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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. Neither has
6 the pcredemo program. There are separate text files for the pcregrep and
7 pcretest commands.
8 -----------------------------------------------------------------------------
11 PCRE(3) PCRE(3)
15 PCRE - Perl-compatible regular expressions
20 The PCRE library is a set of functions that implement regular expres-
21 sion pattern matching using the same syntax and semantics as Perl, with
22 just a few differences. Some features that appeared in Python and PCRE
23 before they appeared in Perl are also available using the Python syn-
24 tax, there is some support for one or two .NET and Oniguruma syntax
25 items, and there is an option for requesting some minor changes that
26 give better JavaScript compatibility.
28 Starting with release 8.30, it is possible to compile two separate PCRE
29 libraries: the original, which supports 8-bit character strings
30 (including UTF-8 strings), and a second library that supports 16-bit
31 character strings (including UTF-16 strings). The build process allows
32 either one or both to be built. The majority of the work to make this
33 possible was done by Zoltan Herczeg.
35 The two libraries contain identical sets of functions, except that the
36 names in the 16-bit library start with pcre16_ instead of pcre_. To
37 avoid over-complication and reduce the documentation maintenance load,
38 most of the documentation describes the 8-bit library, with the differ-
39 ences for the 16-bit library described separately in the pcre16 page.
40 References to functions or structures of the form pcre[16]_xxx should
41 be read as meaning "pcre_xxx when using the 8-bit library and
42 pcre16_xxx when using the 16-bit library".
44 The current implementation of PCRE corresponds approximately with Perl
45 5.12, including support for UTF-8/16 encoded strings and Unicode gen-
46 eral category properties. However, UTF-8/16 and Unicode support has to
47 be explicitly enabled; it is not the default. The Unicode tables corre-
48 spond to Unicode release 6.0.0.
50 In addition to the Perl-compatible matching function, PCRE contains an
51 alternative function that matches the same compiled patterns in a dif-
52 ferent way. In certain circumstances, the alternative function has some
53 advantages. For a discussion of the two matching algorithms, see the
54 pcrematching page.
56 PCRE is written in C and released as a C library. A number of people
57 have written wrappers and interfaces of various kinds. In particular,
58 Google Inc. have provided a comprehensive C++ wrapper for the 8-bit
59 library. This is now included as part of the PCRE distribution. The
60 pcrecpp page has details of this interface. Other people's contribu-
61 tions can be found in the Contrib directory at the primary FTP site,
62 which is:
64 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
66 Details of exactly which Perl regular expression features are and are
67 not supported by PCRE are given in separate documents. See the pcrepat-
68 tern and pcrecompat pages. There is a syntax summary in the pcresyntax
69 page.
71 Some features of PCRE can be included, excluded, or changed when the
72 library is built. The pcre_config() function makes it possible for a
73 client to discover which features are available. The features them-
74 selves are described in the pcrebuild page. Documentation about build-
75 ing PCRE for various operating systems can be found in the README and
76 NON-UNIX-USE files in the source distribution.
78 The libraries contains a number of undocumented internal functions and
79 data tables that are used by more than one of the exported external
80 functions, but which are not intended for use by external callers.
81 Their names all begin with "_pcre_" or "_pcre16_", which hopefully will
82 not provoke any name clashes. In some environments, it is possible to
83 control which external symbols are exported when a shared library is
84 built, and in these cases the undocumented symbols are not exported.
89 The user documentation for PCRE comprises a number of different sec-
90 tions. In the "man" format, each of these is a separate "man page". In
91 the HTML format, each is a separate page, linked from the index page.
92 In the plain text format, all the sections, except the pcredemo sec-
93 tion, are concatenated, for ease of searching. The sections are as fol-
94 lows:
96 pcre this document
97 pcre16 details of the 16-bit library
98 pcre-config show PCRE installation configuration information
99 pcreapi details of PCRE's native C API
100 pcrebuild options for building PCRE
101 pcrecallout details of the callout feature
102 pcrecompat discussion of Perl compatibility
103 pcrecpp details of the C++ wrapper for the 8-bit library
104 pcredemo a demonstration C program that uses PCRE
105 pcregrep description of the pcregrep command (8-bit only)
106 pcrejit discussion of the just-in-time optimization support
107 pcrelimits details of size and other limits
108 pcrematching discussion of the two matching algorithms
109 pcrepartial details of the partial matching facility
110 pcrepattern syntax and semantics of supported
111 regular expressions
112 pcreperform discussion of performance issues
113 pcreposix the POSIX-compatible C API for the 8-bit library
114 pcreprecompile details of saving and re-using precompiled patterns
115 pcresample discussion of the pcredemo program
116 pcrestack discussion of stack usage
117 pcresyntax quick syntax reference
118 pcretest description of the pcretest testing command
119 pcreunicode discussion of Unicode and UTF-8/16 support
121 In addition, in the "man" and HTML formats, there is a short page for
122 each 8-bit C library function, listing its arguments and results.
127 Philip Hazel
128 University Computing Service
129 Cambridge CB2 3QH, England.
131 Putting an actual email address here seems to have been a spam magnet,
132 so I've taken it away. If you want to email me, use my two initials,
133 followed by the two digits 10, at the domain cam.ac.uk.
138 Last updated: 10 January 2012
139 Copyright (c) 1997-2012 University of Cambridge.
140 ------------------------------------------------------------------------------
143 PCRE(3) PCRE(3)
146 NAME
147 PCRE - Perl-compatible regular expressions
149 #include <pcre.h>
154 pcre16 *pcre16_compile(PCRE_SPTR16 pattern, int options,
155 const char **errptr, int *erroffset,
156 const unsigned char *tableptr);
158 pcre16 *pcre16_compile2(PCRE_SPTR16 pattern, int options,
159 int *errorcodeptr,
160 const char **errptr, int *erroffset,
161 const unsigned char *tableptr);
163 pcre16_extra *pcre16_study(const pcre16 *code, int options,
164 const char **errptr);
166 void pcre16_free_study(pcre16_extra *extra);
168 int pcre16_exec(const pcre16 *code, const pcre16_extra *extra,
169 PCRE_SPTR16 subject, int length, int startoffset,
170 int options, int *ovector, int ovecsize);
172 int pcre16_dfa_exec(const pcre16 *code, const pcre16_extra *extra,
173 PCRE_SPTR16 subject, int length, int startoffset,
174 int options, int *ovector, int ovecsize,
175 int *workspace, int wscount);
180 int pcre16_copy_named_substring(const pcre16 *code,
181 PCRE_SPTR16 subject, int *ovector,
182 int stringcount, PCRE_SPTR16 stringname,
183 PCRE_UCHAR16 *buffer, int buffersize);
185 int pcre16_copy_substring(PCRE_SPTR16 subject, int *ovector,
186 int stringcount, int stringnumber, PCRE_UCHAR16 *buffer,
187 int buffersize);
189 int pcre16_get_named_substring(const pcre16 *code,
190 PCRE_SPTR16 subject, int *ovector,
191 int stringcount, PCRE_SPTR16 stringname,
192 PCRE_SPTR16 *stringptr);
194 int pcre16_get_stringnumber(const pcre16 *code,
195 PCRE_SPTR16 name);
197 int pcre16_get_stringtable_entries(const pcre16 *code,
198 PCRE_SPTR16 name, PCRE_UCHAR16 **first, PCRE_UCHAR16 **last);
200 int pcre16_get_substring(PCRE_SPTR16 subject, int *ovector,
201 int stringcount, int stringnumber,
202 PCRE_SPTR16 *stringptr);
204 int pcre16_get_substring_list(PCRE_SPTR16 subject,
205 int *ovector, int stringcount, PCRE_SPTR16 **listptr);
207 void pcre16_free_substring(PCRE_SPTR16 stringptr);
209 void pcre16_free_substring_list(PCRE_SPTR16 *stringptr);
214 pcre16_jit_stack *pcre16_jit_stack_alloc(int startsize, int maxsize);
216 void pcre16_jit_stack_free(pcre16_jit_stack *stack);
218 void pcre16_assign_jit_stack(pcre16_extra *extra,
219 pcre16_jit_callback callback, void *data);
221 const unsigned char *pcre16_maketables(void);
223 int pcre16_fullinfo(const pcre16 *code, const pcre16_extra *extra,
224 int what, void *where);
226 int pcre16_refcount(pcre16 *code, int adjust);
228 int pcre16_config(int what, void *where);
230 const char *pcre16_version(void);
232 int pcre16_pattern_to_host_byte_order(pcre16 *code,
233 pcre16_extra *extra, const unsigned char *tables);
238 void *(*pcre16_malloc)(size_t);
240 void (*pcre16_free)(void *);
242 void *(*pcre16_stack_malloc)(size_t);
244 void (*pcre16_stack_free)(void *);
246 int (*pcre16_callout)(pcre16_callout_block *);
251 int pcre16_utf16_to_host_byte_order(PCRE_UCHAR16 *output,
252 PCRE_SPTR16 input, int length, int *byte_order,
253 int keep_boms);
258 Starting with release 8.30, it is possible to compile a PCRE library
259 that supports 16-bit character strings, including UTF-16 strings, as
260 well as or instead of the original 8-bit library. The majority of the
261 work to make this possible was done by Zoltan Herczeg. The two
262 libraries contain identical sets of functions, used in exactly the same
263 way. Only the names of the functions and the data types of their argu-
264 ments and results are different. To avoid over-complication and reduce
265 the documentation maintenance load, most of the PCRE documentation
266 describes the 8-bit library, with only occasional references to the
267 16-bit library. This page describes what is different when you use the
268 16-bit library.
270 WARNING: A single application can be linked with both libraries, but
271 you must take care when processing any particular pattern to use func-
272 tions from just one library. For example, if you want to study a pat-
273 tern that was compiled with pcre16_compile(), you must do so with
274 pcre16_study(), not pcre_study(), and you must free the study data with
275 pcre16_free_study().
280 There is only one header file, pcre.h. It contains prototypes for all
281 the functions in both libraries, as well as definitions of flags,
282 structures, error codes, etc.
287 In Unix-like systems, the 16-bit library is called libpcre16, and can
288 normally be accesss by adding -lpcre16 to the command for linking an
289 application that uses PCRE.
294 In the 8-bit library, strings are passed to PCRE library functions as
295 vectors of bytes with the C type "char *". In the 16-bit library,
296 strings are passed as vectors of unsigned 16-bit quantities. The macro
297 PCRE_UCHAR16 specifies an appropriate data type, and PCRE_SPTR16 is
298 defined as "const PCRE_UCHAR16 *". In very many environments, "short
299 int" is a 16-bit data type. When PCRE is built, it defines PCRE_UCHAR16
300 as "short int", but checks that it really is a 16-bit data type. If it
301 is not, the build fails with an error message telling the maintainer to
302 modify the definition appropriately.
307 The types of the opaque structures that are used for compiled 16-bit
308 patterns and JIT stacks are pcre16 and pcre16_jit_stack respectively.
309 The type of the user-accessible structure that is returned by
310 pcre16_study() is pcre16_extra, and the type of the structure that is
311 used for passing data to a callout function is pcre16_callout_block.
312 These structures contain the same fields, with the same names, as their
313 8-bit counterparts. The only difference is that pointers to character
314 strings are 16-bit instead of 8-bit types.
319 For every function in the 8-bit library there is a corresponding func-
320 tion in the 16-bit library with a name that starts with pcre16_ instead
321 of pcre_. The prototypes are listed above. In addition, there is one
322 extra function, pcre16_utf16_to_host_byte_order(). This is a utility
323 function that converts a UTF-16 character string to host byte order if
324 necessary. The other 16-bit functions expect the strings they are
325 passed to be in host byte order.
327 The input and output arguments of pcre16_utf16_to_host_byte_order() may
328 point to the same address, that is, conversion in place is supported.
329 The output buffer must be at least as long as the input.
331 The length argument specifies the number of 16-bit data units in the
332 input string; a negative value specifies a zero-terminated string.
334 If byte_order is NULL, it is assumed that the string starts off in host
335 byte order. This may be changed by byte-order marks (BOMs) anywhere in
336 the string (commonly as the first character).
338 If byte_order is not NULL, a non-zero value of the integer to which it
339 points means that the input starts off in host byte order, otherwise
340 the opposite order is assumed. Again, BOMs in the string can change
341 this. The final byte order is passed back at the end of processing.
343 If keep_boms is not zero, byte-order mark characters (0xfeff) are
344 copied into the output string. Otherwise they are discarded.
346 The result of the function is the number of 16-bit units placed into
347 the output buffer, including the zero terminator if the string was
348 zero-terminated.
353 The offsets within subject strings that are returned by the matching
354 functions are in 16-bit units rather than bytes.
359 The name-to-number translation table that is maintained for named sub-
360 patterns uses 16-bit characters. The pcre16_get_stringtable_entries()
361 function returns the length of each entry in the table as the number of
362 16-bit data units.
367 There are two new general option names, PCRE_UTF16 and
368 PCRE_NO_UTF16_CHECK, which correspond to PCRE_UTF8 and
369 PCRE_NO_UTF8_CHECK in the 8-bit library. In fact, these new options
370 define the same bits in the options word.
372 For the pcre16_config() function there is an option PCRE_CONFIG_UTF16
373 that returns 1 if UTF-16 support is configured, otherwise 0. If this
374 option is given to pcre_config(), or if the PCRE_CONFIG_UTF8 option is
375 given to pcre16_config(), the result is the PCRE_ERROR_BADOPTION error.
380 In 16-bit mode, when PCRE_UTF16 is not set, character values are
381 treated in the same way as in 8-bit, non UTF-8 mode, except, of course,
382 that they can range from 0 to 0xffff instead of 0 to 0xff. Character
383 types for characters less than 0xff can therefore be influenced by the
384 locale in the same way as before. Characters greater than 0xff have
385 only one case, and no "type" (such as letter or digit).
387 In UTF-16 mode, the character code is Unicode, in the range 0 to
388 0x10ffff, with the exception of values in the range 0xd800 to 0xdfff
389 because those are "surrogate" values that are used in pairs to encode
390 values greater than 0xffff.
392 A UTF-16 string can indicate its endianness by special code knows as a
393 byte-order mark (BOM). The PCRE functions do not handle this, expecting
394 strings to be in host byte order. A utility function called
395 pcre16_utf16_to_host_byte_order() is provided to help with this (see
396 above).
402 spond to their 8-bit counterparts. The error PCRE_ERROR_BADMODE is
403 given when a compiled pattern is passed to a function that processes
404 patterns in the other mode, for example, if a pattern compiled with
405 pcre_compile() is passed to pcre16_exec().
407 There are new error codes whose names begin with PCRE_UTF16_ERR for
408 invalid UTF-16 strings, corresponding to the PCRE_UTF8_ERR codes for
409 UTF-8 strings that are described in the section entitled "Reason codes
410 for invalid UTF-8 strings" in the main pcreapi page. The UTF-16 errors
411 are:
413 PCRE_UTF16_ERR1 Missing low surrogate at end of string
414 PCRE_UTF16_ERR2 Invalid low surrogate follows high surrogate
415 PCRE_UTF16_ERR3 Isolated low surrogate
416 PCRE_UTF16_ERR4 Invalid character 0xfffe
421 If there is an error while compiling a pattern, the error text that is
422 passed back by pcre16_compile() or pcre16_compile2() is still an 8-bit
423 character string, zero-terminated.
428 The subject and mark fields in the callout block that is passed to a
429 callout function point to 16-bit vectors.
434 The pcretest program continues to operate with 8-bit input and output
435 files, but it can be used for testing the 16-bit library. If it is run
436 with the command line option -16, patterns and subject strings are con-
437 verted from 8-bit to 16-bit before being passed to PCRE, and the 16-bit
438 library functions are used instead of the 8-bit ones. Returned 16-bit
439 strings are converted to 8-bit for output. If the 8-bit library was not
440 compiled, pcretest defaults to 16-bit and the -16 option is ignored.
442 When PCRE is being built, the RunTest script that is called by "make
443 check" uses the pcretest -C option to discover which of the 8-bit and
444 16-bit libraries has been built, and runs the tests appropriately.
449 Not all the features of the 8-bit library are available with the 16-bit
450 library. The C++ and POSIX wrapper functions support only the 8-bit
451 library, and the pcregrep program is at present 8-bit only.
456 Philip Hazel
457 University Computing Service
458 Cambridge CB2 3QH, England.
463 Last updated: 08 January 2012
464 Copyright (c) 1997-2012 University of Cambridge.
465 ------------------------------------------------------------------------------
471 NAME
472 PCRE - Perl-compatible regular expressions
477 This document describes the optional features of PCRE that can be
478 selected when the library is compiled. It assumes use of the configure
479 script, where the optional features are selected or deselected by pro-
480 viding options to configure before running the make command. However,
481 the same options can be selected in both Unix-like and non-Unix-like
482 environments using the GUI facility of cmake-gui if you are using CMake
483 instead of configure to build PCRE.
485 There is a lot more information about building PCRE in non-Unix-like
486 environments in the file called NON_UNIX_USE, which is part of the PCRE
487 distribution. You should consult this file as well as the README file
488 if you are building in a non-Unix-like environment.
490 The complete list of options for configure (which includes the standard
491 ones such as the selection of the installation directory) can be
492 obtained by running
494 ./configure --help
496 The following sections include descriptions of options whose names
497 begin with --enable or --disable. These settings specify changes to the
498 defaults for the configure command. Because of the way that configure
499 works, --enable and --disable always come in pairs, so the complemen-
500 tary option always exists as well, but as it specifies the default, it
501 is not described.
506 By default, a library called libpcre is built, containing functions
507 that take string arguments contained in vectors of bytes, either as
508 single-byte characters, or interpreted as UTF-8 strings. You can also
509 build a separate library, called libpcre16, in which strings are con-
510 tained in vectors of 16-bit data units and interpreted either as sin-
511 gle-unit characters or UTF-16 strings, by adding
513 --enable-pcre16
515 to the configure command. If you do not want the 8-bit library, add
517 --disable-pcre8
519 as well. At least one of the two libraries must be built. Note that the
520 C++ and POSIX wrappers are for the 8-bit library only, and that pcre-
521 grep is an 8-bit program. None of these are built if you select only
522 the 16-bit library.
527 The PCRE building process uses libtool to build both shared and static
528 Unix libraries by default. You can suppress one of these by adding one
529 of
531 --disable-shared
532 --disable-static
534 to the configure command, as required.
539 By default, if the 8-bit library is being built, the configure script
540 will search for a C++ compiler and C++ header files. If it finds them,
541 it automatically builds the C++ wrapper library (which supports only
542 8-bit strings). You can disable this by adding
544 --disable-cpp
546 to the configure command.
549 UTF-8 and UTF-16 SUPPORT
551 To build PCRE with support for UTF Unicode character strings, add
553 --enable-utf
555 to the configure command. This setting applies to both libraries,
556 adding support for UTF-8 to the 8-bit library and support for UTF-16 to
557 the 16-bit library. There are no separate options for enabling UTF-8
558 and UTF-16 independently because that would allow ridiculous settings
559 such as requesting UTF-16 support while building only the 8-bit
560 library. It is not possible to build one library with UTF support and
561 the other without in the same configuration. (For backwards compatibil-
562 ity, --enable-utf8 is a synonym of --enable-utf.)
564 Of itself, this setting does not make PCRE treat strings as UTF-8 or
565 UTF-16. As well as compiling PCRE with this option, you also have have
566 to set the PCRE_UTF8 or PCRE_UTF16 option when you call one of the pat-
567 tern compiling functions.
569 If you set --enable-utf when compiling in an EBCDIC environment, PCRE
570 expects its input to be either ASCII or UTF-8 (depending on the runtime
571 option). It is not possible to support both EBCDIC and UTF-8 codes in
572 the same version of the library. Consequently, --enable-utf and
573 --enable-ebcdic are mutually exclusive.
578 UTF support allows the libraries to process character codepoints up to
579 0x10ffff in the strings that they handle. On its own, however, it does
580 not provide any facilities for accessing the properties of such charac-
581 ters. If you want to be able to use the pattern escapes \P, \p, and \X,
582 which refer to Unicode character properties, you must add
584 --enable-unicode-properties
586 to the configure command. This implies UTF support, even if you have
587 not explicitly requested it.
589 Including Unicode property support adds around 30K of tables to the
590 PCRE library. Only the general category properties such as Lu and Nd
591 are supported. Details are given in the pcrepattern documentation.
596 Just-in-time compiler support is included in the build by specifying
598 --enable-jit
600 This support is available only for certain hardware architectures. If
601 this option is set for an unsupported architecture, a compile time
602 error occurs. See the pcrejit documentation for a discussion of JIT
603 usage. When JIT support is enabled, pcregrep automatically makes use of
604 it, unless you add
606 --disable-pcregrep-jit
608 to the "configure" command.
613 By default, PCRE interprets the linefeed (LF) character as indicating
614 the end of a line. This is the normal newline character on Unix-like
615 systems. You can compile PCRE to use carriage return (CR) instead, by
616 adding
618 --enable-newline-is-cr
620 to the configure command. There is also a --enable-newline-is-lf
621 option, which explicitly specifies linefeed as the newline character.
623 Alternatively, you can specify that line endings are to be indicated by
624 the two character sequence CRLF. If you want this, add
626 --enable-newline-is-crlf
628 to the configure command. There is a fourth option, specified by
630 --enable-newline-is-anycrlf
632 which causes PCRE to recognize any of the three sequences CR, LF, or
633 CRLF as indicating a line ending. Finally, a fifth option, specified by
635 --enable-newline-is-any
637 causes PCRE to recognize any Unicode newline sequence.
639 Whatever line ending convention is selected when PCRE is built can be
640 overridden when the library functions are called. At build time it is
641 conventional to use the standard for your operating system.
646 By default, the sequence \R in a pattern matches any Unicode newline
647 sequence, whatever has been selected as the line ending sequence. If
648 you specify
650 --enable-bsr-anycrlf
652 the default is changed so that \R matches only CR, LF, or CRLF. What-
653 ever is selected when PCRE is built can be overridden when the library
654 functions are called.
659 When the 8-bit library is called through the POSIX interface (see the
660 pcreposix documentation), additional working storage is required for
661 holding the pointers to capturing substrings, because PCRE requires
662 three integers per substring, whereas the POSIX interface provides only
663 two. If the number of expected substrings is small, the wrapper func-
664 tion uses space on the stack, because this is faster than using mal-
665 loc() for each call. The default threshold above which the stack is no
666 longer used is 10; it can be changed by adding a setting such as
668 --with-posix-malloc-threshold=20
670 to the configure command.
675 Within a compiled pattern, offset values are used to point from one
676 part to another (for example, from an opening parenthesis to an alter-
677 nation metacharacter). By default, two-byte values are used for these
678 offsets, leading to a maximum size for a compiled pattern of around
679 64K. This is sufficient to handle all but the most gigantic patterns.
680 Nevertheless, some people do want to process truly enormous patterns,
681 so it is possible to compile PCRE to use three-byte or four-byte off-
682 sets by adding a setting such as
684 --with-link-size=3
686 to the configure command. The value given must be 2, 3, or 4. For the
687 16-bit library, a value of 3 is rounded up to 4. Using longer offsets
688 slows down the operation of PCRE because it has to load additional data
689 when handling them.
694 When matching with the pcre_exec() function, PCRE implements backtrack-
695 ing by making recursive calls to an internal function called match().
696 In environments where the size of the stack is limited, this can se-
697 verely limit PCRE's operation. (The Unix environment does not usually
698 suffer from this problem, but it may sometimes be necessary to increase
699 the maximum stack size. There is a discussion in the pcrestack docu-
700 mentation.) An alternative approach to recursion that uses memory from
701 the heap to remember data, instead of using recursive function calls,
702 has been implemented to work round the problem of limited stack size.
703 If you want to build a version of PCRE that works this way, add
705 --disable-stack-for-recursion
707 to the configure command. With this configuration, PCRE will use the
708 pcre_stack_malloc and pcre_stack_free variables to call memory manage-
709 ment functions. By default these point to malloc() and free(), but you
710 can replace the pointers so that your own functions are used instead.
712 Separate functions are provided rather than using pcre_malloc and
713 pcre_free because the usage is very predictable: the block sizes
714 requested are always the same, and the blocks are always freed in
715 reverse order. A calling program might be able to implement optimized
716 functions that perform better than malloc() and free(). PCRE runs
717 noticeably more slowly when built in this way. This option affects only
718 the pcre_exec() function; it is not relevant for pcre_dfa_exec().
723 Internally, PCRE has a function called match(), which it calls repeat-
724 edly (sometimes recursively) when matching a pattern with the
725 pcre_exec() function. By controlling the maximum number of times this
726 function may be called during a single matching operation, a limit can
727 be placed on the resources used by a single call to pcre_exec(). The
728 limit can be changed at run time, as described in the pcreapi documen-
729 tation. The default is 10 million, but this can be changed by adding a
730 setting such as
732 --with-match-limit=500000
734 to the configure command. This setting has no effect on the
735 pcre_dfa_exec() matching function.
737 In some environments it is desirable to limit the depth of recursive
738 calls of match() more strictly than the total number of calls, in order
739 to restrict the maximum amount of stack (or heap, if --disable-stack-
740 for-recursion is specified) that is used. A second limit controls this;
741 it defaults to the value that is set for --with-match-limit, which
742 imposes no additional constraints. However, you can set a lower limit
743 by adding, for example,
745 --with-match-limit-recursion=10000
747 to the configure command. This value can also be overridden at run
748 time.
753 PCRE uses fixed tables for processing characters whose code values are
754 less than 256. By default, PCRE is built with a set of tables that are
755 distributed in the file pcre_chartables.c.dist. These tables are for
756 ASCII codes only. If you add
758 --enable-rebuild-chartables
760 to the configure command, the distributed tables are no longer used.
761 Instead, a program called dftables is compiled and run. This outputs
762 the source for new set of tables, created in the default locale of your
763 C runtime system. (This method of replacing the tables does not work if
764 you are cross compiling, because dftables is run on the local host. If
765 you need to create alternative tables when cross compiling, you will
766 have to do so "by hand".)
771 PCRE assumes by default that it will run in an environment where the
772 character code is ASCII (or Unicode, which is a superset of ASCII).
773 This is the case for most computer operating systems. PCRE can, how-
774 ever, be compiled to run in an EBCDIC environment by adding
776 --enable-ebcdic
778 to the configure command. This setting implies --enable-rebuild-charta-
779 bles. You should only use it if you know that you are in an EBCDIC
780 environment (for example, an IBM mainframe operating system). The
781 --enable-ebcdic option is incompatible with --enable-utf.
786 By default, pcregrep reads all files as plain text. You can build it so
787 that it recognizes files whose names end in .gz or .bz2, and reads them
788 with libz or libbz2, respectively, by adding one or both of
790 --enable-pcregrep-libz
791 --enable-pcregrep-libbz2
793 to the configure command. These options naturally require that the rel-
794 evant libraries are installed on your system. Configuration will fail
795 if they are not.
800 pcregrep uses an internal buffer to hold a "window" on the file it is
801 scanning, in order to be able to output "before" and "after" lines when
802 it finds a match. The size of the buffer is controlled by a parameter
803 whose default value is 20K. The buffer itself is three times this size,
804 but because of the way it is used for holding "before" lines, the long-
805 est line that is guaranteed to be processable is the parameter size.
806 You can change the default parameter value by adding, for example,
808 --with-pcregrep-bufsize=50K
810 to the configure command. The caller of pcregrep can, however, override
811 this value by specifying a run-time option.
816 If you add
818 --enable-pcretest-libreadline
820 to the configure command, pcretest is linked with the libreadline
821 library, and when its input is from a terminal, it reads it using the
822 readline() function. This provides line-editing and history facilities.
823 Note that libreadline is GPL-licensed, so if you distribute a binary of
824 pcretest linked in this way, there may be licensing issues.
