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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 BYTE</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 character strings. To use this,
65 PCRE must be built to include UTF-8 support, and you must call
66 <b>pcre_compile()</b> or <b>pcre_compile2()</b> with the PCRE_UTF8 option. There
67 is also a special sequence that can be given at the start of a pattern:
68 <pre>
69 (*UTF8)
70 </pre>
71 Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
72 option. This feature is not Perl-compatible. How setting UTF-8 mode affects
73 pattern matching is mentioned in several places below. There is also a summary
74 of UTF-8 features in the
75 <a href="pcre.html#utf8support">section on UTF-8 support</a>
76 in the main
77 <a href="pcre.html"><b>pcre</b></a>
78 page.
79 </P>
80 <P>
81 Another special sequence that may appear at the start of a pattern or in
82 combination with (*UTF8) is:
83 <pre>
84 (*UCP)
85 </pre>
86 This has the same effect as setting the PCRE_UCP option: it causes sequences
87 such as \d and \w to use Unicode properties to determine character types,
88 instead of recognizing only characters with codes less than 128 via a lookup
89 table.
90 </P>
91 <P>
92 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
93 PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
94 also some more of these special sequences that are concerned with the handling
95 of newlines; they are described below.
96 </P>
97 <P>
98 The remainder of this document discusses the patterns that are supported by
99 PCRE when its main matching function, <b>pcre_exec()</b>, is used.
100 From release 6.0, PCRE offers a second matching function,
101 <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
102 Perl-compatible. Some of the features discussed below are not available when
103 <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
104 alternative function, and how it differs from the normal function, are
105 discussed in the
106 <a href="pcrematching.html"><b>pcrematching</b></a>
107 page.
108 <a name="newlines"></a></P>
109 <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
110 <P>
111 PCRE supports five different conventions for indicating line breaks in
112 strings: a single CR (carriage return) character, a single LF (linefeed)
113 character, the two-character sequence CRLF, any of the three preceding, or any
114 Unicode newline sequence. The
115 <a href="pcreapi.html"><b>pcreapi</b></a>
116 page has
117 <a href="pcreapi.html#newlines">further discussion</a>
118 about newlines, and shows how to set the newline convention in the
119 <i>options</i> arguments for the compiling and matching functions.
120 </P>
121 <P>
122 It is also possible to specify a newline convention by starting a pattern
123 string with one of the following five sequences:
124 <pre>
125 (*CR) carriage return
126 (*LF) linefeed
127 (*CRLF) carriage return, followed by linefeed
128 (*ANYCRLF) any of the three above
129 (*ANY) all Unicode newline sequences
130 </pre>
131 These override the default and the options given to <b>pcre_compile()</b> or
132 <b>pcre_compile2()</b>. For example, on a Unix system where LF is the default
133 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 UTF-8 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-8 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 UTF-8 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.
249 <a name="digitsafterbackslash"></a></P>
250 <br><b>
251 Non-printing characters
252 </b><br>
253 <P>
254 A second use of backslash provides a way of encoding non-printing characters
255 in patterns in a visible manner. There is no restriction on the appearance of
256 non-printing characters, apart from the binary zero that terminates a pattern,
257 but when a pattern is being prepared by text editing, it is often easier to use
258 one of the following escape sequences than the binary character it represents:
259 <pre>
260 \a alarm, that is, the BEL character (hex 07)
261 \cx "control-x", where x is any ASCII character
262 \e escape (hex 1B)
263 \f formfeed (hex 0C)
264 \n linefeed (hex 0A)
265 \r carriage return (hex 0D)
266 \t tab (hex 09)
267 \ddd character with octal code ddd, or back reference
268 \xhh character with hex code hh
269 \x{hhh..} character with hex code hhh..
270 </pre>
271 The precise effect of \cx is as follows: if x is a lower case letter, it
272 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
273 Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while
274 \c; becomes hex 7B (; is 3B). If the byte following \c has a value greater
275 than 127, a compile-time error occurs. This locks out non-ASCII characters in
276 both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
277 values are valid. A lower case letter is converted to upper case, and then the
278 0xc0 bits are flipped.)
279 </P>
280 <P>
281 After \x, from zero to two hexadecimal digits are read (letters can be in
282 upper or lower case). Any number of hexadecimal digits may appear between \x{
283 and }, but the value of the character code must be less than 256 in non-UTF-8
284 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
285 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
286 point, which is 10FFFF.
287 </P>
288 <P>
289 If characters other than hexadecimal digits appear between \x{ and }, or if
290 there is no terminating }, this form of escape is not recognized. Instead, the
291 initial \x will be interpreted as a basic hexadecimal escape, with no
292 following digits, giving a character whose value is zero.
293 </P>
294 <P>
295 Characters whose value is less than 256 can be defined by either of the two
296 syntaxes for \x. There is no difference in the way they are handled. For
297 example, \xdc is exactly the same as \x{dc}.
298 </P>
299 <P>
300 After \0 up to two further octal digits are read. If there are fewer than two
301 digits, just those that are present are used. Thus the sequence \0\x\07
302 specifies two binary zeros followed by a BEL character (code value 7). Make
303 sure you supply two digits after the initial zero if the pattern character that
304 follows is itself an octal digit.
305 </P>
306 <P>
307 The handling of a backslash followed by a digit other than 0 is complicated.
308 Outside a character class, PCRE reads it and any following digits as a decimal
309 number. If the number is less than 10, or if there have been at least that many
310 previous capturing left parentheses in the expression, the entire sequence is
311 taken as a <i>back reference</i>. A description of how this works is given
312 <a href="#backreferences">later,</a>
313 following the discussion of
314 <a href="#subpattern">parenthesized subpatterns.</a>
315 </P>
316 <P>
317 Inside a character class, or if the decimal number is greater than 9 and there
318 have not been that many capturing subpatterns, PCRE re-reads up to three octal
319 digits following the backslash, and uses them to generate a data character. Any
320 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
321 character specified in octal must be less than \400. In UTF-8 mode, values up
322 to \777 are permitted. For example:
323 <pre>
324 \040 is another way of writing a space
325 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
326 \7 is always a back reference
327 \11 might be a back reference, or another way of writing a tab
328 \011 is always a tab
329 \0113 is a tab followed by the character "3"
330 \113 might be a back reference, otherwise the character with octal code 113
331 \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
332 \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
333 </pre>
334 Note that octal values of 100 or greater must not be introduced by a leading
335 zero, because no more than three octal digits are ever read.
336 </P>
337 <P>
338 All the sequences that define a single character value can be used both inside
339 and outside character classes. In addition, inside a character class, the
340 sequence \b is interpreted as the backspace character (hex 08). The sequences
341 \B, \N, \R, and \X are not special inside a character class. Like any other
342 unrecognized escape sequences, they are treated as the literal characters "B",
343 "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
344 set. Outside a character class, these sequences have different meanings.
345 </P>
346 <br><b>
347 Absolute and relative back references
348 </b><br>
349 <P>
350 The sequence \g followed by an unsigned or a negative number, optionally
351 enclosed in braces, is an absolute or relative back reference. A named back
352 reference can be coded as \g{name}. Back references are discussed
353 <a href="#backreferences">later,</a>
354 following the discussion of
355 <a href="#subpattern">parenthesized subpatterns.</a>
356 </P>
357 <br><b>
358 Absolute and relative subroutine calls
359 </b><br>
360 <P>
361 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
362 a number enclosed either in angle brackets or single quotes, is an alternative
363 syntax for referencing a subpattern as a "subroutine". Details are discussed
364 <a href="#onigurumasubroutines">later.</a>
365 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
366 synonymous. The former is a back reference; the latter is a
367 <a href="#subpatternsassubroutines">subroutine</a>
368 call.
