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Contents of /code/tags/pcre-2.07/pcre.3

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Revision 38 - (show annotations)
Sat Feb 24 21:39:11 2007 UTC (14 years, 1 month ago) by nigel
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Tag code/trunk as code/tags/pcre-2.07.
1 .TH PCRE 3
3 pcre - Perl-compatible regular expressions.
5 .B #include <pcre.h>
6 .PP
7 .SM
8 .br
9 .B pcre *pcre_compile(const char *\fIpattern\fR, int \fIoptions\fR,
10 .ti +5n
11 .B const char **\fIerrptr\fR, int *\fIerroffset\fR,
12 .ti +5n
13 .B const unsigned char *\fItableptr\fR);
14 .PP
15 .br
16 .B pcre_extra *pcre_study(const pcre *\fIcode\fR, int \fIoptions\fR,
17 .ti +5n
18 .B const char **\fIerrptr\fR);
19 .PP
20 .br
21 .B int pcre_exec(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR,"
22 .ti +5n
23 .B "const char *\fIsubject\fR," int \fIlength\fR, int \fIstartoffset\fR,
24 .ti +5n
25 .B int \fIoptions\fR, int *\fIovector\fR, int \fIovecsize\fR);
26 .PP
27 .br
28 .B int pcre_copy_substring(const char *\fIsubject\fR, int *\fIovector\fR,
29 .ti +5n
30 .B int \fIstringcount\fR, int \fIstringnumber\fR, char *\fIbuffer\fR,
31 .ti +5n
32 .B int \fIbuffersize\fR);
33 .PP
34 .br
35 .B int pcre_get_substring(const char *\fIsubject\fR, int *\fIovector\fR,
36 .ti +5n
37 .B int \fIstringcount\fR, int \fIstringnumber\fR,
38 .ti +5n
39 .B const char **\fIstringptr\fR);
40 .PP
41 .br
42 .B int pcre_get_substring_list(const char *\fIsubject\fR,
43 .ti +5n
44 .B int *\fIovector\fR, int \fIstringcount\fR, "const char ***\fIlistptr\fR);"
45 .PP
46 .br
47 .B const unsigned char *pcre_maketables(void);
48 .PP
49 .br
50 .B int pcre_info(const pcre *\fIcode\fR, int *\fIoptptr\fR, int
51 .B *\fIfirstcharptr\fR);
52 .PP
53 .br
54 .B char *pcre_version(void);
55 .PP
56 .br
57 .B void *(*pcre_malloc)(size_t);
58 .PP
59 .br
60 .B void (*pcre_free)(void *);
65 The PCRE library is a set of functions that implement regular expression
66 pattern matching using the same syntax and semantics as Perl 5, with just a few
67 differences (see below). The current implementation corresponds to Perl 5.005.
69 PCRE has its own native API, which is described in this document. There is also
70 a set of wrapper functions that correspond to the POSIX API. These are
71 described in the \fBpcreposix\fR documentation.
73 The native API function prototypes are defined in the header file \fBpcre.h\fR,
74 and on Unix systems the library itself is called \fBlibpcre.a\fR, so can be
75 accessed by adding \fB-lpcre\fR to the command for linking an application which
76 calls it.
78 The functions \fBpcre_compile()\fR, \fBpcre_study()\fR, and \fBpcre_exec()\fR
79 are used for compiling and matching regular expressions, while
80 \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
81 \fBpcre_get_substring_list()\fR are convenience functions for extracting
82 captured substrings from a matched subject string. The function
83 \fBpcre_maketables()\fR is used (optionally) to build a set of character tables
84 in the current locale for passing to \fBpcre_compile()\fR.
86 The function \fBpcre_info()\fR is used to find out information about a compiled
87 pattern, while the function \fBpcre_version()\fR returns a pointer to a string
88 containing the version of PCRE and its date of release.
90 The global variables \fBpcre_malloc\fR and \fBpcre_free\fR initially contain
91 the entry points of the standard \fBmalloc()\fR and \fBfree()\fR functions
92 respectively. PCRE calls the memory management functions via these variables,
93 so a calling program can replace them if it wishes to intercept the calls. This
94 should be done before calling any PCRE functions.
98 The PCRE functions can be used in multi-threading applications, with the
99 proviso that the memory management functions pointed to by \fBpcre_malloc\fR
100 and \fBpcre_free\fR are shared by all threads.
102 The compiled form of a regular expression is not altered during matching, so
103 the same compiled pattern can safely be used by several threads at once.
107 The function \fBpcre_compile()\fR is called to compile a pattern into an
108 internal form. The pattern is a C string terminated by a binary zero, and
109 is passed in the argument \fIpattern\fR. A pointer to a single block of memory
110 that is obtained via \fBpcre_malloc\fR is returned. This contains the
111 compiled code and related data. The \fBpcre\fR type is defined for this for
112 convenience, but in fact \fBpcre\fR is just a typedef for \fBvoid\fR, since the
113 contents of the block are not externally defined. It is up to the caller to
114 free the memory when it is no longer required.
115 .PP
116 The size of a compiled pattern is roughly proportional to the length of the
117 pattern string, except that each character class (other than those containing
118 just a single character, negated or not) requires 33 bytes, and repeat
119 quantifiers with a minimum greater than one or a bounded maximum cause the
120 relevant portions of the compiled pattern to be replicated.
121 .PP
122 The \fIoptions\fR argument contains independent bits that affect the
123 compilation. It should be zero if no options are required. Some of the options,
124 in particular, those that are compatible with Perl, can also be set and unset
125 from within the pattern (see the detailed description of regular expressions
126 below). For these options, the contents of the \fIoptions\fR argument specifies
127 their initial settings at the start of compilation and execution. The
128 PCRE_ANCHORED option can be set at the time of matching as well as at compile
129 time.
130 .PP
131 If \fIerrptr\fR is NULL, \fBpcre_compile()\fR returns NULL immediately.
132 Otherwise, if compilation of a pattern fails, \fBpcre_compile()\fR returns
133 NULL, and sets the variable pointed to by \fIerrptr\fR to point to a textual
134 error message. The offset from the start of the pattern to the character where
135 the error was discovered is placed in the variable pointed to by
136 \fIerroffset\fR, which must not be NULL. If it is, an immediate error is given.
137 .PP
138 If the final argument, \fItableptr\fR, is NULL, PCRE uses a default set of
139 character tables which are built when it is compiled, using the default C
140 locale. Otherwise, \fItableptr\fR must be the result of a call to
141 \fBpcre_maketables()\fR. See the section on locale support below.
142 .PP
143 The following option bits are defined in the header file:
147 If this bit is set, the pattern is forced to be "anchored", that is, it is
148 constrained to match only at the start of the string which is being searched
149 (the "subject string"). This effect can also be achieved by appropriate
150 constructs in the pattern itself, which is the only way to do it in Perl.
154 If this bit is set, letters in the pattern match both upper and lower case
155 letters. It is equivalent to Perl's /i option.
159 If this bit is set, a dollar metacharacter in the pattern matches only at the
160 end of the subject string. Without this option, a dollar also matches
161 immediately before the final character if it is a newline (but not before any
162 other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
163 set. There is no equivalent to this option in Perl.
167 If this bit is set, a dot metacharater in the pattern matches all characters,
168 including newlines. Without it, newlines are excluded. This option is
169 equivalent to Perl's /s option. A negative class such as [^a] always matches a
170 newline character, independent of the setting of this option.