826 Setting this option causes the -lreadline option to be added to the
827 pcretest build. In many operating environments with a sytem-installed
828 libreadline this is sufficient. However, in some environments (e.g. if
829 an unmodified distribution version of readline is in use), some extra
830 configuration may be necessary. The INSTALL file for libreadline says
831 this:
833 "Readline uses the termcap functions, but does not link with the
834 termcap or curses library itself, allowing applications which link
835 with readline the to choose an appropriate library."
837 If your environment has not been set up so that an appropriate library
838 is automatically included, you may need to add something like
840 LIBS="-ncurses"
842 immediately before the configure command.
847 pcreapi(3), pcre16, pcre_config(3).
852 Philip Hazel
853 University Computing Service
854 Cambridge CB2 3QH, England.
859 Last updated: 07 January 2012
860 Copyright (c) 1997-2012 University of Cambridge.
861 ------------------------------------------------------------------------------
867 NAME
868 PCRE - Perl-compatible regular expressions
873 This document describes the two different algorithms that are available
874 in PCRE for matching a compiled regular expression against a given sub-
875 ject string. The "standard" algorithm is the one provided by the
876 pcre_exec() and pcre16_exec() functions. These work in the same was as
877 Perl's matching function, and provide a Perl-compatible matching opera-
878 tion. The just-in-time (JIT) optimization that is described in the
879 pcrejit documentation is compatible with these functions.
881 An alternative algorithm is provided by the pcre_dfa_exec() and
882 pcre16_dfa_exec() functions; they operate in a different way, and are
883 not Perl-compatible. This alternative has advantages and disadvantages
884 compared with the standard algorithm, and these are described below.
886 When there is only one possible way in which a given subject string can
887 match a pattern, the two algorithms give the same answer. A difference
888 arises, however, when there are multiple possibilities. For example, if
889 the pattern
891 ^<.*>
893 is matched against the string
895 <something> <something else> <something further>
897 there are three possible answers. The standard algorithm finds only one
898 of them, whereas the alternative algorithm finds all three.
903 The set of strings that are matched by a regular expression can be rep-
904 resented as a tree structure. An unlimited repetition in the pattern
905 makes the tree of infinite size, but it is still a tree. Matching the
906 pattern to a given subject string (from a given starting point) can be
907 thought of as a search of the tree. There are two ways to search a
908 tree: depth-first and breadth-first, and these correspond to the two
909 matching algorithms provided by PCRE.
914 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
915 sions", the standard algorithm is an "NFA algorithm". It conducts a
916 depth-first search of the pattern tree. That is, it proceeds along a
917 single path through the tree, checking that the subject matches what is
918 required. When there is a mismatch, the algorithm tries any alterna-
919 tives at the current point, and if they all fail, it backs up to the
920 previous branch point in the tree, and tries the next alternative
921 branch at that level. This often involves backing up (moving to the
922 left) in the subject string as well. The order in which repetition
923 branches are tried is controlled by the greedy or ungreedy nature of
924 the quantifier.
926 If a leaf node is reached, a matching string has been found, and at
927 that point the algorithm stops. Thus, if there is more than one possi-
928 ble match, this algorithm returns the first one that it finds. Whether
929 this is the shortest, the longest, or some intermediate length depends
930 on the way the greedy and ungreedy repetition quantifiers are specified
931 in the pattern.
933 Because it ends up with a single path through the tree, it is rela-
934 tively straightforward for this algorithm to keep track of the sub-
935 strings that are matched by portions of the pattern in parentheses.
936 This provides support for capturing parentheses and back references.
941 This algorithm conducts a breadth-first search of the tree. Starting
942 from the first matching point in the subject, it scans the subject
943 string from left to right, once, character by character, and as it does
944 this, it remembers all the paths through the tree that represent valid
945 matches. In Friedl's terminology, this is a kind of "DFA algorithm",
946 though it is not implemented as a traditional finite state machine (it
947 keeps multiple states active simultaneously).
949 Although the general principle of this matching algorithm is that it
950 scans the subject string only once, without backtracking, there is one
951 exception: when a lookaround assertion is encountered, the characters
952 following or preceding the current point have to be independently
953 inspected.
955 The scan continues until either the end of the subject is reached, or
956 there are no more unterminated paths. At this point, terminated paths
957 represent the different matching possibilities (if there are none, the
958 match has failed). Thus, if there is more than one possible match,
959 this algorithm finds all of them, and in particular, it finds the long-
960 est. The matches are returned in decreasing order of length. There is
961 an option to stop the algorithm after the first match (which is neces-
962 sarily the shortest) is found.
964 Note that all the matches that are found start at the same point in the
965 subject. If the pattern
967 cat(er(pillar)?)?
969 is matched against the string "the caterpillar catchment", the result
970 will be the three strings "caterpillar", "cater", and "cat" that start
971 at the fifth character of the subject. The algorithm does not automati-
972 cally move on to find matches that start at later positions.
974 There are a number of features of PCRE regular expressions that are not
975 supported by the alternative matching algorithm. They are as follows:
977 1. Because the algorithm finds all possible matches, the greedy or
978 ungreedy nature of repetition quantifiers is not relevant. Greedy and
979 ungreedy quantifiers are treated in exactly the same way. However, pos-
980 sessive quantifiers can make a difference when what follows could also
981 match what is quantified, for example in a pattern like this:
983 ^a++\w!
985 This pattern matches "aaab!" but not "aaa!", which would be matched by
986 a non-possessive quantifier. Similarly, if an atomic group is present,
987 it is matched as if it were a standalone pattern at the current point,
988 and the longest match is then "locked in" for the rest of the overall
989 pattern.
991 2. When dealing with multiple paths through the tree simultaneously, it
992 is not straightforward to keep track of captured substrings for the
993 different matching possibilities, and PCRE's implementation of this
994 algorithm does not attempt to do this. This means that no captured sub-
995 strings are available.
997 3. Because no substrings are captured, back references within the pat-
998 tern are not supported, and cause errors if encountered.
1000 4. For the same reason, conditional expressions that use a backrefer-
1001 ence as the condition or test for a specific group recursion are not
1002 supported.
1004 5. Because many paths through the tree may be active, the \K escape
1005 sequence, which resets the start of the match when encountered (but may
1006 be on some paths and not on others), is not supported. It causes an
1007 error if encountered.
1009 6. Callouts are supported, but the value of the capture_top field is
1010 always 1, and the value of the capture_last field is always -1.
1012 7. The \C escape sequence, which (in the standard algorithm) always
1013 matches a single data unit, even in UTF-8 or UTF-16 modes, is not sup-
1014 ported in these modes, because the alternative algorithm moves through
1015 the subject string one character (not data unit) at a time, for all
1016 active paths through the tree.
1018 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
1019 are not supported. (*FAIL) is supported, and behaves like a failing
1020 negative assertion.
1025 Using the alternative matching algorithm provides the following advan-
1026 tages:
1028 1. All possible matches (at a single point in the subject) are automat-
1029 ically found, and in particular, the longest match is found. To find
1030 more than one match using the standard algorithm, you have to do kludgy
1031 things with callouts.
1033 2. Because the alternative algorithm scans the subject string just
1034 once, and never needs to backtrack (except for lookbehinds), it is pos-
1035 sible to pass very long subject strings to the matching function in
1036 several pieces, checking for partial matching each time. Although it is
1037 possible to do multi-segment matching using the standard algorithm by
1038 retaining partially matched substrings, it is more complicated. The
1039 pcrepartial documentation gives details of partial matching and dis-
1040 cusses multi-segment matching.
1045 The alternative algorithm suffers from a number of disadvantages:
1047 1. It is substantially slower than the standard algorithm. This is
1048 partly because it has to search for all possible matches, but is also
1049 because it is less susceptible to optimization.
1051 2. Capturing parentheses and back references are not supported.
1053 3. Although atomic groups are supported, their use does not provide the
1054 performance advantage that it does for the standard algorithm.
1059 Philip Hazel
1060 University Computing Service
1061 Cambridge CB2 3QH, England.
1066 Last updated: 08 January 2012
1067 Copyright (c) 1997-2012 University of Cambridge.
1068 ------------------------------------------------------------------------------
1074 NAME
1075 PCRE - Perl-compatible regular expressions
1077 #include <pcre.h>
1082 pcre *pcre_compile(const char *pattern, int options,
1083 const char **errptr, int *erroffset,
1084 const unsigned char *tableptr);
1086 pcre *pcre_compile2(const char *pattern, int options,
1087 int *errorcodeptr,
1088 const char **errptr, int *erroffset,
1089 const unsigned char *tableptr);
1091 pcre_extra *pcre_study(const pcre *code, int options,
1092 const char **errptr);
1094 void pcre_free_study(pcre_extra *extra);
1096 int pcre_exec(const pcre *code, const pcre_extra *extra,
1097 const char *subject, int length, int startoffset,
1098 int options, int *ovector, int ovecsize);
1100 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
1101 const char *subject, int length, int startoffset,
1102 int options, int *ovector, int ovecsize,
1103 int *workspace, int wscount);
1108 int pcre_copy_named_substring(const pcre *code,
1109 const char *subject, int *ovector,
1110 int stringcount, const char *stringname,
1111 char *buffer, int buffersize);
1113 int pcre_copy_substring(const char *subject, int *ovector,
1114 int stringcount, int stringnumber, char *buffer,
1115 int buffersize);
1117 int pcre_get_named_substring(const pcre *code,
1118 const char *subject, int *ovector,
1119 int stringcount, const char *stringname,
1120 const char **stringptr);
1122 int pcre_get_stringnumber(const pcre *code,
1123 const char *name);
1125 int pcre_get_stringtable_entries(const pcre *code,
1126 const char *name, char **first, char **last);
1128 int pcre_get_substring(const char *subject, int *ovector,
1129 int stringcount, int stringnumber,
1130 const char **stringptr);
1132 int pcre_get_substring_list(const char *subject,
1133 int *ovector, int stringcount, const char ***listptr);
1135 void pcre_free_substring(const char *stringptr);
1137 void pcre_free_substring_list(const char **stringptr);
1142 pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);
1144 void pcre_jit_stack_free(pcre_jit_stack *stack);
1146 void pcre_assign_jit_stack(pcre_extra *extra,
1147 pcre_jit_callback callback, void *data);
1149 const unsigned char *pcre_maketables(void);
1151 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1152 int what, void *where);
1154 int pcre_refcount(pcre *code, int adjust);
1156 int pcre_config(int what, void *where);
1158 const char *pcre_version(void);
1160 int pcre_pattern_to_host_byte_order(pcre *code,
1161 pcre_extra *extra, const unsigned char *tables);
1166 void *(*pcre_malloc)(size_t);
1168 void (*pcre_free)(void *);
1170 void *(*pcre_stack_malloc)(size_t);
1172 void (*pcre_stack_free)(void *);
1174 int (*pcre_callout)(pcre_callout_block *);
1179 From release 8.30, PCRE can be compiled as a library for handling
1180 16-bit character strings as well as, or instead of, the original
1181 library that handles 8-bit character strings. To avoid too much compli-
1182 cation, this document describes the 8-bit versions of the functions,
1183 with only occasional references to the 16-bit library.
1185 The 16-bit functions operate in the same way as their 8-bit counter-
1186 parts; they just use different data types for their arguments and
1187 results, and their names start with pcre16_ instead of pcre_. For every
1188 option that has UTF8 in its name (for example, PCRE_UTF8), there is a
1189 corresponding 16-bit name with UTF8 replaced by UTF16. This facility is
1190 in fact just cosmetic; the 16-bit option names define the same bit val-
1191 ues.
1193 References to bytes and UTF-8 in this document should be read as refer-
1194 ences to 16-bit data quantities and UTF-16 when using the 16-bit
1195 library, unless specified otherwise. More details of the specific dif-
1196 ferences for the 16-bit library are given in the pcre16 page.
1201 PCRE has its own native API, which is described in this document. There
1202 are also some wrapper functions (for the 8-bit library only) that cor-
1203 respond to the POSIX regular expression API, but they do not give
1204 access to all the functionality. They are described in the pcreposix
1205 documentation. Both of these APIs define a set of C function calls. A
1206 C++ wrapper (again for the 8-bit library only) is also distributed with
1207 PCRE. It is documented in the pcrecpp page.
1209 The native API C function prototypes are defined in the header file
1210 pcre.h, and on Unix-like systems the (8-bit) library itself is called
1211 libpcre. It can normally be accessed by adding -lpcre to the command
1212 for linking an application that uses PCRE. The header file defines the
1213 macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release
1214 numbers for the library. Applications can use these to include support
1215 for different releases of PCRE.
1217 In a Windows environment, if you want to statically link an application
1218 program against a non-dll pcre.a file, you must define PCRE_STATIC
1219 before including pcre.h or pcrecpp.h, because otherwise the pcre_mal-
1220 loc() and pcre_free() exported functions will be declared
1221 __declspec(dllimport), with unwanted results.
1223 The functions pcre_compile(), pcre_compile2(), pcre_study(), and
1224 pcre_exec() are used for compiling and matching regular expressions in
1225 a Perl-compatible manner. A sample program that demonstrates the sim-
1226 plest way of using them is provided in the file called pcredemo.c in
1227 the PCRE source distribution. A listing of this program is given in the
1228 pcredemo documentation, and the pcresample documentation describes how
1229 to compile and run it.
1231 Just-in-time compiler support is an optional feature of PCRE that can
1232 be built in appropriate hardware environments. It greatly speeds up the
1233 matching performance of many patterns. Simple programs can easily
1234 request that it be used if available, by setting an option that is
1235 ignored when it is not relevant. More complicated programs might need
1236 to make use of the functions pcre_jit_stack_alloc(),
1237 pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control
1238 the JIT code's memory usage. These functions are discussed in the
1239 pcrejit documentation.
1241 A second matching function, pcre_dfa_exec(), which is not Perl-compati-
1242 ble, is also provided. This uses a different algorithm for the match-
1243 ing. The alternative algorithm finds all possible matches (at a given
1244 point in the subject), and scans the subject just once (unless there
1245 are lookbehind assertions). However, this algorithm does not return
1246 captured substrings. A description of the two matching algorithms and
1247 their advantages and disadvantages is given in the pcrematching docu-
1248 mentation.
1250 In addition to the main compiling and matching functions, there are
1251 convenience functions for extracting captured substrings from a subject
1252 string that is matched by pcre_exec(). They are:
1254 pcre_copy_substring()
1255 pcre_copy_named_substring()
1256 pcre_get_substring()
1257 pcre_get_named_substring()
1258 pcre_get_substring_list()
1259 pcre_get_stringnumber()
1260 pcre_get_stringtable_entries()
1262 pcre_free_substring() and pcre_free_substring_list() are also provided,
1263 to free the memory used for extracted strings.
1265 The function pcre_maketables() is used to build a set of character
1266 tables in the current locale for passing to pcre_compile(),
1267 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
1268 provided for specialist use. Most commonly, no special tables are
1269 passed, in which case internal tables that are generated when PCRE is
1270 built are used.
1272 The function pcre_fullinfo() is used to find out information about a
1273 compiled pattern. The function pcre_version() returns a pointer to a
1274 string containing the version of PCRE and its date of release.
1276 The function pcre_refcount() maintains a reference count in a data
1277 block containing a compiled pattern. This is provided for the benefit
1278 of object-oriented applications.
1280 The global variables pcre_malloc and pcre_free initially contain the
1281 entry points of the standard malloc() and free() functions, respec-
1282 tively. PCRE calls the memory management functions via these variables,
1283 so a calling program can replace them if it wishes to intercept the
1284 calls. This should be done before calling any PCRE functions.
1286 The global variables pcre_stack_malloc and pcre_stack_free are also
1287 indirections to memory management functions. These special functions
1288 are used only when PCRE is compiled to use the heap for remembering
1289 data, instead of recursive function calls, when running the pcre_exec()
1290 function. See the pcrebuild documentation for details of how to do
1291 this. It is a non-standard way of building PCRE, for use in environ-
1292 ments that have limited stacks. Because of the greater use of memory
1293 management, it runs more slowly. Separate functions are provided so
1294 that special-purpose external code can be used for this case. When
1295 used, these functions are always called in a stack-like manner (last
1296 obtained, first freed), and always for memory blocks of the same size.
1297 There is a discussion about PCRE's stack usage in the pcrestack docu-
1298 mentation.
1300 The global variable pcre_callout initially contains NULL. It can be set
1301 by the caller to a "callout" function, which PCRE will then call at
1302 specified points during a matching operation. Details are given in the
1303 pcrecallout documentation.
1308 PCRE supports five different conventions for indicating line breaks in
1309 strings: a single CR (carriage return) character, a single LF (line-
1310 feed) character, the two-character sequence CRLF, any of the three pre-
1311 ceding, or any Unicode newline sequence. The Unicode newline sequences
1312 are the three just mentioned, plus the single characters VT (vertical
1313 tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
1314 separator, U+2028), and PS (paragraph separator, U+2029).
1316 Each of the first three conventions is used by at least one operating
1317 system as its standard newline sequence. When PCRE is built, a default
1318 can be specified. The default default is LF, which is the Unix stan-
1319 dard. When PCRE is run, the default can be overridden, either when a
1320 pattern is compiled, or when it is matched.
1322 At compile time, the newline convention can be specified by the options
1323 argument of pcre_compile(), or it can be specified by special text at
1324 the start of the pattern itself; this overrides any other settings. See
1325 the pcrepattern page for details of the special character sequences.
1327 In the PCRE documentation the word "newline" is used to mean "the char-
1328 acter or pair of characters that indicate a line break". The choice of
1329 newline convention affects the handling of the dot, circumflex, and
1330 dollar metacharacters, the handling of #-comments in /x mode, and, when
1331 CRLF is a recognized line ending sequence, the match position advance-
1332 ment for a non-anchored pattern. There is more detail about this in the
1333 section on pcre_exec() options below.
1335 The choice of newline convention does not affect the interpretation of
1336 the \n or \r escape sequences, nor does it affect what \R matches,
1337 which is controlled in a similar way, but by separate options.
1342 The PCRE functions can be used in multi-threading applications, with
1343 the proviso that the memory management functions pointed to by
1344 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
1345 callout function pointed to by pcre_callout, are shared by all threads.
1347 The compiled form of a regular expression is not altered during match-
1348 ing, so the same compiled pattern can safely be used by several threads
1349 at once.
1351 If the just-in-time optimization feature is being used, it needs sepa-
1352 rate memory stack areas for each thread. See the pcrejit documentation
1353 for more details.
1358 The compiled form of a regular expression can be saved and re-used at a
1359 later time, possibly by a different program, and even on a host other
1360 than the one on which it was compiled. Details are given in the
1361 pcreprecompile documentation, which includes a description of the
1362 pcre_pattern_to_host_byte_order() function. However, compiling a regu-
1363 lar expression with one version of PCRE for use with a different ver-
1364 sion is not guaranteed to work and may cause crashes.
1369 int pcre_config(int what, void *where);
1371 The function pcre_config() makes it possible for a PCRE client to dis-
1372 cover which optional features have been compiled into the PCRE library.
1373 The pcrebuild documentation has more details about these optional fea-
1374 tures.
1376 The first argument for pcre_config() is an integer, specifying which
1377 information is required; the second argument is a pointer to a variable
1378 into which the information is placed. The returned value is zero on
1379 success, or the negative error code PCRE_ERROR_BADOPTION if the value
1380 in the first argument is not recognized. The following information is
1381 available:
1385 The output is an integer that is set to one if UTF-8 support is avail-
1386 able; otherwise it is set to zero. If this option is given to the
1387 16-bit version of this function, pcre16_config(), the result is
1392 The output is an integer that is set to one if UTF-16 support is avail-
1393 able; otherwise it is set to zero. This value should normally be given
1394 to the 16-bit version of this function, pcre16_config(). If it is given
1395 to the 8-bit version of this function, the result is PCRE_ERROR_BADOP-
1396 TION.
1400 The output is an integer that is set to one if support for Unicode
1401 character properties is available; otherwise it is set to zero.
1405 The output is an integer that is set to one if support for just-in-time
1406 compiling is available; otherwise it is set to zero.
1410 The output is a pointer to a zero-terminated "const char *" string. If
1411 JIT support is available, the string contains the name of the architec-
1412 ture for which the JIT compiler is configured, for example "x86 32bit
1413 (little endian + unaligned)". If JIT support is not available, the
1414 result is NULL.
1418 The output is an integer whose value specifies the default character
1419 sequence that is recognized as meaning "newline". The four values that
1420 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
1421 and -1 for ANY. Though they are derived from ASCII, the same values
1422 are returned in EBCDIC environments. The default should normally corre-
1423 spond to the standard sequence for your operating system.
1427 The output is an integer whose value indicates what character sequences
1428 the \R escape sequence matches by default. A value of 0 means that \R
1429 matches any Unicode line ending sequence; a value of 1 means that \R
1430 matches only CR, LF, or CRLF. The default can be overridden when a pat-
1431 tern is compiled or matched.
1435 The output is an integer that contains the number of bytes used for
1436 internal linkage in compiled regular expressions. For the 8-bit
1437 library, the value can be 2, 3, or 4. For the 16-bit library, the value
1438 is either 2 or 4 and is still a number of bytes. The default value of 2
1439 is sufficient for all but the most massive patterns, since it allows
1440 the compiled pattern to be up to 64K in size. Larger values allow
1441 larger regular expressions to be compiled, at the expense of slower
1442 matching.
1446 The output is an integer that contains the threshold above which the
1447 POSIX interface uses malloc() for output vectors. Further details are
1448 given in the pcreposix documentation.
1452 The output is a long integer that gives the default limit for the num-
1453 ber of internal matching function calls in a pcre_exec() execution.
1454 Further details are given with pcre_exec() below.
1458 The output is a long integer that gives the default limit for the depth
1459 of recursion when calling the internal matching function in a
1460 pcre_exec() execution. Further details are given with pcre_exec()
1461 below.
1465 The output is an integer that is set to one if internal recursion when
1466 running pcre_exec() is implemented by recursive function calls that use
1467 the stack to remember their state. This is the usual way that PCRE is
1468 compiled. The output is zero if PCRE was compiled to use blocks of data
1469 on the heap instead of recursive function calls. In this case,
1470 pcre_stack_malloc and pcre_stack_free are called to manage memory
1471 blocks on the heap, thus avoiding the use of the stack.
1476 pcre *pcre_compile(const char *pattern, int options,
1477 const char **errptr, int *erroffset,
1478 const unsigned char *tableptr);
1480 pcre *pcre_compile2(const char *pattern, int options,
1481 int *errorcodeptr,
1482 const char **errptr, int *erroffset,
1483 const unsigned char *tableptr);
1485 Either of the functions pcre_compile() or pcre_compile2() can be called
1486 to compile a pattern into an internal form. The only difference between
1487 the two interfaces is that pcre_compile2() has an additional argument,
1488 errorcodeptr, via which a numerical error code can be returned. To
1489 avoid too much repetition, we refer just to pcre_compile() below, but
1490 the information applies equally to pcre_compile2().
1492 The pattern is a C string terminated by a binary zero, and is passed in
1493 the pattern argument. A pointer to a single block of memory that is
1494 obtained via pcre_malloc is returned. This contains the compiled code
1495 and related data. The pcre type is defined for the returned block; this
1496 is a typedef for a structure whose contents are not externally defined.
1497 It is up to the caller to free the memory (via pcre_free) when it is no
1498 longer required.
1500 Although the compiled code of a PCRE regex is relocatable, that is, it
1501 does not depend on memory location, the complete pcre data block is not
1502 fully relocatable, because it may contain a copy of the tableptr argu-
1503 ment, which is an address (see below).
1505 The options argument contains various bit settings that affect the com-
1506 pilation. It should be zero if no options are required. The available
1507 options are described below. Some of them (in particular, those that
1508 are compatible with Perl, but some others as well) can also be set and
1509 unset from within the pattern (see the detailed description in the
1510 pcrepattern documentation). For those options that can be different in
1511 different parts of the pattern, the contents of the options argument
1512 specifies their settings at the start of compilation and execution. The
1514 PCRE_NO_START_OPTIMIZE options can be set at the time of matching as
1515 well as at compile time.
1517 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1518 if compilation of a pattern fails, pcre_compile() returns NULL, and
1519 sets the variable pointed to by errptr to point to a textual error mes-
1520 sage. This is a static string that is part of the library. You must not
1521 try to free it. Normally, the offset from the start of the pattern to
1522 the byte that was being processed when the error was discovered is
1523 placed in the variable pointed to by erroffset, which must not be NULL
1524 (if it is, an immediate error is given). However, for an invalid UTF-8
1525 string, the offset is that of the first byte of the failing character.
1527 Some errors are not detected until the whole pattern has been scanned;
1528 in these cases, the offset passed back is the length of the pattern.
1529 Note that the offset is in bytes, not characters, even in UTF-8 mode.
1530 It may sometimes point into the middle of a UTF-8 character.
1532 If pcre_compile2() is used instead of pcre_compile(), and the error-
1533 codeptr argument is not NULL, a non-zero error code number is returned
1534 via this argument in the event of an error. This is in addition to the
1535 textual error message. Error codes and messages are listed below.
1537 If the final argument, tableptr, is NULL, PCRE uses a default set of
1538 character tables that are built when PCRE is compiled, using the
1539 default C locale. Otherwise, tableptr must be an address that is the
1540 result of a call to pcre_maketables(). This value is stored with the
1541 compiled pattern, and used again by pcre_exec(), unless another table
1542 pointer is passed to it. For more discussion, see the section on locale
1543 support below.
1545 This code fragment shows a typical straightforward call to pcre_com-
1546 pile():
1548 pcre *re;
1549 const char *error;
1550 int erroffset;
1551 re = pcre_compile(
1552 "^A.*Z", /* the pattern */
1553 0, /* default options */
1554 &error, /* for error message */
1555 &erroffset, /* for error offset */
1556 NULL); /* use default character tables */
1558 The following names for option bits are defined in the pcre.h header
1559 file:
1563 If this bit is set, the pattern is forced to be "anchored", that is, it
1564 is constrained to match only at the first matching point in the string
1565 that is being searched (the "subject string"). This effect can also be
1566 achieved by appropriate constructs in the pattern itself, which is the
1567 only way to do it in Perl.
1571 If this bit is set, pcre_compile() automatically inserts callout items,
1572 all with number 255, before each pattern item. For discussion of the
1573 callout facility, see the pcrecallout documentation.
1578 These options (which are mutually exclusive) control what the \R escape
1579 sequence matches. The choice is either to match only CR, LF, or CRLF,
1580 or to match any Unicode newline sequence. The default is specified when
1581 PCRE is built. It can be overridden from within the pattern, or by set-
1582 ting an option when a compiled pattern is matched.
1586 If this bit is set, letters in the pattern match both upper and lower
1587 case letters. It is equivalent to Perl's /i option, and it can be
1588 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1589 always understands the concept of case for characters whose values are
1590 less than 128, so caseless matching is always possible. For characters
1591 with higher values, the concept of case is supported if PCRE is com-
1592 piled with Unicode property support, but not otherwise. If you want to
1593 use caseless matching for characters 128 and above, you must ensure
1594 that PCRE is compiled with Unicode property support as well as with
1595 UTF-8 support.
1599 If this bit is set, a dollar metacharacter in the pattern matches only
1600 at the end of the subject string. Without this option, a dollar also
1601 matches immediately before a newline at the end of the string (but not
1602 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1603 if PCRE_MULTILINE is set. There is no equivalent to this option in
1604 Perl, and no way to set it within a pattern.
1608 If this bit is set, a dot metacharacter in the pattern matches a char-
1609 acter of any value, including one that indicates a newline. However, it
1610 only ever matches one character, even if newlines are coded as CRLF.
1611 Without this option, a dot does not match when the current position is
1612 at a newline. This option is equivalent to Perl's /s option, and it can
1613 be changed within a pattern by a (?s) option setting. A negative class
1614 such as [^a] always matches newline characters, independent of the set-
1615 ting of this option.
