if
2208 it is greater than zero. If pcre_exec() returned zero, indicating that
2209 it ran out of space in ovector, the value passed as stringcount should
2210 be the number of elements in the vector divided by three.
2211
2212 The functions pcre_copy_substring() and pcre_get_substring() extract a
2213 single substring, whose number is given as stringnumber. A value of
2214 zero extracts the substring that matched the entire pattern, whereas
2215 higher values extract the captured substrings. For pcre_copy_sub-
2216 string(), the string is placed in buffer, whose length is given by
2217 buffersize, while for pcre_get_substring() a new block of memory is
2218 obtained via pcre_malloc, and its address is returned via stringptr.
2219 The yield of the function is the length of the string, not including
2220 the terminating zero, or one of these error codes:
2221
2222 PCRE_ERROR_NOMEMORY (-6)
2223
2224 The buffer was too small for pcre_copy_substring(), or the attempt to
2225 get memory failed for pcre_get_substring().
2226
2227 PCRE_ERROR_NOSUBSTRING (-7)
2228
2229 There is no substring whose number is stringnumber.
2230
2231 The pcre_get_substring_list() function extracts all available sub-
2232 strings and builds a list of pointers to them. All this is done in a
2233 single block of memory that is obtained via pcre_malloc. The address of
2234 the memory block is returned via listptr, which is also the start of
2235 the list of string pointers. The end of the list is marked by a NULL
2236 pointer. The yield of the function is zero if all went well, or the
2237 error code
2238
2239 PCRE_ERROR_NOMEMORY (-6)
2240
2241 if the attempt to get the memory block failed.
2242
2243 When any of these functions encounter a substring that is unset, which
2244 can happen when capturing subpattern number n+1 matches some part of
2245 the subject, but subpattern n has not been used at all, they return an
2246 empty string. This can be distinguished from a genuine zero-length sub-
2247 string by inspecting the appropriate offset in ovector, which is nega-
2248 tive for unset substrings.
2249
2250 The two convenience functions pcre_free_substring() and pcre_free_sub-
2251 string_list() can be used to free the memory returned by a previous
2252 call of pcre_get_substring() or pcre_get_substring_list(), respec-
2253 tively. They do nothing more than call the function pointed to by
2254 pcre_free, which of course could be called directly from a C program.
2255 However, PCRE is used in some situations where it is linked via a spe-
2256 cial interface to another programming language that cannot use
2257 pcre_free directly; it is for these cases that the functions are pro-
2258 vided.
2259
2260
2261 EXTRACTING CAPTURED SUBSTRINGS BY NAME
2262
2263 int pcre_get_stringnumber(const pcre *code,
2264 const char *name);
2265
2266 int pcre_copy_named_substring(const pcre *code,
2267 const char *subject, int *ovector,
2268 int stringcount, const char *stringname,
2269 char *buffer, int buffersize);
2270
2271 int pcre_get_named_substring(const pcre *code,
2272 const char *subject, int *ovector,
2273 int stringcount, const char *stringname,
2274 const char **stringptr);
2275
2276 To extract a substring by name, you first have to find associated num-
2277 ber. For example, for this pattern
2278
2279 (a+)b(?<xxx>\d+)...
2280
2281 the number of the subpattern called "xxx" is 2. If the name is known to
2282 be unique (PCRE_DUPNAMES was not set), you can find the number from the
2283 name by calling pcre_get_stringnumber(). The first argument is the com-
2284 piled pattern, and the second is the name. The yield of the function is
2285 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2286 subpattern of that name.
2287
2288 Given the number, you can extract the substring directly, or use one of
2289 the functions described in the previous section. For convenience, there
2290 are also two functions that do the whole job.
2291
2292 Most of the arguments of pcre_copy_named_substring() and
2293 pcre_get_named_substring() are the same as those for the similarly
2294 named functions that extract by number. As these are described in the
2295 previous section, they are not re-described here. There are just two
2296 differences:
2297
2298 First, instead of a substring number, a substring name is given. Sec-
2299 ond, there is an extra argument, given at the start, which is a pointer
2300 to the compiled pattern. This is needed in order to gain access to the
2301 name-to-number translation table.
2302
2303 These functions call pcre_get_stringnumber(), and if it succeeds, they
2304 then call pcre_copy_substring() or pcre_get_substring(), as appropri-
2305 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
2306 behaviour may not be what you want (see the next section).
2307
2308
2309 DUPLICATE SUBPATTERN NAMES
2310
2311 int pcre_get_stringtable_entries(const pcre *code,
2312 const char *name, char **first, char **last);
2313
2314 When a pattern is compiled with the PCRE_DUPNAMES option, names for
2315 subpatterns are not required to be unique. Normally, patterns with
2316 duplicate names are such that in any one match, only one of the named
2317 subpatterns participates. An example is shown in the pcrepattern docu-
2318 mentation.
2319
2320 When duplicates are present, pcre_copy_named_substring() and
2321 pcre_get_named_substring() return the first substring corresponding to
2322 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
2323 (-7) is returned; no data is returned. The pcre_get_stringnumber()
2324 function returns one of the numbers that are associated with the name,
2325 but it is not defined which it is.
2326
2327 If you want to get full details of all captured substrings for a given
2328 name, you must use the pcre_get_stringtable_entries() function. The
2329 first argument is the compiled pattern, and the second is the name. The
2330 third and fourth are pointers to variables which are updated by the
2331 function. After it has run, they point to the first and last entries in
2332 the name-to-number table for the given name. The function itself
2333 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2334 there are none. The format of the table is described above in the sec-
2335 tion entitled Information about a pattern. Given all the relevant
2336 entries for the name, you can extract each of their numbers, and hence
2337 the captured data, if any.
2338
2339
2340 FINDING ALL POSSIBLE MATCHES
2341
2342 The traditional matching function uses a similar algorithm to Perl,
2343 which stops when it finds the first match, starting at a given point in
2344 the subject. If you want to find all possible matches, or the longest
2345 possible match, consider using the alternative matching function (see
2346 below) instead. If you cannot use the alternative function, but still
2347 need to find all possible matches, you can kludge it up by making use
2348 of the callout facility, which is described in the pcrecallout documen-
2349 tation.
2350
2351 What you have to do is to insert a callout right at the end of the pat-
2352 tern. When your callout function is called, extract and save the cur-
2353 rent matched substring. Then return 1, which forces pcre_exec() to
2354 backtrack and try other alternatives. Ultimately, when it runs out of
2355 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2356
2357
2358 MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2359
2360 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2361 const char *subject, int length, int startoffset,
2362 int options, int *ovector, int ovecsize,
2363 int *workspace, int wscount);
2364
2365 The function pcre_dfa_exec() is called to match a subject string
2366 against a compiled pattern, using a matching algorithm that scans the
2367 subject string just once, and does not backtrack. This has different
2368 characteristics to the normal algorithm, and is not compatible with
2369 Perl. Some of the features of PCRE patterns are not supported. Never-
2370 theless, there are times when this kind of matching can be useful. For
2371 a discussion of the two matching algorithms, see the pcrematching docu-
2372 mentation.
2373
2374 The arguments for the pcre_dfa_exec() function are the same as for
2375 pcre_exec(), plus two extras. The ovector argument is used in a differ-
2376 ent way, and this is described below. The other common arguments are
2377 used in the same way as for pcre_exec(), so their description is not
2378 repeated here.
2379
2380 The two additional arguments provide workspace for the function. The
2381 workspace vector should contain at least 20 elements. It is used for
2382 keeping track of multiple paths through the pattern tree. More
2383 workspace will be needed for patterns and subjects where there are a
2384 lot of potential matches.
2385
2386 Here is an example of a simple call to pcre_dfa_exec():
2387
2388 int rc;
2389 int ovector[10];
2390 int wspace[20];
2391 rc = pcre_dfa_exec(
2392 re, /* result of pcre_compile() */
2393 NULL, /* we didn't study the pattern */
2394 "some string", /* the subject string */
2395 11, /* the length of the subject string */
2396 0, /* start at offset 0 in the subject */
2397 0, /* default options */
2398 ovector, /* vector of integers for substring information */
2399 10, /* number of elements (NOT size in bytes) */
2400 wspace, /* working space vector */
2401 20); /* number of elements (NOT size in bytes) */
2402
2403 Option bits for pcre_dfa_exec()
2404
2405 The unused bits of the options argument for pcre_dfa_exec() must be
2406 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
2407 LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
2408 PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2409 three of these are the same as for pcre_exec(), so their description is
2410 not repeated here.
2411
2412 PCRE_PARTIAL
2413
2414 This has the same general effect as it does for pcre_exec(), but the
2415 details are slightly different. When PCRE_PARTIAL is set for
2416 pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is converted into
2417 PCRE_ERROR_PARTIAL if the end of the subject is reached, there have
2418 been no complete matches, but there is still at least one matching pos-
2419 sibility. The portion of the string that provided the partial match is
2420 set as the first matching string.
2421
2422 PCRE_DFA_SHORTEST
2423
2424 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
2425 stop as soon as it has found one match. Because of the way the alterna-
2426 tive algorithm works, this is necessarily the shortest possible match
2427 at the first possible matching point in the subject string.
2428
2429 PCRE_DFA_RESTART
2430
2431 When pcre_dfa_exec() is called with the PCRE_PARTIAL option, and
2432 returns a partial match, it is possible to call it again, with addi-
2433 tional subject characters, and have it continue with the same match.
2434 The PCRE_DFA_RESTART option requests this action; when it is set, the
2435 workspace and wscount options must reference the same vector as before
2436 because data about the match so far is left in them after a partial
2437 match. There is more discussion of this facility in the pcrepartial
2438 documentation.
2439
2440 Successful returns from pcre_dfa_exec()
2441
2442 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
2443 string in the subject. Note, however, that all the matches from one run
2444 of the function start at the same point in the subject. The shorter
2445 matches are all initial substrings of the longer matches. For example,
2446 if the pattern
2447
2448 <.*>
2449
2450 is matched against the string
2451
2452 This is <something> <something else> <something further> no more
2453
2454 the three matched strings are
2455
2456 <something>
2457 <something> <something else>
2458 <something> <something else> <something further>
2459
2460 On success, the yield of the function is a number greater than zero,
2461 which is the number of matched substrings. The substrings themselves
2462 are returned in ovector. Each string uses two elements; the first is
2463 the offset to the start, and the second is the offset to the end. In
2464 fact, all the strings have the same start offset. (Space could have
2465 been saved by giving this only once, but it was decided to retain some
2466 compatibility with the way pcre_exec() returns data, even though the
2467 meaning of the strings is different.)
2468
2469 The strings are returned in reverse order of length; that is, the long-
2470 est matching string is given first. If there were too many matches to
2471 fit into ovector, the yield of the function is zero, and the vector is
2472 filled with the longest matches.
2473
2474 Error returns from pcre_dfa_exec()
2475
2476 The pcre_dfa_exec() function returns a negative number when it fails.
2477 Many of the errors are the same as for pcre_exec(), and these are
2478 described above. There are in addition the following errors that are
2479 specific to pcre_dfa_exec():
2480
2481 PCRE_ERROR_DFA_UITEM (-16)
2482
2483 This return is given if pcre_dfa_exec() encounters an item in the pat-
2484 tern that it does not support, for instance, the use of \C or a back
2485 reference.
2486
2487 PCRE_ERROR_DFA_UCOND (-17)
2488
2489 This return is given if pcre_dfa_exec() encounters a condition item
2490 that uses a back reference for the condition, or a test for recursion
2491 in a specific group. These are not supported.
2492
2493 PCRE_ERROR_DFA_UMLIMIT (-18)
2494
2495 This return is given if pcre_dfa_exec() is called with an extra block
2496 that contains a setting of the match_limit field. This is not supported
2497 (it is meaningless).
2498
2499 PCRE_ERROR_DFA_WSSIZE (-19)
2500
2501 This return is given if pcre_dfa_exec() runs out of space in the
2502 workspace vector.
2503
2504 PCRE_ERROR_DFA_RECURSE (-20)
2505
2506 When a recursive subpattern is processed, the matching function calls
2507 itself recursively, using private vectors for ovector and workspace.
2508 This error is given if the output vector is not large enough. This
2509 should be extremely rare, as a vector of size 1000 is used.
2510
2511
2512 SEE ALSO
2513
2514 pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2515 tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2516
2517
2518 AUTHOR
2519
2520 Philip Hazel
2521 University Computing Service
2522 Cambridge CB2 3QH, England.
2523
2524
2525 REVISION
2526
2527 Last updated: 11 September 2007
2528 Copyright (c) 1997-2007 University of Cambridge.
2529 ------------------------------------------------------------------------------
2530
2531
2532 PCRECALLOUT(3) PCRECALLOUT(3)
2533
2534
2535 NAME
2536 PCRE - Perl-compatible regular expressions
2537
2538
2539 PCRE CALLOUTS
2540
2541 int (*pcre_callout)(pcre_callout_block *);
2542
2543 PCRE provides a feature called "callout", which is a means of temporar-
2544 ily passing control to the caller of PCRE in the middle of pattern
2545 matching. The caller of PCRE provides an external function by putting
2546 its entry point in the global variable pcre_callout. By default, this
2547 variable contains NULL, which disables all calling out.
2548
2549 Within a regular expression, (?C) indicates the points at which the
2550 external function is to be called. Different callout points can be
2551 identified by putting a number less than 256 after the letter C. The
2552 default value is zero. For example, this pattern has two callout
2553 points:
2554
2555 (?C1)abc(?C2)def
2556
2557 If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() is
2558 called, PCRE automatically inserts callouts, all with number 255,
2559 before each item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2560 used with the pattern
2561
2562 A(\d{2}|--)
2563
2564 it is processed as if it were
2565
2566 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2567
2568 Notice that there is a callout before and after each parenthesis and
2569 alternation bar. Automatic callouts can be used for tracking the
2570 progress of pattern matching. The pcretest command has an option that
2571 sets automatic callouts; when it is used, the output indicates how the
2572 pattern is matched. This is useful information when you are trying to
2573 optimize the performance of a particular pattern.
2574
2575
2576 MISSING CALLOUTS
2577
2578 You should be aware that, because of optimizations in the way PCRE
2579 matches patterns, callouts sometimes do not happen. For example, if the
2580 pattern is
2581
2582 ab(?C4)cd
2583
2584 PCRE knows that any matching string must contain the letter "d". If the
2585 subject string is "abyz", the lack of "d" means that matching doesn't
2586 ever start, and the callout is never reached. However, with "abyd",
2587 though the result is still no match, the callout is obeyed.
2588
2589
2590 THE CALLOUT INTERFACE
2591
2592 During matching, when PCRE reaches a callout point, the external func-
2593 tion defined by pcre_callout is called (if it is set). This applies to
2594 both the pcre_exec() and the pcre_dfa_exec() matching functions. The
2595 only argument to the callout function is a pointer to a pcre_callout
2596 block. This structure contains the following fields:
2597
2598 int version;
2599 int callout_number;
2600 int *offset_vector;
2601 const char *subject;
2602 int subject_length;
2603 int start_match;
2604 int current_position;
2605 int capture_top;
2606 int capture_last;
2607 void *callout_data;
2608 int pattern_position;
2609 int next_item_length;
2610
2611 The version field is an integer containing the version number of the
2612 block format. The initial version was 0; the current version is 1. The
2613 version number will change again in future if additional fields are
2614 added, but the intention is never to remove any of the existing fields.
2615
2616 The callout_number field contains the number of the callout, as com-
2617 piled into the pattern (that is, the number after ?C for manual call-
2618 outs, and 255 for automatically generated callouts).
2619
2620 The offset_vector field is a pointer to the vector of offsets that was
2621 passed by the caller to pcre_exec() or pcre_dfa_exec(). When
2622 pcre_exec() is used, the contents can be inspected in order to extract
2623 substrings that have been matched so far, in the same way as for
2624 extracting substrings after a match has completed. For pcre_dfa_exec()
2625 this field is not useful.
2626
2627 The subject and subject_length fields contain copies of the values that
2628 were passed to pcre_exec().
2629
2630 The start_match field normally contains the offset within the subject
2631 at which the current match attempt started. However, if the escape
2632 sequence \K has been encountered, this value is changed to reflect the
2633 modified starting point. If the pattern is not anchored, the callout
2634 function may be called several times from the same point in the pattern
2635 for different starting points in the subject.
2636
2637 The current_position field contains the offset within the subject of
2638 the current match pointer.
2639
2640 When the pcre_exec() function is used, the capture_top field contains
2641 one more than the number of the highest numbered captured substring so
2642 far. If no substrings have been captured, the value of capture_top is
2643 one. This is always the case when pcre_dfa_exec() is used, because it
2644 does not support captured substrings.
2645
2646 The capture_last field contains the number of the most recently cap-
2647 tured substring. If no substrings have been captured, its value is -1.
2648 This is always the case when pcre_dfa_exec() is used.
2649
2650 The callout_data field contains a value that is passed to pcre_exec()
2651 or pcre_dfa_exec() specifically so that it can be passed back in call-
2652 outs. It is passed in the pcre_callout field of the pcre_extra data
2653 structure. If no such data was passed, the value of callout_data in a
2654 pcre_callout block is NULL. There is a description of the pcre_extra
2655 structure in the pcreapi documentation.
2656
2657 The pattern_position field is present from version 1 of the pcre_call-
2658 out structure. It contains the offset to the next item to be matched in
2659 the pattern string.
2660
2661 The next_item_length field is present from version 1 of the pcre_call-
2662 out structure. It contains the length of the next item to be matched in
2663 the pattern string. When the callout immediately precedes an alterna-
2664 tion bar, a closing parenthesis, or the end of the pattern, the length
2665 is zero. When the callout precedes an opening parenthesis, the length
2666 is that of the entire subpattern.
2667
2668 The pattern_position and next_item_length fields are intended to help
2669 in distinguishing between different automatic callouts, which all have
2670 the same callout number. However, they are set for all callouts.
2671
2672
2673 RETURN VALUES
2674
2675 The external callout function returns an integer to PCRE. If the value
2676 is zero, matching proceeds as normal. If the value is greater than
2677 zero, matching fails at the current point, but the testing of other
2678 matching possibilities goes ahead, just as if a lookahead assertion had
2679 failed. If the value is less than zero, the match is abandoned, and
2680 pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2681
2682 Negative values should normally be chosen from the set of
2683 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2684 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
2685 reserved for use by callout functions; it will never be used by PCRE
2686 itself.
