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1 <html>
2 <head>
3 <title>pcrepattern specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrepattern man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
14 <br>
15 <ul>
16 <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17 <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
18 <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
19 <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
20 <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
21 <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT) AND \N</a>
22 <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE DATA UNIT</a>
24 <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
25 <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
26 <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
27 <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
28 <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
29 <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
30 <li><a name="TOC15" href="#SEC15">REPETITION</a>
32 <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
33 <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
34 <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
35 <li><a name="TOC20" href="#SEC20">COMMENTS</a>
36 <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
37 <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
38 <li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a>
39 <li><a name="TOC24" href="#SEC24">CALLOUTS</a>
40 <li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a>
41 <li><a name="TOC26" href="#SEC26">SEE ALSO</a>
42 <li><a name="TOC27" href="#SEC27">AUTHOR</a>
43 <li><a name="TOC28" href="#SEC28">REVISION</a>
44 </ul>
45 <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
46 <P>
47 The syntax and semantics of the regular expressions that are supported by PCRE
48 are described in detail below. There is a quick-reference syntax summary in the
49 <a href="pcresyntax.html"><b>pcresyntax</b></a>
50 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
51 also supports some alternative regular expression syntax (which does not
52 conflict with the Perl syntax) in order to provide some compatibility with
53 regular expressions in Python, .NET, and Oniguruma.
54 </P>
55 <P>
56 Perl's regular expressions are described in its own documentation, and
57 regular expressions in general are covered in a number of books, some of which
58 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
59 published by O'Reilly, covers regular expressions in great detail. This
60 description of PCRE's regular expressions is intended as reference material.
61 </P>
62 <P>
63 The original operation of PCRE was on strings of one-byte characters. However,
64 there is now also support for UTF-8 strings in the original library, and a
65 second library that supports 16-bit and UTF-16 character strings. To use these
66 features, PCRE must be built to include appropriate support. When using UTF
67 strings you must either call the compiling function with the PCRE_UTF8 or
68 PCRE_UTF16 option, or the pattern must start with one of these special
69 sequences:
70 <pre>
71 (*UTF8)
72 (*UTF16)
73 </pre>
74 Starting a pattern with such a sequence is equivalent to setting the relevant
75 option. This feature is not Perl-compatible. How setting a UTF mode affects
76 pattern matching is mentioned in several places below. There is also a summary
77 of features in the
78 <a href="pcreunicode.html"><b>pcreunicode</b></a>
79 page.
80 </P>
81 <P>
82 Another special sequence that may appear at the start of a pattern or in
83 combination with (*UTF8) or (*UTF16) is:
84 <pre>
85 (*UCP)
86 </pre>
87 This has the same effect as setting the PCRE_UCP option: it causes sequences
88 such as \d and \w to use Unicode properties to determine character types,
89 instead of recognizing only characters with codes less than 128 via a lookup
90 table.
91 </P>
92 <P>
93 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
94 PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
95 also some more of these special sequences that are concerned with the handling
96 of newlines; they are described below.
97 </P>
98 <P>
99 The remainder of this document discusses the patterns that are supported by
100 PCRE when one its main matching functions, <b>pcre_exec()</b> (8-bit) or
101 <b>pcre16_exec()</b> (16-bit), is used. PCRE also has alternative matching
102 functions, <b>pcre_dfa_exec()</b> and <b>pcre16_dfa_exec()</b>, which match using
103 a different algorithm that is not Perl-compatible. Some of the features
104 discussed below are not available when DFA matching is used. The advantages and
105 disadvantages of the alternative functions, and how they differ from the normal
106 functions, are discussed in the
107 <a href="pcrematching.html"><b>pcrematching</b></a>
108 page.
109 <a name="newlines"></a></P>
110 <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
111 <P>
112 PCRE supports five different conventions for indicating line breaks in
113 strings: a single CR (carriage return) character, a single LF (linefeed)
114 character, the two-character sequence CRLF, any of the three preceding, or any
115 Unicode newline sequence. The
116 <a href="pcreapi.html"><b>pcreapi</b></a>
117 page has
118 <a href="pcreapi.html#newlines">further discussion</a>
119 about newlines, and shows how to set the newline convention in the
120 <i>options</i> arguments for the compiling and matching functions.
121 </P>
122 <P>
123 It is also possible to specify a newline convention by starting a pattern
124 string with one of the following five sequences:
125 <pre>
126 (*CR) carriage return
127 (*LF) linefeed
128 (*CRLF) carriage return, followed by linefeed
129 (*ANYCRLF) any of the three above
130 (*ANY) all Unicode newline sequences
131 </pre>
132 These override the default and the options given to the compiling function. For
133 example, on a Unix system where LF is the default newline sequence, the pattern
134 <pre>
135 (*CR)a.b
136 </pre>
137 changes the convention to CR. That pattern matches "a\nb" because LF is no
138 longer a newline. Note that these special settings, which are not
139 Perl-compatible, are recognized only at the very start of a pattern, and that
140 they must be in upper case. If more than one of them is present, the last one
141 is used.
142 </P>
143 <P>
144 The newline convention affects the interpretation of the dot metacharacter when
145 PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not
146 affect what the \R escape sequence matches. By default, this is any Unicode
147 newline sequence, for Perl compatibility. However, this can be changed; see the
148 description of \R in the section entitled
149 <a href="#newlineseq">"Newline sequences"</a>
150 below. A change of \R setting can be combined with a change of newline
151 convention.
152 </P>
153 <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
154 <P>
155 A regular expression is a pattern that is matched against a subject string from
156 left to right. Most characters stand for themselves in a pattern, and match the
157 corresponding characters in the subject. As a trivial example, the pattern
158 <pre>
159 The quick brown fox
160 </pre>
161 matches a portion of a subject string that is identical to itself. When
162 caseless matching is specified (the PCRE_CASELESS option), letters are matched
163 independently of case. In a UTF mode, PCRE always understands the concept of
164 case for characters whose values are less than 128, so caseless matching is
165 always possible. For characters with higher values, the concept of case is
166 supported if PCRE is compiled with Unicode property support, but not otherwise.
167 If you want to use caseless matching for characters 128 and above, you must
168 ensure that PCRE is compiled with Unicode property support as well as with
169 UTF support.
170 </P>
171 <P>
172 The power of regular expressions comes from the ability to include alternatives
173 and repetitions in the pattern. These are encoded in the pattern by the use of
174 <i>metacharacters</i>, which do not stand for themselves but instead are
175 interpreted in some special way.
176 </P>
177 <P>
178 There are two different sets of metacharacters: those that are recognized
179 anywhere in the pattern except within square brackets, and those that are
180 recognized within square brackets. Outside square brackets, the metacharacters
181 are as follows:
182 <pre>
183 \ general escape character with several uses
184 ^ assert start of string (or line, in multiline mode)
185 $ assert end of string (or line, in multiline mode)
186 . match any character except newline (by default)
187 [ start character class definition
188 | start of alternative branch
189 ( start subpattern
190 ) end subpattern
191 ? extends the meaning of (
192 also 0 or 1 quantifier
193 also quantifier minimizer
194 * 0 or more quantifier
195 + 1 or more quantifier
196 also "possessive quantifier"
197 { start min/max quantifier
198 </pre>
199 Part of a pattern that is in square brackets is called a "character class". In
200 a character class the only metacharacters are:
201 <pre>
202 \ general escape character
203 ^ negate the class, but only if the first character
204 - indicates character range
205 [ POSIX character class (only if followed by POSIX syntax)
206 ] terminates the character class
207 </pre>
208 The following sections describe the use of each of the metacharacters.
209 </P>
210 <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
211 <P>
212 The backslash character has several uses. Firstly, if it is followed by a
213 character that is not a number or a letter, it takes away any special meaning
214 that character may have. This use of backslash as an escape character applies
215 both inside and outside character classes.
216 </P>
217 <P>
218 For example, if you want to match a * character, you write \* in the pattern.
219 This escaping action applies whether or not the following character would
220 otherwise be interpreted as a metacharacter, so it is always safe to precede a
221 non-alphanumeric with backslash to specify that it stands for itself. In
222 particular, if you want to match a backslash, you write \\.
223 </P>
224 <P>
225 In a UTF mode, only ASCII numbers and letters have any special meaning after a
226 backslash. All other characters (in particular, those whose codepoints are
227 greater than 127) are treated as literals.
228 </P>
229 <P>
230 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
231 pattern (other than in a character class) and characters between a # outside
232 a character class and the next newline are ignored. An escaping backslash can
233 be used to include a whitespace or # character as part of the pattern.
234 </P>
235 <P>
236 If you want to remove the special meaning from a sequence of characters, you
237 can do so by putting them between \Q and \E. This is different from Perl in
238 that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
239 Perl, $ and @ cause variable interpolation. Note the following examples:
240 <pre>
241 Pattern PCRE matches Perl matches
243 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
244 \Qabc\$xyz\E abc\$xyz abc\$xyz
245 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
246 </pre>
247 The \Q...\E sequence is recognized both inside and outside character classes.
248 An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
249 by \E later in the pattern, the literal interpretation continues to the end of
250 the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
251 a character class, this causes an error, because the character class is not
252 terminated.
253 <a name="digitsafterbackslash"></a></P>
254 <br><b>
255 Non-printing characters
256 </b><br>
257 <P>
258 A second use of backslash provides a way of encoding non-printing characters
259 in patterns in a visible manner. There is no restriction on the appearance of
260 non-printing characters, apart from the binary zero that terminates a pattern,
261 but when a pattern is being prepared by text editing, it is often easier to use
262 one of the following escape sequences than the binary character it represents:
263 <pre>
264 \a alarm, that is, the BEL character (hex 07)
265 \cx "control-x", where x is any ASCII character
266 \e escape (hex 1B)
267 \f formfeed (hex 0C)
268 \n linefeed (hex 0A)
269 \r carriage return (hex 0D)
270 \t tab (hex 09)
271 \ddd character with octal code ddd, or back reference
272 \xhh character with hex code hh
273 \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
274 \uhhhh character with hex code hhhh (JavaScript mode only)
275 </pre>
276 The precise effect of \cx is as follows: if x is a lower case letter, it
277 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
278 Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while
279 \c; becomes hex 7B (; is 3B). If the byte following \c has a value greater
280 than 127, a compile-time error occurs. This locks out non-ASCII characters in
281 all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A
282 lower case letter is converted to upper case, and then the 0xc0 bits are
283 flipped.)
284 </P>
285 <P>
286 By default, after \x, from zero to two hexadecimal digits are read (letters
287 can be in upper or lower case). Any number of hexadecimal digits may appear
288 between \x{ and }, but the character code is constrained as follows:
289 <pre>
290 8-bit non-UTF mode less than 0x100
291 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
292 16-bit non-UTF mode less than 0x10000
293 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
294 </pre>
295 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
296 "surrogate" codepoints).
297 </P>
298 <P>
299 If characters other than hexadecimal digits appear between \x{ and }, or if
300 there is no terminating }, this form of escape is not recognized. Instead, the
301 initial \x will be interpreted as a basic hexadecimal escape, with no
302 following digits, giving a character whose value is zero.
303 </P>
304 <P>
305 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is
306 as just described only when it is followed by two hexadecimal digits.
307 Otherwise, it matches a literal "x" character. In JavaScript mode, support for
308 code points greater than 256 is provided by \u, which must be followed by
309 four hexadecimal digits; otherwise it matches a literal "u" character.
310 </P>
311 <P>
312 Characters whose value is less than 256 can be defined by either of the two
313 syntaxes for \x (or by \u in JavaScript mode). There is no difference in the
314 way they are handled. For example, \xdc is exactly the same as \x{dc} (or
315 \u00dc in JavaScript mode).
316 </P>
317 <P>
318 After \0 up to two further octal digits are read. If there are fewer than two
319 digits, just those that are present are used. Thus the sequence \0\x\07
320 specifies two binary zeros followed by a BEL character (code value 7). Make
321 sure you supply two digits after the initial zero if the pattern character that
322 follows is itself an octal digit.
323 </P>
324 <P>
325 The handling of a backslash followed by a digit other than 0 is complicated.
326 Outside a character class, PCRE reads it and any following digits as a decimal
327 number. If the number is less than 10, or if there have been at least that many
328 previous capturing left parentheses in the expression, the entire sequence is
329 taken as a <i>back reference</i>. A description of how this works is given
330 <a href="#backreferences">later,</a>
331 following the discussion of
332 <a href="#subpattern">parenthesized subpatterns.</a>
333 </P>
334 <P>
335 Inside a character class, or if the decimal number is greater than 9 and there
336 have not been that many capturing subpatterns, PCRE re-reads up to three octal
337 digits following the backslash, and uses them to generate a data character. Any
338 subsequent digits stand for themselves. The value of the character is
339 constrained in the same way as characters specified in hexadecimal.
340 For example:
341 <pre>
342 \040 is another way of writing a space
343 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
344 \7 is always a back reference
345 \11 might be a back reference, or another way of writing a tab
346 \011 is always a tab
347 \0113 is a tab followed by the character "3"
348 \113 might be a back reference, otherwise the character with octal code 113
349 \377 might be a back reference, otherwise the value 255 (decimal)
350 \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
351 </pre>
352 Note that octal values of 100 or greater must not be introduced by a leading
353 zero, because no more than three octal digits are ever read.
354 </P>
355 <P>
356 All the sequences that define a single character value can be used both inside
357 and outside character classes. In addition, inside a character class, \b is
358 interpreted as the backspace character (hex 08).
359 </P>
360 <P>
361 \N is not allowed in a character class. \B, \R, and \X are not special
362 inside a character class. Like other unrecognized escape sequences, they are
363 treated as the literal characters "B", "R", and "X" by default, but cause an
364 error if the PCRE_EXTRA option is set. Outside a character class, these
365 sequences have different meanings.