174 If this bit is set, whitespace data characters in the pattern are totally
175 ignored except when escaped or inside a character class, and characters between
176 an unescaped # outside a character class and the next newline character,
177 inclusive, are also ignored. This is equivalent to Perl's /x option, and makes
178 it possible to include comments inside complicated patterns. Note, however,
179 that this applies only to data characters. Whitespace characters may never
180 appear within special character sequences in a pattern, for example within the
181 sequence (?( which introduces a conditional subpattern.
185 This option turns on additional functionality of PCRE that is incompatible with
186 Perl. Any backslash in a pattern that is followed by a letter that has no
187 special meaning causes an error, thus reserving these combinations for future
188 expansion. By default, as in Perl, a backslash followed by a letter with no
189 special meaning is treated as a literal. There are at present no other features
190 controlled by this option.
194 By default, PCRE treats the subject string as consisting of a single "line" of
195 characters (even if it actually contains several newlines). The "start of line"
196 metacharacter (^) matches only at the start of the string, while the "end of
197 line" metacharacter ($) matches only at the end of the string, or before a
198 terminating newline (unless PCRE_DOLLAR_ENDONLY is set). This is the same as
199 Perl.
201 When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs
202 match immediately following or immediately before any newline in the subject
203 string, respectively, as well as at the very start and end. This is equivalent
204 to Perl's /m option. If there are no "\\n" characters in a subject string, or
205 no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no
206 effect.
210 This option inverts the "greediness" of the quantifiers so that they are not
211 greedy by default, but become greedy if followed by "?". It is not compatible
212 with Perl. It can also be set by a (?U) option setting within the pattern.
216 When a pattern is going to be used several times, it is worth spending more
217 time analyzing it in order to speed up the time taken for matching. The
218 function \fBpcre_study()\fR takes a pointer to a compiled pattern as its first
219 argument, and returns a pointer to a \fBpcre_extra\fR block (another \fBvoid\fR
220 typedef) containing additional information about the pattern; this can be
221 passed to \fBpcre_exec()\fR. If no additional information is available, NULL
222 is returned.
224 The second argument contains option bits. At present, no options are defined
225 for \fBpcre_study()\fR, and this argument should always be zero.
227 The third argument for \fBpcre_study()\fR is a pointer to an error message. If
228 studying succeeds (even if no data is returned), the variable it points to is
229 set to NULL. Otherwise it points to a textual error message.
231 At present, studying a pattern is useful only for non-anchored patterns that do
232 not have a single fixed starting character. A bitmap of possible starting
233 characters is created.
237 PCRE handles caseless matching, and determines whether characters are letters,
238 digits, or whatever, by reference to a set of tables. The library contains a
239 default set of tables which is created in the default C locale when PCRE is
240 compiled. This is used when the final argument of \fBpcre_compile()\fR is NULL,
241 and is sufficient for many applications.
243 An alternative set of tables can, however, be supplied. Such tables are built
244 by calling the \fBpcre_maketables()\fR function, which has no arguments, in the
245 relevant locale. The result can then be passed to \fBpcre_compile()\fR as often
246 as necessary. For example, to build and use tables that are appropriate for the
247 French locale (where accented characters with codes greater than 128 are
248 treated as letters), the following code could be used:
250 setlocale(LC_CTYPE, "fr");
251 tables = pcre_maketables();
252 re = pcre_compile(..., tables);
254 The tables are built in memory that is obtained via \fBpcre_malloc\fR. The
255 pointer that is passed to \fBpcre_compile\fR is saved with the compiled
256 pattern, and the same tables are used via this pointer by \fBpcre_study()\fR
257 and \fBpcre_exec()\fR. Thus for any single pattern, compilation, studying and
258 matching all happen in the same locale, but different patterns can be compiled
259 in different locales. It is the caller's responsibility to ensure that the
260 memory containing the tables remains available for as long as it is needed.
264 The \fBpcre_info()\fR function returns information about a compiled pattern.
265 Its yield is the number of capturing subpatterns, or one of the following
266 negative numbers:
268 PCRE_ERROR_NULL the argument \fIcode\fR was NULL
269 PCRE_ERROR_BADMAGIC the "magic number" was not found
271 If the \fIoptptr\fR argument is not NULL, a copy of the options with which the
272 pattern was compiled is placed in the integer it points to. These option bits
273 are those specified in the call to \fBpcre_compile()\fR, modified by any
274 top-level option settings within the pattern itself, and with the PCRE_ANCHORED
275 bit set if the form of the pattern implies that it can match only at the start
276 of a subject string.
278 If the pattern is not anchored and the \fIfirstcharptr\fR argument is not NULL,
279 it is used to pass back information about the first character of any matched
280 string. If there is a fixed first character, e.g. from a pattern such as
281 (cat|cow|coyote), then it is returned in the integer pointed to by
282 \fIfirstcharptr\fR. Otherwise, if either
284 (a) the pattern was compiled with the PCRE_MULTILINE option, and every branch
285 starts with "^", or
287 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set
288 (if it were set, the pattern would be anchored),
290 then -1 is returned, indicating that the pattern matches only at the
291 start of a subject string or after any "\\n" within the string. Otherwise -2 is
292 returned.
296 The function \fBpcre_exec()\fR is called to match a subject string against a
297 pre-compiled pattern, which is passed in the \fIcode\fR argument. If the
298 pattern has been studied, the result of the study should be passed in the
299 \fIextra\fR argument. Otherwise this must be NULL.
301 The PCRE_ANCHORED option can be passed in the \fIoptions\fR argument, whose
302 unused bits must be zero. However, if a pattern was compiled with
303 PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it
304 cannot be made unachored at matching time.
306 There are also three further options that can be set only at matching time:
310 The first character of the string is not the beginning of a line, so the
311 circumflex metacharacter should not match before it. Setting this without
312 PCRE_MULTILINE (at compile time) causes circumflex never to match.
316 The end of the string is not the end of a line, so the dollar metacharacter
317 should not match it nor (except in multiline mode) a newline immediately before
318 it. Setting this without PCRE_MULTILINE (at compile time) causes dollar never
319 to match.
323 An empty string is not considered to be a valid match if this option is set. If
324 there are alternatives in the pattern, they are tried. If all the alternatives
325 match the empty string, the entire match fails. For example, if the pattern
327 a?b?
329 is applied to a string not beginning with "a" or "b", it matches the empty
330 string at the start of the subject. With PCRE_NOTEMPTY set, this match is not
331 valid, so PCRE searches further into the string for occurrences of "a" or "b".
332 Perl has no direct equivalent of this option, but it makes a special case of
333 a pattern match of the empty string within its \fBsplit()\fR function. Using
334 PCRE_NOTEMPTY it is possible to emulate this behaviour.
336 The subject string is passed as a pointer in \fIsubject\fR, a length in
337 \fIlength\fR, and a starting offset in \fIstartoffset\fR. Unlike the pattern
338 string, it may contain binary zero characters. When the starting offset is
339 zero, the search for a match starts at the beginning of the subject, and this
340 is by far the most common case.
342 A non-zero starting offset is useful when searching for another match in the
343 same subject by calling \fBpcre_exec()\fR again after a previous success.