369 <a name="genericchartypes"></a></P>
370 <br><b>
371 Generic character types
372 </b><br>
373 <P>
374 Another use of backslash is for specifying generic character types:
375 <pre>
376 \d any decimal digit
377 \D any character that is not a decimal digit
378 \h any horizontal whitespace character
379 \H any character that is not a horizontal whitespace character
380 \s any whitespace character
381 \S any character that is not a whitespace character
382 \v any vertical whitespace character
383 \V any character that is not a vertical whitespace character
384 \w any "word" character
385 \W any "non-word" character
386 </pre>
387 There is also the single sequence \N, which matches a non-newline character.
388 This is the same as
389 <a href="#fullstopdot">the "." metacharacter</a>
390 when PCRE_DOTALL is not set.
391 </P>
392 <P>
393 Each pair of lower and upper case escape sequences partitions the complete set
394 of characters into two disjoint sets. Any given character matches one, and only
395 one, of each pair. The sequences can appear both inside and outside character
396 classes. They each match one character of the appropriate type. If the current
397 matching point is at the end of the subject string, all of them fail, because
398 there is no character to match.
399 </P>
400 <P>
401 For compatibility with Perl, \s does not match the VT character (code 11).
402 This makes it different from the the POSIX "space" class. The \s characters
403 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
404 included in a Perl script, \s may match the VT character. In PCRE, it never
405 does.
406 </P>
407 <P>
408 A "word" character is an underscore or any character that is a letter or digit.
409 By default, the definition of letters and digits is controlled by PCRE's
410 low-valued character tables, and may vary if locale-specific matching is taking
411 place (see
412 <a href="pcreapi.html#localesupport">"Locale support"</a>
413 in the
414 <a href="pcreapi.html"><b>pcreapi</b></a>
415 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
416 or "french" in Windows, some character codes greater than 128 are used for
417 accented letters, and these are then matched by \w. The use of locales with
418 Unicode is discouraged.
419 </P>
420 <P>
421 By default, in UTF-8 mode, characters with values greater than 128 never match
422 \d, \s, or \w, and always match \D, \S, and \W. These sequences retain
423 their original meanings from before UTF-8 support was available, mainly for
424 efficiency reasons. However, if PCRE is compiled with Unicode property support,
425 and the PCRE_UCP option is set, the behaviour is changed so that Unicode
426 properties are used to determine character types, as follows:
427 <pre>
428 \d any character that \p{Nd} matches (decimal digit)
429 \s any character that \p{Z} matches, plus HT, LF, FF, CR
430 \w any character that \p{L} or \p{N} matches, plus underscore
431 </pre>
432 The upper case escapes match the inverse sets of characters. Note that \d
433 matches only decimal digits, whereas \w matches any Unicode digit, as well as
434 any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
435 \B because they are defined in terms of \w and \W. Matching these sequences
436 is noticeably slower when PCRE_UCP is set.
437 </P>
438 <P>
439 The sequences \h, \H, \v, and \V are features that were added to Perl at
440 release 5.10. In contrast to the other sequences, which match only ASCII
441 characters by default, these always match certain high-valued codepoints in
442 UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
443 are:
444 <pre>
445 U+0009 Horizontal tab
446 U+0020 Space
447 U+00A0 Non-break space
448 U+1680 Ogham space mark
449 U+180E Mongolian vowel separator
450 U+2000 En quad
451 U+2001 Em quad
452 U+2002 En space
453 U+2003 Em space
454 U+2004 Three-per-em space
455 U+2005 Four-per-em space
456 U+2006 Six-per-em space
457 U+2007 Figure space
458 U+2008 Punctuation space
459 U+2009 Thin space
460 U+200A Hair space
461 U+202F Narrow no-break space
462 U+205F Medium mathematical space
463 U+3000 Ideographic space
464 </pre>
465 The vertical space characters are:
466 <pre>
467 U+000A Linefeed
468 U+000B Vertical tab
469 U+000C Formfeed
470 U+000D Carriage return
471 U+0085 Next line
472 U+2028 Line separator
473 U+2029 Paragraph separator
474 <a name="newlineseq"></a></PRE>
475 </P>
476 <br><b>
477 Newline sequences
478 </b><br>
479 <P>
480 Outside a character class, by default, the escape sequence \R matches any
481 Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the following:
482 <pre>
483 (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
484 </pre>
485 This is an example of an "atomic group", details of which are given
486 <a href="#atomicgroup">below.</a>
487 This particular group matches either the two-character sequence CR followed by
488 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
489 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
490 line, U+0085). The two-character sequence is treated as a single unit that
491 cannot be split.
492 </P>
493 <P>
494 In UTF-8 mode, two additional characters whose codepoints are greater than 255
495 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
496 Unicode character property support is not needed for these characters to be
497 recognized.
498 </P>
499 <P>
500 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
501 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
502 either at compile time or when the pattern is matched. (BSR is an abbrevation
503 for "backslash R".) This can be made the default when PCRE is built; if this is
504 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
505 It is also possible to specify these settings by starting a pattern string with
506 one of the following sequences:
507 <pre>
508 (*BSR_ANYCRLF) CR, LF, or CRLF only
509 (*BSR_UNICODE) any Unicode newline sequence
510 </pre>
511 These override the default and the options given to <b>pcre_compile()</b> or
512 <b>pcre_compile2()</b>, but they can be overridden by options given to
513 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. Note that these special settings,
514 which are not Perl-compatible, are recognized only at the very start of a
515 pattern, and that they must be in upper case. If more than one of them is
516 present, the last one is used. They can be combined with a change of newline
517 convention; for example, a pattern can start with:
518 <pre>
520 </pre>
521 They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
522 a character class, \R is treated as an unrecognized escape sequence, and so
523 matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
524 <a name="uniextseq"></a></P>
525 <br><b>
526 Unicode character properties
527 </b><br>
528 <P>
529 When PCRE is built with Unicode character property support, three additional
530 escape sequences that match characters with specific properties are available.
531 When not in UTF-8 mode, these sequences are of course limited to testing
532 characters whose codepoints are less than 256, but they do work in this mode.
533 The extra escape sequences are:
534 <pre>
535 \p{<i>xx</i>} a character with the <i>xx</i> property
536 \P{<i>xx</i>} a character without the <i>xx</i> property
537 \X an extended Unicode sequence
538 </pre>
539 The property names represented by <i>xx</i> above are limited to the Unicode
540 script names, the general category properties, "Any", which matches any
541 character (including newline), and some special PCRE properties (described
542 in the
543 <a href="#extraprops">next section).</a>
544 Other Perl properties such as "InMusicalSymbols" are not currently supported by
545 PCRE. Note that \P{Any} does not match any characters, so always causes a
546 match failure.