366 </P>
367 <br><b>
368 Unsupported escape sequences
369 </b><br>
370 <P>
371 In Perl, the sequences \l, \L, \u, and \U are recognized by its string
372 handler and used to modify the case of following characters. By default, PCRE
373 does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
374 option is set, \U matches a "U" character, and \u can be used to define a
375 character by code point, as described in the previous section.
376 </P>
377 <br><b>
378 Absolute and relative back references
379 </b><br>
380 <P>
381 The sequence \g followed by an unsigned or a negative number, optionally
382 enclosed in braces, is an absolute or relative back reference. A named back
383 reference can be coded as \g{name}. Back references are discussed
384 <a href="#backreferences">later,</a>
385 following the discussion of
386 <a href="#subpattern">parenthesized subpatterns.</a>
387 </P>
388 <br><b>
389 Absolute and relative subroutine calls
390 </b><br>
391 <P>
392 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
393 a number enclosed either in angle brackets or single quotes, is an alternative
394 syntax for referencing a subpattern as a "subroutine". Details are discussed
395 <a href="#onigurumasubroutines">later.</a>
396 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
397 synonymous. The former is a back reference; the latter is a
398 <a href="#subpatternsassubroutines">subroutine</a>
399 call.
400 <a name="genericchartypes"></a></P>
401 <br><b>
402 Generic character types
403 </b><br>
404 <P>
405 Another use of backslash is for specifying generic character types:
406 <pre>
407 \d any decimal digit
408 \D any character that is not a decimal digit
409 \h any horizontal whitespace character
410 \H any character that is not a horizontal whitespace character
411 \s any whitespace character
412 \S any character that is not a whitespace character
413 \v any vertical whitespace character
414 \V any character that is not a vertical whitespace character
415 \w any "word" character
416 \W any "non-word" character
417 </pre>
418 There is also the single sequence \N, which matches a non-newline character.
419 This is the same as
420 <a href="#fullstopdot">the "." metacharacter</a>
421 when PCRE_DOTALL is not set. Perl also uses \N to match characters by name;
422 PCRE does not support this.
423 </P>
424 <P>
425 Each pair of lower and upper case escape sequences partitions the complete set
426 of characters into two disjoint sets. Any given character matches one, and only
427 one, of each pair. The sequences can appear both inside and outside character
428 classes. They each match one character of the appropriate type. If the current
429 matching point is at the end of the subject string, all of them fail, because
430 there is no character to match.
431 </P>
432 <P>
433 For compatibility with Perl, \s does not match the VT character (code 11).
434 This makes it different from the the POSIX "space" class. The \s characters
435 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
436 included in a Perl script, \s may match the VT character. In PCRE, it never
437 does.
438 </P>
439 <P>
440 A "word" character is an underscore or any character that is a letter or digit.
441 By default, the definition of letters and digits is controlled by PCRE's
442 low-valued character tables, and may vary if locale-specific matching is taking
443 place (see
444 <a href="pcreapi.html#localesupport">"Locale support"</a>
445 in the
446 <a href="pcreapi.html"><b>pcreapi</b></a>
447 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
448 or "french" in Windows, some character codes greater than 128 are used for
449 accented letters, and these are then matched by \w. The use of locales with
450 Unicode is discouraged.
451 </P>
452 <P>
453 By default, in a UTF mode, characters with values greater than 128 never match
454 \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
455 their original meanings from before UTF support was available, mainly for
456 efficiency reasons. However, if PCRE is compiled with Unicode property support,
457 and the PCRE_UCP option is set, the behaviour is changed so that Unicode
458 properties are used to determine character types, as follows:
459 <pre>
460 \d any character that \p{Nd} matches (decimal digit)
461 \s any character that \p{Z} matches, plus HT, LF, FF, CR
462 \w any character that \p{L} or \p{N} matches, plus underscore
463 </pre>
464 The upper case escapes match the inverse sets of characters. Note that \d
465 matches only decimal digits, whereas \w matches any Unicode digit, as well as
466 any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
467 \B because they are defined in terms of \w and \W. Matching these sequences
468 is noticeably slower when PCRE_UCP is set.
469 </P>
470 <P>
471 The sequences \h, \H, \v, and \V are features that were added to Perl at
472 release 5.10. In contrast to the other sequences, which match only ASCII
473 characters by default, these always match certain high-valued codepoints,
474 whether or not PCRE_UCP is set. The horizontal space characters are:
475 <pre>
476 U+0009 Horizontal tab
477 U+0020 Space
478 U+00A0 Non-break space
479 U+1680 Ogham space mark
480 U+180E Mongolian vowel separator
481 U+2000 En quad
482 U+2001 Em quad
483 U+2002 En space
484 U+2003 Em space
485 U+2004 Three-per-em space
486 U+2005 Four-per-em space
487 U+2006 Six-per-em space
488 U+2007 Figure space
489 U+2008 Punctuation space
490 U+2009 Thin space
491 U+200A Hair space
492 U+202F Narrow no-break space
493 U+205F Medium mathematical space
494 U+3000 Ideographic space
495 </pre>
496 The vertical space characters are:
497 <pre>
498 U+000A Linefeed
499 U+000B Vertical tab
500 U+000C Formfeed
501 U+000D Carriage return
502 U+0085 Next line
503 U+2028 Line separator
504 U+2029 Paragraph separator
505 </pre>
506 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
507 relevant.
508 <a name="newlineseq"></a></P>
509 <br><b>
510 Newline sequences
511 </b><br>
512 <P>
513 Outside a character class, by default, the escape sequence \R matches any
514 Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
515 following:
516 <pre>
517 (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
518 </pre>
519 This is an example of an "atomic group", details of which are given
520 <a href="#atomicgroup">below.</a>
521 This particular group matches either the two-character sequence CR followed by
522 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
523 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
524 line, U+0085). The two-character sequence is treated as a single unit that
525 cannot be split.
526 </P>
527 <P>
528 In other modes, two additional characters whose codepoints are greater than 255
529 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
530 Unicode character property support is not needed for these characters to be
531 recognized.
532 </P>
533 <P>
534 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
535 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
536 either at compile time or when the pattern is matched. (BSR is an abbrevation
537 for "backslash R".) This can be made the default when PCRE is built; if this is
538 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
539 It is also possible to specify these settings by starting a pattern string with
540 one of the following sequences:
541 <pre>
542 (*BSR_ANYCRLF) CR, LF, or CRLF only
543 (*BSR_UNICODE) any Unicode newline sequence
544 </pre>
545 These override the default and the options given to the compiling function, but
546 they can themselves be overridden by options given to a matching function. Note
547 that these special settings, which are not Perl-compatible, are recognized only
548 at the very start of a pattern, and that they must be in upper case. If more
549 than one of them is present, the last one is used. They can be combined with a
550 change of newline convention; for example, a pattern can start with:
551 <pre>
553 </pre>
554 They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
555 sequences. Inside a character class, \R is treated as an unrecognized escape
556 sequence, and so matches the letter "R" by default, but causes an error if
557 PCRE_EXTRA is set.
558 <a name="uniextseq"></a></P>
559 <br><b>
560 Unicode character properties
561 </b><br>
562 <P>
563 When PCRE is built with Unicode character property support, three additional
564 escape sequences that match characters with specific properties are available.
565 When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
566 characters whose codepoints are less than 256, but they do work in this mode.
567 The extra escape sequences are:
568 <pre>
569 \p{<i>xx</i>} a character with the <i>xx</i> property
570 \P{<i>xx</i>} a character without the <i>xx</i> property
571 \X an extended Unicode sequence
572 </pre>
573 The property names represented by <i>xx</i> above are limited to the Unicode
574 script names, the general category properties, "Any", which matches any
575 character (including newline), and some special PCRE properties (described
576 in the
577 <a href="#extraprops">next section).</a>
578 Other Perl properties such as "InMusicalSymbols" are not currently supported by
579 PCRE. Note that \P{Any} does not match any characters, so always causes a
580 match failure.
581 </P>
582 <P>
583 Sets of Unicode characters are defined as belonging to certain scripts. A
584 character from one of these sets can be matched using a script name. For
585 example:
586 <pre>
587 \p{Greek}
588 \P{Han}
589 </pre>
590 Those that are not part of an identified script are lumped together as
591 "Common". The current list of scripts is:
592 </P>
593 <P>
594 Arabic,
595 Armenian,
596 Avestan,
597 Balinese,
598 Bamum,
599 Bengali,
600 Bopomofo,
601 Braille,
602 Buginese,
603 Buhid,
604 Canadian_Aboriginal,
605 Carian,
606 Cham,
607 Cherokee,
608 Common,
609 Coptic,
610 Cuneiform,
611 Cypriot,
612 Cyrillic,
613 Deseret,
614 Devanagari,
615 Egyptian_Hieroglyphs,
616 Ethiopic,
617 Georgian,
618 Glagolitic,
619 Gothic,
620 Greek,
621 Gujarati,
622 Gurmukhi,
623 Han,
624 Hangul,
625 Hanunoo,
626 Hebrew,
627 Hiragana,
628 Imperial_Aramaic,
629 Inherited,
630 Inscriptional_Pahlavi,
631 Inscriptional_Parthian,
632 Javanese,
633 Kaithi,
634 Kannada,
635 Katakana,
636 Kayah_Li,
637 Kharoshthi,
638 Khmer,
639 Lao,
640 Latin,
641 Lepcha,
642 Limbu,
643 Linear_B,
644 Lisu,
645 Lycian,
646 Lydian,
647 Malayalam,
648 Meetei_Mayek,
649 Mongolian,
650 Myanmar,
651 New_Tai_Lue,
652 Nko,
653 Ogham,
654 Old_Italic,
655 Old_Persian,
656 Old_South_Arabian,
657 Old_Turkic,
658 Ol_Chiki,
659 Oriya,
660 Osmanya,
661 Phags_Pa,
662 Phoenician,
663 Rejang,
664 Runic,
665 Samaritan,
666 Saurashtra,
667 Shavian,
668 Sinhala,
669 Sundanese,
670 Syloti_Nagri,
671 Syriac,
672 Tagalog,
673 Tagbanwa,
674 Tai_Le,
675 Tai_Tham,
676 Tai_Viet,
677 Tamil,
678 Telugu,
679 Thaana,
680 Thai,
681 Tibetan,
682 Tifinagh,
683 Ugaritic,
684 Vai,
685 Yi.
686 </P>
687 <P>
688 Each character has exactly one Unicode general category property, specified by
689 a two-letter abbreviation. For compatibility with Perl, negation can be
690 specified by including a circumflex between the opening brace and the property
691 name. For example, \p{^Lu} is the same as \P{Lu}.
692 </P>
693 <P>
694 If only one letter is specified with \p or \P, it includes all the general
695 category properties that start with that letter. In this case, in the absence
696 of negation, the curly brackets in the escape sequence are optional; these two
697 examples have the same effect:
698 <pre>
699 \p{L}
700 \pL
701 </pre>
702 The following general category property codes are supported:
703 <pre>
704 C Other
705 Cc Control
706 Cf Format
707 Cn Unassigned
708 Co Private use
709 Cs Surrogate
711 L Letter
712 Ll Lower case letter
713 Lm Modifier letter
714 Lo Other letter
715 Lt Title case letter
716 Lu Upper case letter
718 M Mark
719 Mc Spacing mark
720 Me Enclosing mark
721 Mn Non-spacing mark
723 N Number
724 Nd Decimal number
725 Nl Letter number
726 No Other number
728 P Punctuation
729 Pc Connector punctuation
730 Pd Dash punctuation
731 Pe Close punctuation
732 Pf Final punctuation
733 Pi Initial punctuation
734 Po Other punctuation
735 Ps Open punctuation
737 S Symbol
738 Sc Currency symbol
739 Sk Modifier symbol
740 Sm Mathematical symbol
741 So Other symbol
743 Z Separator
744 Zl Line separator
745 Zp Paragraph separator
746 Zs Space separator
747 </pre>
748 The special property L& is also supported: it matches a character that has
749 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
750 a modifier or "other".
751 </P>
752 <P>
753 The Cs (Surrogate) property applies only to characters in the range U+D800 to
754 U+DFFF. Such characters are not valid in Unicode strings and so
755 cannot be tested by PCRE, unless UTF validity checking has been turned off
756 (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
757 <a href="pcreapi.html"><b>pcreapi</b></a>
758 page). Perl does not support the Cs property.
759 </P>
760 <P>
761 The long synonyms for property names that Perl supports (such as \p{Letter})
762 are not supported by PCRE, nor is it permitted to prefix any of these
763 properties with "Is".
764 </P>
765 <P>
766 No character that is in the Unicode table has the Cn (unassigned) property.
767 Instead, this property is assumed for any code point that is not in the
768 Unicode table.
769 </P>
770 <P>
771 Specifying caseless matching does not affect these escape sequences. For
772 example, \p{Lu} always matches only upper case letters.
773 </P>
774 <P>
775 The \X escape matches any number of Unicode characters that form an extended
776 Unicode sequence. \X is equivalent to
777 <pre>
778 (?&#62;\PM\pM*)
779 </pre>
780 That is, it matches a character without the "mark" property, followed by zero
781 or more characters with the "mark" property, and treats the sequence as an
782 atomic group
783 <a href="#atomicgroup">(see below).</a>
784 Characters with the "mark" property are typically accents that affect the
785 preceding character. None of them have codepoints less than 256, so in
786 8-bit non-UTF-8 mode \X matches any one character.
787 </P>
788 <P>
789 Note that recent versions of Perl have changed \X to match what Unicode calls
790 an "extended grapheme cluster", which has a more complicated definition.