2687
2688
2689 AUTHOR
2690
2691 Philip Hazel
2692 University Computing Service
2693 Cambridge CB2 3QH, England.
2694
2695
2696 REVISION
2697
2698 Last updated: 29 May 2007
2699 Copyright (c) 1997-2007 University of Cambridge.
2700 ------------------------------------------------------------------------------
2701
2702
2703 PCRECOMPAT(3) PCRECOMPAT(3)
2704
2705
2706 NAME
2707 PCRE - Perl-compatible regular expressions
2708
2709
2710 DIFFERENCES BETWEEN PCRE AND PERL
2711
2712 This document describes the differences in the ways that PCRE and Perl
2713 handle regular expressions. The differences described here are mainly
2714 with respect to Perl 5.8, though PCRE versions 7.0 and later contain
2715 some features that are expected to be in the forthcoming Perl 5.10.
2716
2717 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
2718 of what it does have are given in the section on UTF-8 support in the
2719 main pcre page.
2720
2721 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2722 permits them, but they do not mean what you might think. For example,
2723 (?!a){3} does not assert that the next three characters are not "a". It
2724 just asserts that the next character is not "a" three times.
2725
2726 3. Capturing subpatterns that occur inside negative lookahead asser-
2727 tions are counted, but their entries in the offsets vector are never
2728 set. Perl sets its numerical variables from any such patterns that are
2729 matched before the assertion fails to match something (thereby succeed-
2730 ing), but only if the negative lookahead assertion contains just one
2731 branch.
2732
2733 4. Though binary zero characters are supported in the subject string,
2734 they are not allowed in a pattern string because it is passed as a nor-
2735 mal C string, terminated by zero. The escape sequence \0 can be used in
2736 the pattern to represent a binary zero.
2737
2738 5. The following Perl escape sequences are not supported: \l, \u, \L,
2739 \U, and \N. In fact these are implemented by Perl's general string-han-
2740 dling and are not part of its pattern matching engine. If any of these
2741 are encountered by PCRE, an error is generated.
2742
2743 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
2744 is built with Unicode character property support. The properties that
2745 can be tested with \p and \P are limited to the general category prop-
2746 erties such as Lu and Nd, script names such as Greek or Han, and the
2747 derived properties Any and L&.
2748
2749 7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2750 ters in between are treated as literals. This is slightly different
2751 from Perl in that $ and @ are also handled as literals inside the
2752 quotes. In Perl, they cause variable interpolation (but of course PCRE
2753 does not have variables). Note the following examples:
2754
2755 Pattern PCRE matches Perl matches
2756
2757 \Qabc$xyz\E abc$xyz abc followed by the
2758 contents of $xyz
2759 \Qabc\$xyz\E abc\$xyz abc\$xyz
2760 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
2761
2762 The \Q...\E sequence is recognized both inside and outside character
2763 classes.
2764
2765 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2766 constructions. However, there is support for recursive patterns. This
2767 is not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2768 "callout" feature allows an external function to be called during pat-
2769 tern matching. See the pcrecallout documentation for details.
2770
2771 9. Subpatterns that are called recursively or as "subroutines" are
2772 always treated as atomic groups in PCRE. This is like Python, but
2773 unlike Perl.
2774
2775 10. There are some differences that are concerned with the settings of
2776 captured strings when part of a pattern is repeated. For example,
2777 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
2778 unset, but in PCRE it is set to "b".
2779
2780 11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT),
2781 (*FAIL), (*F), (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in
2782 the forms without an argument. PCRE does not support (*MARK). If
2783 (*ACCEPT) is within capturing parentheses, PCRE does not set that cap-
2784 ture group; this is different to Perl.
2785
2786 12. PCRE provides some extensions to the Perl regular expression facil-
2787 ities. Perl 5.10 will include new features that are not in earlier
2788 versions, some of which (such as named parentheses) have been in PCRE
2789 for some time. This list is with respect to Perl 5.10:
2790
2791 (a) Although lookbehind assertions must match fixed length strings,
2792 each alternative branch of a lookbehind assertion can match a different
2793 length of string. Perl requires them all to have the same length.
2794
2795 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
2796 meta-character matches only at the very end of the string.
2797
2798 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2799 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
2800 ignored. (Perl can be made to issue a warning.)
2801
2802 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
2803 fiers is inverted, that is, by default they are not greedy, but if fol-
2804 lowed by a question mark they are.
2805
2806 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2807 tried only at the first matching position in the subject string.
2808
2809 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2810 TURE options for pcre_exec() have no Perl equivalents.
2811
2812 (g) The \R escape sequence can be restricted to match only CR, LF, or
2813 CRLF by the PCRE_BSR_ANYCRLF option.
2814
2815 (h) The callout facility is PCRE-specific.
2816
2817 (i) The partial matching facility is PCRE-specific.
2818
2819 (j) Patterns compiled by PCRE can be saved and re-used at a later time,
2820 even on different hosts that have the other endianness.
2821
2822 (k) The alternative matching function (pcre_dfa_exec()) matches in a
2823 different way and is not Perl-compatible.
2824
2825 (l) PCRE recognizes some special sequences such as (*CR) at the start
2826 of a pattern that set overall options that cannot be changed within the
2827 pattern.
2828
2829
2830 AUTHOR
2831
2832 Philip Hazel
2833 University Computing Service
2834 Cambridge CB2 3QH, England.
2835
2836
2837 REVISION
2838
2839 Last updated: 11 September 2007
2840 Copyright (c) 1997-2007 University of Cambridge.
2841 ------------------------------------------------------------------------------
2842
2843
2844 PCREPATTERN(3) PCREPATTERN(3)
2845
2846
2847 NAME
2848 PCRE - Perl-compatible regular expressions
2849
2850
2851 PCRE REGULAR EXPRESSION DETAILS
2852
2853 The syntax and semantics of the regular expressions that are supported
2854 by PCRE are described in detail below. There is a quick-reference syn-
2855 tax summary in the pcresyntax page. Perl's regular expressions are
2856 described in its own documentation, and regular expressions in general
2857 are covered in a number of books, some of which have copious examples.
2858 Jeffrey Friedl's "Mastering Regular Expressions", published by
2859 O'Reilly, covers regular expressions in great detail. This description
2860 of PCRE's regular expressions is intended as reference material.
2861
2862 The original operation of PCRE was on strings of one-byte characters.
2863 However, there is now also support for UTF-8 character strings. To use
2864 this, you must build PCRE to include UTF-8 support, and then call
2865 pcre_compile() with the PCRE_UTF8 option. How this affects pattern
2866 matching is mentioned in several places below. There is also a summary
2867 of UTF-8 features in the section on UTF-8 support in the main pcre
2868 page.
2869
2870 The remainder of this document discusses the patterns that are sup-
2871 ported by PCRE when its main matching function, pcre_exec(), is used.
2872 From release 6.0, PCRE offers a second matching function,
2873 pcre_dfa_exec(), which matches using a different algorithm that is not
2874 Perl-compatible. Some of the features discussed below are not available
2875 when pcre_dfa_exec() is used. The advantages and disadvantages of the
2876 alternative function, and how it differs from the normal function, are
2877 discussed in the pcrematching page.
2878
2879
2880 NEWLINE CONVENTIONS
2881
2882 PCRE supports five different conventions for indicating line breaks in
2883 strings: a single CR (carriage return) character, a single LF (line-
2884 feed) character, the two-character sequence CRLF, any of the three pre-
2885 ceding, or any Unicode newline sequence. The pcreapi page has further
2886 discussion about newlines, and shows how to set the newline convention
2887 in the options arguments for the compiling and matching functions.
2888
2889 It is also possible to specify a newline convention by starting a pat-
2890 tern string with one of the following five sequences:
2891
2892 (*CR) carriage return
2893 (*LF) linefeed
2894 (*CRLF) carriage return, followed by linefeed
2895 (*ANYCRLF) any of the three above
2896 (*ANY) all Unicode newline sequences
2897
2898 These override the default and the options given to pcre_compile(). For
2899 example, on a Unix system where LF is the default newline sequence, the
2900 pattern
2901
2902 (*CR)a.b
2903
2904 changes the convention to CR. That pattern matches "a\nb" because LF is
2905 no longer a newline. Note that these special settings, which are not
2906 Perl-compatible, are recognized only at the very start of a pattern,
2907 and that they must be in upper case. If more than one of them is
2908 present, the last one is used.
2909
2910 The newline convention does not affect what the \R escape sequence
2911 matches. By default, this is any Unicode newline sequence, for Perl
2912 compatibility. However, this can be changed; see the description of \R
2913 in the section entitled "Newline sequences" below.
2914
2915
2916 CHARACTERS AND METACHARACTERS
2917
2918 A regular expression is a pattern that is matched against a subject
2919 string from left to right. Most characters stand for themselves in a
2920 pattern, and match the corresponding characters in the subject. As a
2921 trivial example, the pattern
2922
2923 The quick brown fox
2924
2925 matches a portion of a subject string that is identical to itself. When
2926 caseless matching is specified (the PCRE_CASELESS option), letters are
2927 matched independently of case. In UTF-8 mode, PCRE always understands
2928 the concept of case for characters whose values are less than 128, so
2929 caseless matching is always possible. For characters with higher val-
2930 ues, the concept of case is supported if PCRE is compiled with Unicode
2931 property support, but not otherwise. If you want to use caseless
2932 matching for characters 128 and above, you must ensure that PCRE is
2933 compiled with Unicode property support as well as with UTF-8 support.
2934
2935 The power of regular expressions comes from the ability to include
2936 alternatives and repetitions in the pattern. These are encoded in the
2937 pattern by the use of metacharacters, which do not stand for themselves
2938 but instead are interpreted in some special way.
2939
2940 There are two different sets of metacharacters: those that are recog-
2941 nized anywhere in the pattern except within square brackets, and those
2942 that are recognized within square brackets. Outside square brackets,
2943 the metacharacters are as follows:
2944
2945 \ general escape character with several uses
2946 ^ assert start of string (or line, in multiline mode)
2947 $ assert end of string (or line, in multiline mode)
2948 . match any character except newline (by default)
2949 [ start character class definition
2950 | start of alternative branch
2951 ( start subpattern
2952 ) end subpattern
2953 ? extends the meaning of (
2954 also 0 or 1 quantifier
2955 also quantifier minimizer
2956 * 0 or more quantifier
2957 + 1 or more quantifier
2958 also "possessive quantifier"
2959 { start min/max quantifier
2960
2961 Part of a pattern that is in square brackets is called a "character
2962 class". In a character class the only metacharacters are:
2963
2964 \ general escape character
2965 ^ negate the class, but only if the first character
2966 - indicates character range
2967 [ POSIX character class (only if followed by POSIX
2968 syntax)
2969 ] terminates the character class
2970
2971 The following sections describe the use of each of the metacharacters.
2972
2973
2974 BACKSLASH
2975
2976 The backslash character has several uses. Firstly, if it is followed by
2977 a non-alphanumeric character, it takes away any special meaning that
2978 character may have. This use of backslash as an escape character
2979 applies both inside and outside character classes.
2980
2981 For example, if you want to match a * character, you write \* in the
2982 pattern. This escaping action applies whether or not the following
2983 character would otherwise be interpreted as a metacharacter, so it is
2984 always safe to precede a non-alphanumeric with backslash to specify
2985 that it stands for itself. In particular, if you want to match a back-
2986 slash, you write \\.
2987
2988 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
2989 the pattern (other than in a character class) and characters between a
2990 # outside a character class and the next newline are ignored. An escap-
2991 ing backslash can be used to include a whitespace or # character as
2992 part of the pattern.
2993
2994 If you want to remove the special meaning from a sequence of charac-
2995 ters, you can do so by putting them between \Q and \E. This is differ-
2996 ent from Perl in that $ and @ are handled as literals in \Q...\E
2997 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
2998 tion. Note the following examples:
2999
3000 Pattern PCRE matches Perl matches
3001
3002 \Qabc$xyz\E abc$xyz abc followed by the
3003 contents of $xyz
3004 \Qabc\$xyz\E abc\$xyz abc\$xyz
3005 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3006
3007 The \Q...\E sequence is recognized both inside and outside character
3008 classes.
3009
3010 Non-printing characters
3011
3012 A second use of backslash provides a way of encoding non-printing char-
3013 acters in patterns in a visible manner. There is no restriction on the
3014 appearance of non-printing characters, apart from the binary zero that
3015 terminates a pattern, but when a pattern is being prepared by text
3016 editing, it is usually easier to use one of the following escape
3017 sequences than the binary character it represents:
3018
3019 \a alarm, that is, the BEL character (hex 07)
3020 \cx "control-x", where x is any character
3021 \e escape (hex 1B)
3022 \f formfeed (hex 0C)
3023 \n linefeed (hex 0A)
3024 \r carriage return (hex 0D)
3025 \t tab (hex 09)
3026 \ddd character with octal code ddd, or backreference
3027 \xhh character with hex code hh
3028 \x{hhh..} character with hex code hhh..
3029
3030 The precise effect of \cx is as follows: if x is a lower case letter,
3031 it is converted to upper case. Then bit 6 of the character (hex 40) is
3032 inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
3033 becomes hex 7B.
3034
3035 After \x, from zero to two hexadecimal digits are read (letters can be
3036 in upper or lower case). Any number of hexadecimal digits may appear
3037 between \x{ and }, but the value of the character code must be less
3038 than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
3039 the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
3040 than the largest Unicode code point, which is 10FFFF.
3041
3042 If characters other than hexadecimal digits appear between \x{ and },
3043 or if there is no terminating }, this form of escape is not recognized.
3044 Instead, the initial \x will be interpreted as a basic hexadecimal
3045 escape, with no following digits, giving a character whose value is
3046 zero.
3047
3048 Characters whose value is less than 256 can be defined by either of the
3049 two syntaxes for \x. There is no difference in the way they are han-
3050 dled. For example, \xdc is exactly the same as \x{dc}.
3051
3052 After \0 up to two further octal digits are read. If there are fewer
3053 than two digits, just those that are present are used. Thus the
3054 sequence \0\x\07 specifies two binary zeros followed by a BEL character
3055 (code value 7). Make sure you supply two digits after the initial zero
3056 if the pattern character that follows is itself an octal digit.
3057
3058 The handling of a backslash followed by a digit other than 0 is compli-
3059 cated. Outside a character class, PCRE reads it and any following dig-
3060 its as a decimal number. If the number is less than 10, or if there
3061 have been at least that many previous capturing left parentheses in the
3062 expression, the entire sequence is taken as a back reference. A
3063 description of how this works is given later, following the discussion
3064 of parenthesized subpatterns.
3065
3066 Inside a character class, or if the decimal number is greater than 9
3067 and there have not been that many capturing subpatterns, PCRE re-reads
3068 up to three octal digits following the backslash, and uses them to gen-
3069 erate a data character. Any subsequent digits stand for themselves. In
3070 non-UTF-8 mode, the value of a character specified in octal must be
3071 less than \400. In UTF-8 mode, values up to \777 are permitted. For
3072 example:
3073
3074 \040 is another way of writing a space
3075 \40 is the same, provided there are fewer than 40
3076 previous capturing subpatterns
3077 \7 is always a back reference
3078 \11 might be a back reference, or another way of
3079 writing a tab
3080 \011 is always a tab
3081 \0113 is a tab followed by the character "3"
3082 \113 might be a back reference, otherwise the
3083 character with octal code 113
3084 \377 might be a back reference, otherwise
3085 the byte consisting entirely of 1 bits
3086 \81 is either a back reference, or a binary zero
3087 followed by the two characters "8" and "1"
3088
3089 Note that octal values of 100 or greater must not be introduced by a
3090 leading zero, because no more than three octal digits are ever read.
3091
3092 All the sequences that define a single character value can be used both
3093 inside and outside character classes. In addition, inside a character
3094 class, the sequence \b is interpreted as the backspace character (hex
3095 08), and the sequences \R and \X are interpreted as the characters "R"
3096 and "X", respectively. Outside a character class, these sequences have
3097 different meanings (see below).
3098
3099 Absolute and relative back references
3100
3101 The sequence \g followed by an unsigned or a negative number, option-
3102 ally enclosed in braces, is an absolute or relative back reference. A
3103 named back reference can be coded as \g{name}. Back references are dis-
3104 cussed later, following the discussion of parenthesized subpatterns.
3105
3106 Generic character types
3107
3108 Another use of backslash is for specifying generic character types. The
3109 following are always recognized:
3110
3111 \d any decimal digit
3112 \D any character that is not a decimal digit
3113 \h any horizontal whitespace character
3114 \H any character that is not a horizontal whitespace character
3115 \s any whitespace character
3116 \S any character that is not a whitespace character
3117 \v any vertical whitespace character
3118 \V any character that is not a vertical whitespace character
3119 \w any "word" character
3120 \W any "non-word" character
3121
3122 Each pair of escape sequences partitions the complete set of characters
3123 into two disjoint sets. Any given character matches one, and only one,
3124 of each pair.
3125
3126 These character type sequences can appear both inside and outside char-
3127 acter classes. They each match one character of the appropriate type.
3128 If the current matching point is at the end of the subject string, all
3129 of them fail, since there is no character to match.
3130
3131 For compatibility with Perl, \s does not match the VT character (code
3132 11). This makes it different from the the POSIX "space" class. The \s
3133 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
3134 "use locale;" is included in a Perl script, \s may match the VT charac-
3135 ter. In PCRE, it never does.
3136
3137 In UTF-8 mode, characters with values greater than 128 never match \d,
3138 \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
3139 code character property support is available. These sequences retain
3140 their original meanings from before UTF-8 support was available, mainly
3141 for efficiency reasons.
3142
3143 The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
3144 the other sequences, these do match certain high-valued codepoints in
3145 UTF-8 mode. The horizontal space characters are:
3146
3147 U+0009 Horizontal tab
3148 U+0020 Space
3149 U+00A0 Non-break space
3150 U+1680 Ogham space mark
3151 U+180E Mongolian vowel separator
3152 U+2000 En quad
3153 U+2001 Em quad
3154 U+2002 En space
3155 U+2003 Em space
3156 U+2004 Three-per-em space
3157 U+2005 Four-per-em space
3158 U+2006 Six-per-em space
3159 U+2007 Figure space
3160 U+2008 Punctuation space
3161 U+2009 Thin space
3162 U+200A Hair space
3163 U+202F Narrow no-break space
3164 U+205F Medium mathematical space
3165 U+3000 Ideographic space
3166
3167 The vertical space characters are:
3168
3169 U+000A Linefeed
3170 U+000B Vertical tab
3171 U+000C Formfeed
3172 U+000D Carriage return
3173 U+0085 Next line
3174 U+2028 Line separator
3175 U+2029 Paragraph separator
3176
3177 A "word" character is an underscore or any character less than 256 that
3178 is a letter or digit. The definition of letters and digits is con-
3179 trolled by PCRE's low-valued character tables, and may vary if locale-
3180 specific matching is taking place (see "Locale support" in the pcreapi
3181 page). For example, in a French locale such as "fr_FR" in Unix-like
3182 systems, or "french" in Windows, some character codes greater than 128
3183 are used for accented letters, and these are matched by \w. The use of
3184 locales with Unicode is discouraged.