547 </P>
548 <P>
549 Sets of Unicode characters are defined as belonging to certain scripts. A
550 character from one of these sets can be matched using a script name. For
551 example:
552 <pre>
553 \p{Greek}
554 \P{Han}
555 </pre>
556 Those that are not part of an identified script are lumped together as
557 "Common". The current list of scripts is:
558 </P>
559 <P>
560 Arabic,
561 Armenian,
562 Avestan,
563 Balinese,
564 Bamum,
565 Bengali,
566 Bopomofo,
567 Braille,
568 Buginese,
569 Buhid,
570 Canadian_Aboriginal,
571 Carian,
572 Cham,
573 Cherokee,
574 Common,
575 Coptic,
576 Cuneiform,
577 Cypriot,
578 Cyrillic,
579 Deseret,
580 Devanagari,
581 Egyptian_Hieroglyphs,
582 Ethiopic,
583 Georgian,
584 Glagolitic,
585 Gothic,
586 Greek,
587 Gujarati,
588 Gurmukhi,
589 Han,
590 Hangul,
591 Hanunoo,
592 Hebrew,
593 Hiragana,
594 Imperial_Aramaic,
595 Inherited,
596 Inscriptional_Pahlavi,
597 Inscriptional_Parthian,
598 Javanese,
599 Kaithi,
600 Kannada,
601 Katakana,
602 Kayah_Li,
603 Kharoshthi,
604 Khmer,
605 Lao,
606 Latin,
607 Lepcha,
608 Limbu,
609 Linear_B,
610 Lisu,
611 Lycian,
612 Lydian,
613 Malayalam,
614 Meetei_Mayek,
615 Mongolian,
616 Myanmar,
617 New_Tai_Lue,
618 Nko,
619 Ogham,
620 Old_Italic,
621 Old_Persian,
622 Old_South_Arabian,
623 Old_Turkic,
624 Ol_Chiki,
625 Oriya,
626 Osmanya,
627 Phags_Pa,
628 Phoenician,
629 Rejang,
630 Runic,
631 Samaritan,
632 Saurashtra,
633 Shavian,
634 Sinhala,
635 Sundanese,
636 Syloti_Nagri,
637 Syriac,
638 Tagalog,
639 Tagbanwa,
640 Tai_Le,
641 Tai_Tham,
642 Tai_Viet,
643 Tamil,
644 Telugu,
645 Thaana,
646 Thai,
647 Tibetan,
648 Tifinagh,
649 Ugaritic,
650 Vai,
651 Yi.
652 </P>
653 <P>
654 Each character has exactly one Unicode general category property, specified by
655 a two-letter abbreviation. For compatibility with Perl, negation can be
656 specified by including a circumflex between the opening brace and the property
657 name. For example, \p{^Lu} is the same as \P{Lu}.
658 </P>
659 <P>
660 If only one letter is specified with \p or \P, it includes all the general
661 category properties that start with that letter. In this case, in the absence
662 of negation, the curly brackets in the escape sequence are optional; these two
663 examples have the same effect:
664 <pre>
665 \p{L}
666 \pL
667 </pre>
668 The following general category property codes are supported:
669 <pre>
670 C Other
671 Cc Control
672 Cf Format
673 Cn Unassigned
674 Co Private use
675 Cs Surrogate
677 L Letter
678 Ll Lower case letter
679 Lm Modifier letter
680 Lo Other letter
681 Lt Title case letter
682 Lu Upper case letter
684 M Mark
685 Mc Spacing mark
686 Me Enclosing mark
687 Mn Non-spacing mark
689 N Number
690 Nd Decimal number
691 Nl Letter number
692 No Other number
694 P Punctuation
695 Pc Connector punctuation
696 Pd Dash punctuation
697 Pe Close punctuation
698 Pf Final punctuation
699 Pi Initial punctuation
700 Po Other punctuation
701 Ps Open punctuation
703 S Symbol
704 Sc Currency symbol
705 Sk Modifier symbol
706 Sm Mathematical symbol
707 So Other symbol
709 Z Separator
710 Zl Line separator
711 Zp Paragraph separator
712 Zs Space separator
713 </pre>
714 The special property L& is also supported: it matches a character that has
715 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
716 a modifier or "other".
717 </P>
718 <P>
719 The Cs (Surrogate) property applies only to characters in the range U+D800 to
720 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
721 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
722 (see the discussion of PCRE_NO_UTF8_CHECK in the
723 <a href="pcreapi.html"><b>pcreapi</b></a>
724 page). Perl does not support the Cs property.
725 </P>
726 <P>
727 The long synonyms for property names that Perl supports (such as \p{Letter})
728 are not supported by PCRE, nor is it permitted to prefix any of these
729 properties with "Is".
730 </P>
731 <P>
732 No character that is in the Unicode table has the Cn (unassigned) property.
733 Instead, this property is assumed for any code point that is not in the
734 Unicode table.
735 </P>
736 <P>
737 Specifying caseless matching does not affect these escape sequences. For
738 example, \p{Lu} always matches only upper case letters.
739 </P>
740 <P>
741 The \X escape matches any number of Unicode characters that form an extended
742 Unicode sequence. \X is equivalent to
743 <pre>
744 (?&#62;\PM\pM*)
745 </pre>
746 That is, it matches a character without the "mark" property, followed by zero
747 or more characters with the "mark" property, and treats the sequence as an
748 atomic group
749 <a href="#atomicgroup">(see below).</a>
750 Characters with the "mark" property are typically accents that affect the
751 preceding character. None of them have codepoints less than 256, so in
752 non-UTF-8 mode \X matches any one character.
753 </P>
754 <P>
755 Matching characters by Unicode property is not fast, because PCRE has to search
756 a structure that contains data for over fifteen thousand characters. That is
757 why the traditional escape sequences such as \d and \w do not use Unicode
758 properties in PCRE by default, though you can make them do so by setting the
759 PCRE_UCP option for <b>pcre_compile()</b> or by starting the pattern with
760 (*UCP).
761 <a name="extraprops"></a></P>
762 <br><b>
763 PCRE's additional properties
764 </b><br>
765 <P>
766 As well as the standard Unicode properties described in the previous
767 section, PCRE supports four more that make it possible to convert traditional
768 escape sequences such as \w and \s and POSIX character classes to use Unicode
769 properties. PCRE uses these non-standard, non-Perl properties internally when
770 PCRE_UCP is set. They are:
771 <pre>
772 Xan Any alphanumeric character
773 Xps Any POSIX space character
774 Xsp Any Perl space character
775 Xwd Any Perl "word" character
776 </pre>
777 Xan matches characters that have either the L (letter) or the N (number)
778 property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
779 carriage return, and any other character that has the Z (separator) property.
780 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
781 same characters as Xan, plus underscore.
782 <a name="resetmatchstart"></a></P>
783 <br><b>
784 Resetting the match start
785 </b><br>
786 <P>
787 The escape sequence \K causes any previously matched characters not to be
788 included in the final matched sequence. For example, the pattern:
789 <pre>
790 foo\Kbar
791 </pre>
792 matches "foobar", but reports that it has matched "bar". This feature is
793 similar to a lookbehind assertion
794 <a href="#lookbehind">(described below).</a>
795 However, in this case, the part of the subject before the real match does not
796 have to be of fixed length, as lookbehind assertions do. The use of \K does
797 not interfere with the setting of
798 <a href="#subpattern">captured substrings.</a>
799 For example, when the pattern
800 <pre>
801 (foo)\Kbar
802 </pre>
803 matches "foobar", the first substring is still set to "foo".
804 </P>
805 <P>
806 Perl documents that the use of \K within assertions is "not well defined". In
807 PCRE, \K is acted upon when it occurs inside positive assertions, but is
808 ignored in negative assertions.