344 Setting \fIstartoffset\fR differs from just passing over a shortened string and
345 setting PCRE_NOTBOL in the case of a pattern that begins with any kind of
346 lookbehind. For example, consider the pattern
348 \\Biss\\B
350 which finds occurrences of "iss" in the middle of words. (\\B matches only if
351 the current position in the subject is not a word boundary.) When applied to
352 the string "Mississipi" the first call to \fBpcre_exec()\fR finds the first
353 occurrence. If \fBpcre_exec()\fR is called again with just the remainder of the
354 subject, namely "issipi", it does not match, because \\B is always false at the
355 start of the subject, which is deemed to be a word boundary. However, if
356 \fBpcre_exec()\fR is passed the entire string again, but with \fIstartoffset\fR
357 set to 4, it finds the second occurrence of "iss" because it is able to look
358 behind the starting point to discover that it is preceded by a letter.
360 If a non-zero starting offset is passed when the pattern is anchored, one
361 attempt to match at the given offset is tried. This can only succeed if the
362 pattern does not require the match to be at the start of the subject.
364 In general, a pattern matches a certain portion of the subject, and in
365 addition, further substrings from the subject may be picked out by parts of the
366 pattern. Following the usage in Jeffrey Friedl's book, this is called
367 "capturing" in what follows, and the phrase "capturing subpattern" is used for
368 a fragment of a pattern that picks out a substring. PCRE supports several other
369 kinds of parenthesized subpattern that do not cause substrings to be captured.
371 Captured substrings are returned to the caller via a vector of integer offsets
372 whose address is passed in \fIovector\fR. The number of elements in the vector
373 is passed in \fIovecsize\fR. The first two-thirds of the vector is used to pass
374 back captured substrings, each substring using a pair of integers. The
375 remaining third of the vector is used as workspace by \fBpcre_exec()\fR while
376 matching capturing subpatterns, and is not available for passing back
377 information. The length passed in \fIovecsize\fR should always be a multiple of
378 three. If it is not, it is rounded down.
380 When a match has been successful, information about captured substrings is
381 returned in pairs of integers, starting at the beginning of \fIovector\fR, and
382 continuing up to two-thirds of its length at the most. The first element of a
383 pair is set to the offset of the first character in a substring, and the second
384 is set to the offset of the first character after the end of a substring. The
385 first pair, \fIovector[0]\fR and \fIovector[1]\fR, identify the portion of the
386 subject string matched by the entire pattern. The next pair is used for the
387 first capturing subpattern, and so on. The value returned by \fBpcre_exec()\fR
388 is the number of pairs that have been set. If there are no capturing
389 subpatterns, the return value from a successful match is 1, indicating that
390 just the first pair of offsets has been set.
392 Some convenience functions are provided for extracting the captured substrings
393 as separate strings. These are described in the following section.
395 It is possible for an capturing subpattern number \fIn+1\fR to match some
396 part of the subject when subpattern \fIn\fR has not been used at all. For
397 example, if the string "abc" is matched against the pattern (a|(z))(bc)
398 subpatterns 1 and 3 are matched, but 2 is not. When this happens, both offset
399 values corresponding to the unused subpattern are set to -1.
401 If a capturing subpattern is matched repeatedly, it is the last portion of the
402 string that it matched that gets returned.
404 If the vector is too small to hold all the captured substrings, it is used as
405 far as possible (up to two-thirds of its length), and the function returns a
406 value of zero. In particular, if the substring offsets are not of interest,
407 \fBpcre_exec()\fR may be called with \fIovector\fR passed as NULL and
408 \fIovecsize\fR as zero. However, if the pattern contains back references and
409 the \fIovector\fR isn't big enough to remember the related substrings, PCRE has
410 to get additional memory for use during matching. Thus it is usually advisable
411 to supply an \fIovector\fR.
413 Note that \fBpcre_info()\fR can be used to find out how many capturing
414 subpatterns there are in a compiled pattern. The smallest size for
415 \fIovector\fR that will allow for \fIn\fR captured substrings in addition to
416 the offsets of the substring matched by the whole pattern is (\fIn\fR+1)*3.
418 If \fBpcre_exec()\fR fails, it returns a negative number. The following are
419 defined in the header file:
423 The subject string did not match the pattern.
427 Either \fIcode\fR or \fIsubject\fR was passed as NULL, or \fIovector\fR was
428 NULL and \fIovecsize\fR was not zero.
432 An unrecognized bit was set in the \fIoptions\fR argument.
436 PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch
437 the case when it is passed a junk pointer. This is the error it gives when the
438 magic number isn't present.
442 While running the pattern match, an unknown item was encountered in the
443 compiled pattern. This error could be caused by a bug in PCRE or by overwriting
444 of the compiled pattern.
448 If a pattern contains back references, but the \fIovector\fR that is passed to
449 \fBpcre_exec()\fR is not big enough to remember the referenced substrings, PCRE
450 gets a block of memory at the start of matching to use for this purpose. If the
451 call via \fBpcre_malloc()\fR fails, this error is given. The memory is freed at
452 the end of matching.
456 Captured substrings can be accessed directly by using the offsets returned by
457 \fBpcre_exec()\fR in \fIovector\fR. For convenience, the functions
458 \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
459 \fBpcre_get_substring_list()\fR are provided for extracting captured substrings
460 as new, separate, zero-terminated strings. A substring that contains a binary
461 zero is correctly extracted and has a further zero added on the end, but the
462 result does not, of course, function as a C string.
464 The first three arguments are the same for all three functions: \fIsubject\fR
465 is the subject string which has just been successfully matched, \fIovector\fR
466 is a pointer to the vector of integer offsets that was passed to
467 \fBpcre_exec()\fR, and \fIstringcount\fR is the number of substrings that
468 were captured by the match, including the substring that matched the entire
469 regular expression. This is the value returned by \fBpcre_exec\fR if it
470 is greater than zero. If \fBpcre_exec()\fR returned zero, indicating that it
471 ran out of space in \fIovector\fR, then the value passed as
472 \fIstringcount\fR should be the size of the vector divided by three.
474 The functions \fBpcre_copy_substring()\fR and \fBpcre_get_substring()\fR
475 extract a single substring, whose number is given as \fIstringnumber\fR. A
476 value of zero extracts the substring that matched the entire pattern, while
477 higher values extract the captured substrings. For \fBpcre_copy_substring()\fR,
478 the string is placed in \fIbuffer\fR, whose length is given by
479 \fIbuffersize\fR, while for \fBpcre_get_substring()\fR a new block of store is
480 obtained via \fBpcre_malloc\fR, and its address is returned via
481 \fIstringptr\fR. The yield of the function is the length of the string, not
482 including the terminating zero, or one of
486 The buffer was too small for \fBpcre_copy_substring()\fR, or the attempt to get
487 memory failed for \fBpcre_get_substring()\fR.
491 There is no substring whose number is \fIstringnumber\fR.
493 The \fBpcre_get_substring_list()\fR function extracts all available substrings
494 and builds a list of pointers to them. All this is done in a single block of
495 memory which is obtained via \fBpcre_malloc\fR. The address of the memory block
496 is returned via \fIlistptr\fR, which is also the start of the list of string
497 pointers. The end of the list is marked by a NULL pointer. The yield of the
498 function is zero if all went well, or
502 if the attempt to get the memory block failed.
504 When any of these functions encounter a substring that is unset, which can
505 happen when capturing subpattern number \fIn+1\fR matches some part of the
506 subject, but subpattern \fIn\fR has not been used at all, they return an empty
507 string. This can be distinguished from a genuine zero-length substring by
508 inspecting the appropriate offset in \fIovector\fR, which is negative for unset
509 substrings.
514 There are some size limitations in PCRE but it is hoped that they will never in
515 practice be relevant.
516 The maximum length of a compiled pattern is 65539 (sic) bytes.
517 All values in repeating quantifiers must be less than 65536.
518 The maximum number of capturing subpatterns is 99.
519 The maximum number of all parenthesized subpatterns, including capturing
520 subpatterns, assertions, and other types of subpattern, is 200.