791 </P>
792 <P>
793 Matching characters by Unicode property is not fast, because PCRE has to search
794 a structure that contains data for over fifteen thousand characters. That is
795 why the traditional escape sequences such as \d and \w do not use Unicode
796 properties in PCRE by default, though you can make them do so by setting the
797 PCRE_UCP option or by starting the pattern with (*UCP).
798 <a name="extraprops"></a></P>
799 <br><b>
800 PCRE's additional properties
801 </b><br>
802 <P>
803 As well as the standard Unicode properties described in the previous
804 section, PCRE supports four more that make it possible to convert traditional
805 escape sequences such as \w and \s and POSIX character classes to use Unicode
806 properties. PCRE uses these non-standard, non-Perl properties internally when
807 PCRE_UCP is set. They are:
808 <pre>
809 Xan Any alphanumeric character
810 Xps Any POSIX space character
811 Xsp Any Perl space character
812 Xwd Any Perl "word" character
813 </pre>
814 Xan matches characters that have either the L (letter) or the N (number)
815 property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
816 carriage return, and any other character that has the Z (separator) property.
817 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
818 same characters as Xan, plus underscore.
819 <a name="resetmatchstart"></a></P>
820 <br><b>
821 Resetting the match start
822 </b><br>
823 <P>
824 The escape sequence \K causes any previously matched characters not to be
825 included in the final matched sequence. For example, the pattern:
826 <pre>
827 foo\Kbar
828 </pre>
829 matches "foobar", but reports that it has matched "bar". This feature is
830 similar to a lookbehind assertion
831 <a href="#lookbehind">(described below).</a>
832 However, in this case, the part of the subject before the real match does not
833 have to be of fixed length, as lookbehind assertions do. The use of \K does
834 not interfere with the setting of
835 <a href="#subpattern">captured substrings.</a>
836 For example, when the pattern
837 <pre>
838 (foo)\Kbar
839 </pre>
840 matches "foobar", the first substring is still set to "foo".
841 </P>
842 <P>
843 Perl documents that the use of \K within assertions is "not well defined". In
844 PCRE, \K is acted upon when it occurs inside positive assertions, but is
845 ignored in negative assertions.
846 <a name="smallassertions"></a></P>
847 <br><b>
848 Simple assertions
849 </b><br>
850 <P>
851 The final use of backslash is for certain simple assertions. An assertion
852 specifies a condition that has to be met at a particular point in a match,
853 without consuming any characters from the subject string. The use of
854 subpatterns for more complicated assertions is described
855 <a href="#bigassertions">below.</a>
856 The backslashed assertions are:
857 <pre>
858 \b matches at a word boundary
859 \B matches when not at a word boundary
860 \A matches at the start of the subject
861 \Z matches at the end of the subject
862 also matches before a newline at the end of the subject
863 \z matches only at the end of the subject
864 \G matches at the first matching position in the subject
865 </pre>
866 Inside a character class, \b has a different meaning; it matches the backspace
867 character. If any other of these assertions appears in a character class, by
868 default it matches the corresponding literal character (for example, \B
869 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
870 escape sequence" error is generated instead.
871 </P>
872 <P>
873 A word boundary is a position in the subject string where the current character
874 and the previous character do not both match \w or \W (i.e. one matches
875 \w and the other matches \W), or the start or end of the string if the
876 first or last character matches \w, respectively. In a UTF mode, the meanings
877 of \w and \W can be changed by setting the PCRE_UCP option. When this is
878 done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
879 of word" or "end of word" metasequence. However, whatever follows \b normally
880 determines which it is. For example, the fragment \ba matches "a" at the start
881 of a word.
882 </P>
883 <P>
884 The \A, \Z, and \z assertions differ from the traditional circumflex and
885 dollar (described in the next section) in that they only ever match at the very
886 start and end of the subject string, whatever options are set. Thus, they are
887 independent of multiline mode. These three assertions are not affected by the
888 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
889 circumflex and dollar metacharacters. However, if the <i>startoffset</i>
890 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
891 at a point other than the beginning of the subject, \A can never match. The
892 difference between \Z and \z is that \Z matches before a newline at the end
893 of the string as well as at the very end, whereas \z matches only at the end.
894 </P>
895 <P>
896 The \G assertion is true only when the current matching position is at the
897 start point of the match, as specified by the <i>startoffset</i> argument of
898 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
899 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
900 arguments, you can mimic Perl's /g option, and it is in this kind of
901 implementation where \G can be useful.
902 </P>
903 <P>
904 Note, however, that PCRE's interpretation of \G, as the start of the current
905 match, is subtly different from Perl's, which defines it as the end of the
906 previous match. In Perl, these can be different when the previously matched
907 string was empty. Because PCRE does just one match at a time, it cannot
908 reproduce this behaviour.
909 </P>
910 <P>
911 If all the alternatives of a pattern begin with \G, the expression is anchored
912 to the starting match position, and the "anchored" flag is set in the compiled
913 regular expression.
914 </P>
915 <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
916 <P>
917 Outside a character class, in the default matching mode, the circumflex
918 character is an assertion that is true only if the current matching point is
919 at the start of the subject string. If the <i>startoffset</i> argument of
920 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
921 option is unset. Inside a character class, circumflex has an entirely different
922 meaning
923 <a href="#characterclass">(see below).</a>
924 </P>
925 <P>
926 Circumflex need not be the first character of the pattern if a number of
927 alternatives are involved, but it should be the first thing in each alternative
928 in which it appears if the pattern is ever to match that branch. If all
929 possible alternatives start with a circumflex, that is, if the pattern is
930 constrained to match only at the start of the subject, it is said to be an
931 "anchored" pattern. (There are also other constructs that can cause a pattern
932 to be anchored.)
933 </P>
934 <P>
935 A dollar character is an assertion that is true only if the current matching
936 point is at the end of the subject string, or immediately before a newline
937 at the end of the string (by default). Dollar need not be the last character of
938 the pattern if a number of alternatives are involved, but it should be the last
939 item in any branch in which it appears. Dollar has no special meaning in a
940 character class.
941 </P>
942 <P>
943 The meaning of dollar can be changed so that it matches only at the very end of
944 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
945 does not affect the \Z assertion.
946 </P>
947 <P>
948 The meanings of the circumflex and dollar characters are changed if the
949 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
950 immediately after internal newlines as well as at the start of the subject
951 string. It does not match after a newline that ends the string. A dollar
952 matches before any newlines in the string, as well as at the very end, when
953 PCRE_MULTILINE is set. When newline is specified as the two-character
954 sequence CRLF, isolated CR and LF characters do not indicate newlines.
955 </P>
956 <P>
957 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
958 \n represents a newline) in multiline mode, but not otherwise. Consequently,
959 patterns that are anchored in single line mode because all branches start with
960 ^ are not anchored in multiline mode, and a match for circumflex is possible
961 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
962 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
963 </P>
964 <P>
965 Note that the sequences \A, \Z, and \z can be used to match the start and
966 end of the subject in both modes, and if all branches of a pattern start with
967 \A it is always anchored, whether or not PCRE_MULTILINE is set.
968 <a name="fullstopdot"></a></P>
969 <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
970 <P>
971 Outside a character class, a dot in the pattern matches any one character in
972 the subject string except (by default) a character that signifies the end of a
973 line.
974 </P>
975 <P>
976 When a line ending is defined as a single character, dot never matches that
977 character; when the two-character sequence CRLF is used, dot does not match CR
978 if it is immediately followed by LF, but otherwise it matches all characters
979 (including isolated CRs and LFs). When any Unicode line endings are being
980 recognized, dot does not match CR or LF or any of the other line ending
981 characters.
982 </P>
983 <P>
984 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
985 option is set, a dot matches any one character, without exception. If the
986 two-character sequence CRLF is present in the subject string, it takes two dots
987 to match it.
988 </P>
989 <P>
990 The handling of dot is entirely independent of the handling of circumflex and
991 dollar, the only relationship being that they both involve newlines. Dot has no
992 special meaning in a character class.
993 </P>
994 <P>
995 The escape sequence \N behaves like a dot, except that it is not affected by
996 the PCRE_DOTALL option. In other words, it matches any character except one
997 that signifies the end of a line. Perl also uses \N to match characters by
998 name; PCRE does not support this.
999 </P>
1000 <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br>
1001 <P>
1002 Outside a character class, the escape sequence \C matches any one data unit,
1003 whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1004 byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \C always
1005 matches line-ending characters. The feature is provided in Perl in order to
1006 match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1007 used. Because \C breaks up characters into individual data units, matching one
1008 unit with \C in a UTF mode means that the rest of the string may start with a
1009 malformed UTF character. This has undefined results, because PCRE assumes that
1010 it is dealing with valid UTF strings (and by default it checks this at the
1011 start of processing unless the PCRE_NO_UTF8_CHECK option is used).
1012 </P>
1013 <P>
1014 PCRE does not allow \C to appear in lookbehind assertions
1015 <a href="#lookbehind">(described below)</a>
1016 in a UTF mode, because this would make it impossible to calculate the length of
1017 the lookbehind.
1018 </P>
1019 <P>
1020 In general, the \C escape sequence is best avoided. However, one
1021 way of using it that avoids the problem of malformed UTF characters is to use a
1022 lookahead to check the length of the next character, as in this pattern, which
1023 could be used with a UTF-8 string (ignore white space and line breaks):
1024 <pre>
1025 (?| (?=[\x00-\x7f])(\C) |
1026 (?=[\x80-\x{7ff}])(\C)(\C) |
1027 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
1028 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
1029 </pre>
1030 A group that starts with (?| resets the capturing parentheses numbers in each
1031 alternative (see
1032 <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
1033 below). The assertions at the start of each branch check the next UTF-8
1034 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1035 character's individual bytes are then captured by the appropriate number of
1036 groups.
1037 <a name="characterclass"></a></P>
1038 <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
1039 <P>
1040 An opening square bracket introduces a character class, terminated by a closing
1041 square bracket. A closing square bracket on its own is not special by default.
1042 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1043 bracket causes a compile-time error. If a closing square bracket is required as
1044 a member of the class, it should be the first data character in the class
1045 (after an initial circumflex, if present) or escaped with a backslash.
1046 </P>
1047 <P>
1048 A character class matches a single character in the subject. In a UTF mode, the
1049 character may be more than one data unit long. A matched character must be in
1050 the set of characters defined by the class, unless the first character in the
1051 class definition is a circumflex, in which case the subject character must not
1052 be in the set defined by the class. If a circumflex is actually required as a
1053 member of the class, ensure it is not the first character, or escape it with a
1054 backslash.
1055 </P>
1056 <P>
1057 For example, the character class [aeiou] matches any lower case vowel, while
1058 [^aeiou] matches any character that is not a lower case vowel. Note that a
1059 circumflex is just a convenient notation for specifying the characters that
1060 are in the class by enumerating those that are not. A class that starts with a
1061 circumflex is not an assertion; it still consumes a character from the subject
1062 string, and therefore it fails if the current pointer is at the end of the
1063 string.
1064 </P>
1065 <P>
1066 In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
1067 included in a class as a literal string of data units, or by using the \x{
1068 escaping mechanism.
1069 </P>
1070 <P>
1071 When caseless matching is set, any letters in a class represent both their
1072 upper case and lower case versions, so for example, a caseless [aeiou] matches
1073 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1074 caseful version would. In a UTF mode, PCRE always understands the concept of
1075 case for characters whose values are less than 128, so caseless matching is
1076 always possible. For characters with higher values, the concept of case is
1077 supported if PCRE is compiled with Unicode property support, but not otherwise.
1078 If you want to use caseless matching in a UTF mode for characters 128 and
1079 above, you must ensure that PCRE is compiled with Unicode property support as
1080 well as with UTF support.
1081 </P>
1082 <P>
1083 Characters that might indicate line breaks are never treated in any special way
1084 when matching character classes, whatever line-ending sequence is in use, and
1085 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1086 such as [^a] always matches one of these characters.
1087 </P>
1088 <P>
1089 The minus (hyphen) character can be used to specify a range of characters in a
1090 character class. For example, [d-m] matches any letter between d and m,
1091 inclusive. If a minus character is required in a class, it must be escaped with
1092 a backslash or appear in a position where it cannot be interpreted as
1093 indicating a range, typically as the first or last character in the class.
1094 </P>
1095 <P>
1096 It is not possible to have the literal character "]" as the end character of a
1097 range. A pattern such as [W-]46] is interpreted as a class of two characters
1098 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1099 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1100 the end of range, so [W-\]46] is interpreted as a class containing a range
1101 followed by two other characters. The octal or hexadecimal representation of
1102 "]" can also be used to end a range.
1103 </P>
1104 <P>
1105 Ranges operate in the collating sequence of character values. They can also be
1106 used for characters specified numerically, for example [\000-\037]. Ranges
1107 can include any characters that are valid for the current mode.
1108 </P>
1109 <P>
1110 If a range that includes letters is used when caseless matching is set, it
1111 matches the letters in either case. For example, [W-c] is equivalent to
1112 [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1113 tables for a French locale are in use, [\xc8-\xcb] matches accented E
1114 characters in both cases. In UTF modes, PCRE supports the concept of case for
1115 characters with values greater than 128 only when it is compiled with Unicode
1116 property support.
1117 </P>
1118 <P>
1119 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
1120 \V, \w, and \W may appear in a character class, and add the characters that
1121 they match to the class. For example, [\dABCDEF] matches any hexadecimal
1122 digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w
1123 and their upper case partners, just as it does when they appear outside a
1124 character class, as described in the section entitled
1125 <a href="#genericchartypes">"Generic character types"</a>
1126 above. The escape sequence \b has a different meaning inside a character
1127 class; it matches the backspace character. The sequences \B, \N, \R, and \X
1128 are not special inside a character class. Like any other unrecognized escape
1129 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1130 default, but cause an error if the PCRE_EXTRA option is set.