809 <a name="smallassertions"></a></P>
810 <br><b>
811 Simple assertions
812 </b><br>
813 <P>
814 The final use of backslash is for certain simple assertions. An assertion
815 specifies a condition that has to be met at a particular point in a match,
816 without consuming any characters from the subject string. The use of
817 subpatterns for more complicated assertions is described
818 <a href="#bigassertions">below.</a>
819 The backslashed assertions are:
820 <pre>
821 \b matches at a word boundary
822 \B matches when not at a word boundary
823 \A matches at the start of the subject
824 \Z matches at the end of the subject
825 also matches before a newline at the end of the subject
826 \z matches only at the end of the subject
827 \G matches at the first matching position in the subject
828 </pre>
829 Inside a character class, \b has a different meaning; it matches the backspace
830 character. If any other of these assertions appears in a character class, by
831 default it matches the corresponding literal character (for example, \B
832 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
833 escape sequence" error is generated instead.
834 </P>
835 <P>
836 A word boundary is a position in the subject string where the current character
837 and the previous character do not both match \w or \W (i.e. one matches
838 \w and the other matches \W), or the start or end of the string if the
839 first or last character matches \w, respectively. In UTF-8 mode, the meanings
840 of \w and \W can be changed by setting the PCRE_UCP option. When this is
841 done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
842 of word" or "end of word" metasequence. However, whatever follows \b normally
843 determines which it is. For example, the fragment \ba matches "a" at the start
844 of a word.
845 </P>
846 <P>
847 The \A, \Z, and \z assertions differ from the traditional circumflex and
848 dollar (described in the next section) in that they only ever match at the very
849 start and end of the subject string, whatever options are set. Thus, they are
850 independent of multiline mode. These three assertions are not affected by the
851 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
852 circumflex and dollar metacharacters. However, if the <i>startoffset</i>
853 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
854 at a point other than the beginning of the subject, \A can never match. The
855 difference between \Z and \z is that \Z matches before a newline at the end
856 of the string as well as at the very end, whereas \z matches only at the end.
857 </P>
858 <P>
859 The \G assertion is true only when the current matching position is at the
860 start point of the match, as specified by the <i>startoffset</i> argument of
861 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
862 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
863 arguments, you can mimic Perl's /g option, and it is in this kind of
864 implementation where \G can be useful.
865 </P>
866 <P>
867 Note, however, that PCRE's interpretation of \G, as the start of the current
868 match, is subtly different from Perl's, which defines it as the end of the
869 previous match. In Perl, these can be different when the previously matched
870 string was empty. Because PCRE does just one match at a time, it cannot
871 reproduce this behaviour.
872 </P>
873 <P>
874 If all the alternatives of a pattern begin with \G, the expression is anchored
875 to the starting match position, and the "anchored" flag is set in the compiled
876 regular expression.
877 </P>
878 <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
879 <P>
880 Outside a character class, in the default matching mode, the circumflex
881 character is an assertion that is true only if the current matching point is
882 at the start of the subject string. If the <i>startoffset</i> argument of
883 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
884 option is unset. Inside a character class, circumflex has an entirely different
885 meaning
886 <a href="#characterclass">(see below).</a>
887 </P>
888 <P>
889 Circumflex need not be the first character of the pattern if a number of
890 alternatives are involved, but it should be the first thing in each alternative
891 in which it appears if the pattern is ever to match that branch. If all
892 possible alternatives start with a circumflex, that is, if the pattern is
893 constrained to match only at the start of the subject, it is said to be an
894 "anchored" pattern. (There are also other constructs that can cause a pattern
895 to be anchored.)
896 </P>
897 <P>
898 A dollar character is an assertion that is true only if the current matching
899 point is at the end of the subject string, or immediately before a newline
900 at the end of the string (by default). Dollar need not be the last character of
901 the pattern if a number of alternatives are involved, but it should be the last
902 item in any branch in which it appears. Dollar has no special meaning in a
903 character class.
904 </P>
905 <P>
906 The meaning of dollar can be changed so that it matches only at the very end of
907 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
908 does not affect the \Z assertion.
909 </P>
910 <P>
911 The meanings of the circumflex and dollar characters are changed if the
912 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
913 immediately after internal newlines as well as at the start of the subject
914 string. It does not match after a newline that ends the string. A dollar
915 matches before any newlines in the string, as well as at the very end, when
916 PCRE_MULTILINE is set. When newline is specified as the two-character
917 sequence CRLF, isolated CR and LF characters do not indicate newlines.
918 </P>
919 <P>
920 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
921 \n represents a newline) in multiline mode, but not otherwise. Consequently,
922 patterns that are anchored in single line mode because all branches start with
923 ^ are not anchored in multiline mode, and a match for circumflex is possible
924 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
925 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
926 </P>
927 <P>
928 Note that the sequences \A, \Z, and \z can be used to match the start and
929 end of the subject in both modes, and if all branches of a pattern start with
930 \A it is always anchored, whether or not PCRE_MULTILINE is set.
931 <a name="fullstopdot"></a></P>
932 <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
933 <P>
934 Outside a character class, a dot in the pattern matches any one character in
935 the subject string except (by default) a character that signifies the end of a
936 line. In UTF-8 mode, the matched character may be more than one byte long.
937 </P>
938 <P>
939 When a line ending is defined as a single character, dot never matches that
940 character; when the two-character sequence CRLF is used, dot does not match CR
941 if it is immediately followed by LF, but otherwise it matches all characters
942 (including isolated CRs and LFs). When any Unicode line endings are being
943 recognized, dot does not match CR or LF or any of the other line ending
944 characters.
945 </P>
946 <P>
947 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
948 option is set, a dot matches any one character, without exception. If the
949 two-character sequence CRLF is present in the subject string, it takes two dots
950 to match it.
951 </P>
952 <P>
953 The handling of dot is entirely independent of the handling of circumflex and
954 dollar, the only relationship being that they both involve newlines. Dot has no
955 special meaning in a character class.
956 </P>
957 <P>
958 The escape sequence \N behaves like a dot, except that it is not affected by
959 the PCRE_DOTALL option. In other words, it matches any character except one
960 that signifies the end of a line.
961 </P>
962 <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
963 <P>
964 Outside a character class, the escape sequence \C matches any one byte, both
965 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
966 characters. The feature is provided in Perl in order to match individual bytes
967 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
968 rest of the string may start with a malformed UTF-8 character. For this reason,
969 the \C escape sequence is best avoided.
970 </P>
971 <P>
972 PCRE does not allow \C to appear in lookbehind assertions
973 <a href="#lookbehind">(described below),</a>
974 because in UTF-8 mode this would make it impossible to calculate the length of
975 the lookbehind.
976 <a name="characterclass"></a></P>
977 <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
978 <P>
979 An opening square bracket introduces a character class, terminated by a closing
980 square bracket. A closing square bracket on its own is not special by default.
981 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
982 bracket causes a compile-time error. If a closing square bracket is required as
983 a member of the class, it should be the first data character in the class
984 (after an initial circumflex, if present) or escaped with a backslash.
985 </P>
986 <P>
987 A character class matches a single character in the subject. In UTF-8 mode, the
988 character may be more than one byte long. A matched character must be in the
989 set of characters defined by the class, unless the first character in the class
990 definition is a circumflex, in which case the subject character must not be in
991 the set defined by the class. If a circumflex is actually required as a member
992 of the class, ensure it is not the first character, or escape it with a
993 backslash.