3185
3186 Newline sequences
3187
3188 Outside a character class, by default, the escape sequence \R matches
3189 any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
3190 mode \R is equivalent to the following:
3191
3192 (?>\r\n|\n|\x0b|\f|\r|\x85)
3193
3194 This is an example of an "atomic group", details of which are given
3195 below. This particular group matches either the two-character sequence
3196 CR followed by LF, or one of the single characters LF (linefeed,
3197 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3198 return, U+000D), or NEL (next line, U+0085). The two-character sequence
3199 is treated as a single unit that cannot be split.
3200
3201 In UTF-8 mode, two additional characters whose codepoints are greater
3202 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3203 rator, U+2029). Unicode character property support is not needed for
3204 these characters to be recognized.
3205
3206 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
3207 the complete set of Unicode line endings) by setting the option
3208 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
3209 This can be made the default when PCRE is built; if this is the case,
3210 the other behaviour can be requested via the PCRE_BSR_UNICODE option.
3211 It is also possible to specify these settings by starting a pattern
3212 string with one of the following sequences:
3213
3214 (*BSR_ANYCRLF) CR, LF, or CRLF only
3215 (*BSR_UNICODE) any Unicode newline sequence
3216
3217 These override the default and the options given to pcre_compile(), but
3218 they can be overridden by options given to pcre_exec(). Note that these
3219 special settings, which are not Perl-compatible, are recognized only at
3220 the very start of a pattern, and that they must be in upper case. If
3221 more than one of them is present, the last one is used.
3222
3223 Inside a character class, \R matches the letter "R".
3224
3225 Unicode character properties
3226
3227 When PCRE is built with Unicode character property support, three addi-
3228 tional escape sequences that match characters with specific properties
3229 are available. When not in UTF-8 mode, these sequences are of course
3230 limited to testing characters whose codepoints are less than 256, but
3231 they do work in this mode. The extra escape sequences are:
3232
3233 \p{xx} a character with the xx property
3234 \P{xx} a character without the xx property
3235 \X an extended Unicode sequence
3236
3237 The property names represented by xx above are limited to the Unicode
3238 script names, the general category properties, and "Any", which matches
3239 any character (including newline). Other properties such as "InMusical-
3240 Symbols" are not currently supported by PCRE. Note that \P{Any} does
3241 not match any characters, so always causes a match failure.
3242
3243 Sets of Unicode characters are defined as belonging to certain scripts.
3244 A character from one of these sets can be matched using a script name.
3245 For example:
3246
3247 \p{Greek}
3248 \P{Han}
3249
3250 Those that are not part of an identified script are lumped together as
3251 "Common". The current list of scripts is:
3252
3253 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
3254 Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
3255 Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
3256 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
3257 gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
3258 Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
3259 Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
3260 Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
3261 Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
3262
3263 Each character has exactly one general category property, specified by
3264 a two-letter abbreviation. For compatibility with Perl, negation can be
3265 specified by including a circumflex between the opening brace and the
3266 property name. For example, \p{^Lu} is the same as \P{Lu}.
3267
3268 If only one letter is specified with \p or \P, it includes all the gen-
3269 eral category properties that start with that letter. In this case, in
3270 the absence of negation, the curly brackets in the escape sequence are
3271 optional; these two examples have the same effect:
3272
3273 \p{L}
3274 \pL
3275
3276 The following general category property codes are supported:
3277
3278 C Other
3279 Cc Control
3280 Cf Format
3281 Cn Unassigned
3282 Co Private use
3283 Cs Surrogate
3284
3285 L Letter
3286 Ll Lower case letter
3287 Lm Modifier letter
3288 Lo Other letter
3289 Lt Title case letter
3290 Lu Upper case letter
3291
3292 M Mark
3293 Mc Spacing mark
3294 Me Enclosing mark
3295 Mn Non-spacing mark
3296
3297 N Number
3298 Nd Decimal number
3299 Nl Letter number
3300 No Other number
3301
3302 P Punctuation
3303 Pc Connector punctuation
3304 Pd Dash punctuation
3305 Pe Close punctuation
3306 Pf Final punctuation
3307 Pi Initial punctuation
3308 Po Other punctuation
3309 Ps Open punctuation
3310
3311 S Symbol
3312 Sc Currency symbol
3313 Sk Modifier symbol
3314 Sm Mathematical symbol
3315 So Other symbol
3316
3317 Z Separator
3318 Zl Line separator
3319 Zp Paragraph separator
3320 Zs Space separator
3321
3322 The special property L& is also supported: it matches a character that
3323 has the Lu, Ll, or Lt property, in other words, a letter that is not
3324 classified as a modifier or "other".
3325
3326 The Cs (Surrogate) property applies only to characters in the range
3327 U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see
3328 RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3329 ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in
3330 the pcreapi page).
3331
3332 The long synonyms for these properties that Perl supports (such as
3333 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
3334 any of these properties with "Is".
3335
3336 No character that is in the Unicode table has the Cn (unassigned) prop-
3337 erty. Instead, this property is assumed for any code point that is not
3338 in the Unicode table.
3339
3340 Specifying caseless matching does not affect these escape sequences.
3341 For example, \p{Lu} always matches only upper case letters.
3342
3343 The \X escape matches any number of Unicode characters that form an
3344 extended Unicode sequence. \X is equivalent to
3345
3346 (?>\PM\pM*)
3347
3348 That is, it matches a character without the "mark" property, followed
3349 by zero or more characters with the "mark" property, and treats the
3350 sequence as an atomic group (see below). Characters with the "mark"
3351 property are typically accents that affect the preceding character.
3352 None of them have codepoints less than 256, so in non-UTF-8 mode \X
3353 matches any one character.
3354
3355 Matching characters by Unicode property is not fast, because PCRE has
3356 to search a structure that contains data for over fifteen thousand
3357 characters. That is why the traditional escape sequences such as \d and
3358 \w do not use Unicode properties in PCRE.
3359
3360 Resetting the match start
3361
3362 The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
3363 ously matched characters not to be included in the final matched
3364 sequence. For example, the pattern:
3365
3366 foo\Kbar
3367
3368 matches "foobar", but reports that it has matched "bar". This feature
3369 is similar to a lookbehind assertion (described below). However, in
3370 this case, the part of the subject before the real match does not have
3371 to be of fixed length, as lookbehind assertions do. The use of \K does
3372 not interfere with the setting of captured substrings. For example,
3373 when the pattern
3374
3375 (foo)\Kbar
3376
3377 matches "foobar", the first substring is still set to "foo".
3378
3379 Simple assertions
3380
3381 The final use of backslash is for certain simple assertions. An asser-
3382 tion specifies a condition that has to be met at a particular point in
3383 a match, without consuming any characters from the subject string. The
3384 use of subpatterns for more complicated assertions is described below.
3385 The backslashed assertions are:
3386
3387 \b matches at a word boundary
3388 \B matches when not at a word boundary
3389 \A matches at the start of the subject
3390 \Z matches at the end of the subject
3391 also matches before a newline at the end of the subject
3392 \z matches only at the end of the subject
3393 \G matches at the first matching position in the subject
3394
3395 These assertions may not appear in character classes (but note that \b
3396 has a different meaning, namely the backspace character, inside a char-
3397 acter class).
3398
3399 A word boundary is a position in the subject string where the current
3400 character and the previous character do not both match \w or \W (i.e.
3401 one matches \w and the other matches \W), or the start or end of the
3402 string if the first or last character matches \w, respectively.
3403
3404 The \A, \Z, and \z assertions differ from the traditional circumflex
3405 and dollar (described in the next section) in that they only ever match
3406 at the very start and end of the subject string, whatever options are
3407 set. Thus, they are independent of multiline mode. These three asser-
3408 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
3409 affect only the behaviour of the circumflex and dollar metacharacters.
3410 However, if the startoffset argument of pcre_exec() is non-zero, indi-
3411 cating that matching is to start at a point other than the beginning of
3412 the subject, \A can never match. The difference between \Z and \z is
3413 that \Z matches before a newline at the end of the string as well as at
3414 the very end, whereas \z matches only at the end.
3415
3416 The \G assertion is true only when the current matching position is at
3417 the start point of the match, as specified by the startoffset argument
3418 of pcre_exec(). It differs from \A when the value of startoffset is
3419 non-zero. By calling pcre_exec() multiple times with appropriate argu-
3420 ments, you can mimic Perl's /g option, and it is in this kind of imple-
3421 mentation where \G can be useful.
3422
3423 Note, however, that PCRE's interpretation of \G, as the start of the
3424 current match, is subtly different from Perl's, which defines it as the
3425 end of the previous match. In Perl, these can be different when the
3426 previously matched string was empty. Because PCRE does just one match
3427 at a time, it cannot reproduce this behaviour.
3428
3429 If all the alternatives of a pattern begin with \G, the expression is
3430 anchored to the starting match position, and the "anchored" flag is set
3431 in the compiled regular expression.
3432
3433
3434 CIRCUMFLEX AND DOLLAR
3435
3436 Outside a character class, in the default matching mode, the circumflex
3437 character is an assertion that is true only if the current matching
3438 point is at the start of the subject string. If the startoffset argu-
3439 ment of pcre_exec() is non-zero, circumflex can never match if the
3440 PCRE_MULTILINE option is unset. Inside a character class, circumflex
3441 has an entirely different meaning (see below).
3442
3443 Circumflex need not be the first character of the pattern if a number
3444 of alternatives are involved, but it should be the first thing in each
3445 alternative in which it appears if the pattern is ever to match that
3446 branch. If all possible alternatives start with a circumflex, that is,
3447 if the pattern is constrained to match only at the start of the sub-
3448 ject, it is said to be an "anchored" pattern. (There are also other
3449 constructs that can cause a pattern to be anchored.)
3450
3451 A dollar character is an assertion that is true only if the current
3452 matching point is at the end of the subject string, or immediately
3453 before a newline at the end of the string (by default). Dollar need not
3454 be the last character of the pattern if a number of alternatives are
3455 involved, but it should be the last item in any branch in which it
3456 appears. Dollar has no special meaning in a character class.
3457
3458 The meaning of dollar can be changed so that it matches only at the
3459 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
3460 compile time. This does not affect the \Z assertion.
3461
3462 The meanings of the circumflex and dollar characters are changed if the
3463 PCRE_MULTILINE option is set. When this is the case, a circumflex
3464 matches immediately after internal newlines as well as at the start of
3465 the subject string. It does not match after a newline that ends the
3466 string. A dollar matches before any newlines in the string, as well as
3467 at the very end, when PCRE_MULTILINE is set. When newline is specified
3468 as the two-character sequence CRLF, isolated CR and LF characters do
3469 not indicate newlines.
3470
3471 For example, the pattern /^abc$/ matches the subject string "def\nabc"
3472 (where \n represents a newline) in multiline mode, but not otherwise.
3473 Consequently, patterns that are anchored in single line mode because
3474 all branches start with ^ are not anchored in multiline mode, and a
3475 match for circumflex is possible when the startoffset argument of
3476 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
3477 PCRE_MULTILINE is set.
3478
3479 Note that the sequences \A, \Z, and \z can be used to match the start
3480 and end of the subject in both modes, and if all branches of a pattern
3481 start with \A it is always anchored, whether or not PCRE_MULTILINE is
3482 set.
3483
3484
3485 FULL STOP (PERIOD, DOT)
3486
3487 Outside a character class, a dot in the pattern matches any one charac-
3488 ter in the subject string except (by default) a character that signi-
3489 fies the end of a line. In UTF-8 mode, the matched character may be
3490 more than one byte long.
3491
3492 When a line ending is defined as a single character, dot never matches
3493 that character; when the two-character sequence CRLF is used, dot does
3494 not match CR if it is immediately followed by LF, but otherwise it
3495 matches all characters (including isolated CRs and LFs). When any Uni-
3496 code line endings are being recognized, dot does not match CR or LF or
3497 any of the other line ending characters.
3498
3499 The behaviour of dot with regard to newlines can be changed. If the
3500 PCRE_DOTALL option is set, a dot matches any one character, without
3501 exception. If the two-character sequence CRLF is present in the subject
3502 string, it takes two dots to match it.
3503
3504 The handling of dot is entirely independent of the handling of circum-
3505 flex and dollar, the only relationship being that they both involve
3506 newlines. Dot has no special meaning in a character class.
3507
3508
3509 MATCHING A SINGLE BYTE
3510
3511 Outside a character class, the escape sequence \C matches any one byte,
3512 both in and out of UTF-8 mode. Unlike a dot, it always matches any
3513 line-ending characters. The feature is provided in Perl in order to
3514 match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
3515 acters into individual bytes, what remains in the string may be a mal-
3516 formed UTF-8 string. For this reason, the \C escape sequence is best
3517 avoided.
3518
3519 PCRE does not allow \C to appear in lookbehind assertions (described
3520 below), because in UTF-8 mode this would make it impossible to calcu-
3521 late the length of the lookbehind.
3522
3523
3524 SQUARE BRACKETS AND CHARACTER CLASSES
3525
3526 An opening square bracket introduces a character class, terminated by a
3527 closing square bracket. A closing square bracket on its own is not spe-
3528 cial. If a closing square bracket is required as a member of the class,
3529 it should be the first data character in the class (after an initial
3530 circumflex, if present) or escaped with a backslash.
3531
3532 A character class matches a single character in the subject. In UTF-8
3533 mode, the character may occupy more than one byte. A matched character
3534 must be in the set of characters defined by the class, unless the first
3535 character in the class definition is a circumflex, in which case the
3536 subject character must not be in the set defined by the class. If a
3537 circumflex is actually required as a member of the class, ensure it is
3538 not the first character, or escape it with a backslash.
3539
3540 For example, the character class [aeiou] matches any lower case vowel,
3541 while [^aeiou] matches any character that is not a lower case vowel.
3542 Note that a circumflex is just a convenient notation for specifying the
3543 characters that are in the class by enumerating those that are not. A
3544 class that starts with a circumflex is not an assertion: it still con-
3545 sumes a character from the subject string, and therefore it fails if
3546 the current pointer is at the end of the string.
3547
3548 In UTF-8 mode, characters with values greater than 255 can be included
3549 in a class as a literal string of bytes, or by using the \x{ escaping
3550 mechanism.
3551
3552 When caseless matching is set, any letters in a class represent both
3553 their upper case and lower case versions, so for example, a caseless
3554 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
3555 match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
3556 understands the concept of case for characters whose values are less
3557 than 128, so caseless matching is always possible. For characters with
3558 higher values, the concept of case is supported if PCRE is compiled
3559 with Unicode property support, but not otherwise. If you want to use
3560 caseless matching for characters 128 and above, you must ensure that
3561 PCRE is compiled with Unicode property support as well as with UTF-8
3562 support.
3563
3564 Characters that might indicate line breaks are never treated in any
3565 special way when matching character classes, whatever line-ending
3566 sequence is in use, and whatever setting of the PCRE_DOTALL and
3567 PCRE_MULTILINE options is used. A class such as [^a] always matches one
3568 of these characters.
3569
3570 The minus (hyphen) character can be used to specify a range of charac-
3571 ters in a character class. For example, [d-m] matches any letter
3572 between d and m, inclusive. If a minus character is required in a
3573 class, it must be escaped with a backslash or appear in a position
3574 where it cannot be interpreted as indicating a range, typically as the
3575 first or last character in the class.
3576
3577 It is not possible to have the literal character "]" as the end charac-
3578 ter of a range. A pattern such as [W-]46] is interpreted as a class of
3579 two characters ("W" and "-") followed by a literal string "46]", so it
3580 would match "W46]" or "-46]". However, if the "]" is escaped with a
3581 backslash it is interpreted as the end of range, so [W-\]46] is inter-
3582 preted as a class containing a range followed by two other characters.
3583 The octal or hexadecimal representation of "]" can also be used to end
3584 a range.
3585
3586 Ranges operate in the collating sequence of character values. They can
3587 also be used for characters specified numerically, for example
3588 [\000-\037]. In UTF-8 mode, ranges can include characters whose values
3589 are greater than 255, for example [\x{100}-\x{2ff}].
3590
3591 If a range that includes letters is used when caseless matching is set,
3592 it matches the letters in either case. For example, [W-c] is equivalent
3593 to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
3594 character tables for a French locale are in use, [\xc8-\xcb] matches
3595 accented E characters in both cases. In UTF-8 mode, PCRE supports the
3596 concept of case for characters with values greater than 128 only when
3597 it is compiled with Unicode property support.
3598
3599 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
3600 in a character class, and add the characters that they match to the
3601 class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
3602 flex can conveniently be used with the upper case character types to
3603 specify a more restricted set of characters than the matching lower
3604 case type. For example, the class [^\W_] matches any letter or digit,
3605 but not underscore.
3606
3607 The only metacharacters that are recognized in character classes are
3608 backslash, hyphen (only where it can be interpreted as specifying a
3609 range), circumflex (only at the start), opening square bracket (only
3610 when it can be interpreted as introducing a POSIX class name - see the
3611 next section), and the terminating closing square bracket. However,
3612 escaping other non-alphanumeric characters does no harm.
