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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 an integer whose value specifies the default character
1411 sequence that is recognized as meaning "newline". The four values that
1412 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
1413 and -1 for ANY. Though they are derived from ASCII, the same values
1414 are returned in EBCDIC environments. The default should normally corre-
1415 spond to the standard sequence for your operating system.
1419 The output is an integer whose value indicates what character sequences
1420 the \R escape sequence matches by default. A value of 0 means that \R
1421 matches any Unicode line ending sequence; a value of 1 means that \R
1422 matches only CR, LF, or CRLF. The default can be overridden when a pat-
1423 tern is compiled or matched.
1427 The output is an integer that contains the number of bytes used for
1428 internal linkage in compiled regular expressions. For the 8-bit
1429 library, the value can be 2, 3, or 4. For the 16-bit library, the value
1430 is either 2 or 4 and is still a number of bytes. The default value of 2
1431 is sufficient for all but the most massive patterns, since it allows
1432 the compiled pattern to be up to 64K in size. Larger values allow
1433 larger regular expressions to be compiled, at the expense of slower
1434 matching.
1438 The output is an integer that contains the threshold above which the
1439 POSIX interface uses malloc() for output vectors. Further details are
1440 given in the pcreposix documentation.
1444 The output is a long integer that gives the default limit for the num-
1445 ber of internal matching function calls in a pcre_exec() execution.
1446 Further details are given with pcre_exec() below.
1450 The output is a long integer that gives the default limit for the depth
1451 of recursion when calling the internal matching function in a
1452 pcre_exec() execution. Further details are given with pcre_exec()
1453 below.
1457 The output is an integer that is set to one if internal recursion when
1458 running pcre_exec() is implemented by recursive function calls that use
1459 the stack to remember their state. This is the usual way that PCRE is
1460 compiled. The output is zero if PCRE was compiled to use blocks of data
1461 on the heap instead of recursive function calls. In this case,
1462 pcre_stack_malloc and pcre_stack_free are called to manage memory
1463 blocks on the heap, thus avoiding the use of the stack.
1468 pcre *pcre_compile(const char *pattern, int options,
1469 const char **errptr, int *erroffset,
1470 const unsigned char *tableptr);
1472 pcre *pcre_compile2(const char *pattern, int options,
1473 int *errorcodeptr,
1474 const char **errptr, int *erroffset,
1475 const unsigned char *tableptr);
1477 Either of the functions pcre_compile() or pcre_compile2() can be called
1478 to compile a pattern into an internal form. The only difference between
1479 the two interfaces is that pcre_compile2() has an additional argument,
1480 errorcodeptr, via which a numerical error code can be returned. To
1481 avoid too much repetition, we refer just to pcre_compile() below, but
1482 the information applies equally to pcre_compile2().
1484 The pattern is a C string terminated by a binary zero, and is passed in
1485 the pattern argument. A pointer to a single block of memory that is
1486 obtained via pcre_malloc is returned. This contains the compiled code
1487 and related data. The pcre type is defined for the returned block; this
1488 is a typedef for a structure whose contents are not externally defined.
1489 It is up to the caller to free the memory (via pcre_free) when it is no
1490 longer required.
1492 Although the compiled code of a PCRE regex is relocatable, that is, it
1493 does not depend on memory location, the complete pcre data block is not
1494 fully relocatable, because it may contain a copy of the tableptr argu-
1495 ment, which is an address (see below).
1497 The options argument contains various bit settings that affect the com-
1498 pilation. It should be zero if no options are required. The available
1499 options are described below. Some of them (in particular, those that
1500 are compatible with Perl, but some others as well) can also be set and
1501 unset from within the pattern (see the detailed description in the
1502 pcrepattern documentation). For those options that can be different in
1503 different parts of the pattern, the contents of the options argument
1504 specifies their settings at the start of compilation and execution. The
1506 PCRE_NO_START_OPT options can be set at the time of matching as well as
1507 at compile time.
1509 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1510 if compilation of a pattern fails, pcre_compile() returns NULL, and
1511 sets the variable pointed to by errptr to point to a textual error mes-
1512 sage. This is a static string that is part of the library. You must not
1513 try to free it. Normally, the offset from the start of the pattern to
1514 the byte that was being processed when the error was discovered is
1515 placed in the variable pointed to by erroffset, which must not be NULL
1516 (if it is, an immediate error is given). However, for an invalid UTF-8
1517 string, the offset is that of the first byte of the failing character.
1519 Some errors are not detected until the whole pattern has been scanned;
1520 in these cases, the offset passed back is the length of the pattern.
1521 Note that the offset is in bytes, not characters, even in UTF-8 mode.
1522 It may sometimes point into the middle of a UTF-8 character.
1524 If pcre_compile2() is used instead of pcre_compile(), and the error-
1525 codeptr argument is not NULL, a non-zero error code number is returned
1526 via this argument in the event of an error. This is in addition to the
1527 textual error message. Error codes and messages are listed below.
1529 If the final argument, tableptr, is NULL, PCRE uses a default set of
1530 character tables that are built when PCRE is compiled, using the
1531 default C locale. Otherwise, tableptr must be an address that is the
1532 result of a call to pcre_maketables(). This value is stored with the
1533 compiled pattern, and used again by pcre_exec(), unless another table
1534 pointer is passed to it. For more discussion, see the section on locale
1535 support below.
1537 This code fragment shows a typical straightforward call to pcre_com-
1538 pile():
1540 pcre *re;
1541 const char *error;
1542 int erroffset;
1543 re = pcre_compile(
1544 "^A.*Z", /* the pattern */
1545 0, /* default options */
1546 &error, /* for error message */
1547 &erroffset, /* for error offset */
1548 NULL); /* use default character tables */
1550 The following names for option bits are defined in the pcre.h header
1551 file:
1555 If this bit is set, the pattern is forced to be "anchored", that is, it
1556 is constrained to match only at the first matching point in the string
1557 that is being searched (the "subject string"). This effect can also be
1558 achieved by appropriate constructs in the pattern itself, which is the
1559 only way to do it in Perl.
1563 If this bit is set, pcre_compile() automatically inserts callout items,
1564 all with number 255, before each pattern item. For discussion of the
1565 callout facility, see the pcrecallout documentation.
1570 These options (which are mutually exclusive) control what the \R escape
1571 sequence matches. The choice is either to match only CR, LF, or CRLF,
1572 or to match any Unicode newline sequence. The default is specified when
1573 PCRE is built. It can be overridden from within the pattern, or by set-
1574 ting an option when a compiled pattern is matched.
1578 If this bit is set, letters in the pattern match both upper and lower
1579 case letters. It is equivalent to Perl's /i option, and it can be
1580 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1581 always understands the concept of case for characters whose values are
1582 less than 128, so caseless matching is always possible. For characters
1583 with higher values, the concept of case is supported if PCRE is com-
1584 piled with Unicode property support, but not otherwise. If you want to
1585 use caseless matching for characters 128 and above, you must ensure
1586 that PCRE is compiled with Unicode property support as well as with
1587 UTF-8 support.
1591 If this bit is set, a dollar metacharacter in the pattern matches only
1592 at the end of the subject string. Without this option, a dollar also
1593 matches immediately before a newline at the end of the string (but not
1594 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1595 if PCRE_MULTILINE is set. There is no equivalent to this option in
1596 Perl, and no way to set it within a pattern.
1600 If this bit is set, a dot metacharacter in the pattern matches a char-
1601 acter of any value, including one that indicates a newline. However, it
1602 only ever matches one character, even if newlines are coded as CRLF.
1603 Without this option, a dot does not match when the current position is
1604 at a newline. This option is equivalent to Perl's /s option, and it can
1605 be changed within a pattern by a (?s) option setting. A negative class
1606 such as [^a] always matches newline characters, independent of the set-
1607 ting of this option.
1611 If this bit is set, names used to identify capturing subpatterns need
1612 not be unique. This can be helpful for certain types of pattern when it
1613 is known that only one instance of the named subpattern can ever be
1614 matched. There are more details of named subpatterns below; see also
1615 the pcrepattern documentation.
1619 If this bit is set, whitespace data characters in the pattern are
1620 totally ignored except when escaped or inside a character class. White-
1621 space does not include the VT character (code 11). In addition, charac-
1622 ters between an unescaped # outside a character class and the next new-
1623 line, inclusive, are also ignored. This is equivalent to Perl's /x
1624 option, and it can be changed within a pattern by a (?x) option set-
1625 ting.
1627 Which characters are interpreted as newlines is controlled by the
1628 options passed to pcre_compile() or by a special sequence at the start
1629 of the pattern, as described in the section entitled "Newline conven-
1630 tions" in the pcrepattern documentation. Note that the end of this type
1631 of comment is a literal newline sequence in the pattern; escape
1632 sequences that happen to represent a newline do not count.
1634 This option makes it possible to include comments inside complicated
1635 patterns. Note, however, that this applies only to data characters.
1636 Whitespace characters may never appear within special character
1637 sequences in a pattern, for example within the sequence (?( that intro-
1638 duces a conditional subpattern.
1642 This option was invented in order to turn on additional functionality
1643 of PCRE that is incompatible with Perl, but it is currently of very
1644 little use. When set, any backslash in a pattern that is followed by a
1645 letter that has no special meaning causes an error, thus reserving
1646 these combinations for future expansion. By default, as in Perl, a
1647 backslash followed by a letter with no special meaning is treated as a
1648 literal. (Perl can, however, be persuaded to give an error for this, by
1649 running it with the -w option.) There are at present no other features
1650 controlled by this option. It can also be set by a (?X) option setting
1651 within a pattern.
1655 If this option is set, an unanchored pattern is required to match
1656 before or at the first newline in the subject string, though the
1657 matched text may continue over the newline.
1661 If this option is set, PCRE's behaviour is changed in some ways so that
1662 it is compatible with JavaScript rather than Perl. The changes are as
1663 follows:
1665 (1) A lone closing square bracket in a pattern causes a compile-time
1666 error, because this is illegal in JavaScript (by default it is treated
1667 as a data character). Thus, the pattern AB]CD becomes illegal when this
1668 option is set.
1670 (2) At run time, a back reference to an unset subpattern group matches
1671 an empty string (by default this causes the current matching alterna-
1672 tive to fail). A pattern such as (\1)(a) succeeds when this option is
1673 set (assuming it can find an "a" in the subject), whereas it fails by
1674 default, for Perl compatibility.
1676 (3) \U matches an upper case "U" character; by default \U causes a com-
1677 pile time error (Perl uses \U to upper case subsequent characters).
1679 (4) \u matches a lower case "u" character unless it is followed by four
1680 hexadecimal digits, in which case the hexadecimal number defines the
1681 code point to match. By default, \u causes a compile time error (Perl
1682 uses it to upper case the following character).
1684 (5) \x matches a lower case "x" character unless it is followed by two
1685 hexadecimal digits, in which case the hexadecimal number defines the
1686 code point to match. By default, as in Perl, a hexadecimal number is
1687 always expected after \x, but it may have zero, one, or two digits (so,
1688 for example, \xz matches a binary zero character followed by z).
1692 By default, PCRE treats the subject string as consisting of a single
1693 line of characters (even if it actually contains newlines). The "start
1694 of line" metacharacter (^) matches only at the start of the string,
1695 while the "end of line" metacharacter ($) matches only at the end of
1696 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1697 is set). This is the same as Perl.
1699 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1700 constructs match immediately following or immediately before internal
1701 newlines in the subject string, respectively, as well as at the very
1702 start and end. This is equivalent to Perl's /m option, and it can be
1703 changed within a pattern by a (?m) option setting. If there are no new-
1704 lines in a subject string, or no occurrences of ^ or $ in a pattern,
1705 setting PCRE_MULTILINE has no effect.
1713 These options override the default newline definition that was chosen
1714 when PCRE was built. Setting the first or the second specifies that a
1715 newline is indicated by a single character (CR or LF, respectively).
1716 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1717 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1718 that any of the three preceding sequences should be recognized. Setting
1719 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1720 recognized. The Unicode newline sequences are the three just mentioned,
1721 plus the single characters VT (vertical tab, U+000B), FF (formfeed,
1722 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1723 (paragraph separator, U+2029). For the 8-bit library, the last two are
1724 recognized only in UTF-8 mode.
1726 The newline setting in the options word uses three bits that are
1727 treated as a number, giving eight possibilities. Currently only six are
1728 used (default plus the five values above). This means that if you set
1729 more than one newline option, the combination may or may not be sensi-
1730 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1731 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1732 cause an error.
1734 The only time that a line break in a pattern is specially recognized
1735 when compiling is when PCRE_EXTENDED is set. CR and LF are whitespace
1736 characters, and so are ignored in this mode. Also, an unescaped # out-
1737 side a character class indicates a comment that lasts until after the
1738 next line break sequence. In other circumstances, line break sequences
1739 in patterns are treated as literal data.
1741 The newline option that is set at compile time becomes the default that
1742 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1746 If this option is set, it disables the use of numbered capturing paren-
1747 theses in the pattern. Any opening parenthesis that is not followed by
1748 ? behaves as if it were followed by ?: but named parentheses can still
1749 be used for capturing (and they acquire numbers in the usual way).
1750 There is no equivalent of this option in Perl.
1754 This is an option that acts at matching time; that is, it is really an
1755 option for pcre_exec() or pcre_dfa_exec(). If it is set at compile
1756 time, it is remembered with the compiled pattern and assumed at match-
1757 ing time. For details see the discussion of PCRE_NO_START_OPTIMIZE
1758 below.
1762 This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W,
1763 \w, and some of the POSIX character classes. By default, only ASCII
1764 characters are recognized, but if PCRE_UCP is set, Unicode properties
1765 are used instead to classify characters. More details are given in the
1766 section on generic character types in the pcrepattern page. If you set
1767 PCRE_UCP, matching one of the items it affects takes much longer. The
1768 option is available only if PCRE has been compiled with Unicode prop-
1769 erty support.
1773 This option inverts the "greediness" of the quantifiers so that they
1774 are not greedy by default, but become greedy if followed by "?". It is
1775 not compatible with Perl. It can also be set by a (?U) option setting
1776 within the pattern.
1778 PCRE_UTF8
1780 This option causes PCRE to regard both the pattern and the subject as
1781 strings of UTF-8 characters instead of single-byte strings. However, it
1782 is available only when PCRE is built to include UTF support. If not,
1783 the use of this option provokes an error. Details of how this option
1784 changes the behaviour of PCRE are given in the pcreunicode page.
1788 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1789 automatically checked. There is a discussion about the validity of
1790 UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence is
1791 found, pcre_compile() returns an error. If you already know that your
1792 pattern is valid, and you want to skip this check for performance rea-
1793 sons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the
1794 effect of passing an invalid UTF-8 string as a pattern is undefined. It
1795 may cause your program to crash. Note that this option can also be
1796 passed to pcre_exec() and pcre_dfa_exec(), to suppress the validity
1797 checking of subject strings.
1802 The following table lists the error codes than may be returned by
1803 pcre_compile2(), along with the error messages that may be returned by
1804 both compiling functions. Note that error messages are always 8-bit
1805 ASCII strings, even in 16-bit mode. As PCRE has developed, some error
1806 codes have fallen out of use. To avoid confusion, they have not been
1807 re-used.
1809 0 no error
1810 1 \ at end of pattern
1811 2 \c at end of pattern
1812 3 unrecognized character follows \
1813 4 numbers out of order in {} quantifier
1814 5 number too big in {} quantifier
1815 6 missing terminating ] for character class
1816 7 invalid escape sequence in character class
1817 8 range out of order in character class
1818 9 nothing to repeat
1819 10 [this code is not in use]
1820 11 internal error: unexpected repeat
1821 12 unrecognized character after (? or (?-
1822 13 POSIX named classes are supported only within a class
1823 14 missing )
1824 15 reference to non-existent subpattern
1825 16 erroffset passed as NULL
1826 17 unknown option bit(s) set
1827 18 missing ) after comment
1828 19 [this code is not in use]
1829 20 regular expression is too large
1830 21 failed to get memory
1831 22 unmatched parentheses
1832 23 internal error: code overflow
1833 24 unrecognized character after (?<
1834 25 lookbehind assertion is not fixed length
1835 26 malformed number or name after (?(
1836 27 conditional group contains more than two branches
1837 28 assertion expected after (?(
1838 29 (?R or (?[+-]digits must be followed by )
1839 30 unknown POSIX class name
1840 31 POSIX collating elements are not supported
1841 32 this version of PCRE is compiled without UTF support
1842 33 [this code is not in use]
1843 34 character value in \x{...} sequence is too large
1844 35 invalid condition (?(0)
1845 36 \C not allowed in lookbehind assertion
1846 37 PCRE does not support \L, \l, \N{name}, \U, or \u
1847 38 number after (?C is > 255
1848 39 closing ) for (?C expected
1849 40 recursive call could loop indefinitely
1850 41 unrecognized character after (?P
1851 42 syntax error in subpattern name (missing terminator)
1852 43 two named subpatterns have the same name
1853 44 invalid UTF-8 string (specifically UTF-8)
1854 45 support for \P, \p, and \X has not been compiled
1855 46 malformed \P or \p sequence
1856 47 unknown property name after \P or \p
1857 48 subpattern name is too long (maximum 32 characters)
1858 49 too many named subpatterns (maximum 10000)
1859 50 [this code is not in use]
1860 51 octal value is greater than \377 in 8-bit non-UTF-8 mode
1861 52 internal error: overran compiling workspace
1862 53 internal error: previously-checked referenced subpattern
1863 not found
1864 54 DEFINE group contains more than one branch
1865 55 repeating a DEFINE group is not allowed
1866 56 inconsistent NEWLINE options
1867 57 \g is not followed by a braced, angle-bracketed, or quoted
1868 name/number or by a plain number
1869 58 a numbered reference must not be zero
1870 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
1871 60 (*VERB) not recognized
1872 61 number is too big
1873 62 subpattern name expected
1874 63 digit expected after (?+
1875 64 ] is an invalid data character in JavaScript compatibility mode
1876 65 different names for subpatterns of the same number are
1877 not allowed
1878 66 (*MARK) must have an argument
1879 67 this version of PCRE is not compiled with Unicode property
1880 support
1881 68 \c must be followed by an ASCII character
1882 69 \k is not followed by a braced, angle-bracketed, or quoted name
1883 70 internal error: unknown opcode in find_fixedlength()
1884 71 \N is not supported in a class
1885 72 too many forward references
1886 73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff)
1887 74 invalid UTF-16 string (specifically UTF-16)
1889 The numbers 32 and 10000 in errors 48 and 49 are defaults; different
1890 values may be used if the limits were changed when PCRE was built.
1895 pcre_extra *pcre_study(const pcre *code, int options
1896 const char **errptr);
1898 If a compiled pattern is going to be used several times, it is worth
1899 spending more time analyzing it in order to speed up the time taken for
1900 matching. The function pcre_study() takes a pointer to a compiled pat-
1901 tern as its first argument. If studying the pattern produces additional
1902 information that will help speed up matching, pcre_study() returns a
1903 pointer to a pcre_extra block, in which the study_data field points to
1904 the results of the study.
1906 The returned value from pcre_study() can be passed directly to
1907 pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con-
1908 tains other fields that can be set by the caller before the block is
1909 passed; these are described below in the section on matching a pattern.
1911 If studying the pattern does not produce any useful information,
1912 pcre_study() returns NULL. In that circumstance, if the calling program
1913 wants to pass any of the other fields to pcre_exec() or
1914 pcre_dfa_exec(), it must set up its own pcre_extra block.
1916 The second argument of pcre_study() contains option bits. There is only
1917 one option: PCRE_STUDY_JIT_COMPILE. If this is set, and the just-in-
1918 time compiler is available, the pattern is further compiled into
1919 machine code that executes much faster than the pcre_exec() matching
1920 function. If the just-in-time compiler is not available, this option is
1921 ignored. All other bits in the options argument must be zero.
1923 JIT compilation is a heavyweight optimization. It can take some time
1924 for patterns to be analyzed, and for one-off matches and simple pat-
1925 terns the benefit of faster execution might be offset by a much slower
1926 study time. Not all patterns can be optimized by the JIT compiler. For
1927 those that cannot be handled, matching automatically falls back to the
1928 pcre_exec() interpreter. For more details, see the pcrejit documenta-
1929 tion.
1931 The third argument for pcre_study() is a pointer for an error message.
1932 If studying succeeds (even if no data is returned), the variable it
1933 points to is set to NULL. Otherwise it is set to point to a textual
1934 error message. This is a static string that is part of the library. You
1935 must not try to free it. You should test the error pointer for NULL
1936 after calling pcre_study(), to be sure that it has run successfully.
1938 When you are finished with a pattern, you can free the memory used for
1939 the study data by calling pcre_free_study(). This function was added to
1940 the API for release 8.20. For earlier versions, the memory could be
1941 freed with pcre_free(), just like the pattern itself. This will still
1942 work in cases where PCRE_STUDY_JIT_COMPILE is not used, but it is
1943 advisable to change to the new function when convenient.
1945 This is a typical way in which pcre_study() is used (except that in a
1946 real application there should be tests for errors):
1948 int rc;
1949 pcre *re;
1950 pcre_extra *sd;
1951 re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
1952 sd = pcre_study(
1953 re, /* result of pcre_compile() */
1954 0, /* no options */
1955 &error); /* set to NULL or points to a message */
1956 rc = pcre_exec( /* see below for details of pcre_exec() options */
1957 re, sd, "subject", 7, 0, 0, ovector, 30);
1958 ...
1959 pcre_free_study(sd);
1960 pcre_free(re);
1962 Studying a pattern does two things: first, a lower bound for the length
1963 of subject string that is needed to match the pattern is computed. This
1964 does not mean that there are any strings of that length that match, but
1965 it does guarantee that no shorter strings match. The value is used by
1966 pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to
1967 match strings that are shorter than the lower bound. You can find out
1968 the value in a calling program via the pcre_fullinfo() function.