994 </P>
995 <P>
996 For example, the character class [aeiou] matches any lower case vowel, while
997 [^aeiou] matches any character that is not a lower case vowel. Note that a
998 circumflex is just a convenient notation for specifying the characters that
999 are in the class by enumerating those that are not. A class that starts with a
1000 circumflex is not an assertion; it still consumes a character from the subject
1001 string, and therefore it fails if the current pointer is at the end of the
1002 string.
1003 </P>
1004 <P>
1005 In UTF-8 mode, characters with values greater than 255 can be included in a
1006 class as a literal string of bytes, or by using the \x{ escaping mechanism.
1007 </P>
1008 <P>
1009 When caseless matching is set, any letters in a class represent both their
1010 upper case and lower case versions, so for example, a caseless [aeiou] matches
1011 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1012 caseful version would. In UTF-8 mode, PCRE always understands the concept of
1013 case for characters whose values are less than 128, so caseless matching is
1014 always possible. For characters with higher values, the concept of case is
1015 supported if PCRE is compiled with Unicode property support, but not otherwise.
1016 If you want to use caseless matching in UTF8-mode for characters 128 and above,
1017 you must ensure that PCRE is compiled with Unicode property support as well as
1018 with UTF-8 support.
1019 </P>
1020 <P>
1021 Characters that might indicate line breaks are never treated in any special way
1022 when matching character classes, whatever line-ending sequence is in use, and
1023 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1024 such as [^a] always matches one of these characters.
1025 </P>
1026 <P>
1027 The minus (hyphen) character can be used to specify a range of characters in a
1028 character class. For example, [d-m] matches any letter between d and m,
1029 inclusive. If a minus character is required in a class, it must be escaped with
1030 a backslash or appear in a position where it cannot be interpreted as
1031 indicating a range, typically as the first or last character in the class.
1032 </P>
1033 <P>
1034 It is not possible to have the literal character "]" as the end character of a
1035 range. A pattern such as [W-]46] is interpreted as a class of two characters
1036 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1037 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1038 the end of range, so [W-\]46] is interpreted as a class containing a range
1039 followed by two other characters. The octal or hexadecimal representation of
1040 "]" can also be used to end a range.
1041 </P>
1042 <P>
1043 Ranges operate in the collating sequence of character values. They can also be
1044 used for characters specified numerically, for example [\000-\037]. In UTF-8
1045 mode, ranges can include characters whose values are greater than 255, for
1046 example [\x{100}-\x{2ff}].
1047 </P>
1048 <P>
1049 If a range that includes letters is used when caseless matching is set, it
1050 matches the letters in either case. For example, [W-c] is equivalent to
1051 [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1052 tables for a French locale are in use, [\xc8-\xcb] matches accented E
1053 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1054 characters with values greater than 128 only when it is compiled with Unicode
1055 property support.
1056 </P>
1057 <P>
1058 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
1059 \V, \w, and \W may appear in a character class, and add the characters that
1060 they match to the class. For example, [\dABCDEF] matches any hexadecimal
1061 digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \d, \s, \w
1062 and their upper case partners, just as it does when they appear outside a
1063 character class, as described in the section entitled
1064 <a href="#genericchartypes">"Generic character types"</a>
1065 above. The escape sequence \b has a different meaning inside a character
1066 class; it matches the backspace character. The sequences \B, \N, \R, and \X
1067 are not special inside a character class. Like any other unrecognized escape
1068 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1069 default, but cause an error if the PCRE_EXTRA option is set.
1070 </P>
1071 <P>
1072 A circumflex can conveniently be used with the upper case character types to
1073 specify a more restricted set of characters than the matching lower case type.
1074 For example, the class [^\W_] matches any letter or digit, but not underscore,
1075 whereas [\w] includes underscore. A positive character class should be read as
1076 "something OR something OR ..." and a negative class as "NOT something AND NOT
1077 something AND NOT ...".
1078 </P>
1079 <P>
1080 The only metacharacters that are recognized in character classes are backslash,
1081 hyphen (only where it can be interpreted as specifying a range), circumflex
1082 (only at the start), opening square bracket (only when it can be interpreted as
1083 introducing a POSIX class name - see the next section), and the terminating
1084 closing square bracket. However, escaping other non-alphanumeric characters
1085 does no harm.
1086 </P>
1087 <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
1088 <P>
1089 Perl supports the POSIX notation for character classes. This uses names
1090 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1091 this notation. For example,
1092 <pre>
1093 [01[:alpha:]%]
1094 </pre>
1095 matches "0", "1", any alphabetic character, or "%". The supported class names
1096 are:
1097 <pre>
1098 alnum letters and digits
1099 alpha letters
1100 ascii character codes 0 - 127
1101 blank space or tab only
1102 cntrl control characters
1103 digit decimal digits (same as \d)
1104 graph printing characters, excluding space
1105 lower lower case letters
1106 print printing characters, including space
1107 punct printing characters, excluding letters and digits and space
1108 space white space (not quite the same as \s)
1109 upper upper case letters
1110 word "word" characters (same as \w)
1111 xdigit hexadecimal digits
1112 </pre>
1113 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1114 space (32). Notice that this list includes the VT character (code 11). This
1115 makes "space" different to \s, which does not include VT (for Perl
1116 compatibility).
1117 </P>
1118 <P>
1119 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1120 5.8. Another Perl extension is negation, which is indicated by a ^ character
1121 after the colon. For example,
1122 <pre>
1123 [12[:^digit:]]
1124 </pre>
1125 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1126 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1127 supported, and an error is given if they are encountered.
1128 </P>
1129 <P>
1130 By default, in UTF-8 mode, characters with values greater than 128 do not match
1131 any of the POSIX character classes. However, if the PCRE_UCP option is passed
1132 to <b>pcre_compile()</b>, some of the classes are changed so that Unicode
1133 character properties are used. This is achieved by replacing the POSIX classes
1134 by other sequences, as follows:
1135 <pre>
1136 [:alnum:] becomes \p{Xan}
1137 [:alpha:] becomes \p{L}
1138 [:blank:] becomes \h
1139 [:digit:] becomes \p{Nd}
1140 [:lower:] becomes \p{Ll}
1141 [:space:] becomes \p{Xps}
1142 [:upper:] becomes \p{Lu}
1143 [:word:] becomes \p{Xwd}
1144 </pre>
1145 Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX
1146 classes are unchanged, and match only characters with code points less than
1147 128.
1148 </P>
1149 <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
1150 <P>
1151 Vertical bar characters are used to separate alternative patterns. For example,
1152 the pattern
1153 <pre>
1154 gilbert|sullivan
1155 </pre>
1156 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1157 and an empty alternative is permitted (matching the empty string). The matching
1158 process tries each alternative in turn, from left to right, and the first one
1159 that succeeds is used. If the alternatives are within a subpattern
1160 <a href="#subpattern">(defined below),</a>
1161 "succeeds" means matching the rest of the main pattern as well as the
1162 alternative in the subpattern.
1163 </P>
1164 <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1165 <P>
1166 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1167 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1168 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1169 The option letters are
1170 <pre>
1171 i for PCRE_CASELESS
1173 s for PCRE_DOTALL
1174 x for PCRE_EXTENDED
1175 </pre>
1176 For example, (?im) sets caseless, multiline matching. It is also possible to
1177 unset these options by preceding the letter with a hyphen, and a combined
1178 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1179 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1180 permitted. If a letter appears both before and after the hyphen, the option is
1181 unset.