3613
3614
3615 POSIX CHARACTER CLASSES
3616
3617 Perl supports the POSIX notation for character classes. This uses names
3618 enclosed by [: and :] within the enclosing square brackets. PCRE also
3619 supports this notation. For example,
3620
3621 [01[:alpha:]%]
3622
3623 matches "0", "1", any alphabetic character, or "%". The supported class
3624 names are
3625
3626 alnum letters and digits
3627 alpha letters
3628 ascii character codes 0 - 127
3629 blank space or tab only
3630 cntrl control characters
3631 digit decimal digits (same as \d)
3632 graph printing characters, excluding space
3633 lower lower case letters
3634 print printing characters, including space
3635 punct printing characters, excluding letters and digits
3636 space white space (not quite the same as \s)
3637 upper upper case letters
3638 word "word" characters (same as \w)
3639 xdigit hexadecimal digits
3640
3641 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
3642 and space (32). Notice that this list includes the VT character (code
3643 11). This makes "space" different to \s, which does not include VT (for
3644 Perl compatibility).
3645
3646 The name "word" is a Perl extension, and "blank" is a GNU extension
3647 from Perl 5.8. Another Perl extension is negation, which is indicated
3648 by a ^ character after the colon. For example,
3649
3650 [12[:^digit:]]
3651
3652 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
3653 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
3654 these are not supported, and an error is given if they are encountered.
3655
3656 In UTF-8 mode, characters with values greater than 128 do not match any
3657 of the POSIX character classes.
3658
3659
3660 VERTICAL BAR
3661
3662 Vertical bar characters are used to separate alternative patterns. For
3663 example, the pattern
3664
3665 gilbert|sullivan
3666
3667 matches either "gilbert" or "sullivan". Any number of alternatives may
3668 appear, and an empty alternative is permitted (matching the empty
3669 string). The matching process tries each alternative in turn, from left
3670 to right, and the first one that succeeds is used. If the alternatives
3671 are within a subpattern (defined below), "succeeds" means matching the
3672 rest of the main pattern as well as the alternative in the subpattern.
3673
3674
3675 INTERNAL OPTION SETTING
3676
3677 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
3678 PCRE_EXTENDED options can be changed from within the pattern by a
3679 sequence of Perl option letters enclosed between "(?" and ")". The
3680 option letters are
3681
3682 i for PCRE_CASELESS
3683 m for PCRE_MULTILINE
3684 s for PCRE_DOTALL
3685 x for PCRE_EXTENDED
3686
3687 For example, (?im) sets caseless, multiline matching. It is also possi-
3688 ble to unset these options by preceding the letter with a hyphen, and a
3689 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
3690 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
3691 is also permitted. If a letter appears both before and after the
3692 hyphen, the option is unset.
3693
3694 When an option change occurs at top level (that is, not inside subpat-
3695 tern parentheses), the change applies to the remainder of the pattern
3696 that follows. If the change is placed right at the start of a pattern,
3697 PCRE extracts it into the global options (and it will therefore show up
3698 in data extracted by the pcre_fullinfo() function).
3699
3700 An option change within a subpattern (see below for a description of
3701 subpatterns) affects only that part of the current pattern that follows
3702 it, so
3703
3704 (a(?i)b)c
3705
3706 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
3707 used). By this means, options can be made to have different settings
3708 in different parts of the pattern. Any changes made in one alternative
3709 do carry on into subsequent branches within the same subpattern. For
3710 example,
3711
3712 (a(?i)b|c)
3713
3714 matches "ab", "aB", "c", and "C", even though when matching "C" the
3715 first branch is abandoned before the option setting. This is because
3716 the effects of option settings happen at compile time. There would be
3717 some very weird behaviour otherwise.
3718
3719 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
3720 can be changed in the same way as the Perl-compatible options by using
3721 the characters J, U and X respectively.
3722
3723
3724 SUBPATTERNS
3725
3726 Subpatterns are delimited by parentheses (round brackets), which can be
3727 nested. Turning part of a pattern into a subpattern does two things:
3728
3729 1. It localizes a set of alternatives. For example, the pattern
3730
3731 cat(aract|erpillar|)
3732
3733 matches one of the words "cat", "cataract", or "caterpillar". Without
3734 the parentheses, it would match "cataract", "erpillar" or an empty
3735 string.
3736
3737 2. It sets up the subpattern as a capturing subpattern. This means
3738 that, when the whole pattern matches, that portion of the subject
3739 string that matched the subpattern is passed back to the caller via the
3740 ovector argument of pcre_exec(). Opening parentheses are counted from
3741 left to right (starting from 1) to obtain numbers for the capturing
3742 subpatterns.
3743
3744 For example, if the string "the red king" is matched against the pat-
3745 tern
3746
3747 the ((red|white) (king|queen))
3748
3749 the captured substrings are "red king", "red", and "king", and are num-
3750 bered 1, 2, and 3, respectively.
3751
3752 The fact that plain parentheses fulfil two functions is not always
3753 helpful. There are often times when a grouping subpattern is required
3754 without a capturing requirement. If an opening parenthesis is followed
3755 by a question mark and a colon, the subpattern does not do any captur-
3756 ing, and is not counted when computing the number of any subsequent
3757 capturing subpatterns. For example, if the string "the white queen" is
3758 matched against the pattern
3759
3760 the ((?:red|white) (king|queen))
3761
3762 the captured substrings are "white queen" and "queen", and are numbered
3763 1 and 2. The maximum number of capturing subpatterns is 65535.
3764
3765 As a convenient shorthand, if any option settings are required at the
3766 start of a non-capturing subpattern, the option letters may appear
3767 between the "?" and the ":". Thus the two patterns
3768
3769 (?i:saturday|sunday)
3770 (?:(?i)saturday|sunday)
3771
3772 match exactly the same set of strings. Because alternative branches are
3773 tried from left to right, and options are not reset until the end of
3774 the subpattern is reached, an option setting in one branch does affect
3775 subsequent branches, so the above patterns match "SUNDAY" as well as
3776 "Saturday".
3777
3778
3779 DUPLICATE SUBPATTERN NUMBERS
3780
3781 Perl 5.10 introduced a feature whereby each alternative in a subpattern
3782 uses the same numbers for its capturing parentheses. Such a subpattern
3783 starts with (?| and is itself a non-capturing subpattern. For example,
3784 consider this pattern:
3785
3786 (?|(Sat)ur|(Sun))day
3787
3788 Because the two alternatives are inside a (?| group, both sets of cap-
3789 turing parentheses are numbered one. Thus, when the pattern matches,
3790 you can look at captured substring number one, whichever alternative
3791 matched. This construct is useful when you want to capture part, but
3792 not all, of one of a number of alternatives. Inside a (?| group, paren-
3793 theses are numbered as usual, but the number is reset at the start of
3794 each branch. The numbers of any capturing buffers that follow the sub-
3795 pattern start after the highest number used in any branch. The follow-
3796 ing example is taken from the Perl documentation. The numbers under-
3797 neath show in which buffer the captured content will be stored.
3798
3799 # before ---------------branch-reset----------- after
3800 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
3801 # 1 2 2 3 2 3 4
3802
3803 A backreference or a recursive call to a numbered subpattern always
3804 refers to the first one in the pattern with the given number.
3805
3806 An alternative approach to using this "branch reset" feature is to use
3807 duplicate named subpatterns, as described in the next section.
3808
3809
3810 NAMED SUBPATTERNS
3811
3812 Identifying capturing parentheses by number is simple, but it can be
3813 very hard to keep track of the numbers in complicated regular expres-
3814 sions. Furthermore, if an expression is modified, the numbers may
3815 change. To help with this difficulty, PCRE supports the naming of sub-
3816 patterns. This feature was not added to Perl until release 5.10. Python
3817 had the feature earlier, and PCRE introduced it at release 4.0, using
3818 the Python syntax. PCRE now supports both the Perl and the Python syn-
3819 tax.
3820
3821 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
3822 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
3823 to capturing parentheses from other parts of the pattern, such as back-
3824 references, recursion, and conditions, can be made by name as well as
3825 by number.
3826
3827 Names consist of up to 32 alphanumeric characters and underscores.
3828 Named capturing parentheses are still allocated numbers as well as
3829 names, exactly as if the names were not present. The PCRE API provides
3830 function calls for extracting the name-to-number translation table from
3831 a compiled pattern. There is also a convenience function for extracting
3832 a captured substring by name.
3833
3834 By default, a name must be unique within a pattern, but it is possible
3835 to relax this constraint by setting the PCRE_DUPNAMES option at compile
3836 time. This can be useful for patterns where only one instance of the
3837 named parentheses can match. Suppose you want to match the name of a
3838 weekday, either as a 3-letter abbreviation or as the full name, and in
3839 both cases you want to extract the abbreviation. This pattern (ignoring
3840 the line breaks) does the job:
3841
3842 (?<DN>Mon|Fri|Sun)(?:day)?|
3843 (?<DN>Tue)(?:sday)?|
3844 (?<DN>Wed)(?:nesday)?|
3845 (?<DN>Thu)(?:rsday)?|
3846 (?<DN>Sat)(?:urday)?
3847
3848 There are five capturing substrings, but only one is ever set after a
3849 match. (An alternative way of solving this problem is to use a "branch
3850 reset" subpattern, as described in the previous section.)
3851
3852 The convenience function for extracting the data by name returns the
3853 substring for the first (and in this example, the only) subpattern of
3854 that name that matched. This saves searching to find which numbered
3855 subpattern it was. If you make a reference to a non-unique named sub-
3856 pattern from elsewhere in the pattern, the one that corresponds to the
3857 lowest number is used. For further details of the interfaces for han-
3858 dling named subpatterns, see the pcreapi documentation.
3859
3860
3861 REPETITION
3862
3863 Repetition is specified by quantifiers, which can follow any of the
3864 following items:
3865
3866 a literal data character
3867 the dot metacharacter
3868 the \C escape sequence
3869 the \X escape sequence (in UTF-8 mode with Unicode properties)
3870 the \R escape sequence
3871 an escape such as \d that matches a single character
3872 a character class
3873 a back reference (see next section)
3874 a parenthesized subpattern (unless it is an assertion)
3875
3876 The general repetition quantifier specifies a minimum and maximum num-
3877 ber of permitted matches, by giving the two numbers in curly brackets
3878 (braces), separated by a comma. The numbers must be less than 65536,
3879 and the first must be less than or equal to the second. For example:
3880
3881 z{2,4}
3882
3883 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
3884 special character. If the second number is omitted, but the comma is
3885 present, there is no upper limit; if the second number and the comma
3886 are both omitted, the quantifier specifies an exact number of required
3887 matches. Thus
3888
3889 [aeiou]{3,}
3890
3891 matches at least 3 successive vowels, but may match many more, while
3892
3893 \d{8}
3894
3895 matches exactly 8 digits. An opening curly bracket that appears in a
3896 position where a quantifier is not allowed, or one that does not match
3897 the syntax of a quantifier, is taken as a literal character. For exam-
3898 ple, {,6} is not a quantifier, but a literal string of four characters.
3899
3900 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
3901 individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
3902 acters, each of which is represented by a two-byte sequence. Similarly,
3903 when Unicode property support is available, \X{3} matches three Unicode
3904 extended sequences, each of which may be several bytes long (and they
3905 may be of different lengths).
3906
3907 The quantifier {0} is permitted, causing the expression to behave as if
3908 the previous item and the quantifier were not present.
3909
3910 For convenience, the three most common quantifiers have single-charac-
3911 ter abbreviations:
3912
3913 * is equivalent to {0,}
3914 + is equivalent to {1,}
3915 ? is equivalent to {0,1}
3916
3917 It is possible to construct infinite loops by following a subpattern
3918 that can match no characters with a quantifier that has no upper limit,
3919 for example:
3920
3921 (a?)*
3922
3923 Earlier versions of Perl and PCRE used to give an error at compile time
3924 for such patterns. However, because there are cases where this can be
3925 useful, such patterns are now accepted, but if any repetition of the
3926 subpattern does in fact match no characters, the loop is forcibly bro-
3927 ken.
3928
3929 By default, the quantifiers are "greedy", that is, they match as much
3930 as possible (up to the maximum number of permitted times), without
3931 causing the rest of the pattern to fail. The classic example of where
3932 this gives problems is in trying to match comments in C programs. These
3933 appear between /* and */ and within the comment, individual * and /
3934 characters may appear. An attempt to match C comments by applying the
3935 pattern
3936
3937 /\*.*\*/
3938
3939 to the string
3940
3941 /* first comment */ not comment /* second comment */
3942
3943 fails, because it matches the entire string owing to the greediness of
3944 the .* item.
3945
3946 However, if a quantifier is followed by a question mark, it ceases to
3947 be greedy, and instead matches the minimum number of times possible, so
3948 the pattern
3949
3950 /\*.*?\*/
3951
3952 does the right thing with the C comments. The meaning of the various
3953 quantifiers is not otherwise changed, just the preferred number of
3954 matches. Do not confuse this use of question mark with its use as a
3955 quantifier in its own right. Because it has two uses, it can sometimes
3956 appear doubled, as in
3957
3958 \d??\d
3959
3960 which matches one digit by preference, but can match two if that is the
3961 only way the rest of the pattern matches.
3962
3963 If the PCRE_UNGREEDY option is set (an option that is not available in
3964 Perl), the quantifiers are not greedy by default, but individual ones
3965 can be made greedy by following them with a question mark. In other
3966 words, it inverts the default behaviour.
3967
3968 When a parenthesized subpattern is quantified with a minimum repeat
3969 count that is greater than 1 or with a limited maximum, more memory is
3970 required for the compiled pattern, in proportion to the size of the
3971 minimum or maximum.
3972
3973 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
3974 alent to Perl's /s) is set, thus allowing the dot to match newlines,
3975 the pattern is implicitly anchored, because whatever follows will be
3976 tried against every character position in the subject string, so there
3977 is no point in retrying the overall match at any position after the
3978 first. PCRE normally treats such a pattern as though it were preceded
3979 by \A.
3980
3981 In cases where it is known that the subject string contains no new-
3982 lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
3983 mization, or alternatively using ^ to indicate anchoring explicitly.
3984
3985 However, there is one situation where the optimization cannot be used.
3986 When .* is inside capturing parentheses that are the subject of a
3987 backreference elsewhere in the pattern, a match at the start may fail
3988 where a later one succeeds. Consider, for example:
3989
3990 (.*)abc\1
3991
3992 If the subject is "xyz123abc123" the match point is the fourth charac-
3993 ter. For this reason, such a pattern is not implicitly anchored.
3994
3995 When a capturing subpattern is repeated, the value captured is the sub-
3996 string that matched the final iteration. For example, after
3997
3998 (tweedle[dume]{3}\s*)+
3999
4000 has matched "tweedledum tweedledee" the value of the captured substring
4001 is "tweedledee". However, if there are nested capturing subpatterns,
4002 the corresponding captured values may have been set in previous itera-
4003 tions. For example, after
4004
4005 /(a|(b))+/
4006
4007 matches "aba" the value of the second captured substring is "b".
4008
4009
4010 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
4011
4012 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
4013 repetition, failure of what follows normally causes the repeated item
4014 to be re-evaluated to see if a different number of repeats allows the
4015 rest of the pattern to match. Sometimes it is useful to prevent this,
4016 either to change the nature of the match, or to cause it fail earlier
4017 than it otherwise might, when the author of the pattern knows there is
4018 no point in carrying on.
4019
4020 Consider, for example, the pattern \d+foo when applied to the subject
4021 line
4022
4023 123456bar
4024
4025 After matching all 6 digits and then failing to match "foo", the normal
4026 action of the matcher is to try again with only 5 digits matching the
4027 \d+ item, and then with 4, and so on, before ultimately failing.
4028 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
4029 the means for specifying that once a subpattern has matched, it is not
4030 to be re-evaluated in this way.
4031
4032 If we use atomic grouping for the previous example, the matcher gives
4033 up immediately on failing to match "foo" the first time. The notation
4034 is a kind of special parenthesis, starting with (?> as in this example:
4035
4036 (?>\d+)foo
4037
4038 This kind of parenthesis "locks up" the part of the pattern it con-
4039 tains once it has matched, and a failure further into the pattern is
4040 prevented from backtracking into it. Backtracking past it to previous
4041 items, however, works as normal.
4042
4043 An alternative description is that a subpattern of this type matches
4044 the string of characters that an identical standalone pattern would
4045 match, if anchored at the current point in the subject string.
4046
4047 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
4048 such as the above example can be thought of as a maximizing repeat that
4049 must swallow everything it can. So, while both \d+ and \d+? are pre-
4050 pared to adjust the number of digits they match in order to make the
4051 rest of the pattern match, (?>\d+) can only match an entire sequence of
4052 digits.
4053
4054 Atomic groups in general can of course contain arbitrarily complicated
4055 subpatterns, and can be nested. However, when the subpattern for an
4056 atomic group is just a single repeated item, as in the example above, a
4057 simpler notation, called a "possessive quantifier" can be used. This
4058 consists of an additional + character following a quantifier. Using
4059 this notation, the previous example can be rewritten as
4060
4061 \d++foo
4062
4063 Note that a possessive quantifier can be used with an entire group, for
4064 example:
4065
4066 (abc|xyz){2,3}+
4067
4068 Possessive quantifiers are always greedy; the setting of the
4069 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
4070 simpler forms of atomic group. However, there is no difference in the
4071 meaning of a possessive quantifier and the equivalent atomic group,
4072 though there may be a performance difference; possessive quantifiers
4073 should be slightly faster.
4074
4075 The possessive quantifier syntax is an extension to the Perl 5.8 syn-
4076 tax. Jeffrey Friedl originated the idea (and the name) in the first
4077 edition of his book. Mike McCloskey liked it, so implemented it when he
4078 built Sun's Java package, and PCRE copied it from there. It ultimately
4079 found its way into Perl at release 5.10.
4080
4081 PCRE has an optimization that automatically "possessifies" certain sim-
4082 ple pattern constructs. For example, the sequence A+B is treated as
4083 A++B because there is no point in backtracking into a sequence of A's
4084 when B must follow.
4085
4086 When a pattern contains an unlimited repeat inside a subpattern that
4087 can itself be repeated an unlimited number of times, the use of an
4088 atomic group is the only way to avoid some failing matches taking a
4089 very long time indeed. The pattern
4090
4091 (\D+|<\d+>)*[!?]
4092
4093 matches an unlimited number of substrings that either consist of non-
4094 digits, or digits enclosed in <>, followed by either ! or ?. When it
4095 matches, it runs quickly. However, if it is applied to
4096
4097 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4098
4099 it takes a long time before reporting failure. This is because the
4100 string can be divided between the internal \D+ repeat and the external
4101 * repeat in a large number of ways, and all have to be tried. (The
4102 example uses [!?] rather than a single character at the end, because
4103 both PCRE and Perl have an optimization that allows for fast failure
4104 when a single character is used. They remember the last single charac-
4105 ter that is required for a match, and fail early if it is not present
4106 in the string.) If the pattern is changed so that it uses an atomic
4107 group, like this:
4108
4109 ((?>\D+)|<\d+>)*[!?]