1970 Studying a pattern is also useful for non-anchored patterns that do not
1971 have a single fixed starting character. A bitmap of possible starting
1972 bytes is created. This speeds up finding a position in the subject at
1973 which to start matching. (In 16-bit mode, the bitmap is used for 16-bit
1974 values less than 256.)
1976 These two optimizations apply to both pcre_exec() and pcre_dfa_exec().
1977 However, they are not used by pcre_exec() if pcre_study() is called
1978 with the PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling is
1979 successful. The optimizations can be disabled by setting the
1980 PCRE_NO_START_OPTIMIZE option when calling pcre_exec() or
1981 pcre_dfa_exec(). You might want to do this if your pattern contains
1982 callouts or (*MARK) (which cannot be handled by the JIT compiler), and
1983 you want to make use of these facilities in cases where matching fails.
1984 See the discussion of PCRE_NO_START_OPTIMIZE below.
1989 PCRE handles caseless matching, and determines whether characters are
1990 letters, digits, or whatever, by reference to a set of tables, indexed
1991 by character value. When running in UTF-8 mode, this applies only to
1992 characters with codes less than 128. By default, higher-valued codes
1993 never match escapes such as \w or \d, but they can be tested with \p if
1994 PCRE is built with Unicode character property support. Alternatively,
1995 the PCRE_UCP option can be set at compile time; this causes \w and
1996 friends to use Unicode property support instead of built-in tables. The
1997 use of locales with Unicode is discouraged. If you are handling charac-
1998 ters with codes greater than 128, you should either use UTF-8 and Uni-
1999 code, or use locales, but not try to mix the two.
2001 PCRE contains an internal set of tables that are used when the final
2002 argument of pcre_compile() is NULL. These are sufficient for many
2003 applications. Normally, the internal tables recognize only ASCII char-
2004 acters. However, when PCRE is built, it is possible to cause the inter-
2005 nal tables to be rebuilt in the default "C" locale of the local system,
2006 which may cause them to be different.
2008 The internal tables can always be overridden by tables supplied by the
2009 application that calls PCRE. These may be created in a different locale
2010 from the default. As more and more applications change to using Uni-
2011 code, the need for this locale support is expected to die away.
2013 External tables are built by calling the pcre_maketables() function,
2014 which has no arguments, in the relevant locale. The result can then be
2015 passed to pcre_compile() or pcre_exec() as often as necessary. For
2016 example, to build and use tables that are appropriate for the French
2017 locale (where accented characters with values greater than 128 are
2018 treated as letters), the following code could be used:
2020 setlocale(LC_CTYPE, "fr_FR");
2021 tables = pcre_maketables();
2022 re = pcre_compile(..., tables);
2024 The locale name "fr_FR" is used on Linux and other Unix-like systems;
2025 if you are using Windows, the name for the French locale is "french".
2027 When pcre_maketables() runs, the tables are built in memory that is
2028 obtained via pcre_malloc. It is the caller's responsibility to ensure
2029 that the memory containing the tables remains available for as long as
2030 it is needed.
2032 The pointer that is passed to pcre_compile() is saved with the compiled
2033 pattern, and the same tables are used via this pointer by pcre_study()
2034 and normally also by pcre_exec(). Thus, by default, for any single pat-
2035 tern, compilation, studying and matching all happen in the same locale,
2036 but different patterns can be compiled in different locales.
2038 It is possible to pass a table pointer or NULL (indicating the use of
2039 the internal tables) to pcre_exec(). Although not intended for this
2040 purpose, this facility could be used to match a pattern in a different
2041 locale from the one in which it was compiled. Passing table pointers at
2042 run time is discussed below in the section on matching a pattern.
2047 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
2048 int what, void *where);
2050 The pcre_fullinfo() function returns information about a compiled pat-
2051 tern. It replaces the pcre_info() function, which was removed from the
2052 library at version 8.30, after more than 10 years of obsolescence.
2054 The first argument for pcre_fullinfo() is a pointer to the compiled
2055 pattern. The second argument is the result of pcre_study(), or NULL if
2056 the pattern was not studied. The third argument specifies which piece
2057 of information is required, and the fourth argument is a pointer to a
2058 variable to receive the data. The yield of the function is zero for
2059 success, or one of the following negative numbers:
2061 PCRE_ERROR_NULL the argument code was NULL
2062 the argument where was NULL
2063 PCRE_ERROR_BADMAGIC the "magic number" was not found
2064 PCRE_ERROR_BADENDIANNESS the pattern was compiled with different
2065 endianness
2066 PCRE_ERROR_BADOPTION the value of what was invalid
2068 The "magic number" is placed at the start of each compiled pattern as
2069 an simple check against passing an arbitrary memory pointer. The endi-
2070 anness error can occur if a compiled pattern is saved and reloaded on a
2071 different host. Here is a typical call of pcre_fullinfo(), to obtain
2072 the length of the compiled pattern:
2074 int rc;
2075 size_t length;
2076 rc = pcre_fullinfo(
2077 re, /* result of pcre_compile() */
2078 sd, /* result of pcre_study(), or NULL */
2079 PCRE_INFO_SIZE, /* what is required */
2080 &length); /* where to put the data */
2082 The possible values for the third argument are defined in pcre.h, and
2083 are as follows:
2087 Return the number of the highest back reference in the pattern. The
2088 fourth argument should point to an int variable. Zero is returned if
2089 there are no back references.
2093 Return the number of capturing subpatterns in the pattern. The fourth
2094 argument should point to an int variable.
2098 Return a pointer to the internal default character tables within PCRE.
2099 The fourth argument should point to an unsigned char * variable. This
2100 information call is provided for internal use by the pcre_study() func-
2101 tion. External callers can cause PCRE to use its internal tables by
2102 passing a NULL table pointer.
2106 Return information about the first data unit of any matched string, for
2107 a non-anchored pattern. (The name of this option refers to the 8-bit
2108 library, where data units are bytes.) The fourth argument should point
2109 to an int variable.
2111 If there is a fixed first value, for example, the letter "c" from a
2112 pattern such as (cat|cow|coyote), its value is returned. In the 8-bit
2113 library, the value is always less than 256; in the 16-bit library the
2114 value can be up to 0xffff.
2116 If there is no fixed first value, and if either
2118 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
2119 branch starts with "^", or
2121 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
2122 set (if it were set, the pattern would be anchored),
2124 -1 is returned, indicating that the pattern matches only at the start
2125 of a subject string or after any newline within the string. Otherwise
2126 -2 is returned. For anchored patterns, -2 is returned.
2130 If the pattern was studied, and this resulted in the construction of a
2131 256-bit table indicating a fixed set of values for the first data unit
2132 in any matching string, a pointer to the table is returned. Otherwise
2133 NULL is returned. The fourth argument should point to an unsigned char
2134 * variable.
2138 Return 1 if the pattern contains any explicit matches for CR or LF
2139 characters, otherwise 0. The fourth argument should point to an int
2140 variable. An explicit match is either a literal CR or LF character, or
2141 \r or \n.
2145 Return 1 if the (?J) or (?-J) option setting is used in the pattern,
2146 otherwise 0. The fourth argument should point to an int variable. (?J)
2147 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
2151 Return 1 if the pattern was studied with the PCRE_STUDY_JIT_COMPILE
2152 option, and just-in-time compiling was successful. The fourth argument
2153 should point to an int variable. A return value of 0 means that JIT
2154 support is not available in this version of PCRE, or that the pattern
2155 was not studied with the PCRE_STUDY_JIT_COMPILE option, or that the JIT
2156 compiler could not handle this particular pattern. See the pcrejit doc-
2157 umentation for details of what can and cannot be handled.
2161 If the pattern was successfully studied with the PCRE_STUDY_JIT_COMPILE
2162 option, return the size of the JIT compiled code, otherwise return
2163 zero. The fourth argument should point to a size_t variable.
2167 Return the value of the rightmost literal data unit that must exist in
2168 any matched string, other than at its start, if such a value has been
2169 recorded. The fourth argument should point to an int variable. If there
2170 is no such value, -1 is returned. For anchored patterns, a last literal
2171 value is recorded only if it follows something of variable length. For
2172 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
2173 /^a\dz\d/ the returned value is -1.
2177 If the pattern was studied and a minimum length for matching subject
2178 strings was computed, its value is returned. Otherwise the returned
2179 value is -1. The value is a number of characters, which in UTF-8 mode
2180 may be different from the number of bytes. The fourth argument should
2181 point to an int variable. A non-negative value is a lower bound to the
2182 length of any matching string. There may not be any strings of that
2183 length that do actually match, but every string that does match is at
2184 least that long.
2190 PCRE supports the use of named as well as numbered capturing parenthe-
2191 ses. The names are just an additional way of identifying the parenthe-
2192 ses, which still acquire numbers. Several convenience functions such as
2193 pcre_get_named_substring() are provided for extracting captured sub-
2194 strings by name. It is also possible to extract the data directly, by
2195 first converting the name to a number in order to access the correct
2196 pointers in the output vector (described with pcre_exec() below). To do
2197 the conversion, you need to use the name-to-number map, which is
2198 described by these three values.
2200 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
2201 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
2202 of each entry; both of these return an int value. The entry size
2203 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
2204 a pointer to the first entry of the table. This is a pointer to char in
2205 the 8-bit library, where the first two bytes of each entry are the num-
2206 ber of the capturing parenthesis, most significant byte first. In the
2207 16-bit library, the pointer points to 16-bit data units, the first of
2208 which contains the parenthesis number. The rest of the entry is the
2209 corresponding name, zero terminated.
2211 The names are in alphabetical order. Duplicate names may appear if (?|
2212 is used to create multiple groups with the same number, as described in
2213 the section on duplicate subpattern numbers in the pcrepattern page.
2214 Duplicate names for subpatterns with different numbers are permitted
2215 only if PCRE_DUPNAMES is set. In all cases of duplicate names, they
2216 appear in the table in the order in which they were found in the pat-
2217 tern. In the absence of (?| this is the order of increasing number;
2218 when (?| is used this is not necessarily the case because later subpat-
2219 terns may have lower numbers.
2221 As a simple example of the name/number table, consider the following
2222 pattern after compilation by the 8-bit library (assume PCRE_EXTENDED is
2223 set, so white space - including newlines - is ignored):
2225 (?<date> (?<year>(\d\d)?\d\d) -
2226 (?<month>\d\d) - (?<day>\d\d) )
2228 There are four named subpatterns, so the table has four entries, and
2229 each entry in the table is eight bytes long. The table is as follows,
2230 with non-printing bytes shows in hexadecimal, and undefined bytes shown
2231 as ??:
2233 00 01 d a t e 00 ??
2234 00 05 d a y 00 ?? ??
2235 00 04 m o n t h 00
2236 00 02 y e a r 00 ??
2238 When writing code to extract data from named subpatterns using the
2239 name-to-number map, remember that the length of the entries is likely
2240 to be different for each compiled pattern.
2244 Return 1 if the pattern can be used for partial matching with
2245 pcre_exec(), otherwise 0. The fourth argument should point to an int
2246 variable. From release 8.00, this always returns 1, because the
2247 restrictions that previously applied to partial matching have been
2248 lifted. The pcrepartial documentation gives details of partial match-
2249 ing.
2253 Return a copy of the options with which the pattern was compiled. The
2254 fourth argument should point to an unsigned long int variable. These
2255 option bits are those specified in the call to pcre_compile(), modified
2256 by any top-level option settings at the start of the pattern itself. In
2257 other words, they are the options that will be in force when matching
2258 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
2259 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
2262 A pattern is automatically anchored by PCRE if all of its top-level
2263 alternatives begin with one of the following:
2265 ^ unless PCRE_MULTILINE is set
2266 \A always
2267 \G always
2268 .* if PCRE_DOTALL is set and there are no back
2269 references to the subpattern in which .* appears
2271 For such patterns, the PCRE_ANCHORED bit is set in the options returned
2272 by pcre_fullinfo().
2276 Return the size of the compiled pattern in bytes (for both libraries).
2277 The fourth argument should point to a size_t variable. This value does
2278 not include the size of the pcre structure that is returned by
2279 pcre_compile(). The value that is passed as the argument to pcre_mal-
2280 loc() when pcre_compile() is getting memory in which to place the com-
2281 piled data is the value returned by this option plus the size of the
2282 pcre structure. Studying a compiled pattern, with or without JIT, does
2283 not alter the value returned by this option.
2287 Return the size in bytes of the data block pointed to by the study_data
2288 field in a pcre_extra block. If pcre_extra is NULL, or there is no
2289 study data, zero is returned. The fourth argument should point to a
2290 size_t variable. The study_data field is set by pcre_study() to record
2291 information that will speed up matching (see the section entitled
2292 "Studying a pattern" above). The format of the study_data block is pri-
2293 vate, but its length is made available via this option so that it can
2294 be saved and restored (see the pcreprecompile documentation for
2295 details).
2300 int pcre_refcount(pcre *code, int adjust);
2302 The pcre_refcount() function is used to maintain a reference count in
2303 the data block that contains a compiled pattern. It is provided for the
2304 benefit of applications that operate in an object-oriented manner,
2305 where different parts of the application may be using the same compiled
2306 pattern, but you want to free the block when they are all done.
2308 When a pattern is compiled, the reference count field is initialized to
2309 zero. It is changed only by calling this function, whose action is to
2310 add the adjust value (which may be positive or negative) to it. The
2311 yield of the function is the new value. However, the value of the count
2312 is constrained to lie between 0 and 65535, inclusive. If the new value
2313 is outside these limits, it is forced to the appropriate limit value.
2315 Except when it is zero, the reference count is not correctly preserved
2316 if a pattern is compiled on one host and then transferred to a host
2317 whose byte-order is different. (This seems a highly unlikely scenario.)
2322 int pcre_exec(const pcre *code, const pcre_extra *extra,
2323 const char *subject, int length, int startoffset,
2324 int options, int *ovector, int ovecsize);
2326 The function pcre_exec() is called to match a subject string against a
2327 compiled pattern, which is passed in the code argument. If the pattern
2328 was studied, the result of the study should be passed in the extra
2329 argument. You can call pcre_exec() with the same code and extra argu-
2330 ments as many times as you like, in order to match different subject
2331 strings with the same pattern.
2333 This function is the main matching facility of the library, and it
2334 operates in a Perl-like manner. For specialist use there is also an
2335 alternative matching function, which is described below in the section
2336 about the pcre_dfa_exec() function.
2338 In most applications, the pattern will have been compiled (and option-
2339 ally studied) in the same process that calls pcre_exec(). However, it
2340 is possible to save compiled patterns and study data, and then use them
2341 later in different processes, possibly even on different hosts. For a
2342 discussion about this, see the pcreprecompile documentation.
2344 Here is an example of a simple call to pcre_exec():
2346 int rc;
2347 int ovector[30];
2348 rc = pcre_exec(
2349 re, /* result of pcre_compile() */
2350 NULL, /* we didn't study the pattern */
2351 "some string", /* the subject string */
2352 11, /* the length of the subject string */
2353 0, /* start at offset 0 in the subject */
2354 0, /* default options */
2355 ovector, /* vector of integers for substring information */
2356 30); /* number of elements (NOT size in bytes) */
2358 Extra data for pcre_exec()
2360 If the extra argument is not NULL, it must point to a pcre_extra data
2361 block. The pcre_study() function returns such a block (when it doesn't
2362 return NULL), but you can also create one for yourself, and pass addi-
2363 tional information in it. The pcre_extra block contains the following
2364 fields (not necessarily in this order):
2366 unsigned long int flags;
2367 void *study_data;
2368 void *executable_jit;
2369 unsigned long int match_limit;
2370 unsigned long int match_limit_recursion;
2371 void *callout_data;
2372 const unsigned char *tables;
2373 unsigned char **mark;
2375 In the 16-bit version of this structure, the mark field has type
2376 "PCRE_UCHAR16 **".
2378 The flags field is a bitmap that specifies which of the other fields
2379 are set. The flag bits are:
2389 Other flag bits should be set to zero. The study_data field and some-
2390 times the executable_jit field are set in the pcre_extra block that is
2391 returned by pcre_study(), together with the appropriate flag bits. You
2392 should not set these yourself, but you may add to the block by setting
2393 the other fields and their corresponding flag bits.
2395 The match_limit field provides a means of preventing PCRE from using up
2396 a vast amount of resources when running patterns that are not going to
2397 match, but which have a very large number of possibilities in their
2398 search trees. The classic example is a pattern that uses nested unlim-
2399 ited repeats.
2401 Internally, pcre_exec() uses a function called match(), which it calls
2402 repeatedly (sometimes recursively). The limit set by match_limit is
2403 imposed on the number of times this function is called during a match,
2404 which has the effect of limiting the amount of backtracking that can
2405 take place. For patterns that are not anchored, the count restarts from
2406 zero for each position in the subject string.
2408 When pcre_exec() is called with a pattern that was successfully studied
2409 with the PCRE_STUDY_JIT_COMPILE option, the way that the matching is
2410 executed is entirely different. However, there is still the possibility
2411 of runaway matching that goes on for a very long time, and so the
2412 match_limit value is also used in this case (but in a different way) to
2413 limit how long the matching can continue.
2415 The default value for the limit can be set when PCRE is built; the
2416 default default is 10 million, which handles all but the most extreme
2417 cases. You can override the default by suppling pcre_exec() with a
2418 pcre_extra block in which match_limit is set, and
2419 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
2420 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
2422 The match_limit_recursion field is similar to match_limit, but instead
2423 of limiting the total number of times that match() is called, it limits
2424 the depth of recursion. The recursion depth is a smaller number than
2425 the total number of calls, because not all calls to match() are recur-
2426 sive. This limit is of use only if it is set smaller than match_limit.
2428 Limiting the recursion depth limits the amount of machine stack that
2429 can be used, or, when PCRE has been compiled to use memory on the heap
2430 instead of the stack, the amount of heap memory that can be used. This
2431 limit is not relevant, and is ignored, if the pattern was successfully
2432 studied with PCRE_STUDY_JIT_COMPILE.
2434 The default value for match_limit_recursion can be set when PCRE is
2435 built; the default default is the same value as the default for
2436 match_limit. You can override the default by suppling pcre_exec() with
2437 a pcre_extra block in which match_limit_recursion is set, and
2438 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
2439 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
2441 The callout_data field is used in conjunction with the "callout" fea-
2442 ture, and is described in the pcrecallout documentation.
2444 The tables field is used to pass a character tables pointer to
2445 pcre_exec(); this overrides the value that is stored with the compiled
2446 pattern. A non-NULL value is stored with the compiled pattern only if
2447 custom tables were supplied to pcre_compile() via its tableptr argu-
2448 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
2449 PCRE's internal tables to be used. This facility is helpful when re-
2450 using patterns that have been saved after compiling with an external
2451 set of tables, because the external tables might be at a different
2452 address when pcre_exec() is called. See the pcreprecompile documenta-
2453 tion for a discussion of saving compiled patterns for later use.
2455 If PCRE_EXTRA_MARK is set in the flags field, the mark field must be
2456 set to point to a suitable variable. If the pattern contains any back-
2457 tracking control verbs such as (*MARK:NAME), and the execution ends up
2458 with a name to pass back, a pointer to the name string (zero termi-
2459 nated) is placed in the variable pointed to by the mark field. The
2460 names are within the compiled pattern; if you wish to retain such a
2461 name you must copy it before freeing the memory of a compiled pattern.
2462 If there is no name to pass back, the variable pointed to by the mark
2463 field is set to NULL. For details of the backtracking control verbs,
2464 see the section entitled "Backtracking control" in the pcrepattern doc-
2465 umentation.
2467 Option bits for pcre_exec()
2469 The unused bits of the options argument for pcre_exec() must be zero.
2470 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
2475 If the pattern was successfully studied with the PCRE_STUDY_JIT_COMPILE
2476 option, the only supported options for JIT execution are
2478 PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
2479 supported. If an unsupported option is used, JIT execution is disabled
2480 and the normal interpretive code in pcre_exec() is run.
2484 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
2485 matching position. If a pattern was compiled with PCRE_ANCHORED, or
2486 turned out to be anchored by virtue of its contents, it cannot be made
2487 unachored at matching time.
2492 These options (which are mutually exclusive) control what the \R escape
2493 sequence matches. The choice is either to match only CR, LF, or CRLF,
2494 or to match any Unicode newline sequence. These options override the
2495 choice that was made or defaulted when the pattern was compiled.
2503 These options override the newline definition that was chosen or
2504 defaulted when the pattern was compiled. For details, see the descrip-
2505 tion of pcre_compile() above. During matching, the newline choice
2506 affects the behaviour of the dot, circumflex, and dollar metacharac-
2507 ters. It may also alter the way the match position is advanced after a
2508 match failure for an unanchored pattern.
2511 set, and a match attempt for an unanchored pattern fails when the cur-
2512 rent position is at a CRLF sequence, and the pattern contains no
2513 explicit matches for CR or LF characters, the match position is
2514 advanced by two characters instead of one, in other words, to after the
2515 CRLF.
2517 The above rule is a compromise that makes the most common cases work as
2518 expected. For example, if the pattern is .+A (and the PCRE_DOTALL
2519 option is not set), it does not match the string "\r\nA" because, after
2520 failing at the start, it skips both the CR and the LF before retrying.
2521 However, the pattern [\r\n]A does match that string, because it con-
2522 tains an explicit CR or LF reference, and so advances only by one char-
2523 acter after the first failure.
2525 An explicit match for CR of LF is either a literal appearance of one of
2526 those characters, or one of the \r or \n escape sequences. Implicit
2527 matches such as [^X] do not count, nor does \s (which includes CR and
2528 LF in the characters that it matches).
2530 Notwithstanding the above, anomalous effects may still occur when CRLF
2531 is a valid newline sequence and explicit \r or \n escapes appear in the
2532 pattern.
2536 This option specifies that first character of the subject string is not
2537 the beginning of a line, so the circumflex metacharacter should not
2538 match before it. Setting this without PCRE_MULTILINE (at compile time)
2539 causes circumflex never to match. This option affects only the behav-
2540 iour of the circumflex metacharacter. It does not affect \A.