1182 </P>
1183 <P>
1184 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1185 changed in the same way as the Perl-compatible options by using the characters
1186 J, U and X respectively.
1187 </P>
1188 <P>
1189 When one of these option changes occurs at top level (that is, not inside
1190 subpattern parentheses), the change applies to the remainder of the pattern
1191 that follows. If the change is placed right at the start of a pattern, PCRE
1192 extracts it into the global options (and it will therefore show up in data
1193 extracted by the <b>pcre_fullinfo()</b> function).
1194 </P>
1195 <P>
1196 An option change within a subpattern (see below for a description of
1197 subpatterns) affects only that part of the subpattern that follows it, so
1198 <pre>
1199 (a(?i)b)c
1200 </pre>
1201 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1202 By this means, options can be made to have different settings in different
1203 parts of the pattern. Any changes made in one alternative do carry on
1204 into subsequent branches within the same subpattern. For example,
1205 <pre>
1206 (a(?i)b|c)
1207 </pre>
1208 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1209 branch is abandoned before the option setting. This is because the effects of
1210 option settings happen at compile time. There would be some very weird
1211 behaviour otherwise.
1212 </P>
1213 <P>
1214 <b>Note:</b> There are other PCRE-specific options that can be set by the
1215 application when the compile or match functions are called. In some cases the
1216 pattern can contain special leading sequences such as (*CRLF) to override what
1217 the application has set or what has been defaulted. Details are given in the
1218 section entitled
1219 <a href="#newlineseq">"Newline sequences"</a>
1220 above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1221 to set UTF-8 and Unicode property modes; they are equivalent to setting the
1222 PCRE_UTF8 and the PCRE_UCP options, respectively.
1223 <a name="subpattern"></a></P>
1224 <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1225 <P>
1226 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1227 Turning part of a pattern into a subpattern does two things:
1228 <br>
1229 <br>
1230 1. It localizes a set of alternatives. For example, the pattern
1231 <pre>
1232 cat(aract|erpillar|)
1233 </pre>
1234 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1235 match "cataract", "erpillar" or an empty string.
1236 <br>
1237 <br>
1238 2. It sets up the subpattern as a capturing subpattern. This means that, when
1239 the whole pattern matches, that portion of the subject string that matched the
1240 subpattern is passed back to the caller via the <i>ovector</i> argument of
1241 <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
1242 from 1) to obtain numbers for the capturing subpatterns. For example, if the
1243 string "the red king" is matched against the pattern
1244 <pre>
1245 the ((red|white) (king|queen))
1246 </pre>
1247 the captured substrings are "red king", "red", and "king", and are numbered 1,
1248 2, and 3, respectively.
1249 </P>
1250 <P>
1251 The fact that plain parentheses fulfil two functions is not always helpful.
1252 There are often times when a grouping subpattern is required without a
1253 capturing requirement. If an opening parenthesis is followed by a question mark
1254 and a colon, the subpattern does not do any capturing, and is not counted when
1255 computing the number of any subsequent capturing subpatterns. For example, if
1256 the string "the white queen" is matched against the pattern
1257 <pre>
1258 the ((?:red|white) (king|queen))
1259 </pre>
1260 the captured substrings are "white queen" and "queen", and are numbered 1 and
1261 2. The maximum number of capturing subpatterns is 65535.
1262 </P>
1263 <P>
1264 As a convenient shorthand, if any option settings are required at the start of
1265 a non-capturing subpattern, the option letters may appear between the "?" and
1266 the ":". Thus the two patterns
1267 <pre>
1268 (?i:saturday|sunday)
1269 (?:(?i)saturday|sunday)
1270 </pre>
1271 match exactly the same set of strings. Because alternative branches are tried
1272 from left to right, and options are not reset until the end of the subpattern
1273 is reached, an option setting in one branch does affect subsequent branches, so
1274 the above patterns match "SUNDAY" as well as "Saturday".
1275 <a name="dupsubpatternnumber"></a></P>
1276 <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1277 <P>
1278 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1279 the same numbers for its capturing parentheses. Such a subpattern starts with
1280 (?| and is itself a non-capturing subpattern. For example, consider this
1281 pattern:
1282 <pre>
1283 (?|(Sat)ur|(Sun))day
1284 </pre>
1285 Because the two alternatives are inside a (?| group, both sets of capturing
1286 parentheses are numbered one. Thus, when the pattern matches, you can look
1287 at captured substring number one, whichever alternative matched. This construct
1288 is useful when you want to capture part, but not all, of one of a number of
1289 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1290 number is reset at the start of each branch. The numbers of any capturing
1291 parentheses that follow the subpattern start after the highest number used in
1292 any branch. The following example is taken from the Perl documentation. The
1293 numbers underneath show in which buffer the captured content will be stored.
1294 <pre>
1295 # before ---------------branch-reset----------- after
1296 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1297 # 1 2 2 3 2 3 4
1298 </pre>
1299 A back reference to a numbered subpattern uses the most recent value that is
1300 set for that number by any subpattern. The following pattern matches "abcabc"
1301 or "defdef":
1302 <pre>
1303 /(?|(abc)|(def))\1/
1304 </pre>
1305 In contrast, a recursive or "subroutine" call to a numbered subpattern always
1306 refers to the first one in the pattern with the given number. The following
1307 pattern matches "abcabc" or "defabc":
1308 <pre>
1309 /(?|(abc)|(def))(?1)/
1310 </pre>
1311 If a
1312 <a href="#conditions">condition test</a>
1313 for a subpattern's having matched refers to a non-unique number, the test is
1314 true if any of the subpatterns of that number have matched.
1315 </P>
1316 <P>
1317 An alternative approach to using this "branch reset" feature is to use
1318 duplicate named subpatterns, as described in the next section.
1319 </P>
1320 <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1321 <P>
1322 Identifying capturing parentheses by number is simple, but it can be very hard
1323 to keep track of the numbers in complicated regular expressions. Furthermore,
1324 if an expression is modified, the numbers may change. To help with this
1325 difficulty, PCRE supports the naming of subpatterns. This feature was not
1326 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1327 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1328 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1329 have different names, but PCRE does not.
1330 </P>
1331 <P>
1332 In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1333 (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1334 parentheses from other parts of the pattern, such as
1335 <a href="#backreferences">back references,</a>
1336 <a href="#recursion">recursion,</a>
1337 and
1338 <a href="#conditions">conditions,</a>
1339 can be made by name as well as by number.
1340 </P>
1341 <P>
1342 Names consist of up to 32 alphanumeric characters and underscores. Named
1343 capturing parentheses are still allocated numbers as well as names, exactly as
1344 if the names were not present. The PCRE API provides function calls for
1345 extracting the name-to-number translation table from a compiled pattern. There
1346 is also a convenience function for extracting a captured substring by name.
1347 </P>
1348 <P>
1349 By default, a name must be unique within a pattern, but it is possible to relax
1350 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1351 names are also always permitted for subpatterns with the same number, set up as
1352 described in the previous section.) Duplicate names can be useful for patterns
1353 where only one instance of the named parentheses can match. Suppose you want to
1354 match the name of a weekday, either as a 3-letter abbreviation or as the full
1355 name, and in both cases you want to extract the abbreviation. This pattern
1356 (ignoring the line breaks) does the job:
1357 <pre>
1358 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1359 (?&#60;DN&#62;Tue)(?:sday)?|
1360 (?&#60;DN&#62;Wed)(?:nesday)?|
1361 (?&#60;DN&#62;Thu)(?:rsday)?|
1362 (?&#60;DN&#62;Sat)(?:urday)?