1131 </P>
1132 <P>
1133 A circumflex can conveniently be used with the upper case character types to
1134 specify a more restricted set of characters than the matching lower case type.
1135 For example, the class [^\W_] matches any letter or digit, but not underscore,
1136 whereas [\w] includes underscore. A positive character class should be read as
1137 "something OR something OR ..." and a negative class as "NOT something AND NOT
1138 something AND NOT ...".
1139 </P>
1140 <P>
1141 The only metacharacters that are recognized in character classes are backslash,
1142 hyphen (only where it can be interpreted as specifying a range), circumflex
1143 (only at the start), opening square bracket (only when it can be interpreted as
1144 introducing a POSIX class name - see the next section), and the terminating
1145 closing square bracket. However, escaping other non-alphanumeric characters
1146 does no harm.
1147 </P>
1148 <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
1149 <P>
1150 Perl supports the POSIX notation for character classes. This uses names
1151 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1152 this notation. For example,
1153 <pre>
1154 [01[:alpha:]%]
1155 </pre>
1156 matches "0", "1", any alphabetic character, or "%". The supported class names
1157 are:
1158 <pre>
1159 alnum letters and digits
1160 alpha letters
1161 ascii character codes 0 - 127
1162 blank space or tab only
1163 cntrl control characters
1164 digit decimal digits (same as \d)
1165 graph printing characters, excluding space
1166 lower lower case letters
1167 print printing characters, including space
1168 punct printing characters, excluding letters and digits and space
1169 space white space (not quite the same as \s)
1170 upper upper case letters
1171 word "word" characters (same as \w)
1172 xdigit hexadecimal digits
1173 </pre>
1174 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1175 space (32). Notice that this list includes the VT character (code 11). This
1176 makes "space" different to \s, which does not include VT (for Perl
1177 compatibility).
1178 </P>
1179 <P>
1180 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1181 5.8. Another Perl extension is negation, which is indicated by a ^ character
1182 after the colon. For example,
1183 <pre>
1184 [12[:^digit:]]
1185 </pre>
1186 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1187 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1188 supported, and an error is given if they are encountered.
1189 </P>
1190 <P>
1191 By default, in UTF modes, characters with values greater than 128 do not match
1192 any of the POSIX character classes. However, if the PCRE_UCP option is passed
1193 to <b>pcre_compile()</b>, some of the classes are changed so that Unicode
1194 character properties are used. This is achieved by replacing the POSIX classes
1195 by other sequences, as follows:
1196 <pre>
1197 [:alnum:] becomes \p{Xan}
1198 [:alpha:] becomes \p{L}
1199 [:blank:] becomes \h
1200 [:digit:] becomes \p{Nd}
1201 [:lower:] becomes \p{Ll}
1202 [:space:] becomes \p{Xps}
1203 [:upper:] becomes \p{Lu}
1204 [:word:] becomes \p{Xwd}
1205 </pre>
1206 Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX
1207 classes are unchanged, and match only characters with code points less than
1208 128.
1209 </P>
1210 <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
1211 <P>
1212 Vertical bar characters are used to separate alternative patterns. For example,
1213 the pattern
1214 <pre>
1215 gilbert|sullivan
1216 </pre>
1217 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1218 and an empty alternative is permitted (matching the empty string). The matching
1219 process tries each alternative in turn, from left to right, and the first one
1220 that succeeds is used. If the alternatives are within a subpattern
1221 <a href="#subpattern">(defined below),</a>
1222 "succeeds" means matching the rest of the main pattern as well as the
1223 alternative in the subpattern.
1224 </P>
1225 <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1226 <P>
1227 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1228 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1229 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1230 The option letters are
1231 <pre>
1232 i for PCRE_CASELESS
1234 s for PCRE_DOTALL
1235 x for PCRE_EXTENDED
1236 </pre>
1237 For example, (?im) sets caseless, multiline matching. It is also possible to
1238 unset these options by preceding the letter with a hyphen, and a combined
1239 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1240 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1241 permitted. If a letter appears both before and after the hyphen, the option is
1242 unset.
1243 </P>
1244 <P>
1245 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1246 changed in the same way as the Perl-compatible options by using the characters
1247 J, U and X respectively.
1248 </P>
1249 <P>
1250 When one of these option changes occurs at top level (that is, not inside
1251 subpattern parentheses), the change applies to the remainder of the pattern
1252 that follows. If the change is placed right at the start of a pattern, PCRE
1253 extracts it into the global options (and it will therefore show up in data
1254 extracted by the <b>pcre_fullinfo()</b> function).
1255 </P>
1256 <P>
1257 An option change within a subpattern (see below for a description of
1258 subpatterns) affects only that part of the subpattern that follows it, so
1259 <pre>
1260 (a(?i)b)c
1261 </pre>
1262 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1263 By this means, options can be made to have different settings in different
1264 parts of the pattern. Any changes made in one alternative do carry on
1265 into subsequent branches within the same subpattern. For example,
1266 <pre>
1267 (a(?i)b|c)
1268 </pre>
1269 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1270 branch is abandoned before the option setting. This is because the effects of
1271 option settings happen at compile time. There would be some very weird
1272 behaviour otherwise.
1273 </P>
1274 <P>
1275 <b>Note:</b> There are other PCRE-specific options that can be set by the
1276 application when the compiling or matching functions are called. In some cases
1277 the pattern can contain special leading sequences such as (*CRLF) to override
1278 what the application has set or what has been defaulted. Details are given in
1279 the section entitled
1280 <a href="#newlineseq">"Newline sequences"</a>
1281 above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1282 can be used to set UTF and Unicode property modes; they are equivalent to
1283 setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1284 <a name="subpattern"></a></P>
1285 <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1286 <P>
1287 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1288 Turning part of a pattern into a subpattern does two things:
1289 <br>
1290 <br>
1291 1. It localizes a set of alternatives. For example, the pattern
1292 <pre>
1293 cat(aract|erpillar|)
1294 </pre>
1295 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1296 match "cataract", "erpillar" or an empty string.
1297 <br>
1298 <br>
1299 2. It sets up the subpattern as a capturing subpattern. This means that, when
1300 the whole pattern matches, that portion of the subject string that matched the
1301 subpattern is passed back to the caller via the <i>ovector</i> argument of the
1302 matching function. (This applies only to the traditional matching functions;
1303 the DFA matching functions do not support capturing.)
1304 </P>
1305 <P>
1306 Opening parentheses are counted from left to right (starting from 1) to obtain
1307 numbers for the capturing subpatterns. For example, if the string "the red
1308 king" is matched against the pattern
1309 <pre>
1310 the ((red|white) (king|queen))
1311 </pre>
1312 the captured substrings are "red king", "red", and "king", and are numbered 1,
1313 2, and 3, respectively.
1314 </P>
1315 <P>
1316 The fact that plain parentheses fulfil two functions is not always helpful.
1317 There are often times when a grouping subpattern is required without a
1318 capturing requirement. If an opening parenthesis is followed by a question mark
1319 and a colon, the subpattern does not do any capturing, and is not counted when
1320 computing the number of any subsequent capturing subpatterns. For example, if
1321 the string "the white queen" is matched against the pattern
1322 <pre>
1323 the ((?:red|white) (king|queen))
1324 </pre>
1325 the captured substrings are "white queen" and "queen", and are numbered 1 and
1326 2. The maximum number of capturing subpatterns is 65535.
1327 </P>
1328 <P>
1329 As a convenient shorthand, if any option settings are required at the start of
1330 a non-capturing subpattern, the option letters may appear between the "?" and
1331 the ":". Thus the two patterns
1332 <pre>
1333 (?i:saturday|sunday)
1334 (?:(?i)saturday|sunday)
1335 </pre>
1336 match exactly the same set of strings. Because alternative branches are tried
1337 from left to right, and options are not reset until the end of the subpattern
1338 is reached, an option setting in one branch does affect subsequent branches, so
1339 the above patterns match "SUNDAY" as well as "Saturday".
1340 <a name="dupsubpatternnumber"></a></P>
1341 <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1342 <P>
1343 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1344 the same numbers for its capturing parentheses. Such a subpattern starts with
1345 (?| and is itself a non-capturing subpattern. For example, consider this
1346 pattern:
1347 <pre>
1348 (?|(Sat)ur|(Sun))day
1349 </pre>
1350 Because the two alternatives are inside a (?| group, both sets of capturing
1351 parentheses are numbered one. Thus, when the pattern matches, you can look
1352 at captured substring number one, whichever alternative matched. This construct
1353 is useful when you want to capture part, but not all, of one of a number of
1354 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1355 number is reset at the start of each branch. The numbers of any capturing
1356 parentheses that follow the subpattern start after the highest number used in
1357 any branch. The following example is taken from the Perl documentation. The
1358 numbers underneath show in which buffer the captured content will be stored.
1359 <pre>
1360 # before ---------------branch-reset----------- after
1361 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1362 # 1 2 2 3 2 3 4
1363 </pre>
1364 A back reference to a numbered subpattern uses the most recent value that is
1365 set for that number by any subpattern. The following pattern matches "abcabc"
1366 or "defdef":
1367 <pre>
1368 /(?|(abc)|(def))\1/
1369 </pre>
1370 In contrast, a subroutine call to a numbered subpattern always refers to the
1371 first one in the pattern with the given number. The following pattern matches
1372 "abcabc" or "defabc":
1373 <pre>
1374 /(?|(abc)|(def))(?1)/
1375 </pre>
1376 If a
1377 <a href="#conditions">condition test</a>
1378 for a subpattern's having matched refers to a non-unique number, the test is
1379 true if any of the subpatterns of that number have matched.
1380 </P>
1381 <P>
1382 An alternative approach to using this "branch reset" feature is to use
1383 duplicate named subpatterns, as described in the next section.
1384 </P>
1385 <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1386 <P>
1387 Identifying capturing parentheses by number is simple, but it can be very hard
1388 to keep track of the numbers in complicated regular expressions. Furthermore,
1389 if an expression is modified, the numbers may change. To help with this
1390 difficulty, PCRE supports the naming of subpatterns. This feature was not
1391 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1392 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1393 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1394 have different names, but PCRE does not.
1395 </P>
1396 <P>
1397 In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1398 (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1399 parentheses from other parts of the pattern, such as
1400 <a href="#backreferences">back references,</a>
1401 <a href="#recursion">recursion,</a>
1402 and
1403 <a href="#conditions">conditions,</a>
1404 can be made by name as well as by number.
1405 </P>
1406 <P>
1407 Names consist of up to 32 alphanumeric characters and underscores. Named
1408 capturing parentheses are still allocated numbers as well as names, exactly as
1409 if the names were not present. The PCRE API provides function calls for
1410 extracting the name-to-number translation table from a compiled pattern. There
1411 is also a convenience function for extracting a captured substring by name.
1412 </P>
1413 <P>
1414 By default, a name must be unique within a pattern, but it is possible to relax
1415 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1416 names are also always permitted for subpatterns with the same number, set up as
1417 described in the previous section.) Duplicate names can be useful for patterns
1418 where only one instance of the named parentheses can match. Suppose you want to
1419 match the name of a weekday, either as a 3-letter abbreviation or as the full
1420 name, and in both cases you want to extract the abbreviation. This pattern
1421 (ignoring the line breaks) does the job:
1422 <pre>
1423 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1424 (?&#60;DN&#62;Tue)(?:sday)?|
1425 (?&#60;DN&#62;Wed)(?:nesday)?|
1426 (?&#60;DN&#62;Thu)(?:rsday)?|
1427 (?&#60;DN&#62;Sat)(?:urday)?
1428 </pre>
1429 There are five capturing substrings, but only one is ever set after a match.
1430 (An alternative way of solving this problem is to use a "branch reset"
1431 subpattern, as described in the previous section.)
1432 </P>
1433 <P>
1434 The convenience function for extracting the data by name returns the substring
1435 for the first (and in this example, the only) subpattern of that name that
1436 matched. This saves searching to find which numbered subpattern it was.
1437 </P>
1438 <P>
1439 If you make a back reference to a non-unique named subpattern from elsewhere in
1440 the pattern, the one that corresponds to the first occurrence of the name is
1441 used. In the absence of duplicate numbers (see the previous section) this is
1442 the one with the lowest number. If you use a named reference in a condition
1443 test (see the
1444 <a href="#conditions">section about conditions</a>
1445 below), either to check whether a subpattern has matched, or to check for
1446 recursion, all subpatterns with the same name are tested. If the condition is
1447 true for any one of them, the overall condition is true. This is the same
1448 behaviour as testing by number. For further details of the interfaces for
1449 handling named subpatterns, see the
1450 <a href="pcreapi.html"><b>pcreapi</b></a>
1451 documentation.
1452 </P>
1453 <P>
1454 <b>Warning:</b> You cannot use different names to distinguish between two
1455 subpatterns with the same number because PCRE uses only the numbers when
1456 matching. For this reason, an error is given at compile time if different names
1457 are given to subpatterns with the same number. However, you can give the same
1458 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1459 </P>
1460 <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1461 <P>
1462 Repetition is specified by quantifiers, which can follow any of the following
1463 items:
1464 <pre>
1465 a literal data character
1466 the dot metacharacter
1467 the \C escape sequence
1468 the \X escape sequence
1469 the \R escape sequence
1470 an escape such as \d or \pL that matches a single character
1471 a character class
1472 a back reference (see next section)
1473 a parenthesized subpattern (including assertions)
1474 a subroutine call to a subpattern (recursive or otherwise)
1475 </pre>
1476 The general repetition quantifier specifies a minimum and maximum number of
1477 permitted matches, by giving the two numbers in curly brackets (braces),
1478 separated by a comma. The numbers must be less than 65536, and the first must
1479 be less than or equal to the second. For example:
1480 <pre>
1481 z{2,4}
1482 </pre>
1483 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1484 character. If the second number is omitted, but the comma is present, there is
1485 no upper limit; if the second number and the comma are both omitted, the
1486 quantifier specifies an exact number of required matches. Thus
1487 <pre>
1488 [aeiou]{3,}
1489 </pre>
1490 matches at least 3 successive vowels, but may match many more, while
1491 <pre>
1492 \d{8}
1493 </pre>
1494 matches exactly 8 digits. An opening curly bracket that appears in a position
1495 where a quantifier is not allowed, or one that does not match the syntax of a
1496 quantifier, is taken as a literal character. For example, {,6} is not a
1497 quantifier, but a literal string of four characters.