2544 This option specifies that the end of the subject string is not the end
2545 of a line, so the dollar metacharacter should not match it nor (except
2546 in multiline mode) a newline immediately before it. Setting this with-
2547 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
2548 option affects only the behaviour of the dollar metacharacter. It does
2549 not affect \Z or \z.
2553 An empty string is not considered to be a valid match if this option is
2554 set. If there are alternatives in the pattern, they are tried. If all
2555 the alternatives match the empty string, the entire match fails. For
2556 example, if the pattern
2558 a?b?
2560 is applied to a string not beginning with "a" or "b", it matches an
2561 empty string at the start of the subject. With PCRE_NOTEMPTY set, this
2562 match is not valid, so PCRE searches further into the string for occur-
2563 rences of "a" or "b".
2567 This is like PCRE_NOTEMPTY, except that an empty string match that is
2568 not at the start of the subject is permitted. If the pattern is
2569 anchored, such a match can occur only if the pattern contains \K.
2571 Perl has no direct equivalent of PCRE_NOTEMPTY or
2572 PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern
2573 match of the empty string within its split() function, and when using
2574 the /g modifier. It is possible to emulate Perl's behaviour after
2575 matching a null string by first trying the match again at the same off-
2576 set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that
2577 fails, by advancing the starting offset (see below) and trying an ordi-
2578 nary match again. There is some code that demonstrates how to do this
2579 in the pcredemo sample program. In the most general case, you have to
2580 check to see if the newline convention recognizes CRLF as a newline,
2581 and if so, and the current character is CR followed by LF, advance the
2582 starting offset by two characters instead of one.
2586 There are a number of optimizations that pcre_exec() uses at the start
2587 of a match, in order to speed up the process. For example, if it is
2588 known that an unanchored match must start with a specific character, it
2589 searches the subject for that character, and fails immediately if it
2590 cannot find it, without actually running the main matching function.
2591 This means that a special item such as (*COMMIT) at the start of a pat-
2592 tern is not considered until after a suitable starting point for the
2593 match has been found. When callouts or (*MARK) items are in use, these
2594 "start-up" optimizations can cause them to be skipped if the pattern is
2595 never actually used. The start-up optimizations are in effect a pre-
2596 scan of the subject that takes place before the pattern is run.
2598 The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations,
2599 possibly causing performance to suffer, but ensuring that in cases
2600 where the result is "no match", the callouts do occur, and that items
2601 such as (*COMMIT) and (*MARK) are considered at every possible starting
2602 position in the subject string. If PCRE_NO_START_OPTIMIZE is set at
2603 compile time, it cannot be unset at matching time.
2605 Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching
2606 operation. Consider the pattern
2610 When this is compiled, PCRE records the fact that a match must start
2611 with the character "A". Suppose the subject string is "DEFABC". The
2612 start-up optimization scans along the subject, finds "A" and runs the
2613 first match attempt from there. The (*COMMIT) item means that the pat-
2614 tern must match the current starting position, which in this case, it
2615 does. However, if the same match is run with PCRE_NO_START_OPTIMIZE
2616 set, the initial scan along the subject string does not happen. The
2617 first match attempt is run starting from "D" and when this fails,
2618 (*COMMIT) prevents any further matches being tried, so the overall
2619 result is "no match". If the pattern is studied, more start-up opti-
2620 mizations may be used. For example, a minimum length for the subject
2621 may be recorded. Consider the pattern
2623 (*MARK:A)(X|Y)
2625 The minimum length for a match is one character. If the subject is
2626 "ABC", there will be attempts to match "ABC", "BC", "C", and then
2627 finally an empty string. If the pattern is studied, the final attempt
2628 does not take place, because PCRE knows that the subject is too short,
2629 and so the (*MARK) is never encountered. In this case, studying the
2630 pattern does not affect the overall match result, which is still "no
2631 match", but it does affect the auxiliary information that is returned.
2635 When PCRE_UTF8 is set at compile time, the validity of the subject as a
2636 UTF-8 string is automatically checked when pcre_exec() is subsequently
2637 called. The value of startoffset is also checked to ensure that it
2638 points to the start of a UTF-8 character. There is a discussion about
2639 the validity of UTF-8 strings in the pcreunicode page. If an invalid
2640 sequence of bytes is found, pcre_exec() returns the error
2641 PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the problem is a
2642 truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8. In
2643 both cases, information about the precise nature of the error may also
2644 be returned (see the descriptions of these errors in the section enti-
2645 tled Error return values from pcre_exec() below). If startoffset con-
2646 tains a value that does not point to the start of a UTF-8 character (or
2647 to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned.
2649 If you already know that your subject is valid, and you want to skip
2650 these checks for performance reasons, you can set the
2651 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
2652 do this for the second and subsequent calls to pcre_exec() if you are
2653 making repeated calls to find all the matches in a single subject
2654 string. However, you should be sure that the value of startoffset
2655 points to the start of a character (or the end of the subject). When
2656 PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid string as a
2657 subject or an invalid value of startoffset is undefined. Your program
2658 may crash.
2663 These options turn on the partial matching feature. For backwards com-
2664 patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial
2665 match occurs if the end of the subject string is reached successfully,
2666 but there are not enough subject characters to complete the match. If
2667 this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set,
2668 matching continues by testing any remaining alternatives. Only if no
2669 complete match can be found is PCRE_ERROR_PARTIAL returned instead of
2670 PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the
2671 caller is prepared to handle a partial match, but only if no complete
2672 match can be found.
2674 If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this
2675 case, if a partial match is found, pcre_exec() immediately returns
2676 PCRE_ERROR_PARTIAL, without considering any other alternatives. In
2677 other words, when PCRE_PARTIAL_HARD is set, a partial match is consid-
2678 ered to be more important that an alternative complete match.
2680 In both cases, the portion of the string that was inspected when the
2681 partial match was found is set as the first matching string. There is a
2682 more detailed discussion of partial and multi-segment matching, with
2683 examples, in the pcrepartial documentation.
2685 The string to be matched by pcre_exec()
2687 The subject string is passed to pcre_exec() as a pointer in subject, a
2688 length in bytes in length, and a starting byte offset in startoffset.
2689 If this is negative or greater than the length of the subject,
2690 pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is
2691 zero, the search for a match starts at the beginning of the subject,
2692 and this is by far the most common case. In UTF-8 mode, the byte offset
2693 must point to the start of a UTF-8 character (or the end of the sub-
2694 ject). Unlike the pattern string, the subject may contain binary zero
2695 bytes.
2697 A non-zero starting offset is useful when searching for another match
2698 in the same subject by calling pcre_exec() again after a previous suc-
2699 cess. Setting startoffset differs from just passing over a shortened
2700 string and setting PCRE_NOTBOL in the case of a pattern that begins
2701 with any kind of lookbehind. For example, consider the pattern
2703 \Biss\B
2705 which finds occurrences of "iss" in the middle of words. (\B matches
2706 only if the current position in the subject is not a word boundary.)
2707 When applied to the string "Mississipi" the first call to pcre_exec()
2708 finds the first occurrence. If pcre_exec() is called again with just
2709 the remainder of the subject, namely "issipi", it does not match,
2710 because \B is always false at the start of the subject, which is deemed
2711 to be a word boundary. However, if pcre_exec() is passed the entire
2712 string again, but with startoffset set to 4, it finds the second occur-
2713 rence of "iss" because it is able to look behind the starting point to
2714 discover that it is preceded by a letter.
2716 Finding all the matches in a subject is tricky when the pattern can
2717 match an empty string. It is possible to emulate Perl's /g behaviour by
2718 first trying the match again at the same offset, with the
2719 PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that
2720 fails, advancing the starting offset and trying an ordinary match
2721 again. There is some code that demonstrates how to do this in the pcre-
2722 demo sample program. In the most general case, you have to check to see
2723 if the newline convention recognizes CRLF as a newline, and if so, and
2724 the current character is CR followed by LF, advance the starting offset
2725 by two characters instead of one.
2727 If a non-zero starting offset is passed when the pattern is anchored,
2728 one attempt to match at the given offset is made. This can only succeed
2729 if the pattern does not require the match to be at the start of the
2730 subject.
2732 How pcre_exec() returns captured substrings
2734 In general, a pattern matches a certain portion of the subject, and in
2735 addition, further substrings from the subject may be picked out by
2736 parts of the pattern. Following the usage in Jeffrey Friedl's book,
2737 this is called "capturing" in what follows, and the phrase "capturing
2738 subpattern" is used for a fragment of a pattern that picks out a sub-
2739 string. PCRE supports several other kinds of parenthesized subpattern
2740 that do not cause substrings to be captured.
2742 Captured substrings are returned to the caller via a vector of integers
2743 whose address is passed in ovector. The number of elements in the vec-
2744 tor is passed in ovecsize, which must be a non-negative number. Note:
2745 this argument is NOT the size of ovector in bytes.
2747 The first two-thirds of the vector is used to pass back captured sub-
2748 strings, each substring using a pair of integers. The remaining third
2749 of the vector is used as workspace by pcre_exec() while matching cap-
2750 turing subpatterns, and is not available for passing back information.
2751 The number passed in ovecsize should always be a multiple of three. If
2752 it is not, it is rounded down.
2754 When a match is successful, information about captured substrings is
2755 returned in pairs of integers, starting at the beginning of ovector,
2756 and continuing up to two-thirds of its length at the most. The first
2757 element of each pair is set to the byte offset of the first character
2758 in a substring, and the second is set to the byte offset of the first
2759 character after the end of a substring. Note: these values are always
2760 byte offsets, even in UTF-8 mode. They are not character counts.
2762 The first pair of integers, ovector[0] and ovector[1], identify the
2763 portion of the subject string matched by the entire pattern. The next
2764 pair is used for the first capturing subpattern, and so on. The value
2765 returned by pcre_exec() is one more than the highest numbered pair that
2766 has been set. For example, if two substrings have been captured, the
2767 returned value is 3. If there are no capturing subpatterns, the return
2768 value from a successful match is 1, indicating that just the first pair
2769 of offsets has been set.
2771 If a capturing subpattern is matched repeatedly, it is the last portion
2772 of the string that it matched that is returned.
2774 If the vector is too small to hold all the captured substring offsets,
2775 it is used as far as possible (up to two-thirds of its length), and the
2776 function returns a value of zero. If neither the actual string matched
2777 not any captured substrings are of interest, pcre_exec() may be called
2778 with ovector passed as NULL and ovecsize as zero. However, if the pat-
2779 tern contains back references and the ovector is not big enough to
2780 remember the related substrings, PCRE has to get additional memory for
2781 use during matching. Thus it is usually advisable to supply an ovector
2782 of reasonable size.
2784 There are some cases where zero is returned (indicating vector over-
2785 flow) when in fact the vector is exactly the right size for the final
2786 match. For example, consider the pattern
2788 (a)(?:(b)c|bd)
2790 If a vector of 6 elements (allowing for only 1 captured substring) is
2791 given with subject string "abd", pcre_exec() will try to set the second
2792 captured string, thereby recording a vector overflow, before failing to
2793 match "c" and backing up to try the second alternative. The zero
2794 return, however, does correctly indicate that the maximum number of
2795 slots (namely 2) have been filled. In similar cases where there is tem-
2796 porary overflow, but the final number of used slots is actually less
2797 than the maximum, a non-zero value is returned.
2799 The pcre_fullinfo() function can be used to find out how many capturing
2800 subpatterns there are in a compiled pattern. The smallest size for
2801 ovector that will allow for n captured substrings, in addition to the
2802 offsets of the substring matched by the whole pattern, is (n+1)*3.
2804 It is possible for capturing subpattern number n+1 to match some part
2805 of the subject when subpattern n has not been used at all. For example,
2806 if the string "abc" is matched against the pattern (a|(z))(bc) the
2807 return from the function is 4, and subpatterns 1 and 3 are matched, but
2808 2 is not. When this happens, both values in the offset pairs corre-
2809 sponding to unused subpatterns are set to -1.
2811 Offset values that correspond to unused subpatterns at the end of the
2812 expression are also set to -1. For example, if the string "abc" is
2813 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2814 matched. The return from the function is 2, because the highest used
2815 capturing subpattern number is 1, and the offsets for for the second
2816 and third capturing subpatterns (assuming the vector is large enough,
2817 of course) are set to -1.
2819 Note: Elements in the first two-thirds of ovector that do not corre-
2820 spond to capturing parentheses in the pattern are never changed. That
2821 is, if a pattern contains n capturing parentheses, no more than ovec-
2822 tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in
2823 the first two-thirds) retain whatever values they previously had.
2825 Some convenience functions are provided for extracting the captured
2826 substrings as separate strings. These are described below.
2828 Error return values from pcre_exec()
2830 If pcre_exec() fails, it returns a negative number. The following are
2831 defined in the header file:
2835 The subject string did not match the pattern.
2839 Either code or subject was passed as NULL, or ovector was NULL and
2840 ovecsize was not zero.
2844 An unrecognized bit was set in the options argument.
2848 PCRE stores a 4-byte "magic number" at the start of the compiled code,
2849 to catch the case when it is passed a junk pointer and to detect when a
2850 pattern that was compiled in an environment of one endianness is run in
2851 an environment with the other endianness. This is the error that PCRE
2852 gives when the magic number is not present.
2856 While running the pattern match, an unknown item was encountered in the
2857 compiled pattern. This error could be caused by a bug in PCRE or by
2858 overwriting of the compiled pattern.
2862 If a pattern contains back references, but the ovector that is passed
2863 to pcre_exec() is not big enough to remember the referenced substrings,
2864 PCRE gets a block of memory at the start of matching to use for this
2865 purpose. If the call via pcre_malloc() fails, this error is given. The
2866 memory is automatically freed at the end of matching.
2868 This error is also given if pcre_stack_malloc() fails in pcre_exec().
2869 This can happen only when PCRE has been compiled with --disable-stack-
2870 for-recursion.
2874 This error is used by the pcre_copy_substring(), pcre_get_substring(),
2875 and pcre_get_substring_list() functions (see below). It is never
2876 returned by pcre_exec().
2880 The backtracking limit, as specified by the match_limit field in a
2881 pcre_extra structure (or defaulted) was reached. See the description
2882 above.
2886 This error is never generated by pcre_exec() itself. It is provided for
2887 use by callout functions that want to yield a distinctive error code.
2888 See the pcrecallout documentation for details.
2892 A string that contains an invalid UTF-8 byte sequence was passed as a
2893 subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of
2894 the output vector (ovecsize) is at least 2, the byte offset to the
2895 start of the the invalid UTF-8 character is placed in the first ele-
2896 ment, and a reason code is placed in the second element. The reason
2897 codes are listed in the following section. For backward compatibility,
2898 if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char-
2899 acter at the end of the subject (reason codes 1 to 5),
2900 PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8.
2904 The UTF-8 byte sequence that was passed as a subject was checked and
2905 found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the
2906 value of startoffset did not point to the beginning of a UTF-8 charac-
2907 ter or the end of the subject.
2911 The subject string did not match, but it did match partially. See the
2912 pcrepartial documentation for details of partial matching.
2916 This code is no longer in use. It was formerly returned when the
2917 PCRE_PARTIAL option was used with a compiled pattern containing items
2918 that were not supported for partial matching. From release 8.00
2919 onwards, there are no restrictions on partial matching.
2923 An unexpected internal error has occurred. This error could be caused
2924 by a bug in PCRE or by overwriting of the compiled pattern.
2928 This error is given if the value of the ovecsize argument is negative.
2932 The internal recursion limit, as specified by the match_limit_recursion
2933 field in a pcre_extra structure (or defaulted) was reached. See the
2934 description above.
2938 An invalid combination of PCRE_NEWLINE_xxx options was given.
2942 The value of startoffset was negative or greater than the length of the
2943 subject, that is, the value in length.
2947 This error is returned instead of PCRE_ERROR_BADUTF8 when the subject
2948 string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD
2949 option is set. Information about the failure is returned as for
2950 PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but
2951 this special error code for PCRE_PARTIAL_HARD precedes the implementa-
2952 tion of returned information; it is retained for backwards compatibil-
2953 ity.
2957 This error is returned when pcre_exec() detects a recursion loop within
2958 the pattern. Specifically, it means that either the whole pattern or a
2959 subpattern has been called recursively for the second time at the same
2960 position in the subject string. Some simple patterns that might do this
2961 are detected and faulted at compile time, but more complicated cases,
2962 in particular mutual recursions between two different subpatterns, can-
2963 not be detected until run time.
2967 This error is returned when a pattern that was successfully studied
2968 using the PCRE_STUDY_JIT_COMPILE option is being matched, but the mem-
2969 ory available for the just-in-time processing stack is not large
2970 enough. See the pcrejit documentation for more details.
2974 This error is given if a pattern that was compiled by the 8-bit library
2975 is passed to a 16-bit library function, or vice versa.
2979 This error is given if a pattern that was compiled and saved is
2980 reloaded on a host with different endianness. The utility function
2981 pcre_pattern_to_host_byte_order() can be used to convert such a pattern
2982 so that it runs on the new host.
2984 Error numbers -16 to -20 and -22 are not used by pcre_exec().
2986 Reason codes for invalid UTF-8 strings
2988 This section applies only to the 8-bit library. The corresponding
2989 information for the 16-bit library is given in the pcre16 page.
2991 When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT-
2992 UTF8, and the size of the output vector (ovecsize) is at least 2, the
2993 offset of the start of the invalid UTF-8 character is placed in the
2994 first output vector element (ovector[0]) and a reason code is placed in
2995 the second element (ovector[1]). The reason codes are given names in
2996 the pcre.h header file:
3004 The string ends with a truncated UTF-8 character; the code specifies
3005 how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8
3006 characters to be no longer than 4 bytes, the encoding scheme (origi-
3007 nally defined by RFC 2279) allows for up to 6 bytes, and this is
3008 checked first; hence the possibility of 4 or 5 missing bytes.
3014 PCRE_UTF8_ERR10
3016 The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
3017 the character do not have the binary value 0b10 (that is, either the
3018 most significant bit is 0, or the next bit is 1).
3020 PCRE_UTF8_ERR11
3021 PCRE_UTF8_ERR12
3023 A character that is valid by the RFC 2279 rules is either 5 or 6 bytes
3024 long; these code points are excluded by RFC 3629.
3026 PCRE_UTF8_ERR13
3028 A 4-byte character has a value greater than 0x10fff; these code points
3029 are excluded by RFC 3629.
3031 PCRE_UTF8_ERR14
3033 A 3-byte character has a value in the range 0xd800 to 0xdfff; this
3034 range of code points are reserved by RFC 3629 for use with UTF-16, and
3035 so are excluded from UTF-8.
3037 PCRE_UTF8_ERR15
3038 PCRE_UTF8_ERR16
3039 PCRE_UTF8_ERR17
3040 PCRE_UTF8_ERR18
3041 PCRE_UTF8_ERR19
3043 A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
3044 for a value that can be represented by fewer bytes, which is invalid.
3045 For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
3046 rect coding uses just one byte.
3048 PCRE_UTF8_ERR20
3050 The two most significant bits of the first byte of a character have the
3051 binary value 0b10 (that is, the most significant bit is 1 and the sec-
3052 ond is 0). Such a byte can only validly occur as the second or subse-
3053 quent byte of a multi-byte character.
3055 PCRE_UTF8_ERR21
3057 The first byte of a character has the value 0xfe or 0xff. These values
3058 can never occur in a valid UTF-8 string.
3063 int pcre_copy_substring(const char *subject, int *ovector,
3064 int stringcount, int stringnumber, char *buffer,
3065 int buffersize);
3067 int pcre_get_substring(const char *subject, int *ovector,
3068 int stringcount, int stringnumber,
3069 const char **stringptr);
3071 int pcre_get_substring_list(const char *subject,
3072 int *ovector, int stringcount, const char ***listptr);
3074 Captured substrings can be accessed directly by using the offsets
3075 returned by pcre_exec() in ovector. For convenience, the functions
3076 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
3077 string_list() are provided for extracting captured substrings as new,
3078 separate, zero-terminated strings. These functions identify substrings
3079 by number. The next section describes functions for extracting named
3080 substrings.
3082 A substring that contains a binary zero is correctly extracted and has
3083 a further zero added on the end, but the result is not, of course, a C
3084 string. However, you can process such a string by referring to the
3085 length that is returned by pcre_copy_substring() and pcre_get_sub-
3086 string(). Unfortunately, the interface to pcre_get_substring_list() is
3087 not adequate for handling strings containing binary zeros, because the
3088 end of the final string is not independently indicated.
3090 The first three arguments are the same for all three of these func-
3091 tions: subject is the subject string that has just been successfully
3092 matched, ovector is a pointer to the vector of integer offsets that was
3093 passed to pcre_exec(), and stringcount is the number of substrings that
3094 were captured by the match, including the substring that matched the
3095 entire regular expression. This is the value returned by pcre_exec() if
3096 it is greater than zero. If pcre_exec() returned zero, indicating that
3097 it ran out of space in ovector, the value passed as stringcount should
3098 be the number of elements in the vector divided by three.
3100 The functions pcre_copy_substring() and pcre_get_substring() extract a
3101 single substring, whose number is given as stringnumber. A value of
3102 zero extracts the substring that matched the entire pattern, whereas
3103 higher values extract the captured substrings. For pcre_copy_sub-
3104 string(), the string is placed in buffer, whose length is given by
3105 buffersize, while for pcre_get_substring() a new block of memory is
3106 obtained via pcre_malloc, and its address is returned via stringptr.
3107 The yield of the function is the length of the string, not including
3108 the terminating zero, or one of these error codes:
3112 The buffer was too small for pcre_copy_substring(), or the attempt to
3113 get memory failed for pcre_get_substring().
3117 There is no substring whose number is stringnumber.
3119 The pcre_get_substring_list() function extracts all available sub-
3120 strings and builds a list of pointers to them. All this is done in a
3121 single block of memory that is obtained via pcre_malloc. The address of
3122 the memory block is returned via listptr, which is also the start of
3123 the list of string pointers. The end of the list is marked by a NULL
3124 pointer. The yield of the function is zero if all went well, or the
3125 error code
3129 if the attempt to get the memory block failed.