4110
4111 sequences of non-digits cannot be broken, and failure happens quickly.
4112
4113
4114 BACK REFERENCES
4115
4116 Outside a character class, a backslash followed by a digit greater than
4117 0 (and possibly further digits) is a back reference to a capturing sub-
4118 pattern earlier (that is, to its left) in the pattern, provided there
4119 have been that many previous capturing left parentheses.
4120
4121 However, if the decimal number following the backslash is less than 10,
4122 it is always taken as a back reference, and causes an error only if
4123 there are not that many capturing left parentheses in the entire pat-
4124 tern. In other words, the parentheses that are referenced need not be
4125 to the left of the reference for numbers less than 10. A "forward back
4126 reference" of this type can make sense when a repetition is involved
4127 and the subpattern to the right has participated in an earlier itera-
4128 tion.
4129
4130 It is not possible to have a numerical "forward back reference" to a
4131 subpattern whose number is 10 or more using this syntax because a
4132 sequence such as \50 is interpreted as a character defined in octal.
4133 See the subsection entitled "Non-printing characters" above for further
4134 details of the handling of digits following a backslash. There is no
4135 such problem when named parentheses are used. A back reference to any
4136 subpattern is possible using named parentheses (see below).
4137
4138 Another way of avoiding the ambiguity inherent in the use of digits
4139 following a backslash is to use the \g escape sequence, which is a fea-
4140 ture introduced in Perl 5.10. This escape must be followed by an
4141 unsigned number or a negative number, optionally enclosed in braces.
4142 These examples are all identical:
4143
4144 (ring), \1
4145 (ring), \g1
4146 (ring), \g{1}
4147
4148 An unsigned number specifies an absolute reference without the ambigu-
4149 ity that is present in the older syntax. It is also useful when literal
4150 digits follow the reference. A negative number is a relative reference.
4151 Consider this example:
4152
4153 (abc(def)ghi)\g{-1}
4154
4155 The sequence \g{-1} is a reference to the most recently started captur-
4156 ing subpattern before \g, that is, is it equivalent to \2. Similarly,
4157 \g{-2} would be equivalent to \1. The use of relative references can be
4158 helpful in long patterns, and also in patterns that are created by
4159 joining together fragments that contain references within themselves.
4160
4161 A back reference matches whatever actually matched the capturing sub-
4162 pattern in the current subject string, rather than anything matching
4163 the subpattern itself (see "Subpatterns as subroutines" below for a way
4164 of doing that). So the pattern
4165
4166 (sens|respons)e and \1ibility
4167
4168 matches "sense and sensibility" and "response and responsibility", but
4169 not "sense and responsibility". If caseful matching is in force at the
4170 time of the back reference, the case of letters is relevant. For exam-
4171 ple,
4172
4173 ((?i)rah)\s+\1
4174
4175 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
4176 original capturing subpattern is matched caselessly.
4177
4178 There are several different ways of writing back references to named
4179 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
4180 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
4181 unified back reference syntax, in which \g can be used for both numeric
4182 and named references, is also supported. We could rewrite the above
4183 example in any of the following ways:
4184
4185 (?<p1>(?i)rah)\s+\k<p1>
4186 (?'p1'(?i)rah)\s+\k{p1}
4187 (?P<p1>(?i)rah)\s+(?P=p1)
4188 (?<p1>(?i)rah)\s+\g{p1}
4189
4190 A subpattern that is referenced by name may appear in the pattern
4191 before or after the reference.
4192
4193 There may be more than one back reference to the same subpattern. If a
4194 subpattern has not actually been used in a particular match, any back
4195 references to it always fail. For example, the pattern
4196
4197 (a|(bc))\2
4198
4199 always fails if it starts to match "a" rather than "bc". Because there
4200 may be many capturing parentheses in a pattern, all digits following
4201 the backslash are taken as part of a potential back reference number.
4202 If the pattern continues with a digit character, some delimiter must be
4203 used to terminate the back reference. If the PCRE_EXTENDED option is
4204 set, this can be whitespace. Otherwise an empty comment (see "Com-
4205 ments" below) can be used.
4206
4207 A back reference that occurs inside the parentheses to which it refers
4208 fails when the subpattern is first used, so, for example, (a\1) never
4209 matches. However, such references can be useful inside repeated sub-
4210 patterns. For example, the pattern
4211
4212 (a|b\1)+
4213
4214 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
4215 ation of the subpattern, the back reference matches the character
4216 string corresponding to the previous iteration. In order for this to
4217 work, the pattern must be such that the first iteration does not need
4218 to match the back reference. This can be done using alternation, as in
4219 the example above, or by a quantifier with a minimum of zero.
4220
4221
4222 ASSERTIONS
4223
4224 An assertion is a test on the characters following or preceding the
4225 current matching point that does not actually consume any characters.
4226 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
4227 described above.
4228
4229 More complicated assertions are coded as subpatterns. There are two
4230 kinds: those that look ahead of the current position in the subject
4231 string, and those that look behind it. An assertion subpattern is
4232 matched in the normal way, except that it does not cause the current
4233 matching position to be changed.
4234
4235 Assertion subpatterns are not capturing subpatterns, and may not be
4236 repeated, because it makes no sense to assert the same thing several
4237 times. If any kind of assertion contains capturing subpatterns within
4238 it, these are counted for the purposes of numbering the capturing sub-
4239 patterns in the whole pattern. However, substring capturing is carried
4240 out only for positive assertions, because it does not make sense for
4241 negative assertions.
4242
4243 Lookahead assertions
4244
4245 Lookahead assertions start with (?= for positive assertions and (?! for
4246 negative assertions. For example,
4247
4248 \w+(?=;)
4249
4250 matches a word followed by a semicolon, but does not include the semi-
4251 colon in the match, and
4252
4253 foo(?!bar)
4254
4255 matches any occurrence of "foo" that is not followed by "bar". Note
4256 that the apparently similar pattern
4257
4258 (?!foo)bar
4259
4260 does not find an occurrence of "bar" that is preceded by something
4261 other than "foo"; it finds any occurrence of "bar" whatsoever, because
4262 the assertion (?!foo) is always true when the next three characters are
4263 "bar". A lookbehind assertion is needed to achieve the other effect.
4264
4265 If you want to force a matching failure at some point in a pattern, the
4266 most convenient way to do it is with (?!) because an empty string
4267 always matches, so an assertion that requires there not to be an empty
4268 string must always fail.
4269
4270 Lookbehind assertions
4271
4272 Lookbehind assertions start with (?<= for positive assertions and (?<!
4273 for negative assertions. For example,
4274
4275 (?<!foo)bar
4276
4277 does find an occurrence of "bar" that is not preceded by "foo". The
4278 contents of a lookbehind assertion are restricted such that all the
4279 strings it matches must have a fixed length. However, if there are sev-
4280 eral top-level alternatives, they do not all have to have the same
4281 fixed length. Thus
4282
4283 (?<=bullock|donkey)
4284
4285 is permitted, but
4286
4287 (?<!dogs?|cats?)
4288
4289 causes an error at compile time. Branches that match different length
4290 strings are permitted only at the top level of a lookbehind assertion.
4291 This is an extension compared with Perl (at least for 5.8), which
4292 requires all branches to match the same length of string. An assertion
4293 such as
4294
4295 (?<=ab(c|de))
4296
4297 is not permitted, because its single top-level branch can match two
4298 different lengths, but it is acceptable if rewritten to use two top-
4299 level branches:
4300
4301 (?<=abc|abde)
4302
4303 In some cases, the Perl 5.10 escape sequence \K (see above) can be used
4304 instead of a lookbehind assertion; this is not restricted to a fixed-
4305 length.
4306
4307 The implementation of lookbehind assertions is, for each alternative,
4308 to temporarily move the current position back by the fixed length and
4309 then try to match. If there are insufficient characters before the cur-
4310 rent position, the assertion fails.
4311
4312 PCRE does not allow the \C escape (which matches a single byte in UTF-8
4313 mode) to appear in lookbehind assertions, because it makes it impossi-
4314 ble to calculate the length of the lookbehind. The \X and \R escapes,
4315 which can match different numbers of bytes, are also not permitted.
4316
4317 Possessive quantifiers can be used in conjunction with lookbehind
4318 assertions to specify efficient matching at the end of the subject
4319 string. Consider a simple pattern such as
4320
4321 abcd$
4322
4323 when applied to a long string that does not match. Because matching
4324 proceeds from left to right, PCRE will look for each "a" in the subject
4325 and then see if what follows matches the rest of the pattern. If the
4326 pattern is specified as
4327
4328 ^.*abcd$
4329
4330 the initial .* matches the entire string at first, but when this fails
4331 (because there is no following "a"), it backtracks to match all but the
4332 last character, then all but the last two characters, and so on. Once
4333 again the search for "a" covers the entire string, from right to left,
4334 so we are no better off. However, if the pattern is written as
4335
4336 ^.*+(?<=abcd)
4337
4338 there can be no backtracking for the .*+ item; it can match only the
4339 entire string. The subsequent lookbehind assertion does a single test
4340 on the last four characters. If it fails, the match fails immediately.
4341 For long strings, this approach makes a significant difference to the
4342 processing time.
4343
4344 Using multiple assertions
4345
4346 Several assertions (of any sort) may occur in succession. For example,
4347
4348 (?<=\d{3})(?<!999)foo
4349
4350 matches "foo" preceded by three digits that are not "999". Notice that
4351 each of the assertions is applied independently at the same point in
4352 the subject string. First there is a check that the previous three
4353 characters are all digits, and then there is a check that the same
4354 three characters are not "999". This pattern does not match "foo" pre-
4355 ceded by six characters, the first of which are digits and the last
4356 three of which are not "999". For example, it doesn't match "123abc-
4357 foo". A pattern to do that is
4358
4359 (?<=\d{3}...)(?<!999)foo
4360
4361 This time the first assertion looks at the preceding six characters,
4362 checking that the first three are digits, and then the second assertion
4363 checks that the preceding three characters are not "999".
4364
4365 Assertions can be nested in any combination. For example,
4366
4367 (?<=(?<!foo)bar)baz
4368
4369 matches an occurrence of "baz" that is preceded by "bar" which in turn
4370 is not preceded by "foo", while
4371
4372 (?<=\d{3}(?!999)...)foo
4373
4374 is another pattern that matches "foo" preceded by three digits and any
4375 three characters that are not "999".
4376
4377
4378 CONDITIONAL SUBPATTERNS
4379
4380 It is possible to cause the matching process to obey a subpattern con-
4381 ditionally or to choose between two alternative subpatterns, depending
4382 on the result of an assertion, or whether a previous capturing subpat-
4383 tern matched or not. The two possible forms of conditional subpattern
4384 are
4385
4386 (?(condition)yes-pattern)
4387 (?(condition)yes-pattern|no-pattern)
4388
4389 If the condition is satisfied, the yes-pattern is used; otherwise the
4390 no-pattern (if present) is used. If there are more than two alterna-
4391 tives in the subpattern, a compile-time error occurs.
4392
4393 There are four kinds of condition: references to subpatterns, refer-
4394 ences to recursion, a pseudo-condition called DEFINE, and assertions.
4395
4396 Checking for a used subpattern by number
4397
4398 If the text between the parentheses consists of a sequence of digits,
4399 the condition is true if the capturing subpattern of that number has
4400 previously matched. An alternative notation is to precede the digits
4401 with a plus or minus sign. In this case, the subpattern number is rela-
4402 tive rather than absolute. The most recently opened parentheses can be
4403 referenced by (?(-1), the next most recent by (?(-2), and so on. In
4404 looping constructs it can also make sense to refer to subsequent groups
4405 with constructs such as (?(+2).
4406
4407 Consider the following pattern, which contains non-significant white
4408 space to make it more readable (assume the PCRE_EXTENDED option) and to
4409 divide it into three parts for ease of discussion:
4410
4411 ( \( )? [^()]+ (?(1) \) )
4412
4413 The first part matches an optional opening parenthesis, and if that
4414 character is present, sets it as the first captured substring. The sec-
4415 ond part matches one or more characters that are not parentheses. The
4416 third part is a conditional subpattern that tests whether the first set
4417 of parentheses matched or not. If they did, that is, if subject started
4418 with an opening parenthesis, the condition is true, and so the yes-pat-
4419 tern is executed and a closing parenthesis is required. Otherwise,
4420 since no-pattern is not present, the subpattern matches nothing. In
4421 other words, this pattern matches a sequence of non-parentheses,
4422 optionally enclosed in parentheses.
4423
4424 If you were embedding this pattern in a larger one, you could use a
4425 relative reference:
4426
4427 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
4428
4429 This makes the fragment independent of the parentheses in the larger
4430 pattern.
4431
4432 Checking for a used subpattern by name
4433
4434 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
4435 used subpattern by name. For compatibility with earlier versions of
4436 PCRE, which had this facility before Perl, the syntax (?(name)...) is
4437 also recognized. However, there is a possible ambiguity with this syn-
4438 tax, because subpattern names may consist entirely of digits. PCRE
4439 looks first for a named subpattern; if it cannot find one and the name
4440 consists entirely of digits, PCRE looks for a subpattern of that num-
4441 ber, which must be greater than zero. Using subpattern names that con-
4442 sist entirely of digits is not recommended.
4443
4444 Rewriting the above example to use a named subpattern gives this:
4445
4446 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
4447
4448
4449 Checking for pattern recursion
4450
4451 If the condition is the string (R), and there is no subpattern with the
4452 name R, the condition is true if a recursive call to the whole pattern
4453 or any subpattern has been made. If digits or a name preceded by amper-
4454 sand follow the letter R, for example:
4455
4456 (?(R3)...) or (?(R&name)...)
4457
4458 the condition is true if the most recent recursion is into the subpat-
4459 tern whose number or name is given. This condition does not check the
4460 entire recursion stack.
4461
4462 At "top level", all these recursion test conditions are false. Recur-
4463 sive patterns are described below.
4464
4465 Defining subpatterns for use by reference only
4466
4467 If the condition is the string (DEFINE), and there is no subpattern
4468 with the name DEFINE, the condition is always false. In this case,
4469 there may be only one alternative in the subpattern. It is always
4470 skipped if control reaches this point in the pattern; the idea of
4471 DEFINE is that it can be used to define "subroutines" that can be ref-
4472 erenced from elsewhere. (The use of "subroutines" is described below.)
4473 For example, a pattern to match an IPv4 address could be written like
4474 this (ignore whitespace and line breaks):
4475
4476 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
4477 \b (?&byte) (\.(?&byte)){3} \b
4478
4479 The first part of the pattern is a DEFINE group inside which a another
4480 group named "byte" is defined. This matches an individual component of
4481 an IPv4 address (a number less than 256). When matching takes place,
4482 this part of the pattern is skipped because DEFINE acts like a false
4483 condition.
4484
4485 The rest of the pattern uses references to the named group to match the
4486 four dot-separated components of an IPv4 address, insisting on a word
4487 boundary at each end.
4488
4489 Assertion conditions
4490
4491 If the condition is not in any of the above formats, it must be an
4492 assertion. This may be a positive or negative lookahead or lookbehind
4493 assertion. Consider this pattern, again containing non-significant
4494 white space, and with the two alternatives on the second line:
4495
4496 (?(?=[^a-z]*[a-z])
4497 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
4498
4499 The condition is a positive lookahead assertion that matches an
4500 optional sequence of non-letters followed by a letter. In other words,
4501 it tests for the presence of at least one letter in the subject. If a
4502 letter is found, the subject is matched against the first alternative;
4503 otherwise it is matched against the second. This pattern matches
4504 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
4505 letters and dd are digits.
4506
4507
4508 COMMENTS
4509
4510 The sequence (?# marks the start of a comment that continues up to the
4511 next closing parenthesis. Nested parentheses are not permitted. The
4512 characters that make up a comment play no part in the pattern matching
4513 at all.
4514
4515 If the PCRE_EXTENDED option is set, an unescaped # character outside a
4516 character class introduces a comment that continues to immediately
4517 after the next newline in the pattern.
4518
4519
4520 RECURSIVE PATTERNS
4521
4522 Consider the problem of matching a string in parentheses, allowing for
4523 unlimited nested parentheses. Without the use of recursion, the best
4524 that can be done is to use a pattern that matches up to some fixed
4525 depth of nesting. It is not possible to handle an arbitrary nesting
4526 depth.
4527
4528 For some time, Perl has provided a facility that allows regular expres-
4529 sions to recurse (amongst other things). It does this by interpolating
4530 Perl code in the expression at run time, and the code can refer to the
4531 expression itself. A Perl pattern using code interpolation to solve the
4532 parentheses problem can be created like this:
4533
4534 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
4535
4536 The (?p{...}) item interpolates Perl code at run time, and in this case
4537 refers recursively to the pattern in which it appears.
4538
4539 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
4540 it supports special syntax for recursion of the entire pattern, and
4541 also for individual subpattern recursion. After its introduction in
4542 PCRE and Python, this kind of recursion was introduced into Perl at
4543 release 5.10.
4544
4545 A special item that consists of (? followed by a number greater than
4546 zero and a closing parenthesis is a recursive call of the subpattern of
4547 the given number, provided that it occurs inside that subpattern. (If
4548 not, it is a "subroutine" call, which is described in the next sec-
4549 tion.) The special item (?R) or (?0) is a recursive call of the entire
4550 regular expression.
4551
4552 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
4553 always treated as an atomic group. That is, once it has matched some of
4554 the subject string, it is never re-entered, even if it contains untried
4555 alternatives and there is a subsequent matching failure.
4556
4557 This PCRE pattern solves the nested parentheses problem (assume the
4558 PCRE_EXTENDED option is set so that white space is ignored):
4559
4560 \( ( (?>[^()]+) | (?R) )* \)
4561
4562 First it matches an opening parenthesis. Then it matches any number of
4563 substrings which can either be a sequence of non-parentheses, or a
4564 recursive match of the pattern itself (that is, a correctly parenthe-
4565 sized substring). Finally there is a closing parenthesis.
4566
4567 If this were part of a larger pattern, you would not want to recurse
4568 the entire pattern, so instead you could use this:
4569
4570 ( \( ( (?>[^()]+) | (?1) )* \) )
4571
4572 We have put the pattern into parentheses, and caused the recursion to
4573 refer to them instead of the whole pattern.