1363 </pre>
1364 There are five capturing substrings, but only one is ever set after a match.
1365 (An alternative way of solving this problem is to use a "branch reset"
1366 subpattern, as described in the previous section.)
1367 </P>
1368 <P>
1369 The convenience function for extracting the data by name returns the substring
1370 for the first (and in this example, the only) subpattern of that name that
1371 matched. This saves searching to find which numbered subpattern it was.
1372 </P>
1373 <P>
1374 If you make a back reference to a non-unique named subpattern from elsewhere in
1375 the pattern, the one that corresponds to the first occurrence of the name is
1376 used. In the absence of duplicate numbers (see the previous section) this is
1377 the one with the lowest number. If you use a named reference in a condition
1378 test (see the
1379 <a href="#conditions">section about conditions</a>
1380 below), either to check whether a subpattern has matched, or to check for
1381 recursion, all subpatterns with the same name are tested. If the condition is
1382 true for any one of them, the overall condition is true. This is the same
1383 behaviour as testing by number. For further details of the interfaces for
1384 handling named subpatterns, see the
1385 <a href="pcreapi.html"><b>pcreapi</b></a>
1386 documentation.
1387 </P>
1388 <P>
1389 <b>Warning:</b> You cannot use different names to distinguish between two
1390 subpatterns with the same number because PCRE uses only the numbers when
1391 matching. For this reason, an error is given at compile time if different names
1392 are given to subpatterns with the same number. However, you can give the same
1393 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1394 </P>
1395 <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1396 <P>
1397 Repetition is specified by quantifiers, which can follow any of the following
1398 items:
1399 <pre>
1400 a literal data character
1401 the dot metacharacter
1402 the \C escape sequence
1403 the \X escape sequence (in UTF-8 mode with Unicode properties)
1404 the \R escape sequence
1405 an escape such as \d or \pL that matches a single character
1406 a character class
1407 a back reference (see next section)
1408 a parenthesized subpattern (unless it is an assertion)
1409 a recursive or "subroutine" call to a subpattern
1410 </pre>
1411 The general repetition quantifier specifies a minimum and maximum number of
1412 permitted matches, by giving the two numbers in curly brackets (braces),
1413 separated by a comma. The numbers must be less than 65536, and the first must
1414 be less than or equal to the second. For example:
1415 <pre>
1416 z{2,4}
1417 </pre>
1418 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1419 character. If the second number is omitted, but the comma is present, there is
1420 no upper limit; if the second number and the comma are both omitted, the
1421 quantifier specifies an exact number of required matches. Thus
1422 <pre>
1423 [aeiou]{3,}
1424 </pre>
1425 matches at least 3 successive vowels, but may match many more, while
1426 <pre>
1427 \d{8}
1428 </pre>
1429 matches exactly 8 digits. An opening curly bracket that appears in a position
1430 where a quantifier is not allowed, or one that does not match the syntax of a
1431 quantifier, is taken as a literal character. For example, {,6} is not a
1432 quantifier, but a literal string of four characters.
1433 </P>
1434 <P>
1435 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1436 bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
1437 which is represented by a two-byte sequence. Similarly, when Unicode property
1438 support is available, \X{3} matches three Unicode extended sequences, each of
1439 which may be several bytes long (and they may be of different lengths).
1440 </P>
1441 <P>
1442 The quantifier {0} is permitted, causing the expression to behave as if the
1443 previous item and the quantifier were not present. This may be useful for
1444 subpatterns that are referenced as
1445 <a href="#subpatternsassubroutines">subroutines</a>
1446 from elsewhere in the pattern (but see also the section entitled
1447 <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1448 below). Items other than subpatterns that have a {0} quantifier are omitted
1449 from the compiled pattern.
1450 </P>
1451 <P>
1452 For convenience, the three most common quantifiers have single-character
1453 abbreviations:
1454 <pre>
1455 * is equivalent to {0,}
1456 + is equivalent to {1,}
1457 ? is equivalent to {0,1}
1458 </pre>
1459 It is possible to construct infinite loops by following a subpattern that can
1460 match no characters with a quantifier that has no upper limit, for example:
1461 <pre>
1462 (a?)*
1463 </pre>
1464 Earlier versions of Perl and PCRE used to give an error at compile time for
1465 such patterns. However, because there are cases where this can be useful, such
1466 patterns are now accepted, but if any repetition of the subpattern does in fact
1467 match no characters, the loop is forcibly broken.
1468 </P>
1469 <P>
1470 By default, the quantifiers are "greedy", that is, they match as much as
1471 possible (up to the maximum number of permitted times), without causing the
1472 rest of the pattern to fail. The classic example of where this gives problems
1473 is in trying to match comments in C programs. These appear between /* and */
1474 and within the comment, individual * and / characters may appear. An attempt to
1475 match C comments by applying the pattern
1476 <pre>
1477 /\*.*\*/
1478 </pre>
1479 to the string
1480 <pre>
1481 /* first comment */ not comment /* second comment */
1482 </pre>
1483 fails, because it matches the entire string owing to the greediness of the .*
1484 item.
1485 </P>
1486 <P>
1487 However, if a quantifier is followed by a question mark, it ceases to be
1488 greedy, and instead matches the minimum number of times possible, so the
1489 pattern
1490 <pre>
1491 /\*.*?\*/
1492 </pre>
1493 does the right thing with the C comments. The meaning of the various
1494 quantifiers is not otherwise changed, just the preferred number of matches.
1495 Do not confuse this use of question mark with its use as a quantifier in its
1496 own right. Because it has two uses, it can sometimes appear doubled, as in
1497 <pre>
1498 \d??\d
1499 </pre>
1500 which matches one digit by preference, but can match two if that is the only
1501 way the rest of the pattern matches.
1502 </P>
1503 <P>
1504 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1505 the quantifiers are not greedy by default, but individual ones can be made
1506 greedy by following them with a question mark. In other words, it inverts the
1507 default behaviour.
1508 </P>
1509 <P>
1510 When a parenthesized subpattern is quantified with a minimum repeat count that
1511 is greater than 1 or with a limited maximum, more memory is required for the
1512 compiled pattern, in proportion to the size of the minimum or maximum.
1513 </P>
1514 <P>
1515 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1516 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1517 implicitly anchored, because whatever follows will be tried against every
1518 character position in the subject string, so there is no point in retrying the
1519 overall match at any position after the first. PCRE normally treats such a
1520 pattern as though it were preceded by \A.
1521 </P>
1522 <P>
1523 In cases where it is known that the subject string contains no newlines, it is
1524 worth setting PCRE_DOTALL in order to obtain this optimization, or
1525 alternatively using ^ to indicate anchoring explicitly.
1526 </P>
1527 <P>
1528 However, there is one situation where the optimization cannot be used. When .*
1529 is inside capturing parentheses that are the subject of a back reference
1530 elsewhere in the pattern, a match at the start may fail where a later one
1531 succeeds. Consider, for example:
1532 <pre>
1533 (.*)abc\1
1534 </pre>
1535 If the subject is "xyz123abc123" the match point is the fourth character. For
1536 this reason, such a pattern is not implicitly anchored.