3131 When any of these functions encounter a substring that is unset, which
3132 can happen when capturing subpattern number n+1 matches some part of
3133 the subject, but subpattern n has not been used at all, they return an
3134 empty string. This can be distinguished from a genuine zero-length sub-
3135 string by inspecting the appropriate offset in ovector, which is nega-
3136 tive for unset substrings.
3138 The two convenience functions pcre_free_substring() and pcre_free_sub-
3139 string_list() can be used to free the memory returned by a previous
3140 call of pcre_get_substring() or pcre_get_substring_list(), respec-
3141 tively. They do nothing more than call the function pointed to by
3142 pcre_free, which of course could be called directly from a C program.
3143 However, PCRE is used in some situations where it is linked via a spe-
3144 cial interface to another programming language that cannot use
3145 pcre_free directly; it is for these cases that the functions are pro-
3146 vided.
3151 int pcre_get_stringnumber(const pcre *code,
3152 const char *name);
3154 int pcre_copy_named_substring(const pcre *code,
3155 const char *subject, int *ovector,
3156 int stringcount, const char *stringname,
3157 char *buffer, int buffersize);
3159 int pcre_get_named_substring(const pcre *code,
3160 const char *subject, int *ovector,
3161 int stringcount, const char *stringname,
3162 const char **stringptr);
3164 To extract a substring by name, you first have to find associated num-
3165 ber. For example, for this pattern
3167 (a+)b(?<xxx>\d+)...
3169 the number of the subpattern called "xxx" is 2. If the name is known to
3170 be unique (PCRE_DUPNAMES was not set), you can find the number from the
3171 name by calling pcre_get_stringnumber(). The first argument is the com-
3172 piled pattern, and the second is the name. The yield of the function is
3173 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
3174 subpattern of that name.
3176 Given the number, you can extract the substring directly, or use one of
3177 the functions described in the previous section. For convenience, there
3178 are also two functions that do the whole job.
3180 Most of the arguments of pcre_copy_named_substring() and
3181 pcre_get_named_substring() are the same as those for the similarly
3182 named functions that extract by number. As these are described in the
3183 previous section, they are not re-described here. There are just two
3184 differences:
3186 First, instead of a substring number, a substring name is given. Sec-
3187 ond, there is an extra argument, given at the start, which is a pointer
3188 to the compiled pattern. This is needed in order to gain access to the
3189 name-to-number translation table.
3191 These functions call pcre_get_stringnumber(), and if it succeeds, they
3192 then call pcre_copy_substring() or pcre_get_substring(), as appropri-
3193 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
3194 behaviour may not be what you want (see the next section).
3196 Warning: If the pattern uses the (?| feature to set up multiple subpat-
3197 terns with the same number, as described in the section on duplicate
3198 subpattern numbers in the pcrepattern page, you cannot use names to
3199 distinguish the different subpatterns, because names are not included
3200 in the compiled code. The matching process uses only numbers. For this
3201 reason, the use of different names for subpatterns of the same number
3202 causes an error at compile time.
3207 int pcre_get_stringtable_entries(const pcre *code,
3208 const char *name, char **first, char **last);
3210 When a pattern is compiled with the PCRE_DUPNAMES option, names for
3211 subpatterns are not required to be unique. (Duplicate names are always
3212 allowed for subpatterns with the same number, created by using the (?|
3213 feature. Indeed, if such subpatterns are named, they are required to
3214 use the same names.)
3216 Normally, patterns with duplicate names are such that in any one match,
3217 only one of the named subpatterns participates. An example is shown in
3218 the pcrepattern documentation.
3220 When duplicates are present, pcre_copy_named_substring() and
3221 pcre_get_named_substring() return the first substring corresponding to
3222 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
3223 (-7) is returned; no data is returned. The pcre_get_stringnumber()
3224 function returns one of the numbers that are associated with the name,
3225 but it is not defined which it is.
3227 If you want to get full details of all captured substrings for a given
3228 name, you must use the pcre_get_stringtable_entries() function. The
3229 first argument is the compiled pattern, and the second is the name. The
3230 third and fourth are pointers to variables which are updated by the
3231 function. After it has run, they point to the first and last entries in
3232 the name-to-number table for the given name. The function itself
3233 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
3234 there are none. The format of the table is described above in the sec-
3235 tion entitled Information about a pattern above. Given all the rele-
3236 vant entries for the name, you can extract each of their numbers, and
3237 hence the captured data, if any.
3242 The traditional matching function uses a similar algorithm to Perl,
3243 which stops when it finds the first match, starting at a given point in
3244 the subject. If you want to find all possible matches, or the longest
3245 possible match, consider using the alternative matching function (see
3246 below) instead. If you cannot use the alternative function, but still
3247 need to find all possible matches, you can kludge it up by making use
3248 of the callout facility, which is described in the pcrecallout documen-
3249 tation.
3251 What you have to do is to insert a callout right at the end of the pat-
3252 tern. When your callout function is called, extract and save the cur-
3253 rent matched substring. Then return 1, which forces pcre_exec() to
3254 backtrack and try other alternatives. Ultimately, when it runs out of
3255 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
3260 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
3261 const char *subject, int length, int startoffset,
3262 int options, int *ovector, int ovecsize,
3263 int *workspace, int wscount);
3265 The function pcre_dfa_exec() is called to match a subject string
3266 against a compiled pattern, using a matching algorithm that scans the
3267 subject string just once, and does not backtrack. This has different
3268 characteristics to the normal algorithm, and is not compatible with
3269 Perl. Some of the features of PCRE patterns are not supported. Never-
3270 theless, there are times when this kind of matching can be useful. For
3271 a discussion of the two matching algorithms, and a list of features
3272 that pcre_dfa_exec() does not support, see the pcrematching documenta-
3273 tion.
3275 The arguments for the pcre_dfa_exec() function are the same as for
3276 pcre_exec(), plus two extras. The ovector argument is used in a differ-
3277 ent way, and this is described below. The other common arguments are
3278 used in the same way as for pcre_exec(), so their description is not
3279 repeated here.
3281 The two additional arguments provide workspace for the function. The
3282 workspace vector should contain at least 20 elements. It is used for
3283 keeping track of multiple paths through the pattern tree. More
3284 workspace will be needed for patterns and subjects where there are a
3285 lot of potential matches.
3287 Here is an example of a simple call to pcre_dfa_exec():
3289 int rc;
3290 int ovector[10];
3291 int wspace[20];
3292 rc = pcre_dfa_exec(
3293 re, /* result of pcre_compile() */
3294 NULL, /* we didn't study the pattern */
3295 "some string", /* the subject string */
3296 11, /* the length of the subject string */
3297 0, /* start at offset 0 in the subject */
3298 0, /* default options */
3299 ovector, /* vector of integers for substring information */
3300 10, /* number of elements (NOT size in bytes) */
3301 wspace, /* working space vector */
3302 20); /* number of elements (NOT size in bytes) */
3304 Option bits for pcre_dfa_exec()
3306 The unused bits of the options argument for pcre_dfa_exec() must be
3307 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
3312 four of these are exactly the same as for pcre_exec(), so their
3313 description is not repeated here.
3318 These have the same general effect as they do for pcre_exec(), but the
3319 details are slightly different. When PCRE_PARTIAL_HARD is set for
3320 pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub-
3321 ject is reached and there is still at least one matching possibility
3322 that requires additional characters. This happens even if some complete
3323 matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
3324 code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
3325 of the subject is reached, there have been no complete matches, but
3326 there is still at least one matching possibility. The portion of the
3327 string that was inspected when the longest partial match was found is
3328 set as the first matching string in both cases. There is a more
3329 detailed discussion of partial and multi-segment matching, with exam-
3330 ples, in the pcrepartial documentation.
3334 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
3335 stop as soon as it has found one match. Because of the way the alterna-
3336 tive algorithm works, this is necessarily the shortest possible match
3337 at the first possible matching point in the subject string.
3341 When pcre_dfa_exec() returns a partial match, it is possible to call it
3342 again, with additional subject characters, and have it continue with
3343 the same match. The PCRE_DFA_RESTART option requests this action; when
3344 it is set, the workspace and wscount options must reference the same
3345 vector as before because data about the match so far is left in them
3346 after a partial match. There is more discussion of this facility in the
3347 pcrepartial documentation.
3349 Successful returns from pcre_dfa_exec()
3351 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
3352 string in the subject. Note, however, that all the matches from one run
3353 of the function start at the same point in the subject. The shorter
3354 matches are all initial substrings of the longer matches. For example,
3355 if the pattern
3357 <.*>
3359 is matched against the string
3361 This is <something> <something else> <something further> no more
3363 the three matched strings are
3365 <something>
3366 <something> <something else>
3367 <something> <something else> <something further>
3369 On success, the yield of the function is a number greater than zero,
3370 which is the number of matched substrings. The substrings themselves
3371 are returned in ovector. Each string uses two elements; the first is
3372 the offset to the start, and the second is the offset to the end. In
3373 fact, all the strings have the same start offset. (Space could have
3374 been saved by giving this only once, but it was decided to retain some
3375 compatibility with the way pcre_exec() returns data, even though the
3376 meaning of the strings is different.)
3378 The strings are returned in reverse order of length; that is, the long-
3379 est matching string is given first. If there were too many matches to
3380 fit into ovector, the yield of the function is zero, and the vector is
3381 filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec()
3382 can use the entire ovector for returning matched strings.
3384 Error returns from pcre_dfa_exec()
3386 The pcre_dfa_exec() function returns a negative number when it fails.
3387 Many of the errors are the same as for pcre_exec(), and these are
3388 described above. There are in addition the following errors that are
3389 specific to pcre_dfa_exec():
3393 This return is given if pcre_dfa_exec() encounters an item in the pat-
3394 tern that it does not support, for instance, the use of \C or a back
3395 reference.
3399 This return is given if pcre_dfa_exec() encounters a condition item
3400 that uses a back reference for the condition, or a test for recursion
3401 in a specific group. These are not supported.
3405 This return is given if pcre_dfa_exec() is called with an extra block
3406 that contains a setting of the match_limit or match_limit_recursion
3407 fields. This is not supported (these fields are meaningless for DFA
3408 matching).
3412 This return is given if pcre_dfa_exec() runs out of space in the
3413 workspace vector.
3417 When a recursive subpattern is processed, the matching function calls
3418 itself recursively, using private vectors for ovector and workspace.
3419 This error is given if the output vector is not large enough. This
3420 should be extremely rare, as a vector of size 1000 is used.
3425 pcre16(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematch-
3426 ing(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcresample(3),
3427 pcrestack(3).
3432 Philip Hazel
3433 University Computing Service
3434 Cambridge CB2 3QH, England.
3439 Last updated: 07 January 2012
3440 Copyright (c) 1997-2012 University of Cambridge.
3441 ------------------------------------------------------------------------------
3447 NAME
3448 PCRE - Perl-compatible regular expressions
3453 int (*pcre_callout)(pcre_callout_block *);
3455 int (*pcre16_callout)(pcre16_callout_block *);
3457 PCRE provides a feature called "callout", which is a means of temporar-
3458 ily passing control to the caller of PCRE in the middle of pattern
3459 matching. The caller of PCRE provides an external function by putting
3460 its entry point in the global variable pcre_callout (pcre16_callout for
3461 the 16-bit library). By default, this variable contains NULL, which
3462 disables all calling out.
3464 Within a regular expression, (?C) indicates the points at which the
3465 external function is to be called. Different callout points can be
3466 identified by putting a number less than 256 after the letter C. The
3467 default value is zero. For example, this pattern has two callout
3468 points:
3470 (?C1)abc(?C2)def
3472 If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled,
3473 PCRE automatically inserts callouts, all with number 255, before each
3474 item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the
3475 pattern
3477 A(\d{2}|--)
3479 it is processed as if it were
3481 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
3483 Notice that there is a callout before and after each parenthesis and
3484 alternation bar. Automatic callouts can be used for tracking the
3485 progress of pattern matching. The pcretest command has an option that
3486 sets automatic callouts; when it is used, the output indicates how the
3487 pattern is matched. This is useful information when you are trying to
3488 optimize the performance of a particular pattern.
3490 The use of callouts in a pattern makes it ineligible for optimization
3491 by the just-in-time compiler. Studying such a pattern with the
3492 PCRE_STUDY_JIT_COMPILE option always fails.
3497 You should be aware that, because of optimizations in the way PCRE
3498 matches patterns by default, callouts sometimes do not happen. For
3499 example, if the pattern is
3501 ab(?C4)cd
3503 PCRE knows that any matching string must contain the letter "d". If the
3504 subject string is "abyz", the lack of "d" means that matching doesn't
3505 ever start, and the callout is never reached. However, with "abyd",
3506 though the result is still no match, the callout is obeyed.
3508 If the pattern is studied, PCRE knows the minimum length of a matching
3509 string, and will immediately give a "no match" return without actually
3510 running a match if the subject is not long enough, or, for unanchored
3511 patterns, if it has been scanned far enough.
3513 You can disable these optimizations by passing the PCRE_NO_START_OPTI-
3514 MIZE option to the matching function, or by starting the pattern with
3515 (*NO_START_OPT). This slows down the matching process, but does ensure
3516 that callouts such as the example above are obeyed.
3521 During matching, when PCRE reaches a callout point, the external func-
3522 tion defined by pcre_callout or pcre16_callout is called (if it is
3523 set). This applies to both normal and DFA matching. The only argument
3524 to the callout function is a pointer to a pcre_callout or pcre16_call-
3525 out block. These structures contains the following fields:
3527 int version;
3528 int callout_number;
3529 int *offset_vector;
3530 const char *subject; (8-bit version)
3531 PCRE_SPTR16 subject; (16-bit version)
3532 int subject_length;
3533 int start_match;
3534 int current_position;
3535 int capture_top;
3536 int capture_last;
3537 void *callout_data;
3538 int pattern_position;
3539 int next_item_length;
3540 const unsigned char *mark; (8-bit version)
3541 const PCRE_UCHAR16 *mark; (16-bit version)
3543 The version field is an integer containing the version number of the
3544 block format. The initial version was 0; the current version is 2. The
3545 version number will change again in future if additional fields are
3546 added, but the intention is never to remove any of the existing fields.
3548 The callout_number field contains the number of the callout, as com-
3549 piled into the pattern (that is, the number after ?C for manual call-
3550 outs, and 255 for automatically generated callouts).
3552 The offset_vector field is a pointer to the vector of offsets that was
3553 passed by the caller to the matching function. When pcre_exec() or
3554 pcre16_exec() is used, the contents can be inspected, in order to
3555 extract substrings that have been matched so far, in the same way as
3556 for extracting substrings after a match has completed. For the DFA
3557 matching functions, this field is not useful.
3559 The subject and subject_length fields contain copies of the values that
3560 were passed to the matching function.
3562 The start_match field normally contains the offset within the subject
3563 at which the current match attempt started. However, if the escape
3564 sequence \K has been encountered, this value is changed to reflect the
3565 modified starting point. If the pattern is not anchored, the callout
3566 function may be called several times from the same point in the pattern
3567 for different starting points in the subject.
3569 The current_position field contains the offset within the subject of
3570 the current match pointer.
3572 When the pcre_exec() or pcre16_exec() is used, the capture_top field
3573 contains one more than the number of the highest numbered captured sub-
3574 string so far. If no substrings have been captured, the value of cap-
3575 ture_top is one. This is always the case when the DFA functions are
3576 used, because they do not support captured substrings.
3578 The capture_last field contains the number of the most recently cap-
3579 tured substring. If no substrings have been captured, its value is -1.
3580 This is always the case for the DFA matching functions.
3582 The callout_data field contains a value that is passed to a matching
3583 function specifically so that it can be passed back in callouts. It is
3584 passed in the callout_data field of a pcre_extra or pcre16_extra data
3585 structure. If no such data was passed, the value of callout_data in a
3586 callout block is NULL. There is a description of the pcre_extra struc-
3587 ture in the pcreapi documentation.
3589 The pattern_position field is present from version 1 of the callout
3590 structure. It contains the offset to the next item to be matched in the
3591 pattern string.
3593 The next_item_length field is present from version 1 of the callout
3594 structure. It contains the length of the next item to be matched in the
3595 pattern string. When the callout immediately precedes an alternation
3596 bar, a closing parenthesis, or the end of the pattern, the length is
3597 zero. When the callout precedes an opening parenthesis, the length is
3598 that of the entire subpattern.
3600 The pattern_position and next_item_length fields are intended to help
3601 in distinguishing between different automatic callouts, which all have
3602 the same callout number. However, they are set for all callouts.
3604 The mark field is present from version 2 of the callout structure. In
3605 callouts from pcre_exec() or pcre16_exec() it contains a pointer to the
3606 zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
3607 (*THEN) item in the match, or NULL if no such items have been passed.
3608 Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
3609 previous (*MARK). In callouts from the DFA matching functions this
3610 field always contains NULL.
3615 The external callout function returns an integer to PCRE. If the value
3616 is zero, matching proceeds as normal. If the value is greater than
3617 zero, matching fails at the current point, but the testing of other
3618 matching possibilities goes ahead, just as if a lookahead assertion had
3619 failed. If the value is less than zero, the match is abandoned, the
3620 matching function returns the negative value.
3622 Negative values should normally be chosen from the set of
3623 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
3624 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
3625 reserved for use by callout functions; it will never be used by PCRE
3626 itself.
3631 Philip Hazel
3632 University Computing Service
3633 Cambridge CB2 3QH, England.
3638 Last updated: 08 Janurary 2012
3639 Copyright (c) 1997-2012 University of Cambridge.
3640 ------------------------------------------------------------------------------
3646 NAME
3647 PCRE - Perl-compatible regular expressions
3652 This document describes the differences in the ways that PCRE and Perl
3653 handle regular expressions. The differences described here are with
3654 respect to Perl versions 5.10 and above.
3656 1. PCRE has only a subset of Perl's Unicode support. Details of what it
3657 does have are given in the pcreunicode page.
3659 2. PCRE allows repeat quantifiers only on parenthesized assertions, but
3660 they do not mean what you might think. For example, (?!a){3} does not
3661 assert that the next three characters are not "a". It just asserts that
3662 the next character is not "a" three times (in principle: PCRE optimizes
3663 this to run the assertion just once). Perl allows repeat quantifiers on
3664 other assertions such as \b, but these do not seem to have any use.
3666 3. Capturing subpatterns that occur inside negative lookahead asser-
3667 tions are counted, but their entries in the offsets vector are never
3668 set. Perl sets its numerical variables from any such patterns that are
3669 matched before the assertion fails to match something (thereby succeed-
3670 ing), but only if the negative lookahead assertion contains just one
3671 branch.
3673 4. Though binary zero characters are supported in the subject string,
3674 they are not allowed in a pattern string because it is passed as a nor-
3675 mal C string, terminated by zero. The escape sequence \0 can be used in
3676 the pattern to represent a binary zero.
3678 5. The following Perl escape sequences are not supported: \l, \u, \L,
3679 \U, and \N when followed by a character name or Unicode value. (\N on
3680 its own, matching a non-newline character, is supported.) In fact these
3681 are implemented by Perl's general string-handling and are not part of
3682 its pattern matching engine. If any of these are encountered by PCRE,
3683 an error is generated by default. However, if the PCRE_JAVASCRIPT_COM-
3684 PAT option is set, \U and \u are interpreted as JavaScript interprets
3685 them.
3687 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
3688 is built with Unicode character property support. The properties that
3689 can be tested with \p and \P are limited to the general category prop-
3690 erties such as Lu and Nd, script names such as Greek or Han, and the
3691 derived properties Any and L&. PCRE does support the Cs (surrogate)
3692 property, which Perl does not; the Perl documentation says "Because
3693 Perl hides the need for the user to understand the internal representa-
3694 tion of Unicode characters, there is no need to implement the somewhat
3695 messy concept of surrogates."
3697 7. PCRE implements a simpler version of \X than Perl, which changed to
3698 make \X match what Unicode calls an "extended grapheme cluster". This
3699 is more complicated than an extended Unicode sequence, which is what
3700 PCRE matches.
3702 8. PCRE does support the \Q...\E escape for quoting substrings. Charac-
3703 ters in between are treated as literals. This is slightly different
3704 from Perl in that $ and @ are also handled as literals inside the
3705 quotes. In Perl, they cause variable interpolation (but of course PCRE
3706 does not have variables). Note the following examples:
3708 Pattern PCRE matches Perl matches
3710 \Qabc$xyz\E abc$xyz abc followed by the
3711 contents of $xyz
3712 \Qabc\$xyz\E abc\$xyz abc\$xyz
3713 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3715 The \Q...\E sequence is recognized both inside and outside character
3716 classes.
3718 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
3719 constructions. However, there is support for recursive patterns. This
3720 is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE
3721 "callout" feature allows an external function to be called during pat-
3722 tern matching. See the pcrecallout documentation for details.
3724 10. Subpatterns that are called as subroutines (whether or not recur-
3725 sively) are always treated as atomic groups in PCRE. This is like
3726 Python, but unlike Perl. Captured values that are set outside a sub-
3727 routine call can be reference from inside in PCRE, but not in Perl.
3728 There is a discussion that explains these differences in more detail in
3729 the section on recursion differences from Perl in the pcrepattern page.
3731 11. If (*THEN) is present in a group that is called as a subroutine,
3732 its action is limited to that group, even if the group does not contain
3733 any | characters.
3735 12. There are some differences that are concerned with the settings of
3736 captured strings when part of a pattern is repeated. For example,
3737 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
3738 unset, but in PCRE it is set to "b".
