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1 .TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31"
3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 The syntax and semantics of the regular expressions that are supported by PCRE
8 are described in detail below. There is a quick-reference syntax summary in the
9 .\" HREF
10 \fBpcresyntax\fP
11 .\"
12 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13 also supports some alternative regular expression syntax (which does not
14 conflict with the Perl syntax) in order to provide some compatibility with
15 regular expressions in Python, .NET, and Oniguruma.
16 .P
17 Perl's regular expressions are described in its own documentation, and
18 regular expressions in general are covered in a number of books, some of which
19 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20 published by O'Reilly, covers regular expressions in great detail. This
21 description of PCRE's regular expressions is intended as reference material.
22 .P
23 The original operation of PCRE was on strings of one-byte characters. However,
24 there is now also support for UTF-8 strings in the original library, and a
25 second library that supports 16-bit and UTF-16 character strings. To use these
26 features, PCRE must be built to include appropriate support. When using UTF
27 strings you must either call the compiling function with the PCRE_UTF8 or
28 PCRE_UTF16 option, or the pattern must start with one of these special
29 sequences:
30 .sp
31 (*UTF8)
32 (*UTF16)
33 .sp
34 Starting a pattern with such a sequence is equivalent to setting the relevant
35 option. This feature is not Perl-compatible. How setting a UTF mode affects
36 pattern matching is mentioned in several places below. There is also a summary
37 of features in the
38 .\" HREF
39 \fBpcreunicode\fP
40 .\"
41 page.
42 .P
43 Another special sequence that may appear at the start of a pattern or in
44 combination with (*UTF8) or (*UTF16) is:
45 .sp
46 (*UCP)
47 .sp
48 This has the same effect as setting the PCRE_UCP option: it causes sequences
49 such as \ed and \ew to use Unicode properties to determine character types,
50 instead of recognizing only characters with codes less than 128 via a lookup
51 table.
52 .P
53 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
54 PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
55 also some more of these special sequences that are concerned with the handling
56 of newlines; they are described below.
57 .P
58 The remainder of this document discusses the patterns that are supported by
59 PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
60 \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching
61 functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using
62 a different algorithm that is not Perl-compatible. Some of the features
63 discussed below are not available when DFA matching is used. The advantages and
64 disadvantages of the alternative functions, and how they differ from the normal
65 functions, are discussed in the
66 .\" HREF
67 \fBpcrematching\fP
68 .\"
69 page.
70 .
71 .
72 .\" HTML <a name="newlines"></a>
74 .rs
75 .sp
76 PCRE supports five different conventions for indicating line breaks in
77 strings: a single CR (carriage return) character, a single LF (linefeed)
78 character, the two-character sequence CRLF, any of the three preceding, or any
79 Unicode newline sequence. The
80 .\" HREF
81 \fBpcreapi\fP
82 .\"
83 page has
84 .\" HTML <a href="pcreapi.html#newlines">
85 .\" </a>
86 further discussion
87 .\"
88 about newlines, and shows how to set the newline convention in the
89 \fIoptions\fP arguments for the compiling and matching functions.
90 .P
91 It is also possible to specify a newline convention by starting a pattern
92 string with one of the following five sequences:
93 .sp
94 (*CR) carriage return
95 (*LF) linefeed
96 (*CRLF) carriage return, followed by linefeed
97 (*ANYCRLF) any of the three above
98 (*ANY) all Unicode newline sequences
99 .sp
100 These override the default and the options given to the compiling function. For
101 example, on a Unix system where LF is the default newline sequence, the pattern
102 .sp
103 (*CR)a.b
104 .sp
105 changes the convention to CR. That pattern matches "a\enb" because LF is no
106 longer a newline. Note that these special settings, which are not
107 Perl-compatible, are recognized only at the very start of a pattern, and that
108 they must be in upper case. If more than one of them is present, the last one
109 is used.
110 .P
111 The newline convention affects the interpretation of the dot metacharacter when
112 PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
113 affect what the \eR escape sequence matches. By default, this is any Unicode
114 newline sequence, for Perl compatibility. However, this can be changed; see the
115 description of \eR in the section entitled
116 .\" HTML <a href="#newlineseq">
117 .\" </a>
118 "Newline sequences"
119 .\"
120 below. A change of \eR setting can be combined with a change of newline
121 convention.
122 .
123 .
125 .rs
126 .sp
127 A regular expression is a pattern that is matched against a subject string from
128 left to right. Most characters stand for themselves in a pattern, and match the
129 corresponding characters in the subject. As a trivial example, the pattern
130 .sp
131 The quick brown fox
132 .sp
133 matches a portion of a subject string that is identical to itself. When
134 caseless matching is specified (the PCRE_CASELESS option), letters are matched
135 independently of case. In a UTF mode, PCRE always understands the concept of
136 case for characters whose values are less than 128, so caseless matching is
137 always possible. For characters with higher values, the concept of case is
138 supported if PCRE is compiled with Unicode property support, but not otherwise.
139 If you want to use caseless matching for characters 128 and above, you must
140 ensure that PCRE is compiled with Unicode property support as well as with
141 UTF support.
142 .P
143 The power of regular expressions comes from the ability to include alternatives
144 and repetitions in the pattern. These are encoded in the pattern by the use of
145 \fImetacharacters\fP, which do not stand for themselves but instead are
146 interpreted in some special way.
147 .P
148 There are two different sets of metacharacters: those that are recognized
149 anywhere in the pattern except within square brackets, and those that are
150 recognized within square brackets. Outside square brackets, the metacharacters
151 are as follows:
152 .sp
153 \e general escape character with several uses
154 ^ assert start of string (or line, in multiline mode)
155 $ assert end of string (or line, in multiline mode)
156 . match any character except newline (by default)
157 [ start character class definition
158 | start of alternative branch
159 ( start subpattern
160 ) end subpattern
161 ? extends the meaning of (
162 also 0 or 1 quantifier
163 also quantifier minimizer
164 * 0 or more quantifier
165 + 1 or more quantifier
166 also "possessive quantifier"
167 { start min/max quantifier
168 .sp
169 Part of a pattern that is in square brackets is called a "character class". In
170 a character class the only metacharacters are:
171 .sp
172 \e general escape character
173 ^ negate the class, but only if the first character
174 - indicates character range
175 .\" JOIN
176 [ POSIX character class (only if followed by POSIX
177 syntax)
178 ] terminates the character class
179 .sp
180 The following sections describe the use of each of the metacharacters.
181 .
182 .
184 .rs
185 .sp
186 The backslash character has several uses. Firstly, if it is followed by a
187 character that is not a number or a letter, it takes away any special meaning
188 that character may have. This use of backslash as an escape character applies
189 both inside and outside character classes.
190 .P
191 For example, if you want to match a * character, you write \e* in the pattern.
192 This escaping action applies whether or not the following character would
193 otherwise be interpreted as a metacharacter, so it is always safe to precede a
194 non-alphanumeric with backslash to specify that it stands for itself. In
195 particular, if you want to match a backslash, you write \e\e.
196 .P
197 In a UTF mode, only ASCII numbers and letters have any special meaning after a
198 backslash. All other characters (in particular, those whose codepoints are
199 greater than 127) are treated as literals.
200 .P
201 If a pattern is compiled with the PCRE_EXTENDED option, white space in the
202 pattern (other than in a character class) and characters between a # outside
203 a character class and the next newline are ignored. An escaping backslash can
204 be used to include a white space or # character as part of the pattern.
205 .P
206 If you want to remove the special meaning from a sequence of characters, you
207 can do so by putting them between \eQ and \eE. This is different from Perl in
208 that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
209 Perl, $ and @ cause variable interpolation. Note the following examples:
210 .sp
211 Pattern PCRE matches Perl matches
212 .sp
213 .\" JOIN
214 \eQabc$xyz\eE abc$xyz abc followed by the
215 contents of $xyz
216 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
217 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
218 .sp
219 The \eQ...\eE sequence is recognized both inside and outside character classes.
220 An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
221 by \eE later in the pattern, the literal interpretation continues to the end of
222 the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
223 a character class, this causes an error, because the character class is not
224 terminated.
225 .
226 .
227 .\" HTML <a name="digitsafterbackslash"></a>
228 .SS "Non-printing characters"
229 .rs
230 .sp
231 A second use of backslash provides a way of encoding non-printing characters
232 in patterns in a visible manner. There is no restriction on the appearance of
233 non-printing characters, apart from the binary zero that terminates a pattern,
234 but when a pattern is being prepared by text editing, it is often easier to use
235 one of the following escape sequences than the binary character it represents:
236 .sp
237 \ea alarm, that is, the BEL character (hex 07)
238 \ecx "control-x", where x is any ASCII character
239 \ee escape (hex 1B)
240 \ef form feed (hex 0C)
241 \en linefeed (hex 0A)
242 \er carriage return (hex 0D)
243 \et tab (hex 09)
244 \eddd character with octal code ddd, or back reference
245 \exhh character with hex code hh
246 \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
247 \euhhhh character with hex code hhhh (JavaScript mode only)
248 .sp
249 The precise effect of \ecx on ASCII characters is as follows: if x is a lower
250 case letter, it is converted to upper case. Then bit 6 of the character (hex
251 40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
252 but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
253 data item (byte or 16-bit value) following \ec has a value greater than 127, a
254 compile-time error occurs. This locks out non-ASCII characters in all modes.
255 .P
256 The \ec facility was designed for use with ASCII characters, but with the
257 extension to Unicode it is even less useful than it once was. It is, however,
258 recognized when PCRE is compiled in EBCDIC mode, where data items are always
259 bytes. In this mode, all values are valid after \ec. If the next character is a
260 lower case letter, it is converted to upper case. Then the 0xc0 bits of the
261 byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
262 the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
263 characters also generate different values.
264 .P
265 By default, after \ex, from zero to two hexadecimal digits are read (letters
266 can be in upper or lower case). Any number of hexadecimal digits may appear
267 between \ex{ and }, but the character code is constrained as follows:
268 .sp
269 8-bit non-UTF mode less than 0x100
270 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
271 16-bit non-UTF mode less than 0x10000
272 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
273 .sp
274 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
275 "surrogate" codepoints).
276 .P
277 If characters other than hexadecimal digits appear between \ex{ and }, or if
278 there is no terminating }, this form of escape is not recognized. Instead, the
279 initial \ex will be interpreted as a basic hexadecimal escape, with no
280 following digits, giving a character whose value is zero.
281 .P
282 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
283 as just described only when it is followed by two hexadecimal digits.
284 Otherwise, it matches a literal "x" character. In JavaScript mode, support for
285 code points greater than 256 is provided by \eu, which must be followed by
286 four hexadecimal digits; otherwise it matches a literal "u" character.
287 Character codes specified by \eu in JavaScript mode are constrained in the same
288 was as those specified by \ex in non-JavaScript mode.
289 .P
290 Characters whose value is less than 256 can be defined by either of the two
291 syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
292 way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
293 \eu00dc in JavaScript mode).
294 .P
295 After \e0 up to two further octal digits are read. If there are fewer than two
296 digits, just those that are present are used. Thus the sequence \e0\ex\e07
297 specifies two binary zeros followed by a BEL character (code value 7). Make
298 sure you supply two digits after the initial zero if the pattern character that
299 follows is itself an octal digit.
300 .P
301 The handling of a backslash followed by a digit other than 0 is complicated.
302 Outside a character class, PCRE reads it and any following digits as a decimal
303 number. If the number is less than 10, or if there have been at least that many
304 previous capturing left parentheses in the expression, the entire sequence is
305 taken as a \fIback reference\fP. A description of how this works is given
306 .\" HTML <a href="#backreferences">
307 .\" </a>
308 later,
309 .\"
310 following the discussion of
311 .\" HTML <a href="#subpattern">
312 .\" </a>
313 parenthesized subpatterns.
314 .\"
315 .P
316 Inside a character class, or if the decimal number is greater than 9 and there
317 have not been that many capturing subpatterns, PCRE re-reads up to three octal
318 digits following the backslash, and uses them to generate a data character. Any
319 subsequent digits stand for themselves. The value of the character is
320 constrained in the same way as characters specified in hexadecimal.
321 For example:
322 .sp
323 \e040 is another way of writing a space
324 .\" JOIN
325 \e40 is the same, provided there are fewer than 40
326 previous capturing subpatterns
327 \e7 is always a back reference
328 .\" JOIN
329 \e11 might be a back reference, or another way of
330 writing a tab
331 \e011 is always a tab
332 \e0113 is a tab followed by the character "3"
333 .\" JOIN
334 \e113 might be a back reference, otherwise the
335 character with octal code 113
336 .\" JOIN
337 \e377 might be a back reference, otherwise
338 the value 255 (decimal)
339 .\" JOIN
340 \e81 is either a back reference, or a binary zero
341 followed by the two characters "8" and "1"
342 .sp
343 Note that octal values of 100 or greater must not be introduced by a leading
344 zero, because no more than three octal digits are ever read.
345 .P
346 All the sequences that define a single character value can be used both inside
347 and outside character classes. In addition, inside a character class, \eb is
348 interpreted as the backspace character (hex 08).
349 .P
350 \eN is not allowed in a character class. \eB, \eR, and \eX are not special
351 inside a character class. Like other unrecognized escape sequences, they are
352 treated as the literal characters "B", "R", and "X" by default, but cause an
353 error if the PCRE_EXTRA option is set. Outside a character class, these
354 sequences have different meanings.
355 .
356 .
357 .SS "Unsupported escape sequences"
358 .rs
359 .sp
360 In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
361 handler and used to modify the case of following characters. By default, PCRE
362 does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
363 option is set, \eU matches a "U" character, and \eu can be used to define a
364 character by code point, as described in the previous section.
365 .
366 .