4574
4575 In a larger pattern, keeping track of parenthesis numbers can be
4576 tricky. This is made easier by the use of relative references. (A Perl
4577 5.10 feature.) Instead of (?1) in the pattern above you can write
4578 (?-2) to refer to the second most recently opened parentheses preceding
4579 the recursion. In other words, a negative number counts capturing
4580 parentheses leftwards from the point at which it is encountered.
4581
4582 It is also possible to refer to subsequently opened parentheses, by
4583 writing references such as (?+2). However, these cannot be recursive
4584 because the reference is not inside the parentheses that are refer-
4585 enced. They are always "subroutine" calls, as described in the next
4586 section.
4587
4588 An alternative approach is to use named parentheses instead. The Perl
4589 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
4590 supported. We could rewrite the above example as follows:
4591
4592 (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
4593
4594 If there is more than one subpattern with the same name, the earliest
4595 one is used.
4596
4597 This particular example pattern that we have been looking at contains
4598 nested unlimited repeats, and so the use of atomic grouping for match-
4599 ing strings of non-parentheses is important when applying the pattern
4600 to strings that do not match. For example, when this pattern is applied
4601 to
4602
4603 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
4604
4605 it yields "no match" quickly. However, if atomic grouping is not used,
4606 the match runs for a very long time indeed because there are so many
4607 different ways the + and * repeats can carve up the subject, and all
4608 have to be tested before failure can be reported.
4609
4610 At the end of a match, the values set for any capturing subpatterns are
4611 those from the outermost level of the recursion at which the subpattern
4612 value is set. If you want to obtain intermediate values, a callout
4613 function can be used (see below and the pcrecallout documentation). If
4614 the pattern above is matched against
4615
4616 (ab(cd)ef)
4617
4618 the value for the capturing parentheses is "ef", which is the last
4619 value taken on at the top level. If additional parentheses are added,
4620 giving
4621
4622 \( ( ( (?>[^()]+) | (?R) )* ) \)
4623 ^ ^
4624 ^ ^
4625
4626 the string they capture is "ab(cd)ef", the contents of the top level
4627 parentheses. If there are more than 15 capturing parentheses in a pat-
4628 tern, PCRE has to obtain extra memory to store data during a recursion,
4629 which it does by using pcre_malloc, freeing it via pcre_free after-
4630 wards. If no memory can be obtained, the match fails with the
4631 PCRE_ERROR_NOMEMORY error.
4632
4633 Do not confuse the (?R) item with the condition (R), which tests for
4634 recursion. Consider this pattern, which matches text in angle brack-
4635 ets, allowing for arbitrary nesting. Only digits are allowed in nested
4636 brackets (that is, when recursing), whereas any characters are permit-
4637 ted at the outer level.
4638
4639 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
4640
4641 In this pattern, (?(R) is the start of a conditional subpattern, with
4642 two different alternatives for the recursive and non-recursive cases.
4643 The (?R) item is the actual recursive call.
4644
4645
4646 SUBPATTERNS AS SUBROUTINES
4647
4648 If the syntax for a recursive subpattern reference (either by number or
4649 by name) is used outside the parentheses to which it refers, it oper-
4650 ates like a subroutine in a programming language. The "called" subpat-
4651 tern may be defined before or after the reference. A numbered reference
4652 can be absolute or relative, as in these examples:
4653
4654 (...(absolute)...)...(?2)...
4655 (...(relative)...)...(?-1)...
4656 (...(?+1)...(relative)...
4657
4658 An earlier example pointed out that the pattern
4659
4660 (sens|respons)e and \1ibility
4661
4662 matches "sense and sensibility" and "response and responsibility", but
4663 not "sense and responsibility". If instead the pattern
4664
4665 (sens|respons)e and (?1)ibility
4666
4667 is used, it does match "sense and responsibility" as well as the other
4668 two strings. Another example is given in the discussion of DEFINE
4669 above.
4670
4671 Like recursive subpatterns, a "subroutine" call is always treated as an
4672 atomic group. That is, once it has matched some of the subject string,
4673 it is never re-entered, even if it contains untried alternatives and
4674 there is a subsequent matching failure.
4675
4676 When a subpattern is used as a subroutine, processing options such as
4677 case-independence are fixed when the subpattern is defined. They cannot
4678 be changed for different calls. For example, consider this pattern:
4679
4680 (abc)(?i:(?-1))
4681
4682 It matches "abcabc". It does not match "abcABC" because the change of
4683 processing option does not affect the called subpattern.
4684
4685
4686 CALLOUTS
4687
4688 Perl has a feature whereby using the sequence (?{...}) causes arbitrary
4689 Perl code to be obeyed in the middle of matching a regular expression.
4690 This makes it possible, amongst other things, to extract different sub-
4691 strings that match the same pair of parentheses when there is a repeti-
4692 tion.
4693
4694 PCRE provides a similar feature, but of course it cannot obey arbitrary
4695 Perl code. The feature is called "callout". The caller of PCRE provides
4696 an external function by putting its entry point in the global variable
4697 pcre_callout. By default, this variable contains NULL, which disables
4698 all calling out.
4699
4700 Within a regular expression, (?C) indicates the points at which the
4701 external function is to be called. If you want to identify different
4702 callout points, you can put a number less than 256 after the letter C.
4703 The default value is zero. For example, this pattern has two callout
4704 points:
4705
4706 (?C1)abc(?C2)def
4707
4708 If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
4709 automatically installed before each item in the pattern. They are all
4710 numbered 255.
4711
4712 During matching, when PCRE reaches a callout point (and pcre_callout is
4713 set), the external function is called. It is provided with the number
4714 of the callout, the position in the pattern, and, optionally, one item
4715 of data originally supplied by the caller of pcre_exec(). The callout
4716 function may cause matching to proceed, to backtrack, or to fail alto-
4717 gether. A complete description of the interface to the callout function
4718 is given in the pcrecallout documentation.
4719
4720
4721 BACTRACKING CONTROL
4722
4723 Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
4724 which are described in the Perl documentation as "experimental and sub-
4725 ject to change or removal in a future version of Perl". It goes on to
4726 say: "Their usage in production code should be noted to avoid problems
4727 during upgrades." The same remarks apply to the PCRE features described
4728 in this section.
4729
4730 Since these verbs are specifically related to backtracking, they can be
4731 used only when the pattern is to be matched using pcre_exec(), which
4732 uses a backtracking algorithm. They cause an error if encountered by
4733 pcre_dfa_exec().
4734
4735 The new verbs make use of what was previously invalid syntax: an open-
4736 ing parenthesis followed by an asterisk. In Perl, they are generally of
4737 the form (*VERB:ARG) but PCRE does not support the use of arguments, so
4738 its general form is just (*VERB). Any number of these verbs may occur
4739 in a pattern. There are two kinds:
4740
4741 Verbs that act immediately
4742
4743 The following verbs act as soon as they are encountered:
4744
4745 (*ACCEPT)
4746
4747 This verb causes the match to end successfully, skipping the remainder
4748 of the pattern. When inside a recursion, only the innermost pattern is
4749 ended immediately. PCRE differs from Perl in what happens if the
4750 (*ACCEPT) is inside capturing parentheses. In Perl, the data so far is
4751 captured: in PCRE no data is captured. For example:
4752
4753 A(A|B(*ACCEPT)|C)D
4754
4755 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data
4756 is captured.
4757
4758 (*FAIL) or (*F)
4759
4760 This verb causes the match to fail, forcing backtracking to occur. It
4761 is equivalent to (?!) but easier to read. The Perl documentation notes
4762 that it is probably useful only when combined with (?{}) or (??{}).
4763 Those are, of course, Perl features that are not present in PCRE. The
4764 nearest equivalent is the callout feature, as for example in this pat-
4765 tern:
4766
4767 a+(?C)(*FAIL)
4768
4769 A match with the string "aaaa" always fails, but the callout is taken
4770 before each backtrack happens (in this example, 10 times).
4771
4772 Verbs that act after backtracking
4773
4774 The following verbs do nothing when they are encountered. Matching con-
4775 tinues with what follows, but if there is no subsequent match, a fail-
4776 ure is forced. The verbs differ in exactly what kind of failure
4777 occurs.
4778
4779 (*COMMIT)
4780
4781 This verb causes the whole match to fail outright if the rest of the
4782 pattern does not match. Even if the pattern is unanchored, no further
4783 attempts to find a match by advancing the start point take place. Once
4784 (*COMMIT) has been passed, pcre_exec() is committed to finding a match
4785 at the current starting point, or not at all. For example:
4786
4787 a+(*COMMIT)b
4788
4789 This matches "xxaab" but not "aacaab". It can be thought of as a kind
4790 of dynamic anchor, or "I've started, so I must finish."
4791
4792 (*PRUNE)
4793
4794 This verb causes the match to fail at the current position if the rest
4795 of the pattern does not match. If the pattern is unanchored, the normal
4796 "bumpalong" advance to the next starting character then happens. Back-
4797 tracking can occur as usual to the left of (*PRUNE), or when matching
4798 to the right of (*PRUNE), but if there is no match to the right, back-
4799 tracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE)
4800 is just an alternative to an atomic group or possessive quantifier, but
4801 there are some uses of (*PRUNE) that cannot be expressed in any other
4802 way.
4803
4804 (*SKIP)
4805
4806 This verb is like (*PRUNE), except that if the pattern is unanchored,
4807 the "bumpalong" advance is not to the next character, but to the posi-
4808 tion in the subject where (*SKIP) was encountered. (*SKIP) signifies
4809 that whatever text was matched leading up to it cannot be part of a
4810 successful match. Consider:
4811
4812 a+(*SKIP)b
4813
4814 If the subject is "aaaac...", after the first match attempt fails
4815 (starting at the first character in the string), the starting point
4816 skips on to start the next attempt at "c". Note that a possessive quan-
4817 tifer does not have the same effect in this example; although it would
4818 suppress backtracking during the first match attempt, the second
4819 attempt would start at the second character instead of skipping on to
4820 "c".
4821
4822 (*THEN)
4823
4824 This verb causes a skip to the next alternation if the rest of the pat-
4825 tern does not match. That is, it cancels pending backtracking, but only
4826 within the current alternation. Its name comes from the observation
4827 that it can be used for a pattern-based if-then-else block:
4828
4829 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
4830
4831 If the COND1 pattern matches, FOO is tried (and possibly further items
4832 after the end of the group if FOO succeeds); on failure the matcher
4833 skips to the second alternative and tries COND2, without backtracking
4834 into COND1. If (*THEN) is used outside of any alternation, it acts
4835 exactly like (*PRUNE).
4836
4837
4838 SEE ALSO
4839
4840 pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
4841
4842
4843 AUTHOR
4844
4845 Philip Hazel
4846 University Computing Service
4847 Cambridge CB2 3QH, England.
4848
4849
4850 REVISION
4851
4852 Last updated: 11 September 2007
4853 Copyright (c) 1997-2007 University of Cambridge.
4854 ------------------------------------------------------------------------------
4855
4856
4857 PCRESYNTAX(3) PCRESYNTAX(3)
4858
4859
4860 NAME
4861 PCRE - Perl-compatible regular expressions
4862
4863
4864 PCRE REGULAR EXPRESSION SYNTAX SUMMARY
4865
4866 The full syntax and semantics of the regular expressions that are sup-
4867 ported by PCRE are described in the pcrepattern documentation. This
4868 document contains just a quick-reference summary of the syntax.
4869
4870
4871 QUOTING
4872
4873 \x where x is non-alphanumeric is a literal x
4874 \Q...\E treat enclosed characters as literal
4875
4876
4877 CHARACTERS
4878
4879 \a alarm, that is, the BEL character (hex 07)
4880 \cx "control-x", where x is any character
4881 \e escape (hex 1B)
4882 \f formfeed (hex 0C)
4883 \n newline (hex 0A)
4884 \r carriage return (hex 0D)
4885 \t tab (hex 09)
4886 \ddd character with octal code ddd, or backreference
4887 \xhh character with hex code hh
4888 \x{hhh..} character with hex code hhh..
4889
4890
4891 CHARACTER TYPES
4892
4893 . any character except newline;
4894 in dotall mode, any character whatsoever
4895 \C one byte, even in UTF-8 mode (best avoided)
4896 \d a decimal digit
4897 \D a character that is not a decimal digit
4898 \h a horizontal whitespace character
4899 \H a character that is not a horizontal whitespace character
4900 \p{xx} a character with the xx property
4901 \P{xx} a character without the xx property
4902 \R a newline sequence
4903 \s a whitespace character
4904 \S a character that is not a whitespace character
4905 \v a vertical whitespace character
4906 \V a character that is not a vertical whitespace character
4907 \w a "word" character
4908 \W a "non-word" character
4909 \X an extended Unicode sequence
4910
4911 In PCRE, \d, \D, \s, \S, \w, and \W recognize only ASCII characters.
4912
4913
4914 GENERAL CATEGORY PROPERTY CODES FOR \p and \P
4915
4916 C Other
4917 Cc Control
4918 Cf Format
4919 Cn Unassigned
4920 Co Private use
4921 Cs Surrogate
4922
4923 L Letter
4924 Ll Lower case letter
4925 Lm Modifier letter
4926 Lo Other letter
4927 Lt Title case letter
4928 Lu Upper case letter
4929 L& Ll, Lu, or Lt
4930
4931 M Mark
4932 Mc Spacing mark
4933 Me Enclosing mark
4934 Mn Non-spacing mark
4935
4936 N Number
4937 Nd Decimal number
4938 Nl Letter number
4939 No Other number
4940
4941 P Punctuation
4942 Pc Connector punctuation
4943 Pd Dash punctuation
4944 Pe Close punctuation
4945 Pf Final punctuation
4946 Pi Initial punctuation
4947 Po Other punctuation
4948 Ps Open punctuation
4949
4950 S Symbol
4951 Sc Currency symbol
4952 Sk Modifier symbol
4953 Sm Mathematical symbol
4954 So Other symbol
4955
4956 Z Separator
4957 Zl Line separator
4958 Zp Paragraph separator
4959 Zs Space separator
4960
4961
4962 SCRIPT NAMES FOR \p AND \P
4963
4964 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
4965 Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
4966 Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
4967 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
4968 gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
4969 Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
4970 Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
4971 Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
4972 Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
4973
4974
4975 CHARACTER CLASSES
4976
4977 [...] positive character class
4978 [^...] negative character class
4979 [x-y] range (can be used for hex characters)
4980 [[:xxx:]] positive POSIX named set
4981 [[^:xxx:]] negative POSIX named set
4982
4983 alnum alphanumeric
4984 alpha alphabetic
4985 ascii 0-127
4986 blank space or tab
4987 cntrl control character
4988 digit decimal digit
4989 graph printing, excluding space
4990 lower lower case letter
4991 print printing, including space
4992 punct printing, excluding alphanumeric
4993 space whitespace
4994 upper upper case letter
4995 word same as \w
4996 xdigit hexadecimal digit
4997
4998 In PCRE, POSIX character set names recognize only ASCII characters. You
4999 can use \Q...\E inside a character class.
5000
5001
5002 QUANTIFIERS
5003
5004 ? 0 or 1, greedy
5005 ?+ 0 or 1, possessive
5006 ?? 0 or 1, lazy
5007 * 0 or more, greedy
5008 *+ 0 or more, possessive
5009 *? 0 or more, lazy
5010 + 1 or more, greedy
5011 ++ 1 or more, possessive
5012 +? 1 or more, lazy
5013 {n} exactly n
5014 {n,m} at least n, no more than m, greedy
5015 {n,m}+ at least n, no more than m, possessive
5016 {n,m}? at least n, no more than m, lazy
5017 {n,} n or more, greedy
5018 {n,}+ n or more, possessive
5019 {n,}? n or more, lazy
5020
5021
5022 ANCHORS AND SIMPLE ASSERTIONS
5023
5024 \b word boundary
5025 \B not a word boundary
5026 ^ start of subject
5027 also after internal newline in multiline mode
5028 \A start of subject
5029 $ end of subject
5030 also before newline at end of subject
5031 also before internal newline in multiline mode
5032 \Z end of subject
5033 also before newline at end of subject
5034 \z end of subject
5035 \G first matching position in subject
5036
5037
5038 MATCH POINT RESET
5039
5040 \K reset start of match
5041
5042
5043 ALTERNATION
5044
5045 expr|expr|expr...
5046
5047
5048 CAPTURING
5049
5050 (...) capturing group
5051 (?<name>...) named capturing group (Perl)
5052 (?'name'...) named capturing group (Perl)
5053 (?P<name>...) named capturing group (Python)
5054 (?:...) non-capturing group
5055 (?|...) non-capturing group; reset group numbers for
5056 capturing groups in each alternative
5057
5058
5059 ATOMIC GROUPS
5060
5061 (?>...) atomic, non-capturing group
5062
5063
5064 COMMENT
5065
5066 (?#....) comment (not nestable)
5067
5068
5069 OPTION SETTING
5070
5071 (?i) caseless
5072 (?J) allow duplicate names
5073 (?m) multiline
5074 (?s) single line (dotall)
5075 (?U) default ungreedy (lazy)
5076 (?x) extended (ignore white space)
5077 (?-...) unset option(s)
5078
5079
5080 LOOKAHEAD AND LOOKBEHIND ASSERTIONS
5081
5082 (?=...) positive look ahead
5083 (?!...) negative look ahead
5084 (?<=...) positive look behind
5085 (?<!...) negative look behind
5086
5087 Each top-level branch of a look behind must be of a fixed length.
5088
5089
5090 BACKREFERENCES
5091
5092 \n reference by number (can be ambiguous)
5093 \gn reference by number
5094 \g{n} reference by number
5095 \g{-n} relative reference by number
5096 \k<name> reference by name (Perl)
5097 \k'name' reference by name (Perl)
5098 \g{name} reference by name (Perl)
5099 \k{name} reference by name (.NET)
5100 (?P=name) reference by name (Python)
5101
5102
5103 SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)
5104
5105 (?R) recurse whole pattern
5106 (?n) call subpattern by absolute number
5107 (?+n) call subpattern by relative number
5108 (?-n) call subpattern by relative number
5109 (?&name) call subpattern by name (Perl)
5110 (?P>name) call subpattern by name (Python)
5111
5112
5113 CONDITIONAL PATTERNS
5114
5115 (?(condition)yes-pattern)
5116 (?(condition)yes-pattern|no-pattern)
5117
5118 (?(n)... absolute reference condition
5119 (?(+n)... relative reference condition
5120 (?(-n)... relative reference condition
5121 (?(<name>)... named reference condition (Perl)
5122 (?('name')... named reference condition (Perl)
5123 (?(name)... named reference condition (PCRE)
5124 (?(R)... overall recursion condition
5125 (?(Rn)... specific group recursion condition
5126 (?(R&name)... specific recursion condition
5127 (?(DEFINE)... define subpattern for reference
5128 (?(assert)... assertion condition
5129
5130
5131 BACKTRACKING CONTROL
5132
5133 The following act immediately they are reached:
5134
5135 (*ACCEPT) force successful match
5136 (*FAIL) force backtrack; synonym (*F)
5137
5138 The following act only when a subsequent match failure causes a back-
5139 track to reach them. They all force a match failure, but they differ in
5140 what happens afterwards. Those that advance the start-of-match point do
5141 so only if the pattern is not anchored.