3740 13. PCRE's handling of duplicate subpattern numbers and duplicate sub-
3741 pattern names is not as general as Perl's. This is a consequence of the
3742 fact the PCRE works internally just with numbers, using an external ta-
3743 ble to translate between numbers and names. In particular, a pattern
3744 such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have
3745 the same number but different names, is not supported, and causes an
3746 error at compile time. If it were allowed, it would not be possible to
3747 distinguish which parentheses matched, because both names map to cap-
3748 turing subpattern number 1. To avoid this confusing situation, an error
3749 is given at compile time.
3751 14. Perl recognizes comments in some places that PCRE does not, for
3752 example, between the ( and ? at the start of a subpattern. If the /x
3753 modifier is set, Perl allows whitespace between ( and ? but PCRE never
3754 does, even if the PCRE_EXTENDED option is set.
3756 15. PCRE provides some extensions to the Perl regular expression facil-
3757 ities. Perl 5.10 includes new features that are not in earlier ver-
3758 sions of Perl, some of which (such as named parentheses) have been in
3759 PCRE for some time. This list is with respect to Perl 5.10:
3761 (a) Although lookbehind assertions in PCRE must match fixed length
3762 strings, each alternative branch of a lookbehind assertion can match a
3763 different length of string. Perl requires them all to have the same
3764 length.
3766 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
3767 meta-character matches only at the very end of the string.
3769 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
3770 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
3771 ignored. (Perl can be made to issue a warning.)
3773 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
3774 fiers is inverted, that is, by default they are not greedy, but if fol-
3775 lowed by a question mark they are.
3777 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
3778 tried only at the first matching position in the subject string.
3781 and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva-
3782 lents.
3784 (g) The \R escape sequence can be restricted to match only CR, LF, or
3785 CRLF by the PCRE_BSR_ANYCRLF option.
3787 (h) The callout facility is PCRE-specific.
3789 (i) The partial matching facility is PCRE-specific.
3791 (j) Patterns compiled by PCRE can be saved and re-used at a later time,
3792 even on different hosts that have the other endianness. However, this
3793 does not apply to optimized data created by the just-in-time compiler.
3795 (k) The alternative matching functions (pcre_dfa_exec() and
3796 pcre16_dfa_exec()) match in a different way and are not Perl-compati-
3797 ble.
3799 (l) PCRE recognizes some special sequences such as (*CR) at the start
3800 of a pattern that set overall options that cannot be changed within the
3801 pattern.
3806 Philip Hazel
3807 University Computing Service
3808 Cambridge CB2 3QH, England.
3813 Last updated: 08 Januray 2012
3814 Copyright (c) 1997-2012 University of Cambridge.
3815 ------------------------------------------------------------------------------
3821 NAME
3822 PCRE - Perl-compatible regular expressions
3827 The syntax and semantics of the regular expressions that are supported
3828 by PCRE are described in detail below. There is a quick-reference syn-
3829 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
3830 semantics as closely as it can. PCRE also supports some alternative
3831 regular expression syntax (which does not conflict with the Perl syn-
3832 tax) in order to provide some compatibility with regular expressions in
3833 Python, .NET, and Oniguruma.
3835 Perl's regular expressions are described in its own documentation, and
3836 regular expressions in general are covered in a number of books, some
3837 of which have copious examples. Jeffrey Friedl's "Mastering Regular
3838 Expressions", published by O'Reilly, covers regular expressions in
3839 great detail. This description of PCRE's regular expressions is
3840 intended as reference material.
3842 The original operation of PCRE was on strings of one-byte characters.
3843 However, there is now also support for UTF-8 strings in the original
3844 library, and a second library that supports 16-bit and UTF-16 character
3845 strings. To use these features, PCRE must be built to include appropri-
3846 ate support. When using UTF strings you must either call the compiling
3847 function with the PCRE_UTF8 or PCRE_UTF16 option, or the pattern must
3848 start with one of these special sequences:
3850 (*UTF8)
3851 (*UTF16)
3853 Starting a pattern with such a sequence is equivalent to setting the
3854 relevant option. This feature is not Perl-compatible. How setting a UTF
3855 mode affects pattern matching is mentioned in several places below.
3856 There is also a summary of features in the pcreunicode page.
3858 Another special sequence that may appear at the start of a pattern or
3859 in combination with (*UTF8) or (*UTF16) is:
3861 (*UCP)
3863 This has the same effect as setting the PCRE_UCP option: it causes
3864 sequences such as \d and \w to use Unicode properties to determine
3865 character types, instead of recognizing only characters with codes less
3866 than 128 via a lookup table.
3868 If a pattern starts with (*NO_START_OPT), it has the same effect as
3869 setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
3870 time. There are also some more of these special sequences that are con-
3871 cerned with the handling of newlines; they are described below.
3873 The remainder of this document discusses the patterns that are sup-
3874 ported by PCRE when one its main matching functions, pcre_exec()
3875 (8-bit) or pcre16_exec() (16-bit), is used. PCRE also has alternative
3876 matching functions, pcre_dfa_exec() and pcre16_dfa_exec(), which match
3877 using a different algorithm that is not Perl-compatible. Some of the
3878 features discussed below are not available when DFA matching is used.
3879 The advantages and disadvantages of the alternative functions, and how
3880 they differ from the normal functions, are discussed in the pcrematch-
3881 ing page.
3886 PCRE supports five different conventions for indicating line breaks in
3887 strings: a single CR (carriage return) character, a single LF (line-
3888 feed) character, the two-character sequence CRLF, any of the three pre-
3889 ceding, or any Unicode newline sequence. The pcreapi page has further
3890 discussion about newlines, and shows how to set the newline convention
3891 in the options arguments for the compiling and matching functions.
3893 It is also possible to specify a newline convention by starting a pat-
3894 tern string with one of the following five sequences:
3896 (*CR) carriage return
3897 (*LF) linefeed
3898 (*CRLF) carriage return, followed by linefeed
3899 (*ANYCRLF) any of the three above
3900 (*ANY) all Unicode newline sequences
3902 These override the default and the options given to the compiling func-
3903 tion. For example, on a Unix system where LF is the default newline
3904 sequence, the pattern
3906 (*CR)a.b
3908 changes the convention to CR. That pattern matches "a\nb" because LF is
3909 no longer a newline. Note that these special settings, which are not
3910 Perl-compatible, are recognized only at the very start of a pattern,
3911 and that they must be in upper case. If more than one of them is
3912 present, the last one is used.
3914 The newline convention affects the interpretation of the dot metachar-
3915 acter when PCRE_DOTALL is not set, and also the behaviour of \N. How-
3916 ever, it does not affect what the \R escape sequence matches. By
3917 default, this is any Unicode newline sequence, for Perl compatibility.
3918 However, this can be changed; see the description of \R in the section
3919 entitled "Newline sequences" below. A change of \R setting can be com-
3920 bined with a change of newline convention.
3925 A regular expression is a pattern that is matched against a subject
3926 string from left to right. Most characters stand for themselves in a
3927 pattern, and match the corresponding characters in the subject. As a
3928 trivial example, the pattern
3930 The quick brown fox
3932 matches a portion of a subject string that is identical to itself. When
3933 caseless matching is specified (the PCRE_CASELESS option), letters are
3934 matched independently of case. In a UTF mode, PCRE always understands
3935 the concept of case for characters whose values are less than 128, so
3936 caseless matching is always possible. For characters with higher val-
3937 ues, the concept of case is supported if PCRE is compiled with Unicode
3938 property support, but not otherwise. If you want to use caseless
3939 matching for characters 128 and above, you must ensure that PCRE is
3940 compiled with Unicode property support as well as with UTF support.
3942 The power of regular expressions comes from the ability to include
3943 alternatives and repetitions in the pattern. These are encoded in the
3944 pattern by the use of metacharacters, which do not stand for themselves
3945 but instead are interpreted in some special way.
3947 There are two different sets of metacharacters: those that are recog-
3948 nized anywhere in the pattern except within square brackets, and those
3949 that are recognized within square brackets. Outside square brackets,
3950 the metacharacters are as follows:
3952 \ general escape character with several uses
3953 ^ assert start of string (or line, in multiline mode)
3954 $ assert end of string (or line, in multiline mode)
3955 . match any character except newline (by default)
3956 [ start character class definition
3957 | start of alternative branch
3958 ( start subpattern
3959 ) end subpattern
3960 ? extends the meaning of (
3961 also 0 or 1 quantifier
3962 also quantifier minimizer
3963 * 0 or more quantifier
3964 + 1 or more quantifier
3965 also "possessive quantifier"
3966 { start min/max quantifier
3968 Part of a pattern that is in square brackets is called a "character
3969 class". In a character class the only metacharacters are:
3971 \ general escape character
3972 ^ negate the class, but only if the first character
3973 - indicates character range
3974 [ POSIX character class (only if followed by POSIX
3975 syntax)
3976 ] terminates the character class
3978 The following sections describe the use of each of the metacharacters.
3983 The backslash character has several uses. Firstly, if it is followed by
3984 a character that is not a number or a letter, it takes away any special
3985 meaning that character may have. This use of backslash as an escape
3986 character applies both inside and outside character classes.
3988 For example, if you want to match a * character, you write \* in the
3989 pattern. This escaping action applies whether or not the following
3990 character would otherwise be interpreted as a metacharacter, so it is
3991 always safe to precede a non-alphanumeric with backslash to specify
3992 that it stands for itself. In particular, if you want to match a back-
3993 slash, you write \\.
3995 In a UTF mode, only ASCII numbers and letters have any special meaning
3996 after a backslash. All other characters (in particular, those whose
3997 codepoints are greater than 127) are treated as literals.
3999 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
4000 the pattern (other than in a character class) and characters between a
4001 # outside a character class and the next newline are ignored. An escap-
4002 ing backslash can be used to include a whitespace or # character as
4003 part of the pattern.
4005 If you want to remove the special meaning from a sequence of charac-
4006 ters, you can do so by putting them between \Q and \E. This is differ-
4007 ent from Perl in that $ and @ are handled as literals in \Q...\E
4008 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
4009 tion. Note the following examples:
4011 Pattern PCRE matches Perl matches
4013 \Qabc$xyz\E abc$xyz abc followed by the
4014 contents of $xyz
4015 \Qabc\$xyz\E abc\$xyz abc\$xyz
4016 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
4018 The \Q...\E sequence is recognized both inside and outside character
4019 classes. An isolated \E that is not preceded by \Q is ignored. If \Q
4020 is not followed by \E later in the pattern, the literal interpretation
4021 continues to the end of the pattern (that is, \E is assumed at the
4022 end). If the isolated \Q is inside a character class, this causes an
4023 error, because the character class is not terminated.
4025 Non-printing characters
4027 A second use of backslash provides a way of encoding non-printing char-
4028 acters in patterns in a visible manner. There is no restriction on the
4029 appearance of non-printing characters, apart from the binary zero that
4030 terminates a pattern, but when a pattern is being prepared by text
4031 editing, it is often easier to use one of the following escape
4032 sequences than the binary character it represents:
4034 \a alarm, that is, the BEL character (hex 07)
4035 \cx "control-x", where x is any ASCII character
4036 \e escape (hex 1B)
4037 \f formfeed (hex 0C)
4038 \n linefeed (hex 0A)
4039 \r carriage return (hex 0D)
4040 \t tab (hex 09)
4041 \ddd character with octal code ddd, or back reference
4042 \xhh character with hex code hh
4043 \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
4044 \uhhhh character with hex code hhhh (JavaScript mode only)
4046 The precise effect of \cx is as follows: if x is a lower case letter,
4047 it is converted to upper case. Then bit 6 of the character (hex 40) is
4048 inverted. Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
4049 is 7B), while \c; becomes hex 7B (; is 3B). If the byte following \c
4050 has a value greater than 127, a compile-time error occurs. This locks
4051 out non-ASCII characters in all modes. (When PCRE is compiled in EBCDIC
4052 mode, all byte values are valid. A lower case letter is converted to
4053 upper case, and then the 0xc0 bits are flipped.)
4055 By default, after \x, from zero to two hexadecimal digits are read
4056 (letters can be in upper or lower case). Any number of hexadecimal dig-
4057 its may appear between \x{ and }, but the character code is constrained
4058 as follows:
4060 8-bit non-UTF mode less than 0x100
4061 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
4062 16-bit non-UTF mode less than 0x10000
4063 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
4065 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-
4066 called "surrogate" codepoints).
4068 If characters other than hexadecimal digits appear between \x{ and },
4069 or if there is no terminating }, this form of escape is not recognized.
4070 Instead, the initial \x will be interpreted as a basic hexadecimal
4071 escape, with no following digits, giving a character whose value is
4072 zero.
4074 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
4075 is as just described only when it is followed by two hexadecimal dig-
4076 its. Otherwise, it matches a literal "x" character. In JavaScript
4077 mode, support for code points greater than 256 is provided by \u, which
4078 must be followed by four hexadecimal digits; otherwise it matches a
4079 literal "u" character.
4081 Characters whose value is less than 256 can be defined by either of the
4082 two syntaxes for \x (or by \u in JavaScript mode). There is no differ-
4083 ence in the way they are handled. For example, \xdc is exactly the same
4084 as \x{dc} (or \u00dc in JavaScript mode).
4086 After \0 up to two further octal digits are read. If there are fewer
4087 than two digits, just those that are present are used. Thus the
4088 sequence \0\x\07 specifies two binary zeros followed by a BEL character
4089 (code value 7). Make sure you supply two digits after the initial zero
4090 if the pattern character that follows is itself an octal digit.
4092 The handling of a backslash followed by a digit other than 0 is compli-
4093 cated. Outside a character class, PCRE reads it and any following dig-
4094 its as a decimal number. If the number is less than 10, or if there
4095 have been at least that many previous capturing left parentheses in the
4096 expression, the entire sequence is taken as a back reference. A
4097 description of how this works is given later, following the discussion
4098 of parenthesized subpatterns.
4100 Inside a character class, or if the decimal number is greater than 9
4101 and there have not been that many capturing subpatterns, PCRE re-reads
4102 up to three octal digits following the backslash, and uses them to gen-
4103 erate a data character. Any subsequent digits stand for themselves. The
4104 value of the character is constrained in the same way as characters
4105 specified in hexadecimal. For example:
4107 \040 is another way of writing a space
4108 \40 is the same, provided there are fewer than 40
4109 previous capturing subpatterns
4110 \7 is always a back reference
4111 \11 might be a back reference, or another way of
4112 writing a tab
4113 \011 is always a tab
4114 \0113 is a tab followed by the character "3"
4115 \113 might be a back reference, otherwise the
4116 character with octal code 113
4117 \377 might be a back reference, otherwise
4118 the value 255 (decimal)
4119 \81 is either a back reference, or a binary zero
4120 followed by the two characters "8" and "1"
4122 Note that octal values of 100 or greater must not be introduced by a
4123 leading zero, because no more than three octal digits are ever read.
4125 All the sequences that define a single character value can be used both
4126 inside and outside character classes. In addition, inside a character
4127 class, \b is interpreted as the backspace character (hex 08).
4129 \N is not allowed in a character class. \B, \R, and \X are not special
4130 inside a character class. Like other unrecognized escape sequences,
4131 they are treated as the literal characters "B", "R", and "X" by
4132 default, but cause an error if the PCRE_EXTRA option is set. Outside a
4133 character class, these sequences have different meanings.
4135 Unsupported escape sequences
4137 In Perl, the sequences \l, \L, \u, and \U are recognized by its string
4138 handler and used to modify the case of following characters. By
4139 default, PCRE does not support these escape sequences. However, if the
4140 PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and
4141 \u can be used to define a character by code point, as described in the
4142 previous section.
4144 Absolute and relative back references
4146 The sequence \g followed by an unsigned or a negative number, option-
4147 ally enclosed in braces, is an absolute or relative back reference. A
4148 named back reference can be coded as \g{name}. Back references are dis-
4149 cussed later, following the discussion of parenthesized subpatterns.
4151 Absolute and relative subroutine calls
4153 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
4154 name or a number enclosed either in angle brackets or single quotes, is
4155 an alternative syntax for referencing a subpattern as a "subroutine".
4156 Details are discussed later. Note that \g{...} (Perl syntax) and
4157 \g<...> (Oniguruma syntax) are not synonymous. The former is a back
4158 reference; the latter is a subroutine call.
4160 Generic character types
4162 Another use of backslash is for specifying generic character types:
4164 \d any decimal digit
4165 \D any character that is not a decimal digit
4166 \h any horizontal whitespace character
4167 \H any character that is not a horizontal whitespace character
4168 \s any whitespace character
4169 \S any character that is not a whitespace character
4170 \v any vertical whitespace character
4171 \V any character that is not a vertical whitespace character
4172 \w any "word" character
4173 \W any "non-word" character
4175 There is also the single sequence \N, which matches a non-newline char-
4176 acter. This is the same as the "." metacharacter when PCRE_DOTALL is
4177 not set. Perl also uses \N to match characters by name; PCRE does not
4178 support this.
4180 Each pair of lower and upper case escape sequences partitions the com-
4181 plete set of characters into two disjoint sets. Any given character
4182 matches one, and only one, of each pair. The sequences can appear both
4183 inside and outside character classes. They each match one character of
4184 the appropriate type. If the current matching point is at the end of
4185 the subject string, all of them fail, because there is no character to
4186 match.
4188 For compatibility with Perl, \s does not match the VT character (code
4189 11). This makes it different from the the POSIX "space" class. The \s
4190 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
4191 "use locale;" is included in a Perl script, \s may match the VT charac-
4192 ter. In PCRE, it never does.
4194 A "word" character is an underscore or any character that is a letter
4195 or digit. By default, the definition of letters and digits is con-
4196 trolled by PCRE's low-valued character tables, and may vary if locale-
4197 specific matching is taking place (see "Locale support" in the pcreapi
4198 page). For example, in a French locale such as "fr_FR" in Unix-like
4199 systems, or "french" in Windows, some character codes greater than 128
4200 are used for accented letters, and these are then matched by \w. The
4201 use of locales with Unicode is discouraged.
4203 By default, in a UTF mode, characters with values greater than 128
4204 never match \d, \s, or \w, and always match \D, \S, and \W. These
4205 sequences retain their original meanings from before UTF support was
4206 available, mainly for efficiency reasons. However, if PCRE is compiled
4207 with Unicode property support, and the PCRE_UCP option is set, the be-
4208 haviour is changed so that Unicode properties are used to determine
4209 character types, as follows:
4211 \d any character that \p{Nd} matches (decimal digit)
4212 \s any character that \p{Z} matches, plus HT, LF, FF, CR
4213 \w any character that \p{L} or \p{N} matches, plus underscore
4215 The upper case escapes match the inverse sets of characters. Note that
4216 \d matches only decimal digits, whereas \w matches any Unicode digit,
4217 as well as any Unicode letter, and underscore. Note also that PCRE_UCP
4218 affects \b, and \B because they are defined in terms of \w and \W.
4219 Matching these sequences is noticeably slower when PCRE_UCP is set.
4221 The sequences \h, \H, \v, and \V are features that were added to Perl
4222 at release 5.10. In contrast to the other sequences, which match only
4223 ASCII characters by default, these always match certain high-valued
4224 codepoints, whether or not PCRE_UCP is set. The horizontal space char-
4225 acters are:
4227 U+0009 Horizontal tab
4228 U+0020 Space
4229 U+00A0 Non-break space
4230 U+1680 Ogham space mark
4231 U+180E Mongolian vowel separator
4232 U+2000 En quad
4233 U+2001 Em quad
4234 U+2002 En space
4235 U+2003 Em space
4236 U+2004 Three-per-em space
4237 U+2005 Four-per-em space
4238 U+2006 Six-per-em space
4239 U+2007 Figure space
4240 U+2008 Punctuation space
4241 U+2009 Thin space
4242 U+200A Hair space
4243 U+202F Narrow no-break space
4244 U+205F Medium mathematical space
4245 U+3000 Ideographic space
4247 The vertical space characters are:
4249 U+000A Linefeed
4250 U+000B Vertical tab
4251 U+000C Formfeed
4252 U+000D Carriage return
4253 U+0085 Next line
4254 U+2028 Line separator
4255 U+2029 Paragraph separator
4257 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
4258 256 are relevant.
4260 Newline sequences
4262 Outside a character class, by default, the escape sequence \R matches
4263 any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent
4264 to the following:
4266 (?>\r\n|\n|\x0b|\f|\r|\x85)
4268 This is an example of an "atomic group", details of which are given
4269 below. This particular group matches either the two-character sequence
4270 CR followed by LF, or one of the single characters LF (linefeed,
4271 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
4272 return, U+000D), or NEL (next line, U+0085). The two-character sequence
4273 is treated as a single unit that cannot be split.
4275 In other modes, two additional characters whose codepoints are greater
4276 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
4277 rator, U+2029). Unicode character property support is not needed for
4278 these characters to be recognized.
4280 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
4281 the complete set of Unicode line endings) by setting the option
4282 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
4283 (BSR is an abbrevation for "backslash R".) This can be made the default
4284 when PCRE is built; if this is the case, the other behaviour can be
4285 requested via the PCRE_BSR_UNICODE option. It is also possible to
4286 specify these settings by starting a pattern string with one of the
4287 following sequences:
4289 (*BSR_ANYCRLF) CR, LF, or CRLF only
4290 (*BSR_UNICODE) any Unicode newline sequence
4292 These override the default and the options given to the compiling func-
4293 tion, but they can themselves be overridden by options given to a
4294 matching function. Note that these special settings, which are not
4295 Perl-compatible, are recognized only at the very start of a pattern,
4296 and that they must be in upper case. If more than one of them is
4297 present, the last one is used. They can be combined with a change of
4298 newline convention; for example, a pattern can start with:
4302 They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
4303 sequences. Inside a character class, \R is treated as an unrecognized
4304 escape sequence, and so matches the letter "R" by default, but causes
4305 an error if PCRE_EXTRA is set.