367 .SS "Absolute and relative back references"
368 .rs
369 .sp
370 The sequence \eg followed by an unsigned or a negative number, optionally
371 enclosed in braces, is an absolute or relative back reference. A named back
372 reference can be coded as \eg{name}. Back references are discussed
373 .\" HTML <a href="#backreferences">
374 .\" </a>
375 later,
376 .\"
377 following the discussion of
378 .\" HTML <a href="#subpattern">
379 .\" </a>
380 parenthesized subpatterns.
381 .\"
382 .
383 .
384 .SS "Absolute and relative subroutine calls"
385 .rs
386 .sp
387 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
388 a number enclosed either in angle brackets or single quotes, is an alternative
389 syntax for referencing a subpattern as a "subroutine". Details are discussed
390 .\" HTML <a href="#onigurumasubroutines">
391 .\" </a>
392 later.
393 .\"
394 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
395 synonymous. The former is a back reference; the latter is a
396 .\" HTML <a href="#subpatternsassubroutines">
397 .\" </a>
398 subroutine
399 .\"
400 call.
401 .
402 .
403 .\" HTML <a name="genericchartypes"></a>
404 .SS "Generic character types"
405 .rs
406 .sp
407 Another use of backslash is for specifying generic character types:
408 .sp
409 \ed any decimal digit
410 \eD any character that is not a decimal digit
411 \eh any horizontal white space character
412 \eH any character that is not a horizontal white space character
413 \es any white space character
414 \eS any character that is not a white space character
415 \ev any vertical white space character
416 \eV any character that is not a vertical white space character
417 \ew any "word" character
418 \eW any "non-word" character
419 .sp
420 There is also the single sequence \eN, which matches a non-newline character.
421 This is the same as
422 .\" HTML <a href="#fullstopdot">
423 .\" </a>
424 the "." metacharacter
425 .\"
426 when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
427 PCRE does not support this.
428 .P
429 Each pair of lower and upper case escape sequences partitions the complete set
430 of characters into two disjoint sets. Any given character matches one, and only
431 one, of each pair. The sequences can appear both inside and outside character
432 classes. They each match one character of the appropriate type. If the current
433 matching point is at the end of the subject string, all of them fail, because
434 there is no character to match.
435 .P
436 For compatibility with Perl, \es does not match the VT character (code 11).
437 This makes it different from the the POSIX "space" class. The \es characters
438 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
439 included in a Perl script, \es may match the VT character. In PCRE, it never
440 does.
441 .P
442 A "word" character is an underscore or any character that is a letter or digit.
443 By default, the definition of letters and digits is controlled by PCRE's
444 low-valued character tables, and may vary if locale-specific matching is taking
445 place (see
446 .\" HTML <a href="pcreapi.html#localesupport">
447 .\" </a>
448 "Locale support"
449 .\"
450 in the
451 .\" HREF
452 \fBpcreapi\fP
453 .\"
454 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
455 or "french" in Windows, some character codes greater than 128 are used for
456 accented letters, and these are then matched by \ew. The use of locales with
457 Unicode is discouraged.
458 .P
459 By default, in a UTF mode, characters with values greater than 128 never match
460 \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
461 their original meanings from before UTF support was available, mainly for
462 efficiency reasons. However, if PCRE is compiled with Unicode property support,
463 and the PCRE_UCP option is set, the behaviour is changed so that Unicode
464 properties are used to determine character types, as follows:
465 .sp
466 \ed any character that \ep{Nd} matches (decimal digit)
467 \es any character that \ep{Z} matches, plus HT, LF, FF, CR
468 \ew any character that \ep{L} or \ep{N} matches, plus underscore
469 .sp
470 The upper case escapes match the inverse sets of characters. Note that \ed
471 matches only decimal digits, whereas \ew matches any Unicode digit, as well as
472 any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
473 \eB because they are defined in terms of \ew and \eW. Matching these sequences
474 is noticeably slower when PCRE_UCP is set.
475 .P
476 The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
477 release 5.10. In contrast to the other sequences, which match only ASCII
478 characters by default, these always match certain high-valued codepoints,
479 whether or not PCRE_UCP is set. The horizontal space characters are:
480 .sp
481 U+0009 Horizontal tab
482 U+0020 Space
483 U+00A0 Non-break space
484 U+1680 Ogham space mark
485 U+180E Mongolian vowel separator
486 U+2000 En quad
487 U+2001 Em quad
488 U+2002 En space
489 U+2003 Em space
490 U+2004 Three-per-em space
491 U+2005 Four-per-em space
492 U+2006 Six-per-em space
493 U+2007 Figure space
494 U+2008 Punctuation space
495 U+2009 Thin space
496 U+200A Hair space
497 U+202F Narrow no-break space
498 U+205F Medium mathematical space
499 U+3000 Ideographic space
500 .sp
501 The vertical space characters are:
502 .sp
503 U+000A Linefeed
504 U+000B Vertical tab
505 U+000C Form feed
506 U+000D Carriage return
507 U+0085 Next line
508 U+2028 Line separator
509 U+2029 Paragraph separator
510 .sp
511 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
512 relevant.
513 .
514 .
515 .\" HTML <a name="newlineseq"></a>
516 .SS "Newline sequences"
517 .rs
518 .sp
519 Outside a character class, by default, the escape sequence \eR matches any
520 Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
521 following:
522 .sp
523 (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
524 .sp
525 This is an example of an "atomic group", details of which are given
526 .\" HTML <a href="#atomicgroup">
527 .\" </a>
528 below.
529 .\"
530 This particular group matches either the two-character sequence CR followed by
531 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
532 U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
533 line, U+0085). The two-character sequence is treated as a single unit that
534 cannot be split.
535 .P
536 In other modes, two additional characters whose codepoints are greater than 255
537 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
538 Unicode character property support is not needed for these characters to be
539 recognized.
540 .P
541 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
542 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
543 either at compile time or when the pattern is matched. (BSR is an abbrevation
544 for "backslash R".) This can be made the default when PCRE is built; if this is
545 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
546 It is also possible to specify these settings by starting a pattern string with
547 one of the following sequences:
548 .sp
549 (*BSR_ANYCRLF) CR, LF, or CRLF only
550 (*BSR_UNICODE) any Unicode newline sequence
551 .sp
552 These override the default and the options given to the compiling function, but
553 they can themselves be overridden by options given to a matching function. Note
554 that these special settings, which are not Perl-compatible, are recognized only
555 at the very start of a pattern, and that they must be in upper case. If more
556 than one of them is present, the last one is used. They can be combined with a
557 change of newline convention; for example, a pattern can start with:
558 .sp
560 .sp
561 They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
562 sequences. Inside a character class, \eR is treated as an unrecognized escape
563 sequence, and so matches the letter "R" by default, but causes an error if
564 PCRE_EXTRA is set.
565 .
566 .
567 .\" HTML <a name="uniextseq"></a>
568 .SS Unicode character properties
569 .rs
570 .sp
571 When PCRE is built with Unicode character property support, three additional
572 escape sequences that match characters with specific properties are available.
573 When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
574 characters whose codepoints are less than 256, but they do work in this mode.
575 The extra escape sequences are:
576 .sp
577 \ep{\fIxx\fP} a character with the \fIxx\fP property
578 \eP{\fIxx\fP} a character without the \fIxx\fP property
579 \eX an extended Unicode sequence
580 .sp
581 The property names represented by \fIxx\fP above are limited to the Unicode
582 script names, the general category properties, "Any", which matches any
583 character (including newline), and some special PCRE properties (described
584 in the
585 .\" HTML <a href="#extraprops">
586 .\" </a>
587 next section).
588 .\"
589 Other Perl properties such as "InMusicalSymbols" are not currently supported by
590 PCRE. Note that \eP{Any} does not match any characters, so always causes a
591 match failure.
592 .P
593 Sets of Unicode characters are defined as belonging to certain scripts. A
594 character from one of these sets can be matched using a script name. For
595 example:
596 .sp
597 \ep{Greek}
598 \eP{Han}
599 .sp
600 Those that are not part of an identified script are lumped together as
601 "Common". The current list of scripts is:
602 .P
603 Arabic,
604 Armenian,
605 Avestan,
606 Balinese,
607 Bamum,
608 Batak,
609 Bengali,
610 Bopomofo,
611 Brahmi,
612 Braille,
613 Buginese,
614 Buhid,
615 Canadian_Aboriginal,
616 Carian,
617 Chakma,
618 Cham,
619 Cherokee,
620 Common,
621 Coptic,
622 Cuneiform,
623 Cypriot,
624 Cyrillic,
625 Deseret,
626 Devanagari,
627 Egyptian_Hieroglyphs,
628 Ethiopic,
629 Georgian,
630 Glagolitic,
631 Gothic,
632 Greek,
633 Gujarati,
634 Gurmukhi,
635 Han,
636 Hangul,
637 Hanunoo,
638 Hebrew,
639 Hiragana,
640 Imperial_Aramaic,
641 Inherited,
642 Inscriptional_Pahlavi,
643 Inscriptional_Parthian,
644 Javanese,
645 Kaithi,
646 Kannada,
647 Katakana,
648 Kayah_Li,
649 Kharoshthi,
650 Khmer,
651 Lao,
652 Latin,
653 Lepcha,
654 Limbu,
655 Linear_B,
656 Lisu,
657 Lycian,
658 Lydian,
659 Malayalam,
660 Mandaic,
661 Meetei_Mayek,
662 Meroitic_Cursive,
663 Meroitic_Hieroglyphs,
664 Miao,
665 Mongolian,
666 Myanmar,
667 New_Tai_Lue,
668 Nko,
669 Ogham,
670 Old_Italic,
671 Old_Persian,
672 Old_South_Arabian,
673 Old_Turkic,
674 Ol_Chiki,
675 Oriya,
676 Osmanya,
677 Phags_Pa,
678 Phoenician,
679 Rejang,
680 Runic,
681 Samaritan,
682 Saurashtra,
683 Sharada,
684 Shavian,
685 Sinhala,
686 Sora_Sompeng,
687 Sundanese,
688 Syloti_Nagri,
689 Syriac,
690 Tagalog,
691 Tagbanwa,
692 Tai_Le,
693 Tai_Tham,
694 Tai_Viet,
695 Takri,
696 Tamil,
697 Telugu,
698 Thaana,
699 Thai,
700 Tibetan,
701 Tifinagh,
702 Ugaritic,
703 Vai,
704 Yi.
705 .P
706 Each character has exactly one Unicode general category property, specified by
707 a two-letter abbreviation. For compatibility with Perl, negation can be
708 specified by including a circumflex between the opening brace and the property
709 name. For example, \ep{^Lu} is the same as \eP{Lu}.
710 .P
711 If only one letter is specified with \ep or \eP, it includes all the general
712 category properties that start with that letter. In this case, in the absence
713 of negation, the curly brackets in the escape sequence are optional; these two
714 examples have the same effect:
715 .sp
716 \ep{L}
717 \epL
718 .sp
719 The following general category property codes are supported:
720 .sp
721 C Other
722 Cc Control
723 Cf Format
724 Cn Unassigned
725 Co Private use
726 Cs Surrogate
727 .sp
728 L Letter
729 Ll Lower case letter
730 Lm Modifier letter
731 Lo Other letter
732 Lt Title case letter
733 Lu Upper case letter
734 .sp
735 M Mark
736 Mc Spacing mark
737 Me Enclosing mark
738 Mn Non-spacing mark
739 .sp
740 N Number
741 Nd Decimal number
742 Nl Letter number
743 No Other number
744 .sp
745 P Punctuation
746 Pc Connector punctuation
747 Pd Dash punctuation
748 Pe Close punctuation
749 Pf Final punctuation
750 Pi Initial punctuation
751 Po Other punctuation
752 Ps Open punctuation
753 .sp
754 S Symbol
755 Sc Currency symbol
756 Sk Modifier symbol
757 Sm Mathematical symbol
758 So Other symbol
759 .sp
760 Z Separator
761 Zl Line separator
762 Zp Paragraph separator
763 Zs Space separator
764 .sp
765 The special property L& is also supported: it matches a character that has
766 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
767 a modifier or "other".
768 .P
769 The Cs (Surrogate) property applies only to characters in the range U+D800 to
770 U+DFFF. Such characters are not valid in Unicode strings and so
771 cannot be tested by PCRE, unless UTF validity checking has been turned off
772 (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
773 .\" HREF
774 \fBpcreapi\fP
775 .\"
776 page). Perl does not support the Cs property.
777 .P
778 The long synonyms for property names that Perl supports (such as \ep{Letter})
779 are not supported by PCRE, nor is it permitted to prefix any of these
780 properties with "Is".
781 .P
782 No character that is in the Unicode table has the Cn (unassigned) property.
783 Instead, this property is assumed for any code point that is not in the
784 Unicode table.
785 .P
786 Specifying caseless matching does not affect these escape sequences. For
787 example, \ep{Lu} always matches only upper case letters.
788 .P
789 The \eX escape matches any number of Unicode characters that form an extended
790 Unicode sequence. \eX is equivalent to
791 .sp
792 (?>\ePM\epM*)
793 .sp
794 That is, it matches a character without the "mark" property, followed by zero
795 or more characters with the "mark" property, and treats the sequence as an
796 atomic group
797 .\" HTML <a href="#atomicgroup">
798 .\" </a>
799 (see below).
800 .\"
801 Characters with the "mark" property are typically accents that affect the
802 preceding character. None of them have codepoints less than 256, so in
803 8-bit non-UTF-8 mode \eX matches any one character.
804 .P
805 Note that recent versions of Perl have changed \eX to match what Unicode calls
806 an "extended grapheme cluster", which has a more complicated definition.