522 The maximum length of a subject string is the largest positive number that an
523 integer variable can hold. However, PCRE uses recursion to handle subpatterns
524 and indefinite repetition. This means that the available stack space may limit
525 the size of a subject string that can be processed by certain patterns.
529 The differences described here are with respect to Perl 5.005.
531 1. By default, a whitespace character is any character that the C library
532 function \fBisspace()\fR recognizes, though it is possible to compile PCRE with
533 alternative character type tables. Normally \fBisspace()\fR matches space,
534 formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5
535 no longer includes vertical tab in its set of whitespace characters. The \\v
536 escape that was in the Perl documentation for a long time was never in fact
537 recognized. However, the character itself was treated as whitespace at least
538 up to 5.002. In 5.004 and 5.005 it does not match \\s.
540 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
541 them, but they do not mean what you might think. For example, (?!a){3} does
542 not assert that the next three characters are not "a". It just asserts that the
543 next character is not "a" three times.
545 3. Capturing subpatterns that occur inside negative lookahead assertions are
546 counted, but their entries in the offsets vector are never set. Perl sets its
547 numerical variables from any such patterns that are matched before the
548 assertion fails to match something (thereby succeeding), but only if the
549 negative lookahead assertion contains just one branch.
551 4. Though binary zero characters are supported in the subject string, they are
552 not allowed in a pattern string because it is passed as a normal C string,
553 terminated by zero. The escape sequence "\\0" can be used in the pattern to
554 represent a binary zero.
556 5. The following Perl escape sequences are not supported: \\l, \\u, \\L, \\U,
557 \\E, \\Q. In fact these are implemented by Perl's general string-handling and
558 are not part of its pattern matching engine.
560 6. The Perl \\G assertion is not supported as it is not relevant to single
561 pattern matches.
563 7. Fairly obviously, PCRE does not support the (?{code}) construction.
565 8. There are at the time of writing some oddities in Perl 5.005_02 concerned
566 with the settings of captured strings when part of a pattern is repeated. For
567 example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value
568 "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if
569 the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) get set.
571 In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the
572 future Perl changes to a consistent state that is different, PCRE may change to
573 follow.
575 9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern
576 /^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not.
577 However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.
579 10. PCRE provides some extensions to the Perl regular expression facilities:
581 (a) Although lookbehind assertions must match fixed length strings, each
582 alternative branch of a lookbehind assertion can match a different length of
583 string. Perl 5.005 requires them all to have the same length.
585 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-
586 character matches only at the very end of the string.
588 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
589 meaning is faulted.
591 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
592 inverted, that is, by default they are not greedy, but if followed by a
593 question mark they are.
595 (e) PCRE_ANCHORED can be used to force a pattern to be tried only at the start
596 of the subject.
598 (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options for
599 \fBpcre_exec()\fR have no Perl equivalents.
603 The syntax and semantics of the regular expressions supported by PCRE are
604 described below. Regular expressions are also described in the Perl
605 documentation and in a number of other books, some of which have copious
606 examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
607 O'Reilly (ISBN 1-56592-257-3), covers them in great detail. The description
608 here is intended as reference documentation.
610 A regular expression is a pattern that is matched against a subject string from
611 left to right. Most characters stand for themselves in a pattern, and match the
612 corresponding characters in the subject. As a trivial example, the pattern
614 The quick brown fox
616 matches a portion of a subject string that is identical to itself. The power of
617 regular expressions comes from the ability to include alternatives and
618 repetitions in the pattern. These are encoded in the pattern by the use of
619 \fImeta-characters\fR, which do not stand for themselves but instead are
620 interpreted in some special way.
622 There are two different sets of meta-characters: those that are recognized
623 anywhere in the pattern except within square brackets, and those that are
624 recognized in square brackets. Outside square brackets, the meta-characters are
625 as follows:
627 \\ general escape character with several uses
628 ^ assert start of subject (or line, in multiline mode)
629 $ assert end of subject (or line, in multiline mode)
630 . match any character except newline (by default)
631 [ start character class definition
632 | start of alternative branch
633 ( start subpattern
634 ) end subpattern
635 ? extends the meaning of (
636 also 0 or 1 quantifier
637 also quantifier minimizer
638 * 0 or more quantifier
639 + 1 or more quantifier
640 { start min/max quantifier
642 Part of a pattern that is in square brackets is called a "character class". In
643 a character class the only meta-characters are:
645 \\ general escape character
646 ^ negate the class, but only if the first character
647 - indicates character range
648 ] terminates the character class
650 The following sections describe the use of each of the meta-characters.
654 The backslash character has several uses. Firstly, if it is followed by a
655 non-alphameric character, it takes away any special meaning that character may
656 have. This use of backslash as an escape character applies both inside and
657 outside character classes.
659 For example, if you want to match a "*" character, you write "\\*" in the
660 pattern. This applies whether or not the following character would otherwise be
661 interpreted as a meta-character, so it is always safe to precede a
662 non-alphameric with "\\" to specify that it stands for itself. In particular,
663 if you want to match a backslash, you write "\\\\".
665 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
666 pattern (other than in a character class) and characters between a "#" outside
667 a character class and the next newline character are ignored. An escaping
668 backslash can be used to include a whitespace or "#" character as part of the
669 pattern.
671 A second use of backslash provides a way of encoding non-printing characters
672 in patterns in a visible manner. There is no restriction on the appearance of
673 non-printing characters, apart from the binary zero that terminates a pattern,
674 but when a pattern is being prepared by text editing, it is usually easier to
675 use one of the following escape sequences than the binary character it
676 represents:
678 \\a alarm, that is, the BEL character (hex 07)
679 \\cx "control-x", where x is any character
680 \\e escape (hex 1B)
681 \\f formfeed (hex 0C)
682 \\n newline (hex 0A)
683 \\r carriage return (hex 0D)
684 \\t tab (hex 09)
685 \\xhh character with hex code hh
686 \\ddd character with octal code ddd, or backreference
688 The precise effect of "\\cx" is as follows: if "x" is a lower case letter, it
689 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
690 Thus "\\cz" becomes hex 1A, but "\\c{" becomes hex 3B, while "\\c;" becomes hex
691 7B.
693 After "\\x", up to two hexadecimal digits are read (letters can be in upper or
694 lower case).
696 After "\\0" up to two further octal digits are read. In both cases, if there
697 are fewer than two digits, just those that are present are used. Thus the
698 sequence "\\0\\x\\07" specifies two binary zeros followed by a BEL character.
699 Make sure you supply two digits after the initial zero if the character that
700 follows is itself an octal digit.
702 The handling of a backslash followed by a digit other than 0 is complicated.
703 Outside a character class, PCRE reads it and any following digits as a decimal
704 number. If the number is less than 10, or if there have been at least that many
705 previous capturing left parentheses in the expression, the entire sequence is
706 taken as a \fIback reference\fR. A description of how this works is given
707 later, following the discussion of parenthesized subpatterns.
709 Inside a character class, or if the decimal number is greater than 9 and there
710 have not been that many capturing subpatterns, PCRE re-reads up to three octal
711 digits following the backslash, and generates a single byte from the least
712 significant 8 bits of the value. Any subsequent digits stand for themselves.