1498 </P>
1499 <P>
1500 In UTF modes, quantifiers apply to characters rather than to individual data
1501 units. Thus, for example, \x{100}{2} matches two characters, each of
1502 which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1503 \X{3} matches three Unicode extended sequences, each of which may be several
1504 data units long (and they may be of different lengths).
1505 </P>
1506 <P>
1507 The quantifier {0} is permitted, causing the expression to behave as if the
1508 previous item and the quantifier were not present. This may be useful for
1509 subpatterns that are referenced as
1510 <a href="#subpatternsassubroutines">subroutines</a>
1511 from elsewhere in the pattern (but see also the section entitled
1512 <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1513 below). Items other than subpatterns that have a {0} quantifier are omitted
1514 from the compiled pattern.
1515 </P>
1516 <P>
1517 For convenience, the three most common quantifiers have single-character
1518 abbreviations:
1519 <pre>
1520 * is equivalent to {0,}
1521 + is equivalent to {1,}
1522 ? is equivalent to {0,1}
1523 </pre>
1524 It is possible to construct infinite loops by following a subpattern that can
1525 match no characters with a quantifier that has no upper limit, for example:
1526 <pre>
1527 (a?)*
1528 </pre>
1529 Earlier versions of Perl and PCRE used to give an error at compile time for
1530 such patterns. However, because there are cases where this can be useful, such
1531 patterns are now accepted, but if any repetition of the subpattern does in fact
1532 match no characters, the loop is forcibly broken.
1533 </P>
1534 <P>
1535 By default, the quantifiers are "greedy", that is, they match as much as
1536 possible (up to the maximum number of permitted times), without causing the
1537 rest of the pattern to fail. The classic example of where this gives problems
1538 is in trying to match comments in C programs. These appear between /* and */
1539 and within the comment, individual * and / characters may appear. An attempt to
1540 match C comments by applying the pattern
1541 <pre>
1542 /\*.*\*/
1543 </pre>
1544 to the string
1545 <pre>
1546 /* first comment */ not comment /* second comment */
1547 </pre>
1548 fails, because it matches the entire string owing to the greediness of the .*
1549 item.
1550 </P>
1551 <P>
1552 However, if a quantifier is followed by a question mark, it ceases to be
1553 greedy, and instead matches the minimum number of times possible, so the
1554 pattern
1555 <pre>
1556 /\*.*?\*/
1557 </pre>
1558 does the right thing with the C comments. The meaning of the various
1559 quantifiers is not otherwise changed, just the preferred number of matches.
1560 Do not confuse this use of question mark with its use as a quantifier in its
1561 own right. Because it has two uses, it can sometimes appear doubled, as in
1562 <pre>
1563 \d??\d
1564 </pre>
1565 which matches one digit by preference, but can match two if that is the only
1566 way the rest of the pattern matches.
1567 </P>
1568 <P>
1569 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1570 the quantifiers are not greedy by default, but individual ones can be made
1571 greedy by following them with a question mark. In other words, it inverts the
1572 default behaviour.
1573 </P>
1574 <P>
1575 When a parenthesized subpattern is quantified with a minimum repeat count that
1576 is greater than 1 or with a limited maximum, more memory is required for the
1577 compiled pattern, in proportion to the size of the minimum or maximum.
1578 </P>
1579 <P>
1580 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1581 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1582 implicitly anchored, because whatever follows will be tried against every
1583 character position in the subject string, so there is no point in retrying the
1584 overall match at any position after the first. PCRE normally treats such a
1585 pattern as though it were preceded by \A.
1586 </P>
1587 <P>
1588 In cases where it is known that the subject string contains no newlines, it is
1589 worth setting PCRE_DOTALL in order to obtain this optimization, or
1590 alternatively using ^ to indicate anchoring explicitly.
1591 </P>
1592 <P>
1593 However, there is one situation where the optimization cannot be used. When .*
1594 is inside capturing parentheses that are the subject of a back reference
1595 elsewhere in the pattern, a match at the start may fail where a later one
1596 succeeds. Consider, for example:
1597 <pre>
1598 (.*)abc\1
1599 </pre>
1600 If the subject is "xyz123abc123" the match point is the fourth character. For
1601 this reason, such a pattern is not implicitly anchored.
1602 </P>
1603 <P>
1604 When a capturing subpattern is repeated, the value captured is the substring
1605 that matched the final iteration. For example, after
1606 <pre>
1607 (tweedle[dume]{3}\s*)+
1608 </pre>
1609 has matched "tweedledum tweedledee" the value of the captured substring is
1610 "tweedledee". However, if there are nested capturing subpatterns, the
1611 corresponding captured values may have been set in previous iterations. For
1612 example, after
1613 <pre>
1614 /(a|(b))+/
1615 </pre>
1616 matches "aba" the value of the second captured substring is "b".
1617 <a name="atomicgroup"></a></P>
1618 <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1619 <P>
1620 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1621 repetition, failure of what follows normally causes the repeated item to be
1622 re-evaluated to see if a different number of repeats allows the rest of the
1623 pattern to match. Sometimes it is useful to prevent this, either to change the
1624 nature of the match, or to cause it fail earlier than it otherwise might, when
1625 the author of the pattern knows there is no point in carrying on.
1626 </P>
1627 <P>
1628 Consider, for example, the pattern \d+foo when applied to the subject line
1629 <pre>
1630 123456bar
1631 </pre>
1632 After matching all 6 digits and then failing to match "foo", the normal
1633 action of the matcher is to try again with only 5 digits matching the \d+
1634 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1635 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1636 that once a subpattern has matched, it is not to be re-evaluated in this way.
1637 </P>
1638 <P>
1639 If we use atomic grouping for the previous example, the matcher gives up
1640 immediately on failing to match "foo" the first time. The notation is a kind of
1641 special parenthesis, starting with (?&#62; as in this example:
1642 <pre>
1643 (?&#62;\d+)foo
1644 </pre>
1645 This kind of parenthesis "locks up" the part of the pattern it contains once
1646 it has matched, and a failure further into the pattern is prevented from
1647 backtracking into it. Backtracking past it to previous items, however, works as
1648 normal.
1649 </P>
1650 <P>
1651 An alternative description is that a subpattern of this type matches the string
1652 of characters that an identical standalone pattern would match, if anchored at
1653 the current point in the subject string.
1654 </P>
1655 <P>
1656 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1657 the above example can be thought of as a maximizing repeat that must swallow
1658 everything it can. So, while both \d+ and \d+? are prepared to adjust the
1659 number of digits they match in order to make the rest of the pattern match,
1660 (?&#62;\d+) can only match an entire sequence of digits.
1661 </P>
1662 <P>
1663 Atomic groups in general can of course contain arbitrarily complicated
1664 subpatterns, and can be nested. However, when the subpattern for an atomic
1665 group is just a single repeated item, as in the example above, a simpler
1666 notation, called a "possessive quantifier" can be used. This consists of an
1667 additional + character following a quantifier. Using this notation, the
1668 previous example can be rewritten as
1669 <pre>
1670 \d++foo
1671 </pre>
1672 Note that a possessive quantifier can be used with an entire group, for
1673 example:
1674 <pre>
1675 (abc|xyz){2,3}+
1676 </pre>
1677 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1678 option is ignored. They are a convenient notation for the simpler forms of
1679 atomic group. However, there is no difference in the meaning of a possessive
1680 quantifier and the equivalent atomic group, though there may be a performance
1681 difference; possessive quantifiers should be slightly faster.
1682 </P>
1683 <P>
1684 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1685 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1686 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1687 package, and PCRE copied it from there. It ultimately found its way into Perl
1688 at release 5.10.
1689 </P>
1690 <P>
1691 PCRE has an optimization that automatically "possessifies" certain simple
1692 pattern constructs. For example, the sequence A+B is treated as A++B because
1693 there is no point in backtracking into a sequence of A's when B must follow.
1694 </P>
1695 <P>
1696 When a pattern contains an unlimited repeat inside a subpattern that can itself
1697 be repeated an unlimited number of times, the use of an atomic group is the
1698 only way to avoid some failing matches taking a very long time indeed. The
1699 pattern
1700 <pre>
1701 (\D+|&#60;\d+&#62;)*[!?]
1702 </pre>
1703 matches an unlimited number of substrings that either consist of non-digits, or
1704 digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1705 quickly. However, if it is applied to
1706 <pre>
1707 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1708 </pre>
1709 it takes a long time before reporting failure. This is because the string can
1710 be divided between the internal \D+ repeat and the external * repeat in a
1711 large number of ways, and all have to be tried. (The example uses [!?] rather
1712 than a single character at the end, because both PCRE and Perl have an
1713 optimization that allows for fast failure when a single character is used. They
1714 remember the last single character that is required for a match, and fail early
1715 if it is not present in the string.) If the pattern is changed so that it uses
1716 an atomic group, like this:
1717 <pre>
1718 ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1719 </pre>
1720 sequences of non-digits cannot be broken, and failure happens quickly.
1721 <a name="backreferences"></a></P>
1722 <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1723 <P>
1724 Outside a character class, a backslash followed by a digit greater than 0 (and
1725 possibly further digits) is a back reference to a capturing subpattern earlier
1726 (that is, to its left) in the pattern, provided there have been that many
1727 previous capturing left parentheses.
1728 </P>
1729 <P>
1730 However, if the decimal number following the backslash is less than 10, it is
1731 always taken as a back reference, and causes an error only if there are not
1732 that many capturing left parentheses in the entire pattern. In other words, the
1733 parentheses that are referenced need not be to the left of the reference for
1734 numbers less than 10. A "forward back reference" of this type can make sense
1735 when a repetition is involved and the subpattern to the right has participated
1736 in an earlier iteration.
1737 </P>
1738 <P>
1739 It is not possible to have a numerical "forward back reference" to a subpattern
1740 whose number is 10 or more using this syntax because a sequence such as \50 is
1741 interpreted as a character defined in octal. See the subsection entitled
1742 "Non-printing characters"
1743 <a href="#digitsafterbackslash">above</a>
1744 for further details of the handling of digits following a backslash. There is
1745 no such problem when named parentheses are used. A back reference to any
1746 subpattern is possible using named parentheses (see below).
1747 </P>
1748 <P>
1749 Another way of avoiding the ambiguity inherent in the use of digits following a
1750 backslash is to use the \g escape sequence. This escape must be followed by an
1751 unsigned number or a negative number, optionally enclosed in braces. These
1752 examples are all identical:
1753 <pre>
1754 (ring), \1
1755 (ring), \g1
1756 (ring), \g{1}
1757 </pre>
1758 An unsigned number specifies an absolute reference without the ambiguity that
1759 is present in the older syntax. It is also useful when literal digits follow
1760 the reference. A negative number is a relative reference. Consider this
1761 example:
1762 <pre>
1763 (abc(def)ghi)\g{-1}
1764 </pre>
1765 The sequence \g{-1} is a reference to the most recently started capturing
1766 subpattern before \g, that is, is it equivalent to \2 in this example.
1767 Similarly, \g{-2} would be equivalent to \1. The use of relative references
1768 can be helpful in long patterns, and also in patterns that are created by
1769 joining together fragments that contain references within themselves.
1770 </P>
1771 <P>
1772 A back reference matches whatever actually matched the capturing subpattern in
1773 the current subject string, rather than anything matching the subpattern
1774 itself (see
1775 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1776 below for a way of doing that). So the pattern
1777 <pre>
1778 (sens|respons)e and \1ibility
1779 </pre>
1780 matches "sense and sensibility" and "response and responsibility", but not
1781 "sense and responsibility". If caseful matching is in force at the time of the
1782 back reference, the case of letters is relevant. For example,
1783 <pre>
1784 ((?i)rah)\s+\1
1785 </pre>
1786 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1787 capturing subpattern is matched caselessly.
1788 </P>
1789 <P>
1790 There are several different ways of writing back references to named
1791 subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1792 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1793 back reference syntax, in which \g can be used for both numeric and named
1794 references, is also supported. We could rewrite the above example in any of
1795 the following ways:
1796 <pre>
1797 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1798 (?'p1'(?i)rah)\s+\k{p1}
1799 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1800 (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1801 </pre>
1802 A subpattern that is referenced by name may appear in the pattern before or
1803 after the reference.
1804 </P>
1805 <P>
1806 There may be more than one back reference to the same subpattern. If a
1807 subpattern has not actually been used in a particular match, any back
1808 references to it always fail by default. For example, the pattern
1809 <pre>
1810 (a|(bc))\2
1811 </pre>
1812 always fails if it starts to match "a" rather than "bc". However, if the
1813 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1814 unset value matches an empty string.
1815 </P>
1816 <P>
1817 Because there may be many capturing parentheses in a pattern, all digits
1818 following a backslash are taken as part of a potential back reference number.
1819 If the pattern continues with a digit character, some delimiter must be used to
1820 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1821 whitespace. Otherwise, the \g{ syntax or an empty comment (see
1822 <a href="#comments">"Comments"</a>
1823 below) can be used.