807 .P
808 Matching characters by Unicode property is not fast, because PCRE has to search
809 a structure that contains data for over fifteen thousand characters. That is
810 why the traditional escape sequences such as \ed and \ew do not use Unicode
811 properties in PCRE by default, though you can make them do so by setting the
812 PCRE_UCP option or by starting the pattern with (*UCP).
813 .
814 .
815 .\" HTML <a name="extraprops"></a>
816 .SS PCRE's additional properties
817 .rs
818 .sp
819 As well as the standard Unicode properties described in the previous
820 section, PCRE supports four more that make it possible to convert traditional
821 escape sequences such as \ew and \es and POSIX character classes to use Unicode
822 properties. PCRE uses these non-standard, non-Perl properties internally when
823 PCRE_UCP is set. They are:
824 .sp
825 Xan Any alphanumeric character
826 Xps Any POSIX space character
827 Xsp Any Perl space character
828 Xwd Any Perl "word" character
829 .sp
830 Xan matches characters that have either the L (letter) or the N (number)
831 property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
832 carriage return, and any other character that has the Z (separator) property.
833 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
834 same characters as Xan, plus underscore.
835 .
836 .
837 .\" HTML <a name="resetmatchstart"></a>
838 .SS "Resetting the match start"
839 .rs
840 .sp
841 The escape sequence \eK causes any previously matched characters not to be
842 included in the final matched sequence. For example, the pattern:
843 .sp
844 foo\eKbar
845 .sp
846 matches "foobar", but reports that it has matched "bar". This feature is
847 similar to a lookbehind assertion
848 .\" HTML <a href="#lookbehind">
849 .\" </a>
850 (described below).
851 .\"
852 However, in this case, the part of the subject before the real match does not
853 have to be of fixed length, as lookbehind assertions do. The use of \eK does
854 not interfere with the setting of
855 .\" HTML <a href="#subpattern">
856 .\" </a>
857 captured substrings.
858 .\"
859 For example, when the pattern
860 .sp
861 (foo)\eKbar
862 .sp
863 matches "foobar", the first substring is still set to "foo".
864 .P
865 Perl documents that the use of \eK within assertions is "not well defined". In
866 PCRE, \eK is acted upon when it occurs inside positive assertions, but is
867 ignored in negative assertions.
868 .
869 .
870 .\" HTML <a name="smallassertions"></a>
871 .SS "Simple assertions"
872 .rs
873 .sp
874 The final use of backslash is for certain simple assertions. An assertion
875 specifies a condition that has to be met at a particular point in a match,
876 without consuming any characters from the subject string. The use of
877 subpatterns for more complicated assertions is described
878 .\" HTML <a href="#bigassertions">
879 .\" </a>
880 below.
881 .\"
882 The backslashed assertions are:
883 .sp
884 \eb matches at a word boundary
885 \eB matches when not at a word boundary
886 \eA matches at the start of the subject
887 \eZ matches at the end of the subject
888 also matches before a newline at the end of the subject
889 \ez matches only at the end of the subject
890 \eG matches at the first matching position in the subject
891 .sp
892 Inside a character class, \eb has a different meaning; it matches the backspace
893 character. If any other of these assertions appears in a character class, by
894 default it matches the corresponding literal character (for example, \eB
895 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
896 escape sequence" error is generated instead.
897 .P
898 A word boundary is a position in the subject string where the current character
899 and the previous character do not both match \ew or \eW (i.e. one matches
900 \ew and the other matches \eW), or the start or end of the string if the
901 first or last character matches \ew, respectively. In a UTF mode, the meanings
902 of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
903 done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
904 of word" or "end of word" metasequence. However, whatever follows \eb normally
905 determines which it is. For example, the fragment \eba matches "a" at the start
906 of a word.
907 .P
908 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
909 dollar (described in the next section) in that they only ever match at the very
910 start and end of the subject string, whatever options are set. Thus, they are
911 independent of multiline mode. These three assertions are not affected by the
912 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
913 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
914 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
915 at a point other than the beginning of the subject, \eA can never match. The
916 difference between \eZ and \ez is that \eZ matches before a newline at the end
917 of the string as well as at the very end, whereas \ez matches only at the end.
918 .P
919 The \eG assertion is true only when the current matching position is at the
920 start point of the match, as specified by the \fIstartoffset\fP argument of
921 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
922 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
923 arguments, you can mimic Perl's /g option, and it is in this kind of
924 implementation where \eG can be useful.
925 .P
926 Note, however, that PCRE's interpretation of \eG, as the start of the current
927 match, is subtly different from Perl's, which defines it as the end of the
928 previous match. In Perl, these can be different when the previously matched
929 string was empty. Because PCRE does just one match at a time, it cannot
930 reproduce this behaviour.
931 .P
932 If all the alternatives of a pattern begin with \eG, the expression is anchored
933 to the starting match position, and the "anchored" flag is set in the compiled
934 regular expression.
935 .
936 .
938 .rs
939 .sp
940 Outside a character class, in the default matching mode, the circumflex
941 character is an assertion that is true only if the current matching point is
942 at the start of the subject string. If the \fIstartoffset\fP argument of
943 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
944 option is unset. Inside a character class, circumflex has an entirely different
945 meaning
946 .\" HTML <a href="#characterclass">
947 .\" </a>
948 (see below).
949 .\"
950 .P
951 Circumflex need not be the first character of the pattern if a number of
952 alternatives are involved, but it should be the first thing in each alternative
953 in which it appears if the pattern is ever to match that branch. If all
954 possible alternatives start with a circumflex, that is, if the pattern is
955 constrained to match only at the start of the subject, it is said to be an
956 "anchored" pattern. (There are also other constructs that can cause a pattern
957 to be anchored.)
958 .P
959 A dollar character is an assertion that is true only if the current matching
960 point is at the end of the subject string, or immediately before a newline
961 at the end of the string (by default). Dollar need not be the last character of
962 the pattern if a number of alternatives are involved, but it should be the last
963 item in any branch in which it appears. Dollar has no special meaning in a
964 character class.
965 .P
966 The meaning of dollar can be changed so that it matches only at the very end of
967 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
968 does not affect the \eZ assertion.
969 .P
970 The meanings of the circumflex and dollar characters are changed if the
971 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
972 immediately after internal newlines as well as at the start of the subject
973 string. It does not match after a newline that ends the string. A dollar
974 matches before any newlines in the string, as well as at the very end, when
975 PCRE_MULTILINE is set. When newline is specified as the two-character
976 sequence CRLF, isolated CR and LF characters do not indicate newlines.
977 .P
978 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
979 \en represents a newline) in multiline mode, but not otherwise. Consequently,
980 patterns that are anchored in single line mode because all branches start with
981 ^ are not anchored in multiline mode, and a match for circumflex is possible
982 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
983 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
984 .P
985 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
986 end of the subject in both modes, and if all branches of a pattern start with
987 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
988 .
989 .
990 .\" HTML <a name="fullstopdot"></a>
992 .rs
993 .sp
994 Outside a character class, a dot in the pattern matches any one character in
995 the subject string except (by default) a character that signifies the end of a
996 line.
997 .P
998 When a line ending is defined as a single character, dot never matches that
999 character; when the two-character sequence CRLF is used, dot does not match CR
1000 if it is immediately followed by LF, but otherwise it matches all characters
1001 (including isolated CRs and LFs). When any Unicode line endings are being
1002 recognized, dot does not match CR or LF or any of the other line ending
1003 characters.
1004 .P
1005 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
1006 option is set, a dot matches any one character, without exception. If the
1007 two-character sequence CRLF is present in the subject string, it takes two dots
1008 to match it.
1009 .P
1010 The handling of dot is entirely independent of the handling of circumflex and
1011 dollar, the only relationship being that they both involve newlines. Dot has no
1012 special meaning in a character class.
1013 .P
1014 The escape sequence \eN behaves like a dot, except that it is not affected by
1015 the PCRE_DOTALL option. In other words, it matches any character except one
1016 that signifies the end of a line. Perl also uses \eN to match characters by
1017 name; PCRE does not support this.
1018 .
1019 .
1021 .rs
1022 .sp
1023 Outside a character class, the escape sequence \eC matches any one data unit,
1024 whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1025 byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
1026 matches line-ending characters. The feature is provided in Perl in order to
1027 match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1028 used. Because \eC breaks up characters into individual data units, matching one
1029 unit with \eC in a UTF mode means that the rest of the string may start with a
1030 malformed UTF character. This has undefined results, because PCRE assumes that
1031 it is dealing with valid UTF strings (and by default it checks this at the
1032 start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option
1033 is used).
1034 .P
1035 PCRE does not allow \eC to appear in lookbehind assertions
1036 .\" HTML <a href="#lookbehind">
1037 .\" </a>
1038 (described below)
1039 .\"
1040 in a UTF mode, because this would make it impossible to calculate the length of
1041 the lookbehind.
1042 .P
1043 In general, the \eC escape sequence is best avoided. However, one
1044 way of using it that avoids the problem of malformed UTF characters is to use a
1045 lookahead to check the length of the next character, as in this pattern, which
1046 could be used with a UTF-8 string (ignore white space and line breaks):
1047 .sp
1048 (?| (?=[\ex00-\ex7f])(\eC) |
1049 (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1050 (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1051 (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1052 .sp
1053 A group that starts with (?| resets the capturing parentheses numbers in each
1054 alternative (see
1055 .\" HTML <a href="#dupsubpatternnumber">
1056 .\" </a>
1057 "Duplicate Subpattern Numbers"
1058 .\"
1059 below). The assertions at the start of each branch check the next UTF-8
1060 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1061 character's individual bytes are then captured by the appropriate number of
1062 groups.
1063 .
1064 .
1065 .\" HTML <a name="characterclass"></a>
1067 .rs
1068 .sp
1069 An opening square bracket introduces a character class, terminated by a closing
1070 square bracket. A closing square bracket on its own is not special by default.
1071 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1072 bracket causes a compile-time error. If a closing square bracket is required as
1073 a member of the class, it should be the first data character in the class
1074 (after an initial circumflex, if present) or escaped with a backslash.
1075 .P
1076 A character class matches a single character in the subject. In a UTF mode, the
1077 character may be more than one data unit long. A matched character must be in
1078 the set of characters defined by the class, unless the first character in the
1079 class definition is a circumflex, in which case the subject character must not
1080 be in the set defined by the class. If a circumflex is actually required as a
1081 member of the class, ensure it is not the first character, or escape it with a
1082 backslash.
1083 .P
1084 For example, the character class [aeiou] matches any lower case vowel, while
1085 [^aeiou] matches any character that is not a lower case vowel. Note that a
1086 circumflex is just a convenient notation for specifying the characters that
1087 are in the class by enumerating those that are not. A class that starts with a
1088 circumflex is not an assertion; it still consumes a character from the subject
1089 string, and therefore it fails if the current pointer is at the end of the
1090 string.
1091 .P
1092 In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
1093 included in a class as a literal string of data units, or by using the \ex{
1094 escaping mechanism.
1095 .P
1096 When caseless matching is set, any letters in a class represent both their
1097 upper case and lower case versions, so for example, a caseless [aeiou] matches
1098 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1099 caseful version would. In a UTF mode, PCRE always understands the concept of
1100 case for characters whose values are less than 128, so caseless matching is
1101 always possible. For characters with higher values, the concept of case is
1102 supported if PCRE is compiled with Unicode property support, but not otherwise.
1103 If you want to use caseless matching in a UTF mode for characters 128 and
1104 above, you must ensure that PCRE is compiled with Unicode property support as
1105 well as with UTF support.
1106 .P
1107 Characters that might indicate line breaks are never treated in any special way
1108 when matching character classes, whatever line-ending sequence is in use, and
1109 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1110 such as [^a] always matches one of these characters.
1111 .P
1112 The minus (hyphen) character can be used to specify a range of characters in a
1113 character class. For example, [d-m] matches any letter between d and m,
1114 inclusive. If a minus character is required in a class, it must be escaped with
1115 a backslash or appear in a position where it cannot be interpreted as
1116 indicating a range, typically as the first or last character in the class.
1117 .P
1118 It is not possible to have the literal character "]" as the end character of a
1119 range. A pattern such as [W-]46] is interpreted as a class of two characters
1120 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1121 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1122 the end of range, so [W-\e]46] is interpreted as a class containing a range
1123 followed by two other characters. The octal or hexadecimal representation of
1124 "]" can also be used to end a range.
1125 .P
1126 Ranges operate in the collating sequence of character values. They can also be
1127 used for characters specified numerically, for example [\e000-\e037]. Ranges
1128 can include any characters that are valid for the current mode.
1129 .P
1130 If a range that includes letters is used when caseless matching is set, it
1131 matches the letters in either case. For example, [W-c] is equivalent to
1132 [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1133 tables for a French locale are in use, [\exc8-\excb] matches accented E
1134 characters in both cases. In UTF modes, PCRE supports the concept of case for
1135 characters with values greater than 128 only when it is compiled with Unicode
1136 property support.
1137 .P
1138 The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1139 \eV, \ew, and \eW may appear in a character class, and add the characters that
1140 they match to the class. For example, [\edABCDEF] matches any hexadecimal
1141 digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1142 and their upper case partners, just as it does when they appear outside a
1143 character class, as described in the section entitled
1144 .\" HTML <a href="#genericchartypes">
1145 .\" </a>
1146 "Generic character types"
1147 .\"
1148 above. The escape sequence \eb has a different meaning inside a character
1149 class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1150 are not special inside a character class. Like any other unrecognized escape
1151 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1152 default, but cause an error if the PCRE_EXTRA option is set.
1153 .P
1154 A circumflex can conveniently be used with the upper case character types to
1155 specify a more restricted set of characters than the matching lower case type.