713 For example:
715 \\040 is another way of writing a space
716 \\40 is the same, provided there are fewer than 40
717 previous capturing subpatterns
718 \\7 is always a back reference
719 \\11 might be a back reference, or another way of
720 writing a tab
721 \\011 is always a tab
722 \\0113 is a tab followed by the character "3"
723 \\113 is the character with octal code 113 (since there
724 can be no more than 99 back references)
725 \\377 is a byte consisting entirely of 1 bits
726 \\81 is either a back reference, or a binary zero
727 followed by the two characters "8" and "1"
729 Note that octal values of 100 or greater must not be introduced by a leading
730 zero, because no more than three octal digits are ever read.
732 All the sequences that define a single byte value can be used both inside and
733 outside character classes. In addition, inside a character class, the sequence
734 "\\b" is interpreted as the backspace character (hex 08). Outside a character
735 class it has a different meaning (see below).
737 The third use of backslash is for specifying generic character types:
739 \\d any decimal digit
740 \\D any character that is not a decimal digit
741 \\s any whitespace character
742 \\S any character that is not a whitespace character
743 \\w any "word" character
744 \\W any "non-word" character
746 Each pair of escape sequences partitions the complete set of characters into
747 two disjoint sets. Any given character matches one, and only one, of each pair.
749 A "word" character is any letter or digit or the underscore character, that is,
750 any character which can be part of a Perl "word". The definition of letters and
751 digits is controlled by PCRE's character tables, and may vary if locale-
752 specific matching is taking place (see "Locale support" above). For example, in
753 the "fr" (French) locale, some character codes greater than 128 are used for
754 accented letters, and these are matched by \\w.
756 These character type sequences can appear both inside and outside character
757 classes. They each match one character of the appropriate type. If the current
758 matching point is at the end of the subject string, all of them fail, since
759 there is no character to match.
761 The fourth use of backslash is for certain simple assertions. An assertion
762 specifies a condition that has to be met at a particular point in a match,
763 without consuming any characters from the subject string. The use of
764 subpatterns for more complicated assertions is described below. The backslashed
765 assertions are
767 \\b word boundary
768 \\B not a word boundary
769 \\A start of subject (independent of multiline mode)
770 \\Z end of subject or newline at end (independent of multiline mode)
771 \\z end of subject (independent of multiline mode)
773 These assertions may not appear in character classes (but note that "\\b" has a
774 different meaning, namely the backspace character, inside a character class).
776 A word boundary is a position in the subject string where the current character
777 and the previous character do not both match \\w or \\W (i.e. one matches
778 \\w and the other matches \\W), or the start or end of the string if the
779 first or last character matches \\w, respectively.
781 The \\A, \\Z, and \\z assertions differ from the traditional circumflex and
782 dollar (described below) in that they only ever match at the very start and end
783 of the subject string, whatever options are set. They are not affected by the
784 PCRE_NOTBOL or PCRE_NOTEOL options. If the \fIstartoffset\fR argument of
785 \fBpcre_exec()\fR is non-zero, \\A can never match. The difference between \\Z
786 and \\z is that \\Z matches before a newline that is the last character of the
787 string as well as at the end of the string, whereas \\z matches only at the
788 end.
792 Outside a character class, in the default matching mode, the circumflex
793 character is an assertion which is true only if the current matching point is
794 at the start of the subject string. If the \fIstartoffset\fR argument of
795 \fBpcre_exec()\fR is non-zero, circumflex can never match. Inside a character
796 class, circumflex has an entirely different meaning (see below).
798 Circumflex need not be the first character of the pattern if a number of
799 alternatives are involved, but it should be the first thing in each alternative
800 in which it appears if the pattern is ever to match that branch. If all
801 possible alternatives start with a circumflex, that is, if the pattern is
802 constrained to match only at the start of the subject, it is said to be an
803 "anchored" pattern. (There are also other constructs that can cause a pattern
804 to be anchored.)
806 A dollar character is an assertion which is true only if the current matching
807 point is at the end of the subject string, or immediately before a newline
808 character that is the last character in the string (by default). Dollar need
809 not be the last character of the pattern if a number of alternatives are
810 involved, but it should be the last item in any branch in which it appears.
811 Dollar has no special meaning in a character class.
813 The meaning of dollar can be changed so that it matches only at the very end of
814 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching
815 time. This does not affect the \\Z assertion.
817 The meanings of the circumflex and dollar characters are changed if the
818 PCRE_MULTILINE option is set. When this is the case, they match immediately
819 after and immediately before an internal "\\n" character, respectively, in
820 addition to matching at the start and end of the subject string. For example,
821 the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,
822 but not otherwise. Consequently, patterns that are anchored in single line mode
823 because all branches start with "^" are not anchored in multiline mode, and a
824 match for circumflex is possible when the \fIstartoffset\fR argument of
825 \fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
826 PCRE_MULTILINE is set.
828 Note that the sequences \\A, \\Z, and \\z can be used to match the start and
829 end of the subject in both modes, and if all branches of a pattern start with
830 \\A is it always anchored, whether PCRE_MULTILINE is set or not.
834 Outside a character class, a dot in the pattern matches any one character in
835 the subject, including a non-printing character, but not (by default) newline.
836 If the PCRE_DOTALL option is set, then dots match newlines as well. The
837 handling of dot is entirely independent of the handling of circumflex and
838 dollar, the only relationship being that they both involve newline characters.
839 Dot has no special meaning in a character class.
843 An opening square bracket introduces a character class, terminated by a closing
844 square bracket. A closing square bracket on its own is not special. If a
845 closing square bracket is required as a member of the class, it should be the
846 first data character in the class (after an initial circumflex, if present) or
847 escaped with a backslash.
849 A character class matches a single character in the subject; the character must
850 be in the set of characters defined by the class, unless the first character in
851 the class is a circumflex, in which case the subject character must not be in
852 the set defined by the class. If a circumflex is actually required as a member
853 of the class, ensure it is not the first character, or escape it with a
854 backslash.
856 For example, the character class [aeiou] matches any lower case vowel, while
857 [^aeiou] matches any character that is not a lower case vowel. Note that a
858 circumflex is just a convenient notation for specifying the characters which
859 are in the class by enumerating those that are not. It is not an assertion: it
860 still consumes a character from the subject string, and fails if the current
861 pointer is at the end of the string.
863 When caseless matching is set, any letters in a class represent both their
864 upper case and lower case versions, so for example, a caseless [aeiou] matches
865 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
866 caseful version would.
868 The newline character is never treated in any special way in character classes,
869 whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
870 such as [^a] will always match a newline.
872 The minus (hyphen) character can be used to specify a range of characters in a
873 character class. For example, [d-m] matches any letter between d and m,
874 inclusive. If a minus character is required in a class, it must be escaped with
875 a backslash or appear in a position where it cannot be interpreted as
876 indicating a range, typically as the first or last character in the class.
878 It is not possible to have the literal character "]" as the end character of a
879 range. A pattern such as [W-]46] is interpreted as a class of two characters
880 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
881 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
882 the end of range, so [W-\\]46] is interpreted as a single class containing a
883 range followed by two separate characters. The octal or hexadecimal
884 representation of "]" can also be used to end a range.
886 Ranges operate in ASCII collating sequence. They can also be used for
887 characters specified numerically, for example [\\000-\\037]. If a range that
888 includes letters is used when caseless matching is set, it matches the letters
889 in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched
890 caselessly, and if character tables for the "fr" locale are in use,
891 [\\xc8-\\xcb] matches accented E characters in both cases.
893 The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a
894 character class, and add the characters that they match to the class. For
895 example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can
896 conveniently be used with the upper case character types to specify a more
897 restricted set of characters than the matching lower case type. For example,
898 the class [^\\W_] matches any letter or digit, but not underscore.