4307 Unicode character properties
4309 When PCRE is built with Unicode character property support, three addi-
4310 tional escape sequences that match characters with specific properties
4311 are available. When in 8-bit non-UTF-8 mode, these sequences are of
4312 course limited to testing characters whose codepoints are less than
4313 256, but they do work in this mode. The extra escape sequences are:
4315 \p{xx} a character with the xx property
4316 \P{xx} a character without the xx property
4317 \X an extended Unicode sequence
4319 The property names represented by xx above are limited to the Unicode
4320 script names, the general category properties, "Any", which matches any
4321 character (including newline), and some special PCRE properties
4322 (described in the next section). Other Perl properties such as "InMu-
4323 sicalSymbols" are not currently supported by PCRE. Note that \P{Any}
4324 does not match any characters, so always causes a match failure.
4326 Sets of Unicode characters are defined as belonging to certain scripts.
4327 A character from one of these sets can be matched using a script name.
4328 For example:
4330 \p{Greek}
4331 \P{Han}
4333 Those that are not part of an identified script are lumped together as
4334 "Common". The current list of scripts is:
4336 Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
4337 Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common,
4338 Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp-
4339 tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek,
4340 Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe-
4341 rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
4342 Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
4343 Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam,
4344 Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
4345 Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya,
4346 Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian,
4347 Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le,
4348 Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,
4349 Ugaritic, Vai, Yi.
4351 Each character has exactly one Unicode general category property, spec-
4352 ified by a two-letter abbreviation. For compatibility with Perl, nega-
4353 tion can be specified by including a circumflex between the opening
4354 brace and the property name. For example, \p{^Lu} is the same as
4355 \P{Lu}.
4357 If only one letter is specified with \p or \P, it includes all the gen-
4358 eral category properties that start with that letter. In this case, in
4359 the absence of negation, the curly brackets in the escape sequence are
4360 optional; these two examples have the same effect:
4362 \p{L}
4363 \pL
4365 The following general category property codes are supported:
4367 C Other
4368 Cc Control
4369 Cf Format
4370 Cn Unassigned
4371 Co Private use
4372 Cs Surrogate
4374 L Letter
4375 Ll Lower case letter
4376 Lm Modifier letter
4377 Lo Other letter
4378 Lt Title case letter
4379 Lu Upper case letter
4381 M Mark
4382 Mc Spacing mark
4383 Me Enclosing mark
4384 Mn Non-spacing mark
4386 N Number
4387 Nd Decimal number
4388 Nl Letter number
4389 No Other number
4391 P Punctuation
4392 Pc Connector punctuation
4393 Pd Dash punctuation
4394 Pe Close punctuation
4395 Pf Final punctuation
4396 Pi Initial punctuation
4397 Po Other punctuation
4398 Ps Open punctuation
4400 S Symbol
4401 Sc Currency symbol
4402 Sk Modifier symbol
4403 Sm Mathematical symbol
4404 So Other symbol
4406 Z Separator
4407 Zl Line separator
4408 Zp Paragraph separator
4409 Zs Space separator
4411 The special property L& is also supported: it matches a character that
4412 has the Lu, Ll, or Lt property, in other words, a letter that is not
4413 classified as a modifier or "other".
4415 The Cs (Surrogate) property applies only to characters in the range
4416 U+D800 to U+DFFF. Such characters are not valid in Unicode strings and
4417 so cannot be tested by PCRE, unless UTF validity checking has been
4418 turned off (see the discussion of PCRE_NO_UTF8_CHECK and
4419 PCRE_NO_UTF16_CHECK in the pcreapi page). Perl does not support the Cs
4420 property.
4422 The long synonyms for property names that Perl supports (such as
4423 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
4424 any of these properties with "Is".
4426 No character that is in the Unicode table has the Cn (unassigned) prop-
4427 erty. Instead, this property is assumed for any code point that is not
4428 in the Unicode table.
4430 Specifying caseless matching does not affect these escape sequences.
4431 For example, \p{Lu} always matches only upper case letters.
4433 The \X escape matches any number of Unicode characters that form an
4434 extended Unicode sequence. \X is equivalent to
4436 (?>\PM\pM*)
4438 That is, it matches a character without the "mark" property, followed
4439 by zero or more characters with the "mark" property, and treats the
4440 sequence as an atomic group (see below). Characters with the "mark"
4441 property are typically accents that affect the preceding character.
4442 None of them have codepoints less than 256, so in 8-bit non-UTF-8 mode
4443 \X matches any one character.
4445 Note that recent versions of Perl have changed \X to match what Unicode
4446 calls an "extended grapheme cluster", which has a more complicated def-
4447 inition.
4449 Matching characters by Unicode property is not fast, because PCRE has
4450 to search a structure that contains data for over fifteen thousand
4451 characters. That is why the traditional escape sequences such as \d and
4452 \w do not use Unicode properties in PCRE by default, though you can
4453 make them do so by setting the PCRE_UCP option or by starting the pat-
4454 tern with (*UCP).
4456 PCRE's additional properties
4458 As well as the standard Unicode properties described in the previous
4459 section, PCRE supports four more that make it possible to convert tra-
4460 ditional escape sequences such as \w and \s and POSIX character classes
4461 to use Unicode properties. PCRE uses these non-standard, non-Perl prop-
4462 erties internally when PCRE_UCP is set. They are:
4464 Xan Any alphanumeric character
4465 Xps Any POSIX space character
4466 Xsp Any Perl space character
4467 Xwd Any Perl "word" character
4469 Xan matches characters that have either the L (letter) or the N (num-
4470 ber) property. Xps matches the characters tab, linefeed, vertical tab,
4471 formfeed, or carriage return, and any other character that has the Z
4472 (separator) property. Xsp is the same as Xps, except that vertical tab
4473 is excluded. Xwd matches the same characters as Xan, plus underscore.
4475 Resetting the match start
4477 The escape sequence \K causes any previously matched characters not to
4478 be included in the final matched sequence. For example, the pattern:
4480 foo\Kbar
4482 matches "foobar", but reports that it has matched "bar". This feature
4483 is similar to a lookbehind assertion (described below). However, in
4484 this case, the part of the subject before the real match does not have
4485 to be of fixed length, as lookbehind assertions do. The use of \K does
4486 not interfere with the setting of captured substrings. For example,
4487 when the pattern
4489 (foo)\Kbar
4491 matches "foobar", the first substring is still set to "foo".
4493 Perl documents that the use of \K within assertions is "not well
4494 defined". In PCRE, \K is acted upon when it occurs inside positive
4495 assertions, but is ignored in negative assertions.
4497 Simple assertions
4499 The final use of backslash is for certain simple assertions. An asser-
4500 tion specifies a condition that has to be met at a particular point in
4501 a match, without consuming any characters from the subject string. The
4502 use of subpatterns for more complicated assertions is described below.
4503 The backslashed assertions are:
4505 \b matches at a word boundary
4506 \B matches when not at a word boundary
4507 \A matches at the start of the subject
4508 \Z matches at the end of the subject
4509 also matches before a newline at the end of the subject
4510 \z matches only at the end of the subject
4511 \G matches at the first matching position in the subject
4513 Inside a character class, \b has a different meaning; it matches the
4514 backspace character. If any other of these assertions appears in a
4515 character class, by default it matches the corresponding literal char-
4516 acter (for example, \B matches the letter B). However, if the
4517 PCRE_EXTRA option is set, an "invalid escape sequence" error is gener-
4518 ated instead.
4520 A word boundary is a position in the subject string where the current
4521 character and the previous character do not both match \w or \W (i.e.
4522 one matches \w and the other matches \W), or the start or end of the
4523 string if the first or last character matches \w, respectively. In a
4524 UTF mode, the meanings of \w and \W can be changed by setting the
4525 PCRE_UCP option. When this is done, it also affects \b and \B. Neither
4526 PCRE nor Perl has a separate "start of word" or "end of word" metase-
4527 quence. However, whatever follows \b normally determines which it is.
4528 For example, the fragment \ba matches "a" at the start of a word.
4530 The \A, \Z, and \z assertions differ from the traditional circumflex
4531 and dollar (described in the next section) in that they only ever match
4532 at the very start and end of the subject string, whatever options are
4533 set. Thus, they are independent of multiline mode. These three asser-
4534 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
4535 affect only the behaviour of the circumflex and dollar metacharacters.
4536 However, if the startoffset argument of pcre_exec() is non-zero, indi-
4537 cating that matching is to start at a point other than the beginning of
4538 the subject, \A can never match. The difference between \Z and \z is
4539 that \Z matches before a newline at the end of the string as well as at
4540 the very end, whereas \z matches only at the end.
4542 The \G assertion is true only when the current matching position is at
4543 the start point of the match, as specified by the startoffset argument
4544 of pcre_exec(). It differs from \A when the value of startoffset is
4545 non-zero. By calling pcre_exec() multiple times with appropriate argu-
4546 ments, you can mimic Perl's /g option, and it is in this kind of imple-
4547 mentation where \G can be useful.
4549 Note, however, that PCRE's interpretation of \G, as the start of the
4550 current match, is subtly different from Perl's, which defines it as the
4551 end of the previous match. In Perl, these can be different when the
4552 previously matched string was empty. Because PCRE does just one match
4553 at a time, it cannot reproduce this behaviour.
4555 If all the alternatives of a pattern begin with \G, the expression is
4556 anchored to the starting match position, and the "anchored" flag is set
4557 in the compiled regular expression.
4562 Outside a character class, in the default matching mode, the circumflex
4563 character is an assertion that is true only if the current matching
4564 point is at the start of the subject string. If the startoffset argu-
4565 ment of pcre_exec() is non-zero, circumflex can never match if the
4566 PCRE_MULTILINE option is unset. Inside a character class, circumflex
4567 has an entirely different meaning (see below).
4569 Circumflex need not be the first character of the pattern if a number
4570 of alternatives are involved, but it should be the first thing in each
4571 alternative in which it appears if the pattern is ever to match that
4572 branch. If all possible alternatives start with a circumflex, that is,
4573 if the pattern is constrained to match only at the start of the sub-
4574 ject, it is said to be an "anchored" pattern. (There are also other
4575 constructs that can cause a pattern to be anchored.)
4577 A dollar character is an assertion that is true only if the current
4578 matching point is at the end of the subject string, or immediately
4579 before a newline at the end of the string (by default). Dollar need not
4580 be the last character of the pattern if a number of alternatives are
4581 involved, but it should be the last item in any branch in which it
4582 appears. Dollar has no special meaning in a character class.
4584 The meaning of dollar can be changed so that it matches only at the
4585 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
4586 compile time. This does not affect the \Z assertion.
4588 The meanings of the circumflex and dollar characters are changed if the
4589 PCRE_MULTILINE option is set. When this is the case, a circumflex
4590 matches immediately after internal newlines as well as at the start of
4591 the subject string. It does not match after a newline that ends the
4592 string. A dollar matches before any newlines in the string, as well as
4593 at the very end, when PCRE_MULTILINE is set. When newline is specified
4594 as the two-character sequence CRLF, isolated CR and LF characters do
4595 not indicate newlines.
4597 For example, the pattern /^abc$/ matches the subject string "def\nabc"
4598 (where \n represents a newline) in multiline mode, but not otherwise.
4599 Consequently, patterns that are anchored in single line mode because
4600 all branches start with ^ are not anchored in multiline mode, and a
4601 match for circumflex is possible when the startoffset argument of
4602 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
4603 PCRE_MULTILINE is set.
4605 Note that the sequences \A, \Z, and \z can be used to match the start
4606 and end of the subject in both modes, and if all branches of a pattern
4607 start with \A it is always anchored, whether or not PCRE_MULTILINE is
4608 set.
4613 Outside a character class, a dot in the pattern matches any one charac-
4614 ter in the subject string except (by default) a character that signi-
4615 fies the end of a line.
4617 When a line ending is defined as a single character, dot never matches
4618 that character; when the two-character sequence CRLF is used, dot does
4619 not match CR if it is immediately followed by LF, but otherwise it
4620 matches all characters (including isolated CRs and LFs). When any Uni-
4621 code line endings are being recognized, dot does not match CR or LF or
4622 any of the other line ending characters.
4624 The behaviour of dot with regard to newlines can be changed. If the
4625 PCRE_DOTALL option is set, a dot matches any one character, without
4626 exception. If the two-character sequence CRLF is present in the subject
4627 string, it takes two dots to match it.
4629 The handling of dot is entirely independent of the handling of circum-
4630 flex and dollar, the only relationship being that they both involve
4631 newlines. Dot has no special meaning in a character class.
4633 The escape sequence \N behaves like a dot, except that it is not
4634 affected by the PCRE_DOTALL option. In other words, it matches any
4635 character except one that signifies the end of a line. Perl also uses
4636 \N to match characters by name; PCRE does not support this.
4641 Outside a character class, the escape sequence \C matches any one data
4642 unit, whether or not a UTF mode is set. In the 8-bit library, one data
4643 unit is one byte; in the 16-bit library it is a 16-bit unit. Unlike a
4644 dot, \C always matches line-ending characters. The feature is provided
4645 in Perl in order to match individual bytes in UTF-8 mode, but it is
4646 unclear how it can usefully be used. Because \C breaks up characters
4647 into individual data units, matching one unit with \C in a UTF mode
4648 means that the rest of the string may start with a malformed UTF char-
4649 acter. This has undefined results, because PCRE assumes that it is
4650 dealing with valid UTF strings (and by default it checks this at the
4651 start of processing unless the PCRE_NO_UTF8_CHECK option is used).
4653 PCRE does not allow \C to appear in lookbehind assertions (described
4654 below) in a UTF mode, because this would make it impossible to calcu-
4655 late the length of the lookbehind.
4657 In general, the \C escape sequence is best avoided. However, one way of
4658 using it that avoids the problem of malformed UTF characters is to use
4659 a lookahead to check the length of the next character, as in this pat-
4660 tern, which could be used with a UTF-8 string (ignore white space and
4661 line breaks):
4663 (?| (?=[\x00-\x7f])(\C) |
4664 (?=[\x80-\x{7ff}])(\C)(\C) |
4665 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
4666 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
4668 A group that starts with (?| resets the capturing parentheses numbers
4669 in each alternative (see "Duplicate Subpattern Numbers" below). The
4670 assertions at the start of each branch check the next UTF-8 character
4671 for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
4672 character's individual bytes are then captured by the appropriate num-
4673 ber of groups.
4678 An opening square bracket introduces a character class, terminated by a
4679 closing square bracket. A closing square bracket on its own is not spe-
4680 cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
4681 a lone closing square bracket causes a compile-time error. If a closing
4682 square bracket is required as a member of the class, it should be the
4683 first data character in the class (after an initial circumflex, if
4684 present) or escaped with a backslash.
4686 A character class matches a single character in the subject. In a UTF
4687 mode, the character may be more than one data unit long. A matched
4688 character must be in the set of characters defined by the class, unless
4689 the first character in the class definition is a circumflex, in which
4690 case the subject character must not be in the set defined by the class.
4691 If a circumflex is actually required as a member of the class, ensure
4692 it is not the first character, or escape it with a backslash.
4694 For example, the character class [aeiou] matches any lower case vowel,
4695 while [^aeiou] matches any character that is not a lower case vowel.
4696 Note that a circumflex is just a convenient notation for specifying the
4697 characters that are in the class by enumerating those that are not. A
4698 class that starts with a circumflex is not an assertion; it still con-
4699 sumes a character from the subject string, and therefore it fails if
4700 the current pointer is at the end of the string.
4702 In UTF-8 (UTF-16) mode, characters with values greater than 255
4703 (0xffff) can be included in a class as a literal string of data units,
4704 or by using the \x{ escaping mechanism.
4706 When caseless matching is set, any letters in a class represent both
4707 their upper case and lower case versions, so for example, a caseless
4708 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
4709 match "A", whereas a caseful version would. In a UTF mode, PCRE always
4710 understands the concept of case for characters whose values are less
4711 than 128, so caseless matching is always possible. For characters with
4712 higher values, the concept of case is supported if PCRE is compiled
4713 with Unicode property support, but not otherwise. If you want to use
4714 caseless matching in a UTF mode for characters 128 and above, you must
4715 ensure that PCRE is compiled with Unicode property support as well as
4716 with UTF support.
4718 Characters that might indicate line breaks are never treated in any
4719 special way when matching character classes, whatever line-ending
4720 sequence is in use, and whatever setting of the PCRE_DOTALL and
4721 PCRE_MULTILINE options is used. A class such as [^a] always matches one
4722 of these characters.
4724 The minus (hyphen) character can be used to specify a range of charac-
4725 ters in a character class. For example, [d-m] matches any letter
4726 between d and m, inclusive. If a minus character is required in a
4727 class, it must be escaped with a backslash or appear in a position
4728 where it cannot be interpreted as indicating a range, typically as the
4729 first or last character in the class.
4731 It is not possible to have the literal character "]" as the end charac-
4732 ter of a range. A pattern such as [W-]46] is interpreted as a class of
4733 two characters ("W" and "-") followed by a literal string "46]", so it
4734 would match "W46]" or "-46]". However, if the "]" is escaped with a
4735 backslash it is interpreted as the end of range, so [W-\]46] is inter-
4736 preted as a class containing a range followed by two other characters.
4737 The octal or hexadecimal representation of "]" can also be used to end
4738 a range.
4740 Ranges operate in the collating sequence of character values. They can
4741 also be used for characters specified numerically, for example
4742 [\000-\037]. Ranges can include any characters that are valid for the
4743 current mode.
4745 If a range that includes letters is used when caseless matching is set,
4746 it matches the letters in either case. For example, [W-c] is equivalent
4747 to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if
4748 character tables for a French locale are in use, [\xc8-\xcb] matches
4749 accented E characters in both cases. In UTF modes, PCRE supports the
4750 concept of case for characters with values greater than 128 only when
4751 it is compiled with Unicode property support.
4753 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
4754 \w, and \W may appear in a character class, and add the characters that
4755 they match to the class. For example, [\dABCDEF] matches any hexadeci-
4756 mal digit. In UTF modes, the PCRE_UCP option affects the meanings of
4757 \d, \s, \w and their upper case partners, just as it does when they
4758 appear outside a character class, as described in the section entitled
4759 "Generic character types" above. The escape sequence \b has a different
4760 meaning inside a character class; it matches the backspace character.
4761 The sequences \B, \N, \R, and \X are not special inside a character
4762 class. Like any other unrecognized escape sequences, they are treated
4763 as the literal characters "B", "N", "R", and "X" by default, but cause
4764 an error if the PCRE_EXTRA option is set.
4766 A circumflex can conveniently be used with the upper case character
4767 types to specify a more restricted set of characters than the matching
4768 lower case type. For example, the class [^\W_] matches any letter or
4769 digit, but not underscore, whereas [\w] includes underscore. A positive
4770 character class should be read as "something OR something OR ..." and a
4771 negative class as "NOT something AND NOT something AND NOT ...".
4773 The only metacharacters that are recognized in character classes are
4774 backslash, hyphen (only where it can be interpreted as specifying a
4775 range), circumflex (only at the start), opening square bracket (only
4776 when it can be interpreted as introducing a POSIX class name - see the
4777 next section), and the terminating closing square bracket. However,
4778 escaping other non-alphanumeric characters does no harm.
4783 Perl supports the POSIX notation for character classes. This uses names
4784 enclosed by [: and :] within the enclosing square brackets. PCRE also
4785 supports this notation. For example,
4787 [01[:alpha:]%]
4789 matches "0", "1", any alphabetic character, or "%". The supported class
4790 names are:
4792 alnum letters and digits
4793 alpha letters
4794 ascii character codes 0 - 127
4795 blank space or tab only
4796 cntrl control characters
4797 digit decimal digits (same as \d)
4798 graph printing characters, excluding space
4799 lower lower case letters
4800 print printing characters, including space
4801 punct printing characters, excluding letters and digits and space
4802 space white space (not quite the same as \s)
4803 upper upper case letters
4804 word "word" characters (same as \w)
4805 xdigit hexadecimal digits
4807 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
4808 and space (32). Notice that this list includes the VT character (code
4809 11). This makes "space" different to \s, which does not include VT (for
4810 Perl compatibility).
4812 The name "word" is a Perl extension, and "blank" is a GNU extension
4813 from Perl 5.8. Another Perl extension is negation, which is indicated
4814 by a ^ character after the colon. For example,
4816 [12[:^digit:]]
4818 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
4819 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
4820 these are not supported, and an error is given if they are encountered.
4822 By default, in UTF modes, characters with values greater than 128 do
4823 not match any of the POSIX character classes. However, if the PCRE_UCP
4824 option is passed to pcre_compile(), some of the classes are changed so
4825 that Unicode character properties are used. This is achieved by replac-
4826 ing the POSIX classes by other sequences, as follows:
4828 [:alnum:] becomes \p{Xan}
4829 [:alpha:] becomes \p{L}
4830 [:blank:] becomes \h
4831 [:digit:] becomes \p{Nd}
4832 [:lower:] becomes \p{Ll}
4833 [:space:] becomes \p{Xps}
4834 [:upper:] becomes \p{Lu}
4835 [:word:] becomes \p{Xwd}
4837 Negated versions, such as [:^alpha:] use \P instead of \p. The other
4838 POSIX classes are unchanged, and match only characters with code points
4839 less than 128.
4844 Vertical bar characters are used to separate alternative patterns. For
4845 example, the pattern
4847 gilbert|sullivan
4849 matches either "gilbert" or "sullivan". Any number of alternatives may
4850 appear, and an empty alternative is permitted (matching the empty
4851 string). The matching process tries each alternative in turn, from left
4852 to right, and the first one that succeeds is used. If the alternatives
4853 are within a subpattern (defined below), "succeeds" means matching the
4854 rest of the main pattern as well as the alternative in the subpattern.
4859 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
4860 PCRE_EXTENDED options (which are Perl-compatible) can be changed from
4861 within the pattern by a sequence of Perl option letters enclosed
4862 between "(?" and ")". The option letters are
4864 i for PCRE_CASELESS
4866 s for PCRE_DOTALL
4867 x for PCRE_EXTENDED
4869 For example, (?im) sets caseless, multiline matching. It is also possi-
4870 ble to unset these options by preceding the letter with a hyphen, and a
4871 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
4873 is also permitted. If a letter appears both before and after the
4874 hyphen, the option is unset.