1156 For example, the class [^\eW_] matches any letter or digit, but not underscore,
1157 whereas [\ew] includes underscore. A positive character class should be read as
1158 "something OR something OR ..." and a negative class as "NOT something AND NOT
1159 something AND NOT ...".
1160 .P
1161 The only metacharacters that are recognized in character classes are backslash,
1162 hyphen (only where it can be interpreted as specifying a range), circumflex
1163 (only at the start), opening square bracket (only when it can be interpreted as
1164 introducing a POSIX class name - see the next section), and the terminating
1165 closing square bracket. However, escaping other non-alphanumeric characters
1166 does no harm.
1167 .
1168 .
1170 .rs
1171 .sp
1172 Perl supports the POSIX notation for character classes. This uses names
1173 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1174 this notation. For example,
1175 .sp
1176 [01[:alpha:]%]
1177 .sp
1178 matches "0", "1", any alphabetic character, or "%". The supported class names
1179 are:
1180 .sp
1181 alnum letters and digits
1182 alpha letters
1183 ascii character codes 0 - 127
1184 blank space or tab only
1185 cntrl control characters
1186 digit decimal digits (same as \ed)
1187 graph printing characters, excluding space
1188 lower lower case letters
1189 print printing characters, including space
1190 punct printing characters, excluding letters and digits and space
1191 space white space (not quite the same as \es)
1192 upper upper case letters
1193 word "word" characters (same as \ew)
1194 xdigit hexadecimal digits
1195 .sp
1196 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1197 space (32). Notice that this list includes the VT character (code 11). This
1198 makes "space" different to \es, which does not include VT (for Perl
1199 compatibility).
1200 .P
1201 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1202 5.8. Another Perl extension is negation, which is indicated by a ^ character
1203 after the colon. For example,
1204 .sp
1205 [12[:^digit:]]
1206 .sp
1207 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1208 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1209 supported, and an error is given if they are encountered.
1210 .P
1211 By default, in UTF modes, characters with values greater than 128 do not match
1212 any of the POSIX character classes. However, if the PCRE_UCP option is passed
1213 to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1214 character properties are used. This is achieved by replacing the POSIX classes
1215 by other sequences, as follows:
1216 .sp
1217 [:alnum:] becomes \ep{Xan}
1218 [:alpha:] becomes \ep{L}
1219 [:blank:] becomes \eh
1220 [:digit:] becomes \ep{Nd}
1221 [:lower:] becomes \ep{Ll}
1222 [:space:] becomes \ep{Xps}
1223 [:upper:] becomes \ep{Lu}
1224 [:word:] becomes \ep{Xwd}
1225 .sp
1226 Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1227 classes are unchanged, and match only characters with code points less than
1228 128.
1229 .
1230 .
1232 .rs
1233 .sp
1234 Vertical bar characters are used to separate alternative patterns. For example,
1235 the pattern
1236 .sp
1237 gilbert|sullivan
1238 .sp
1239 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1240 and an empty alternative is permitted (matching the empty string). The matching
1241 process tries each alternative in turn, from left to right, and the first one
1242 that succeeds is used. If the alternatives are within a subpattern
1243 .\" HTML <a href="#subpattern">
1244 .\" </a>
1245 (defined below),
1246 .\"
1247 "succeeds" means matching the rest of the main pattern as well as the
1248 alternative in the subpattern.
1249 .
1250 .
1252 .rs
1253 .sp
1254 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1255 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1256 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1257 The option letters are
1258 .sp
1259 i for PCRE_CASELESS
1261 s for PCRE_DOTALL
1262 x for PCRE_EXTENDED
1263 .sp
1264 For example, (?im) sets caseless, multiline matching. It is also possible to
1265 unset these options by preceding the letter with a hyphen, and a combined
1266 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1267 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1268 permitted. If a letter appears both before and after the hyphen, the option is
1269 unset.
1270 .P
1271 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1272 changed in the same way as the Perl-compatible options by using the characters
1273 J, U and X respectively.
1274 .P
1275 When one of these option changes occurs at top level (that is, not inside
1276 subpattern parentheses), the change applies to the remainder of the pattern
1277 that follows. If the change is placed right at the start of a pattern, PCRE
1278 extracts it into the global options (and it will therefore show up in data
1279 extracted by the \fBpcre_fullinfo()\fP function).
1280 .P
1281 An option change within a subpattern (see below for a description of
1282 subpatterns) affects only that part of the subpattern that follows it, so
1283 .sp
1284 (a(?i)b)c
1285 .sp
1286 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1287 By this means, options can be made to have different settings in different
1288 parts of the pattern. Any changes made in one alternative do carry on
1289 into subsequent branches within the same subpattern. For example,
1290 .sp
1291 (a(?i)b|c)
1292 .sp
1293 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1294 branch is abandoned before the option setting. This is because the effects of
1295 option settings happen at compile time. There would be some very weird
1296 behaviour otherwise.
1297 .P
1298 \fBNote:\fP There are other PCRE-specific options that can be set by the
1299 application when the compiling or matching functions are called. In some cases
1300 the pattern can contain special leading sequences such as (*CRLF) to override
1301 what the application has set or what has been defaulted. Details are given in
1302 the section entitled
1303 .\" HTML <a href="#newlineseq">
1304 .\" </a>
1305 "Newline sequences"
1306 .\"
1307 above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1308 can be used to set UTF and Unicode property modes; they are equivalent to
1309 setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1310 .
1311 .
1312 .\" HTML <a name="subpattern"></a>
1314 .rs
1315 .sp
1316 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1317 Turning part of a pattern into a subpattern does two things:
1318 .sp
1319 1. It localizes a set of alternatives. For example, the pattern
1320 .sp
1321 cat(aract|erpillar|)
1322 .sp
1323 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1324 match "cataract", "erpillar" or an empty string.
1325 .sp
1326 2. It sets up the subpattern as a capturing subpattern. This means that, when
1327 the whole pattern matches, that portion of the subject string that matched the
1328 subpattern is passed back to the caller via the \fIovector\fP argument of the
1329 matching function. (This applies only to the traditional matching functions;
1330 the DFA matching functions do not support capturing.)
1331 .P
1332 Opening parentheses are counted from left to right (starting from 1) to obtain
1333 numbers for the capturing subpatterns. For example, if the string "the red
1334 king" is matched against the pattern
1335 .sp
1336 the ((red|white) (king|queen))
1337 .sp
1338 the captured substrings are "red king", "red", and "king", and are numbered 1,
1339 2, and 3, respectively.
1340 .P
1341 The fact that plain parentheses fulfil two functions is not always helpful.
1342 There are often times when a grouping subpattern is required without a
1343 capturing requirement. If an opening parenthesis is followed by a question mark
1344 and a colon, the subpattern does not do any capturing, and is not counted when
1345 computing the number of any subsequent capturing subpatterns. For example, if
1346 the string "the white queen" is matched against the pattern
1347 .sp
1348 the ((?:red|white) (king|queen))
1349 .sp
1350 the captured substrings are "white queen" and "queen", and are numbered 1 and
1351 2. The maximum number of capturing subpatterns is 65535.
1352 .P
1353 As a convenient shorthand, if any option settings are required at the start of
1354 a non-capturing subpattern, the option letters may appear between the "?" and
1355 the ":". Thus the two patterns
1356 .sp
1357 (?i:saturday|sunday)
1358 (?:(?i)saturday|sunday)
1359 .sp
1360 match exactly the same set of strings. Because alternative branches are tried
1361 from left to right, and options are not reset until the end of the subpattern
1362 is reached, an option setting in one branch does affect subsequent branches, so
1363 the above patterns match "SUNDAY" as well as "Saturday".
1364 .
1365 .
1366 .\" HTML <a name="dupsubpatternnumber"></a>
1368 .rs
1369 .sp
1370 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1371 the same numbers for its capturing parentheses. Such a subpattern starts with
1372 (?| and is itself a non-capturing subpattern. For example, consider this
1373 pattern:
1374 .sp
1375 (?|(Sat)ur|(Sun))day
1376 .sp
1377 Because the two alternatives are inside a (?| group, both sets of capturing
1378 parentheses are numbered one. Thus, when the pattern matches, you can look
1379 at captured substring number one, whichever alternative matched. This construct
1380 is useful when you want to capture part, but not all, of one of a number of
1381 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1382 number is reset at the start of each branch. The numbers of any capturing
1383 parentheses that follow the subpattern start after the highest number used in
1384 any branch. The following example is taken from the Perl documentation. The
1385 numbers underneath show in which buffer the captured content will be stored.
1386 .sp
1387 # before ---------------branch-reset----------- after
1388 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1389 # 1 2 2 3 2 3 4
1390 .sp
1391 A back reference to a numbered subpattern uses the most recent value that is
1392 set for that number by any subpattern. The following pattern matches "abcabc"
1393 or "defdef":
1394 .sp
1395 /(?|(abc)|(def))\e1/
1396 .sp
1397 In contrast, a subroutine call to a numbered subpattern always refers to the
1398 first one in the pattern with the given number. The following pattern matches
1399 "abcabc" or "defabc":
1400 .sp
1401 /(?|(abc)|(def))(?1)/
1402 .sp
1403 If a
1404 .\" HTML <a href="#conditions">
1405 .\" </a>
1406 condition test
1407 .\"
1408 for a subpattern's having matched refers to a non-unique number, the test is
1409 true if any of the subpatterns of that number have matched.
1410 .P
1411 An alternative approach to using this "branch reset" feature is to use
1412 duplicate named subpatterns, as described in the next section.
1413 .
1414 .
1416 .rs
1417 .sp
1418 Identifying capturing parentheses by number is simple, but it can be very hard
1419 to keep track of the numbers in complicated regular expressions. Furthermore,
1420 if an expression is modified, the numbers may change. To help with this
1421 difficulty, PCRE supports the naming of subpatterns. This feature was not
1422 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1423 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1424 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1425 have different names, but PCRE does not.
1426 .P
1427 In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1428 (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1429 parentheses from other parts of the pattern, such as
1430 .\" HTML <a href="#backreferences">
1431 .\" </a>
1432 back references,
1433 .\"
1434 .\" HTML <a href="#recursion">
1435 .\" </a>
1436 recursion,
1437 .\"
1438 and
1439 .\" HTML <a href="#conditions">
1440 .\" </a>
1441 conditions,
1442 .\"
1443 can be made by name as well as by number.
1444 .P
1445 Names consist of up to 32 alphanumeric characters and underscores. Named
1446 capturing parentheses are still allocated numbers as well as names, exactly as
1447 if the names were not present. The PCRE API provides function calls for
1448 extracting the name-to-number translation table from a compiled pattern. There
1449 is also a convenience function for extracting a captured substring by name.
1450 .P
1451 By default, a name must be unique within a pattern, but it is possible to relax
1452 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1453 names are also always permitted for subpatterns with the same number, set up as
1454 described in the previous section.) Duplicate names can be useful for patterns
1455 where only one instance of the named parentheses can match. Suppose you want to
1456 match the name of a weekday, either as a 3-letter abbreviation or as the full
1457 name, and in both cases you want to extract the abbreviation. This pattern
1458 (ignoring the line breaks) does the job:
1459 .sp
1460 (?<DN>Mon|Fri|Sun)(?:day)?|
1461 (?<DN>Tue)(?:sday)?|
1462 (?<DN>Wed)(?:nesday)?|
1463 (?<DN>Thu)(?:rsday)?|
1464 (?<DN>Sat)(?:urday)?
1465 .sp
1466 There are five capturing substrings, but only one is ever set after a match.
1467 (An alternative way of solving this problem is to use a "branch reset"
1468 subpattern, as described in the previous section.)
1469 .P
1470 The convenience function for extracting the data by name returns the substring
1471 for the first (and in this example, the only) subpattern of that name that
1472 matched. This saves searching to find which numbered subpattern it was.
1473 .P
1474 If you make a back reference to a non-unique named subpattern from elsewhere in
1475 the pattern, the one that corresponds to the first occurrence of the name is
1476 used. In the absence of duplicate numbers (see the previous section) this is
1477 the one with the lowest number. If you use a named reference in a condition
1478 test (see the
1479 .\"
1480 .\" HTML <a href="#conditions">
1481 .\" </a>
1482 section about conditions
1483 .\"
1484 below), either to check whether a subpattern has matched, or to check for
1485 recursion, all subpatterns with the same name are tested. If the condition is
1486 true for any one of them, the overall condition is true. This is the same
1487 behaviour as testing by number. For further details of the interfaces for
1488 handling named subpatterns, see the
1489 .\" HREF
1490 \fBpcreapi\fP
1491 .\"
1492 documentation.
1493 .P
1494 \fBWarning:\fP You cannot use different names to distinguish between two
1495 subpatterns with the same number because PCRE uses only the numbers when
1496 matching. For this reason, an error is given at compile time if different names
1497 are given to subpatterns with the same number. However, you can give the same
1498 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1499 .
1500 .
1502 .rs
1503 .sp
1504 Repetition is specified by quantifiers, which can follow any of the following
1505 items:
1506 .sp
1507 a literal data character
1508 the dot metacharacter
1509 the \eC escape sequence
1510 the \eX escape sequence
1511 the \eR escape sequence
1512 an escape such as \ed or \epL that matches a single character
1513 a character class
1514 a back reference (see next section)
1515 a parenthesized subpattern (including assertions)
1516 a subroutine call to a subpattern (recursive or otherwise)
1517 .sp
1518 The general repetition quantifier specifies a minimum and maximum number of
1519 permitted matches, by giving the two numbers in curly brackets (braces),
1520 separated by a comma. The numbers must be less than 65536, and the first must
1521 be less than or equal to the second. For example:
1522 .sp
1523 z{2,4}
1524 .sp
1525 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1526 character. If the second number is omitted, but the comma is present, there is
1527 no upper limit; if the second number and the comma are both omitted, the
1528 quantifier specifies an exact number of required matches. Thus
1529 .sp
1530 [aeiou]{3,}
1531 .sp
1532 matches at least 3 successive vowels, but may match many more, while
1533 .sp
1534 \ed{8}
1535 .sp
1536 matches exactly 8 digits. An opening curly bracket that appears in a position
1537 where a quantifier is not allowed, or one that does not match the syntax of a
1538 quantifier, is taken as a literal character. For example, {,6} is not a
1539 quantifier, but a literal string of four characters.