900 All non-alphameric characters other than \\, -, ^ (at the start) and the
901 terminating ] are non-special in character classes, but it does no harm if they
902 are escaped.
906 Vertical bar characters are used to separate alternative patterns. For example,
907 the pattern
909 gilbert|sullivan
911 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
912 and an empty alternative is permitted (matching the empty string).
913 The matching process tries each alternative in turn, from left to right,
914 and the first one that succeeds is used. If the alternatives are within a
915 subpattern (defined below), "succeeds" means matching the rest of the main
916 pattern as well as the alternative in the subpattern.
921 can be changed from within the pattern by a sequence of Perl option letters
922 enclosed between "(?" and ")". The option letters are
926 s for PCRE_DOTALL
929 For example, (?im) sets caseless, multiline matching. It is also possible to
930 unset these options by preceding the letter with a hyphen, and a combined
931 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
932 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
933 permitted. If a letter appears both before and after the hyphen, the option is
934 unset.
936 The scope of these option changes depends on where in the pattern the setting
937 occurs. For settings that are outside any subpattern (defined below), the
938 effect is the same as if the options were set or unset at the start of
939 matching. The following patterns all behave in exactly the same way:
941 (?i)abc
942 a(?i)bc
943 ab(?i)c
944 abc(?i)
946 which in turn is the same as compiling the pattern abc with PCRE_CASELESS set.
947 In other words, such "top level" settings apply to the whole pattern (unless
948 there are other changes inside subpatterns). If there is more than one setting
949 of the same option at top level, the rightmost setting is used.
951 If an option change occurs inside a subpattern, the effect is different. This
952 is a change of behaviour in Perl 5.005. An option change inside a subpattern
953 affects only that part of the subpattern that follows it, so
955 (a(?i)b)c
957 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
958 By this means, options can be made to have different settings in different
959 parts of the pattern. Any changes made in one alternative do carry on
960 into subsequent branches within the same subpattern. For example,
962 (a(?i)b|c)
964 matches "ab", "aB", "c", and "C", even though when matching "C" the first
965 branch is abandoned before the option setting. This is because the effects of
966 option settings happen at compile time. There would be some very weird
967 behaviour otherwise.
969 The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
970 same way as the Perl-compatible options by using the characters U and X
971 respectively. The (?X) flag setting is special in that it must always occur
972 earlier in the pattern than any of the additional features it turns on, even
973 when it is at top level. It is best put at the start.
977 Subpatterns are delimited by parentheses (round brackets), which can be nested.
978 Marking part of a pattern as a subpattern does two things:
980 1. It localizes a set of alternatives. For example, the pattern
982 cat(aract|erpillar|)
984 matches one of the words "cat", "cataract", or "caterpillar". Without the
985 parentheses, it would match "cataract", "erpillar" or the empty string.
987 2. It sets up the subpattern as a capturing subpattern (as defined above).
988 When the whole pattern matches, that portion of the subject string that matched
989 the subpattern is passed back to the caller via the \fIovector\fR argument of
990 \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting
991 from 1) to obtain the numbers of the capturing subpatterns.
993 For example, if the string "the red king" is matched against the pattern
995 the ((red|white) (king|queen))
997 the captured substrings are "red king", "red", and "king", and are numbered 1,
998 2, and 3.
1000 The fact that plain parentheses fulfil two functions is not always helpful.
1001 There are often times when a grouping subpattern is required without a
1002 capturing requirement. If an opening parenthesis is followed by "?:", the
1003 subpattern does not do any capturing, and is not counted when computing the
1004 number of any subsequent capturing subpatterns. For example, if the string "the
1005 white queen" is matched against the pattern
1007 the ((?:red|white) (king|queen))
1009 the captured substrings are "white queen" and "queen", and are numbered 1 and
1010 2. The maximum number of captured substrings is 99, and the maximum number of
1011 all subpatterns, both capturing and non-capturing, is 200.
1013 As a convenient shorthand, if any option settings are required at the start of
1014 a non-capturing subpattern, the option letters may appear between the "?" and
1015 the ":". Thus the two patterns
1017 (?i:saturday|sunday)
1018 (?:(?i)saturday|sunday)
1020 match exactly the same set of strings. Because alternative branches are tried
1021 from left to right, and options are not reset until the end of the subpattern
1022 is reached, an option setting in one branch does affect subsequent branches, so
1023 the above patterns match "SUNDAY" as well as "Saturday".
1027 Repetition is specified by quantifiers, which can follow any of the following
1028 items:
1030 a single character, possibly escaped
1031 the . metacharacter
1032 a character class
1033 a back reference (see next section)
1034 a parenthesized subpattern (unless it is an assertion - see below)
1036 The general repetition quantifier specifies a minimum and maximum number of
1037 permitted matches, by giving the two numbers in curly brackets (braces),
1038 separated by a comma. The numbers must be less than 65536, and the first must
1039 be less than or equal to the second. For example:
1041 z{2,4}
1043 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1044 character. If the second number is omitted, but the comma is present, there is
1045 no upper limit; if the second number and the comma are both omitted, the
1046 quantifier specifies an exact number of required matches. Thus
1048 [aeiou]{3,}
1050 matches at least 3 successive vowels, but may match many more, while
1052 \\d{8}
1054 matches exactly 8 digits. An opening curly bracket that appears in a position
1055 where a quantifier is not allowed, or one that does not match the syntax of a
1056 quantifier, is taken as a literal character. For example, {,6} is not a
1057 quantifier, but a literal string of four characters.
1059 The quantifier {0} is permitted, causing the expression to behave as if the
1060 previous item and the quantifier were not present.
1062 For convenience (and historical compatibility) the three most common
1063 quantifiers have single-character abbreviations:
1065 * is equivalent to {0,}
1066 + is equivalent to {1,}
1067 ? is equivalent to {0,1}
1069 It is possible to construct infinite loops by following a subpattern that can
1070 match no characters with a quantifier that has no upper limit, for example:
1072 (a?)*
1074 Earlier versions of Perl and PCRE used to give an error at compile time for
1075 such patterns. However, because there are cases where this can be useful, such
1076 patterns are now accepted, but if any repetition of the subpattern does in fact
1077 match no characters, the loop is forcibly broken.
1079 By default, the quantifiers are "greedy", that is, they match as much as
1080 possible (up to the maximum number of permitted times), without causing the
1081 rest of the pattern to fail. The classic example of where this gives problems
1082 is in trying to match comments in C programs. These appear between the
1083 sequences /* and */ and within the sequence, individual * and / characters may
1084 appear. An attempt to match C comments by applying the pattern
1086 /\\*.*\\*/
1088 to the string
1090 /* first command */ not comment /* second comment */
1092 fails, because it matches the entire string due to the greediness of the .*
1093 item.
1095 However, if a quantifier is followed by a question mark, then it ceases to be
1096 greedy, and instead matches the minimum number of times possible, so the
1097 pattern
1099 /\\*.*?\\*/
1101 does the right thing with the C comments. The meaning of the various
1102 quantifiers is not otherwise changed, just the preferred number of matches.
1103 Do not confuse this use of question mark with its use as a quantifier in its
1104 own right. Because it has two uses, it can sometimes appear doubled, as in
1106 \\d??\\d
1108 which matches one digit by preference, but can match two if that is the only
1109 way the rest of the pattern matches.