4877 can be changed in the same way as the Perl-compatible options by using
4878 the characters J, U and X respectively.
4880 When one of these option changes occurs at top level (that is, not
4881 inside subpattern parentheses), the change applies to the remainder of
4882 the pattern that follows. If the change is placed right at the start of
4883 a pattern, PCRE extracts it into the global options (and it will there-
4884 fore show up in data extracted by the pcre_fullinfo() function).
4886 An option change within a subpattern (see below for a description of
4887 subpatterns) affects only that part of the subpattern that follows it,
4888 so
4890 (a(?i)b)c
4892 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
4893 used). By this means, options can be made to have different settings
4894 in different parts of the pattern. Any changes made in one alternative
4895 do carry on into subsequent branches within the same subpattern. For
4896 example,
4898 (a(?i)b|c)
4900 matches "ab", "aB", "c", and "C", even though when matching "C" the
4901 first branch is abandoned before the option setting. This is because
4902 the effects of option settings happen at compile time. There would be
4903 some very weird behaviour otherwise.
4905 Note: There are other PCRE-specific options that can be set by the
4906 application when the compiling or matching functions are called. In
4907 some cases the pattern can contain special leading sequences such as
4908 (*CRLF) to override what the application has set or what has been
4909 defaulted. Details are given in the section entitled "Newline
4910 sequences" above. There are also the (*UTF8), (*UTF16), and (*UCP)
4911 leading sequences that can be used to set UTF and Unicode property
4912 modes; they are equivalent to setting the PCRE_UTF8, PCRE_UTF16, and
4913 the PCRE_UCP options, respectively.
4918 Subpatterns are delimited by parentheses (round brackets), which can be
4919 nested. Turning part of a pattern into a subpattern does two things:
4921 1. It localizes a set of alternatives. For example, the pattern
4923 cat(aract|erpillar|)
4925 matches "cataract", "caterpillar", or "cat". Without the parentheses,
4926 it would match "cataract", "erpillar" or an empty string.
4928 2. It sets up the subpattern as a capturing subpattern. This means
4929 that, when the whole pattern matches, that portion of the subject
4930 string that matched the subpattern is passed back to the caller via the
4931 ovector argument of the matching function. (This applies only to the
4932 traditional matching functions; the DFA matching functions do not sup-
4933 port capturing.)
4935 Opening parentheses are counted from left to right (starting from 1) to
4936 obtain numbers for the capturing subpatterns. For example, if the
4937 string "the red king" is matched against the pattern
4939 the ((red|white) (king|queen))
4941 the captured substrings are "red king", "red", and "king", and are num-
4942 bered 1, 2, and 3, respectively.
4944 The fact that plain parentheses fulfil two functions is not always
4945 helpful. There are often times when a grouping subpattern is required
4946 without a capturing requirement. If an opening parenthesis is followed
4947 by a question mark and a colon, the subpattern does not do any captur-
4948 ing, and is not counted when computing the number of any subsequent
4949 capturing subpatterns. For example, if the string "the white queen" is
4950 matched against the pattern
4952 the ((?:red|white) (king|queen))
4954 the captured substrings are "white queen" and "queen", and are numbered
4955 1 and 2. The maximum number of capturing subpatterns is 65535.
4957 As a convenient shorthand, if any option settings are required at the
4958 start of a non-capturing subpattern, the option letters may appear
4959 between the "?" and the ":". Thus the two patterns
4961 (?i:saturday|sunday)
4962 (?:(?i)saturday|sunday)
4964 match exactly the same set of strings. Because alternative branches are
4965 tried from left to right, and options are not reset until the end of
4966 the subpattern is reached, an option setting in one branch does affect
4967 subsequent branches, so the above patterns match "SUNDAY" as well as
4968 "Saturday".
4973 Perl 5.10 introduced a feature whereby each alternative in a subpattern
4974 uses the same numbers for its capturing parentheses. Such a subpattern
4975 starts with (?| and is itself a non-capturing subpattern. For example,
4976 consider this pattern:
4978 (?|(Sat)ur|(Sun))day
4980 Because the two alternatives are inside a (?| group, both sets of cap-
4981 turing parentheses are numbered one. Thus, when the pattern matches,
4982 you can look at captured substring number one, whichever alternative
4983 matched. This construct is useful when you want to capture part, but
4984 not all, of one of a number of alternatives. Inside a (?| group, paren-
4985 theses are numbered as usual, but the number is reset at the start of
4986 each branch. The numbers of any capturing parentheses that follow the
4987 subpattern start after the highest number used in any branch. The fol-
4988 lowing example is taken from the Perl documentation. The numbers under-
4989 neath show in which buffer the captured content will be stored.
4991 # before ---------------branch-reset----------- after
4992 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
4993 # 1 2 2 3 2 3 4
4995 A back reference to a numbered subpattern uses the most recent value
4996 that is set for that number by any subpattern. The following pattern
4997 matches "abcabc" or "defdef":
4999 /(?|(abc)|(def))\1/
5001 In contrast, a subroutine call to a numbered subpattern always refers
5002 to the first one in the pattern with the given number. The following
5003 pattern matches "abcabc" or "defabc":
5005 /(?|(abc)|(def))(?1)/
5007 If a condition test for a subpattern's having matched refers to a non-
5008 unique number, the test is true if any of the subpatterns of that num-
5009 ber have matched.
5011 An alternative approach to using this "branch reset" feature is to use
5012 duplicate named subpatterns, as described in the next section.
5017 Identifying capturing parentheses by number is simple, but it can be
5018 very hard to keep track of the numbers in complicated regular expres-
5019 sions. Furthermore, if an expression is modified, the numbers may
5020 change. To help with this difficulty, PCRE supports the naming of sub-
5021 patterns. This feature was not added to Perl until release 5.10. Python
5022 had the feature earlier, and PCRE introduced it at release 4.0, using
5023 the Python syntax. PCRE now supports both the Perl and the Python syn-
5024 tax. Perl allows identically numbered subpatterns to have different
5025 names, but PCRE does not.
5027 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
5028 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
5029 to capturing parentheses from other parts of the pattern, such as back
5030 references, recursion, and conditions, can be made by name as well as
5031 by number.
5033 Names consist of up to 32 alphanumeric characters and underscores.
5034 Named capturing parentheses are still allocated numbers as well as
5035 names, exactly as if the names were not present. The PCRE API provides
5036 function calls for extracting the name-to-number translation table from
5037 a compiled pattern. There is also a convenience function for extracting
5038 a captured substring by name.
5040 By default, a name must be unique within a pattern, but it is possible
5041 to relax this constraint by setting the PCRE_DUPNAMES option at compile
5042 time. (Duplicate names are also always permitted for subpatterns with
5043 the same number, set up as described in the previous section.) Dupli-
5044 cate names can be useful for patterns where only one instance of the
5045 named parentheses can match. Suppose you want to match the name of a
5046 weekday, either as a 3-letter abbreviation or as the full name, and in
5047 both cases you want to extract the abbreviation. This pattern (ignoring
5048 the line breaks) does the job:
5050 (?<DN>Mon|Fri|Sun)(?:day)?|
5051 (?<DN>Tue)(?:sday)?|
5052 (?<DN>Wed)(?:nesday)?|
5053 (?<DN>Thu)(?:rsday)?|
5054 (?<DN>Sat)(?:urday)?
5056 There are five capturing substrings, but only one is ever set after a
5057 match. (An alternative way of solving this problem is to use a "branch
5058 reset" subpattern, as described in the previous section.)
5060 The convenience function for extracting the data by name returns the
5061 substring for the first (and in this example, the only) subpattern of
5062 that name that matched. This saves searching to find which numbered
5063 subpattern it was.
5065 If you make a back reference to a non-unique named subpattern from
5066 elsewhere in the pattern, the one that corresponds to the first occur-
5067 rence of the name is used. In the absence of duplicate numbers (see the
5068 previous section) this is the one with the lowest number. If you use a
5069 named reference in a condition test (see the section about conditions
5070 below), either to check whether a subpattern has matched, or to check
5071 for recursion, all subpatterns with the same name are tested. If the
5072 condition is true for any one of them, the overall condition is true.
5073 This is the same behaviour as testing by number. For further details of
5074 the interfaces for handling named subpatterns, see the pcreapi documen-
5075 tation.
5077 Warning: You cannot use different names to distinguish between two sub-
5078 patterns with the same number because PCRE uses only the numbers when
5079 matching. For this reason, an error is given at compile time if differ-
5080 ent names are given to subpatterns with the same number. However, you
5081 can give the same name to subpatterns with the same number, even when
5082 PCRE_DUPNAMES is not set.
5087 Repetition is specified by quantifiers, which can follow any of the
5088 following items:
5090 a literal data character
5091 the dot metacharacter
5092 the \C escape sequence
5093 the \X escape sequence
5094 the \R escape sequence
5095 an escape such as \d or \pL that matches a single character
5096 a character class
5097 a back reference (see next section)
5098 a parenthesized subpattern (including assertions)
5099 a subroutine call to a subpattern (recursive or otherwise)
5101 The general repetition quantifier specifies a minimum and maximum num-
5102 ber of permitted matches, by giving the two numbers in curly brackets
5103 (braces), separated by a comma. The numbers must be less than 65536,
5104 and the first must be less than or equal to the second. For example:
5106 z{2,4}
5108 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
5109 special character. If the second number is omitted, but the comma is
5110 present, there is no upper limit; if the second number and the comma
5111 are both omitted, the quantifier specifies an exact number of required
5112 matches. Thus
5114 [aeiou]{3,}
5116 matches at least 3 successive vowels, but may match many more, while
5118 \d{8}
5120 matches exactly 8 digits. An opening curly bracket that appears in a
5121 position where a quantifier is not allowed, or one that does not match
5122 the syntax of a quantifier, is taken as a literal character. For exam-
5123 ple, {,6} is not a quantifier, but a literal string of four characters.
5125 In UTF modes, quantifiers apply to characters rather than to individual
5126 data units. Thus, for example, \x{100}{2} matches two characters, each
5127 of which is represented by a two-byte sequence in a UTF-8 string. Simi-
5128 larly, \X{3} matches three Unicode extended sequences, each of which
5129 may be several data units long (and they may be of different lengths).
5131 The quantifier {0} is permitted, causing the expression to behave as if
5132 the previous item and the quantifier were not present. This may be use-
5133 ful for subpatterns that are referenced as subroutines from elsewhere
5134 in the pattern (but see also the section entitled "Defining subpatterns
5135 for use by reference only" below). Items other than subpatterns that
5136 have a {0} quantifier are omitted from the compiled pattern.
5138 For convenience, the three most common quantifiers have single-charac-
5139 ter abbreviations:
5141 * is equivalent to {0,}
5142 + is equivalent to {1,}
5143 ? is equivalent to {0,1}
5145 It is possible to construct infinite loops by following a subpattern
5146 that can match no characters with a quantifier that has no upper limit,
5147 for example:
5149 (a?)*
5151 Earlier versions of Perl and PCRE used to give an error at compile time
5152 for such patterns. However, because there are cases where this can be
5153 useful, such patterns are now accepted, but if any repetition of the
5154 subpattern does in fact match no characters, the loop is forcibly bro-
5155 ken.
5157 By default, the quantifiers are "greedy", that is, they match as much
5158 as possible (up to the maximum number of permitted times), without
5159 causing the rest of the pattern to fail. The classic example of where
5160 this gives problems is in trying to match comments in C programs. These
5161 appear between /* and */ and within the comment, individual * and /
5162 characters may appear. An attempt to match C comments by applying the
5163 pattern
5165 /\*.*\*/
5167 to the string
5169 /* first comment */ not comment /* second comment */
5171 fails, because it matches the entire string owing to the greediness of
5172 the .* item.
5174 However, if a quantifier is followed by a question mark, it ceases to
5175 be greedy, and instead matches the minimum number of times possible, so
5176 the pattern
5178 /\*.*?\*/
5180 does the right thing with the C comments. The meaning of the various
5181 quantifiers is not otherwise changed, just the preferred number of
5182 matches. Do not confuse this use of question mark with its use as a
5183 quantifier in its own right. Because it has two uses, it can sometimes
5184 appear doubled, as in
5186 \d??\d
5188 which matches one digit by preference, but can match two if that is the
5189 only way the rest of the pattern matches.
5191 If the PCRE_UNGREEDY option is set (an option that is not available in
5192 Perl), the quantifiers are not greedy by default, but individual ones
5193 can be made greedy by following them with a question mark. In other
5194 words, it inverts the default behaviour.
5196 When a parenthesized subpattern is quantified with a minimum repeat
5197 count that is greater than 1 or with a limited maximum, more memory is
5198 required for the compiled pattern, in proportion to the size of the
5199 minimum or maximum.
5201 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
5202 alent to Perl's /s) is set, thus allowing the dot to match newlines,
5203 the pattern is implicitly anchored, because whatever follows will be
5204 tried against every character position in the subject string, so there
5205 is no point in retrying the overall match at any position after the
5206 first. PCRE normally treats such a pattern as though it were preceded
5207 by \A.
5209 In cases where it is known that the subject string contains no new-
5210 lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
5211 mization, or alternatively using ^ to indicate anchoring explicitly.
5213 However, there is one situation where the optimization cannot be used.
5214 When .* is inside capturing parentheses that are the subject of a back
5215 reference elsewhere in the pattern, a match at the start may fail where
5216 a later one succeeds. Consider, for example:
5218 (.*)abc\1
5220 If the subject is "xyz123abc123" the match point is the fourth charac-
5221 ter. For this reason, such a pattern is not implicitly anchored.
5223 When a capturing subpattern is repeated, the value captured is the sub-
5224 string that matched the final iteration. For example, after
5226 (tweedle[dume]{3}\s*)+
5228 has matched "tweedledum tweedledee" the value of the captured substring
5229 is "tweedledee". However, if there are nested capturing subpatterns,
5230 the corresponding captured values may have been set in previous itera-
5231 tions. For example, after
5233 /(a|(b))+/
5235 matches "aba" the value of the second captured substring is "b".
5240 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
5241 repetition, failure of what follows normally causes the repeated item
5242 to be re-evaluated to see if a different number of repeats allows the
5243 rest of the pattern to match. Sometimes it is useful to prevent this,
5244 either to change the nature of the match, or to cause it fail earlier
5245 than it otherwise might, when the author of the pattern knows there is
5246 no point in carrying on.
5248 Consider, for example, the pattern \d+foo when applied to the subject
5249 line
5251 123456bar
5253 After matching all 6 digits and then failing to match "foo", the normal
5254 action of the matcher is to try again with only 5 digits matching the
5255 \d+ item, and then with 4, and so on, before ultimately failing.
5256 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
5257 the means for specifying that once a subpattern has matched, it is not
5258 to be re-evaluated in this way.
5260 If we use atomic grouping for the previous example, the matcher gives
5261 up immediately on failing to match "foo" the first time. The notation
5262 is a kind of special parenthesis, starting with (?> as in this example:
5264 (?>\d+)foo
5266 This kind of parenthesis "locks up" the part of the pattern it con-
5267 tains once it has matched, and a failure further into the pattern is
5268 prevented from backtracking into it. Backtracking past it to previous
5269 items, however, works as normal.
5271 An alternative description is that a subpattern of this type matches
5272 the string of characters that an identical standalone pattern would
5273 match, if anchored at the current point in the subject string.
5275 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
5276 such as the above example can be thought of as a maximizing repeat that
5277 must swallow everything it can. So, while both \d+ and \d+? are pre-
5278 pared to adjust the number of digits they match in order to make the
5279 rest of the pattern match, (?>\d+) can only match an entire sequence of
5280 digits.
5282 Atomic groups in general can of course contain arbitrarily complicated
5283 subpatterns, and can be nested. However, when the subpattern for an
5284 atomic group is just a single repeated item, as in the example above, a
5285 simpler notation, called a "possessive quantifier" can be used. This
5286 consists of an additional + character following a quantifier. Using
5287 this notation, the previous example can be rewritten as
5289 \d++foo
5291 Note that a possessive quantifier can be used with an entire group, for
5292 example:
5294 (abc|xyz){2,3}+
5296 Possessive quantifiers are always greedy; the setting of the
5297 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
5298 simpler forms of atomic group. However, there is no difference in the
5299 meaning of a possessive quantifier and the equivalent atomic group,
5300 though there may be a performance difference; possessive quantifiers
5301 should be slightly faster.
5303 The possessive quantifier syntax is an extension to the Perl 5.8 syn-
5304 tax. Jeffrey Friedl originated the idea (and the name) in the first
5305 edition of his book. Mike McCloskey liked it, so implemented it when he
5306 built Sun's Java package, and PCRE copied it from there. It ultimately
5307 found its way into Perl at release 5.10.
5309 PCRE has an optimization that automatically "possessifies" certain sim-
5310 ple pattern constructs. For example, the sequence A+B is treated as
5311 A++B because there is no point in backtracking into a sequence of A's
5312 when B must follow.
5314 When a pattern contains an unlimited repeat inside a subpattern that
5315 can itself be repeated an unlimited number of times, the use of an
5316 atomic group is the only way to avoid some failing matches taking a
5317 very long time indeed. The pattern
5319 (\D+|<\d+>)*[!?]
5321 matches an unlimited number of substrings that either consist of non-
5322 digits, or digits enclosed in <>, followed by either ! or ?. When it
5323 matches, it runs quickly. However, if it is applied to
5325 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
5327 it takes a long time before reporting failure. This is because the
5328 string can be divided between the internal \D+ repeat and the external
5329 * repeat in a large number of ways, and all have to be tried. (The
5330 example uses [!?] rather than a single character at the end, because
5331 both PCRE and Perl have an optimization that allows for fast failure
5332 when a single character is used. They remember the last single charac-
5333 ter that is required for a match, and fail early if it is not present
5334 in the string.) If the pattern is changed so that it uses an atomic
5335 group, like this:
5337 ((?>\D+)|<\d+>)*[!?]
5339 sequences of non-digits cannot be broken, and failure happens quickly.
5344 Outside a character class, a backslash followed by a digit greater than
5345 0 (and possibly further digits) is a back reference to a capturing sub-
5346 pattern earlier (that is, to its left) in the pattern, provided there
5347 have been that many previous capturing left parentheses.
5349 However, if the decimal number following the backslash is less than 10,
5350 it is always taken as a back reference, and causes an error only if
5351 there are not that many capturing left parentheses in the entire pat-
5352 tern. In other words, the parentheses that are referenced need not be
5353 to the left of the reference for numbers less than 10. A "forward back
5354 reference" of this type can make sense when a repetition is involved
5355 and the subpattern to the right has participated in an earlier itera-
5356 tion.
5358 It is not possible to have a numerical "forward back reference" to a
5359 subpattern whose number is 10 or more using this syntax because a
5360 sequence such as \50 is interpreted as a character defined in octal.
5361 See the subsection entitled "Non-printing characters" above for further
5362 details of the handling of digits following a backslash. There is no
5363 such problem when named parentheses are used. A back reference to any
5364 subpattern is possible using named parentheses (see below).
5366 Another way of avoiding the ambiguity inherent in the use of digits
5367 following a backslash is to use the \g escape sequence. This escape
5368 must be followed by an unsigned number or a negative number, optionally
5369 enclosed in braces. These examples are all identical:
5371 (ring), \1
5372 (ring), \g1
5373 (ring), \g{1}
5375 An unsigned number specifies an absolute reference without the ambigu-
5376 ity that is present in the older syntax. It is also useful when literal
5377 digits follow the reference. A negative number is a relative reference.
5378 Consider this example:
5380 (abc(def)ghi)\g{-1}
5382 The sequence \g{-1} is a reference to the most recently started captur-
5383 ing subpattern before \g, that is, is it equivalent to \2 in this exam-
5384 ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
5385 references can be helpful in long patterns, and also in patterns that
5386 are created by joining together fragments that contain references
5387 within themselves.
5389 A back reference matches whatever actually matched the capturing sub-
5390 pattern in the current subject string, rather than anything matching
5391 the subpattern itself (see "Subpatterns as subroutines" below for a way
5392 of doing that). So the pattern
5394 (sens|respons)e and \1ibility
5396 matches "sense and sensibility" and "response and responsibility", but
5397 not "sense and responsibility". If caseful matching is in force at the
5398 time of the back reference, the case of letters is relevant. For exam-
5399 ple,
5401 ((?i)rah)\s+\1
5403 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
5404 original capturing subpattern is matched caselessly.
5406 There are several different ways of writing back references to named
5407 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
5408 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
5409 unified back reference syntax, in which \g can be used for both numeric
5410 and named references, is also supported. We could rewrite the above
5411 example in any of the following ways:
5413 (?<p1>(?i)rah)\s+\k<p1>
5414 (?'p1'(?i)rah)\s+\k{p1}
5415 (?P<p1>(?i)rah)\s+(?P=p1)
5416 (?<p1>(?i)rah)\s+\g{p1}
5418 A subpattern that is referenced by name may appear in the pattern
5419 before or after the reference.
5421 There may be more than one back reference to the same subpattern. If a
5422 subpattern has not actually been used in a particular match, any back
5423 references to it always fail by default. For example, the pattern
5425 (a|(bc))\2
5427 always fails if it starts to match "a" rather than "bc". However, if
5428 the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
5429 ence to an unset value matches an empty string.
5431 Because there may be many capturing parentheses in a pattern, all dig-
5432 its following a backslash are taken as part of a potential back refer-
5433 ence number. If the pattern continues with a digit character, some
5434 delimiter must be used to terminate the back reference. If the
5435 PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{
5436 syntax or an empty comment (see "Comments" below) can be used.
5438 Recursive back references
5440 A back reference that occurs inside the parentheses to which it refers
5441 fails when the subpattern is first used, so, for example, (a\1) never
5442 matches. However, such references can be useful inside repeated sub-
5443 patterns. For example, the pattern
5445 (a|b\1)+
5447 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
5448 ation of the subpattern, the back reference matches the character
5449 string corresponding to the previous iteration. In order for this to
5450 work, the pattern must be such that the first iteration does not need
5451 to match the back reference. This can be done using alternation, as in
5452 the example above, or by a quantifier with a minimum of zero.