1540 .P
1541 In UTF modes, quantifiers apply to characters rather than to individual data
1542 units. Thus, for example, \ex{100}{2} matches two characters, each of
1543 which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1544 \eX{3} matches three Unicode extended sequences, each of which may be several
1545 data units long (and they may be of different lengths).
1546 .P
1547 The quantifier {0} is permitted, causing the expression to behave as if the
1548 previous item and the quantifier were not present. This may be useful for
1549 subpatterns that are referenced as
1550 .\" HTML <a href="#subpatternsassubroutines">
1551 .\" </a>
1552 subroutines
1553 .\"
1554 from elsewhere in the pattern (but see also the section entitled
1555 .\" HTML <a href="#subdefine">
1556 .\" </a>
1557 "Defining subpatterns for use by reference only"
1558 .\"
1559 below). Items other than subpatterns that have a {0} quantifier are omitted
1560 from the compiled pattern.
1561 .P
1562 For convenience, the three most common quantifiers have single-character
1563 abbreviations:
1564 .sp
1565 * is equivalent to {0,}
1566 + is equivalent to {1,}
1567 ? is equivalent to {0,1}
1568 .sp
1569 It is possible to construct infinite loops by following a subpattern that can
1570 match no characters with a quantifier that has no upper limit, for example:
1571 .sp
1572 (a?)*
1573 .sp
1574 Earlier versions of Perl and PCRE used to give an error at compile time for
1575 such patterns. However, because there are cases where this can be useful, such
1576 patterns are now accepted, but if any repetition of the subpattern does in fact
1577 match no characters, the loop is forcibly broken.
1578 .P
1579 By default, the quantifiers are "greedy", that is, they match as much as
1580 possible (up to the maximum number of permitted times), without causing the
1581 rest of the pattern to fail. The classic example of where this gives problems
1582 is in trying to match comments in C programs. These appear between /* and */
1583 and within the comment, individual * and / characters may appear. An attempt to
1584 match C comments by applying the pattern
1585 .sp
1586 /\e*.*\e*/
1587 .sp
1588 to the string
1589 .sp
1590 /* first comment */ not comment /* second comment */
1591 .sp
1592 fails, because it matches the entire string owing to the greediness of the .*
1593 item.
1594 .P
1595 However, if a quantifier is followed by a question mark, it ceases to be
1596 greedy, and instead matches the minimum number of times possible, so the
1597 pattern
1598 .sp
1599 /\e*.*?\e*/
1600 .sp
1601 does the right thing with the C comments. The meaning of the various
1602 quantifiers is not otherwise changed, just the preferred number of matches.
1603 Do not confuse this use of question mark with its use as a quantifier in its
1604 own right. Because it has two uses, it can sometimes appear doubled, as in
1605 .sp
1606 \ed??\ed
1607 .sp
1608 which matches one digit by preference, but can match two if that is the only
1609 way the rest of the pattern matches.
1610 .P
1611 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1612 the quantifiers are not greedy by default, but individual ones can be made
1613 greedy by following them with a question mark. In other words, it inverts the
1614 default behaviour.
1615 .P
1616 When a parenthesized subpattern is quantified with a minimum repeat count that
1617 is greater than 1 or with a limited maximum, more memory is required for the
1618 compiled pattern, in proportion to the size of the minimum or maximum.
1619 .P
1620 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1621 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1622 implicitly anchored, because whatever follows will be tried against every
1623 character position in the subject string, so there is no point in retrying the
1624 overall match at any position after the first. PCRE normally treats such a
1625 pattern as though it were preceded by \eA.
1626 .P
1627 In cases where it is known that the subject string contains no newlines, it is
1628 worth setting PCRE_DOTALL in order to obtain this optimization, or
1629 alternatively using ^ to indicate anchoring explicitly.
1630 .P
1631 However, there are some cases where the optimization cannot be used. When .*
1632 is inside capturing parentheses that are the subject of a back reference
1633 elsewhere in the pattern, a match at the start may fail where a later one
1634 succeeds. Consider, for example:
1635 .sp
1636 (.*)abc\e1
1637 .sp
1638 If the subject is "xyz123abc123" the match point is the fourth character. For
1639 this reason, such a pattern is not implicitly anchored.
1640 .P
1641 Another case where implicit anchoring is not applied is when the leading .* is
1642 inside an atomic group. Once again, a match at the start may fail where a later
1643 one succeeds. Consider this pattern:
1644 .sp
1645 (?>.*?a)b
1646 .sp
1647 It matches "ab" in the subject "aab". The use of the backtracking control verbs
1648 (*PRUNE) and (*SKIP) also disable this optimization.
1649 .P
1650 When a capturing subpattern is repeated, the value captured is the substring
1651 that matched the final iteration. For example, after
1652 .sp
1653 (tweedle[dume]{3}\es*)+
1654 .sp
1655 has matched "tweedledum tweedledee" the value of the captured substring is
1656 "tweedledee". However, if there are nested capturing subpatterns, the
1657 corresponding captured values may have been set in previous iterations. For
1658 example, after
1659 .sp
1660 /(a|(b))+/
1661 .sp
1662 matches "aba" the value of the second captured substring is "b".
1663 .
1664 .
1665 .\" HTML <a name="atomicgroup"></a>
1667 .rs
1668 .sp
1669 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1670 repetition, failure of what follows normally causes the repeated item to be
1671 re-evaluated to see if a different number of repeats allows the rest of the
1672 pattern to match. Sometimes it is useful to prevent this, either to change the
1673 nature of the match, or to cause it fail earlier than it otherwise might, when
1674 the author of the pattern knows there is no point in carrying on.
1675 .P
1676 Consider, for example, the pattern \ed+foo when applied to the subject line
1677 .sp
1678 123456bar
1679 .sp
1680 After matching all 6 digits and then failing to match "foo", the normal
1681 action of the matcher is to try again with only 5 digits matching the \ed+
1682 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1683 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1684 that once a subpattern has matched, it is not to be re-evaluated in this way.
1685 .P
1686 If we use atomic grouping for the previous example, the matcher gives up
1687 immediately on failing to match "foo" the first time. The notation is a kind of
1688 special parenthesis, starting with (?> as in this example:
1689 .sp
1690 (?>\ed+)foo
1691 .sp
1692 This kind of parenthesis "locks up" the part of the pattern it contains once
1693 it has matched, and a failure further into the pattern is prevented from
1694 backtracking into it. Backtracking past it to previous items, however, works as
1695 normal.
1696 .P
1697 An alternative description is that a subpattern of this type matches the string
1698 of characters that an identical standalone pattern would match, if anchored at
1699 the current point in the subject string.
1700 .P
1701 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1702 the above example can be thought of as a maximizing repeat that must swallow
1703 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1704 number of digits they match in order to make the rest of the pattern match,
1705 (?>\ed+) can only match an entire sequence of digits.
1706 .P
1707 Atomic groups in general can of course contain arbitrarily complicated
1708 subpatterns, and can be nested. However, when the subpattern for an atomic
1709 group is just a single repeated item, as in the example above, a simpler
1710 notation, called a "possessive quantifier" can be used. This consists of an
1711 additional + character following a quantifier. Using this notation, the
1712 previous example can be rewritten as
1713 .sp
1714 \ed++foo
1715 .sp
1716 Note that a possessive quantifier can be used with an entire group, for
1717 example:
1718 .sp
1719 (abc|xyz){2,3}+
1720 .sp
1721 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1722 option is ignored. They are a convenient notation for the simpler forms of
1723 atomic group. However, there is no difference in the meaning of a possessive
1724 quantifier and the equivalent atomic group, though there may be a performance
1725 difference; possessive quantifiers should be slightly faster.
1726 .P
1727 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1728 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1729 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1730 package, and PCRE copied it from there. It ultimately found its way into Perl
1731 at release 5.10.
1732 .P
1733 PCRE has an optimization that automatically "possessifies" certain simple
1734 pattern constructs. For example, the sequence A+B is treated as A++B because
1735 there is no point in backtracking into a sequence of A's when B must follow.
1736 .P
1737 When a pattern contains an unlimited repeat inside a subpattern that can itself
1738 be repeated an unlimited number of times, the use of an atomic group is the
1739 only way to avoid some failing matches taking a very long time indeed. The
1740 pattern
1741 .sp
1742 (\eD+|<\ed+>)*[!?]
1743 .sp
1744 matches an unlimited number of substrings that either consist of non-digits, or
1745 digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1746 quickly. However, if it is applied to
1747 .sp
1748 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1749 .sp
1750 it takes a long time before reporting failure. This is because the string can
1751 be divided between the internal \eD+ repeat and the external * repeat in a
1752 large number of ways, and all have to be tried. (The example uses [!?] rather
1753 than a single character at the end, because both PCRE and Perl have an
1754 optimization that allows for fast failure when a single character is used. They
1755 remember the last single character that is required for a match, and fail early
1756 if it is not present in the string.) If the pattern is changed so that it uses
1757 an atomic group, like this:
1758 .sp
1759 ((?>\eD+)|<\ed+>)*[!?]
1760 .sp
1761 sequences of non-digits cannot be broken, and failure happens quickly.
1762 .
1763 .
1764 .\" HTML <a name="backreferences"></a>
1766 .rs
1767 .sp
1768 Outside a character class, a backslash followed by a digit greater than 0 (and
1769 possibly further digits) is a back reference to a capturing subpattern earlier
1770 (that is, to its left) in the pattern, provided there have been that many
1771 previous capturing left parentheses.
1772 .P
1773 However, if the decimal number following the backslash is less than 10, it is
1774 always taken as a back reference, and causes an error only if there are not
1775 that many capturing left parentheses in the entire pattern. In other words, the
1776 parentheses that are referenced need not be to the left of the reference for
1777 numbers less than 10. A "forward back reference" of this type can make sense
1778 when a repetition is involved and the subpattern to the right has participated
1779 in an earlier iteration.
1780 .P
1781 It is not possible to have a numerical "forward back reference" to a subpattern
1782 whose number is 10 or more using this syntax because a sequence such as \e50 is
1783 interpreted as a character defined in octal. See the subsection entitled
1784 "Non-printing characters"
1785 .\" HTML <a href="#digitsafterbackslash">
1786 .\" </a>
1787 above
1788 .\"
1789 for further details of the handling of digits following a backslash. There is
1790 no such problem when named parentheses are used. A back reference to any
1791 subpattern is possible using named parentheses (see below).
1792 .P
1793 Another way of avoiding the ambiguity inherent in the use of digits following a
1794 backslash is to use the \eg escape sequence. This escape must be followed by an
1795 unsigned number or a negative number, optionally enclosed in braces. These
1796 examples are all identical:
1797 .sp
1798 (ring), \e1
1799 (ring), \eg1
1800 (ring), \eg{1}
1801 .sp
1802 An unsigned number specifies an absolute reference without the ambiguity that
1803 is present in the older syntax. It is also useful when literal digits follow
1804 the reference. A negative number is a relative reference. Consider this
1805 example:
1806 .sp
1807 (abc(def)ghi)\eg{-1}
1808 .sp
1809 The sequence \eg{-1} is a reference to the most recently started capturing
1810 subpattern before \eg, that is, is it equivalent to \e2 in this example.
1811 Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1812 can be helpful in long patterns, and also in patterns that are created by
1813 joining together fragments that contain references within themselves.
1814 .P
1815 A back reference matches whatever actually matched the capturing subpattern in
1816 the current subject string, rather than anything matching the subpattern
1817 itself (see
1818 .\" HTML <a href="#subpatternsassubroutines">
1819 .\" </a>
1820 "Subpatterns as subroutines"
1821 .\"
1822 below for a way of doing that). So the pattern
1823 .sp
1824 (sens|respons)e and \e1ibility
1825 .sp
1826 matches "sense and sensibility" and "response and responsibility", but not
1827 "sense and responsibility". If caseful matching is in force at the time of the
1828 back reference, the case of letters is relevant. For example,
1829 .sp
1830 ((?i)rah)\es+\e1
1831 .sp
1832 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1833 capturing subpattern is matched caselessly.
1834 .P
1835 There are several different ways of writing back references to named
1836 subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1837 \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1838 back reference syntax, in which \eg can be used for both numeric and named
1839 references, is also supported. We could rewrite the above example in any of
1840 the following ways:
1841 .sp
1842 (?<p1>(?i)rah)\es+\ek<p1>
1843 (?'p1'(?i)rah)\es+\ek{p1}
1844 (?P<p1>(?i)rah)\es+(?P=p1)
1845 (?<p1>(?i)rah)\es+\eg{p1}
1846 .sp
1847 A subpattern that is referenced by name may appear in the pattern before or
1848 after the reference.
1849 .P
1850 There may be more than one back reference to the same subpattern. If a
1851 subpattern has not actually been used in a particular match, any back
1852 references to it always fail by default. For example, the pattern
1853 .sp
1854 (a|(bc))\e2
1855 .sp
1856 always fails if it starts to match "a" rather than "bc". However, if the
1857 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1858 unset value matches an empty string.
1859 .P
1860 Because there may be many capturing parentheses in a pattern, all digits
1861 following a backslash are taken as part of a potential back reference number.