1824 </P>
1825 <br><b>
1826 Recursive back references
1827 </b><br>
1828 <P>
1829 A back reference that occurs inside the parentheses to which it refers fails
1830 when the subpattern is first used, so, for example, (a\1) never matches.
1831 However, such references can be useful inside repeated subpatterns. For
1832 example, the pattern
1833 <pre>
1834 (a|b\1)+
1835 </pre>
1836 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1837 the subpattern, the back reference matches the character string corresponding
1838 to the previous iteration. In order for this to work, the pattern must be such
1839 that the first iteration does not need to match the back reference. This can be
1840 done using alternation, as in the example above, or by a quantifier with a
1841 minimum of zero.
1842 </P>
1843 <P>
1844 Back references of this type cause the group that they reference to be treated
1845 as an
1846 <a href="#atomicgroup">atomic group.</a>
1847 Once the whole group has been matched, a subsequent matching failure cannot
1848 cause backtracking into the middle of the group.
1849 <a name="bigassertions"></a></P>
1850 <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1851 <P>
1852 An assertion is a test on the characters following or preceding the current
1853 matching point that does not actually consume any characters. The simple
1854 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1855 <a href="#smallassertions">above.</a>
1856 </P>
1857 <P>
1858 More complicated assertions are coded as subpatterns. There are two kinds:
1859 those that look ahead of the current position in the subject string, and those
1860 that look behind it. An assertion subpattern is matched in the normal way,
1861 except that it does not cause the current matching position to be changed.
1862 </P>
1863 <P>
1864 Assertion subpatterns are not capturing subpatterns. If such an assertion
1865 contains capturing subpatterns within it, these are counted for the purposes of
1866 numbering the capturing subpatterns in the whole pattern. However, substring
1867 capturing is carried out only for positive assertions, because it does not make
1868 sense for negative assertions.
1869 </P>
1870 <P>
1871 For compatibility with Perl, assertion subpatterns may be repeated; though
1872 it makes no sense to assert the same thing several times, the side effect of
1873 capturing parentheses may occasionally be useful. In practice, there only three
1874 cases:
1875 <br>
1876 <br>
1877 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1878 However, it may contain internal capturing parenthesized groups that are called
1879 from elsewhere via the
1880 <a href="#subpatternsassubroutines">subroutine mechanism.</a>
1881 <br>
1882 <br>
1883 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1884 were {0,1}. At run time, the rest of the pattern match is tried with and
1885 without the assertion, the order depending on the greediness of the quantifier.
1886 <br>
1887 <br>
1888 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1889 The assertion is obeyed just once when encountered during matching.
1890 </P>
1891 <br><b>
1892 Lookahead assertions
1893 </b><br>
1894 <P>
1895 Lookahead assertions start with (?= for positive assertions and (?! for
1896 negative assertions. For example,
1897 <pre>
1898 \w+(?=;)
1899 </pre>
1900 matches a word followed by a semicolon, but does not include the semicolon in
1901 the match, and
1902 <pre>
1903 foo(?!bar)
1904 </pre>
1905 matches any occurrence of "foo" that is not followed by "bar". Note that the
1906 apparently similar pattern
1907 <pre>
1908 (?!foo)bar
1909 </pre>
1910 does not find an occurrence of "bar" that is preceded by something other than
1911 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1912 (?!foo) is always true when the next three characters are "bar". A
1913 lookbehind assertion is needed to achieve the other effect.
1914 </P>
1915 <P>
1916 If you want to force a matching failure at some point in a pattern, the most
1917 convenient way to do it is with (?!) because an empty string always matches, so
1918 an assertion that requires there not to be an empty string must always fail.
1919 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1920 <a name="lookbehind"></a></P>
1921 <br><b>
1922 Lookbehind assertions
1923 </b><br>
1924 <P>
1925 Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1926 negative assertions. For example,
1927 <pre>
1928 (?&#60;!foo)bar
1929 </pre>
1930 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1931 a lookbehind assertion are restricted such that all the strings it matches must
1932 have a fixed length. However, if there are several top-level alternatives, they
1933 do not all have to have the same fixed length. Thus
1934 <pre>
1935 (?&#60;=bullock|donkey)
1936 </pre>
1937 is permitted, but
1938 <pre>
1939 (?&#60;!dogs?|cats?)
1940 </pre>
1941 causes an error at compile time. Branches that match different length strings
1942 are permitted only at the top level of a lookbehind assertion. This is an
1943 extension compared with Perl, which requires all branches to match the same
1944 length of string. An assertion such as
1945 <pre>
1946 (?&#60;=ab(c|de))
1947 </pre>
1948 is not permitted, because its single top-level branch can match two different
1949 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1950 branches:
1951 <pre>
1952 (?&#60;=abc|abde)
1953 </pre>
1954 In some cases, the escape sequence \K
1955 <a href="#resetmatchstart">(see above)</a>
1956 can be used instead of a lookbehind assertion to get round the fixed-length
1957 restriction.
1958 </P>
1959 <P>
1960 The implementation of lookbehind assertions is, for each alternative, to
1961 temporarily move the current position back by the fixed length and then try to
1962 match. If there are insufficient characters before the current position, the
1963 assertion fails.
1964 </P>
1965 <P>
1966 In a UTF mode, PCRE does not allow the \C escape (which matches a single data
1967 unit even in a UTF mode) to appear in lookbehind assertions, because it makes
1968 it impossible to calculate the length of the lookbehind. The \X and \R
1969 escapes, which can match different numbers of data units, are also not
1970 permitted.
1971 </P>
1972 <P>
1973 <a href="#subpatternsassubroutines">"Subroutine"</a>
1974 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1975 as the subpattern matches a fixed-length string.
1976 <a href="#recursion">Recursion,</a>
1977 however, is not supported.
1978 </P>
1979 <P>
1980 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1981 specify efficient matching of fixed-length strings at the end of subject
1982 strings. Consider a simple pattern such as
1983 <pre>
1984 abcd$
1985 </pre>
1986 when applied to a long string that does not match. Because matching proceeds
1987 from left to right, PCRE will look for each "a" in the subject and then see if
1988 what follows matches the rest of the pattern. If the pattern is specified as
1989 <pre>
1990 ^.*abcd$
1991 </pre>
1992 the initial .* matches the entire string at first, but when this fails (because
1993 there is no following "a"), it backtracks to match all but the last character,
1994 then all but the last two characters, and so on. Once again the search for "a"
1995 covers the entire string, from right to left, so we are no better off. However,
1996 if the pattern is written as
1997 <pre>
1998 ^.*+(?&#60;=abcd)
1999 </pre>
2000 there can be no backtracking for the .*+ item; it can match only the entire
2001 string. The subsequent lookbehind assertion does a single test on the last four
2002 characters. If it fails, the match fails immediately. For long strings, this
2003 approach makes a significant difference to the processing time.
2004 </P>
2005 <br><b>
2006 Using multiple assertions
2007 </b><br>
2008 <P>
2009 Several assertions (of any sort) may occur in succession. For example,
2010 <pre>
2011 (?&#60;=\d{3})(?&#60;!999)foo
2012 </pre>
2013 matches "foo" preceded by three digits that are not "999". Notice that each of
2014 the assertions is applied independently at the same point in the subject
2015 string. First there is a check that the previous three characters are all
2016 digits, and then there is a check that the same three characters are not "999".
2017 This pattern does <i>not</i> match "foo" preceded by six characters, the first
2018 of which are digits and the last three of which are not "999". For example, it
2019 doesn't match "123abcfoo". A pattern to do that is
2020 <pre>
2021 (?&#60;=\d{3}...)(?&#60;!999)foo
2022 </pre>
2023 This time the first assertion looks at the preceding six characters, checking
2024 that the first three are digits, and then the second assertion checks that the
2025 preceding three characters are not "999".
2026 </P>
2027 <P>
2028 Assertions can be nested in any combination. For example,
2029 <pre>
2030 (?&#60;=(?&#60;!foo)bar)baz
2031 </pre>
2032 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2033 preceded by "foo", while
2034 <pre>
2035 (?&#60;=\d{3}(?!999)...)foo
2036 </pre>
2037 is another pattern that matches "foo" preceded by three digits and any three
2038 characters that are not "999".
2039 <a name="conditions"></a></P>
2040 <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
2041 <P>
2042 It is possible to cause the matching process to obey a subpattern
2043 conditionally or to choose between two alternative subpatterns, depending on
2044 the result of an assertion, or whether a specific capturing subpattern has
2045 already been matched. The two possible forms of conditional subpattern are:
2046 <pre>
2047 (?(condition)yes-pattern)
2048 (?(condition)yes-pattern|no-pattern)
2049 </pre>
2050 If the condition is satisfied, the yes-pattern is used; otherwise the
2051 no-pattern (if present) is used. If there are more than two alternatives in the
2052 subpattern, a compile-time error occurs. Each of the two alternatives may
2053 itself contain nested subpatterns of any form, including conditional
2054 subpatterns; the restriction to two alternatives applies only at the level of
2055 the condition. This pattern fragment is an example where the alternatives are
2056 complex:
2057 <pre>
2058 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2060 </PRE>
2061 </P>
2062 <P>
2063 There are four kinds of condition: references to subpatterns, references to
2064 recursion, a pseudo-condition called DEFINE, and assertions.
2065 </P>
2066 <br><b>
2067 Checking for a used subpattern by number
2068 </b><br>
2069 <P>
2070 If the text between the parentheses consists of a sequence of digits, the
2071 condition is true if a capturing subpattern of that number has previously
2072 matched. If there is more than one capturing subpattern with the same number
2073 (see the earlier
2074 <a href="#recursion">section about duplicate subpattern numbers),</a>
2075 the condition is true if any of them have matched. An alternative notation is
2076 to precede the digits with a plus or minus sign. In this case, the subpattern
2077 number is relative rather than absolute. The most recently opened parentheses
2078 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2079 loops it can also make sense to refer to subsequent groups. The next
2080 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2081 zero in any of these forms is not used; it provokes a compile-time error.)
2082 </P>
2083 <P>
2084 Consider the following pattern, which contains non-significant white space to
2085 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2086 three parts for ease of discussion:
2087 <pre>
2088 ( \( )? [^()]+ (?(1) \) )
2089 </pre>
2090 The first part matches an optional opening parenthesis, and if that
2091 character is present, sets it as the first captured substring. The second part
2092 matches one or more characters that are not parentheses. The third part is a
2093 conditional subpattern that tests whether or not the first set of parentheses
2094 matched. If they did, that is, if subject started with an opening parenthesis,
2095 the condition is true, and so the yes-pattern is executed and a closing
2096 parenthesis is required. Otherwise, since no-pattern is not present, the
2097 subpattern matches nothing. In other words, this pattern matches a sequence of
2098 non-parentheses, optionally enclosed in parentheses.
2099 </P>
2100 <P>
2101 If you were embedding this pattern in a larger one, you could use a relative
2102 reference:
2103 <pre>
2104 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
2105 </pre>
2106 This makes the fragment independent of the parentheses in the larger pattern.
2107 </P>
2108 <br><b>
2109 Checking for a used subpattern by name
2110 </b><br>
2111 <P>
2112 Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
2113 subpattern by name. For compatibility with earlier versions of PCRE, which had
2114 this facility before Perl, the syntax (?(name)...) is also recognized. However,
2115 there is a possible ambiguity with this syntax, because subpattern names may
2116 consist entirely of digits. PCRE looks first for a named subpattern; if it
2117 cannot find one and the name consists entirely of digits, PCRE looks for a
2118 subpattern of that number, which must be greater than zero. Using subpattern
2119 names that consist entirely of digits is not recommended.
2120 </P>
2121 <P>
2122 Rewriting the above example to use a named subpattern gives this:
2123 <pre>
2124 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
2125 </pre>
2126 If the name used in a condition of this kind is a duplicate, the test is
2127 applied to all subpatterns of the same name, and is true if any one of them has
2128 matched.
2129 </P>
2130 <br><b>
2131 Checking for pattern recursion
2132 </b><br>
2133 <P>
2134 If the condition is the string (R), and there is no subpattern with the name R,
2135 the condition is true if a recursive call to the whole pattern or any
2136 subpattern has been made. If digits or a name preceded by ampersand follow the
2137 letter R, for example:
2138 <pre>
2139 (?(R3)...) or (?(R&name)...)
2140 </pre>
2141 the condition is true if the most recent recursion is into a subpattern whose
2142 number or name is given. This condition does not check the entire recursion
2143 stack. If the name used in a condition of this kind is a duplicate, the test is
2144 applied to all subpatterns of the same name, and is true if any one of them is
2145 the most recent recursion.
2146 </P>
2147 <P>
2148 At "top level", all these recursion test conditions are false.
2149 <a href="#recursion">The syntax for recursive patterns</a>
2150 is described below.
2151 <a name="subdefine"></a></P>
2152 <br><b>
2153 Defining subpatterns for use by reference only
2154 </b><br>
2155 <P>
2156 If the condition is the string (DEFINE), and there is no subpattern with the
2157 name DEFINE, the condition is always false. In this case, there may be only one
2158 alternative in the subpattern. It is always skipped if control reaches this
2159 point in the pattern; the idea of DEFINE is that it can be used to define
2160 subroutines that can be referenced from elsewhere. (The use of
2161 <a href="#subpatternsassubroutines">subroutines</a>
2162 is described below.) For example, a pattern to match an IPv4 address such as
2163 "" could be written like this (ignore whitespace and line
2164 breaks):
2165 <pre>
2166 (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2167 \b (?&byte) (\.(?&byte)){3} \b
2168 </pre>
2169 The first part of the pattern is a DEFINE group inside which a another group
2170 named "byte" is defined. This matches an individual component of an IPv4
2171 address (a number less than 256). When matching takes place, this part of the
2172 pattern is skipped because DEFINE acts like a false condition. The rest of the
2173 pattern uses references to the named group to match the four dot-separated
2174 components of an IPv4 address, insisting on a word boundary at each end.