1619 If this bit is set, names used to identify capturing subpatterns need
1620 not be unique. This can be helpful for certain types of pattern when it
1621 is known that only one instance of the named subpattern can ever be
1622 matched. There are more details of named subpatterns below; see also
1623 the pcrepattern documentation.
1627 If this bit is set, whitespace data characters in the pattern are
1628 totally ignored except when escaped or inside a character class. White-
1629 space does not include the VT character (code 11). In addition, charac-
1630 ters between an unescaped # outside a character class and the next new-
1631 line, inclusive, are also ignored. This is equivalent to Perl's /x
1632 option, and it can be changed within a pattern by a (?x) option set-
1633 ting.
1635 Which characters are interpreted as newlines is controlled by the
1636 options passed to pcre_compile() or by a special sequence at the start
1637 of the pattern, as described in the section entitled "Newline conven-
1638 tions" in the pcrepattern documentation. Note that the end of this type
1639 of comment is a literal newline sequence in the pattern; escape
1640 sequences that happen to represent a newline do not count.
1642 This option makes it possible to include comments inside complicated
1643 patterns. Note, however, that this applies only to data characters.
1644 Whitespace characters may never appear within special character
1645 sequences in a pattern, for example within the sequence (?( that intro-
1646 duces a conditional subpattern.
1650 This option was invented in order to turn on additional functionality
1651 of PCRE that is incompatible with Perl, but it is currently of very
1652 little use. When set, any backslash in a pattern that is followed by a
1653 letter that has no special meaning causes an error, thus reserving
1654 these combinations for future expansion. By default, as in Perl, a
1655 backslash followed by a letter with no special meaning is treated as a
1656 literal. (Perl can, however, be persuaded to give an error for this, by
1657 running it with the -w option.) There are at present no other features
1658 controlled by this option. It can also be set by a (?X) option setting
1659 within a pattern.
1663 If this option is set, an unanchored pattern is required to match
1664 before or at the first newline in the subject string, though the
1665 matched text may continue over the newline.
1669 If this option is set, PCRE's behaviour is changed in some ways so that
1670 it is compatible with JavaScript rather than Perl. The changes are as
1671 follows:
1673 (1) A lone closing square bracket in a pattern causes a compile-time
1674 error, because this is illegal in JavaScript (by default it is treated
1675 as a data character). Thus, the pattern AB]CD becomes illegal when this
1676 option is set.
1678 (2) At run time, a back reference to an unset subpattern group matches
1679 an empty string (by default this causes the current matching alterna-
1680 tive to fail). A pattern such as (\1)(a) succeeds when this option is
1681 set (assuming it can find an "a" in the subject), whereas it fails by
1682 default, for Perl compatibility.
1684 (3) \U matches an upper case "U" character; by default \U causes a com-
1685 pile time error (Perl uses \U to upper case subsequent characters).
1687 (4) \u matches a lower case "u" character unless it is followed by four
1688 hexadecimal digits, in which case the hexadecimal number defines the
1689 code point to match. By default, \u causes a compile time error (Perl
1690 uses it to upper case the following character).
1692 (5) \x matches a lower case "x" character unless it is followed by two
1693 hexadecimal digits, in which case the hexadecimal number defines the
1694 code point to match. By default, as in Perl, a hexadecimal number is
1695 always expected after \x, but it may have zero, one, or two digits (so,
1696 for example, \xz matches a binary zero character followed by z).
1700 By default, PCRE treats the subject string as consisting of a single
1701 line of characters (even if it actually contains newlines). The "start
1702 of line" metacharacter (^) matches only at the start of the string,
1703 while the "end of line" metacharacter ($) matches only at the end of
1704 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1705 is set). This is the same as Perl.
1707 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1708 constructs match immediately following or immediately before internal
1709 newlines in the subject string, respectively, as well as at the very
1710 start and end. This is equivalent to Perl's /m option, and it can be
1711 changed within a pattern by a (?m) option setting. If there are no new-
1712 lines in a subject string, or no occurrences of ^ or $ in a pattern,
1713 setting PCRE_MULTILINE has no effect.
1721 These options override the default newline definition that was chosen
1722 when PCRE was built. Setting the first or the second specifies that a
1723 newline is indicated by a single character (CR or LF, respectively).
1724 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1725 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1726 that any of the three preceding sequences should be recognized. Setting
1727 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1728 recognized. The Unicode newline sequences are the three just mentioned,
1729 plus the single characters VT (vertical tab, U+000B), FF (formfeed,
1730 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1731 (paragraph separator, U+2029). For the 8-bit library, the last two are
1732 recognized only in UTF-8 mode.
1734 The newline setting in the options word uses three bits that are
1735 treated as a number, giving eight possibilities. Currently only six are
1736 used (default plus the five values above). This means that if you set
1737 more than one newline option, the combination may or may not be sensi-
1738 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1739 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1740 cause an error.
1742 The only time that a line break in a pattern is specially recognized
1743 when compiling is when PCRE_EXTENDED is set. CR and LF are whitespace
1744 characters, and so are ignored in this mode. Also, an unescaped # out-
1745 side a character class indicates a comment that lasts until after the
1746 next line break sequence. In other circumstances, line break sequences
1747 in patterns are treated as literal data.
1749 The newline option that is set at compile time becomes the default that
1750 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1754 If this option is set, it disables the use of numbered capturing paren-
1755 theses in the pattern. Any opening parenthesis that is not followed by
1756 ? behaves as if it were followed by ?: but named parentheses can still
1757 be used for capturing (and they acquire numbers in the usual way).
1758 There is no equivalent of this option in Perl.
1762 This is an option that acts at matching time; that is, it is really an
1763 option for pcre_exec() or pcre_dfa_exec(). If it is set at compile
1764 time, it is remembered with the compiled pattern and assumed at match-
1765 ing time. For details see the discussion of PCRE_NO_START_OPTIMIZE
1766 below.
1770 This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W,
1771 \w, and some of the POSIX character classes. By default, only ASCII
1772 characters are recognized, but if PCRE_UCP is set, Unicode properties
1773 are used instead to classify characters. More details are given in the
1774 section on generic character types in the pcrepattern page. If you set
1775 PCRE_UCP, matching one of the items it affects takes much longer. The
1776 option is available only if PCRE has been compiled with Unicode prop-
1777 erty support.
1781 This option inverts the "greediness" of the quantifiers so that they
1782 are not greedy by default, but become greedy if followed by "?". It is
1783 not compatible with Perl. It can also be set by a (?U) option setting
1784 within the pattern.
1786 PCRE_UTF8
1788 This option causes PCRE to regard both the pattern and the subject as
1789 strings of UTF-8 characters instead of single-byte strings. However, it
1790 is available only when PCRE is built to include UTF support. If not,
1791 the use of this option provokes an error. Details of how this option
1792 changes the behaviour of PCRE are given in the pcreunicode page.
1796 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1797 automatically checked. There is a discussion about the validity of
1798 UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence is
1799 found, pcre_compile() returns an error. If you already know that your
1800 pattern is valid, and you want to skip this check for performance rea-
1801 sons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the
1802 effect of passing an invalid UTF-8 string as a pattern is undefined. It
1803 may cause your program to crash. Note that this option can also be
1804 passed to pcre_exec() and pcre_dfa_exec(), to suppress the validity
1805 checking of subject strings.
1810 The following table lists the error codes than may be returned by
1811 pcre_compile2(), along with the error messages that may be returned by
1812 both compiling functions. Note that error messages are always 8-bit
1813 ASCII strings, even in 16-bit mode. As PCRE has developed, some error
1814 codes have fallen out of use. To avoid confusion, they have not been
1815 re-used.
1817 0 no error
1818 1 \ at end of pattern
1819 2 \c at end of pattern
1820 3 unrecognized character follows \
1821 4 numbers out of order in {} quantifier
1822 5 number too big in {} quantifier
1823 6 missing terminating ] for character class
1824 7 invalid escape sequence in character class
1825 8 range out of order in character class
1826 9 nothing to repeat
1827 10 [this code is not in use]
1828 11 internal error: unexpected repeat
1829 12 unrecognized character after (? or (?-
1830 13 POSIX named classes are supported only within a class
1831 14 missing )
1832 15 reference to non-existent subpattern
1833 16 erroffset passed as NULL
1834 17 unknown option bit(s) set
1835 18 missing ) after comment
1836 19 [this code is not in use]
1837 20 regular expression is too large
1838 21 failed to get memory
1839 22 unmatched parentheses
1840 23 internal error: code overflow
1841 24 unrecognized character after (?<
1842 25 lookbehind assertion is not fixed length
1843 26 malformed number or name after (?(
1844 27 conditional group contains more than two branches
1845 28 assertion expected after (?(
1846 29 (?R or (?[+-]digits must be followed by )
1847 30 unknown POSIX class name
1848 31 POSIX collating elements are not supported
1849 32 this version of PCRE is compiled without UTF support
1850 33 [this code is not in use]
1851 34 character value in \x{...} sequence is too large
1852 35 invalid condition (?(0)
1853 36 \C not allowed in lookbehind assertion
1854 37 PCRE does not support \L, \l, \N{name}, \U, or \u
1855 38 number after (?C is > 255
1856 39 closing ) for (?C expected
1857 40 recursive call could loop indefinitely
1858 41 unrecognized character after (?P
1859 42 syntax error in subpattern name (missing terminator)
1860 43 two named subpatterns have the same name
1861 44 invalid UTF-8 string (specifically UTF-8)
1862 45 support for \P, \p, and \X has not been compiled
1863 46 malformed \P or \p sequence
1864 47 unknown property name after \P or \p
1865 48 subpattern name is too long (maximum 32 characters)
1866 49 too many named subpatterns (maximum 10000)
1867 50 [this code is not in use]
1868 51 octal value is greater than \377 in 8-bit non-UTF-8 mode
1869 52 internal error: overran compiling workspace
1870 53 internal error: previously-checked referenced subpattern
1871 not found
1872 54 DEFINE group contains more than one branch
1873 55 repeating a DEFINE group is not allowed
1874 56 inconsistent NEWLINE options
1875 57 \g is not followed by a braced, angle-bracketed, or quoted
1876 name/number or by a plain number
1877 58 a numbered reference must not be zero
1878 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
1879 60 (*VERB) not recognized
1880 61 number is too big
1881 62 subpattern name expected
1882 63 digit expected after (?+
1883 64 ] is an invalid data character in JavaScript compatibility mode
1884 65 different names for subpatterns of the same number are
1885 not allowed
1886 66 (*MARK) must have an argument
1887 67 this version of PCRE is not compiled with Unicode property
1888 support
1889 68 \c must be followed by an ASCII character
1890 69 \k is not followed by a braced, angle-bracketed, or quoted name
1891 70 internal error: unknown opcode in find_fixedlength()
1892 71 \N is not supported in a class
1893 72 too many forward references
1894 73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff)
1895 74 invalid UTF-16 string (specifically UTF-16)
1897 The numbers 32 and 10000 in errors 48 and 49 are defaults; different
1898 values may be used if the limits were changed when PCRE was built.
1903 pcre_extra *pcre_study(const pcre *code, int options
1904 const char **errptr);
1906 If a compiled pattern is going to be used several times, it is worth
1907 spending more time analyzing it in order to speed up the time taken for
1908 matching. The function pcre_study() takes a pointer to a compiled pat-
1909 tern as its first argument. If studying the pattern produces additional
1910 information that will help speed up matching, pcre_study() returns a
1911 pointer to a pcre_extra block, in which the study_data field points to
1912 the results of the study.
1914 The returned value from pcre_study() can be passed directly to
1915 pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con-
1916 tains other fields that can be set by the caller before the block is
1917 passed; these are described below in the section on matching a pattern.
1919 If studying the pattern does not produce any useful information,
1920 pcre_study() returns NULL. In that circumstance, if the calling program
1921 wants to pass any of the other fields to pcre_exec() or
1922 pcre_dfa_exec(), it must set up its own pcre_extra block.
1924 The second argument of pcre_study() contains option bits. There are
1925 three options:
1931 If any of these are set, and the just-in-time compiler is available,
1932 the pattern is further compiled into machine code that executes much
1933 faster than the pcre_exec() interpretive matching function. If the
1934 just-in-time compiler is not available, these options are ignored. All
1935 other bits in the options argument must be zero.
1937 JIT compilation is a heavyweight optimization. It can take some time
1938 for patterns to be analyzed, and for one-off matches and simple pat-
1939 terns the benefit of faster execution might be offset by a much slower
1940 study time. Not all patterns can be optimized by the JIT compiler. For
1941 those that cannot be handled, matching automatically falls back to the
1942 pcre_exec() interpreter. For more details, see the pcrejit documenta-
1943 tion.
1945 The third argument for pcre_study() is a pointer for an error message.
1946 If studying succeeds (even if no data is returned), the variable it
1947 points to is set to NULL. Otherwise it is set to point to a textual
1948 error message. This is a static string that is part of the library. You
1949 must not try to free it. You should test the error pointer for NULL
1950 after calling pcre_study(), to be sure that it has run successfully.
1952 When you are finished with a pattern, you can free the memory used for
1953 the study data by calling pcre_free_study(). This function was added to
1954 the API for release 8.20. For earlier versions, the memory could be
1955 freed with pcre_free(), just like the pattern itself. This will still
1956 work in cases where JIT optimization is not used, but it is advisable
1957 to change to the new function when convenient.
1959 This is a typical way in which pcre_study() is used (except that in a
1960 real application there should be tests for errors):
1962 int rc;
1963 pcre *re;
1964 pcre_extra *sd;
1965 re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
1966 sd = pcre_study(
1967 re, /* result of pcre_compile() */
1968 0, /* no options */
1969 &error); /* set to NULL or points to a message */
1970 rc = pcre_exec( /* see below for details of pcre_exec() options */
1971 re, sd, "subject", 7, 0, 0, ovector, 30);
1972 ...
1973 pcre_free_study(sd);
1974 pcre_free(re);
1976 Studying a pattern does two things: first, a lower bound for the length
1977 of subject string that is needed to match the pattern is computed. This
1978 does not mean that there are any strings of that length that match, but
1979 it does guarantee that no shorter strings match. The value is used by
1980 pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to
1981 match strings that are shorter than the lower bound. You can find out
1982 the value in a calling program via the pcre_fullinfo() function.
1984 Studying a pattern is also useful for non-anchored patterns that do not
1985 have a single fixed starting character. A bitmap of possible starting
1986 bytes is created. This speeds up finding a position in the subject at
1987 which to start matching. (In 16-bit mode, the bitmap is used for 16-bit
1988 values less than 256.)
1990 These two optimizations apply to both pcre_exec() and pcre_dfa_exec(),
1991 and the information is also used by the JIT compiler. The optimiza-
1992 tions can be disabled by setting the PCRE_NO_START_OPTIMIZE option when
1993 calling pcre_exec() or pcre_dfa_exec(), but if this is done, JIT execu-
1994 tion is also disabled. You might want to do this if your pattern con-
1995 tains callouts or (*MARK) and you want to make use of these facilities
1996 in cases where matching fails. See the discussion of
2002 PCRE handles caseless matching, and determines whether characters are
2003 letters, digits, or whatever, by reference to a set of tables, indexed
2004 by character value. When running in UTF-8 mode, this applies only to
2005 characters with codes less than 128. By default, higher-valued codes
2006 never match escapes such as \w or \d, but they can be tested with \p if
2007 PCRE is built with Unicode character property support. Alternatively,
2008 the PCRE_UCP option can be set at compile time; this causes \w and
2009 friends to use Unicode property support instead of built-in tables. The
2010 use of locales with Unicode is discouraged. If you are handling charac-
2011 ters with codes greater than 128, you should either use UTF-8 and Uni-
2012 code, or use locales, but not try to mix the two.
2014 PCRE contains an internal set of tables that are used when the final
2015 argument of pcre_compile() is NULL. These are sufficient for many
2016 applications. Normally, the internal tables recognize only ASCII char-
2017 acters. However, when PCRE is built, it is possible to cause the inter-
2018 nal tables to be rebuilt in the default "C" locale of the local system,
2019 which may cause them to be different.
2021 The internal tables can always be overridden by tables supplied by the
2022 application that calls PCRE. These may be created in a different locale
2023 from the default. As more and more applications change to using Uni-
2024 code, the need for this locale support is expected to die away.
2026 External tables are built by calling the pcre_maketables() function,
2027 which has no arguments, in the relevant locale. The result can then be
2028 passed to pcre_compile() or pcre_exec() as often as necessary. For
2029 example, to build and use tables that are appropriate for the French
2030 locale (where accented characters with values greater than 128 are
2031 treated as letters), the following code could be used:
2033 setlocale(LC_CTYPE, "fr_FR");
2034 tables = pcre_maketables();
2035 re = pcre_compile(..., tables);
2037 The locale name "fr_FR" is used on Linux and other Unix-like systems;
2038 if you are using Windows, the name for the French locale is "french".
2040 When pcre_maketables() runs, the tables are built in memory that is
2041 obtained via pcre_malloc. It is the caller's responsibility to ensure
2042 that the memory containing the tables remains available for as long as
2043 it is needed.
2045 The pointer that is passed to pcre_compile() is saved with the compiled
2046 pattern, and the same tables are used via this pointer by pcre_study()
2047 and normally also by pcre_exec(). Thus, by default, for any single pat-
2048 tern, compilation, studying and matching all happen in the same locale,
2049 but different patterns can be compiled in different locales.
2051 It is possible to pass a table pointer or NULL (indicating the use of
2052 the internal tables) to pcre_exec(). Although not intended for this
2053 purpose, this facility could be used to match a pattern in a different
2054 locale from the one in which it was compiled. Passing table pointers at
2055 run time is discussed below in the section on matching a pattern.
2060 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
2061 int what, void *where);
2063 The pcre_fullinfo() function returns information about a compiled pat-
2064 tern. It replaces the pcre_info() function, which was removed from the
2065 library at version 8.30, after more than 10 years of obsolescence.
2067 The first argument for pcre_fullinfo() is a pointer to the compiled
2068 pattern. The second argument is the result of pcre_study(), or NULL if
2069 the pattern was not studied. The third argument specifies which piece
2070 of information is required, and the fourth argument is a pointer to a
2071 variable to receive the data. The yield of the function is zero for
2072 success, or one of the following negative numbers:
2074 PCRE_ERROR_NULL the argument code was NULL
2075 the argument where was NULL
2076 PCRE_ERROR_BADMAGIC the "magic number" was not found
2077 PCRE_ERROR_BADENDIANNESS the pattern was compiled with different
2078 endianness
2079 PCRE_ERROR_BADOPTION the value of what was invalid
2081 The "magic number" is placed at the start of each compiled pattern as
2082 an simple check against passing an arbitrary memory pointer. The endi-
2083 anness error can occur if a compiled pattern is saved and reloaded on a
2084 different host. Here is a typical call of pcre_fullinfo(), to obtain
2085 the length of the compiled pattern:
2087 int rc;
2088 size_t length;
2089 rc = pcre_fullinfo(
2090 re, /* result of pcre_compile() */
2091 sd, /* result of pcre_study(), or NULL */
2092 PCRE_INFO_SIZE, /* what is required */
2093 &length); /* where to put the data */
2095 The possible values for the third argument are defined in pcre.h, and
2096 are as follows:
2100 Return the number of the highest back reference in the pattern. The
2101 fourth argument should point to an int variable. Zero is returned if
2102 there are no back references.
2106 Return the number of capturing subpatterns in the pattern. The fourth
2107 argument should point to an int variable.
2111 Return a pointer to the internal default character tables within PCRE.
2112 The fourth argument should point to an unsigned char * variable. This
2113 information call is provided for internal use by the pcre_study() func-
2114 tion. External callers can cause PCRE to use its internal tables by
2115 passing a NULL table pointer.
2119 Return information about the first data unit of any matched string, for
2120 a non-anchored pattern. (The name of this option refers to the 8-bit
2121 library, where data units are bytes.) The fourth argument should point
2122 to an int variable.
2124 If there is a fixed first value, for example, the letter "c" from a
2125 pattern such as (cat|cow|coyote), its value is returned. In the 8-bit
2126 library, the value is always less than 256; in the 16-bit library the
2127 value can be up to 0xffff.
2129 If there is no fixed first value, and if either
2131 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
2132 branch starts with "^", or
2134 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
2135 set (if it were set, the pattern would be anchored),
2137 -1 is returned, indicating that the pattern matches only at the start
2138 of a subject string or after any newline within the string. Otherwise
2139 -2 is returned. For anchored patterns, -2 is returned.
2143 If the pattern was studied, and this resulted in the construction of a
2144 256-bit table indicating a fixed set of values for the first data unit
2145 in any matching string, a pointer to the table is returned. Otherwise
2146 NULL is returned. The fourth argument should point to an unsigned char
2147 * variable.
2151 Return 1 if the pattern contains any explicit matches for CR or LF
2152 characters, otherwise 0. The fourth argument should point to an int
2153 variable. An explicit match is either a literal CR or LF character, or
2154 \r or \n.
2158 Return 1 if the (?J) or (?-J) option setting is used in the pattern,
2159 otherwise 0. The fourth argument should point to an int variable. (?J)
2160 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
2164 Return 1 if the pattern was studied with one of the JIT options, and
2165 just-in-time compiling was successful. The fourth argument should point
2166 to an int variable. A return value of 0 means that JIT support is not
2167 available in this version of PCRE, or that the pattern was not studied
2168 with a JIT option, or that the JIT compiler could not handle this par-
2169 ticular pattern. See the pcrejit documentation for details of what can
2170 and cannot be handled.
2174 If the pattern was successfully studied with a JIT option, return the
2175 size of the JIT compiled code, otherwise return zero. The fourth argu-
2176 ment should point to a size_t variable.
2180 Return the value of the rightmost literal data unit that must exist in
2181 any matched string, other than at its start, if such a value has been
2182 recorded. The fourth argument should point to an int variable. If there
2183 is no such value, -1 is returned. For anchored patterns, a last literal
2184 value is recorded only if it follows something of variable length. For
2185 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
2186 /^a\dz\d/ the returned value is -1.
2190 Return the number of characters (NB not bytes) in the longest lookbe-
2191 hind assertion in the pattern. Note that the simple assertions \b and
2192 \B require a one-character lookbehind. This information is useful when
2193 doing multi-segment matching using the partial matching facilities.
2197 If the pattern was studied and a minimum length for matching subject
2198 strings was computed, its value is returned. Otherwise the returned
2199 value is -1. The value is a number of characters, which in UTF-8 mode
2200 may be different from the number of bytes. The fourth argument should
2201 point to an int variable. A non-negative value is a lower bound to the
2202 length of any matching string. There may not be any strings of that
2203 length that do actually match, but every string that does match is at
2204 least that long.
2210 PCRE supports the use of named as well as numbered capturing parenthe-
2211 ses. The names are just an additional way of identifying the parenthe-
2212 ses, which still acquire numbers. Several convenience functions such as
2213 pcre_get_named_substring() are provided for extracting captured sub-
2214 strings by name. It is also possible to extract the data directly, by
2215 first converting the name to a number in order to access the correct
2216 pointers in the output vector (described with pcre_exec() below). To do
2217 the conversion, you need to use the name-to-number map, which is
2218 described by these three values.
2220 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
2221 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
2222 of each entry; both of these return an int value. The entry size
2223 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
2224 a pointer to the first entry of the table. This is a pointer to char in
2225 the 8-bit library, where the first two bytes of each entry are the num-
2226 ber of the capturing parenthesis, most significant byte first. In the
2227 16-bit library, the pointer points to 16-bit data units, the first of
2228 which contains the parenthesis number. The rest of the entry is the
2229 corresponding name, zero terminated.
2231 The names are in alphabetical order. Duplicate names may appear if (?|
2232 is used to create multiple groups with the same number, as described in
2233 the section on duplicate subpattern numbers in the pcrepattern page.
2234 Duplicate names for subpatterns with different numbers are permitted
2235 only if PCRE_DUPNAMES is set. In all cases of duplicate names, they
2236 appear in the table in the order in which they were found in the pat-
2237 tern. In the absence of (?| this is the order of increasing number;
2238 when (?| is used this is not necessarily the case because later subpat-
2239 terns may have lower numbers.
2241 As a simple example of the name/number table, consider the following
2242 pattern after compilation by the 8-bit library (assume PCRE_EXTENDED is
2243 set, so white space - including newlines - is ignored):
2245 (?<date> (?<year>(\d\d)?\d\d) -
2246 (?<month>\d\d) - (?<day>\d\d) )
2248 There are four named subpatterns, so the table has four entries, and
2249 each entry in the table is eight bytes long. The table is as follows,
2250 with non-printing bytes shows in hexadecimal, and undefined bytes shown
2251 as ??:
2253 00 01 d a t e 00 ??
2254 00 05 d a y 00 ?? ??
2255 00 04 m o n t h 00
2256 00 02 y e a r 00 ??
2258 When writing code to extract data from named subpatterns using the
2259 name-to-number map, remember that the length of the entries is likely
2260 to be different for each compiled pattern.
2264 Return 1 if the pattern can be used for partial matching with
2265 pcre_exec(), otherwise 0. The fourth argument should point to an int
2266 variable. From release 8.00, this always returns 1, because the
2267 restrictions that previously applied to partial matching have been
2268 lifted. The pcrepartial documentation gives details of partial match-
2269 ing.
2273 Return a copy of the options with which the pattern was compiled. The
2274 fourth argument should point to an unsigned long int variable. These
2275 option bits are those specified in the call to pcre_compile(), modified
2276 by any top-level option settings at the start of the pattern itself. In
2277 other words, they are the options that will be in force when matching
2278 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
2279 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
2282 A pattern is automatically anchored by PCRE if all of its top-level
2283 alternatives begin with one of the following:
2285 ^ unless PCRE_MULTILINE is set
2286 \A always
2287 \G always
2288 .* if PCRE_DOTALL is set and there are no back
2289 references to the subpattern in which .* appears
2291 For such patterns, the PCRE_ANCHORED bit is set in the options returned
2292 by pcre_fullinfo().
2296 Return the size of the compiled pattern in bytes (for both libraries).
2297 The fourth argument should point to a size_t variable. This value does
2298 not include the size of the pcre structure that is returned by
2299 pcre_compile(). The value that is passed as the argument to pcre_mal-
2300 loc() when pcre_compile() is getting memory in which to place the com-
2301 piled data is the value returned by this option plus the size of the
2302 pcre structure. Studying a compiled pattern, with or without JIT, does
2303 not alter the value returned by this option.
2307 Return the size in bytes of the data block pointed to by the study_data
2308 field in a pcre_extra block. If pcre_extra is NULL, or there is no
2309 study data, zero is returned. The fourth argument should point to a
2310 size_t variable. The study_data field is set by pcre_study() to record
2311 information that will speed up matching (see the section entitled
2312 "Studying a pattern" above). The format of the study_data block is pri-
2313 vate, but its length is made available via this option so that it can
2314 be saved and restored (see the pcreprecompile documentation for
2315 details).
2320 int pcre_refcount(pcre *code, int adjust);
2322 The pcre_refcount() function is used to maintain a reference count in
2323 the data block that contains a compiled pattern. It is provided for the
2324 benefit of applications that operate in an object-oriented manner,
2325 where different parts of the application may be using the same compiled
2326 pattern, but you want to free the block when they are all done.
2328 When a pattern is compiled, the reference count field is initialized to
2329 zero. It is changed only by calling this function, whose action is to
2330 add the adjust value (which may be positive or negative) to it. The
2331 yield of the function is the new value. However, the value of the count
2332 is constrained to lie between 0 and 65535, inclusive. If the new value
2333 is outside these limits, it is forced to the appropriate limit value.
2335 Except when it is zero, the reference count is not correctly preserved
2336 if a pattern is compiled on one host and then transferred to a host
2337 whose byte-order is different. (This seems a highly unlikely scenario.)