1862 If the pattern continues with a digit character, some delimiter must be used to
1863 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1864 white space. Otherwise, the \eg{ syntax or an empty comment (see
1865 .\" HTML <a href="#comments">
1866 .\" </a>
1867 "Comments"
1868 .\"
1869 below) can be used.
1870 .
1871 .SS "Recursive back references"
1872 .rs
1873 .sp
1874 A back reference that occurs inside the parentheses to which it refers fails
1875 when the subpattern is first used, so, for example, (a\e1) never matches.
1876 However, such references can be useful inside repeated subpatterns. For
1877 example, the pattern
1878 .sp
1879 (a|b\e1)+
1880 .sp
1881 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1882 the subpattern, the back reference matches the character string corresponding
1883 to the previous iteration. In order for this to work, the pattern must be such
1884 that the first iteration does not need to match the back reference. This can be
1885 done using alternation, as in the example above, or by a quantifier with a
1886 minimum of zero.
1887 .P
1888 Back references of this type cause the group that they reference to be treated
1889 as an
1890 .\" HTML <a href="#atomicgroup">
1891 .\" </a>
1892 atomic group.
1893 .\"
1894 Once the whole group has been matched, a subsequent matching failure cannot
1895 cause backtracking into the middle of the group.
1896 .
1897 .
1898 .\" HTML <a name="bigassertions"></a>
1900 .rs
1901 .sp
1902 An assertion is a test on the characters following or preceding the current
1903 matching point that does not actually consume any characters. The simple
1904 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1905 .\" HTML <a href="#smallassertions">
1906 .\" </a>
1907 above.
1908 .\"
1909 .P
1910 More complicated assertions are coded as subpatterns. There are two kinds:
1911 those that look ahead of the current position in the subject string, and those
1912 that look behind it. An assertion subpattern is matched in the normal way,
1913 except that it does not cause the current matching position to be changed.
1914 .P
1915 Assertion subpatterns are not capturing subpatterns. If such an assertion
1916 contains capturing subpatterns within it, these are counted for the purposes of
1917 numbering the capturing subpatterns in the whole pattern. However, substring
1918 capturing is carried out only for positive assertions, because it does not make
1919 sense for negative assertions.
1920 .P
1921 For compatibility with Perl, assertion subpatterns may be repeated; though
1922 it makes no sense to assert the same thing several times, the side effect of
1923 capturing parentheses may occasionally be useful. In practice, there only three
1924 cases:
1925 .sp
1926 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1927 However, it may contain internal capturing parenthesized groups that are called
1928 from elsewhere via the
1929 .\" HTML <a href="#subpatternsassubroutines">
1930 .\" </a>
1931 subroutine mechanism.
1932 .\"
1933 .sp
1934 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1935 were {0,1}. At run time, the rest of the pattern match is tried with and
1936 without the assertion, the order depending on the greediness of the quantifier.
1937 .sp
1938 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1939 The assertion is obeyed just once when encountered during matching.
1940 .
1941 .
1942 .SS "Lookahead assertions"
1943 .rs
1944 .sp
1945 Lookahead assertions start with (?= for positive assertions and (?! for
1946 negative assertions. For example,
1947 .sp
1948 \ew+(?=;)
1949 .sp
1950 matches a word followed by a semicolon, but does not include the semicolon in
1951 the match, and
1952 .sp
1953 foo(?!bar)
1954 .sp
1955 matches any occurrence of "foo" that is not followed by "bar". Note that the
1956 apparently similar pattern
1957 .sp
1958 (?!foo)bar
1959 .sp
1960 does not find an occurrence of "bar" that is preceded by something other than
1961 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1962 (?!foo) is always true when the next three characters are "bar". A
1963 lookbehind assertion is needed to achieve the other effect.
1964 .P
1965 If you want to force a matching failure at some point in a pattern, the most
1966 convenient way to do it is with (?!) because an empty string always matches, so
1967 an assertion that requires there not to be an empty string must always fail.
1968 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1969 .
1970 .
1971 .\" HTML <a name="lookbehind"></a>
1972 .SS "Lookbehind assertions"
1973 .rs
1974 .sp
1975 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1976 negative assertions. For example,
1977 .sp
1978 (?<!foo)bar
1979 .sp
1980 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1981 a lookbehind assertion are restricted such that all the strings it matches must
1982 have a fixed length. However, if there are several top-level alternatives, they
1983 do not all have to have the same fixed length. Thus
1984 .sp
1985 (?<=bullock|donkey)
1986 .sp
1987 is permitted, but
1988 .sp
1989 (?<!dogs?|cats?)
1990 .sp
1991 causes an error at compile time. Branches that match different length strings
1992 are permitted only at the top level of a lookbehind assertion. This is an
1993 extension compared with Perl, which requires all branches to match the same
1994 length of string. An assertion such as
1995 .sp
1996 (?<=ab(c|de))
1997 .sp
1998 is not permitted, because its single top-level branch can match two different
1999 lengths, but it is acceptable to PCRE if rewritten to use two top-level
2000 branches:
2001 .sp
2002 (?<=abc|abde)
2003 .sp
2004 In some cases, the escape sequence \eK
2005 .\" HTML <a href="#resetmatchstart">
2006 .\" </a>
2007 (see above)
2008 .\"
2009 can be used instead of a lookbehind assertion to get round the fixed-length
2010 restriction.
2011 .P
2012 The implementation of lookbehind assertions is, for each alternative, to
2013 temporarily move the current position back by the fixed length and then try to
2014 match. If there are insufficient characters before the current position, the
2015 assertion fails.
2016 .P
2017 In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
2018 unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2019 it impossible to calculate the length of the lookbehind. The \eX and \eR
2020 escapes, which can match different numbers of data units, are also not
2021 permitted.
2022 .P
2023 .\" HTML <a href="#subpatternsassubroutines">
2024 .\" </a>
2025 "Subroutine"
2026 .\"
2027 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2028 as the subpattern matches a fixed-length string.
2029 .\" HTML <a href="#recursion">
2030 .\" </a>
2031 Recursion,
2032 .\"
2033 however, is not supported.
2034 .P
2035 Possessive quantifiers can be used in conjunction with lookbehind assertions to
2036 specify efficient matching of fixed-length strings at the end of subject
2037 strings. Consider a simple pattern such as
2038 .sp
2039 abcd$
2040 .sp
2041 when applied to a long string that does not match. Because matching proceeds
2042 from left to right, PCRE will look for each "a" in the subject and then see if
2043 what follows matches the rest of the pattern. If the pattern is specified as
2044 .sp
2045 ^.*abcd$
2046 .sp
2047 the initial .* matches the entire string at first, but when this fails (because
2048 there is no following "a"), it backtracks to match all but the last character,
2049 then all but the last two characters, and so on. Once again the search for "a"
2050 covers the entire string, from right to left, so we are no better off. However,
2051 if the pattern is written as
2052 .sp
2053 ^.*+(?<=abcd)
2054 .sp
2055 there can be no backtracking for the .*+ item; it can match only the entire
2056 string. The subsequent lookbehind assertion does a single test on the last four
2057 characters. If it fails, the match fails immediately. For long strings, this
2058 approach makes a significant difference to the processing time.
2059 .
2060 .
2061 .SS "Using multiple assertions"
2062 .rs
2063 .sp
2064 Several assertions (of any sort) may occur in succession. For example,
2065 .sp
2066 (?<=\ed{3})(?<!999)foo
2067 .sp
2068 matches "foo" preceded by three digits that are not "999". Notice that each of
2069 the assertions is applied independently at the same point in the subject
2070 string. First there is a check that the previous three characters are all
2071 digits, and then there is a check that the same three characters are not "999".
2072 This pattern does \fInot\fP match "foo" preceded by six characters, the first
2073 of which are digits and the last three of which are not "999". For example, it
2074 doesn't match "123abcfoo". A pattern to do that is
2075 .sp
2076 (?<=\ed{3}...)(?<!999)foo
2077 .sp
2078 This time the first assertion looks at the preceding six characters, checking
2079 that the first three are digits, and then the second assertion checks that the
2080 preceding three characters are not "999".
2081 .P
2082 Assertions can be nested in any combination. For example,
2083 .sp
2084 (?<=(?<!foo)bar)baz
2085 .sp
2086 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2087 preceded by "foo", while
2088 .sp
2089 (?<=\ed{3}(?!999)...)foo
2090 .sp
2091 is another pattern that matches "foo" preceded by three digits and any three
2092 characters that are not "999".
2093 .
2094 .
2095 .\" HTML <a name="conditions"></a>
2097 .rs
2098 .sp
2099 It is possible to cause the matching process to obey a subpattern
2100 conditionally or to choose between two alternative subpatterns, depending on
2101 the result of an assertion, or whether a specific capturing subpattern has
2102 already been matched. The two possible forms of conditional subpattern are:
2103 .sp
2104 (?(condition)yes-pattern)
2105 (?(condition)yes-pattern|no-pattern)
2106 .sp
2107 If the condition is satisfied, the yes-pattern is used; otherwise the
2108 no-pattern (if present) is used. If there are more than two alternatives in the
2109 subpattern, a compile-time error occurs. Each of the two alternatives may
2110 itself contain nested subpatterns of any form, including conditional
2111 subpatterns; the restriction to two alternatives applies only at the level of
2112 the condition. This pattern fragment is an example where the alternatives are
2113 complex:
2114 .sp
2115 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2116 .sp
2117 .P
2118 There are four kinds of condition: references to subpatterns, references to
2119 recursion, a pseudo-condition called DEFINE, and assertions.
2120 .
2121 .SS "Checking for a used subpattern by number"
2122 .rs
2123 .sp
2124 If the text between the parentheses consists of a sequence of digits, the
2125 condition is true if a capturing subpattern of that number has previously
2126 matched. If there is more than one capturing subpattern with the same number
2127 (see the earlier
2128 .\"
2129 .\" HTML <a href="#recursion">
2130 .\" </a>
2131 section about duplicate subpattern numbers),
2132 .\"
2133 the condition is true if any of them have matched. An alternative notation is
2134 to precede the digits with a plus or minus sign. In this case, the subpattern
2135 number is relative rather than absolute. The most recently opened parentheses
2136 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2137 loops it can also make sense to refer to subsequent groups. The next
2138 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2139 zero in any of these forms is not used; it provokes a compile-time error.)
2140 .P
2141 Consider the following pattern, which contains non-significant white space to
2142 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2143 three parts for ease of discussion:
2144 .sp
2145 ( \e( )? [^()]+ (?(1) \e) )
2146 .sp
2147 The first part matches an optional opening parenthesis, and if that
2148 character is present, sets it as the first captured substring. The second part
2149 matches one or more characters that are not parentheses. The third part is a
2150 conditional subpattern that tests whether or not the first set of parentheses
2151 matched. If they did, that is, if subject started with an opening parenthesis,
2152 the condition is true, and so the yes-pattern is executed and a closing
2153 parenthesis is required. Otherwise, since no-pattern is not present, the
2154 subpattern matches nothing. In other words, this pattern matches a sequence of
2155 non-parentheses, optionally enclosed in parentheses.
2156 .P
2157 If you were embedding this pattern in a larger one, you could use a relative
2158 reference:
2159 .sp
2160 ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
2161 .sp
2162 This makes the fragment independent of the parentheses in the larger pattern.
2163 .
2164 .SS "Checking for a used subpattern by name"
2165 .rs
2166 .sp
2167 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2168 subpattern by name. For compatibility with earlier versions of PCRE, which had
2169 this facility before Perl, the syntax (?(name)...) is also recognized. However,
2170 there is a possible ambiguity with this syntax, because subpattern names may
2171 consist entirely of digits. PCRE looks first for a named subpattern; if it
2172 cannot find one and the name consists entirely of digits, PCRE looks for a
2173 subpattern of that number, which must be greater than zero. Using subpattern
2174 names that consist entirely of digits is not recommended.
2175 .P
2176 Rewriting the above example to use a named subpattern gives this:
2177 .sp
2178 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
2179 .sp
2180 If the name used in a condition of this kind is a duplicate, the test is
2181 applied to all subpatterns of the same name, and is true if any one of them has
2182 matched.
2183 .
2184 .SS "Checking for pattern recursion"
2185 .rs
2186 .sp
2187 If the condition is the string (R), and there is no subpattern with the name R,
2188 the condition is true if a recursive call to the whole pattern or any
2189 subpattern has been made. If digits or a name preceded by ampersand follow the
2190 letter R, for example:
2191 .sp
2192 (?(R3)...) or (?(R&name)...)
2193 .sp
2194 the condition is true if the most recent recursion is into a subpattern whose
2195 number or name is given. This condition does not check the entire recursion
2196 stack. If the name used in a condition of this kind is a duplicate, the test is
2197 applied to all subpatterns of the same name, and is true if any one of them is
2198 the most recent recursion.
2199 .P
2200 At "top level", all these recursion test conditions are false.
2201 .\" HTML <a href="#recursion">
2202 .\" </a>
2203 The syntax for recursive patterns
2204 .\"
2205 is described below.
2206 .
2207 .\" HTML <a name="subdefine"></a>
2208 .SS "Defining subpatterns for use by reference only"
2209 .rs
2210 .sp
2211 If the condition is the string (DEFINE), and there is no subpattern with the
2212 name DEFINE, the condition is always false. In this case, there may be only one
2213 alternative in the subpattern. It is always skipped if control reaches this
2214 point in the pattern; the idea of DEFINE is that it can be used to define
2215 subroutines that can be referenced from elsewhere. (The use of
2216 .\" HTML <a href="#subpatternsassubroutines">
2217 .\" </a>
2218 subroutines
2219 .\"
2220 is described below.) For example, a pattern to match an IPv4 address such as
2221 "" could be written like this (ignore white space and line
2222 breaks):
2223 .sp
2224 (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2225 \eb (?&byte) (\e.(?&byte)){3} \eb
2226 .sp
2227 The first part of the pattern is a DEFINE group inside which a another group
2228 named "byte" is defined. This matches an individual component of an IPv4
2229 address (a number less than 256). When matching takes place, this part of the
2230 pattern is skipped because DEFINE acts like a false condition. The rest of the
2231 pattern uses references to the named group to match the four dot-separated
2232 components of an IPv4 address, insisting on a word boundary at each end.