5454 Back references of this type cause the group that they reference to be
5455 treated as an atomic group. Once the whole group has been matched, a
5456 subsequent matching failure cannot cause backtracking into the middle
5457 of the group.
5462 An assertion is a test on the characters following or preceding the
5463 current matching point that does not actually consume any characters.
5464 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
5465 described above.
5467 More complicated assertions are coded as subpatterns. There are two
5468 kinds: those that look ahead of the current position in the subject
5469 string, and those that look behind it. An assertion subpattern is
5470 matched in the normal way, except that it does not cause the current
5471 matching position to be changed.
5473 Assertion subpatterns are not capturing subpatterns. If such an asser-
5474 tion contains capturing subpatterns within it, these are counted for
5475 the purposes of numbering the capturing subpatterns in the whole pat-
5476 tern. However, substring capturing is carried out only for positive
5477 assertions, because it does not make sense for negative assertions.
5479 For compatibility with Perl, assertion subpatterns may be repeated;
5480 though it makes no sense to assert the same thing several times, the
5481 side effect of capturing parentheses may occasionally be useful. In
5482 practice, there only three cases:
5484 (1) If the quantifier is {0}, the assertion is never obeyed during
5485 matching. However, it may contain internal capturing parenthesized
5486 groups that are called from elsewhere via the subroutine mechanism.
5488 (2) If quantifier is {0,n} where n is greater than zero, it is treated
5489 as if it were {0,1}. At run time, the rest of the pattern match is
5490 tried with and without the assertion, the order depending on the greed-
5491 iness of the quantifier.
5493 (3) If the minimum repetition is greater than zero, the quantifier is
5494 ignored. The assertion is obeyed just once when encountered during
5495 matching.
5497 Lookahead assertions
5499 Lookahead assertions start with (?= for positive assertions and (?! for
5500 negative assertions. For example,
5502 \w+(?=;)
5504 matches a word followed by a semicolon, but does not include the semi-
5505 colon in the match, and
5507 foo(?!bar)
5509 matches any occurrence of "foo" that is not followed by "bar". Note
5510 that the apparently similar pattern
5512 (?!foo)bar
5514 does not find an occurrence of "bar" that is preceded by something
5515 other than "foo"; it finds any occurrence of "bar" whatsoever, because
5516 the assertion (?!foo) is always true when the next three characters are
5517 "bar". A lookbehind assertion is needed to achieve the other effect.
5519 If you want to force a matching failure at some point in a pattern, the
5520 most convenient way to do it is with (?!) because an empty string
5521 always matches, so an assertion that requires there not to be an empty
5522 string must always fail. The backtracking control verb (*FAIL) or (*F)
5523 is a synonym for (?!).
5525 Lookbehind assertions
5527 Lookbehind assertions start with (?<= for positive assertions and (?<!
5528 for negative assertions. For example,
5530 (?<!foo)bar
5532 does find an occurrence of "bar" that is not preceded by "foo". The
5533 contents of a lookbehind assertion are restricted such that all the
5534 strings it matches must have a fixed length. However, if there are sev-
5535 eral top-level alternatives, they do not all have to have the same
5536 fixed length. Thus
5538 (?<=bullock|donkey)
5540 is permitted, but
5542 (?<!dogs?|cats?)
5544 causes an error at compile time. Branches that match different length
5545 strings are permitted only at the top level of a lookbehind assertion.
5546 This is an extension compared with Perl, which requires all branches to
5547 match the same length of string. An assertion such as
5549 (?<=ab(c|de))
5551 is not permitted, because its single top-level branch can match two
5552 different lengths, but it is acceptable to PCRE if rewritten to use two
5553 top-level branches:
5555 (?<=abc|abde)
5557 In some cases, the escape sequence \K (see above) can be used instead
5558 of a lookbehind assertion to get round the fixed-length restriction.
5560 The implementation of lookbehind assertions is, for each alternative,
5561 to temporarily move the current position back by the fixed length and
5562 then try to match. If there are insufficient characters before the cur-
5563 rent position, the assertion fails.
5565 In a UTF mode, PCRE does not allow the \C escape (which matches a sin-
5566 gle data unit even in a UTF mode) to appear in lookbehind assertions,
5567 because it makes it impossible to calculate the length of the lookbe-
5568 hind. The \X and \R escapes, which can match different numbers of data
5569 units, are also not permitted.
5571 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
5572 lookbehinds, as long as the subpattern matches a fixed-length string.
5573 Recursion, however, is not supported.
5575 Possessive quantifiers can be used in conjunction with lookbehind
5576 assertions to specify efficient matching of fixed-length strings at the
5577 end of subject strings. Consider a simple pattern such as
5579 abcd$
5581 when applied to a long string that does not match. Because matching
5582 proceeds from left to right, PCRE will look for each "a" in the subject
5583 and then see if what follows matches the rest of the pattern. If the
5584 pattern is specified as
5586 ^.*abcd$
5588 the initial .* matches the entire string at first, but when this fails
5589 (because there is no following "a"), it backtracks to match all but the
5590 last character, then all but the last two characters, and so on. Once
5591 again the search for "a" covers the entire string, from right to left,
5592 so we are no better off. However, if the pattern is written as
5594 ^.*+(?<=abcd)
5596 there can be no backtracking for the .*+ item; it can match only the
5597 entire string. The subsequent lookbehind assertion does a single test
5598 on the last four characters. If it fails, the match fails immediately.
5599 For long strings, this approach makes a significant difference to the
5600 processing time.
5602 Using multiple assertions
5604 Several assertions (of any sort) may occur in succession. For example,
5606 (?<=\d{3})(?<!999)foo
5608 matches "foo" preceded by three digits that are not "999". Notice that
5609 each of the assertions is applied independently at the same point in
5610 the subject string. First there is a check that the previous three
5611 characters are all digits, and then there is a check that the same
5612 three characters are not "999". This pattern does not match "foo" pre-
5613 ceded by six characters, the first of which are digits and the last
5614 three of which are not "999". For example, it doesn't match "123abc-
5615 foo". A pattern to do that is
5617 (?<=\d{3}...)(?<!999)foo
5619 This time the first assertion looks at the preceding six characters,
5620 checking that the first three are digits, and then the second assertion
5621 checks that the preceding three characters are not "999".
5623 Assertions can be nested in any combination. For example,
5625 (?<=(?<!foo)bar)baz
5627 matches an occurrence of "baz" that is preceded by "bar" which in turn
5628 is not preceded by "foo", while
5630 (?<=\d{3}(?!999)...)foo
5632 is another pattern that matches "foo" preceded by three digits and any
5633 three characters that are not "999".
5638 It is possible to cause the matching process to obey a subpattern con-
5639 ditionally or to choose between two alternative subpatterns, depending
5640 on the result of an assertion, or whether a specific capturing subpat-
5641 tern has already been matched. The two possible forms of conditional
5642 subpattern are:
5644 (?(condition)yes-pattern)
5645 (?(condition)yes-pattern|no-pattern)
5647 If the condition is satisfied, the yes-pattern is used; otherwise the
5648 no-pattern (if present) is used. If there are more than two alterna-
5649 tives in the subpattern, a compile-time error occurs. Each of the two
5650 alternatives may itself contain nested subpatterns of any form, includ-
5651 ing conditional subpatterns; the restriction to two alternatives
5652 applies only at the level of the condition. This pattern fragment is an
5653 example where the alternatives are complex:
5655 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
5658 There are four kinds of condition: references to subpatterns, refer-
5659 ences to recursion, a pseudo-condition called DEFINE, and assertions.
5661 Checking for a used subpattern by number
5663 If the text between the parentheses consists of a sequence of digits,
5664 the condition is true if a capturing subpattern of that number has pre-
5665 viously matched. If there is more than one capturing subpattern with
5666 the same number (see the earlier section about duplicate subpattern
5667 numbers), the condition is true if any of them have matched. An alter-
5668 native notation is to precede the digits with a plus or minus sign. In
5669 this case, the subpattern number is relative rather than absolute. The
5670 most recently opened parentheses can be referenced by (?(-1), the next
5671 most recent by (?(-2), and so on. Inside loops it can also make sense
5672 to refer to subsequent groups. The next parentheses to be opened can be
5673 referenced as (?(+1), and so on. (The value zero in any of these forms
5674 is not used; it provokes a compile-time error.)
5676 Consider the following pattern, which contains non-significant white
5677 space to make it more readable (assume the PCRE_EXTENDED option) and to
5678 divide it into three parts for ease of discussion:
5680 ( \( )? [^()]+ (?(1) \) )
5682 The first part matches an optional opening parenthesis, and if that
5683 character is present, sets it as the first captured substring. The sec-
5684 ond part matches one or more characters that are not parentheses. The
5685 third part is a conditional subpattern that tests whether or not the
5686 first set of parentheses matched. If they did, that is, if subject
5687 started with an opening parenthesis, the condition is true, and so the
5688 yes-pattern is executed and a closing parenthesis is required. Other-
5689 wise, since no-pattern is not present, the subpattern matches nothing.
5690 In other words, this pattern matches a sequence of non-parentheses,
5691 optionally enclosed in parentheses.
5693 If you were embedding this pattern in a larger one, you could use a
5694 relative reference:
5696 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
5698 This makes the fragment independent of the parentheses in the larger
5699 pattern.
5701 Checking for a used subpattern by name
5703 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
5704 used subpattern by name. For compatibility with earlier versions of
5705 PCRE, which had this facility before Perl, the syntax (?(name)...) is
5706 also recognized. However, there is a possible ambiguity with this syn-
5707 tax, because subpattern names may consist entirely of digits. PCRE
5708 looks first for a named subpattern; if it cannot find one and the name
5709 consists entirely of digits, PCRE looks for a subpattern of that num-
5710 ber, which must be greater than zero. Using subpattern names that con-
5711 sist entirely of digits is not recommended.
5713 Rewriting the above example to use a named subpattern gives this:
5715 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
5717 If the name used in a condition of this kind is a duplicate, the test
5718 is applied to all subpatterns of the same name, and is true if any one
5719 of them has matched.
5721 Checking for pattern recursion
5723 If the condition is the string (R), and there is no subpattern with the
5724 name R, the condition is true if a recursive call to the whole pattern
5725 or any subpattern has been made. If digits or a name preceded by amper-
5726 sand follow the letter R, for example:
5728 (?(R3)...) or (?(R&name)...)
5730 the condition is true if the most recent recursion is into a subpattern
5731 whose number or name is given. This condition does not check the entire
5732 recursion stack. If the name used in a condition of this kind is a
5733 duplicate, the test is applied to all subpatterns of the same name, and
5734 is true if any one of them is the most recent recursion.
5736 At "top level", all these recursion test conditions are false. The
5737 syntax for recursive patterns is described below.
5739 Defining subpatterns for use by reference only
5741 If the condition is the string (DEFINE), and there is no subpattern
5742 with the name DEFINE, the condition is always false. In this case,
5743 there may be only one alternative in the subpattern. It is always
5744 skipped if control reaches this point in the pattern; the idea of
5745 DEFINE is that it can be used to define subroutines that can be refer-
5746 enced from elsewhere. (The use of subroutines is described below.) For
5747 example, a pattern to match an IPv4 address such as ""
5748 could be written like this (ignore whitespace and line breaks):
5750 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
5751 \b (?&byte) (\.(?&byte)){3} \b
5753 The first part of the pattern is a DEFINE group inside which a another
5754 group named "byte" is defined. This matches an individual component of
5755 an IPv4 address (a number less than 256). When matching takes place,
5756 this part of the pattern is skipped because DEFINE acts like a false
5757 condition. The rest of the pattern uses references to the named group
5758 to match the four dot-separated components of an IPv4 address, insist-
5759 ing on a word boundary at each end.
5761 Assertion conditions
5763 If the condition is not in any of the above formats, it must be an
5764 assertion. This may be a positive or negative lookahead or lookbehind
5765 assertion. Consider this pattern, again containing non-significant
5766 white space, and with the two alternatives on the second line:
5768 (?(?=[^a-z]*[a-z])
5769 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
5771 The condition is a positive lookahead assertion that matches an
5772 optional sequence of non-letters followed by a letter. In other words,
5773 it tests for the presence of at least one letter in the subject. If a
5774 letter is found, the subject is matched against the first alternative;
5775 otherwise it is matched against the second. This pattern matches
5776 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
5777 letters and dd are digits.
5782 There are two ways of including comments in patterns that are processed
5783 by PCRE. In both cases, the start of the comment must not be in a char-
5784 acter class, nor in the middle of any other sequence of related charac-
5785 ters such as (?: or a subpattern name or number. The characters that
5786 make up a comment play no part in the pattern matching.
5788 The sequence (?# marks the start of a comment that continues up to the
5789 next closing parenthesis. Nested parentheses are not permitted. If the
5790 PCRE_EXTENDED option is set, an unescaped # character also introduces a
5791 comment, which in this case continues to immediately after the next
5792 newline character or character sequence in the pattern. Which charac-
5793 ters are interpreted as newlines is controlled by the options passed to
5794 a compiling function or by a special sequence at the start of the pat-
5795 tern, as described in the section entitled "Newline conventions" above.
5796 Note that the end of this type of comment is a literal newline sequence
5797 in the pattern; escape sequences that happen to represent a newline do
5798 not count. For example, consider this pattern when PCRE_EXTENDED is
5799 set, and the default newline convention is in force:
5801 abc #comment \n still comment
5803 On encountering the # character, pcre_compile() skips along, looking
5804 for a newline in the pattern. The sequence \n is still literal at this
5805 stage, so it does not terminate the comment. Only an actual character
5806 with the code value 0x0a (the default newline) does so.
5811 Consider the problem of matching a string in parentheses, allowing for
5812 unlimited nested parentheses. Without the use of recursion, the best
5813 that can be done is to use a pattern that matches up to some fixed
5814 depth of nesting. It is not possible to handle an arbitrary nesting
5815 depth.
5817 For some time, Perl has provided a facility that allows regular expres-
5818 sions to recurse (amongst other things). It does this by interpolating
5819 Perl code in the expression at run time, and the code can refer to the
5820 expression itself. A Perl pattern using code interpolation to solve the
5821 parentheses problem can be created like this:
5823 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
5825 The (?p{...}) item interpolates Perl code at run time, and in this case
5826 refers recursively to the pattern in which it appears.
5828 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
5829 it supports special syntax for recursion of the entire pattern, and
5830 also for individual subpattern recursion. After its introduction in
5831 PCRE and Python, this kind of recursion was subsequently introduced
5832 into Perl at release 5.10.
5834 A special item that consists of (? followed by a number greater than
5835 zero and a closing parenthesis is a recursive subroutine call of the
5836 subpattern of the given number, provided that it occurs inside that
5837 subpattern. (If not, it is a non-recursive subroutine call, which is
5838 described in the next section.) The special item (?R) or (?0) is a
5839 recursive call of the entire regular expression.
5841 This PCRE pattern solves the nested parentheses problem (assume the
5842 PCRE_EXTENDED option is set so that white space is ignored):
5844 \( ( [^()]++ | (?R) )* \)
5846 First it matches an opening parenthesis. Then it matches any number of
5847 substrings which can either be a sequence of non-parentheses, or a
5848 recursive match of the pattern itself (that is, a correctly parenthe-
5849 sized substring). Finally there is a closing parenthesis. Note the use
5850 of a possessive quantifier to avoid backtracking into sequences of non-
5851 parentheses.
5853 If this were part of a larger pattern, you would not want to recurse
5854 the entire pattern, so instead you could use this:
5856 ( \( ( [^()]++ | (?1) )* \) )
5858 We have put the pattern into parentheses, and caused the recursion to
5859 refer to them instead of the whole pattern.
5861 In a larger pattern, keeping track of parenthesis numbers can be
5862 tricky. This is made easier by the use of relative references. Instead
5863 of (?1) in the pattern above you can write (?-2) to refer to the second
5864 most recently opened parentheses preceding the recursion. In other
5865 words, a negative number counts capturing parentheses leftwards from
5866 the point at which it is encountered.
5868 It is also possible to refer to subsequently opened parentheses, by
5869 writing references such as (?+2). However, these cannot be recursive
5870 because the reference is not inside the parentheses that are refer-
5871 enced. They are always non-recursive subroutine calls, as described in
5872 the next section.
5874 An alternative approach is to use named parentheses instead. The Perl
5875 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
5876 supported. We could rewrite the above example as follows:
5878 (?<pn> \( ( [^()]++ | (?&pn) )* \) )
5880 If there is more than one subpattern with the same name, the earliest
5881 one is used.
5883 This particular example pattern that we have been looking at contains
5884 nested unlimited repeats, and so the use of a possessive quantifier for
5885 matching strings of non-parentheses is important when applying the pat-
5886 tern to strings that do not match. For example, when this pattern is
5887 applied to
5889 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
5891 it yields "no match" quickly. However, if a possessive quantifier is
5892 not used, the match runs for a very long time indeed because there are
5893 so many different ways the + and * repeats can carve up the subject,
5894 and all have to be tested before failure can be reported.
5896 At the end of a match, the values of capturing parentheses are those
5897 from the outermost level. If you want to obtain intermediate values, a
5898 callout function can be used (see below and the pcrecallout documenta-
5899 tion). If the pattern above is matched against
5901 (ab(cd)ef)
5903 the value for the inner capturing parentheses (numbered 2) is "ef",
5904 which is the last value taken on at the top level. If a capturing sub-
5905 pattern is not matched at the top level, its final captured value is
5906 unset, even if it was (temporarily) set at a deeper level during the
5907 matching process.
5909 If there are more than 15 capturing parentheses in a pattern, PCRE has
5910 to obtain extra memory to store data during a recursion, which it does
5911 by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
5912 can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
5914 Do not confuse the (?R) item with the condition (R), which tests for
5915 recursion. Consider this pattern, which matches text in angle brack-
5916 ets, allowing for arbitrary nesting. Only digits are allowed in nested
5917 brackets (that is, when recursing), whereas any characters are permit-
5918 ted at the outer level.
5920 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
5922 In this pattern, (?(R) is the start of a conditional subpattern, with
5923 two different alternatives for the recursive and non-recursive cases.
5924 The (?R) item is the actual recursive call.
5926 Differences in recursion processing between PCRE and Perl
5928 Recursion processing in PCRE differs from Perl in two important ways.
5929 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
5930 always treated as an atomic group. That is, once it has matched some of
5931 the subject string, it is never re-entered, even if it contains untried
5932 alternatives and there is a subsequent matching failure. This can be
5933 illustrated by the following pattern, which purports to match a palin-
5934 dromic string that contains an odd number of characters (for example,
5935 "a", "aba", "abcba", "abcdcba"):
5937 ^(.|(.)(?1)\2)$
5939 The idea is that it either matches a single character, or two identical
5940 characters surrounding a sub-palindrome. In Perl, this pattern works;
5941 in PCRE it does not if the pattern is longer than three characters.
5942 Consider the subject string "abcba":
5944 At the top level, the first character is matched, but as it is not at
5945 the end of the string, the first alternative fails; the second alterna-
5946 tive is taken and the recursion kicks in. The recursive call to subpat-
5947 tern 1 successfully matches the next character ("b"). (Note that the
5948 beginning and end of line tests are not part of the recursion).
5950 Back at the top level, the next character ("c") is compared with what
5951 subpattern 2 matched, which was "a". This fails. Because the recursion
5952 is treated as an atomic group, there are now no backtracking points,
5953 and so the entire match fails. (Perl is able, at this point, to re-
5954 enter the recursion and try the second alternative.) However, if the
5955 pattern is written with the alternatives in the other order, things are
5956 different:
5958 ^((.)(?1)\2|.)$
5960 This time, the recursing alternative is tried first, and continues to
5961 recurse until it runs out of characters, at which point the recursion
5962 fails. But this time we do have another alternative to try at the
5963 higher level. That is the big difference: in the previous case the
5964 remaining alternative is at a deeper recursion level, which PCRE cannot
5965 use.
5967 To change the pattern so that it matches all palindromic strings, not
5968 just those with an odd number of characters, it is tempting to change
5969 the pattern to this:
5971 ^((.)(?1)\2|.?)$
5973 Again, this works in Perl, but not in PCRE, and for the same reason.
5974 When a deeper recursion has matched a single character, it cannot be
5975 entered again in order to match an empty string. The solution is to
5976 separate the two cases, and write out the odd and even cases as alter-
5977 natives at the higher level:
5979 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
5981 If you want to match typical palindromic phrases, the pattern has to
5982 ignore all non-word characters, which can be done like this:
5984 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
5986 If run with the PCRE_CASELESS option, this pattern matches phrases such
5987 as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
5988 Perl. Note the use of the possessive quantifier *+ to avoid backtrack-
5989 ing into sequences of non-word characters. Without this, PCRE takes a
5990 great deal longer (ten times or more) to match typical phrases, and
5991 Perl takes so long that you think it has gone into a loop.
5993 WARNING: The palindrome-matching patterns above work only if the sub-
5994 ject string does not start with a palindrome that is shorter than the
5995 entire string. For example, although "abcba" is correctly matched, if
5996 the subject is "ababa", PCRE finds the palindrome "aba" at the start,
5997 then fails at top level because the end of the string does not follow.
5998 Once again, it cannot jump back into the recursion to try other alter-
5999 natives, so the entire match fails.
6001 The second way in which PCRE and Perl differ in their recursion pro-
6002 cessing is in the handling of captured values. In Perl, when a subpat-
6003 tern is called recursively or as a subpattern (see the next section),
6004 it has no access to any values that were captured outside the recur-
6005 sion, whereas in PCRE these values can be referenced. Consider this
6006 pattern:
6008 ^(.)(\1|a(?2))
6010 In PCRE, this pattern matches "bab". The first capturing parentheses
6011 match "b", then in the second group, when the back reference \1 fails
6012 to match "b", the second alternative matches "a" and then recurses. In
6013 the recursion, \1 does now match "b" and so the whole match succeeds.