2342 int pcre_exec(const pcre *code, const pcre_extra *extra,
2343 const char *subject, int length, int startoffset,
2344 int options, int *ovector, int ovecsize);
2346 The function pcre_exec() is called to match a subject string against a
2347 compiled pattern, which is passed in the code argument. If the pattern
2348 was studied, the result of the study should be passed in the extra
2349 argument. You can call pcre_exec() with the same code and extra argu-
2350 ments as many times as you like, in order to match different subject
2351 strings with the same pattern.
2353 This function is the main matching facility of the library, and it
2354 operates in a Perl-like manner. For specialist use there is also an
2355 alternative matching function, which is described below in the section
2356 about the pcre_dfa_exec() function.
2358 In most applications, the pattern will have been compiled (and option-
2359 ally studied) in the same process that calls pcre_exec(). However, it
2360 is possible to save compiled patterns and study data, and then use them
2361 later in different processes, possibly even on different hosts. For a
2362 discussion about this, see the pcreprecompile documentation.
2364 Here is an example of a simple call to pcre_exec():
2366 int rc;
2367 int ovector[30];
2368 rc = pcre_exec(
2369 re, /* result of pcre_compile() */
2370 NULL, /* we didn't study the pattern */
2371 "some string", /* the subject string */
2372 11, /* the length of the subject string */
2373 0, /* start at offset 0 in the subject */
2374 0, /* default options */
2375 ovector, /* vector of integers for substring information */
2376 30); /* number of elements (NOT size in bytes) */
2378 Extra data for pcre_exec()
2380 If the extra argument is not NULL, it must point to a pcre_extra data
2381 block. The pcre_study() function returns such a block (when it doesn't
2382 return NULL), but you can also create one for yourself, and pass addi-
2383 tional information in it. The pcre_extra block contains the following
2384 fields (not necessarily in this order):
2386 unsigned long int flags;
2387 void *study_data;
2388 void *executable_jit;
2389 unsigned long int match_limit;
2390 unsigned long int match_limit_recursion;
2391 void *callout_data;
2392 const unsigned char *tables;
2393 unsigned char **mark;
2395 In the 16-bit version of this structure, the mark field has type
2396 "PCRE_UCHAR16 **".
2398 The flags field is used to specify which of the other fields are set.
2399 The flag bits are:
2409 Other flag bits should be set to zero. The study_data field and some-
2410 times the executable_jit field are set in the pcre_extra block that is
2411 returned by pcre_study(), together with the appropriate flag bits. You
2412 should not set these yourself, but you may add to the block by setting
2413 other fields and their corresponding flag bits.
2415 The match_limit field provides a means of preventing PCRE from using up
2416 a vast amount of resources when running patterns that are not going to
2417 match, but which have a very large number of possibilities in their
2418 search trees. The classic example is a pattern that uses nested unlim-
2419 ited repeats.
2421 Internally, pcre_exec() uses a function called match(), which it calls
2422 repeatedly (sometimes recursively). The limit set by match_limit is
2423 imposed on the number of times this function is called during a match,
2424 which has the effect of limiting the amount of backtracking that can
2425 take place. For patterns that are not anchored, the count restarts from
2426 zero for each position in the subject string.
2428 When pcre_exec() is called with a pattern that was successfully studied
2429 with a JIT option, the way that the matching is executed is entirely
2430 different. However, there is still the possibility of runaway matching
2431 that goes on for a very long time, and so the match_limit value is also
2432 used in this case (but in a different way) to limit how long the match-
2433 ing can continue.
2435 The default value for the limit can be set when PCRE is built; the
2436 default default is 10 million, which handles all but the most extreme
2437 cases. You can override the default by suppling pcre_exec() with a
2438 pcre_extra block in which match_limit is set, and
2439 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
2440 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
2442 The match_limit_recursion field is similar to match_limit, but instead
2443 of limiting the total number of times that match() is called, it limits
2444 the depth of recursion. The recursion depth is a smaller number than
2445 the total number of calls, because not all calls to match() are recur-
2446 sive. This limit is of use only if it is set smaller than match_limit.
2448 Limiting the recursion depth limits the amount of machine stack that
2449 can be used, or, when PCRE has been compiled to use memory on the heap
2450 instead of the stack, the amount of heap memory that can be used. This
2451 limit is not relevant, and is ignored, when matching is done using JIT
2452 compiled code.
2454 The default value for match_limit_recursion can be set when PCRE is
2455 built; the default default is the same value as the default for
2456 match_limit. You can override the default by suppling pcre_exec() with
2457 a pcre_extra block in which match_limit_recursion is set, and
2458 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
2459 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
2461 The callout_data field is used in conjunction with the "callout" fea-
2462 ture, and is described in the pcrecallout documentation.
2464 The tables field is used to pass a character tables pointer to
2465 pcre_exec(); this overrides the value that is stored with the compiled
2466 pattern. A non-NULL value is stored with the compiled pattern only if
2467 custom tables were supplied to pcre_compile() via its tableptr argu-
2468 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
2469 PCRE's internal tables to be used. This facility is helpful when re-
2470 using patterns that have been saved after compiling with an external
2471 set of tables, because the external tables might be at a different
2472 address when pcre_exec() is called. See the pcreprecompile documenta-
2473 tion for a discussion of saving compiled patterns for later use.
2475 If PCRE_EXTRA_MARK is set in the flags field, the mark field must be
2476 set to point to a suitable variable. If the pattern contains any back-
2477 tracking control verbs such as (*MARK:NAME), and the execution ends up
2478 with a name to pass back, a pointer to the name string (zero termi-
2479 nated) is placed in the variable pointed to by the mark field. The
2480 names are within the compiled pattern; if you wish to retain such a
2481 name you must copy it before freeing the memory of a compiled pattern.
2482 If there is no name to pass back, the variable pointed to by the mark
2483 field is set to NULL. For details of the backtracking control verbs,
2484 see the section entitled "Backtracking control" in the pcrepattern doc-
2485 umentation.
2487 Option bits for pcre_exec()
2489 The unused bits of the options argument for pcre_exec() must be zero.
2490 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
2495 If the pattern was successfully studied with one of the just-in-time
2496 (JIT) compile options, the only supported options for JIT execution are
2499 unsupported option is used, JIT execution is disabled and the normal
2500 interpretive code in pcre_exec() is run.
2504 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
2505 matching position. If a pattern was compiled with PCRE_ANCHORED, or
2506 turned out to be anchored by virtue of its contents, it cannot be made
2507 unachored at matching time.
2512 These options (which are mutually exclusive) control what the \R escape
2513 sequence matches. The choice is either to match only CR, LF, or CRLF,
2514 or to match any Unicode newline sequence. These options override the
2515 choice that was made or defaulted when the pattern was compiled.
2523 These options override the newline definition that was chosen or
2524 defaulted when the pattern was compiled. For details, see the descrip-
2525 tion of pcre_compile() above. During matching, the newline choice
2526 affects the behaviour of the dot, circumflex, and dollar metacharac-
2527 ters. It may also alter the way the match position is advanced after a
2528 match failure for an unanchored pattern.
2531 set, and a match attempt for an unanchored pattern fails when the cur-
2532 rent position is at a CRLF sequence, and the pattern contains no
2533 explicit matches for CR or LF characters, the match position is
2534 advanced by two characters instead of one, in other words, to after the
2535 CRLF.
2537 The above rule is a compromise that makes the most common cases work as
2538 expected. For example, if the pattern is .+A (and the PCRE_DOTALL
2539 option is not set), it does not match the string "\r\nA" because, after
2540 failing at the start, it skips both the CR and the LF before retrying.
2541 However, the pattern [\r\n]A does match that string, because it con-
2542 tains an explicit CR or LF reference, and so advances only by one char-
2543 acter after the first failure.
2545 An explicit match for CR of LF is either a literal appearance of one of
2546 those characters, or one of the \r or \n escape sequences. Implicit
2547 matches such as [^X] do not count, nor does \s (which includes CR and
2548 LF in the characters that it matches).
2550 Notwithstanding the above, anomalous effects may still occur when CRLF
2551 is a valid newline sequence and explicit \r or \n escapes appear in the
2552 pattern.
2556 This option specifies that first character of the subject string is not
2557 the beginning of a line, so the circumflex metacharacter should not
2558 match before it. Setting this without PCRE_MULTILINE (at compile time)
2559 causes circumflex never to match. This option affects only the behav-
2560 iour of the circumflex metacharacter. It does not affect \A.
2564 This option specifies that the end of the subject string is not the end
2565 of a line, so the dollar metacharacter should not match it nor (except
2566 in multiline mode) a newline immediately before it. Setting this with-
2567 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
2568 option affects only the behaviour of the dollar metacharacter. It does
2569 not affect \Z or \z.
2573 An empty string is not considered to be a valid match if this option is
2574 set. If there are alternatives in the pattern, they are tried. If all
2575 the alternatives match the empty string, the entire match fails. For
2576 example, if the pattern
2578 a?b?
2580 is applied to a string not beginning with "a" or "b", it matches an
2581 empty string at the start of the subject. With PCRE_NOTEMPTY set, this
2582 match is not valid, so PCRE searches further into the string for occur-
2583 rences of "a" or "b".
2587 This is like PCRE_NOTEMPTY, except that an empty string match that is
2588 not at the start of the subject is permitted. If the pattern is
2589 anchored, such a match can occur only if the pattern contains \K.
2591 Perl has no direct equivalent of PCRE_NOTEMPTY or
2592 PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern
2593 match of the empty string within its split() function, and when using
2594 the /g modifier. It is possible to emulate Perl's behaviour after
2595 matching a null string by first trying the match again at the same off-
2596 set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that
2597 fails, by advancing the starting offset (see below) and trying an ordi-
2598 nary match again. There is some code that demonstrates how to do this
2599 in the pcredemo sample program. In the most general case, you have to
2600 check to see if the newline convention recognizes CRLF as a newline,
2601 and if so, and the current character is CR followed by LF, advance the
2602 starting offset by two characters instead of one.
2606 There are a number of optimizations that pcre_exec() uses at the start
2607 of a match, in order to speed up the process. For example, if it is
2608 known that an unanchored match must start with a specific character, it
2609 searches the subject for that character, and fails immediately if it
2610 cannot find it, without actually running the main matching function.
2611 This means that a special item such as (*COMMIT) at the start of a pat-
2612 tern is not considered until after a suitable starting point for the
2613 match has been found. When callouts or (*MARK) items are in use, these
2614 "start-up" optimizations can cause them to be skipped if the pattern is
2615 never actually used. The start-up optimizations are in effect a pre-
2616 scan of the subject that takes place before the pattern is run.
2618 The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations,
2619 possibly causing performance to suffer, but ensuring that in cases
2620 where the result is "no match", the callouts do occur, and that items
2621 such as (*COMMIT) and (*MARK) are considered at every possible starting
2622 position in the subject string. If PCRE_NO_START_OPTIMIZE is set at
2623 compile time, it cannot be unset at matching time. The use of
2624 PCRE_NO_START_OPTIMIZE disables JIT execution; when it is set, matching
2625 is always done using interpretively.
2627 Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching
2628 operation. Consider the pattern
2632 When this is compiled, PCRE records the fact that a match must start
2633 with the character "A". Suppose the subject string is "DEFABC". The
2634 start-up optimization scans along the subject, finds "A" and runs the
2635 first match attempt from there. The (*COMMIT) item means that the pat-
2636 tern must match the current starting position, which in this case, it
2637 does. However, if the same match is run with PCRE_NO_START_OPTIMIZE
2638 set, the initial scan along the subject string does not happen. The
2639 first match attempt is run starting from "D" and when this fails,
2640 (*COMMIT) prevents any further matches being tried, so the overall
2641 result is "no match". If the pattern is studied, more start-up opti-
2642 mizations may be used. For example, a minimum length for the subject
2643 may be recorded. Consider the pattern
2645 (*MARK:A)(X|Y)
2647 The minimum length for a match is one character. If the subject is
2648 "ABC", there will be attempts to match "ABC", "BC", "C", and then
2649 finally an empty string. If the pattern is studied, the final attempt
2650 does not take place, because PCRE knows that the subject is too short,
2651 and so the (*MARK) is never encountered. In this case, studying the
2652 pattern does not affect the overall match result, which is still "no
2653 match", but it does affect the auxiliary information that is returned.
2657 When PCRE_UTF8 is set at compile time, the validity of the subject as a
2658 UTF-8 string is automatically checked when pcre_exec() is subsequently
2659 called. The value of startoffset is also checked to ensure that it
2660 points to the start of a UTF-8 character. There is a discussion about
2661 the validity of UTF-8 strings in the pcreunicode page. If an invalid
2662 sequence of bytes is found, pcre_exec() returns the error
2663 PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the problem is a
2664 truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8. In
2665 both cases, information about the precise nature of the error may also
2666 be returned (see the descriptions of these errors in the section enti-
2667 tled Error return values from pcre_exec() below). If startoffset con-
2668 tains a value that does not point to the start of a UTF-8 character (or
2669 to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned.
2671 If you already know that your subject is valid, and you want to skip
2672 these checks for performance reasons, you can set the
2673 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
2674 do this for the second and subsequent calls to pcre_exec() if you are
2675 making repeated calls to find all the matches in a single subject
2676 string. However, you should be sure that the value of startoffset
2677 points to the start of a character (or the end of the subject). When
2678 PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid string as a
2679 subject or an invalid value of startoffset is undefined. Your program
2680 may crash.
2685 These options turn on the partial matching feature. For backwards com-
2686 patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial
2687 match occurs if the end of the subject string is reached successfully,
2688 but there are not enough subject characters to complete the match. If
2689 this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set,
2690 matching continues by testing any remaining alternatives. Only if no
2691 complete match can be found is PCRE_ERROR_PARTIAL returned instead of
2692 PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the
2693 caller is prepared to handle a partial match, but only if no complete
2694 match can be found.
2696 If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this
2697 case, if a partial match is found, pcre_exec() immediately returns
2698 PCRE_ERROR_PARTIAL, without considering any other alternatives. In
2699 other words, when PCRE_PARTIAL_HARD is set, a partial match is consid-
2700 ered to be more important that an alternative complete match.
2702 In both cases, the portion of the string that was inspected when the
2703 partial match was found is set as the first matching string. There is a
2704 more detailed discussion of partial and multi-segment matching, with
2705 examples, in the pcrepartial documentation.
2707 The string to be matched by pcre_exec()
2709 The subject string is passed to pcre_exec() as a pointer in subject, a
2710 length in bytes in length, and a starting byte offset in startoffset.
2711 If this is negative or greater than the length of the subject,
2712 pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is
2713 zero, the search for a match starts at the beginning of the subject,
2714 and this is by far the most common case. In UTF-8 mode, the byte offset
2715 must point to the start of a UTF-8 character (or the end of the sub-
2716 ject). Unlike the pattern string, the subject may contain binary zero
2717 bytes.
2719 A non-zero starting offset is useful when searching for another match
2720 in the same subject by calling pcre_exec() again after a previous suc-
2721 cess. Setting startoffset differs from just passing over a shortened
2722 string and setting PCRE_NOTBOL in the case of a pattern that begins
2723 with any kind of lookbehind. For example, consider the pattern
2725 \Biss\B
2727 which finds occurrences of "iss" in the middle of words. (\B matches
2728 only if the current position in the subject is not a word boundary.)
2729 When applied to the string "Mississipi" the first call to pcre_exec()
2730 finds the first occurrence. If pcre_exec() is called again with just
2731 the remainder of the subject, namely "issipi", it does not match,
2732 because \B is always false at the start of the subject, which is deemed
2733 to be a word boundary. However, if pcre_exec() is passed the entire
2734 string again, but with startoffset set to 4, it finds the second occur-
2735 rence of "iss" because it is able to look behind the starting point to
2736 discover that it is preceded by a letter.
2738 Finding all the matches in a subject is tricky when the pattern can
2739 match an empty string. It is possible to emulate Perl's /g behaviour by
2740 first trying the match again at the same offset, with the
2741 PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that
2742 fails, advancing the starting offset and trying an ordinary match
2743 again. There is some code that demonstrates how to do this in the pcre-
2744 demo sample program. In the most general case, you have to check to see
2745 if the newline convention recognizes CRLF as a newline, and if so, and
2746 the current character is CR followed by LF, advance the starting offset
2747 by two characters instead of one.
2749 If a non-zero starting offset is passed when the pattern is anchored,
2750 one attempt to match at the given offset is made. This can only succeed
2751 if the pattern does not require the match to be at the start of the
2752 subject.
2754 How pcre_exec() returns captured substrings
2756 In general, a pattern matches a certain portion of the subject, and in
2757 addition, further substrings from the subject may be picked out by
2758 parts of the pattern. Following the usage in Jeffrey Friedl's book,
2759 this is called "capturing" in what follows, and the phrase "capturing
2760 subpattern" is used for a fragment of a pattern that picks out a sub-
2761 string. PCRE supports several other kinds of parenthesized subpattern
2762 that do not cause substrings to be captured.
2764 Captured substrings are returned to the caller via a vector of integers
2765 whose address is passed in ovector. The number of elements in the vec-
2766 tor is passed in ovecsize, which must be a non-negative number. Note:
2767 this argument is NOT the size of ovector in bytes.
2769 The first two-thirds of the vector is used to pass back captured sub-
2770 strings, each substring using a pair of integers. The remaining third
2771 of the vector is used as workspace by pcre_exec() while matching cap-
2772 turing subpatterns, and is not available for passing back information.
2773 The number passed in ovecsize should always be a multiple of three. If
2774 it is not, it is rounded down.
2776 When a match is successful, information about captured substrings is
2777 returned in pairs of integers, starting at the beginning of ovector,
2778 and continuing up to two-thirds of its length at the most. The first
2779 element of each pair is set to the byte offset of the first character
2780 in a substring, and the second is set to the byte offset of the first
2781 character after the end of a substring. Note: these values are always
2782 byte offsets, even in UTF-8 mode. They are not character counts.
2784 The first pair of integers, ovector[0] and ovector[1], identify the
2785 portion of the subject string matched by the entire pattern. The next
2786 pair is used for the first capturing subpattern, and so on. The value
2787 returned by pcre_exec() is one more than the highest numbered pair that
2788 has been set. For example, if two substrings have been captured, the
2789 returned value is 3. If there are no capturing subpatterns, the return
2790 value from a successful match is 1, indicating that just the first pair
2791 of offsets has been set.
2793 If a capturing subpattern is matched repeatedly, it is the last portion
2794 of the string that it matched that is returned.
2796 If the vector is too small to hold all the captured substring offsets,
2797 it is used as far as possible (up to two-thirds of its length), and the
2798 function returns a value of zero. If neither the actual string matched
2799 nor any captured substrings are of interest, pcre_exec() may be called
2800 with ovector passed as NULL and ovecsize as zero. However, if the pat-
2801 tern contains back references and the ovector is not big enough to
2802 remember the related substrings, PCRE has to get additional memory for
2803 use during matching. Thus it is usually advisable to supply an ovector
2804 of reasonable size.
2806 There are some cases where zero is returned (indicating vector over-
2807 flow) when in fact the vector is exactly the right size for the final
2808 match. For example, consider the pattern
2810 (a)(?:(b)c|bd)
2812 If a vector of 6 elements (allowing for only 1 captured substring) is
2813 given with subject string "abd", pcre_exec() will try to set the second
2814 captured string, thereby recording a vector overflow, before failing to
2815 match "c" and backing up to try the second alternative. The zero
2816 return, however, does correctly indicate that the maximum number of
2817 slots (namely 2) have been filled. In similar cases where there is tem-
2818 porary overflow, but the final number of used slots is actually less
2819 than the maximum, a non-zero value is returned.
2821 The pcre_fullinfo() function can be used to find out how many capturing
2822 subpatterns there are in a compiled pattern. The smallest size for
2823 ovector that will allow for n captured substrings, in addition to the
2824 offsets of the substring matched by the whole pattern, is (n+1)*3.
2826 It is possible for capturing subpattern number n+1 to match some part
2827 of the subject when subpattern n has not been used at all. For example,
2828 if the string "abc" is matched against the pattern (a|(z))(bc) the
2829 return from the function is 4, and subpatterns 1 and 3 are matched, but
2830 2 is not. When this happens, both values in the offset pairs corre-
2831 sponding to unused subpatterns are set to -1.
2833 Offset values that correspond to unused subpatterns at the end of the
2834 expression are also set to -1. For example, if the string "abc" is
2835 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2836 matched. The return from the function is 2, because the highest used
2837 capturing subpattern number is 1, and the offsets for for the second
2838 and third capturing subpatterns (assuming the vector is large enough,
2839 of course) are set to -1.
2841 Note: Elements in the first two-thirds of ovector that do not corre-
2842 spond to capturing parentheses in the pattern are never changed. That
2843 is, if a pattern contains n capturing parentheses, no more than ovec-
2844 tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in
2845 the first two-thirds) retain whatever values they previously had.
2847 Some convenience functions are provided for extracting the captured
2848 substrings as separate strings. These are described below.
2850 Error return values from pcre_exec()
2852 If pcre_exec() fails, it returns a negative number. The following are
2853 defined in the header file:
2857 The subject string did not match the pattern.
2861 Either code or subject was passed as NULL, or ovector was NULL and
2862 ovecsize was not zero.
2866 An unrecognized bit was set in the options argument.
2870 PCRE stores a 4-byte "magic number" at the start of the compiled code,
2871 to catch the case when it is passed a junk pointer and to detect when a
2872 pattern that was compiled in an environment of one endianness is run in
2873 an environment with the other endianness. This is the error that PCRE
2874 gives when the magic number is not present.
2878 While running the pattern match, an unknown item was encountered in the
2879 compiled pattern. This error could be caused by a bug in PCRE or by
2880 overwriting of the compiled pattern.
2884 If a pattern contains back references, but the ovector that is passed
2885 to pcre_exec() is not big enough to remember the referenced substrings,
2886 PCRE gets a block of memory at the start of matching to use for this
2887 purpose. If the call via pcre_malloc() fails, this error is given. The
2888 memory is automatically freed at the end of matching.
2890 This error is also given if pcre_stack_malloc() fails in pcre_exec().
2891 This can happen only when PCRE has been compiled with --disable-stack-
2892 for-recursion.
2896 This error is used by the pcre_copy_substring(), pcre_get_substring(),
2897 and pcre_get_substring_list() functions (see below). It is never
2898 returned by pcre_exec().
2902 The backtracking limit, as specified by the match_limit field in a
2903 pcre_extra structure (or defaulted) was reached. See the description
2904 above.
2908 This error is never generated by pcre_exec() itself. It is provided for
2909 use by callout functions that want to yield a distinctive error code.
2910 See the pcrecallout documentation for details.
2914 A string that contains an invalid UTF-8 byte sequence was passed as a
2915 subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of
2916 the output vector (ovecsize) is at least 2, the byte offset to the
2917 start of the the invalid UTF-8 character is placed in the first ele-
2918 ment, and a reason code is placed in the second element. The reason
2919 codes are listed in the following section. For backward compatibility,
2920 if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char-
2921 acter at the end of the subject (reason codes 1 to 5),
2922 PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8.
2926 The UTF-8 byte sequence that was passed as a subject was checked and
2927 found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the
2928 value of startoffset did not point to the beginning of a UTF-8 charac-
2929 ter or the end of the subject.
2933 The subject string did not match, but it did match partially. See the
2934 pcrepartial documentation for details of partial matching.
2938 This code is no longer in use. It was formerly returned when the
2939 PCRE_PARTIAL option was used with a compiled pattern containing items
2940 that were not supported for partial matching. From release 8.00
2941 onwards, there are no restrictions on partial matching.
2945 An unexpected internal error has occurred. This error could be caused
2946 by a bug in PCRE or by overwriting of the compiled pattern.
2950 This error is given if the value of the ovecsize argument is negative.
2954 The internal recursion limit, as specified by the match_limit_recursion
2955 field in a pcre_extra structure (or defaulted) was reached. See the
2956 description above.
2960 An invalid combination of PCRE_NEWLINE_xxx options was given.
2964 The value of startoffset was negative or greater than the length of the
2965 subject, that is, the value in length.
2969 This error is returned instead of PCRE_ERROR_BADUTF8 when the subject
2970 string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD
2971 option is set. Information about the failure is returned as for
2972 PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but
2973 this special error code for PCRE_PARTIAL_HARD precedes the implementa-
2974 tion of returned information; it is retained for backwards compatibil-
2975 ity.
2979 This error is returned when pcre_exec() detects a recursion loop within
2980 the pattern. Specifically, it means that either the whole pattern or a
2981 subpattern has been called recursively for the second time at the same
2982 position in the subject string. Some simple patterns that might do this
2983 are detected and faulted at compile time, but more complicated cases,
2984 in particular mutual recursions between two different subpatterns, can-
2985 not be detected until run time.
2989 This error is returned when a pattern that was successfully studied
2990 using a JIT compile option is being matched, but the memory available
2991 for the just-in-time processing stack is not large enough. See the
2992 pcrejit documentation for more details.
2996 This error is given if a pattern that was compiled by the 8-bit library
2997 is passed to a 16-bit library function, or vice versa.
3001 This error is given if a pattern that was compiled and saved is
3002 reloaded on a host with different endianness. The utility function
3003 pcre_pattern_to_host_byte_order() can be used to convert such a pattern
3004 so that it runs on the new host.
3006 Error numbers -16 to -20 and -22 are not used by pcre_exec().
3008 Reason codes for invalid UTF-8 strings
3010 This section applies only to the 8-bit library. The corresponding
3011 information for the 16-bit library is given in the pcre16 page.
3013 When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT-
3014 UTF8, and the size of the output vector (ovecsize) is at least 2, the
3015 offset of the start of the invalid UTF-8 character is placed in the
3016 first output vector element (ovector[0]) and a reason code is placed in
3017 the second element (ovector[1]). The reason codes are given names in
3018 the pcre.h header file:
3026 The string ends with a truncated UTF-8 character; the code specifies
3027 how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8
3028 characters to be no longer than 4 bytes, the encoding scheme (origi-
3029 nally defined by RFC 2279) allows for up to 6 bytes, and this is
3030 checked first; hence the possibility of 4 or 5 missing bytes.
3036 PCRE_UTF8_ERR10
3038 The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
3039 the character do not have the binary value 0b10 (that is, either the
3040 most significant bit is 0, or the next bit is 1).
3042 PCRE_UTF8_ERR11
3043 PCRE_UTF8_ERR12
3045 A character that is valid by the RFC 2279 rules is either 5 or 6 bytes
3046 long; these code points are excluded by RFC 3629.
3048 PCRE_UTF8_ERR13
3050 A 4-byte character has a value greater than 0x10fff; these code points
3051 are excluded by RFC 3629.
3053 PCRE_UTF8_ERR14
3055 A 3-byte character has a value in the range 0xd800 to 0xdfff; this
3056 range of code points are reserved by RFC 3629 for use with UTF-16, and
3057 so are excluded from UTF-8.
3059 PCRE_UTF8_ERR15
3060 PCRE_UTF8_ERR16
3061 PCRE_UTF8_ERR17
3062 PCRE_UTF8_ERR18
3063 PCRE_UTF8_ERR19
3065 A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
3066 for a value that can be represented by fewer bytes, which is invalid.
3067 For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
3068 rect coding uses just one byte.
3070 PCRE_UTF8_ERR20
3072 The two most significant bits of the first byte of a character have the
3073 binary value 0b10 (that is, the most significant bit is 1 and the sec-
3074 ond is 0). Such a byte can only validly occur as the second or subse-
3075 quent byte of a multi-byte character.
3077 PCRE_UTF8_ERR21
3079 The first byte of a character has the value 0xfe or 0xff. These values
3080 can never occur in a valid UTF-8 string.