2175 </P>
2176 <br><b>
2177 Assertion conditions
2178 </b><br>
2179 <P>
2180 If the condition is not in any of the above formats, it must be an assertion.
2181 This may be a positive or negative lookahead or lookbehind assertion. Consider
2182 this pattern, again containing non-significant white space, and with the two
2183 alternatives on the second line:
2184 <pre>
2185 (?(?=[^a-z]*[a-z])
2186 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2187 </pre>
2188 The condition is a positive lookahead assertion that matches an optional
2189 sequence of non-letters followed by a letter. In other words, it tests for the
2190 presence of at least one letter in the subject. If a letter is found, the
2191 subject is matched against the first alternative; otherwise it is matched
2192 against the second. This pattern matches strings in one of the two forms
2193 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2194 <a name="comments"></a></P>
2195 <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
2196 <P>
2197 There are two ways of including comments in patterns that are processed by
2198 PCRE. In both cases, the start of the comment must not be in a character class,
2199 nor in the middle of any other sequence of related characters such as (?: or a
2200 subpattern name or number. The characters that make up a comment play no part
2201 in the pattern matching.
2202 </P>
2203 <P>
2204 The sequence (?# marks the start of a comment that continues up to the next
2205 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2206 option is set, an unescaped # character also introduces a comment, which in
2207 this case continues to immediately after the next newline character or
2208 character sequence in the pattern. Which characters are interpreted as newlines
2209 is controlled by the options passed to a compiling function or by a special
2210 sequence at the start of the pattern, as described in the section entitled
2211 <a href="#newlines">"Newline conventions"</a>
2212 above. Note that the end of this type of comment is a literal newline sequence
2213 in the pattern; escape sequences that happen to represent a newline do not
2214 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2215 default newline convention is in force:
2216 <pre>
2217 abc #comment \n still comment
2218 </pre>
2219 On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2220 a newline in the pattern. The sequence \n is still literal at this stage, so
2221 it does not terminate the comment. Only an actual character with the code value
2222 0x0a (the default newline) does so.
2223 <a name="recursion"></a></P>
2224 <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
2225 <P>
2226 Consider the problem of matching a string in parentheses, allowing for
2227 unlimited nested parentheses. Without the use of recursion, the best that can
2228 be done is to use a pattern that matches up to some fixed depth of nesting. It
2229 is not possible to handle an arbitrary nesting depth.
2230 </P>
2231 <P>
2232 For some time, Perl has provided a facility that allows regular expressions to
2233 recurse (amongst other things). It does this by interpolating Perl code in the
2234 expression at run time, and the code can refer to the expression itself. A Perl
2235 pattern using code interpolation to solve the parentheses problem can be
2236 created like this:
2237 <pre>
2238 $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
2239 </pre>
2240 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2241 recursively to the pattern in which it appears.
2242 </P>
2243 <P>
2244 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2245 supports special syntax for recursion of the entire pattern, and also for
2246 individual subpattern recursion. After its introduction in PCRE and Python,
2247 this kind of recursion was subsequently introduced into Perl at release 5.10.
2248 </P>
2249 <P>
2250 A special item that consists of (? followed by a number greater than zero and a
2251 closing parenthesis is a recursive subroutine call of the subpattern of the
2252 given number, provided that it occurs inside that subpattern. (If not, it is a
2253 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2254 call, which is described in the next section.) The special item (?R) or (?0) is
2255 a recursive call of the entire regular expression.
2256 </P>
2257 <P>
2258 This PCRE pattern solves the nested parentheses problem (assume the
2259 PCRE_EXTENDED option is set so that white space is ignored):
2260 <pre>
2261 \( ( [^()]++ | (?R) )* \)
2262 </pre>
2263 First it matches an opening parenthesis. Then it matches any number of
2264 substrings which can either be a sequence of non-parentheses, or a recursive
2265 match of the pattern itself (that is, a correctly parenthesized substring).
2266 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2267 to avoid backtracking into sequences of non-parentheses.
2268 </P>
2269 <P>
2270 If this were part of a larger pattern, you would not want to recurse the entire
2271 pattern, so instead you could use this:
2272 <pre>
2273 ( \( ( [^()]++ | (?1) )* \) )
2274 </pre>
2275 We have put the pattern into parentheses, and caused the recursion to refer to
2276 them instead of the whole pattern.
2277 </P>
2278 <P>
2279 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2280 is made easier by the use of relative references. Instead of (?1) in the
2281 pattern above you can write (?-2) to refer to the second most recently opened
2282 parentheses preceding the recursion. In other words, a negative number counts
2283 capturing parentheses leftwards from the point at which it is encountered.
2284 </P>
2285 <P>
2286 It is also possible to refer to subsequently opened parentheses, by writing
2287 references such as (?+2). However, these cannot be recursive because the
2288 reference is not inside the parentheses that are referenced. They are always
2289 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2290 calls, as described in the next section.
2291 </P>
2292 <P>
2293 An alternative approach is to use named parentheses instead. The Perl syntax
2294 for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
2295 could rewrite the above example as follows:
2296 <pre>
2297 (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
2298 </pre>
2299 If there is more than one subpattern with the same name, the earliest one is
2300 used.
2301 </P>
2302 <P>
2303 This particular example pattern that we have been looking at contains nested
2304 unlimited repeats, and so the use of a possessive quantifier for matching
2305 strings of non-parentheses is important when applying the pattern to strings
2306 that do not match. For example, when this pattern is applied to
2307 <pre>
2308 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2309 </pre>
2310 it yields "no match" quickly. However, if a possessive quantifier is not used,
2311 the match runs for a very long time indeed because there are so many different
2312 ways the + and * repeats can carve up the subject, and all have to be tested
2313 before failure can be reported.
2314 </P>
2315 <P>
2316 At the end of a match, the values of capturing parentheses are those from
2317 the outermost level. If you want to obtain intermediate values, a callout
2318 function can be used (see below and the
2319 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2320 documentation). If the pattern above is matched against
2321 <pre>
2322 (ab(cd)ef)
2323 </pre>
2324 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2325 the last value taken on at the top level. If a capturing subpattern is not
2326 matched at the top level, its final captured value is unset, even if it was
2327 (temporarily) set at a deeper level during the matching process.
2328 </P>
2329 <P>
2330 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2331 obtain extra memory to store data during a recursion, which it does by using
2332 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2333 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2334 </P>
2335 <P>
2336 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2337 Consider this pattern, which matches text in angle brackets, allowing for
2338 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2339 recursing), whereas any characters are permitted at the outer level.
2340 <pre>
2341 &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
2342 </pre>
2343 In this pattern, (?(R) is the start of a conditional subpattern, with two
2344 different alternatives for the recursive and non-recursive cases. The (?R) item
2345 is the actual recursive call.
2346 <a name="recursiondifference"></a></P>
2347 <br><b>
2348 Differences in recursion processing between PCRE and Perl
2349 </b><br>
2350 <P>
2351 Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2352 (like Python, but unlike Perl), a recursive subpattern call is always treated
2353 as an atomic group. That is, once it has matched some of the subject string, it
2354 is never re-entered, even if it contains untried alternatives and there is a
2355 subsequent matching failure. This can be illustrated by the following pattern,
2356 which purports to match a palindromic string that contains an odd number of
2357 characters (for example, "a", "aba", "abcba", "abcdcba"):
2358 <pre>
2359 ^(.|(.)(?1)\2)$
2360 </pre>
2361 The idea is that it either matches a single character, or two identical
2362 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2363 it does not if the pattern is longer than three characters. Consider the
2364 subject string "abcba":
2365 </P>
2366 <P>
2367 At the top level, the first character is matched, but as it is not at the end
2368 of the string, the first alternative fails; the second alternative is taken
2369 and the recursion kicks in. The recursive call to subpattern 1 successfully
2370 matches the next character ("b"). (Note that the beginning and end of line
2371 tests are not part of the recursion).
2372 </P>
2373 <P>
2374 Back at the top level, the next character ("c") is compared with what
2375 subpattern 2 matched, which was "a". This fails. Because the recursion is
2376 treated as an atomic group, there are now no backtracking points, and so the
2377 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2378 try the second alternative.) However, if the pattern is written with the
2379 alternatives in the other order, things are different:
2380 <pre>
2381 ^((.)(?1)\2|.)$
2382 </pre>
2383 This time, the recursing alternative is tried first, and continues to recurse
2384 until it runs out of characters, at which point the recursion fails. But this
2385 time we do have another alternative to try at the higher level. That is the big
2386 difference: in the previous case the remaining alternative is at a deeper
2387 recursion level, which PCRE cannot use.
2388 </P>
2389 <P>
2390 To change the pattern so that it matches all palindromic strings, not just
2391 those with an odd number of characters, it is tempting to change the pattern to
2392 this:
2393 <pre>
2394 ^((.)(?1)\2|.?)$
2395 </pre>
2396 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2397 deeper recursion has matched a single character, it cannot be entered again in
2398 order to match an empty string. The solution is to separate the two cases, and
2399 write out the odd and even cases as alternatives at the higher level:
2400 <pre>
2401 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2402 </pre>
2403 If you want to match typical palindromic phrases, the pattern has to ignore all
2404 non-word characters, which can be done like this:
2405 <pre>
2406 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2407 </pre>
2408 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2409 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2410 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2411 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2412 more) to match typical phrases, and Perl takes so long that you think it has
2413 gone into a loop.
2414 </P>
2415 <P>
2416 <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2417 string does not start with a palindrome that is shorter than the entire string.
2418 For example, although "abcba" is correctly matched, if the subject is "ababa",
2419 PCRE finds the palindrome "aba" at the start, then fails at top level because
2420 the end of the string does not follow. Once again, it cannot jump back into the
2421 recursion to try other alternatives, so the entire match fails.
2422 </P>
2423 <P>
2424 The second way in which PCRE and Perl differ in their recursion processing is
2425 in the handling of captured values. In Perl, when a subpattern is called
2426 recursively or as a subpattern (see the next section), it has no access to any
2427 values that were captured outside the recursion, whereas in PCRE these values
2428 can be referenced. Consider this pattern:
2429 <pre>
2430 ^(.)(\1|a(?2))
2431 </pre>
2432 In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2433 then in the second group, when the back reference \1 fails to match "b", the
2434 second alternative matches "a" and then recurses. In the recursion, \1 does
2435 now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2436 match because inside the recursive call \1 cannot access the externally set
2437 value.
2438 <a name="subpatternsassubroutines"></a></P>
2439 <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2440 <P>
2441 If the syntax for a recursive subpattern call (either by number or by
2442 name) is used outside the parentheses to which it refers, it operates like a
2443 subroutine in a programming language. The called subpattern may be defined
2444 before or after the reference. A numbered reference can be absolute or
2445 relative, as in these examples:
2446 <pre>
2447 (...(absolute)...)...(?2)...
2448 (...(relative)...)...(?-1)...
2449 (...(?+1)...(relative)...
2450 </pre>
2451 An earlier example pointed out that the pattern
2452 <pre>
2453 (sens|respons)e and \1ibility
2454 </pre>
2455 matches "sense and sensibility" and "response and responsibility", but not
2456 "sense and responsibility". If instead the pattern
2457 <pre>
2458 (sens|respons)e and (?1)ibility
2459 </pre>
2460 is used, it does match "sense and responsibility" as well as the other two
2461 strings. Another example is given in the discussion of DEFINE above.
2462 </P>
2463 <P>
2464 All subroutine calls, whether recursive or not, are always treated as atomic
2465 groups. That is, once a subroutine has matched some of the subject string, it
2466 is never re-entered, even if it contains untried alternatives and there is a
2467 subsequent matching failure. Any capturing parentheses that are set during the
2468 subroutine call revert to their previous values afterwards.
2469 </P>
2470 <P>
2471 Processing options such as case-independence are fixed when a subpattern is
2472 defined, so if it is used as a subroutine, such options cannot be changed for
2473 different calls. For example, consider this pattern:
2474 <pre>
2475 (abc)(?i:(?-1))
2476 </pre>
2477 It matches "abcabc". It does not match "abcABC" because the change of
2478 processing option does not affect the called subpattern.
2479 <a name="onigurumasubroutines"></a></P>
2480 <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2481 <P>
2482 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2483 a number enclosed either in angle brackets or single quotes, is an alternative
2484 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2485 are two of the examples used above, rewritten using this syntax:
2486 <pre>
2487 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
2488 (sens|respons)e and \g'1'ibility
2489 </pre>
2490 PCRE supports an extension to Oniguruma: if a number is preceded by a
2491 plus or a minus sign it is taken as a relative reference. For example:
2492 <pre>
2493 (abc)(?i:\g&#60;-1&#62;)
2494 </pre>
2495 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
2496 synonymous. The former is a back reference; the latter is a subroutine call.
2497 </P>
2498 <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
2499 <P>
2500 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2501 code to be obeyed in the middle of matching a regular expression. This makes it
2502 possible, amongst other things, to extract different substrings that match the
2503 same pair of parentheses when there is a repetition.
2504 </P>
2505 <P>
2506 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2507 code. The feature is called "callout". The caller of PCRE provides an external
2508 function by putting its entry point in the global variable <i>pcre_callout</i>
2509 (8-bit library) or <i>pcre16_callout</i> (16-bit library). By default, this
2510 variable contains NULL, which disables all calling out.
2511 </P>
2512 <P>
2513 Within a regular expression, (?C) indicates the points at which the external
2514 function is to be called. If you want to identify different callout points, you
2515 can put a number less than 256 after the letter C. The default value is zero.