2233 .
2234 .SS "Assertion conditions"
2235 .rs
2236 .sp
2237 If the condition is not in any of the above formats, it must be an assertion.
2238 This may be a positive or negative lookahead or lookbehind assertion. Consider
2239 this pattern, again containing non-significant white space, and with the two
2240 alternatives on the second line:
2241 .sp
2242 (?(?=[^a-z]*[a-z])
2243 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2244 .sp
2245 The condition is a positive lookahead assertion that matches an optional
2246 sequence of non-letters followed by a letter. In other words, it tests for the
2247 presence of at least one letter in the subject. If a letter is found, the
2248 subject is matched against the first alternative; otherwise it is matched
2249 against the second. This pattern matches strings in one of the two forms
2250 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2251 .
2252 .
2253 .\" HTML <a name="comments"></a>
2255 .rs
2256 .sp
2257 There are two ways of including comments in patterns that are processed by
2258 PCRE. In both cases, the start of the comment must not be in a character class,
2259 nor in the middle of any other sequence of related characters such as (?: or a
2260 subpattern name or number. The characters that make up a comment play no part
2261 in the pattern matching.
2262 .P
2263 The sequence (?# marks the start of a comment that continues up to the next
2264 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2265 option is set, an unescaped # character also introduces a comment, which in
2266 this case continues to immediately after the next newline character or
2267 character sequence in the pattern. Which characters are interpreted as newlines
2268 is controlled by the options passed to a compiling function or by a special
2269 sequence at the start of the pattern, as described in the section entitled
2270 .\" HTML <a href="#newlines">
2271 .\" </a>
2272 "Newline conventions"
2273 .\"
2274 above. Note that the end of this type of comment is a literal newline sequence
2275 in the pattern; escape sequences that happen to represent a newline do not
2276 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2277 default newline convention is in force:
2278 .sp
2279 abc #comment \en still comment
2280 .sp
2281 On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2282 a newline in the pattern. The sequence \en is still literal at this stage, so
2283 it does not terminate the comment. Only an actual character with the code value
2284 0x0a (the default newline) does so.
2285 .
2286 .
2287 .\" HTML <a name="recursion"></a>
2289 .rs
2290 .sp
2291 Consider the problem of matching a string in parentheses, allowing for
2292 unlimited nested parentheses. Without the use of recursion, the best that can
2293 be done is to use a pattern that matches up to some fixed depth of nesting. It
2294 is not possible to handle an arbitrary nesting depth.
2295 .P
2296 For some time, Perl has provided a facility that allows regular expressions to
2297 recurse (amongst other things). It does this by interpolating Perl code in the
2298 expression at run time, and the code can refer to the expression itself. A Perl
2299 pattern using code interpolation to solve the parentheses problem can be
2300 created like this:
2301 .sp
2302 $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2303 .sp
2304 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2305 recursively to the pattern in which it appears.
2306 .P
2307 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2308 supports special syntax for recursion of the entire pattern, and also for
2309 individual subpattern recursion. After its introduction in PCRE and Python,
2310 this kind of recursion was subsequently introduced into Perl at release 5.10.
2311 .P
2312 A special item that consists of (? followed by a number greater than zero and a
2313 closing parenthesis is a recursive subroutine call of the subpattern of the
2314 given number, provided that it occurs inside that subpattern. (If not, it is a
2315 .\" HTML <a href="#subpatternsassubroutines">
2316 .\" </a>
2317 non-recursive subroutine
2318 .\"
2319 call, which is described in the next section.) The special item (?R) or (?0) is
2320 a recursive call of the entire regular expression.
2321 .P
2322 This PCRE pattern solves the nested parentheses problem (assume the
2323 PCRE_EXTENDED option is set so that white space is ignored):
2324 .sp
2325 \e( ( [^()]++ | (?R) )* \e)
2326 .sp
2327 First it matches an opening parenthesis. Then it matches any number of
2328 substrings which can either be a sequence of non-parentheses, or a recursive
2329 match of the pattern itself (that is, a correctly parenthesized substring).
2330 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2331 to avoid backtracking into sequences of non-parentheses.
2332 .P
2333 If this were part of a larger pattern, you would not want to recurse the entire
2334 pattern, so instead you could use this:
2335 .sp
2336 ( \e( ( [^()]++ | (?1) )* \e) )
2337 .sp
2338 We have put the pattern into parentheses, and caused the recursion to refer to
2339 them instead of the whole pattern.
2340 .P
2341 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2342 is made easier by the use of relative references. Instead of (?1) in the
2343 pattern above you can write (?-2) to refer to the second most recently opened
2344 parentheses preceding the recursion. In other words, a negative number counts
2345 capturing parentheses leftwards from the point at which it is encountered.
2346 .P
2347 It is also possible to refer to subsequently opened parentheses, by writing
2348 references such as (?+2). However, these cannot be recursive because the
2349 reference is not inside the parentheses that are referenced. They are always
2350 .\" HTML <a href="#subpatternsassubroutines">
2351 .\" </a>
2352 non-recursive subroutine
2353 .\"
2354 calls, as described in the next section.
2355 .P
2356 An alternative approach is to use named parentheses instead. The Perl syntax
2357 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2358 could rewrite the above example as follows:
2359 .sp
2360 (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2361 .sp
2362 If there is more than one subpattern with the same name, the earliest one is
2363 used.
2364 .P
2365 This particular example pattern that we have been looking at contains nested
2366 unlimited repeats, and so the use of a possessive quantifier for matching
2367 strings of non-parentheses is important when applying the pattern to strings
2368 that do not match. For example, when this pattern is applied to
2369 .sp
2370 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2371 .sp
2372 it yields "no match" quickly. However, if a possessive quantifier is not used,
2373 the match runs for a very long time indeed because there are so many different
2374 ways the + and * repeats can carve up the subject, and all have to be tested
2375 before failure can be reported.
2376 .P
2377 At the end of a match, the values of capturing parentheses are those from
2378 the outermost level. If you want to obtain intermediate values, a callout
2379 function can be used (see below and the
2380 .\" HREF
2381 \fBpcrecallout\fP
2382 .\"
2383 documentation). If the pattern above is matched against
2384 .sp
2385 (ab(cd)ef)
2386 .sp
2387 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2388 the last value taken on at the top level. If a capturing subpattern is not
2389 matched at the top level, its final captured value is unset, even if it was
2390 (temporarily) set at a deeper level during the matching process.
2391 .P
2392 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2393 obtain extra memory to store data during a recursion, which it does by using
2394 \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2395 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2396 .P
2397 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2398 Consider this pattern, which matches text in angle brackets, allowing for
2399 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2400 recursing), whereas any characters are permitted at the outer level.
2401 .sp
2402 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2403 .sp
2404 In this pattern, (?(R) is the start of a conditional subpattern, with two
2405 different alternatives for the recursive and non-recursive cases. The (?R) item
2406 is the actual recursive call.
2407 .
2408 .
2409 .\" HTML <a name="recursiondifference"></a>
2410 .SS "Differences in recursion processing between PCRE and Perl"
2411 .rs
2412 .sp
2413 Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2414 (like Python, but unlike Perl), a recursive subpattern call is always treated
2415 as an atomic group. That is, once it has matched some of the subject string, it
2416 is never re-entered, even if it contains untried alternatives and there is a
2417 subsequent matching failure. This can be illustrated by the following pattern,
2418 which purports to match a palindromic string that contains an odd number of
2419 characters (for example, "a", "aba", "abcba", "abcdcba"):
2420 .sp
2421 ^(.|(.)(?1)\e2)$
2422 .sp
2423 The idea is that it either matches a single character, or two identical
2424 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2425 it does not if the pattern is longer than three characters. Consider the
2426 subject string "abcba":
2427 .P
2428 At the top level, the first character is matched, but as it is not at the end
2429 of the string, the first alternative fails; the second alternative is taken
2430 and the recursion kicks in. The recursive call to subpattern 1 successfully
2431 matches the next character ("b"). (Note that the beginning and end of line
2432 tests are not part of the recursion).
2433 .P
2434 Back at the top level, the next character ("c") is compared with what
2435 subpattern 2 matched, which was "a". This fails. Because the recursion is
2436 treated as an atomic group, there are now no backtracking points, and so the
2437 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2438 try the second alternative.) However, if the pattern is written with the
2439 alternatives in the other order, things are different:
2440 .sp
2441 ^((.)(?1)\e2|.)$
2442 .sp
2443 This time, the recursing alternative is tried first, and continues to recurse
2444 until it runs out of characters, at which point the recursion fails. But this
2445 time we do have another alternative to try at the higher level. That is the big
2446 difference: in the previous case the remaining alternative is at a deeper
2447 recursion level, which PCRE cannot use.
2448 .P
2449 To change the pattern so that it matches all palindromic strings, not just
2450 those with an odd number of characters, it is tempting to change the pattern to
2451 this:
2452 .sp
2453 ^((.)(?1)\e2|.?)$
2454 .sp
2455 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2456 deeper recursion has matched a single character, it cannot be entered again in
2457 order to match an empty string. The solution is to separate the two cases, and
2458 write out the odd and even cases as alternatives at the higher level:
2459 .sp
2460 ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2461 .sp
2462 If you want to match typical palindromic phrases, the pattern has to ignore all
2463 non-word characters, which can be done like this:
2464 .sp
2465 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2466 .sp
2467 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2468 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2469 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2470 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2471 more) to match typical phrases, and Perl takes so long that you think it has
2472 gone into a loop.
2473 .P
2474 \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2475 string does not start with a palindrome that is shorter than the entire string.
2476 For example, although "abcba" is correctly matched, if the subject is "ababa",
2477 PCRE finds the palindrome "aba" at the start, then fails at top level because
2478 the end of the string does not follow. Once again, it cannot jump back into the
2479 recursion to try other alternatives, so the entire match fails.
2480 .P
2481 The second way in which PCRE and Perl differ in their recursion processing is
2482 in the handling of captured values. In Perl, when a subpattern is called
2483 recursively or as a subpattern (see the next section), it has no access to any
2484 values that were captured outside the recursion, whereas in PCRE these values
2485 can be referenced. Consider this pattern:
2486 .sp
2487 ^(.)(\e1|a(?2))
2488 .sp
2489 In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2490 then in the second group, when the back reference \e1 fails to match "b", the
2491 second alternative matches "a" and then recurses. In the recursion, \e1 does
2492 now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2493 match because inside the recursive call \e1 cannot access the externally set
2494 value.
2495 .
2496 .
2497 .\" HTML <a name="subpatternsassubroutines"></a>
2499 .rs
2500 .sp
2501 If the syntax for a recursive subpattern call (either by number or by
2502 name) is used outside the parentheses to which it refers, it operates like a
2503 subroutine in a programming language. The called subpattern may be defined
2504 before or after the reference. A numbered reference can be absolute or
2505 relative, as in these examples:
2506 .sp
2507 (...(absolute)...)...(?2)...
2508 (...(relative)...)...(?-1)...
2509 (...(?+1)...(relative)...
2510 .sp
2511 An earlier example pointed out that the pattern
2512 .sp
2513 (sens|respons)e and \e1ibility
2514 .sp
2515 matches "sense and sensibility" and "response and responsibility", but not
2516 "sense and responsibility". If instead the pattern
2517 .sp
2518 (sens|respons)e and (?1)ibility
2519 .sp
2520 is used, it does match "sense and responsibility" as well as the other two
2521 strings. Another example is given in the discussion of DEFINE above.
2522 .P
2523 All subroutine calls, whether recursive or not, are always treated as atomic
2524 groups. That is, once a subroutine has matched some of the subject string, it
2525 is never re-entered, even if it contains untried alternatives and there is a
2526 subsequent matching failure. Any capturing parentheses that are set during the
2527 subroutine call revert to their previous values afterwards.
2528 .P
2529 Processing options such as case-independence are fixed when a subpattern is
2530 defined, so if it is used as a subroutine, such options cannot be changed for
2531 different calls. For example, consider this pattern:
2532 .sp
2533 (abc)(?i:(?-1))
2534 .sp
2535 It matches "abcabc". It does not match "abcABC" because the change of
2536 processing option does not affect the called subpattern.
2537 .
2538 .
2539 .\" HTML <a name="onigurumasubroutines"></a>
2541 .rs
2542 .sp
2543 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2544 a number enclosed either in angle brackets or single quotes, is an alternative
2545 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2546 are two of the examples used above, rewritten using this syntax:
2547 .sp
2548 (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2549 (sens|respons)e and \eg'1'ibility
2550 .sp
2551 PCRE supports an extension to Oniguruma: if a number is preceded by a
2552 plus or a minus sign it is taken as a relative reference. For example:
2553 .sp
2554 (abc)(?i:\eg<-1>)
2555 .sp
2556 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2557 synonymous. The former is a back reference; the latter is a subroutine call.
2558 .
2559 .
2561 .rs
2562 .sp
2563 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2564 code to be obeyed in the middle of matching a regular expression. This makes it
2565 possible, amongst other things, to extract different substrings that match the
2566 same pair of parentheses when there is a repetition.
2567 .P
2568 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2569 code. The feature is called "callout". The caller of PCRE provides an external
2570 function by putting its entry point in the global variable \fIpcre_callout\fP
2571 (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2572 variable contains NULL, which disables all calling out.