3085 int pcre_copy_substring(const char *subject, int *ovector,
3086 int stringcount, int stringnumber, char *buffer,
3087 int buffersize);
3089 int pcre_get_substring(const char *subject, int *ovector,
3090 int stringcount, int stringnumber,
3091 const char **stringptr);
3093 int pcre_get_substring_list(const char *subject,
3094 int *ovector, int stringcount, const char ***listptr);
3096 Captured substrings can be accessed directly by using the offsets
3097 returned by pcre_exec() in ovector. For convenience, the functions
3098 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
3099 string_list() are provided for extracting captured substrings as new,
3100 separate, zero-terminated strings. These functions identify substrings
3101 by number. The next section describes functions for extracting named
3102 substrings.
3104 A substring that contains a binary zero is correctly extracted and has
3105 a further zero added on the end, but the result is not, of course, a C
3106 string. However, you can process such a string by referring to the
3107 length that is returned by pcre_copy_substring() and pcre_get_sub-
3108 string(). Unfortunately, the interface to pcre_get_substring_list() is
3109 not adequate for handling strings containing binary zeros, because the
3110 end of the final string is not independently indicated.
3112 The first three arguments are the same for all three of these func-
3113 tions: subject is the subject string that has just been successfully
3114 matched, ovector is a pointer to the vector of integer offsets that was
3115 passed to pcre_exec(), and stringcount is the number of substrings that
3116 were captured by the match, including the substring that matched the
3117 entire regular expression. This is the value returned by pcre_exec() if
3118 it is greater than zero. If pcre_exec() returned zero, indicating that
3119 it ran out of space in ovector, the value passed as stringcount should
3120 be the number of elements in the vector divided by three.
3122 The functions pcre_copy_substring() and pcre_get_substring() extract a
3123 single substring, whose number is given as stringnumber. A value of
3124 zero extracts the substring that matched the entire pattern, whereas
3125 higher values extract the captured substrings. For pcre_copy_sub-
3126 string(), the string is placed in buffer, whose length is given by
3127 buffersize, while for pcre_get_substring() a new block of memory is
3128 obtained via pcre_malloc, and its address is returned via stringptr.
3129 The yield of the function is the length of the string, not including
3130 the terminating zero, or one of these error codes:
3134 The buffer was too small for pcre_copy_substring(), or the attempt to
3135 get memory failed for pcre_get_substring().
3139 There is no substring whose number is stringnumber.
3141 The pcre_get_substring_list() function extracts all available sub-
3142 strings and builds a list of pointers to them. All this is done in a
3143 single block of memory that is obtained via pcre_malloc. The address of
3144 the memory block is returned via listptr, which is also the start of
3145 the list of string pointers. The end of the list is marked by a NULL
3146 pointer. The yield of the function is zero if all went well, or the
3147 error code
3151 if the attempt to get the memory block failed.
3153 When any of these functions encounter a substring that is unset, which
3154 can happen when capturing subpattern number n+1 matches some part of
3155 the subject, but subpattern n has not been used at all, they return an
3156 empty string. This can be distinguished from a genuine zero-length sub-
3157 string by inspecting the appropriate offset in ovector, which is nega-
3158 tive for unset substrings.
3160 The two convenience functions pcre_free_substring() and pcre_free_sub-
3161 string_list() can be used to free the memory returned by a previous
3162 call of pcre_get_substring() or pcre_get_substring_list(), respec-
3163 tively. They do nothing more than call the function pointed to by
3164 pcre_free, which of course could be called directly from a C program.
3165 However, PCRE is used in some situations where it is linked via a spe-
3166 cial interface to another programming language that cannot use
3167 pcre_free directly; it is for these cases that the functions are pro-
3168 vided.
3173 int pcre_get_stringnumber(const pcre *code,
3174 const char *name);
3176 int pcre_copy_named_substring(const pcre *code,
3177 const char *subject, int *ovector,
3178 int stringcount, const char *stringname,
3179 char *buffer, int buffersize);
3181 int pcre_get_named_substring(const pcre *code,
3182 const char *subject, int *ovector,
3183 int stringcount, const char *stringname,
3184 const char **stringptr);
3186 To extract a substring by name, you first have to find associated num-
3187 ber. For example, for this pattern
3189 (a+)b(?<xxx>\d+)...
3191 the number of the subpattern called "xxx" is 2. If the name is known to
3192 be unique (PCRE_DUPNAMES was not set), you can find the number from the
3193 name by calling pcre_get_stringnumber(). The first argument is the com-
3194 piled pattern, and the second is the name. The yield of the function is
3195 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
3196 subpattern of that name.
3198 Given the number, you can extract the substring directly, or use one of
3199 the functions described in the previous section. For convenience, there
3200 are also two functions that do the whole job.
3202 Most of the arguments of pcre_copy_named_substring() and
3203 pcre_get_named_substring() are the same as those for the similarly
3204 named functions that extract by number. As these are described in the
3205 previous section, they are not re-described here. There are just two
3206 differences:
3208 First, instead of a substring number, a substring name is given. Sec-
3209 ond, there is an extra argument, given at the start, which is a pointer
3210 to the compiled pattern. This is needed in order to gain access to the
3211 name-to-number translation table.
3213 These functions call pcre_get_stringnumber(), and if it succeeds, they
3214 then call pcre_copy_substring() or pcre_get_substring(), as appropri-
3215 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
3216 behaviour may not be what you want (see the next section).
3218 Warning: If the pattern uses the (?| feature to set up multiple subpat-
3219 terns with the same number, as described in the section on duplicate
3220 subpattern numbers in the pcrepattern page, you cannot use names to
3221 distinguish the different subpatterns, because names are not included
3222 in the compiled code. The matching process uses only numbers. For this
3223 reason, the use of different names for subpatterns of the same number
3224 causes an error at compile time.
3229 int pcre_get_stringtable_entries(const pcre *code,
3230 const char *name, char **first, char **last);
3232 When a pattern is compiled with the PCRE_DUPNAMES option, names for
3233 subpatterns are not required to be unique. (Duplicate names are always
3234 allowed for subpatterns with the same number, created by using the (?|
3235 feature. Indeed, if such subpatterns are named, they are required to
3236 use the same names.)
3238 Normally, patterns with duplicate names are such that in any one match,
3239 only one of the named subpatterns participates. An example is shown in
3240 the pcrepattern documentation.
3242 When duplicates are present, pcre_copy_named_substring() and
3243 pcre_get_named_substring() return the first substring corresponding to
3244 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
3245 (-7) is returned; no data is returned. The pcre_get_stringnumber()
3246 function returns one of the numbers that are associated with the name,
3247 but it is not defined which it is.
3249 If you want to get full details of all captured substrings for a given
3250 name, you must use the pcre_get_stringtable_entries() function. The
3251 first argument is the compiled pattern, and the second is the name. The
3252 third and fourth are pointers to variables which are updated by the
3253 function. After it has run, they point to the first and last entries in
3254 the name-to-number table for the given name. The function itself
3255 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
3256 there are none. The format of the table is described above in the sec-
3257 tion entitled Information about a pattern above. Given all the rele-
3258 vant entries for the name, you can extract each of their numbers, and
3259 hence the captured data, if any.
3264 The traditional matching function uses a similar algorithm to Perl,
3265 which stops when it finds the first match, starting at a given point in
3266 the subject. If you want to find all possible matches, or the longest
3267 possible match, consider using the alternative matching function (see
3268 below) instead. If you cannot use the alternative function, but still
3269 need to find all possible matches, you can kludge it up by making use
3270 of the callout facility, which is described in the pcrecallout documen-
3271 tation.
3273 What you have to do is to insert a callout right at the end of the pat-
3274 tern. When your callout function is called, extract and save the cur-
3275 rent matched substring. Then return 1, which forces pcre_exec() to
3276 backtrack and try other alternatives. Ultimately, when it runs out of
3277 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
3282 Matching certain patterns using pcre_exec() can use a lot of process
3283 stack, which in certain environments can be rather limited in size.
3284 Some users find it helpful to have an estimate of the amount of stack
3285 that is used by pcre_exec(), to help them set recursion limits, as
3286 described in the pcrestack documentation. The estimate that is output
3287 by pcretest when called with the -m and -C options is obtained by call-
3288 ing pcre_exec with the values NULL, NULL, NULL, -999, and -999 for its
3289 first five arguments.
3291 Normally, if its first argument is NULL, pcre_exec() immediately
3292 returns the negative error code PCRE_ERROR_NULL, but with this special
3293 combination of arguments, it returns instead a negative number whose
3294 absolute value is the approximate stack frame size in bytes. (A nega-
3295 tive number is used so that it is clear that no match has happened.)
3296 The value is approximate because in some cases, recursive calls to
3297 pcre_exec() occur when there are one or two additional variables on the
3298 stack.
3300 If PCRE has been compiled to use the heap instead of the stack for
3301 recursion, the value returned is the size of each block that is
3302 obtained from the heap.
3307 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
3308 const char *subject, int length, int startoffset,
3309 int options, int *ovector, int ovecsize,
3310 int *workspace, int wscount);
3312 The function pcre_dfa_exec() is called to match a subject string
3313 against a compiled pattern, using a matching algorithm that scans the
3314 subject string just once, and does not backtrack. This has different
3315 characteristics to the normal algorithm, and is not compatible with
3316 Perl. Some of the features of PCRE patterns are not supported. Never-
3317 theless, there are times when this kind of matching can be useful. For
3318 a discussion of the two matching algorithms, and a list of features
3319 that pcre_dfa_exec() does not support, see the pcrematching documenta-
3320 tion.
3322 The arguments for the pcre_dfa_exec() function are the same as for
3323 pcre_exec(), plus two extras. The ovector argument is used in a differ-
3324 ent way, and this is described below. The other common arguments are
3325 used in the same way as for pcre_exec(), so their description is not
3326 repeated here.
3328 The two additional arguments provide workspace for the function. The
3329 workspace vector should contain at least 20 elements. It is used for
3330 keeping track of multiple paths through the pattern tree. More
3331 workspace will be needed for patterns and subjects where there are a
3332 lot of potential matches.
3334 Here is an example of a simple call to pcre_dfa_exec():
3336 int rc;
3337 int ovector[10];
3338 int wspace[20];
3339 rc = pcre_dfa_exec(
3340 re, /* result of pcre_compile() */
3341 NULL, /* we didn't study the pattern */
3342 "some string", /* the subject string */
3343 11, /* the length of the subject string */
3344 0, /* start at offset 0 in the subject */
3345 0, /* default options */
3346 ovector, /* vector of integers for substring information */
3347 10, /* number of elements (NOT size in bytes) */
3348 wspace, /* working space vector */
3349 20); /* number of elements (NOT size in bytes) */
3351 Option bits for pcre_dfa_exec()
3353 The unused bits of the options argument for pcre_dfa_exec() must be
3354 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
3359 four of these are exactly the same as for pcre_exec(), so their
3360 description is not repeated here.
3365 These have the same general effect as they do for pcre_exec(), but the
3366 details are slightly different. When PCRE_PARTIAL_HARD is set for
3367 pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub-
3368 ject is reached and there is still at least one matching possibility
3369 that requires additional characters. This happens even if some complete
3370 matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
3371 code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
3372 of the subject is reached, there have been no complete matches, but
3373 there is still at least one matching possibility. The portion of the
3374 string that was inspected when the longest partial match was found is
3375 set as the first matching string in both cases. There is a more
3376 detailed discussion of partial and multi-segment matching, with exam-
3377 ples, in the pcrepartial documentation.
3381 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
3382 stop as soon as it has found one match. Because of the way the alterna-
3383 tive algorithm works, this is necessarily the shortest possible match
3384 at the first possible matching point in the subject string.
3388 When pcre_dfa_exec() returns a partial match, it is possible to call it
3389 again, with additional subject characters, and have it continue with
3390 the same match. The PCRE_DFA_RESTART option requests this action; when
3391 it is set, the workspace and wscount options must reference the same
3392 vector as before because data about the match so far is left in them
3393 after a partial match. There is more discussion of this facility in the
3394 pcrepartial documentation.
3396 Successful returns from pcre_dfa_exec()
3398 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
3399 string in the subject. Note, however, that all the matches from one run
3400 of the function start at the same point in the subject. The shorter
3401 matches are all initial substrings of the longer matches. For example,
3402 if the pattern
3404 <.*>
3406 is matched against the string
3408 This is <something> <something else> <something further> no more
3410 the three matched strings are
3412 <something>
3413 <something> <something else>
3414 <something> <something else> <something further>
3416 On success, the yield of the function is a number greater than zero,
3417 which is the number of matched substrings. The substrings themselves
3418 are returned in ovector. Each string uses two elements; the first is
3419 the offset to the start, and the second is the offset to the end. In
3420 fact, all the strings have the same start offset. (Space could have
3421 been saved by giving this only once, but it was decided to retain some
3422 compatibility with the way pcre_exec() returns data, even though the
3423 meaning of the strings is different.)
3425 The strings are returned in reverse order of length; that is, the long-
3426 est matching string is given first. If there were too many matches to
3427 fit into ovector, the yield of the function is zero, and the vector is
3428 filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec()
3429 can use the entire ovector for returning matched strings.
3431 Error returns from pcre_dfa_exec()
3433 The pcre_dfa_exec() function returns a negative number when it fails.
3434 Many of the errors are the same as for pcre_exec(), and these are
3435 described above. There are in addition the following errors that are
3436 specific to pcre_dfa_exec():
3440 This return is given if pcre_dfa_exec() encounters an item in the pat-
3441 tern that it does not support, for instance, the use of \C or a back
3442 reference.
3446 This return is given if pcre_dfa_exec() encounters a condition item
3447 that uses a back reference for the condition, or a test for recursion
3448 in a specific group. These are not supported.
3452 This return is given if pcre_dfa_exec() is called with an extra block
3453 that contains a setting of the match_limit or match_limit_recursion
3454 fields. This is not supported (these fields are meaningless for DFA
3455 matching).
3459 This return is given if pcre_dfa_exec() runs out of space in the
3460 workspace vector.
3464 When a recursive subpattern is processed, the matching function calls
3465 itself recursively, using private vectors for ovector and workspace.
3466 This error is given if the output vector is not large enough. This
3467 should be extremely rare, as a vector of size 1000 is used.
3472 pcre16(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematch-
3473 ing(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcresample(3),
3474 pcrestack(3).
3479 Philip Hazel
3480 University Computing Service
3481 Cambridge CB2 3QH, England.
3486 Last updated: 24 February 2012
3487 Copyright (c) 1997-2012 University of Cambridge.
3488 ------------------------------------------------------------------------------
3494 NAME
3495 PCRE - Perl-compatible regular expressions
3500 int (*pcre_callout)(pcre_callout_block *);
3502 int (*pcre16_callout)(pcre16_callout_block *);
3504 PCRE provides a feature called "callout", which is a means of temporar-
3505 ily passing control to the caller of PCRE in the middle of pattern
3506 matching. The caller of PCRE provides an external function by putting
3507 its entry point in the global variable pcre_callout (pcre16_callout for
3508 the 16-bit library). By default, this variable contains NULL, which
3509 disables all calling out.
3511 Within a regular expression, (?C) indicates the points at which the
3512 external function is to be called. Different callout points can be
3513 identified by putting a number less than 256 after the letter C. The
3514 default value is zero. For example, this pattern has two callout
3515 points:
3517 (?C1)abc(?C2)def
3519 If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled,
3520 PCRE automatically inserts callouts, all with number 255, before each
3521 item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the
3522 pattern
3524 A(\d{2}|--)
3526 it is processed as if it were
3528 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
3530 Notice that there is a callout before and after each parenthesis and
3531 alternation bar. Automatic callouts can be used for tracking the
3532 progress of pattern matching. The pcretest command has an option that
3533 sets automatic callouts; when it is used, the output indicates how the
3534 pattern is matched. This is useful information when you are trying to
3535 optimize the performance of a particular pattern.
3537 The use of callouts in a pattern makes it ineligible for optimization
3538 by the just-in-time compiler. Studying such a pattern with the
3539 PCRE_STUDY_JIT_COMPILE option always fails.
3544 You should be aware that, because of optimizations in the way PCRE
3545 matches patterns by default, callouts sometimes do not happen. For
3546 example, if the pattern is
3548 ab(?C4)cd
3550 PCRE knows that any matching string must contain the letter "d". If the
3551 subject string is "abyz", the lack of "d" means that matching doesn't
3552 ever start, and the callout is never reached. However, with "abyd",
3553 though the result is still no match, the callout is obeyed.
3555 If the pattern is studied, PCRE knows the minimum length of a matching
3556 string, and will immediately give a "no match" return without actually
3557 running a match if the subject is not long enough, or, for unanchored
3558 patterns, if it has been scanned far enough.
3560 You can disable these optimizations by passing the PCRE_NO_START_OPTI-
3561 MIZE option to the matching function, or by starting the pattern with
3562 (*NO_START_OPT). This slows down the matching process, but does ensure
3563 that callouts such as the example above are obeyed.
3568 During matching, when PCRE reaches a callout point, the external func-
3569 tion defined by pcre_callout or pcre16_callout is called (if it is
3570 set). This applies to both normal and DFA matching. The only argument
3571 to the callout function is a pointer to a pcre_callout or pcre16_call-
3572 out block. These structures contains the following fields:
3574 int version;
3575 int callout_number;
3576 int *offset_vector;
3577 const char *subject; (8-bit version)
3578 PCRE_SPTR16 subject; (16-bit version)
3579 int subject_length;
3580 int start_match;
3581 int current_position;
3582 int capture_top;
3583 int capture_last;
3584 void *callout_data;
3585 int pattern_position;
3586 int next_item_length;
3587 const unsigned char *mark; (8-bit version)
3588 const PCRE_UCHAR16 *mark; (16-bit version)
3590 The version field is an integer containing the version number of the
3591 block format. The initial version was 0; the current version is 2. The
3592 version number will change again in future if additional fields are
3593 added, but the intention is never to remove any of the existing fields.
3595 The callout_number field contains the number of the callout, as com-
3596 piled into the pattern (that is, the number after ?C for manual call-
3597 outs, and 255 for automatically generated callouts).
3599 The offset_vector field is a pointer to the vector of offsets that was
3600 passed by the caller to the matching function. When pcre_exec() or
3601 pcre16_exec() is used, the contents can be inspected, in order to
3602 extract substrings that have been matched so far, in the same way as
3603 for extracting substrings after a match has completed. For the DFA
3604 matching functions, this field is not useful.
3606 The subject and subject_length fields contain copies of the values that
3607 were passed to the matching function.
3609 The start_match field normally contains the offset within the subject
3610 at which the current match attempt started. However, if the escape
3611 sequence \K has been encountered, this value is changed to reflect the
3612 modified starting point. If the pattern is not anchored, the callout
3613 function may be called several times from the same point in the pattern
3614 for different starting points in the subject.
3616 The current_position field contains the offset within the subject of
3617 the current match pointer.
3619 When the pcre_exec() or pcre16_exec() is used, the capture_top field
3620 contains one more than the number of the highest numbered captured sub-
3621 string so far. If no substrings have been captured, the value of cap-
3622 ture_top is one. This is always the case when the DFA functions are
3623 used, because they do not support captured substrings.
3625 The capture_last field contains the number of the most recently cap-
3626 tured substring. If no substrings have been captured, its value is -1.
3627 This is always the case for the DFA matching functions.
3629 The callout_data field contains a value that is passed to a matching
3630 function specifically so that it can be passed back in callouts. It is
3631 passed in the callout_data field of a pcre_extra or pcre16_extra data
3632 structure. If no such data was passed, the value of callout_data in a
3633 callout block is NULL. There is a description of the pcre_extra struc-
3634 ture in the pcreapi documentation.
3636 The pattern_position field is present from version 1 of the callout
3637 structure. It contains the offset to the next item to be matched in the
3638 pattern string.
3640 The next_item_length field is present from version 1 of the callout
3641 structure. It contains the length of the next item to be matched in the
3642 pattern string. When the callout immediately precedes an alternation
3643 bar, a closing parenthesis, or the end of the pattern, the length is
3644 zero. When the callout precedes an opening parenthesis, the length is
3645 that of the entire subpattern.
3647 The pattern_position and next_item_length fields are intended to help
3648 in distinguishing between different automatic callouts, which all have
3649 the same callout number. However, they are set for all callouts.
3651 The mark field is present from version 2 of the callout structure. In
3652 callouts from pcre_exec() or pcre16_exec() it contains a pointer to the
3653 zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
3654 (*THEN) item in the match, or NULL if no such items have been passed.
3655 Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
3656 previous (*MARK). In callouts from the DFA matching functions this
3657 field always contains NULL.
3662 The external callout function returns an integer to PCRE. If the value
3663 is zero, matching proceeds as normal. If the value is greater than
3664 zero, matching fails at the current point, but the testing of other
3665 matching possibilities goes ahead, just as if a lookahead assertion had
3666 failed. If the value is less than zero, the match is abandoned, the
3667 matching function returns the negative value.
3669 Negative values should normally be chosen from the set of
3670 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
3671 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
3672 reserved for use by callout functions; it will never be used by PCRE
3673 itself.
3678 Philip Hazel
3679 University Computing Service
3680 Cambridge CB2 3QH, England.
3685 Last updated: 08 Janurary 2012
3686 Copyright (c) 1997-2012 University of Cambridge.
3687 ------------------------------------------------------------------------------
3693 NAME
3694 PCRE - Perl-compatible regular expressions
3699 This document describes the differences in the ways that PCRE and Perl
3700 handle regular expressions. The differences described here are with
3701 respect to Perl versions 5.10 and above.
3703 1. PCRE has only a subset of Perl's Unicode support. Details of what it
3704 does have are given in the pcreunicode page.
3706 2. PCRE allows repeat quantifiers only on parenthesized assertions, but
3707 they do not mean what you might think. For example, (?!a){3} does not
3708 assert that the next three characters are not "a". It just asserts that
3709 the next character is not "a" three times (in principle: PCRE optimizes
3710 this to run the assertion just once). Perl allows repeat quantifiers on
3711 other assertions such as \b, but these do not seem to have any use.
3713 3. Capturing subpatterns that occur inside negative lookahead asser-
3714 tions are counted, but their entries in the offsets vector are never
3715 set. Perl sets its numerical variables from any such patterns that are
3716 matched before the assertion fails to match something (thereby succeed-
3717 ing), but only if the negative lookahead assertion contains just one
3718 branch.
3720 4. Though binary zero characters are supported in the subject string,
3721 they are not allowed in a pattern string because it is passed as a nor-
3722 mal C string, terminated by zero. The escape sequence \0 can be used in
3723 the pattern to represent a binary zero.
3725 5. The following Perl escape sequences are not supported: \l, \u, \L,
3726 \U, and \N when followed by a character name or Unicode value. (\N on
3727 its own, matching a non-newline character, is supported.) In fact these
3728 are implemented by Perl's general string-handling and are not part of
3729 its pattern matching engine. If any of these are encountered by PCRE,
3730 an error is generated by default. However, if the PCRE_JAVASCRIPT_COM-
3731 PAT option is set, \U and \u are interpreted as JavaScript interprets
3732 them.
3734 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
3735 is built with Unicode character property support. The properties that
3736 can be tested with \p and \P are limited to the general category prop-
3737 erties such as Lu and Nd, script names such as Greek or Han, and the
3738 derived properties Any and L&. PCRE does support the Cs (surrogate)
3739 property, which Perl does not; the Perl documentation says "Because
3740 Perl hides the need for the user to understand the internal representa-
3741 tion of Unicode characters, there is no need to implement the somewhat
3742 messy concept of surrogates."
3744 7. PCRE implements a simpler version of \X than Perl, which changed to
3745 make \X match what Unicode calls an "extended grapheme cluster". This
3746 is more complicated than an extended Unicode sequence, which is what
3747 PCRE matches.
3749 8. PCRE does support the \Q...\E escape for quoting substrings. Charac-
3750 ters in between are treated as literals. This is slightly different
3751 from Perl in that $ and @ are also handled as literals inside the
3752 quotes. In Perl, they cause variable interpolation (but of course PCRE
3753 does not have variables). Note the following examples:
3755 Pattern PCRE matches Perl matches
3757 \Qabc$xyz\E abc$xyz abc followed by the
3758 contents of $xyz
3759 \Qabc\$xyz\E abc\$xyz abc\$xyz
3760 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3762 The \Q...\E sequence is recognized both inside and outside character
3763 classes.
3765 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
3766 constructions. However, there is support for recursive patterns. This
3767 is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE
3768 "callout" feature allows an external function to be called during pat-
3769 tern matching. See the pcrecallout documentation for details.
3771 10. Subpatterns that are called as subroutines (whether or not recur-
3772 sively) are always treated as atomic groups in PCRE. This is like
3773 Python, but unlike Perl. Captured values that are set outside a sub-
3774 routine call can be reference from inside in PCRE, but not in Perl.
3775 There is a discussion that explains these differences in more detail in
3776 the section on recursion differences from Perl in the pcrepattern page.
3778 11. If (*THEN) is present in a group that is called as a subroutine,
3779 its action is limited to that group, even if the group does not contain
3780 any | characters.
3782 12. There are some differences that are concerned with the settings of
3783 captured strings when part of a pattern is repeated. For example,
3784 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
3785 unset, but in PCRE it is set to "b".
3787 13. PCRE's handling of duplicate subpattern numbers and duplicate sub-
3788 pattern names is not as general as Perl's. This is a consequence of the
3789 fact the PCRE works internally just with numbers, using an external ta-
3790 ble to translate between numbers and names. In particular, a pattern
3791 such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have
3792 the same number but different names, is not supported, and causes an
3793 error at compile time. If it were allowed, it would not be possible to
3794 distinguish which parentheses matched, because both names map to cap-
3795 turing subpattern number 1. To avoid this confusing situation, an error
3796 is given at compile time.
3798 14. Perl recognizes comments in some places that PCRE does not, for
3799 example, between the ( and ? at the start of a subpattern. If the /x
3800 modifier is set, Perl allows whitespace between ( and ? but PCRE never
3801 does, even if the PCRE_EXTENDED option is set.
3803 15. PCRE provides some extensions to the Perl regular expression facil-
3804 ities. Perl 5.10 includes new features that are not in earlier ver-
3805 sions of Perl, some of which (such as named parentheses) have been in
3806 PCRE for some time. This list is with respect to Perl 5.10:
3808 (a) Although lookbehind assertions in PCRE must match fixed length
3809 strings, each alternative branch of a lookbehind assertion can match a
3810 different length of string. Perl requires them all to have the same
3811 length.
3813 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
3814 meta-character matches only at the very end of the string.
3816 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
3817 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
3818 ignored. (Perl can be made to issue a warning.)
3820 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
3821 fiers is inverted, that is, by default they are not greedy, but if fol-
3822 lowed by a question mark they are.
3824 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
3825 tried only at the first matching position in the subject string.
3828 and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva-
3829 lents.
3831 (g) The \R escape sequence can be restricted to match only CR, LF, or
3832 CRLF by the PCRE_BSR_ANYCRLF option.
3834 (h) The callout facility is PCRE-specific.
3836 (i) The partial matching facility is PCRE-specific.
3838 (j) Patterns compiled by PCRE can be saved and re-used at a later time,
3839 even on different hosts that have the other endianness. However, this
3840 does not apply to optimized data created by the just-in-time compiler.
3842 (k) The alternative matching functions (pcre_dfa_exec() and
3843 pcre16_dfa_exec()) match in a different way and are not Perl-compati-
3844 ble.
3846 (l) PCRE recognizes some special sequences such as (*CR) at the start
3847 of a pattern that set overall options that cannot be changed within the
3848 pattern.