5142
5143 (*COMMIT) overall failure, no advance of starting point
5144 (*PRUNE) advance to next starting character
5145 (*SKIP) advance start to current matching position
5146 (*THEN) local failure, backtrack to next alternation
5147
5148
5149 NEWLINE CONVENTIONS
5150
5151 These are recognized only at the very start of a pattern.
5152
5153 (*CR)
5154 (*LF)
5155 (*CRLF)
5156 (*ANYCRLF)
5157 (*ANY)
5158
5159
5160 WHAT \R MATCHES
5161
5162 These are recognized only at the very start of a pattern.
5163
5164 (*BSR_ANYCRLF)
5165 (*BSR_UNICODE)
5166
5167
5168 CALLOUTS
5169
5170 (?C) callout
5171 (?Cn) callout with data n
5172
5173
5174 SEE ALSO
5175
5176 pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
5177
5178
5179 AUTHOR
5180
5181 Philip Hazel
5182 University Computing Service
5183 Cambridge CB2 3QH, England.
5184
5185
5186 REVISION
5187
5188 Last updated: 11 September 2007
5189 Copyright (c) 1997-2007 University of Cambridge.
5190 ------------------------------------------------------------------------------
5191
5192
5193 PCREPARTIAL(3) PCREPARTIAL(3)
5194
5195
5196 NAME
5197 PCRE - Perl-compatible regular expressions
5198
5199
5200 PARTIAL MATCHING IN PCRE
5201
5202 In normal use of PCRE, if the subject string that is passed to
5203 pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too
5204 short to match the entire pattern, PCRE_ERROR_NOMATCH is returned.
5205 There are circumstances where it might be helpful to distinguish this
5206 case from other cases in which there is no match.
5207
5208 Consider, for example, an application where a human is required to type
5209 in data for a field with specific formatting requirements. An example
5210 might be a date in the form ddmmmyy, defined by this pattern:
5211
5212 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
5213
5214 If the application sees the user's keystrokes one by one, and can check
5215 that what has been typed so far is potentially valid, it is able to
5216 raise an error as soon as a mistake is made, possibly beeping and not
5217 reflecting the character that has been typed. This immediate feedback
5218 is likely to be a better user interface than a check that is delayed
5219 until the entire string has been entered.
5220
5221 PCRE supports the concept of partial matching by means of the PCRE_PAR-
5222 TIAL option, which can be set when calling pcre_exec() or
5223 pcre_dfa_exec(). When this flag is set for pcre_exec(), the return code
5224 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
5225 during the matching process the last part of the subject string matched
5226 part of the pattern. Unfortunately, for non-anchored matching, it is
5227 not possible to obtain the position of the start of the partial match.
5228 No captured data is set when PCRE_ERROR_PARTIAL is returned.
5229
5230 When PCRE_PARTIAL is set for pcre_dfa_exec(), the return code
5231 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of
5232 the subject is reached, there have been no complete matches, but there
5233 is still at least one matching possibility. The portion of the string
5234 that provided the partial match is set as the first matching string.
5235
5236 Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers
5237 the last literal byte in a pattern, and abandons matching immediately
5238 if such a byte is not present in the subject string. This optimization
5239 cannot be used for a subject string that might match only partially.
5240
5241
5242 RESTRICTED PATTERNS FOR PCRE_PARTIAL
5243
5244 Because of the way certain internal optimizations are implemented in
5245 the pcre_exec() function, the PCRE_PARTIAL option cannot be used with
5246 all patterns. These restrictions do not apply when pcre_dfa_exec() is
5247 used. For pcre_exec(), repeated single characters such as
5248
5249 a{2,4}
5250
5251 and repeated single metasequences such as
5252
5253 \d+
5254
5255 are not permitted if the maximum number of occurrences is greater than
5256 one. Optional items such as \d? (where the maximum is one) are permit-
5257 ted. Quantifiers with any values are permitted after parentheses, so
5258 the invalid examples above can be coded thus:
5259
5260 (a){2,4}
5261 (\d)+
5262
5263 These constructions run more slowly, but for the kinds of application
5264 that are envisaged for this facility, this is not felt to be a major
5265 restriction.
5266
5267 If PCRE_PARTIAL is set for a pattern that does not conform to the
5268 restrictions, pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL
5269 (-13). You can use the PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to
5270 find out if a compiled pattern can be used for partial matching.
5271
5272
5273 EXAMPLE OF PARTIAL MATCHING USING PCRETEST
5274
5275 If the escape sequence \P is present in a pcretest data line, the
5276 PCRE_PARTIAL flag is used for the match. Here is a run of pcretest that
5277 uses the date example quoted above:
5278
5279 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5280 data> 25jun04\P
5281 0: 25jun04
5282 1: jun
5283 data> 25dec3\P
5284 Partial match
5285 data> 3ju\P
5286 Partial match
5287 data> 3juj\P
5288 No match
5289 data> j\P
5290 No match
5291
5292 The first data string is matched completely, so pcretest shows the
5293 matched substrings. The remaining four strings do not match the com-
5294 plete pattern, but the first two are partial matches. The same test,
5295 using pcre_dfa_exec() matching (by means of the \D escape sequence),
5296 produces the following output:
5297
5298 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5299 data> 25jun04\P\D
5300 0: 25jun04
5301 data> 23dec3\P\D
5302 Partial match: 23dec3
5303 data> 3ju\P\D
5304 Partial match: 3ju
5305 data> 3juj\P\D
5306 No match
5307 data> j\P\D
5308 No match
5309
5310 Notice that in this case the portion of the string that was matched is
5311 made available.
5312
5313
5314 MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()
5315
5316 When a partial match has been found using pcre_dfa_exec(), it is possi-
5317 ble to continue the match by providing additional subject data and
5318 calling pcre_dfa_exec() again with the same compiled regular expres-
5319 sion, this time setting the PCRE_DFA_RESTART option. You must also pass
5320 the same working space as before, because this is where details of the
5321 previous partial match are stored. Here is an example using pcretest,
5322 using the \R escape sequence to set the PCRE_DFA_RESTART option (\P and
5323 \D are as above):
5324
5325 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5326 data> 23ja\P\D
5327 Partial match: 23ja
5328 data> n05\R\D
5329 0: n05
5330
5331 The first call has "23ja" as the subject, and requests partial match-
5332 ing; the second call has "n05" as the subject for the continued
5333 (restarted) match. Notice that when the match is complete, only the
5334 last part is shown; PCRE does not retain the previously partially-
5335 matched string. It is up to the calling program to do that if it needs
5336 to.
5337
5338 You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial
5339 matching over multiple segments. This facility can be used to pass very
5340 long subject strings to pcre_dfa_exec(). However, some care is needed
5341 for certain types of pattern.
5342
5343 1. If the pattern contains tests for the beginning or end of a line,
5344 you need to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropri-
5345 ate, when the subject string for any call does not contain the begin-
5346 ning or end of a line.
5347
5348 2. If the pattern contains backward assertions (including \b or \B),
5349 you need to arrange for some overlap in the subject strings to allow
5350 for this. For example, you could pass the subject in chunks that are
5351 500 bytes long, but in a buffer of 700 bytes, with the starting offset
5352 set to 200 and the previous 200 bytes at the start of the buffer.
5353
5354 3. Matching a subject string that is split into multiple segments does
5355 not always produce exactly the same result as matching over one single
5356 long string. The difference arises when there are multiple matching
5357 possibilities, because a partial match result is given only when there
5358 are no completed matches in a call to pcre_dfa_exec(). This means that
5359 as soon as the shortest match has been found, continuation to a new
5360 subject segment is no longer possible. Consider this pcretest example:
5361
5362 re> /dog(sbody)?/
5363 data> do\P\D
5364 Partial match: do
5365 data> gsb\R\P\D
5366 0: g
5367 data> dogsbody\D
5368 0: dogsbody
5369 1: dog
5370
5371 The pattern matches the words "dog" or "dogsbody". When the subject is
5372 presented in several parts ("do" and "gsb" being the first two) the
5373 match stops when "dog" has been found, and it is not possible to con-
5374 tinue. On the other hand, if "dogsbody" is presented as a single
5375 string, both matches are found.
5376
5377 Because of this phenomenon, it does not usually make sense to end a
5378 pattern that is going to be matched in this way with a variable repeat.
5379
5380 4. Patterns that contain alternatives at the top level which do not all
5381 start with the same pattern item may not work as expected. For example,
5382 consider this pattern:
5383
5384 1234|3789
5385
5386 If the first part of the subject is "ABC123", a partial match of the
5387 first alternative is found at offset 3. There is no partial match for
5388 the second alternative, because such a match does not start at the same
5389 point in the subject string. Attempting to continue with the string
5390 "789" does not yield a match because only those alternatives that match
5391 at one point in the subject are remembered. The problem arises because
5392 the start of the second alternative matches within the first alterna-
5393 tive. There is no problem with anchored patterns or patterns such as:
5394
5395 1234|ABCD
5396
5397 where no string can be a partial match for both alternatives.
5398
5399
5400 AUTHOR
5401
5402 Philip Hazel
5403 University Computing Service
5404 Cambridge CB2 3QH, England.
5405
5406
5407 REVISION
5408
5409 Last updated: 04 June 2007
5410 Copyright (c) 1997-2007 University of Cambridge.
5411 ------------------------------------------------------------------------------
5412
5413
5414 PCREPRECOMPILE(3) PCREPRECOMPILE(3)
5415
5416
5417 NAME
5418 PCRE - Perl-compatible regular expressions
5419
5420
5421 SAVING AND RE-USING PRECOMPILED PCRE PATTERNS
5422
5423 If you are running an application that uses a large number of regular
5424 expression patterns, it may be useful to store them in a precompiled
5425 form instead of having to compile them every time the application is
5426 run. If you are not using any private character tables (see the
5427 pcre_maketables() documentation), this is relatively straightforward.
5428 If you are using private tables, it is a little bit more complicated.
5429
5430 If you save compiled patterns to a file, you can copy them to a differ-
5431 ent host and run them there. This works even if the new host has the
5432 opposite endianness to the one on which the patterns were compiled.
5433 There may be a small performance penalty, but it should be insignifi-
5434 cant. However, compiling regular expressions with one version of PCRE
5435 for use with a different version is not guaranteed to work and may
5436 cause crashes.
5437
5438
5439 SAVING A COMPILED PATTERN
5440 The value returned by pcre_compile() points to a single block of memory
5441 that holds the compiled pattern and associated data. You can find the
5442 length of this block in bytes by calling pcre_fullinfo() with an argu-
5443 ment of PCRE_INFO_SIZE. You can then save the data in any appropriate
5444 manner. Here is sample code that compiles a pattern and writes it to a
5445 file. It assumes that the variable fd refers to a file that is open for
5446 output:
5447
5448 int erroroffset, rc, size;
5449 char *error;
5450 pcre *re;
5451
5452 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
5453 if (re == NULL) { ... handle errors ... }
5454 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
5455 if (rc < 0) { ... handle errors ... }
5456 rc = fwrite(re, 1, size, fd);
5457 if (rc != size) { ... handle errors ... }
5458
5459 In this example, the bytes that comprise the compiled pattern are
5460 copied exactly. Note that this is binary data that may contain any of
5461 the 256 possible byte values. On systems that make a distinction
5462 between binary and non-binary data, be sure that the file is opened for
5463 binary output.
5464
5465 If you want to write more than one pattern to a file, you will have to
5466 devise a way of separating them. For binary data, preceding each pat-
5467 tern with its length is probably the most straightforward approach.
5468 Another possibility is to write out the data in hexadecimal instead of
5469 binary, one pattern to a line.
5470
5471 Saving compiled patterns in a file is only one possible way of storing
5472 them for later use. They could equally well be saved in a database, or
5473 in the memory of some daemon process that passes them via sockets to
5474 the processes that want them.
5475
5476 If the pattern has been studied, it is also possible to save the study
5477 data in a similar way to the compiled pattern itself. When studying
5478 generates additional information, pcre_study() returns a pointer to a
5479 pcre_extra data block. Its format is defined in the section on matching
5480 a pattern in the pcreapi documentation. The study_data field points to
5481 the binary study data, and this is what you must save (not the
5482 pcre_extra block itself). The length of the study data can be obtained
5483 by calling pcre_fullinfo() with an argument of PCRE_INFO_STUDYSIZE.
5484 Remember to check that pcre_study() did return a non-NULL value before
5485 trying to save the study data.
5486
5487
5488 RE-USING A PRECOMPILED PATTERN
5489
5490 Re-using a precompiled pattern is straightforward. Having reloaded it
5491 into main memory, you pass its pointer to pcre_exec() or
5492 pcre_dfa_exec() in the usual way. This should work even on another
5493 host, and even if that host has the opposite endianness to the one
5494 where the pattern was compiled.
5495
5496 However, if you passed a pointer to custom character tables when the
5497 pattern was compiled (the tableptr argument of pcre_compile()), you
5498 must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(),
5499 because the value saved with the compiled pattern will obviously be
5500 nonsense. A field in a pcre_extra() block is used to pass this data, as
5501 described in the section on matching a pattern in the pcreapi documen-
5502 tation.
5503
5504 If you did not provide custom character tables when the pattern was
5505 compiled, the pointer in the compiled pattern is NULL, which causes
5506 pcre_exec() to use PCRE's internal tables. Thus, you do not need to
5507 take any special action at run time in this case.
5508
5509 If you saved study data with the compiled pattern, you need to create
5510 your own pcre_extra data block and set the study_data field to point to
5511 the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA
5512 bit in the flags field to indicate that study data is present. Then
5513 pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the
5514 usual way.
5515
5516
5517 COMPATIBILITY WITH DIFFERENT PCRE RELEASES
5518
5519 In general, it is safest to recompile all saved patterns when you
5520 update to a new PCRE release, though not all updates actually require
5521 this. Recompiling is definitely needed for release 7.2.
5522
5523
5524 AUTHOR
5525
5526 Philip Hazel
5527 University Computing Service
5528 Cambridge CB2 3QH, England.
5529
5530
5531 REVISION
5532
5533 Last updated: 13 June 2007
5534 Copyright (c) 1997-2007 University of Cambridge.
5535 ------------------------------------------------------------------------------
5536
5537
5538 PCREPERFORM(3) PCREPERFORM(3)
5539
5540
5541 NAME
5542 PCRE - Perl-compatible regular expressions
5543
5544
5545 PCRE PERFORMANCE
5546
5547 Two aspects of performance are discussed below: memory usage and pro-
5548 cessing time. The way you express your pattern as a regular expression
5549 can affect both of them.
5550
5551
5552 MEMORY USAGE
5553
5554 Patterns are compiled by PCRE into a reasonably efficient byte code, so
5555 that most simple patterns do not use much memory. However, there is one
5556 case where memory usage can be unexpectedly large. When a parenthesized
5557 subpattern has a quantifier with a minimum greater than 1 and/or a lim-
5558 ited maximum, the whole subpattern is repeated in the compiled code.
5559 For example, the pattern
5560
5561 (abc|def){2,4}
5562
5563 is compiled as if it were
5564
5565 (abc|def)(abc|def)((abc|def)(abc|def)?)?
5566
5567 (Technical aside: It is done this way so that backtrack points within
5568 each of the repetitions can be independently maintained.)
5569
5570 For regular expressions whose quantifiers use only small numbers, this
5571 is not usually a problem. However, if the numbers are large, and par-
5572 ticularly if such repetitions are nested, the memory usage can become
5573 an embarrassment. For example, the very simple pattern
5574
5575 ((ab){1,1000}c){1,3}
5576
5577 uses 51K bytes when compiled. When PCRE is compiled with its default
5578 internal pointer size of two bytes, the size limit on a compiled pat-
5579 tern is 64K, and this is reached with the above pattern if the outer
5580 repetition is increased from 3 to 4. PCRE can be compiled to use larger
5581 internal pointers and thus handle larger compiled patterns, but it is
5582 better to try to rewrite your pattern to use less memory if you can.
5583
5584 One way of reducing the memory usage for such patterns is to make use
5585 of PCRE's "subroutine" facility. Re-writing the above pattern as
5586
5587 ((ab)(?2){0,999}c)(?1){0,2}
5588
5589 reduces the memory requirements to 18K, and indeed it remains under 20K
5590 even with the outer repetition increased to 100. However, this pattern
5591 is not exactly equivalent, because the "subroutine" calls are treated
5592 as atomic groups into which there can be no backtracking if there is a
5593 subsequent matching failure. Therefore, PCRE cannot do this kind of
5594 rewriting automatically. Furthermore, there is a noticeable loss of
5595 speed when executing the modified pattern. Nevertheless, if the atomic
5596 grouping is not a problem and the loss of speed is acceptable, this
5597 kind of rewriting will allow you to process patterns that PCRE cannot
5598 otherwise handle.
5599
5600
5601 PROCESSING TIME
5602
5603 Certain items in regular expression patterns are processed more effi-
5604 ciently than others. It is more efficient to use a character class like
5605 [aeiou] than a set of single-character alternatives such as
5606 (a|e|i|o|u). In general, the simplest construction that provides the
5607 required behaviour is usually the most efficient. Jeffrey Friedl's book
5608 contains a lot of useful general discussion about optimizing regular
5609 expressions for efficient performance. This document contains a few
5610 observations about PCRE.
5611
5612 Using Unicode character properties (the \p, \P, and \X escapes) is
5613 slow, because PCRE has to scan a structure that contains data for over
5614 fifteen thousand characters whenever it needs a character's property.
5615 If you can find an alternative pattern that does not use character
5616 properties, it will probably be faster.