1111 If the PCRE_UNGREEDY option is set (an option which is not available in Perl)
1112 then the quantifiers are not greedy by default, but individual ones can be made
1113 greedy by following them with a question mark. In other words, it inverts the
1114 default behaviour.
1116 When a parenthesized subpattern is quantified with a minimum repeat count that
1117 is greater than 1 or with a limited maximum, more store is required for the
1118 compiled pattern, in proportion to the size of the minimum or maximum.
1120 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1121 to Perl's /s) is set, thus allowing the . to match newlines, then the pattern
1122 is implicitly anchored, because whatever follows will be tried against every
1123 character position in the subject string, so there is no point in retrying the
1124 overall match at any position after the first. PCRE treats such a pattern as
1125 though it were preceded by \\A. In cases where it is known that the subject
1126 string contains no newlines, it is worth setting PCRE_DOTALL when the pattern
1127 begins with .* in order to obtain this optimization, or alternatively using ^
1128 to indicate anchoring explicitly.
1130 When a capturing subpattern is repeated, the value captured is the substring
1131 that matched the final iteration. For example, after
1133 (tweedle[dume]{3}\\s*)+
1135 has matched "tweedledum tweedledee" the value of the captured substring is
1136 "tweedledee". However, if there are nested capturing subpatterns, the
1137 corresponding captured values may have been set in previous iterations. For
1138 example, after
1140 /(a|(b))+/
1142 matches "aba" the value of the second captured substring is "b".
1146 Outside a character class, a backslash followed by a digit greater than 0 (and
1147 possibly further digits) is a back reference to a capturing subpattern earlier
1148 (i.e. to its left) in the pattern, provided there have been that many previous
1149 capturing left parentheses.
1151 However, if the decimal number following the backslash is less than 10, it is
1152 always taken as a back reference, and causes an error only if there are not
1153 that many capturing left parentheses in the entire pattern. In other words, the
1154 parentheses that are referenced need not be to the left of the reference for
1155 numbers less than 10. See the section entitled "Backslash" above for further
1156 details of the handling of digits following a backslash.
1158 A back reference matches whatever actually matched the capturing subpattern in
1159 the current subject string, rather than anything matching the subpattern
1160 itself. So the pattern
1162 (sens|respons)e and \\1ibility
1164 matches "sense and sensibility" and "response and responsibility", but not
1165 "sense and responsibility". If caseful matching is in force at the time of the
1166 back reference, then the case of letters is relevant. For example,
1168 ((?i)rah)\\s+\\1
1170 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1171 capturing subpattern is matched caselessly.
1173 There may be more than one back reference to the same subpattern. If a
1174 subpattern has not actually been used in a particular match, then any back
1175 references to it always fail. For example, the pattern
1177 (a|(bc))\\2
1179 always fails if it starts to match "a" rather than "bc". Because there may be
1180 up to 99 back references, all digits following the backslash are taken
1181 as part of a potential back reference number. If the pattern continues with a
1182 digit character, then some delimiter must be used to terminate the back
1183 reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1184 Otherwise an empty comment can be used.
1186 A back reference that occurs inside the parentheses to which it refers fails
1187 when the subpattern is first used, so, for example, (a\\1) never matches.
1188 However, such references can be useful inside repeated subpatterns. For
1189 example, the pattern
1191 (a|b\\1)+
1193 matches any number of "a"s and also "aba", "ababaa" etc. At each iteration of
1194 the subpattern, the back reference matches the character string corresponding
1195 to the previous iteration. In order for this to work, the pattern must be such
1196 that the first iteration does not need to match the back reference. This can be
1197 done using alternation, as in the example above, or by a quantifier with a
1198 minimum of zero.
1202 An assertion is a test on the characters following or preceding the current
1203 matching point that does not actually consume any characters. The simple
1204 assertions coded as \\b, \\B, \\A, \\Z, \\z, ^ and $ are described above. More
1205 complicated assertions are coded as subpatterns. There are two kinds: those
1206 that look ahead of the current position in the subject string, and those that
1207 look behind it.
1209 An assertion subpattern is matched in the normal way, except that it does not
1210 cause the current matching position to be changed. Lookahead assertions start
1211 with (?= for positive assertions and (?! for negative assertions. For example,
1213 \\w+(?=;)
1215 matches a word followed by a semicolon, but does not include the semicolon in
1216 the match, and
1218 foo(?!bar)
1220 matches any occurrence of "foo" that is not followed by "bar". Note that the
1221 apparently similar pattern
1223 (?!foo)bar
1225 does not find an occurrence of "bar" that is preceded by something other than
1226 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1227 (?!foo) is always true when the next three characters are "bar". A
1228 lookbehind assertion is needed to achieve this effect.
1230 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1231 negative assertions. For example,
1233 (?<!foo)bar
1235 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1236 a lookbehind assertion are restricted such that all the strings it matches must
1237 have a fixed length. However, if there are several alternatives, they do not
1238 all have to have the same fixed length. Thus
1240 (?<=bullock|donkey)
1242 is permitted, but
1244 (?<!dogs?|cats?)
1246 causes an error at compile time. Branches that match different length strings
1247 are permitted only at the top level of a lookbehind assertion. This is an
1248 extension compared with Perl 5.005, which requires all branches to match the
1249 same length of string. An assertion such as
1251 (?<=ab(c|de))
1253 is not permitted, because its single top-level branch can match two different
1254 lengths, but it is acceptable if rewritten to use two top-level branches:
1256 (?<=abc|abde)
1258 The implementation of lookbehind assertions is, for each alternative, to
1259 temporarily move the current position back by the fixed width and then try to
1260 match. If there are insufficient characters before the current position, the
1261 match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns
1262 can be particularly useful for matching at the ends of strings; an example is
1263 given at the end of the section on once-only subpatterns.
1265 Several assertions (of any sort) may occur in succession. For example,
1267 (?<=\\d{3})(?<!999)foo
1269 matches "foo" preceded by three digits that are not "999". Notice that each of
1270 the assertions is applied independently at the same point in the subject
1271 string. First there is a check that the previous three characters are all
1272 digits, then there is a check that the same three characters are not "999".
1273 This pattern does \fInot\fR match "foo" preceded by six characters, the first
1274 of which are digits and the last three of which are not "999". For example, it
1275 doesn't match "123abcfoo". A pattern to do that is
1277 (?<=\\d{3}...)(?<!999)foo
1279 This time the first assertion looks at the preceding six characters, checking
1280 that the first three are digits, and then the second assertion checks that the
1281 preceding three characters are not "999".
1283 Assertions can be nested in any combination. For example,
1285 (?<=(?<!foo)bar)baz
1287 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1288 preceded by "foo", while
1290 (?<=\\d{3}(?!999)...)foo
1292 is another pattern which matches "foo" preceded by three digits and any three
1293 characters that are not "999".
1295 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1296 because it makes no sense to assert the same thing several times. If any kind
1297 of assertion contains capturing subpatterns within it, these are counted for
1298 the purposes of numbering the capturing subpatterns in the whole pattern.
1299 However, substring capturing is carried out only for positive assertions,
1300 because it does not make sense for negative assertions.
1302 Assertions count towards the maximum of 200 parenthesized subpatterns.
1306 With both maximizing and minimizing repetition, failure of what follows
1307 normally causes the repeated item to be re-evaluated to see if a different
1308 number of repeats allows the rest of the pattern to match. Sometimes it is
1309 useful to prevent this, either to change the nature of the match, or to cause
1310 it fail earlier than it otherwise might, when the author of the pattern knows
1311 there is no point in carrying on.