3853 Philip Hazel
3854 University Computing Service
3855 Cambridge CB2 3QH, England.
3860 Last updated: 08 Januray 2012
3861 Copyright (c) 1997-2012 University of Cambridge.
3862 ------------------------------------------------------------------------------
3868 NAME
3869 PCRE - Perl-compatible regular expressions
3874 The syntax and semantics of the regular expressions that are supported
3875 by PCRE are described in detail below. There is a quick-reference syn-
3876 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
3877 semantics as closely as it can. PCRE also supports some alternative
3878 regular expression syntax (which does not conflict with the Perl syn-
3879 tax) in order to provide some compatibility with regular expressions in
3880 Python, .NET, and Oniguruma.
3882 Perl's regular expressions are described in its own documentation, and
3883 regular expressions in general are covered in a number of books, some
3884 of which have copious examples. Jeffrey Friedl's "Mastering Regular
3885 Expressions", published by O'Reilly, covers regular expressions in
3886 great detail. This description of PCRE's regular expressions is
3887 intended as reference material.
3889 The original operation of PCRE was on strings of one-byte characters.
3890 However, there is now also support for UTF-8 strings in the original
3891 library, and a second library that supports 16-bit and UTF-16 character
3892 strings. To use these features, PCRE must be built to include appropri-
3893 ate support. When using UTF strings you must either call the compiling
3894 function with the PCRE_UTF8 or PCRE_UTF16 option, or the pattern must
3895 start with one of these special sequences:
3897 (*UTF8)
3898 (*UTF16)
3900 Starting a pattern with such a sequence is equivalent to setting the
3901 relevant option. This feature is not Perl-compatible. How setting a UTF
3902 mode affects pattern matching is mentioned in several places below.
3903 There is also a summary of features in the pcreunicode page.
3905 Another special sequence that may appear at the start of a pattern or
3906 in combination with (*UTF8) or (*UTF16) is:
3908 (*UCP)
3910 This has the same effect as setting the PCRE_UCP option: it causes
3911 sequences such as \d and \w to use Unicode properties to determine
3912 character types, instead of recognizing only characters with codes less
3913 than 128 via a lookup table.
3915 If a pattern starts with (*NO_START_OPT), it has the same effect as
3916 setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
3917 time. There are also some more of these special sequences that are con-
3918 cerned with the handling of newlines; they are described below.
3920 The remainder of this document discusses the patterns that are sup-
3921 ported by PCRE when one its main matching functions, pcre_exec()
3922 (8-bit) or pcre16_exec() (16-bit), is used. PCRE also has alternative
3923 matching functions, pcre_dfa_exec() and pcre16_dfa_exec(), which match
3924 using a different algorithm that is not Perl-compatible. Some of the
3925 features discussed below are not available when DFA matching is used.
3926 The advantages and disadvantages of the alternative functions, and how
3927 they differ from the normal functions, are discussed in the pcrematch-
3928 ing page.
3933 PCRE supports five different conventions for indicating line breaks in
3934 strings: a single CR (carriage return) character, a single LF (line-
3935 feed) character, the two-character sequence CRLF, any of the three pre-
3936 ceding, or any Unicode newline sequence. The pcreapi page has further
3937 discussion about newlines, and shows how to set the newline convention
3938 in the options arguments for the compiling and matching functions.
3940 It is also possible to specify a newline convention by starting a pat-
3941 tern string with one of the following five sequences:
3943 (*CR) carriage return
3944 (*LF) linefeed
3945 (*CRLF) carriage return, followed by linefeed
3946 (*ANYCRLF) any of the three above
3947 (*ANY) all Unicode newline sequences
3949 These override the default and the options given to the compiling func-
3950 tion. For example, on a Unix system where LF is the default newline
3951 sequence, the pattern
3953 (*CR)a.b
3955 changes the convention to CR. That pattern matches "a\nb" because LF is
3956 no longer a newline. Note that these special settings, which are not
3957 Perl-compatible, are recognized only at the very start of a pattern,
3958 and that they must be in upper case. If more than one of them is
3959 present, the last one is used.
3961 The newline convention affects the interpretation of the dot metachar-
3962 acter when PCRE_DOTALL is not set, and also the behaviour of \N. How-
3963 ever, it does not affect what the \R escape sequence matches. By
3964 default, this is any Unicode newline sequence, for Perl compatibility.
3965 However, this can be changed; see the description of \R in the section
3966 entitled "Newline sequences" below. A change of \R setting can be com-
3967 bined with a change of newline convention.
3972 A regular expression is a pattern that is matched against a subject
3973 string from left to right. Most characters stand for themselves in a
3974 pattern, and match the corresponding characters in the subject. As a
3975 trivial example, the pattern
3977 The quick brown fox
3979 matches a portion of a subject string that is identical to itself. When
3980 caseless matching is specified (the PCRE_CASELESS option), letters are
3981 matched independently of case. In a UTF mode, PCRE always understands
3982 the concept of case for characters whose values are less than 128, so
3983 caseless matching is always possible. For characters with higher val-
3984 ues, the concept of case is supported if PCRE is compiled with Unicode
3985 property support, but not otherwise. If you want to use caseless
3986 matching for characters 128 and above, you must ensure that PCRE is
3987 compiled with Unicode property support as well as with UTF support.
3989 The power of regular expressions comes from the ability to include
3990 alternatives and repetitions in the pattern. These are encoded in the
3991 pattern by the use of metacharacters, which do not stand for themselves
3992 but instead are interpreted in some special way.
3994 There are two different sets of metacharacters: those that are recog-
3995 nized anywhere in the pattern except within square brackets, and those
3996 that are recognized within square brackets. Outside square brackets,
3997 the metacharacters are as follows:
3999 \ general escape character with several uses
4000 ^ assert start of string (or line, in multiline mode)
4001 $ assert end of string (or line, in multiline mode)
4002 . match any character except newline (by default)
4003 [ start character class definition
4004 | start of alternative branch
4005 ( start subpattern
4006 ) end subpattern
4007 ? extends the meaning of (
4008 also 0 or 1 quantifier
4009 also quantifier minimizer
4010 * 0 or more quantifier
4011 + 1 or more quantifier
4012 also "possessive quantifier"
4013 { start min/max quantifier
4015 Part of a pattern that is in square brackets is called a "character
4016 class". In a character class the only metacharacters are:
4018 \ general escape character
4019 ^ negate the class, but only if the first character
4020 - indicates character range
4021 [ POSIX character class (only if followed by POSIX
4022 syntax)
4023 ] terminates the character class
4025 The following sections describe the use of each of the metacharacters.
4030 The backslash character has several uses. Firstly, if it is followed by
4031 a character that is not a number or a letter, it takes away any special
4032 meaning that character may have. This use of backslash as an escape
4033 character applies both inside and outside character classes.
4035 For example, if you want to match a * character, you write \* in the
4036 pattern. This escaping action applies whether or not the following
4037 character would otherwise be interpreted as a metacharacter, so it is
4038 always safe to precede a non-alphanumeric with backslash to specify
4039 that it stands for itself. In particular, if you want to match a back-
4040 slash, you write \\.
4042 In a UTF mode, only ASCII numbers and letters have any special meaning
4043 after a backslash. All other characters (in particular, those whose
4044 codepoints are greater than 127) are treated as literals.
4046 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
4047 the pattern (other than in a character class) and characters between a
4048 # outside a character class and the next newline are ignored. An escap-
4049 ing backslash can be used to include a whitespace or # character as
4050 part of the pattern.
4052 If you want to remove the special meaning from a sequence of charac-
4053 ters, you can do so by putting them between \Q and \E. This is differ-
4054 ent from Perl in that $ and @ are handled as literals in \Q...\E
4055 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
4056 tion. Note the following examples:
4058 Pattern PCRE matches Perl matches
4060 \Qabc$xyz\E abc$xyz abc followed by the
4061 contents of $xyz
4062 \Qabc\$xyz\E abc\$xyz abc\$xyz
4063 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
4065 The \Q...\E sequence is recognized both inside and outside character
4066 classes. An isolated \E that is not preceded by \Q is ignored. If \Q
4067 is not followed by \E later in the pattern, the literal interpretation
4068 continues to the end of the pattern (that is, \E is assumed at the
4069 end). If the isolated \Q is inside a character class, this causes an
4070 error, because the character class is not terminated.
4072 Non-printing characters
4074 A second use of backslash provides a way of encoding non-printing char-
4075 acters in patterns in a visible manner. There is no restriction on the
4076 appearance of non-printing characters, apart from the binary zero that
4077 terminates a pattern, but when a pattern is being prepared by text
4078 editing, it is often easier to use one of the following escape
4079 sequences than the binary character it represents:
4081 \a alarm, that is, the BEL character (hex 07)
4082 \cx "control-x", where x is any ASCII character
4083 \e escape (hex 1B)
4084 \f formfeed (hex 0C)
4085 \n linefeed (hex 0A)
4086 \r carriage return (hex 0D)
4087 \t tab (hex 09)
4088 \ddd character with octal code ddd, or back reference
4089 \xhh character with hex code hh
4090 \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
4091 \uhhhh character with hex code hhhh (JavaScript mode only)
4093 The precise effect of \cx is as follows: if x is a lower case letter,
4094 it is converted to upper case. Then bit 6 of the character (hex 40) is
4095 inverted. Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
4096 is 7B), while \c; becomes hex 7B (; is 3B). If the byte following \c
4097 has a value greater than 127, a compile-time error occurs. This locks
4098 out non-ASCII characters in all modes. (When PCRE is compiled in EBCDIC
4099 mode, all byte values are valid. A lower case letter is converted to
4100 upper case, and then the 0xc0 bits are flipped.)
4102 By default, after \x, from zero to two hexadecimal digits are read
4103 (letters can be in upper or lower case). Any number of hexadecimal dig-
4104 its may appear between \x{ and }, but the character code is constrained
4105 as follows:
4107 8-bit non-UTF mode less than 0x100
4108 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
4109 16-bit non-UTF mode less than 0x10000
4110 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
4112 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-
4113 called "surrogate" codepoints).
4115 If characters other than hexadecimal digits appear between \x{ and },
4116 or if there is no terminating }, this form of escape is not recognized.
4117 Instead, the initial \x will be interpreted as a basic hexadecimal
4118 escape, with no following digits, giving a character whose value is
4119 zero.
4121 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
4122 is as just described only when it is followed by two hexadecimal dig-
4123 its. Otherwise, it matches a literal "x" character. In JavaScript
4124 mode, support for code points greater than 256 is provided by \u, which
4125 must be followed by four hexadecimal digits; otherwise it matches a
4126 literal "u" character.
4128 Characters whose value is less than 256 can be defined by either of the
4129 two syntaxes for \x (or by \u in JavaScript mode). There is no differ-
4130 ence in the way they are handled. For example, \xdc is exactly the same
4131 as \x{dc} (or \u00dc in JavaScript mode).
4133 After \0 up to two further octal digits are read. If there are fewer
4134 than two digits, just those that are present are used. Thus the
4135 sequence \0\x\07 specifies two binary zeros followed by a BEL character
4136 (code value 7). Make sure you supply two digits after the initial zero
4137 if the pattern character that follows is itself an octal digit.
4139 The handling of a backslash followed by a digit other than 0 is compli-
4140 cated. Outside a character class, PCRE reads it and any following dig-
4141 its as a decimal number. If the number is less than 10, or if there
4142 have been at least that many previous capturing left parentheses in the
4143 expression, the entire sequence is taken as a back reference. A
4144 description of how this works is given later, following the discussion
4145 of parenthesized subpatterns.
4147 Inside a character class, or if the decimal number is greater than 9
4148 and there have not been that many capturing subpatterns, PCRE re-reads
4149 up to three octal digits following the backslash, and uses them to gen-
4150 erate a data character. Any subsequent digits stand for themselves. The
4151 value of the character is constrained in the same way as characters
4152 specified in hexadecimal. For example:
4154 \040 is another way of writing a space
4155 \40 is the same, provided there are fewer than 40
4156 previous capturing subpatterns
4157 \7 is always a back reference
4158 \11 might be a back reference, or another way of
4159 writing a tab
4160 \011 is always a tab
4161 \0113 is a tab followed by the character "3"
4162 \113 might be a back reference, otherwise the
4163 character with octal code 113
4164 \377 might be a back reference, otherwise
4165 the value 255 (decimal)
4166 \81 is either a back reference, or a binary zero
4167 followed by the two characters "8" and "1"
4169 Note that octal values of 100 or greater must not be introduced by a
4170 leading zero, because no more than three octal digits are ever read.
4172 All the sequences that define a single character value can be used both
4173 inside and outside character classes. In addition, inside a character
4174 class, \b is interpreted as the backspace character (hex 08).
4176 \N is not allowed in a character class. \B, \R, and \X are not special
4177 inside a character class. Like other unrecognized escape sequences,
4178 they are treated as the literal characters "B", "R", and "X" by
4179 default, but cause an error if the PCRE_EXTRA option is set. Outside a
4180 character class, these sequences have different meanings.
4182 Unsupported escape sequences
4184 In Perl, the sequences \l, \L, \u, and \U are recognized by its string
4185 handler and used to modify the case of following characters. By
4186 default, PCRE does not support these escape sequences. However, if the
4187 PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and
4188 \u can be used to define a character by code point, as described in the
4189 previous section.
4191 Absolute and relative back references
4193 The sequence \g followed by an unsigned or a negative number, option-
4194 ally enclosed in braces, is an absolute or relative back reference. A
4195 named back reference can be coded as \g{name}. Back references are dis-
4196 cussed later, following the discussion of parenthesized subpatterns.
4198 Absolute and relative subroutine calls
4200 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
4201 name or a number enclosed either in angle brackets or single quotes, is
4202 an alternative syntax for referencing a subpattern as a "subroutine".
4203 Details are discussed later. Note that \g{...} (Perl syntax) and
4204 \g<...> (Oniguruma syntax) are not synonymous. The former is a back
4205 reference; the latter is a subroutine call.
4207 Generic character types
4209 Another use of backslash is for specifying generic character types:
4211 \d any decimal digit
4212 \D any character that is not a decimal digit
4213 \h any horizontal whitespace character
4214 \H any character that is not a horizontal whitespace character
4215 \s any whitespace character
4216 \S any character that is not a whitespace character
4217 \v any vertical whitespace character
4218 \V any character that is not a vertical whitespace character
4219 \w any "word" character
4220 \W any "non-word" character
4222 There is also the single sequence \N, which matches a non-newline char-
4223 acter. This is the same as the "." metacharacter when PCRE_DOTALL is
4224 not set. Perl also uses \N to match characters by name; PCRE does not
4225 support this.
4227 Each pair of lower and upper case escape sequences partitions the com-
4228 plete set of characters into two disjoint sets. Any given character
4229 matches one, and only one, of each pair. The sequences can appear both
4230 inside and outside character classes. They each match one character of
4231 the appropriate type. If the current matching point is at the end of
4232 the subject string, all of them fail, because there is no character to
4233 match.
4235 For compatibility with Perl, \s does not match the VT character (code
4236 11). This makes it different from the the POSIX "space" class. The \s
4237 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
4238 "use locale;" is included in a Perl script, \s may match the VT charac-
4239 ter. In PCRE, it never does.
4241 A "word" character is an underscore or any character that is a letter
4242 or digit. By default, the definition of letters and digits is con-
4243 trolled by PCRE's low-valued character tables, and may vary if locale-
4244 specific matching is taking place (see "Locale support" in the pcreapi
4245 page). For example, in a French locale such as "fr_FR" in Unix-like
4246 systems, or "french" in Windows, some character codes greater than 128
4247 are used for accented letters, and these are then matched by \w. The
4248 use of locales with Unicode is discouraged.
4250 By default, in a UTF mode, characters with values greater than 128
4251 never match \d, \s, or \w, and always match \D, \S, and \W. These
4252 sequences retain their original meanings from before UTF support was
4253 available, mainly for efficiency reasons. However, if PCRE is compiled
4254 with Unicode property support, and the PCRE_UCP option is set, the be-
4255 haviour is changed so that Unicode properties are used to determine
4256 character types, as follows:
4258 \d any character that \p{Nd} matches (decimal digit)
4259 \s any character that \p{Z} matches, plus HT, LF, FF, CR
4260 \w any character that \p{L} or \p{N} matches, plus underscore
4262 The upper case escapes match the inverse sets of characters. Note that
4263 \d matches only decimal digits, whereas \w matches any Unicode digit,
4264 as well as any Unicode letter, and underscore. Note also that PCRE_UCP
4265 affects \b, and \B because they are defined in terms of \w and \W.
4266 Matching these sequences is noticeably slower when PCRE_UCP is set.
4268 The sequences \h, \H, \v, and \V are features that were added to Perl
4269 at release 5.10. In contrast to the other sequences, which match only
4270 ASCII characters by default, these always match certain high-valued
4271 codepoints, whether or not PCRE_UCP is set. The horizontal space char-
4272 acters are:
4274 U+0009 Horizontal tab
4275 U+0020 Space
4276 U+00A0 Non-break space
4277 U+1680 Ogham space mark
4278 U+180E Mongolian vowel separator
4279 U+2000 En quad
4280 U+2001 Em quad
4281 U+2002 En space
4282 U+2003 Em space
4283 U+2004 Three-per-em space
4284 U+2005 Four-per-em space
4285 U+2006 Six-per-em space
4286 U+2007 Figure space
4287 U+2008 Punctuation space
4288 U+2009 Thin space
4289 U+200A Hair space
4290 U+202F Narrow no-break space
4291 U+205F Medium mathematical space
4292 U+3000 Ideographic space
4294 The vertical space characters are:
4296 U+000A Linefeed
4297 U+000B Vertical tab
4298 U+000C Formfeed
4299 U+000D Carriage return
4300 U+0085 Next line
4301 U+2028 Line separator
4302 U+2029 Paragraph separator
4304 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
4305 256 are relevant.
4307 Newline sequences
4309 Outside a character class, by default, the escape sequence \R matches
4310 any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent
4311 to the following:
4313 (?>\r\n|\n|\x0b|\f|\r|\x85)
4315 This is an example of an "atomic group", details of which are given
4316 below. This particular group matches either the two-character sequence
4317 CR followed by LF, or one of the single characters LF (linefeed,
4318 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
4319 return, U+000D), or NEL (next line, U+0085). The two-character sequence
4320 is treated as a single unit that cannot be split.
4322 In other modes, two additional characters whose codepoints are greater
4323 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
4324 rator, U+2029). Unicode character property support is not needed for
4325 these characters to be recognized.
4327 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
4328 the complete set of Unicode line endings) by setting the option
4329 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
4330 (BSR is an abbrevation for "backslash R".) This can be made the default
4331 when PCRE is built; if this is the case, the other behaviour can be
4332 requested via the PCRE_BSR_UNICODE option. It is also possible to
4333 specify these settings by starting a pattern string with one of the
4334 following sequences:
4336 (*BSR_ANYCRLF) CR, LF, or CRLF only
4337 (*BSR_UNICODE) any Unicode newline sequence
4339 These override the default and the options given to the compiling func-
4340 tion, but they can themselves be overridden by options given to a
4341 matching function. Note that these special settings, which are not
4342 Perl-compatible, are recognized only at the very start of a pattern,
4343 and that they must be in upper case. If more than one of them is
4344 present, the last one is used. They can be combined with a change of
4345 newline convention; for example, a pattern can start with:
4349 They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
4350 sequences. Inside a character class, \R is treated as an unrecognized
4351 escape sequence, and so matches the letter "R" by default, but causes
4352 an error if PCRE_EXTRA is set.
4354 Unicode character properties
4356 When PCRE is built with Unicode character property support, three addi-
4357 tional escape sequences that match characters with specific properties
4358 are available. When in 8-bit non-UTF-8 mode, these sequences are of
4359 course limited to testing characters whose codepoints are less than
4360 256, but they do work in this mode. The extra escape sequences are:
4362 \p{xx} a character with the xx property
4363 \P{xx} a character without the xx property
4364 \X an extended Unicode sequence
4366 The property names represented by xx above are limited to the Unicode
4367 script names, the general category properties, "Any", which matches any
4368 character (including newline), and some special PCRE properties
4369 (described in the next section). Other Perl properties such as "InMu-
4370 sicalSymbols" are not currently supported by PCRE. Note that \P{Any}
4371 does not match any characters, so always causes a match failure.
4373 Sets of Unicode characters are defined as belonging to certain scripts.
4374 A character from one of these sets can be matched using a script name.
4375 For example:
4377 \p{Greek}
4378 \P{Han}
4380 Those that are not part of an identified script are lumped together as
4381 "Common". The current list of scripts is:
4383 Arabic, Armenian, Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo,
4384 Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Chakma,
4385 Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret,
4386 Devanagari, Egyptian_Hieroglyphs, Ethiopic, Georgian, Glagolitic,
4387 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
4388 gana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip-
4389 tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li,
4390 Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian,
4391 Lydian, Malayalam, Mandaic, Meetei_Mayek, Meroitic_Cursive,
4392 Meroitic_Hieroglyphs, Miao, Mongolian, Myanmar, New_Tai_Lue, Nko,
4393 Ogham, Old_Italic, Old_Persian, Old_South_Arabian, Old_Turkic,
4394 Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samari-
4395 tan, Saurashtra, Sharada, Shavian, Sinhala, Sora_Sompeng, Sundanese,
4396 Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet,
4397 Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai,
4398 Yi.
4400 Each character has exactly one Unicode general category property, spec-
4401 ified by a two-letter abbreviation. For compatibility with Perl, nega-
4402 tion can be specified by including a circumflex between the opening
4403 brace and the property name. For example, \p{^Lu} is the same as
4404 \P{Lu}.
4406 If only one letter is specified with \p or \P, it includes all the gen-
4407 eral category properties that start with that letter. In this case, in
4408 the absence of negation, the curly brackets in the escape sequence are
4409 optional; these two examples have the same effect:
4411 \p{L}
4412 \pL
4414 The following general category property codes are supported:
4416 C Other
4417 Cc Control
4418 Cf Format
4419 Cn Unassigned
4420 Co Private use
4421 Cs Surrogate
4423 L Letter
4424 Ll Lower case letter
4425 Lm Modifier letter
4426 Lo Other letter
4427 Lt Title case letter
4428 Lu Upper case letter
4430 M Mark
4431 Mc Spacing mark
4432 Me Enclosing mark
4433 Mn Non-spacing mark
4435 N Number
4436 Nd Decimal number
4437 Nl Letter number
4438 No Other number
4440 P Punctuation
4441 Pc Connector punctuation
4442 Pd Dash punctuation
4443 Pe Close punctuation
4444 Pf Final punctuation
4445 Pi Initial punctuation
4446 Po Other punctuation
4447 Ps Open punctuation
4449 S Symbol
4450 Sc Currency symbol
4451 Sk Modifier symbol
4452 Sm Mathematical symbol
4453 So Other symbol
4455 Z Separator
4456 Zl Line separator
4457 Zp Paragraph separator
4458 Zs Space separator
4460 The special property L& is also supported: it matches a character that
4461 has the Lu, Ll, or Lt property, in other words, a letter that is not
4462 classified as a modifier or "other".
4464 The Cs (Surrogate) property applies only to characters in the range
4465 U+D800 to U+DFFF. Such characters are not valid in Unicode strings and
4466 so cannot be tested by PCRE, unless UTF validity checking has been
4467 turned off (see the discussion of PCRE_NO_UTF8_CHECK and
4468 PCRE_NO_UTF16_CHECK in the pcreapi page). Perl does not support the Cs
4469 property.
4471 The long synonyms for property names that Perl supports (such as
4472 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
4473 any of these properties with "Is".
4475 No character that is in the Unicode table has the Cn (unassigned) prop-
4476 erty. Instead, this property is assumed for any code point that is not
4477 in the Unicode table.
4479 Specifying caseless matching does not affect these escape sequences.
4480 For example, \p{Lu} always matches only upper case letters.
4482 The \X escape matches any number of Unicode characters that form an
4483 extended Unicode sequence. \X is equivalent to
4485 (?>\PM\pM*)
4487 That is, it matches a character without the "mark" property, followed
4488 by zero or more characters with the "mark" property, and treats the
4489 sequence as an atomic group (see below). Characters with the "mark"
4490 property are typically accents that affect the preceding character.
4491 None of them have codepoints less than 256, so in 8-bit non-UTF-8 mode
4492 \X matches any one character.
4494 Note that recent versions of Perl have changed \X to match what Unicode
4495 calls an "extended grapheme cluster", which has a more complicated def-
4496 inition.
4498 Matching characters by Unicode property is not fast, because PCRE has
4499 to search a structure that contains data for over fifteen thousand
4500 characters. That is why the traditional escape sequences such as \d and
4501 \w do not use Unicode properties in PCRE by default, though you can
4502 make them do so by setting the PCRE_UCP option or by starting the pat-
4503 tern with (*UCP).
4505 PCRE's additional properties
4507 As well as the standard Unicode properties described in the previous
4508 section, PCRE supports four more that make it possible to convert tra-
4509 ditional escape sequences such as \w and \s and POSIX character classes
4510 to use Unicode properties. PCRE uses these non-standard, non-Perl prop-
4511 erties internally when PCRE_UCP is set. They are:
4513 Xan Any alphanumeric character
4514 Xps Any POSIX space character
4515 Xsp Any Perl space character
4516 Xwd Any Perl "word" character
4518 Xan matches characters that have either the L (letter) or the N (num-
4519 ber) property. Xps matches the characters tab, linefeed, vertical tab,
4520 formfeed, or carriage return, and any other character that has the Z
4521 (separator) property. Xsp is the same as Xps, except that vertical tab
4522 is excluded. Xwd matches the same characters as Xan, plus underscore.
4524 Resetting the match start
4526 The escape sequence \K causes any previously matched characters not to
4527 be included in the final matched sequence. For example, the pattern:
4529 foo\Kbar
4531 matches "foobar", but reports that it has matched "bar". This feature
4532 is similar to a lookbehind assertion (described below). However, in
4533 this case, the part of the subject before the real match does not have
4534 to be of fixed length, as lookbehind assertions do. The use of \K does
4535 not interfere with the setting of captured substrings. For example,
4536 when the pattern
4538 (foo)\Kbar
4540 matches "foobar", the first substring is still set to "foo".
4542 Perl documents that the use of \K within assertions is "not well
4543 defined". In PCRE, \K is acted upon when it occurs inside positive
4544 assertions, but is ignored in negative assertions.
4546 Simple assertions
4548 The final use of backslash is for certain simple assertions. An asser-
4549 tion specifies a condition that has to be met at a particular point in
4550 a match, without consuming any characters from the subject string. The
4551 use of subpatterns for more complicated assertions is described below.
4552 The backslashed assertions are:
4554 \b matches at a word boundary
4555 \B matches when not at a word boundary
4556 \A matches at the start of the subject
4557 \Z matches at the end of the subject
4558 also matches before a newline at the end of the subject
4559 \z matches only at the end of the subject
4560 \G matches at the first matching position in the subject
4562 Inside a character class, \b has a different meaning; it matches the
4563 backspace character. If any other of these assertions appears in a
4564 character class, by default it matches the corresponding literal char-
4565 acter (for example, \B matches the letter B). However, if the
4566 PCRE_EXTRA option is set, an "invalid escape sequence" error is gener-
4567 ated instead.
4569 A word boundary is a position in the subject string where the current
4570 character and the previous character do not both match \w or \W (i.e.
4571 one matches \w and the other matches \W), or the start or end of the
4572 string if the first or last character matches \w, respectively. In a
4573 UTF mode, the meanings of \w and \W can be changed by setting the
4574 PCRE_UCP option. When this is done, it also affects \b and \B. Neither
4575 PCRE nor Perl has a separate "start of word" or "end of word" metase-
4576 quence. However, whatever follows \b normally determines which it is.
4577 For example, the fragment \ba matches "a" at the start of a word.
4579 The \A, \Z, and \z assertions differ from the traditional circumflex
4580 and dollar (described in the next section) in that they only ever match
4581 at the very start and end of the subject string, whatever options are
4582 set. Thus, they are independent of multiline mode. These three asser-
4583 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
4584 affect only the behaviour of the circumflex and dollar metacharacters.
4585 However, if the startoffset argument of pcre_exec() is non-zero, indi-
4586 cating that matching is to start at a point other than the beginning of
4587 the subject, \A can never match. The difference between \Z and \z is
4588 that \Z matches before a newline at the end of the string as well as at
4589 the very end, whereas \z matches only at the end.