2573 .P
2574 Within a regular expression, (?C) indicates the points at which the external
2575 function is to be called. If you want to identify different callout points, you
2576 can put a number less than 256 after the letter C. The default value is zero.
2577 For example, this pattern has two callout points:
2578 .sp
2579 (?C1)abc(?C2)def
2580 .sp
2581 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2582 automatically installed before each item in the pattern. They are all numbered
2583 255.
2584 .P
2585 During matching, when PCRE reaches a callout point, the external function is
2586 called. It is provided with the number of the callout, the position in the
2587 pattern, and, optionally, one item of data originally supplied by the caller of
2588 the matching function. The callout function may cause matching to proceed, to
2589 backtrack, or to fail altogether. A complete description of the interface to
2590 the callout function is given in the
2591 .\" HREF
2592 \fBpcrecallout\fP
2593 .\"
2594 documentation.
2595 .
2596 .
2597 .\" HTML <a name="backtrackcontrol"></a>
2599 .rs
2600 .sp
2601 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2602 are described in the Perl documentation as "experimental and subject to change
2603 or removal in a future version of Perl". It goes on to say: "Their usage in
2604 production code should be noted to avoid problems during upgrades." The same
2605 remarks apply to the PCRE features described in this section.
2606 .P
2607 Since these verbs are specifically related to backtracking, most of them can be
2608 used only when the pattern is to be matched using one of the traditional
2609 matching functions, which use a backtracking algorithm. With the exception of
2610 (*FAIL), which behaves like a failing negative assertion, they cause an error
2611 if encountered by a DFA matching function.
2612 .P
2613 If any of these verbs are used in an assertion or in a subpattern that is
2614 called as a subroutine (whether or not recursively), their effect is confined
2615 to that subpattern; it does not extend to the surrounding pattern, with one
2616 exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2617 a successful positive assertion \fIis\fP passed back when a match succeeds
2618 (compare capturing parentheses in assertions). Note that such subpatterns are
2619 processed as anchored at the point where they are tested. Note also that Perl's
2620 treatment of subroutines and assertions is different in some cases.
2621 .P
2622 The new verbs make use of what was previously invalid syntax: an opening
2623 parenthesis followed by an asterisk. They are generally of the form
2624 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2625 depending on whether or not an argument is present. A name is any sequence of
2626 characters that does not include a closing parenthesis. The maximum length of
2627 name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name
2628 is empty, that is, if the closing parenthesis immediately follows the colon,
2629 the effect is as if the colon were not there. Any number of these verbs may
2630 occur in a pattern.
2631 .
2632 .
2633 .\" HTML <a name="nooptimize"></a>
2634 .SS "Optimizations that affect backtracking verbs"
2635 .rs
2636 .sp
2637 PCRE contains some optimizations that are used to speed up matching by running
2638 some checks at the start of each match attempt. For example, it may know the
2639 minimum length of matching subject, or that a particular character must be
2640 present. When one of these optimizations suppresses the running of a match, any
2641 included backtracking verbs will not, of course, be processed. You can suppress
2642 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2643 when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2644 pattern with (*NO_START_OPT). There is more discussion of this option in the
2645 section entitled
2646 .\" HTML <a href="pcreapi.html#execoptions">
2647 .\" </a>
2648 "Option bits for \fBpcre_exec()\fP"
2649 .\"
2650 in the
2651 .\" HREF
2652 \fBpcreapi\fP
2653 .\"
2654 documentation.
2655 .P
2656 Experiments with Perl suggest that it too has similar optimizations, sometimes
2657 leading to anomalous results.
2658 .
2659 .
2660 .SS "Verbs that act immediately"
2661 .rs
2662 .sp
2663 The following verbs act as soon as they are encountered. They may not be
2664 followed by a name.
2665 .sp
2666 (*ACCEPT)
2667 .sp
2668 This verb causes the match to end successfully, skipping the remainder of the
2669 pattern. However, when it is inside a subpattern that is called as a
2670 subroutine, only that subpattern is ended successfully. Matching then continues
2671 at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2672 far is captured. For example:
2673 .sp
2674 A((?:A|B(*ACCEPT)|C)D)
2675 .sp
2676 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2677 the outer parentheses.
2678 .sp
2679 (*FAIL) or (*F)
2680 .sp
2681 This verb causes a matching failure, forcing backtracking to occur. It is
2682 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2683 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2684 Perl features that are not present in PCRE. The nearest equivalent is the
2685 callout feature, as for example in this pattern:
2686 .sp
2687 a+(?C)(*FAIL)
2688 .sp
2689 A match with the string "aaaa" always fails, but the callout is taken before
2690 each backtrack happens (in this example, 10 times).
2691 .
2692 .
2693 .SS "Recording which path was taken"
2694 .rs
2695 .sp
2696 There is one verb whose main purpose is to track how a match was arrived at,
2697 though it also has a secondary use in conjunction with advancing the match
2698 starting point (see (*SKIP) below).
2699 .sp
2700 (*MARK:NAME) or (*:NAME)
2701 .sp
2702 A name is always required with this verb. There may be as many instances of
2703 (*MARK) as you like in a pattern, and their names do not have to be unique.
2704 .P
2705 When a match succeeds, the name of the last-encountered (*MARK) on the matching
2706 path is passed back to the caller as described in the section entitled
2707 .\" HTML <a href="pcreapi.html#extradata">
2708 .\" </a>
2709 "Extra data for \fBpcre_exec()\fP"
2710 .\"
2711 in the
2712 .\" HREF
2713 \fBpcreapi\fP
2714 .\"
2715 documentation. Here is an example of \fBpcretest\fP output, where the /K
2716 modifier requests the retrieval and outputting of (*MARK) data:
2717 .sp
2718 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2719 data> XY
2720 0: XY
2721 MK: A
2722 XZ
2723 0: XZ
2724 MK: B
2725 .sp
2726 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2727 indicates which of the two alternatives matched. This is a more efficient way
2728 of obtaining this information than putting each alternative in its own
2729 capturing parentheses.
2730 .P
2731 If (*MARK) is encountered in a positive assertion, its name is recorded and
2732 passed back if it is the last-encountered. This does not happen for negative
2733 assertions.
2734 .P
2735 After a partial match or a failed match, the name of the last encountered
2736 (*MARK) in the entire match process is returned. For example:
2737 .sp
2738 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2739 data> XP
2740 No match, mark = B
2741 .sp
2742 Note that in this unanchored example the mark is retained from the match
2743 attempt that started at the letter "X" in the subject. Subsequent match
2744 attempts starting at "P" and then with an empty string do not get as far as the
2745 (*MARK) item, but nevertheless do not reset it.
2746 .P
2747 If you are interested in (*MARK) values after failed matches, you should
2748 probably set the PCRE_NO_START_OPTIMIZE option
2749 .\" HTML <a href="#nooptimize">
2750 .\" </a>
2751 (see above)
2752 .\"
2753 to ensure that the match is always attempted.
2754 .
2755 .
2756 .SS "Verbs that act after backtracking"
2757 .rs
2758 .sp
2759 The following verbs do nothing when they are encountered. Matching continues
2760 with what follows, but if there is no subsequent match, causing a backtrack to
2761 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2762 the verb. However, when one of these verbs appears inside an atomic group, its
2763 effect is confined to that group, because once the group has been matched,
2764 there is never any backtracking into it. In this situation, backtracking can
2765 "jump back" to the left of the entire atomic group. (Remember also, as stated
2766 above, that this localization also applies in subroutine calls and assertions.)
2767 .P
2768 These verbs differ in exactly what kind of failure occurs when backtracking
2769 reaches them.
2770 .sp
2771 (*COMMIT)
2772 .sp
2773 This verb, which may not be followed by a name, causes the whole match to fail
2774 outright if the rest of the pattern does not match. Even if the pattern is
2775 unanchored, no further attempts to find a match by advancing the starting point
2776 take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2777 finding a match at the current starting point, or not at all. For example:
2778 .sp
2779 a+(*COMMIT)b
2780 .sp
2781 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2782 dynamic anchor, or "I've started, so I must finish." The name of the most
2783 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2784 match failure.
2785 .P
2786 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2787 unless PCRE's start-of-match optimizations are turned off, as shown in this
2788 \fBpcretest\fP example:
2789 .sp
2790 re> /(*COMMIT)abc/
2791 data> xyzabc
2792 0: abc
2793 xyzabc\eY
2794 No match
2795 .sp
2796 PCRE knows that any match must start with "a", so the optimization skips along
2797 the subject to "a" before running the first match attempt, which succeeds. When
2798 the optimization is disabled by the \eY escape in the second subject, the match
2799 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2800 starting points.
2801 .sp
2802 (*PRUNE) or (*PRUNE:NAME)
2803 .sp
2804 This verb causes the match to fail at the current starting position in the
2805 subject if the rest of the pattern does not match. If the pattern is
2806 unanchored, the normal "bumpalong" advance to the next starting character then
2807 happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2808 reached, or when matching to the right of (*PRUNE), but if there is no match to
2809 the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2810 (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2811 but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2812 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2813 anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2814 .sp
2815 (*SKIP)
2816 .sp
2817 This verb, when given without a name, is like (*PRUNE), except that if the
2818 pattern is unanchored, the "bumpalong" advance is not to the next character,
2819 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2820 signifies that whatever text was matched leading up to it cannot be part of a
2821 successful match. Consider:
2822 .sp
2823 a+(*SKIP)b
2824 .sp
2825 If the subject is "aaaac...", after the first match attempt fails (starting at
2826 the first character in the string), the starting point skips on to start the
2827 next attempt at "c". Note that a possessive quantifer does not have the same
2828 effect as this example; although it would suppress backtracking during the
2829 first match attempt, the second attempt would start at the second character
2830 instead of skipping on to "c".
2831 .sp
2832 (*SKIP:NAME)
2833 .sp
2834 When (*SKIP) has an associated name, its behaviour is modified. If the
2835 following pattern fails to match, the previous path through the pattern is
2836 searched for the most recent (*MARK) that has the same name. If one is found,
2837 the "bumpalong" advance is to the subject position that corresponds to that
2838 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2839 matching name is found, the (*SKIP) is ignored.
2840 .sp
2841 (*THEN) or (*THEN:NAME)
2842 .sp
2843 This verb causes a skip to the next innermost alternative if the rest of the
2844 pattern does not match. That is, it cancels pending backtracking, but only
2845 within the current alternative. Its name comes from the observation that it can
2846 be used for a pattern-based if-then-else block:
2847 .sp
2848 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2849 .sp
2850 If the COND1 pattern matches, FOO is tried (and possibly further items after
2851 the end of the group if FOO succeeds); on failure, the matcher skips to the
2852 second alternative and tries COND2, without backtracking into COND1. The
2853 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2854 If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2855 .P
2856 Note that a subpattern that does not contain a | character is just a part of
2857 the enclosing alternative; it is not a nested alternation with only one
2858 alternative. The effect of (*THEN) extends beyond such a subpattern to the
2859 enclosing alternative. Consider this pattern, where A, B, etc. are complex
2860 pattern fragments that do not contain any | characters at this level:
2861 .sp
2862 A (B(*THEN)C) | D
2863 .sp
2864 If A and B are matched, but there is a failure in C, matching does not
2865 backtrack into A; instead it moves to the next alternative, that is, D.
2866 However, if the subpattern containing (*THEN) is given an alternative, it
2867 behaves differently:
2868 .sp
2869 A (B(*THEN)C | (*FAIL)) | D
2870 .sp
2871 The effect of (*THEN) is now confined to the inner subpattern. After a failure
2872 in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2873 because there are no more alternatives to try. In this case, matching does now
2874 backtrack into A.
2875 .P
2876 Note also that a conditional subpattern is not considered as having two
2877 alternatives, because only one is ever used. In other words, the | character in
2878 a conditional subpattern has a different meaning. Ignoring white space,
2879 consider:
2880 .sp
2881 ^.*? (?(?=a) a | b(*THEN)c )
2882 .sp
2883 If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2884 it initially matches zero characters. The condition (?=a) then fails, the
2885 character "b" is matched, but "c" is not. At this point, matching does not
2886 backtrack to .*? as might perhaps be expected from the presence of the |
2887 character. The conditional subpattern is part of the single alternative that
2888 comprises the whole pattern, and so the match fails. (If there was a backtrack
2889 into .*?, allowing it to match "b", the match would succeed.)
2890 .P
2891 The verbs just described provide four different "strengths" of control when
2892 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2893 next alternative. (*PRUNE) comes next, failing the match at the current
2894 starting position, but allowing an advance to the next character (for an
2895 unanchored pattern). (*SKIP) is similar, except that the advance may be more
2896 than one character. (*COMMIT) is the strongest, causing the entire match to
2897 fail.
2898 .P
2899 If more than one such verb is present in a pattern, the "strongest" one wins.
2900 For example, consider this pattern, where A, B, etc. are complex pattern
2901 fragments:
2902 .sp
2903 (A(*COMMIT)B(*THEN)C|D)
2904 .sp
2905 Once A has matched, PCRE is committed to this match, at the current starting
2906 position. If subsequently B matches, but C does not, the normal (*THEN) action
2907 of trying the next alternative (that is, D) does not happen because (*COMMIT)
2908 overrides.
2909 .
2910 .
2911 .SH "SEE ALSO"
2912 .rs
2913 .sp
2914 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2915 \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
2916 .
2917 .
2919 .rs
2920 .sp
2921 .nf
2922 Philip Hazel
2923 University Computing Service
2924 Cambridge CB2 3QH, England.
2925 .fi
2926 .
2927 .
2929 .rs
2930 .sp
2931 .nf
2932 Last updated: 08 August 2012
2933 Copyright (c) 1997-2012 University of Cambridge.
2934 .fi


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