1313 Consider, for example, the pattern \\d+foo when applied to the subject line
1315 123456bar
1317 After matching all 6 digits and then failing to match "foo", the normal
1318 action of the matcher is to try again with only 5 digits matching the \\d+
1319 item, and then with 4, and so on, before ultimately failing. Once-only
1320 subpatterns provide the means for specifying that once a portion of the pattern
1321 has matched, it is not to be re-evaluated in this way, so the matcher would
1322 give up immediately on failing to match "foo" the first time. The notation is
1323 another kind of special parenthesis, starting with (?> as in this example:
1325 (?>\\d+)bar
1327 This kind of parenthesis "locks up" the part of the pattern it contains once
1328 it has matched, and a failure further into the pattern is prevented from
1329 backtracking into it. Backtracking past it to previous items, however, works as
1330 normal.
1332 An alternative description is that a subpattern of this type matches the string
1333 of characters that an identical standalone pattern would match, if anchored at
1334 the current point in the subject string.
1336 Once-only subpatterns are not capturing subpatterns. Simple cases such as the
1337 above example can be thought of as a maximizing repeat that must swallow
1338 everything it can. So, while both \\d+ and \\d+? are prepared to adjust the
1339 number of digits they match in order to make the rest of the pattern match,
1340 (?>\\d+) can only match an entire sequence of digits.
1342 This construction can of course contain arbitrarily complicated subpatterns,
1343 and it can be nested.
1345 Once-only subpatterns can be used in conjunction with lookbehind assertions to
1346 specify efficient matching at the end of the subject string. Consider a simple
1347 pattern such as
1349 abcd$
1351 when applied to a long string which does not match it. Because matching
1352 proceeds from left to right, PCRE will look for each "a" in the subject and
1353 then see if what follows matches the rest of the pattern. If the pattern is
1354 specified as
1356 ^.*abcd$
1358 then the initial .* matches the entire string at first, but when this fails, it
1359 backtracks to match all but the last character, then all but the last two
1360 characters, and so on. Once again the search for "a" covers the entire string,
1361 from right to left, so we are no better off. However, if the pattern is written
1362 as
1364 ^(?>.*)(?<=abcd)
1366 then there can be no backtracking for the .* item; it can match only the entire
1367 string. The subsequent lookbehind assertion does a single test on the last four
1368 characters. If it fails, the match fails immediately. For long strings, this
1369 approach makes a significant difference to the processing time.
1373 It is possible to cause the matching process to obey a subpattern
1374 conditionally or to choose between two alternative subpatterns, depending on
1375 the result of an assertion, or whether a previous capturing subpattern matched
1376 or not. The two possible forms of conditional subpattern are
1378 (?(condition)yes-pattern)
1379 (?(condition)yes-pattern|no-pattern)
1381 If the condition is satisfied, the yes-pattern is used; otherwise the
1382 no-pattern (if present) is used. If there are more than two alternatives in the
1383 subpattern, a compile-time error occurs.
1385 There are two kinds of condition. If the text between the parentheses consists
1386 of a sequence of digits, then the condition is satisfied if the capturing
1387 subpattern of that number has previously matched. Consider the following
1388 pattern, which contains non-significant white space to make it more readable
1389 (assume the PCRE_EXTENDED option) and to divide it into three parts for ease
1390 of discussion:
1392 ( \\( )? [^()]+ (?(1) \\) )
1394 The first part matches an optional opening parenthesis, and if that
1395 character is present, sets it as the first captured substring. The second part
1396 matches one or more characters that are not parentheses. The third part is a
1397 conditional subpattern that tests whether the first set of parentheses matched
1398 or not. If they did, that is, if subject started with an opening parenthesis,
1399 the condition is true, and so the yes-pattern is executed and a closing
1400 parenthesis is required. Otherwise, since no-pattern is not present, the
1401 subpattern matches nothing. In other words, this pattern matches a sequence of
1402 non-parentheses, optionally enclosed in parentheses.
1404 If the condition is not a sequence of digits, it must be an assertion. This may
1405 be a positive or negative lookahead or lookbehind assertion. Consider this
1406 pattern, again containing non-significant white space, and with the two
1407 alternatives on the second line:
1409 (?(?=[^a-z]*[a-z])
1410 \\d{2}[a-z]{3}-\\d{2} | \\d{2}-\\d{2}-\\d{2} )
1412 The condition is a positive lookahead assertion that matches an optional
1413 sequence of non-letters followed by a letter. In other words, it tests for the
1414 presence of at least one letter in the subject. If a letter is found, the
1415 subject is matched against the first alternative; otherwise it is matched
1416 against the second. This pattern matches strings in one of the two forms
1417 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1421 The sequence (?# marks the start of a comment which continues up to the next
1422 closing parenthesis. Nested parentheses are not permitted. The characters
1423 that make up a comment play no part in the pattern matching at all.
1425 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1426 character class introduces a comment that continues up to the next newline
1427 character in the pattern.
1431 Certain items that may appear in patterns are more efficient than others. It is
1432 more efficient to use a character class like [aeiou] than a set of alternatives
1433 such as (a|e|i|o|u). In general, the simplest construction that provides the
1434 required behaviour is usually the most efficient. Jeffrey Friedl's book
1435 contains a lot of discussion about optimizing regular expressions for efficient
1436 performance.
1438 When a pattern begins with .* and the PCRE_DOTALL option is set, the pattern is
1439 implicitly anchored by PCRE, since it can match only at the start of a subject
1440 string. However, if PCRE_DOTALL is not set, PCRE cannot make this optimization,
1441 because the . metacharacter does not then match a newline, and if the subject
1442 string contains newlines, the pattern may match from the character immediately
1443 following one of them instead of from the very start. For example, the pattern
1445 (.*) second
1447 matches the subject "first\\nand second" (where \\n stands for a newline
1448 character) with the first captured substring being "and". In order to do this,
1449 PCRE has to retry the match starting after every newline in the subject.
1451 If you are using such a pattern with subject strings that do not contain
1452 newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
1453 the pattern with ^.* to indicate explicit anchoring. That saves PCRE from
1454 having to scan along the subject looking for a newline to restart at.
1456 Beware of patterns that contain nested indefinite repeats. These can take a
1457 long time to run when applied to a string that does not match. Consider the
1458 pattern fragment
1460 (a+)*
1462 This can match "aaaa" in 33 different ways, and this number increases very
1463 rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
1464 times, and for each of those cases other than 0, the + repeats can match
1465 different numbers of times.) When the remainder of the pattern is such that the
1466 entire match is going to fail, PCRE has in principle to try every possible
1467 variation, and this can take an extremely long time.
1469 An optimization catches some of the more simple cases such as
1471 (a+)*b
1473 where a literal character follows. Before embarking on the standard matching
1474 procedure, PCRE checks that there is a "b" later in the subject string, and if
1475 there is not, it fails the match immediately. However, when there is no
1476 following literal this optimization cannot be used. You can see the difference
1477 by comparing the behaviour of
1479 (a+)*\\d
1481 with the pattern above. The former gives a failure almost instantly when
1482 applied to a whole line of "a" characters, whereas the latter takes an
1483 appreciable time with strings longer than about 20 characters.
1486 Philip Hazel <ph10@cam.ac.uk>
1487 .br
1488 University Computing Service,
1489 .br
1490 New Museums Site,
1491 .br
1492 Cambridge CB2 3QG, England.
1493 .br
1494 Phone: +44 1223 334714
1496 Last updated: 29 July 1999
1497 .br
1498 Copyright (c) 1997-1999 University of Cambridge.

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