4591 The \G assertion is true only when the current matching position is at
4592 the start point of the match, as specified by the startoffset argument
4593 of pcre_exec(). It differs from \A when the value of startoffset is
4594 non-zero. By calling pcre_exec() multiple times with appropriate argu-
4595 ments, you can mimic Perl's /g option, and it is in this kind of imple-
4596 mentation where \G can be useful.
4598 Note, however, that PCRE's interpretation of \G, as the start of the
4599 current match, is subtly different from Perl's, which defines it as the
4600 end of the previous match. In Perl, these can be different when the
4601 previously matched string was empty. Because PCRE does just one match
4602 at a time, it cannot reproduce this behaviour.
4604 If all the alternatives of a pattern begin with \G, the expression is
4605 anchored to the starting match position, and the "anchored" flag is set
4606 in the compiled regular expression.
4611 Outside a character class, in the default matching mode, the circumflex
4612 character is an assertion that is true only if the current matching
4613 point is at the start of the subject string. If the startoffset argu-
4614 ment of pcre_exec() is non-zero, circumflex can never match if the
4615 PCRE_MULTILINE option is unset. Inside a character class, circumflex
4616 has an entirely different meaning (see below).
4618 Circumflex need not be the first character of the pattern if a number
4619 of alternatives are involved, but it should be the first thing in each
4620 alternative in which it appears if the pattern is ever to match that
4621 branch. If all possible alternatives start with a circumflex, that is,
4622 if the pattern is constrained to match only at the start of the sub-
4623 ject, it is said to be an "anchored" pattern. (There are also other
4624 constructs that can cause a pattern to be anchored.)
4626 A dollar character is an assertion that is true only if the current
4627 matching point is at the end of the subject string, or immediately
4628 before a newline at the end of the string (by default). Dollar need not
4629 be the last character of the pattern if a number of alternatives are
4630 involved, but it should be the last item in any branch in which it
4631 appears. Dollar has no special meaning in a character class.
4633 The meaning of dollar can be changed so that it matches only at the
4634 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
4635 compile time. This does not affect the \Z assertion.
4637 The meanings of the circumflex and dollar characters are changed if the
4638 PCRE_MULTILINE option is set. When this is the case, a circumflex
4639 matches immediately after internal newlines as well as at the start of
4640 the subject string. It does not match after a newline that ends the
4641 string. A dollar matches before any newlines in the string, as well as
4642 at the very end, when PCRE_MULTILINE is set. When newline is specified
4643 as the two-character sequence CRLF, isolated CR and LF characters do
4644 not indicate newlines.
4646 For example, the pattern /^abc$/ matches the subject string "def\nabc"
4647 (where \n represents a newline) in multiline mode, but not otherwise.
4648 Consequently, patterns that are anchored in single line mode because
4649 all branches start with ^ are not anchored in multiline mode, and a
4650 match for circumflex is possible when the startoffset argument of
4651 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
4652 PCRE_MULTILINE is set.
4654 Note that the sequences \A, \Z, and \z can be used to match the start
4655 and end of the subject in both modes, and if all branches of a pattern
4656 start with \A it is always anchored, whether or not PCRE_MULTILINE is
4657 set.
4662 Outside a character class, a dot in the pattern matches any one charac-
4663 ter in the subject string except (by default) a character that signi-
4664 fies the end of a line.
4666 When a line ending is defined as a single character, dot never matches
4667 that character; when the two-character sequence CRLF is used, dot does
4668 not match CR if it is immediately followed by LF, but otherwise it
4669 matches all characters (including isolated CRs and LFs). When any Uni-
4670 code line endings are being recognized, dot does not match CR or LF or
4671 any of the other line ending characters.
4673 The behaviour of dot with regard to newlines can be changed. If the
4674 PCRE_DOTALL option is set, a dot matches any one character, without
4675 exception. If the two-character sequence CRLF is present in the subject
4676 string, it takes two dots to match it.
4678 The handling of dot is entirely independent of the handling of circum-
4679 flex and dollar, the only relationship being that they both involve
4680 newlines. Dot has no special meaning in a character class.
4682 The escape sequence \N behaves like a dot, except that it is not
4683 affected by the PCRE_DOTALL option. In other words, it matches any
4684 character except one that signifies the end of a line. Perl also uses
4685 \N to match characters by name; PCRE does not support this.
4690 Outside a character class, the escape sequence \C matches any one data
4691 unit, whether or not a UTF mode is set. In the 8-bit library, one data
4692 unit is one byte; in the 16-bit library it is a 16-bit unit. Unlike a
4693 dot, \C always matches line-ending characters. The feature is provided
4694 in Perl in order to match individual bytes in UTF-8 mode, but it is
4695 unclear how it can usefully be used. Because \C breaks up characters
4696 into individual data units, matching one unit with \C in a UTF mode
4697 means that the rest of the string may start with a malformed UTF char-
4698 acter. This has undefined results, because PCRE assumes that it is
4699 dealing with valid UTF strings (and by default it checks this at the
4700 start of processing unless the PCRE_NO_UTF8_CHECK option is used).
4702 PCRE does not allow \C to appear in lookbehind assertions (described
4703 below) in a UTF mode, because this would make it impossible to calcu-
4704 late the length of the lookbehind.
4706 In general, the \C escape sequence is best avoided. However, one way of
4707 using it that avoids the problem of malformed UTF characters is to use
4708 a lookahead to check the length of the next character, as in this pat-
4709 tern, which could be used with a UTF-8 string (ignore white space and
4710 line breaks):
4712 (?| (?=[\x00-\x7f])(\C) |
4713 (?=[\x80-\x{7ff}])(\C)(\C) |
4714 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
4715 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
4717 A group that starts with (?| resets the capturing parentheses numbers
4718 in each alternative (see "Duplicate Subpattern Numbers" below). The
4719 assertions at the start of each branch check the next UTF-8 character
4720 for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
4721 character's individual bytes are then captured by the appropriate num-
4722 ber of groups.
4727 An opening square bracket introduces a character class, terminated by a
4728 closing square bracket. A closing square bracket on its own is not spe-
4729 cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
4730 a lone closing square bracket causes a compile-time error. If a closing
4731 square bracket is required as a member of the class, it should be the
4732 first data character in the class (after an initial circumflex, if
4733 present) or escaped with a backslash.
4735 A character class matches a single character in the subject. In a UTF
4736 mode, the character may be more than one data unit long. A matched
4737 character must be in the set of characters defined by the class, unless
4738 the first character in the class definition is a circumflex, in which
4739 case the subject character must not be in the set defined by the class.
4740 If a circumflex is actually required as a member of the class, ensure
4741 it is not the first character, or escape it with a backslash.
4743 For example, the character class [aeiou] matches any lower case vowel,
4744 while [^aeiou] matches any character that is not a lower case vowel.
4745 Note that a circumflex is just a convenient notation for specifying the
4746 characters that are in the class by enumerating those that are not. A
4747 class that starts with a circumflex is not an assertion; it still con-
4748 sumes a character from the subject string, and therefore it fails if
4749 the current pointer is at the end of the string.
4751 In UTF-8 (UTF-16) mode, characters with values greater than 255
4752 (0xffff) can be included in a class as a literal string of data units,
4753 or by using the \x{ escaping mechanism.
4755 When caseless matching is set, any letters in a class represent both
4756 their upper case and lower case versions, so for example, a caseless
4757 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
4758 match "A", whereas a caseful version would. In a UTF mode, PCRE always
4759 understands the concept of case for characters whose values are less
4760 than 128, so caseless matching is always possible. For characters with
4761 higher values, the concept of case is supported if PCRE is compiled
4762 with Unicode property support, but not otherwise. If you want to use
4763 caseless matching in a UTF mode for characters 128 and above, you must
4764 ensure that PCRE is compiled with Unicode property support as well as
4765 with UTF support.
4767 Characters that might indicate line breaks are never treated in any
4768 special way when matching character classes, whatever line-ending
4769 sequence is in use, and whatever setting of the PCRE_DOTALL and
4770 PCRE_MULTILINE options is used. A class such as [^a] always matches one
4771 of these characters.
4773 The minus (hyphen) character can be used to specify a range of charac-
4774 ters in a character class. For example, [d-m] matches any letter
4775 between d and m, inclusive. If a minus character is required in a
4776 class, it must be escaped with a backslash or appear in a position
4777 where it cannot be interpreted as indicating a range, typically as the
4778 first or last character in the class.
4780 It is not possible to have the literal character "]" as the end charac-
4781 ter of a range. A pattern such as [W-]46] is interpreted as a class of
4782 two characters ("W" and "-") followed by a literal string "46]", so it
4783 would match "W46]" or "-46]". However, if the "]" is escaped with a
4784 backslash it is interpreted as the end of range, so [W-\]46] is inter-
4785 preted as a class containing a range followed by two other characters.
4786 The octal or hexadecimal representation of "]" can also be used to end
4787 a range.
4789 Ranges operate in the collating sequence of character values. They can
4790 also be used for characters specified numerically, for example
4791 [\000-\037]. Ranges can include any characters that are valid for the
4792 current mode.
4794 If a range that includes letters is used when caseless matching is set,
4795 it matches the letters in either case. For example, [W-c] is equivalent
4796 to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if
4797 character tables for a French locale are in use, [\xc8-\xcb] matches
4798 accented E characters in both cases. In UTF modes, PCRE supports the
4799 concept of case for characters with values greater than 128 only when
4800 it is compiled with Unicode property support.
4802 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
4803 \w, and \W may appear in a character class, and add the characters that
4804 they match to the class. For example, [\dABCDEF] matches any hexadeci-
4805 mal digit. In UTF modes, the PCRE_UCP option affects the meanings of
4806 \d, \s, \w and their upper case partners, just as it does when they
4807 appear outside a character class, as described in the section entitled
4808 "Generic character types" above. The escape sequence \b has a different
4809 meaning inside a character class; it matches the backspace character.
4810 The sequences \B, \N, \R, and \X are not special inside a character
4811 class. Like any other unrecognized escape sequences, they are treated
4812 as the literal characters "B", "N", "R", and "X" by default, but cause
4813 an error if the PCRE_EXTRA option is set.
4815 A circumflex can conveniently be used with the upper case character
4816 types to specify a more restricted set of characters than the matching
4817 lower case type. For example, the class [^\W_] matches any letter or
4818 digit, but not underscore, whereas [\w] includes underscore. A positive
4819 character class should be read as "something OR something OR ..." and a
4820 negative class as "NOT something AND NOT something AND NOT ...".
4822 The only metacharacters that are recognized in character classes are
4823 backslash, hyphen (only where it can be interpreted as specifying a
4824 range), circumflex (only at the start), opening square bracket (only
4825 when it can be interpreted as introducing a POSIX class name - see the
4826 next section), and the terminating closing square bracket. However,
4827 escaping other non-alphanumeric characters does no harm.
4832 Perl supports the POSIX notation for character classes. This uses names
4833 enclosed by [: and :] within the enclosing square brackets. PCRE also
4834 supports this notation. For example,
4836 [01[:alpha:]%]
4838 matches "0", "1", any alphabetic character, or "%". The supported class
4839 names are:
4841 alnum letters and digits
4842 alpha letters
4843 ascii character codes 0 - 127
4844 blank space or tab only
4845 cntrl control characters
4846 digit decimal digits (same as \d)
4847 graph printing characters, excluding space
4848 lower lower case letters
4849 print printing characters, including space
4850 punct printing characters, excluding letters and digits and space
4851 space white space (not quite the same as \s)
4852 upper upper case letters
4853 word "word" characters (same as \w)
4854 xdigit hexadecimal digits
4856 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
4857 and space (32). Notice that this list includes the VT character (code
4858 11). This makes "space" different to \s, which does not include VT (for
4859 Perl compatibility).
4861 The name "word" is a Perl extension, and "blank" is a GNU extension
4862 from Perl 5.8. Another Perl extension is negation, which is indicated
4863 by a ^ character after the colon. For example,
4865 [12[:^digit:]]
4867 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
4868 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
4869 these are not supported, and an error is given if they are encountered.
4871 By default, in UTF modes, characters with values greater than 128 do
4872 not match any of the POSIX character classes. However, if the PCRE_UCP
4873 option is passed to pcre_compile(), some of the classes are changed so
4874 that Unicode character properties are used. This is achieved by replac-
4875 ing the POSIX classes by other sequences, as follows:
4877 [:alnum:] becomes \p{Xan}
4878 [:alpha:] becomes \p{L}
4879 [:blank:] becomes \h
4880 [:digit:] becomes \p{Nd}
4881 [:lower:] becomes \p{Ll}
4882 [:space:] becomes \p{Xps}
4883 [:upper:] becomes \p{Lu}
4884 [:word:] becomes \p{Xwd}
4886 Negated versions, such as [:^alpha:] use \P instead of \p. The other
4887 POSIX classes are unchanged, and match only characters with code points
4888 less than 128.
4893 Vertical bar characters are used to separate alternative patterns. For
4894 example, the pattern
4896 gilbert|sullivan
4898 matches either "gilbert" or "sullivan". Any number of alternatives may
4899 appear, and an empty alternative is permitted (matching the empty
4900 string). The matching process tries each alternative in turn, from left
4901 to right, and the first one that succeeds is used. If the alternatives
4902 are within a subpattern (defined below), "succeeds" means matching the
4903 rest of the main pattern as well as the alternative in the subpattern.
4908 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
4909 PCRE_EXTENDED options (which are Perl-compatible) can be changed from
4910 within the pattern by a sequence of Perl option letters enclosed
4911 between "(?" and ")". The option letters are
4913 i for PCRE_CASELESS
4915 s for PCRE_DOTALL
4916 x for PCRE_EXTENDED
4918 For example, (?im) sets caseless, multiline matching. It is also possi-
4919 ble to unset these options by preceding the letter with a hyphen, and a
4920 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
4922 is also permitted. If a letter appears both before and after the
4923 hyphen, the option is unset.
4926 can be changed in the same way as the Perl-compatible options by using
4927 the characters J, U and X respectively.
4929 When one of these option changes occurs at top level (that is, not
4930 inside subpattern parentheses), the change applies to the remainder of
4931 the pattern that follows. If the change is placed right at the start of
4932 a pattern, PCRE extracts it into the global options (and it will there-
4933 fore show up in data extracted by the pcre_fullinfo() function).
4935 An option change within a subpattern (see below for a description of
4936 subpatterns) affects only that part of the subpattern that follows it,
4937 so
4939 (a(?i)b)c
4941 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
4942 used). By this means, options can be made to have different settings
4943 in different parts of the pattern. Any changes made in one alternative
4944 do carry on into subsequent branches within the same subpattern. For
4945 example,
4947 (a(?i)b|c)
4949 matches "ab", "aB", "c", and "C", even though when matching "C" the
4950 first branch is abandoned before the option setting. This is because
4951 the effects of option settings happen at compile time. There would be
4952 some very weird behaviour otherwise.
4954 Note: There are other PCRE-specific options that can be set by the
4955 application when the compiling or matching functions are called. In
4956 some cases the pattern can contain special leading sequences such as
4957 (*CRLF) to override what the application has set or what has been
4958 defaulted. Details are given in the section entitled "Newline
4959 sequences" above. There are also the (*UTF8), (*UTF16), and (*UCP)
4960 leading sequences that can be used to set UTF and Unicode property
4961 modes; they are equivalent to setting the PCRE_UTF8, PCRE_UTF16, and
4962 the PCRE_UCP options, respectively.
4967 Subpatterns are delimited by parentheses (round brackets), which can be
4968 nested. Turning part of a pattern into a subpattern does two things:
4970 1. It localizes a set of alternatives. For example, the pattern
4972 cat(aract|erpillar|)
4974 matches "cataract", "caterpillar", or "cat". Without the parentheses,
4975 it would match "cataract", "erpillar" or an empty string.
4977 2. It sets up the subpattern as a capturing subpattern. This means
4978 that, when the whole pattern matches, that portion of the subject
4979 string that matched the subpattern is passed back to the caller via the
4980 ovector argument of the matching function. (This applies only to the
4981 traditional matching functions; the DFA matching functions do not sup-
4982 port capturing.)
4984 Opening parentheses are counted from left to right (starting from 1) to
4985 obtain numbers for the capturing subpatterns. For example, if the
4986 string "the red king" is matched against the pattern
4988 the ((red|white) (king|queen))
4990 the captured substrings are "red king", "red", and "king", and are num-
4991 bered 1, 2, and 3, respectively.
4993 The fact that plain parentheses fulfil two functions is not always
4994 helpful. There are often times when a grouping subpattern is required
4995 without a capturing requirement. If an opening parenthesis is followed
4996 by a question mark and a colon, the subpattern does not do any captur-
4997 ing, and is not counted when computing the number of any subsequent
4998 capturing subpatterns. For example, if the string "the white queen" is
4999 matched against the pattern
5001 the ((?:red|white) (king|queen))
5003 the captured substrings are "white queen" and "queen", and are numbered
5004 1 and 2. The maximum number of capturing subpatterns is 65535.
5006 As a convenient shorthand, if any option settings are required at the
5007 start of a non-capturing subpattern, the option letters may appear
5008 between the "?" and the ":". Thus the two patterns
5010 (?i:saturday|sunday)
5011 (?:(?i)saturday|sunday)
5013 match exactly the same set of strings. Because alternative branches are
5014 tried from left to right, and options are not reset until the end of
5015 the subpattern is reached, an option setting in one branch does affect
5016 subsequent branches, so the above patterns match "SUNDAY" as well as
5017 "Saturday".
5022 Perl 5.10 introduced a feature whereby each alternative in a subpattern
5023 uses the same numbers for its capturing parentheses. Such a subpattern
5024 starts with (?| and is itself a non-capturing subpattern. For example,
5025 consider this pattern:
5027 (?|(Sat)ur|(Sun))day
5029 Because the two alternatives are inside a (?| group, both sets of cap-
5030 turing parentheses are numbered one. Thus, when the pattern matches,
5031 you can look at captured substring number one, whichever alternative
5032 matched. This construct is useful when you want to capture part, but
5033 not all, of one of a number of alternatives. Inside a (?| group, paren-
5034 theses are numbered as usual, but the number is reset at the start of
5035 each branch. The numbers of any capturing parentheses that follow the
5036 subpattern start after the highest number used in any branch. The fol-
5037 lowing example is taken from the Perl documentation. The numbers under-
5038 neath show in which buffer the captured content will be stored.
5040 # before ---------------branch-reset----------- after
5041 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
5042 # 1 2 2 3 2 3 4
5044 A back reference to a numbered subpattern uses the most recent value
5045 that is set for that number by any subpattern. The following pattern
5046 matches "abcabc" or "defdef":
5048 /(?|(abc)|(def))\1/
5050 In contrast, a subroutine call to a numbered subpattern always refers
5051 to the first one in the pattern with the given number. The following
5052 pattern matches "abcabc" or "defabc":
5054 /(?|(abc)|(def))(?1)/
5056 If a condition test for a subpattern's having matched refers to a non-
5057 unique number, the test is true if any of the subpatterns of that num-
5058 ber have matched.
5060 An alternative approach to using this "branch reset" feature is to use
5061 duplicate named subpatterns, as described in the next section.
5066 Identifying capturing parentheses by number is simple, but it can be
5067 very hard to keep track of the numbers in complicated regular expres-
5068 sions. Furthermore, if an expression is modified, the numbers may
5069 change. To help with this difficulty, PCRE supports the naming of sub-
5070 patterns. This feature was not added to Perl until release 5.10. Python
5071 had the feature earlier, and PCRE introduced it at release 4.0, using
5072 the Python syntax. PCRE now supports both the Perl and the Python syn-
5073 tax. Perl allows identically numbered subpatterns to have different
5074 names, but PCRE does not.
5076 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
5077 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
5078 to capturing parentheses from other parts of the pattern, such as back
5079 references, recursion, and conditions, can be made by name as well as
5080 by number.
5082 Names consist of up to 32 alphanumeric characters and underscores.
5083 Named capturing parentheses are still allocated numbers as well as
5084 names, exactly as if the names were not present. The PCRE API provides
5085 function calls for extracting the name-to-number translation table from
5086 a compiled pattern. There is also a convenience function for extracting
5087 a captured substring by name.
5089 By default, a name must be unique within a pattern, but it is possible
5090 to relax this constraint by setting the PCRE_DUPNAMES option at compile
5091 time. (Duplicate names are also always permitted for subpatterns with
5092 the same number, set up as described in the previous section.) Dupli-
5093 cate names can be useful for patterns where only one instance of the
5094 named parentheses can match. Suppose you want to match the name of a
5095 weekday, either as a 3-letter abbreviation or as the full name, and in
5096 both cases you want to extract the abbreviation. This pattern (ignoring
5097 the line breaks) does the job:
5099 (?<DN>Mon|Fri|Sun)(?:day)?|
5100 (?<DN>Tue)(?:sday)?|
5101 (?<DN>Wed)(?:nesday)?|
5102 (?<DN>Thu)(?:rsday)?|
5103 (?<DN>Sat)(?:urday)?
5105 There are five capturing substrings, but only one is ever set after a
5106 match. (An alternative way of solving this problem is to use a "branch
5107 reset" subpattern, as described in the previous section.)
5109 The convenience function for extracting the data by name returns the
5110 substring for the first (and in this example, the only) subpattern of
5111 that name that matched. This saves searching to find which numbered
5112 subpattern it was.
5114 If you make a back reference to a non-unique named subpattern from
5115 elsewhere in the pattern, the one that corresponds to the first occur-
5116 rence of the name is used. In the absence of duplicate numbers (see the
5117 previous section) this is the one with the lowest number. If you use a
5118 named reference in a condition test (see the section about conditions
5119 below), either to check whether a subpattern has matched, or to check
5120 for recursion, all subpatterns with the same name are tested. If the
5121 condition is true for any one of them, the overall condition is true.
5122 This is the same behaviour as testing by number. For further details of
5123 the interfaces for handling named subpatterns, see the pcreapi documen-
5124 tation.
5126 Warning: You cannot use different names to distinguish between two sub-
5127 patterns with the same number because PCRE uses only the numbers when
5128 matching. For this reason, an error is given at compile time if differ-
5129 ent names are given to subpatterns with the same number. However, you
5130 can give the same name to subpatterns with the same number, even when
5131 PCRE_DUPNAMES is not set.
5136 Repetition is specified by quantifiers, which can follow any of the
5137 following items:
5139 a literal data character
5140 the dot metacharacter
5141 the \C escape sequence
5142 the \X escape sequence
5143 the \R escape sequence
5144 an escape such as \d or \pL that matches a single character
5145 a character class
5146 a back reference (see next section)
5147 a parenthesized subpattern (including assertions)
5148 a subroutine call to a subpattern (recursive or otherwise)
5150 The general repetition quantifier specifies a minimum and maximum num-
5151 ber of permitted matches, by giving the two numbers in curly brackets
5152 (braces), separated by a comma. The numbers must be less than 65536,
5153 and the first must be less than or equal to the second. For example:
5155 z{2,4}
5157 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
5158 special character. If the second number is omitted, but the comma is
5159 present, there is no upper limit; if the second number and the comma
5160 are both omitted, the quantifier specifies an exact number of required
5161 matches. Thus
5163 [aeiou]{3,}
5165 matches at least 3 successive vowels, but may match many more, while
5167 \d{8}
5169 matches exactly 8 digits. An opening curly bracket that appears in a
5170 position where a quantifier is not allowed, or one that does not match
5171 the syntax of a quantifier, is taken as a literal character. For exam-
5172 ple, {,6} is not a quantifier, but a literal string of four characters.
5174 In UTF modes, quantifiers apply to characters rather than to individual
5175 data units. Thus, for example, \x{100}{2} matches two characters, each
5176 of which is represented by a two-byte sequence in a UTF-8 string. Simi-
5177 larly, \X{3} matches three Unicode extended sequences, each of which
5178 may be several data units long (and they may be of different lengths).
5180 The quantifier {0} is permitted, causing the expression to behave as if
5181 the previous item and the quantifier were not present. This may be use-
5182 ful for subpatterns that are referenced as subroutines from elsewhere
5183 in the pattern (but see also the section entitled "Defining subpatterns
5184 for use by reference only" below). Items other than subpatterns that
5185 have a {0} quantifier are omitted from the compiled pattern.
5187 For convenience, the three most common quantifiers have single-charac-
5188 ter abbreviations:
5190 * is equivalent to {0,}
5191 + is equivalent to {1,}
5192 ? is equivalent to {0,1}
5194 It is possible to construct infinite loops by following a subpattern
5195 that can match no characters with a quantifier that has no upper limit,
5196 for example:
5198 (a?)*
5200 Earlier versions of Perl and PCRE used to give an error at compile time
5201 for such patterns. However, because there are cases where this can be
5202 useful, such patterns are now accepted, but if any repetition of the
5203 subpattern does in fact match no characters, the loop is forcibly bro-
5204 ken.
5206 By default, the quantifiers are "greedy", that is, they match as much
5207 as possible (up to the maximum number of permitted times), without
5208 causing the rest of the pattern to fail. The classic example of where
5209 this gives problems is in trying to match comments in C programs. These
5210 appear between /* and */ and within the comment, individual * and /
5211 characters may appear. An attempt to match C comments by applying the
5212 pattern
5214 /\*.*\*/
5216 to the string
5218 /* first comment */ not comment /* second comment */
5220 fails, because it matches the entire string owing to the greediness of
5221 the .* item.
5223 However, if a quantifier is followed by a question mark, it ceases to
5224 be greedy, and instead matches the minimum number of times possible, so
5225 the pattern
5227 /\*.*?\*/
5229 does the right thing with the C comments. The meaning of the various
5230 quantifiers is not otherwise changed, just the preferred number of
5231 matches. Do not confuse this use of question mark with its use as a
5232 quantifier in its own right. Because it has two uses, it can sometimes
5233 appear doubled, as in
5235 \d??\d
5237 which matches one digit by preference, but can match two if that is the
5238 only way the rest of the pattern matches.
5240 If the PCRE_UNGREEDY option is set (an option that is not available in
5241 Perl), the quantifiers are not greedy by default, but individual ones
5242 can be made greedy by following them with a question mark. In other
5243 words, it inverts the default behaviour.
5245 When a parenthesized subpattern is quantified with a minimum repeat
5246 count that is greater than 1 or with a limited maximum, more memory is
5247 required for the compiled pattern, in proportion to the size of the
5248 minimum or maximum.
5250 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
5251 alent to Perl's /s) is set, thus allowing the dot to match newlines,
5252 the pattern is implicitly anchored, because whatever follows will be
5253 tried against every character position in the subject string, so there
5254 is no point in retrying the overall match at any position after the
5255 first. PCRE normally treats such a pattern as though it were preceded
5256 by \A.
5258 In cases where it is known that the subject string contains no new-
5259 lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
5260 mization, or alternatively using ^ to indicate anchoring explicitly.
5262 However, there is one situation where the optimization cannot be used.
5263 When .* is inside capturing parentheses that are the subject of a back
5264 reference elsewhere in the pattern, a match at the start may fail where
5265 a later one succeeds. Consider, for example:
5267 (.*)abc\1
5269 If the subject is "xyz123abc123" the match point is the fourth charac-
5270 ter. For this reason, such a pattern is not implicitly anchored.
5272 When a capturing subpattern is repeated, the value captured is the sub-
5273 string that matched the final iteration. For example, after
5275 (tweedle[dume]{3}\s*)+
5277 has matched "tweedledum tweedledee" the value of the captured substring
5278 is "tweedledee". However, if there are nested capturing subpatterns,
5279 the corresponding captured values may have been set in previous itera-
5280 tions. For example, after
5282 /(a|(b))+/
5284 matches "aba" the value of the second captured substring is "b".