2516 For example, this pattern has two callout points:
2517 <pre>
2518 (?C1)abc(?C2)def
2519 </pre>
2520 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2521 automatically installed before each item in the pattern. They are all numbered
2522 255.
2523 </P>
2524 <P>
2525 During matching, when PCRE reaches a callout point, the external function is
2526 called. It is provided with the number of the callout, the position in the
2527 pattern, and, optionally, one item of data originally supplied by the caller of
2528 the matching function. The callout function may cause matching to proceed, to
2529 backtrack, or to fail altogether. A complete description of the interface to
2530 the callout function is given in the
2531 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2532 documentation.
2533 <a name="backtrackcontrol"></a></P>
2534 <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
2535 <P>
2536 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2537 are described in the Perl documentation as "experimental and subject to change
2538 or removal in a future version of Perl". It goes on to say: "Their usage in
2539 production code should be noted to avoid problems during upgrades." The same
2540 remarks apply to the PCRE features described in this section.
2541 </P>
2542 <P>
2543 Since these verbs are specifically related to backtracking, most of them can be
2544 used only when the pattern is to be matched using one of the traditional
2545 matching functions, which use a backtracking algorithm. With the exception of
2546 (*FAIL), which behaves like a failing negative assertion, they cause an error
2547 if encountered by a DFA matching function.
2548 </P>
2549 <P>
2550 If any of these verbs are used in an assertion or in a subpattern that is
2551 called as a subroutine (whether or not recursively), their effect is confined
2552 to that subpattern; it does not extend to the surrounding pattern, with one
2553 exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2554 a successful positive assertion <i>is</i> passed back when a match succeeds
2555 (compare capturing parentheses in assertions). Note that such subpatterns are
2556 processed as anchored at the point where they are tested. Note also that Perl's
2557 treatment of subroutines is different in some cases.
2558 </P>
2559 <P>
2560 The new verbs make use of what was previously invalid syntax: an opening
2561 parenthesis followed by an asterisk. They are generally of the form
2562 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2563 depending on whether or not an argument is present. A name is any sequence of
2564 characters that does not include a closing parenthesis. If the name is empty,
2565 that is, if the closing parenthesis immediately follows the colon, the effect
2566 is as if the colon were not there. Any number of these verbs may occur in a
2567 pattern.
2568 <a name="nooptimize"></a></P>
2569 <br><b>
2570 Optimizations that affect backtracking verbs
2571 </b><br>
2572 <P>
2573 PCRE contains some optimizations that are used to speed up matching by running
2574 some checks at the start of each match attempt. For example, it may know the
2575 minimum length of matching subject, or that a particular character must be
2576 present. When one of these optimizations suppresses the running of a match, any
2577 included backtracking verbs will not, of course, be processed. You can suppress
2578 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2579 when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
2580 pattern with (*NO_START_OPT). There is more discussion of this option in the
2581 section entitled
2582 <a href="pcreapi.html#execoptions">"Option bits for <b>pcre_exec()</b>"</a>
2583 in the
2584 <a href="pcreapi.html"><b>pcreapi</b></a>
2585 documentation.
2586 </P>
2587 <P>
2588 Experiments with Perl suggest that it too has similar optimizations, sometimes
2589 leading to anomalous results.
2590 </P>
2591 <br><b>
2592 Verbs that act immediately
2593 </b><br>
2594 <P>
2595 The following verbs act as soon as they are encountered. They may not be
2596 followed by a name.
2597 <pre>
2598 (*ACCEPT)
2599 </pre>
2600 This verb causes the match to end successfully, skipping the remainder of the
2601 pattern. However, when it is inside a subpattern that is called as a
2602 subroutine, only that subpattern is ended successfully. Matching then continues
2603 at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2604 far is captured. For example:
2605 <pre>
2606 A((?:A|B(*ACCEPT)|C)D)
2607 </pre>
2608 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2609 the outer parentheses.
2610 <pre>
2611 (*FAIL) or (*F)
2612 </pre>
2613 This verb causes a matching failure, forcing backtracking to occur. It is
2614 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2615 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2616 Perl features that are not present in PCRE. The nearest equivalent is the
2617 callout feature, as for example in this pattern:
2618 <pre>
2619 a+(?C)(*FAIL)
2620 </pre>
2621 A match with the string "aaaa" always fails, but the callout is taken before
2622 each backtrack happens (in this example, 10 times).
2623 </P>
2624 <br><b>
2625 Recording which path was taken
2626 </b><br>
2627 <P>
2628 There is one verb whose main purpose is to track how a match was arrived at,
2629 though it also has a secondary use in conjunction with advancing the match
2630 starting point (see (*SKIP) below).
2631 <pre>
2632 (*MARK:NAME) or (*:NAME)
2633 </pre>
2634 A name is always required with this verb. There may be as many instances of
2635 (*MARK) as you like in a pattern, and their names do not have to be unique.
2636 </P>
2637 <P>
2638 When a match succeeds, the name of the last-encountered (*MARK) on the matching
2639 path is passed back to the caller as described in the section entitled
2640 <a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a>
2641 in the
2642 <a href="pcreapi.html"><b>pcreapi</b></a>
2643 documentation. Here is an example of <b>pcretest</b> output, where the /K
2644 modifier requests the retrieval and outputting of (*MARK) data:
2645 <pre>
2646 re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
2647 data&#62; XY
2648 0: XY
2649 MK: A
2650 XZ
2651 0: XZ
2652 MK: B
2653 </pre>
2654 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2655 indicates which of the two alternatives matched. This is a more efficient way
2656 of obtaining this information than putting each alternative in its own
2657 capturing parentheses.
2658 </P>
2659 <P>
2660 If (*MARK) is encountered in a positive assertion, its name is recorded and
2661 passed back if it is the last-encountered. This does not happen for negative
2662 assertions.
2663 </P>
2664 <P>
2665 After a partial match or a failed match, the name of the last encountered
2666 (*MARK) in the entire match process is returned. For example:
2667 <pre>
2668 re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
2669 data&#62; XP
2670 No match, mark = B
2671 </pre>
2672 Note that in this unanchored example the mark is retained from the match
2673 attempt that started at the letter "X" in the subject. Subsequent match
2674 attempts starting at "P" and then with an empty string do not get as far as the
2675 (*MARK) item, but nevertheless do not reset it.
2676 </P>
2677 <P>
2678 If you are interested in (*MARK) values after failed matches, you should
2679 probably set the PCRE_NO_START_OPTIMIZE option
2680 <a href="#nooptimize">(see above)</a>
2681 to ensure that the match is always attempted.
2682 </P>
2683 <br><b>
2684 Verbs that act after backtracking
2685 </b><br>
2686 <P>
2687 The following verbs do nothing when they are encountered. Matching continues
2688 with what follows, but if there is no subsequent match, causing a backtrack to
2689 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2690 the verb. However, when one of these verbs appears inside an atomic group, its
2691 effect is confined to that group, because once the group has been matched,
2692 there is never any backtracking into it. In this situation, backtracking can
2693 "jump back" to the left of the entire atomic group. (Remember also, as stated
2694 above, that this localization also applies in subroutine calls and assertions.)
2695 </P>
2696 <P>
2697 These verbs differ in exactly what kind of failure occurs when backtracking
2698 reaches them.
2699 <pre>
2700 (*COMMIT)
2701 </pre>
2702 This verb, which may not be followed by a name, causes the whole match to fail
2703 outright if the rest of the pattern does not match. Even if the pattern is
2704 unanchored, no further attempts to find a match by advancing the starting point
2705 take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to
2706 finding a match at the current starting point, or not at all. For example:
2707 <pre>
2708 a+(*COMMIT)b
2709 </pre>
2710 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2711 dynamic anchor, or "I've started, so I must finish." The name of the most
2712 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2713 match failure.
2714 </P>
2715 <P>
2716 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2717 unless PCRE's start-of-match optimizations are turned off, as shown in this
2718 <b>pcretest</b> example:
2719 <pre>
2720 re&#62; /(*COMMIT)abc/
2721 data&#62; xyzabc
2722 0: abc
2723 xyzabc\Y
2724 No match
2725 </pre>
2726 PCRE knows that any match must start with "a", so the optimization skips along
2727 the subject to "a" before running the first match attempt, which succeeds. When
2728 the optimization is disabled by the \Y escape in the second subject, the match
2729 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2730 starting points.
2731 <pre>
2732 (*PRUNE) or (*PRUNE:NAME)
2733 </pre>
2734 This verb causes the match to fail at the current starting position in the
2735 subject if the rest of the pattern does not match. If the pattern is
2736 unanchored, the normal "bumpalong" advance to the next starting character then
2737 happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2738 reached, or when matching to the right of (*PRUNE), but if there is no match to
2739 the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2740 (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2741 but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2742 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2743 anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2744 <pre>
2745 (*SKIP)
2746 </pre>
2747 This verb, when given without a name, is like (*PRUNE), except that if the
2748 pattern is unanchored, the "bumpalong" advance is not to the next character,
2749 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2750 signifies that whatever text was matched leading up to it cannot be part of a
2751 successful match. Consider:
2752 <pre>
2753 a+(*SKIP)b
2754 </pre>
2755 If the subject is "aaaac...", after the first match attempt fails (starting at
2756 the first character in the string), the starting point skips on to start the
2757 next attempt at "c". Note that a possessive quantifer does not have the same
2758 effect as this example; although it would suppress backtracking during the
2759 first match attempt, the second attempt would start at the second character
2760 instead of skipping on to "c".
2761 <pre>
2762 (*SKIP:NAME)
2763 </pre>
2764 When (*SKIP) has an associated name, its behaviour is modified. If the
2765 following pattern fails to match, the previous path through the pattern is
2766 searched for the most recent (*MARK) that has the same name. If one is found,
2767 the "bumpalong" advance is to the subject position that corresponds to that
2768 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2769 matching name is found, the (*SKIP) is ignored.
2770 <pre>
2771 (*THEN) or (*THEN:NAME)
2772 </pre>
2773 This verb causes a skip to the next innermost alternative if the rest of the
2774 pattern does not match. That is, it cancels pending backtracking, but only
2775 within the current alternative. Its name comes from the observation that it can
2776 be used for a pattern-based if-then-else block:
2777 <pre>
2778 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2779 </pre>
2780 If the COND1 pattern matches, FOO is tried (and possibly further items after
2781 the end of the group if FOO succeeds); on failure, the matcher skips to the
2782 second alternative and tries COND2, without backtracking into COND1. The
2783 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2784 If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2785 </P>
2786 <P>
2787 Note that a subpattern that does not contain a | character is just a part of
2788 the enclosing alternative; it is not a nested alternation with only one
2789 alternative. The effect of (*THEN) extends beyond such a subpattern to the
2790 enclosing alternative. Consider this pattern, where A, B, etc. are complex
2791 pattern fragments that do not contain any | characters at this level:
2792 <pre>
2793 A (B(*THEN)C) | D
2794 </pre>
2795 If A and B are matched, but there is a failure in C, matching does not
2796 backtrack into A; instead it moves to the next alternative, that is, D.
2797 However, if the subpattern containing (*THEN) is given an alternative, it
2798 behaves differently:
2799 <pre>
2800 A (B(*THEN)C | (*FAIL)) | D
2801 </pre>
2802 The effect of (*THEN) is now confined to the inner subpattern. After a failure
2803 in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2804 because there are no more alternatives to try. In this case, matching does now
2805 backtrack into A.
2806 </P>
2807 <P>
2808 Note also that a conditional subpattern is not considered as having two
2809 alternatives, because only one is ever used. In other words, the | character in
2810 a conditional subpattern has a different meaning. Ignoring white space,
2811 consider:
2812 <pre>
2813 ^.*? (?(?=a) a | b(*THEN)c )
2814 </pre>
2815 If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2816 it initially matches zero characters. The condition (?=a) then fails, the
2817 character "b" is matched, but "c" is not. At this point, matching does not
2818 backtrack to .*? as might perhaps be expected from the presence of the |
2819 character. The conditional subpattern is part of the single alternative that
2820 comprises the whole pattern, and so the match fails. (If there was a backtrack
2821 into .*?, allowing it to match "b", the match would succeed.)
2822 </P>
2823 <P>
2824 The verbs just described provide four different "strengths" of control when
2825 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2826 next alternative. (*PRUNE) comes next, failing the match at the current
2827 starting position, but allowing an advance to the next character (for an
2828 unanchored pattern). (*SKIP) is similar, except that the advance may be more
2829 than one character. (*COMMIT) is the strongest, causing the entire match to
2830 fail.
2831 </P>
2832 <P>
2833 If more than one such verb is present in a pattern, the "strongest" one wins.
2834 For example, consider this pattern, where A, B, etc. are complex pattern
2835 fragments:
2836 <pre>
2837 (A(*COMMIT)B(*THEN)C|D)
2838 </pre>
2839 Once A has matched, PCRE is committed to this match, at the current starting
2840 position. If subsequently B matches, but C does not, the normal (*THEN) action
2841 of trying the next alternative (that is, D) does not happen because (*COMMIT)
2842 overrides.
2843 </P>
2844 <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
2845 <P>
2846 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
2847 <b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>.
2848 </P>
2849 <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
2850 <P>
2851 Philip Hazel
2852 <br>
2853 University Computing Service
2854 <br>
2855 Cambridge CB2 3QH, England.
2856 <br>
2857 </P>
2858 <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2859 <P>
2860 Last updated: 24 February 2012
2861 <br>
2862 Copyright &copy; 1997-2012 University of Cambridge.
2863 <br>
2864 <p>
2865 Return to the <a href="index.html">PCRE index page</a>.
2866 </p>


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