5617
5618 When a pattern begins with .* not in parentheses, or in parentheses
5619 that are not the subject of a backreference, and the PCRE_DOTALL option
5620 is set, the pattern is implicitly anchored by PCRE, since it can match
5621 only at the start of a subject string. However, if PCRE_DOTALL is not
5622 set, PCRE cannot make this optimization, because the . metacharacter
5623 does not then match a newline, and if the subject string contains new-
5624 lines, the pattern may match from the character immediately following
5625 one of them instead of from the very start. For example, the pattern
5626
5627 .*second
5628
5629 matches the subject "first\nand second" (where \n stands for a newline
5630 character), with the match starting at the seventh character. In order
5631 to do this, PCRE has to retry the match starting after every newline in
5632 the subject.
5633
5634 If you are using such a pattern with subject strings that do not con-
5635 tain newlines, the best performance is obtained by setting PCRE_DOTALL,
5636 or starting the pattern with ^.* or ^.*? to indicate explicit anchor-
5637 ing. That saves PCRE from having to scan along the subject looking for
5638 a newline to restart at.
5639
5640 Beware of patterns that contain nested indefinite repeats. These can
5641 take a long time to run when applied to a string that does not match.
5642 Consider the pattern fragment
5643
5644 ^(a+)*
5645
5646 This can match "aaaa" in 16 different ways, and this number increases
5647 very rapidly as the string gets longer. (The * repeat can match 0, 1,
5648 2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
5649 repeats can match different numbers of times.) When the remainder of
5650 the pattern is such that the entire match is going to fail, PCRE has in
5651 principle to try every possible variation, and this can take an
5652 extremely long time, even for relatively short strings.
5653
5654 An optimization catches some of the more simple cases such as
5655
5656 (a+)*b
5657
5658 where a literal character follows. Before embarking on the standard
5659 matching procedure, PCRE checks that there is a "b" later in the sub-
5660 ject string, and if there is not, it fails the match immediately. How-
5661 ever, when there is no following literal this optimization cannot be
5662 used. You can see the difference by comparing the behaviour of
5663
5664 (a+)*\d
5665
5666 with the pattern above. The former gives a failure almost instantly
5667 when applied to a whole line of "a" characters, whereas the latter
5668 takes an appreciable time with strings longer than about 20 characters.
5669
5670 In many cases, the solution to this kind of performance issue is to use
5671 an atomic group or a possessive quantifier.
5672
5673
5674 AUTHOR
5675
5676 Philip Hazel
5677 University Computing Service
5678 Cambridge CB2 3QH, England.
5679
5680
5681 REVISION
5682
5683 Last updated: 06 March 2007
5684 Copyright (c) 1997-2007 University of Cambridge.
5685 ------------------------------------------------------------------------------
5686
5687
5688 PCREPOSIX(3) PCREPOSIX(3)
5689
5690
5691 NAME
5692 PCRE - Perl-compatible regular expressions.
5693
5694
5695 SYNOPSIS OF POSIX API
5696
5697 #include <pcreposix.h>
5698
5699 int regcomp(regex_t *preg, const char *pattern,
5700 int cflags);
5701
5702 int regexec(regex_t *preg, const char *string,
5703 size_t nmatch, regmatch_t pmatch[], int eflags);
5704
5705 size_t regerror(int errcode, const regex_t *preg,
5706 char *errbuf, size_t errbuf_size);
5707
5708 void regfree(regex_t *preg);
5709
5710
5711 DESCRIPTION
5712
5713 This set of functions provides a POSIX-style API to the PCRE regular
5714 expression package. See the pcreapi documentation for a description of
5715 PCRE's native API, which contains much additional functionality.
5716
5717 The functions described here are just wrapper functions that ultimately
5718 call the PCRE native API. Their prototypes are defined in the
5719 pcreposix.h header file, and on Unix systems the library itself is
5720 called pcreposix.a, so can be accessed by adding -lpcreposix to the
5721 command for linking an application that uses them. Because the POSIX
5722 functions call the native ones, it is also necessary to add -lpcre.
5723
5724 I have implemented only those option bits that can be reasonably mapped
5725 to PCRE native options. In addition, the option REG_EXTENDED is defined
5726 with the value zero. This has no effect, but since programs that are
5727 written to the POSIX interface often use it, this makes it easier to
5728 slot in PCRE as a replacement library. Other POSIX options are not even
5729 defined.
5730
5731 When PCRE is called via these functions, it is only the API that is
5732 POSIX-like in style. The syntax and semantics of the regular expres-
5733 sions themselves are still those of Perl, subject to the setting of
5734 various PCRE options, as described below. "POSIX-like in style" means
5735 that the API approximates to the POSIX definition; it is not fully
5736 POSIX-compatible, and in multi-byte encoding domains it is probably
5737 even less compatible.
5738
5739 The header for these functions is supplied as pcreposix.h to avoid any
5740 potential clash with other POSIX libraries. It can, of course, be
5741 renamed or aliased as regex.h, which is the "correct" name. It provides
5742 two structure types, regex_t for compiled internal forms, and reg-
5743 match_t for returning captured substrings. It also defines some con-
5744 stants whose names start with "REG_"; these are used for setting
5745 options and identifying error codes.
5746
5747
5748 COMPILING A PATTERN
5749
5750 The function regcomp() is called to compile a pattern into an internal
5751 form. The pattern is a C string terminated by a binary zero, and is
5752 passed in the argument pattern. The preg argument is a pointer to a
5753 regex_t structure that is used as a base for storing information about
5754 the compiled regular expression.
5755
5756 The argument cflags is either zero, or contains one or more of the bits
5757 defined by the following macros:
5758
5759 REG_DOTALL
5760
5761 The PCRE_DOTALL option is set when the regular expression is passed for
5762 compilation to the native function. Note that REG_DOTALL is not part of
5763 the POSIX standard.
5764
5765 REG_ICASE
5766
5767 The PCRE_CASELESS option is set when the regular expression is passed
5768 for compilation to the native function.
5769
5770 REG_NEWLINE
5771
5772 The PCRE_MULTILINE option is set when the regular expression is passed
5773 for compilation to the native function. Note that this does not mimic
5774 the defined POSIX behaviour for REG_NEWLINE (see the following sec-
5775 tion).
5776
5777 REG_NOSUB
5778
5779 The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is
5780 passed for compilation to the native function. In addition, when a pat-
5781 tern that is compiled with this flag is passed to regexec() for match-
5782 ing, the nmatch and pmatch arguments are ignored, and no captured
5783 strings are returned.
5784
5785 REG_UTF8
5786
5787 The PCRE_UTF8 option is set when the regular expression is passed for
5788 compilation to the native function. This causes the pattern itself and
5789 all data strings used for matching it to be treated as UTF-8 strings.
5790 Note that REG_UTF8 is not part of the POSIX standard.
5791
5792 In the absence of these flags, no options are passed to the native
5793 function. This means the the regex is compiled with PCRE default
5794 semantics. In particular, the way it handles newline characters in the
5795 subject string is the Perl way, not the POSIX way. Note that setting
5796 PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE.
5797 It does not affect the way newlines are matched by . (they aren't) or
5798 by a negative class such as [^a] (they are).
5799
5800 The yield of regcomp() is zero on success, and non-zero otherwise. The
5801 preg structure is filled in on success, and one member of the structure
5802 is public: re_nsub contains the number of capturing subpatterns in the
5803 regular expression. Various error codes are defined in the header file.
5804
5805
5806 MATCHING NEWLINE CHARACTERS
5807
5808 This area is not simple, because POSIX and Perl take different views of
5809 things. It is not possible to get PCRE to obey POSIX semantics, but
5810 then PCRE was never intended to be a POSIX engine. The following table
5811 lists the different possibilities for matching newline characters in
5812 PCRE:
5813
5814 Default Change with
5815
5816 . matches newline no PCRE_DOTALL
5817 newline matches [^a] yes not changeable
5818 $ matches \n at end yes PCRE_DOLLARENDONLY
5819 $ matches \n in middle no PCRE_MULTILINE
5820 ^ matches \n in middle no PCRE_MULTILINE
5821
5822 This is the equivalent table for POSIX:
5823
5824 Default Change with
5825
5826 . matches newline yes REG_NEWLINE
5827 newline matches [^a] yes REG_NEWLINE
5828 $ matches \n at end no REG_NEWLINE
5829 $ matches \n in middle no REG_NEWLINE
5830 ^ matches \n in middle no REG_NEWLINE
5831
5832 PCRE's behaviour is the same as Perl's, except that there is no equiva-
5833 lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
5834 no way to stop newline from matching [^a].
5835
5836 The default POSIX newline handling can be obtained by setting
5837 PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
5838 behave exactly as for the REG_NEWLINE action.
5839
5840
5841 MATCHING A PATTERN
5842
5843 The function regexec() is called to match a compiled pattern preg
5844 against a given string, which is terminated by a zero byte, subject to
5845 the options in eflags. These can be:
5846
5847 REG_NOTBOL
5848
5849 The PCRE_NOTBOL option is set when calling the underlying PCRE matching
5850 function.
5851
5852 REG_NOTEOL
5853
5854 The PCRE_NOTEOL option is set when calling the underlying PCRE matching
5855 function.
5856
5857 If the pattern was compiled with the REG_NOSUB flag, no data about any
5858 matched strings is returned. The nmatch and pmatch arguments of
5859 regexec() are ignored.
5860
5861 Otherwise,the portion of the string that was matched, and also any cap-
5862 tured substrings, are returned via the pmatch argument, which points to
5863 an array of nmatch structures of type regmatch_t, containing the mem-
5864 bers rm_so and rm_eo. These contain the offset to the first character
5865 of each substring and the offset to the first character after the end
5866 of each substring, respectively. The 0th element of the vector relates
5867 to the entire portion of string that was matched; subsequent elements
5868 relate to the capturing subpatterns of the regular expression. Unused
5869 entries in the array have both structure members set to -1.
5870
5871 A successful match yields a zero return; various error codes are
5872 defined in the header file, of which REG_NOMATCH is the "expected"
5873 failure code.
5874
5875
5876 ERROR MESSAGES
5877
5878 The regerror() function maps a non-zero errorcode from either regcomp()
5879 or regexec() to a printable message. If preg is not NULL, the error
5880 should have arisen from the use of that structure. A message terminated
5881 by a binary zero is placed in errbuf. The length of the message,
5882 including the zero, is limited to errbuf_size. The yield of the func-
5883 tion is the size of buffer needed to hold the whole message.
5884
5885
5886 MEMORY USAGE
5887
5888 Compiling a regular expression causes memory to be allocated and asso-
5889 ciated with the preg structure. The function regfree() frees all such
5890 memory, after which preg may no longer be used as a compiled expres-
5891 sion.
5892
5893
5894 AUTHOR
5895
5896 Philip Hazel
5897 University Computing Service
5898 Cambridge CB2 3QH, England.
5899
5900
5901 REVISION
5902
5903 Last updated: 06 March 2007
5904 Copyright (c) 1997-2007 University of Cambridge.
5905 ------------------------------------------------------------------------------
5906
5907
5908 PCRECPP(3) PCRECPP(3)
5909
5910
5911 NAME
5912 PCRE - Perl-compatible regular expressions.
5913
5914
5915 SYNOPSIS OF C++ WRAPPER
5916
5917 #include <pcrecpp.h>
5918
5919
5920 DESCRIPTION
5921
5922 The C++ wrapper for PCRE was provided by Google Inc. Some additional
5923 functionality was added by Giuseppe Maxia. This brief man page was con-
5924 structed from the notes in the pcrecpp.h file, which should be con-
5925 sulted for further details.
5926
5927
5928 MATCHING INTERFACE
5929
5930 The "FullMatch" operation checks that supplied text matches a supplied
5931 pattern exactly. If pointer arguments are supplied, it copies matched
5932 sub-strings that match sub-patterns into them.
5933
5934 Example: successful match
5935 pcrecpp::RE re("h.*o");
5936 re.FullMatch("hello");
5937
5938 Example: unsuccessful match (requires full match):
5939 pcrecpp::RE re("e");
5940 !re.FullMatch("hello");
5941
5942 Example: creating a temporary RE object:
5943 pcrecpp::RE("h.*o").FullMatch("hello");
5944
5945 You can pass in a "const char*" or a "string" for "text". The examples
5946 below tend to use a const char*. You can, as in the different examples
5947 above, store the RE object explicitly in a variable or use a temporary
5948 RE object. The examples below use one mode or the other arbitrarily.
5949 Either could correctly be used for any of these examples.
5950
5951 You must supply extra pointer arguments to extract matched subpieces.
5952
5953 Example: extracts "ruby" into "s" and 1234 into "i"
5954 int i;
5955 string s;
5956 pcrecpp::RE re("(\\w+):(\\d+)");
5957 re.FullMatch("ruby:1234", &s, &i);
5958
5959 Example: does not try to extract any extra sub-patterns
5960 re.FullMatch("ruby:1234", &s);
5961
5962 Example: does not try to extract into NULL
5963 re.FullMatch("ruby:1234", NULL, &i);
5964
5965 Example: integer overflow causes failure
5966 !re.FullMatch("ruby:1234567891234", NULL, &i);
5967
5968 Example: fails because there aren't enough sub-patterns:
5969 !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);
5970
5971 Example: fails because string cannot be stored in integer
5972 !pcrecpp::RE("(.*)").FullMatch("ruby", &i);
5973
5974 The provided pointer arguments can be pointers to any scalar numeric
5975 type, or one of:
5976
5977 string (matched piece is copied to string)
5978 StringPiece (StringPiece is mutated to point to matched piece)
5979 T (where "bool T::ParseFrom(const char*, int)" exists)
5980 NULL (the corresponding matched sub-pattern is not copied)
5981
5982 The function returns true iff all of the following conditions are sat-
5983 isfied:
5984
5985 a. "text" matches "pattern" exactly;
5986
5987 b. The number of matched sub-patterns is >= number of supplied
5988 pointers;
5989
5990 c. The "i"th argument has a suitable type for holding the
5991 string captured as the "i"th sub-pattern. If you pass in
5992 NULL for the "i"th argument, or pass fewer arguments than
5993 number of sub-patterns, "i"th captured sub-pattern is
5994 ignored.
5995
5996 CAVEAT: An optional sub-pattern that does not exist in the matched
5997 string is assigned the empty string. Therefore, the following will
5998 return false (because the empty string is not a valid number):
5999
6000 int number;
6001 pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number);
6002
6003 The matching interface supports at most 16 arguments per call. If you
6004 need more, consider using the more general interface
6005 pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.
6006
6007
6008 QUOTING METACHARACTERS
6009
6010 You can use the "QuoteMeta" operation to insert backslashes before all
6011 potentially meaningful characters in a string. The returned string,
6012 used as a regular expression, will exactly match the original string.
6013
6014 Example:
6015 string quoted = RE::QuoteMeta(unquoted);
6016
6017 Note that it's legal to escape a character even if it has no special
6018 meaning in a regular expression -- so this function does that. (This
6019 also makes it identical to the perl function of the same name; see
6020 "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes
6021 "1\.5\-2\.0\?".
6022
6023
6024 PARTIAL MATCHES
6025
6026 You can use the "PartialMatch" operation when you want the pattern to
6027 match any substring of the text.
6028
6029 Example: simple search for a string:
6030 pcrecpp::RE("ell").PartialMatch("hello");
6031
6032 Example: find first number in a string:
6033 int number;
6034 pcrecpp::RE re("(\\d+)");
6035 re.PartialMatch("x*100 + 20", &number);
6036 assert(number == 100);
6037
6038
6039 UTF-8 AND THE MATCHING INTERFACE
6040
6041 By default, pattern and text are plain text, one byte per character.
6042 The UTF8 flag, passed to the constructor, causes both pattern and
6043 string to be treated as UTF-8 text, still a byte stream but potentially
6044 multiple bytes per character. In practice, the text is likelier to be
6045 UTF-8 than the pattern, but the match returned may depend on the UTF8
6046 flag, so always use it when matching UTF8 text. For example, "." will
6047 match one byte normally but with UTF8 set may match up to three bytes
6048 of a multi-byte character.
6049
6050 Example:
6051 pcrecpp::RE_Options options;
6052 options.set_utf8();
6053 pcrecpp::RE re(utf8_pattern, options);
6054 re.FullMatch(utf8_string);
6055
6056 Example: using the convenience function UTF8():
6057 pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
6058 re.FullMatch(utf8_string);
6059
6060 NOTE: The UTF8 flag is ignored if pcre was not configured with the
6061 --enable-utf8 flag.
6062
6063
6064 PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE
6065
6066 PCRE defines some modifiers to change the behavior of the regular
6067 expression engine. The C++ wrapper defines an auxiliary class,
6068 RE_Options, as a vehicle to pass such modifiers to a RE class. Cur-
6069 rently, the following modifiers are supported:
6070
6071 modifier description Perl corresponding
6072
6073 PCRE_CASELESS case insensitive match /i
6074 PCRE_MULTILINE multiple lines match /m
6075 PCRE_DOTALL dot matches newlines /s
6076 PCRE_DOLLAR_ENDONLY $ matches only at end N/A
6077 PCRE_EXTRA strict escape parsing N/A
6078 PCRE_EXTENDED ignore whitespaces /x
6079 PCRE_UTF8 handles UTF8 chars built-in
6080 PCRE_UNGREEDY reverses * and *? N/A
6081 PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*)
6082
6083 (*) Both Perl and PCRE allow non capturing parentheses by means of the
6084 "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap-
6085 ture, while (ab|cd) does.
6086
6087 For a full account on how each modifier works, please check the PCRE
6088 API reference page.
6089
6090 For each modifier, there are two member functions whose name is made
6091 out of the modifier in lowercase, without the "PCRE_" prefix. For
6092 instance, PCRE_CASELESS is handled by
6093
6094 bool caseless()
6095
6096 which returns true if the modifier is set, and
6097
6098 RE_Options & set_caseless(bool)
6099
6100 which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
6101 be accessed through the set_match_limit() and match_limit() member
6102 functions. Setting match_limit to a non-zero value will limit the exe-
6103 cution of pcre to keep it from doing bad things like blowing the stack
6104 or taking an eternity to return a result. A value of 5000 is good
6105 enough to stop stack blowup in a 2MB thread stack. Setting match_limit
6106 to zero disables match limiting. Alternatively, you can call
6107 match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to
6108 limit how much PCRE recurses. match_limit() limits the number of
6109 matches PCRE does; match_limit_recursion() limits the depth of internal
6110 recursion, and therefo