5289 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
5290 repetition, failure of what follows normally causes the repeated item
5291 to be re-evaluated to see if a different number of repeats allows the
5292 rest of the pattern to match. Sometimes it is useful to prevent this,
5293 either to change the nature of the match, or to cause it fail earlier
5294 than it otherwise might, when the author of the pattern knows there is
5295 no point in carrying on.
5297 Consider, for example, the pattern \d+foo when applied to the subject
5298 line
5300 123456bar
5302 After matching all 6 digits and then failing to match "foo", the normal
5303 action of the matcher is to try again with only 5 digits matching the
5304 \d+ item, and then with 4, and so on, before ultimately failing.
5305 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
5306 the means for specifying that once a subpattern has matched, it is not
5307 to be re-evaluated in this way.
5309 If we use atomic grouping for the previous example, the matcher gives
5310 up immediately on failing to match "foo" the first time. The notation
5311 is a kind of special parenthesis, starting with (?> as in this example:
5313 (?>\d+)foo
5315 This kind of parenthesis "locks up" the part of the pattern it con-
5316 tains once it has matched, and a failure further into the pattern is
5317 prevented from backtracking into it. Backtracking past it to previous
5318 items, however, works as normal.
5320 An alternative description is that a subpattern of this type matches
5321 the string of characters that an identical standalone pattern would
5322 match, if anchored at the current point in the subject string.
5324 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
5325 such as the above example can be thought of as a maximizing repeat that
5326 must swallow everything it can. So, while both \d+ and \d+? are pre-
5327 pared to adjust the number of digits they match in order to make the
5328 rest of the pattern match, (?>\d+) can only match an entire sequence of
5329 digits.
5331 Atomic groups in general can of course contain arbitrarily complicated
5332 subpatterns, and can be nested. However, when the subpattern for an
5333 atomic group is just a single repeated item, as in the example above, a
5334 simpler notation, called a "possessive quantifier" can be used. This
5335 consists of an additional + character following a quantifier. Using
5336 this notation, the previous example can be rewritten as
5338 \d++foo
5340 Note that a possessive quantifier can be used with an entire group, for
5341 example:
5343 (abc|xyz){2,3}+
5345 Possessive quantifiers are always greedy; the setting of the
5346 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
5347 simpler forms of atomic group. However, there is no difference in the
5348 meaning of a possessive quantifier and the equivalent atomic group,
5349 though there may be a performance difference; possessive quantifiers
5350 should be slightly faster.
5352 The possessive quantifier syntax is an extension to the Perl 5.8 syn-
5353 tax. Jeffrey Friedl originated the idea (and the name) in the first
5354 edition of his book. Mike McCloskey liked it, so implemented it when he
5355 built Sun's Java package, and PCRE copied it from there. It ultimately
5356 found its way into Perl at release 5.10.
5358 PCRE has an optimization that automatically "possessifies" certain sim-
5359 ple pattern constructs. For example, the sequence A+B is treated as
5360 A++B because there is no point in backtracking into a sequence of A's
5361 when B must follow.
5363 When a pattern contains an unlimited repeat inside a subpattern that
5364 can itself be repeated an unlimited number of times, the use of an
5365 atomic group is the only way to avoid some failing matches taking a
5366 very long time indeed. The pattern
5368 (\D+|<\d+>)*[!?]
5370 matches an unlimited number of substrings that either consist of non-
5371 digits, or digits enclosed in <>, followed by either ! or ?. When it
5372 matches, it runs quickly. However, if it is applied to
5374 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
5376 it takes a long time before reporting failure. This is because the
5377 string can be divided between the internal \D+ repeat and the external
5378 * repeat in a large number of ways, and all have to be tried. (The
5379 example uses [!?] rather than a single character at the end, because
5380 both PCRE and Perl have an optimization that allows for fast failure
5381 when a single character is used. They remember the last single charac-
5382 ter that is required for a match, and fail early if it is not present
5383 in the string.) If the pattern is changed so that it uses an atomic
5384 group, like this:
5386 ((?>\D+)|<\d+>)*[!?]
5388 sequences of non-digits cannot be broken, and failure happens quickly.
5393 Outside a character class, a backslash followed by a digit greater than
5394 0 (and possibly further digits) is a back reference to a capturing sub-
5395 pattern earlier (that is, to its left) in the pattern, provided there
5396 have been that many previous capturing left parentheses.
5398 However, if the decimal number following the backslash is less than 10,
5399 it is always taken as a back reference, and causes an error only if
5400 there are not that many capturing left parentheses in the entire pat-
5401 tern. In other words, the parentheses that are referenced need not be
5402 to the left of the reference for numbers less than 10. A "forward back
5403 reference" of this type can make sense when a repetition is involved
5404 and the subpattern to the right has participated in an earlier itera-
5405 tion.
5407 It is not possible to have a numerical "forward back reference" to a
5408 subpattern whose number is 10 or more using this syntax because a
5409 sequence such as \50 is interpreted as a character defined in octal.
5410 See the subsection entitled "Non-printing characters" above for further
5411 details of the handling of digits following a backslash. There is no
5412 such problem when named parentheses are used. A back reference to any
5413 subpattern is possible using named parentheses (see below).
5415 Another way of avoiding the ambiguity inherent in the use of digits
5416 following a backslash is to use the \g escape sequence. This escape
5417 must be followed by an unsigned number or a negative number, optionally
5418 enclosed in braces. These examples are all identical:
5420 (ring), \1
5421 (ring), \g1
5422 (ring), \g{1}
5424 An unsigned number specifies an absolute reference without the ambigu-
5425 ity that is present in the older syntax. It is also useful when literal
5426 digits follow the reference. A negative number is a relative reference.
5427 Consider this example:
5429 (abc(def)ghi)\g{-1}
5431 The sequence \g{-1} is a reference to the most recently started captur-
5432 ing subpattern before \g, that is, is it equivalent to \2 in this exam-
5433 ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
5434 references can be helpful in long patterns, and also in patterns that
5435 are created by joining together fragments that contain references
5436 within themselves.
5438 A back reference matches whatever actually matched the capturing sub-
5439 pattern in the current subject string, rather than anything matching
5440 the subpattern itself (see "Subpatterns as subroutines" below for a way
5441 of doing that). So the pattern
5443 (sens|respons)e and \1ibility
5445 matches "sense and sensibility" and "response and responsibility", but
5446 not "sense and responsibility". If caseful matching is in force at the
5447 time of the back reference, the case of letters is relevant. For exam-
5448 ple,
5450 ((?i)rah)\s+\1
5452 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
5453 original capturing subpattern is matched caselessly.
5455 There are several different ways of writing back references to named
5456 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
5457 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
5458 unified back reference syntax, in which \g can be used for both numeric
5459 and named references, is also supported. We could rewrite the above
5460 example in any of the following ways:
5462 (?<p1>(?i)rah)\s+\k<p1>
5463 (?'p1'(?i)rah)\s+\k{p1}
5464 (?P<p1>(?i)rah)\s+(?P=p1)
5465 (?<p1>(?i)rah)\s+\g{p1}
5467 A subpattern that is referenced by name may appear in the pattern
5468 before or after the reference.
5470 There may be more than one back reference to the same subpattern. If a
5471 subpattern has not actually been used in a particular match, any back
5472 references to it always fail by default. For example, the pattern
5474 (a|(bc))\2
5476 always fails if it starts to match "a" rather than "bc". However, if
5477 the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
5478 ence to an unset value matches an empty string.
5480 Because there may be many capturing parentheses in a pattern, all dig-
5481 its following a backslash are taken as part of a potential back refer-
5482 ence number. If the pattern continues with a digit character, some
5483 delimiter must be used to terminate the back reference. If the
5484 PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{
5485 syntax or an empty comment (see "Comments" below) can be used.
5487 Recursive back references
5489 A back reference that occurs inside the parentheses to which it refers
5490 fails when the subpattern is first used, so, for example, (a\1) never
5491 matches. However, such references can be useful inside repeated sub-
5492 patterns. For example, the pattern
5494 (a|b\1)+
5496 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
5497 ation of the subpattern, the back reference matches the character
5498 string corresponding to the previous iteration. In order for this to
5499 work, the pattern must be such that the first iteration does not need
5500 to match the back reference. This can be done using alternation, as in
5501 the example above, or by a quantifier with a minimum of zero.
5503 Back references of this type cause the group that they reference to be
5504 treated as an atomic group. Once the whole group has been matched, a
5505 subsequent matching failure cannot cause backtracking into the middle
5506 of the group.
5511 An assertion is a test on the characters following or preceding the
5512 current matching point that does not actually consume any characters.
5513 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
5514 described above.
5516 More complicated assertions are coded as subpatterns. There are two
5517 kinds: those that look ahead of the current position in the subject
5518 string, and those that look behind it. An assertion subpattern is
5519 matched in the normal way, except that it does not cause the current
5520 matching position to be changed.
5522 Assertion subpatterns are not capturing subpatterns. If such an asser-
5523 tion contains capturing subpatterns within it, these are counted for
5524 the purposes of numbering the capturing subpatterns in the whole pat-
5525 tern. However, substring capturing is carried out only for positive
5526 assertions, because it does not make sense for negative assertions.
5528 For compatibility with Perl, assertion subpatterns may be repeated;
5529 though it makes no sense to assert the same thing several times, the
5530 side effect of capturing parentheses may occasionally be useful. In
5531 practice, there only three cases:
5533 (1) If the quantifier is {0}, the assertion is never obeyed during
5534 matching. However, it may contain internal capturing parenthesized
5535 groups that are called from elsewhere via the subroutine mechanism.
5537 (2) If quantifier is {0,n} where n is greater than zero, it is treated
5538 as if it were {0,1}. At run time, the rest of the pattern match is
5539 tried with and without the assertion, the order depending on the greed-
5540 iness of the quantifier.
5542 (3) If the minimum repetition is greater than zero, the quantifier is
5543 ignored. The assertion is obeyed just once when encountered during
5544 matching.
5546 Lookahead assertions
5548 Lookahead assertions start with (?= for positive assertions and (?! for
5549 negative assertions. For example,
5551 \w+(?=;)
5553 matches a word followed by a semicolon, but does not include the semi-
5554 colon in the match, and
5556 foo(?!bar)
5558 matches any occurrence of "foo" that is not followed by "bar". Note
5559 that the apparently similar pattern
5561 (?!foo)bar
5563 does not find an occurrence of "bar" that is preceded by something
5564 other than "foo"; it finds any occurrence of "bar" whatsoever, because
5565 the assertion (?!foo) is always true when the next three characters are
5566 "bar". A lookbehind assertion is needed to achieve the other effect.
5568 If you want to force a matching failure at some point in a pattern, the
5569 most convenient way to do it is with (?!) because an empty string
5570 always matches, so an assertion that requires there not to be an empty
5571 string must always fail. The backtracking control verb (*FAIL) or (*F)
5572 is a synonym for (?!).
5574 Lookbehind assertions
5576 Lookbehind assertions start with (?<= for positive assertions and (?<!
5577 for negative assertions. For example,
5579 (?<!foo)bar
5581 does find an occurrence of "bar" that is not preceded by "foo". The
5582 contents of a lookbehind assertion are restricted such that all the
5583 strings it matches must have a fixed length. However, if there are sev-
5584 eral top-level alternatives, they do not all have to have the same
5585 fixed length. Thus
5587 (?<=bullock|donkey)
5589 is permitted, but
5591 (?<!dogs?|cats?)
5593 causes an error at compile time. Branches that match different length
5594 strings are permitted only at the top level of a lookbehind assertion.
5595 This is an extension compared with Perl, which requires all branches to
5596 match the same length of string. An assertion such as
5598 (?<=ab(c|de))
5600 is not permitted, because its single top-level branch can match two
5601 different lengths, but it is acceptable to PCRE if rewritten to use two
5602 top-level branches:
5604 (?<=abc|abde)
5606 In some cases, the escape sequence \K (see above) can be used instead
5607 of a lookbehind assertion to get round the fixed-length restriction.
5609 The implementation of lookbehind assertions is, for each alternative,
5610 to temporarily move the current position back by the fixed length and
5611 then try to match. If there are insufficient characters before the cur-
5612 rent position, the assertion fails.
5614 In a UTF mode, PCRE does not allow the \C escape (which matches a sin-
5615 gle data unit even in a UTF mode) to appear in lookbehind assertions,
5616 because it makes it impossible to calculate the length of the lookbe-
5617 hind. The \X and \R escapes, which can match different numbers of data
5618 units, are also not permitted.
5620 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
5621 lookbehinds, as long as the subpattern matches a fixed-length string.
5622 Recursion, however, is not supported.
5624 Possessive quantifiers can be used in conjunction with lookbehind
5625 assertions to specify efficient matching of fixed-length strings at the
5626 end of subject strings. Consider a simple pattern such as
5628 abcd$
5630 when applied to a long string that does not match. Because matching
5631 proceeds from left to right, PCRE will look for each "a" in the subject
5632 and then see if what follows matches the rest of the pattern. If the
5633 pattern is specified as
5635 ^.*abcd$
5637 the initial .* matches the entire string at first, but when this fails
5638 (because there is no following "a"), it backtracks to match all but the
5639 last character, then all but the last two characters, and so on. Once
5640 again the search for "a" covers the entire string, from right to left,
5641 so we are no better off. However, if the pattern is written as
5643 ^.*+(?<=abcd)
5645 there can be no backtracking for the .*+ item; it can match only the
5646 entire string. The subsequent lookbehind assertion does a single test
5647 on the last four characters. If it fails, the match fails immediately.
5648 For long strings, this approach makes a significant difference to the
5649 processing time.
5651 Using multiple assertions
5653 Several assertions (of any sort) may occur in succession. For example,
5655 (?<=\d{3})(?<!999)foo
5657 matches "foo" preceded by three digits that are not "999". Notice that
5658 each of the assertions is applied independently at the same point in
5659 the subject string. First there is a check that the previous three
5660 characters are all digits, and then there is a check that the same
5661 three characters are not "999". This pattern does not match "foo" pre-
5662 ceded by six characters, the first of which are digits and the last
5663 three of which are not "999". For example, it doesn't match "123abc-
5664 foo". A pattern to do that is
5666 (?<=\d{3}...)(?<!999)foo
5668 This time the first assertion looks at the preceding six characters,
5669 checking that the first three are digits, and then the second assertion
5670 checks that the preceding three characters are not "999".
5672 Assertions can be nested in any combination. For example,
5674 (?<=(?<!foo)bar)baz
5676 matches an occurrence of "baz" that is preceded by "bar" which in turn
5677 is not preceded by "foo", while
5679 (?<=\d{3}(?!999)...)foo
5681 is another pattern that matches "foo" preceded by three digits and any
5682 three characters that are not "999".
5687 It is possible to cause the matching process to obey a subpattern con-
5688 ditionally or to choose between two alternative subpatterns, depending
5689 on the result of an assertion, or whether a specific capturing subpat-
5690 tern has already been matched. The two possible forms of conditional
5691 subpattern are:
5693 (?(condition)yes-pattern)
5694 (?(condition)yes-pattern|no-pattern)
5696 If the condition is satisfied, the yes-pattern is used; otherwise the
5697 no-pattern (if present) is used. If there are more than two alterna-
5698 tives in the subpattern, a compile-time error occurs. Each of the two
5699 alternatives may itself contain nested subpatterns of any form, includ-
5700 ing conditional subpatterns; the restriction to two alternatives
5701 applies only at the level of the condition. This pattern fragment is an
5702 example where the alternatives are complex:
5704 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
5707 There are four kinds of condition: references to subpatterns, refer-
5708 ences to recursion, a pseudo-condition called DEFINE, and assertions.
5710 Checking for a used subpattern by number
5712 If the text between the parentheses consists of a sequence of digits,
5713 the condition is true if a capturing subpattern of that number has pre-
5714 viously matched. If there is more than one capturing subpattern with
5715 the same number (see the earlier section about duplicate subpattern
5716 numbers), the condition is true if any of them have matched. An alter-
5717 native notation is to precede the digits with a plus or minus sign. In
5718 this case, the subpattern number is relative rather than absolute. The
5719 most recently opened parentheses can be referenced by (?(-1), the next
5720 most recent by (?(-2), and so on. Inside loops it can also make sense
5721 to refer to subsequent groups. The next parentheses to be opened can be
5722 referenced as (?(+1), and so on. (The value zero in any of these forms
5723 is not used; it provokes a compile-time error.)
5725 Consider the following pattern, which contains non-significant white
5726 space to make it more readable (assume the PCRE_EXTENDED option) and to
5727 divide it into three parts for ease of discussion:
5729 ( \( )? [^()]+ (?(1) \) )
5731 The first part matches an optional opening parenthesis, and if that
5732 character is present, sets it as the first captured substring. The sec-
5733 ond part matches one or more characters that are not parentheses. The
5734 third part is a conditional subpattern that tests whether or not the
5735 first set of parentheses matched. If they did, that is, if subject
5736 started with an opening parenthesis, the condition is true, and so the
5737 yes-pattern is executed and a closing parenthesis is required. Other-
5738 wise, since no-pattern is not present, the subpattern matches nothing.
5739 In other words, this pattern matches a sequence of non-parentheses,
5740 optionally enclosed in parentheses.
5742 If you were embedding this pattern in a larger one, you could use a
5743 relative reference:
5745 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
5747 This makes the fragment independent of the parentheses in the larger
5748 pattern.
5750 Checking for a used subpattern by name
5752 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
5753 used subpattern by name. For compatibility with earlier versions of
5754 PCRE, which had this facility before Perl, the syntax (?(name)...) is
5755 also recognized. However, there is a possible ambiguity with this syn-
5756 tax, because subpattern names may consist entirely of digits. PCRE
5757 looks first for a named subpattern; if it cannot find one and the name
5758 consists entirely of digits, PCRE looks for a subpattern of that num-
5759 ber, which must be greater than zero. Using subpattern names that con-
5760 sist entirely of digits is not recommended.
5762 Rewriting the above example to use a named subpattern gives this:
5764 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
5766 If the name used in a condition of this kind is a duplicate, the test
5767 is applied to all subpatterns of the same name, and is true if any one
5768 of them has matched.
5770 Checking for pattern recursion
5772 If the condition is the string (R), and there is no subpattern with the
5773 name R, the condition is true if a recursive call to the whole pattern
5774 or any subpattern has been made. If digits or a name preceded by amper-
5775 sand follow the letter R, for example:
5777 (?(R3)...) or (?(R&name)...)
5779 the condition is true if the most recent recursion is into a subpattern
5780 whose number or name is given. This condition does not check the entire
5781 recursion stack. If the name used in a condition of this kind is a
5782 duplicate, the test is applied to all subpatterns of the same name, and
5783 is true if any one of them is the most recent recursion.
5785 At "top level", all these recursion test conditions are false. The
5786 syntax for recursive patterns is described below.
5788 Defining subpatterns for use by reference only
5790 If the condition is the string (DEFINE), and there is no subpattern
5791 with the name DEFINE, the condition is always false. In this case,
5792 there may be only one alternative in the subpattern. It is always
5793 skipped if control reaches this point in the pattern; the idea of
5794 DEFINE is that it can be used to define subroutines that can be refer-
5795 enced from elsewhere. (The use of subroutines is described below.) For
5796 example, a pattern to match an IPv4 address such as ""
5797 could be written like this (ignore whitespace and line breaks):
5799 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
5800 \b (?&byte) (\.(?&byte)){3} \b
5802 The first part of the pattern is a DEFINE group inside which a another
5803 group named "byte" is defined. This matches an individual component of
5804 an IPv4 address (a number less than 256). When matching takes place,
5805 this part of the pattern is skipped because DEFINE acts like a false
5806 condition. The rest of the pattern uses references to the named group
5807 to match the four dot-separated components of an IPv4 address, insist-
5808 ing on a word boundary at each end.
5810 Assertion conditions
5812 If the condition is not in any of the above formats, it must be an
5813 assertion. This may be a positive or negative lookahead or lookbehind
5814 assertion. Consider this pattern, again containing non-significant
5815 white space, and with the two alternatives on the second line:
5817 (?(?=[^a-z]*[a-z])
5818 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
5820 The condition is a positive lookahead assertion that matches an
5821 optional sequence of non-letters followed by a letter. In other words,
5822 it tests for the presence of at least one letter in the subject. If a
5823 letter is found, the subject is matched against the first alternative;
5824 otherwise it is matched against the second. This pattern matches
5825 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
5826 letters and dd are digits.
5831 There are two ways of including comments in patterns that are processed
5832 by PCRE. In both cases, the start of the comment must not be in a char-
5833 acter class, nor in the middle of any other sequence of related charac-
5834 ters such as (?: or a subpattern name or number. The characters that
5835 make up a comment play no part in the pattern matching.
5837 The sequence (?# marks the start of a comment that continues up to the
5838 next closing parenthesis. Nested parentheses are not permitted. If the
5839 PCRE_EXTENDED option is set, an unescaped # character also introduces a
5840 comment, which in this case continues to immediately after the next
5841 newline character or character sequence in the pattern. Which charac-
5842 ters are interpreted as newlines is controlled by the options passed to
5843 a compiling function or by a special sequence at the start of the pat-
5844 tern, as described in the section entitled "Newline conventions" above.
5845 Note that the end of this type of comment is a literal newline sequence
5846 in the pattern; escape sequences that happen to represent a newline do
5847 not count. For example, consider this pattern when PCRE_EXTENDED is
5848 set, and the default newline convention is in force:
5850 abc #comment \n still comment
5852 On encountering the # character, pcre_compile() skips along, looking
5853 for a newline in the pattern. The sequence \n is still literal at this
5854 stage, so it does not terminate the comment. Only an actual character
5855 with the code value 0x0a (the default newline) does so.
5860 Consider the problem of matching a string in parentheses, allowing for
5861 unlimited nested parentheses. Without the use of recursion, the best
5862 that can be done is to use a pattern that matches up to some fixed
5863 depth of nesting. It is not possible to handle an arbitrary nesting
5864 depth.
5866 For some time, Perl has provided a facility that allows regular expres-
5867 sions to recurse (amongst other things). It does this by interpolating
5868 Perl code in the expression at run time, and the code can refer to the
5869 expression itself. A Perl pattern using code interpolation to solve the
5870 parentheses problem can be created like this:
5872 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
5874 The (?p{...}) item interpolates Perl code at run time, and in this case
5875 refers recursively to the pattern in which it appears.
5877 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
5878 it supports special syntax for recursion of the entire pattern, and
5879 also for individual subpattern recursion. After its introduction in
5880 PCRE and Python, this kind of recursion was subsequently introduced
5881 into Perl at release 5.10.
5883 A special item that consists of (? followed by a number greater than
5884 zero and a closing parenthesis is a recursive subroutine call of the
5885 subpattern of the given number, provided that it occurs inside that
5886 subpattern. (If not, it is a non-recursive subroutine call, which is
5887 described in the next section.) The special item (?R) or (?0) is a
5888 recursive call of the entire regular expression.
5890 This PCRE pattern solves the nested parentheses problem (assume the
5891 PCRE_EXTENDED option is set so that white space is ignored):
5893 \( ( [^()]++ | (?R) )* \)
5895 First it matches an opening parenthesis. Then it matches any number of
5896 substrings which can either be a sequence of non-parentheses, or a
5897 recursive match of the pattern itself (that is, a correctly parenthe-
5898 sized substring). Finally there is a closing parenthesis. Note the use
5899 of a possessive quantifier to avoid backtracking into sequences of non-
5900 parentheses.
5902 If this were part of a larger pattern, you would not want to recurse
5903 the entire pattern, so instead you could use this:
5905 ( \( ( [^()]++ | (?1) )* \) )
5907 We have put the pattern into parentheses, and caused the recursion to
5908 refer to them instead of the whole pattern.
5910 In a larger pattern, keeping track of parenthesis numbers can be
5911 tricky. This is made easier by the use of relative references. Instead
5912 of (?1) in the pattern above you can write (?-2) to refer to the second
5913 most recently opened parentheses preceding the recursion. In other
5914 words, a negative number counts capturing parentheses leftwards from
5915 the point at which it is encountered.
5917 It is also possible to refer to subsequently opened parentheses, by
5918 writing references such as (?+2). However, these cannot be recursive
5919 because the reference is not inside the parentheses that are refer-
5920 enced. They are always non-recursive subroutine calls, as described in
5921 the next section.
5923 An alternative approach is to use named parentheses instead. The Perl
5924 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
5925 supported. We could rewrite the above example as follows:
5927 (?<pn> \( ( [^()]++ | (?&pn) )* \) )
5929 If there is more than one subpattern with the same name, the earliest
5930 one is used.
5932 This particular example pattern that we have been looking at contains
5933 nested unlimited repeats, and so the use of a possessive quantifier for
5934 matching strings of non-parentheses is important when applying the pat-
5935 tern to strings that do not match. For example, when this pattern is
5936 applied to
5938 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
5940 it yields "no match" quickly. However, if a possessive quantifier is
5941 not used, the match runs for a very long time indeed because there are
5942 so many different ways the + and * repeats can carve up the subject,
5943 and all have to be tested before failure can be reported.
5945 At the end of a match, the values of capturing parentheses are those
5946 from the outermost level. If you want to obtain intermediate values, a
5947 callout function can be used (see below and the pcrecallout documenta-
5948 tion). If the pattern above is matched against
5950 (ab(cd)ef)
5952 the value for the inner capturing parentheses (numbered 2) is "ef",
5953 which is the last value taken on at the top level. If a capturing sub-
5954 pattern is not matched at the top level, its final captured value is
5955 unset, even if it was (temporarily) set at a deeper level during the
5956 matching process.
5958 If there are more than 15 capturing parentheses in a pattern, PCRE has
5959 to obtain extra memory to store data during a recursion, which it does
5960 by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
5961 can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
5963 Do not confuse the (?R) item with the condition (R), which tests for
5964 recursion. Consider this pattern, which matches text in angle brack-
5965 ets, allowing for arbitrary nesting. Only digits are allowed in nested
5966 brackets (that is, when recursing), whereas any characters are permit-
5967 ted at the outer level.
5969 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
5971 In this pattern, (?(R) is the start of a conditional subpattern, with
5972 two different alternatives for the recursive and non-recursive cases.
5973 The (?R) item is the actual recursive call.
5975 Differences in recursion processing between PCRE and Perl
5977 Recursion processing in PCRE differs from Perl in two important ways.
5978 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
5979 always treated as an atomic group. That is, once it has matched some of
5980 the subject string, it is never re-entered, even if it contains untried
5981 alternatives and there is a subsequent matching failure. This can be
5982 illustrated by the following pattern, which purports to match a palin-
5983 dromic string that contains an odd number of characters (for example,
5984 "a", "aba", "abcba", "abcdcba"):
5986 ^(.|(.)(?1)\2)$
5988 The idea is that it either matches a single character, or two identical
5989 characters surrounding a sub-palindrome. In Perl, this pattern works;
5990 in PCRE it does not if the pattern is longer than three characters.
5991 Consider the subject string "abcba":
5993 At the top level, the first character is matched, but as it is not at
5994 the end of the string, the first alternative fails; the second alterna-
5995 tive is taken and the recursion kicks in. The recursive call to subpat-
5996 tern 1 successfully matches the next character ("b"). (Note that the
5997 beginning and end of line tests are not part of the recursion).
5999 Back at the top level, the next character ("c") is compared with what
6000 subpattern 2 matched, which was "a". This fails. Because the recursion
6001 is treated as an atomic group, there are now no backtracking points,
6002 and so the entire match fails. (Perl is able, at this point, to re-
6003 enter the recursion and try the second alternative.) However, if the
6004 pattern is written with the alternatives in the other order, things are
6005 different:
6007 ^((.)(?1)\2|.)$
6009 This time, the recursing alternative is tried first, and continues to
6010 recurse until it runs out of characters, at which point the recursion
6011 fails. But this time we do have another alternative to try at the
6012 higher level. That is the big difference: in the previous case the
6013 remaining alternative is at a deeper recursion level, which PCRE cannot
6014 use.
6016 To change the pattern so that it m