1537 </P>
1538 <P>
1539 When a capturing subpattern is repeated, the value captured is the substring
1540 that matched the final iteration. For example, after
1541 <pre>
1542 (tweedle[dume]{3}\s*)+
1543 </pre>
1544 has matched "tweedledum tweedledee" the value of the captured substring is
1545 "tweedledee". However, if there are nested capturing subpatterns, the
1546 corresponding captured values may have been set in previous iterations. For
1547 example, after
1548 <pre>
1549 /(a|(b))+/
1550 </pre>
1551 matches "aba" the value of the second captured substring is "b".
1552 <a name="atomicgroup"></a></P>
1553 <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1554 <P>
1555 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1556 repetition, failure of what follows normally causes the repeated item to be
1557 re-evaluated to see if a different number of repeats allows the rest of the
1558 pattern to match. Sometimes it is useful to prevent this, either to change the
1559 nature of the match, or to cause it fail earlier than it otherwise might, when
1560 the author of the pattern knows there is no point in carrying on.
1561 </P>
1562 <P>
1563 Consider, for example, the pattern \d+foo when applied to the subject line
1564 <pre>
1565 123456bar
1566 </pre>
1567 After matching all 6 digits and then failing to match "foo", the normal
1568 action of the matcher is to try again with only 5 digits matching the \d+
1569 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1570 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1571 that once a subpattern has matched, it is not to be re-evaluated in this way.
1572 </P>
1573 <P>
1574 If we use atomic grouping for the previous example, the matcher gives up
1575 immediately on failing to match "foo" the first time. The notation is a kind of
1576 special parenthesis, starting with (?&#62; as in this example:
1577 <pre>
1578 (?&#62;\d+)foo
1579 </pre>
1580 This kind of parenthesis "locks up" the part of the pattern it contains once
1581 it has matched, and a failure further into the pattern is prevented from
1582 backtracking into it. Backtracking past it to previous items, however, works as
1583 normal.
1584 </P>
1585 <P>
1586 An alternative description is that a subpattern of this type matches the string
1587 of characters that an identical standalone pattern would match, if anchored at
1588 the current point in the subject string.
1589 </P>
1590 <P>
1591 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1592 the above example can be thought of as a maximizing repeat that must swallow
1593 everything it can. So, while both \d+ and \d+? are prepared to adjust the
1594 number of digits they match in order to make the rest of the pattern match,
1595 (?&#62;\d+) can only match an entire sequence of digits.
1596 </P>
1597 <P>
1598 Atomic groups in general can of course contain arbitrarily complicated
1599 subpatterns, and can be nested. However, when the subpattern for an atomic
1600 group is just a single repeated item, as in the example above, a simpler
1601 notation, called a "possessive quantifier" can be used. This consists of an
1602 additional + character following a quantifier. Using this notation, the
1603 previous example can be rewritten as
1604 <pre>
1605 \d++foo
1606 </pre>
1607 Note that a possessive quantifier can be used with an entire group, for
1608 example:
1609 <pre>
1610 (abc|xyz){2,3}+
1611 </pre>
1612 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1613 option is ignored. They are a convenient notation for the simpler forms of
1614 atomic group. However, there is no difference in the meaning of a possessive
1615 quantifier and the equivalent atomic group, though there may be a performance
1616 difference; possessive quantifiers should be slightly faster.
1617 </P>
1618 <P>
1619 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1620 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1621 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1622 package, and PCRE copied it from there. It ultimately found its way into Perl
1623 at release 5.10.
1624 </P>
1625 <P>
1626 PCRE has an optimization that automatically "possessifies" certain simple
1627 pattern constructs. For example, the sequence A+B is treated as A++B because
1628 there is no point in backtracking into a sequence of A's when B must follow.
1629 </P>
1630 <P>
1631 When a pattern contains an unlimited repeat inside a subpattern that can itself
1632 be repeated an unlimited number of times, the use of an atomic group is the
1633 only way to avoid some failing matches taking a very long time indeed. The
1634 pattern
1635 <pre>
1636 (\D+|&#60;\d+&#62;)*[!?]
1637 </pre>
1638 matches an unlimited number of substrings that either consist of non-digits, or
1639 digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1640 quickly. However, if it is applied to
1641 <pre>
1642 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1643 </pre>
1644 it takes a long time before reporting failure. This is because the string can
1645 be divided between the internal \D+ repeat and the external * repeat in a
1646 large number of ways, and all have to be tried. (The example uses [!?] rather
1647 than a single character at the end, because both PCRE and Perl have an
1648 optimization that allows for fast failure when a single character is used. They
1649 remember the last single character that is required for a match, and fail early
1650 if it is not present in the string.) If the pattern is changed so that it uses
1651 an atomic group, like this:
1652 <pre>
1653 ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1654 </pre>
1655 sequences of non-digits cannot be broken, and failure happens quickly.
1656 <a name="backreferences"></a></P>
1657 <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1658 <P>
1659 Outside a character class, a backslash followed by a digit greater than 0 (and
1660 possibly further digits) is a back reference to a capturing subpattern earlier
1661 (that is, to its left) in the pattern, provided there have been that many
1662 previous capturing left parentheses.
1663 </P>
1664 <P>
1665 However, if the decimal number following the backslash is less than 10, it is
1666 always taken as a back reference, and causes an error only if there are not
1667 that many capturing left parentheses in the entire pattern. In other words, the
1668 parentheses that are referenced need not be to the left of the reference for
1669 numbers less than 10. A "forward back reference" of this type can make sense
1670 when a repetition is involved and the subpattern to the right has participated
1671 in an earlier iteration.
1672 </P>
1673 <P>
1674 It is not possible to have a numerical "forward back reference" to a subpattern
1675 whose number is 10 or more using this syntax because a sequence such as \50 is
1676 interpreted as a character defined in octal. See the subsection entitled
1677 "Non-printing characters"
1678 <a href="#digitsafterbackslash">above</a>
1679 for further details of the handling of digits following a backslash. There is
1680 no such problem when named parentheses are used. A back reference to any
1681 subpattern is possible using named parentheses (see below).
1682 </P>
1683 <P>
1684 Another way of avoiding the ambiguity inherent in the use of digits following a
1685 backslash is to use the \g escape sequence. This escape must be followed by an
1686 unsigned number or a negative number, optionally enclosed in braces. These
1687 examples are all identical:
1688 <pre>
1689 (ring), \1
1690 (ring), \g1
1691 (ring), \g{1}
1692 </pre>
1693 An unsigned number specifies an absolute reference without the ambiguity that
1694 is present in the older syntax. It is also useful when literal digits follow
1695 the reference. A negative number is a relative reference. Consider this
1696 example:
1697 <pre>
1698 (abc(def)ghi)\g{-1}
1699 </pre>
1700 The sequence \g{-1} is a reference to the most recently started capturing
1701 subpattern before \g, that is, is it equivalent to \2 in this example.
1702 Similarly, \g{-2} would be equivalent to \1. The use of relative references
1703 can be helpful in long patterns, and also in patterns that are created by
1704 joining together fragments that contain references within themselves.
1705 </P>
1706 <P>
1707 A back reference matches whatever actually matched the capturing subpattern in
1708 the current subject string, rather than anything matching the subpattern
1709 itself (see
1710 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1711 below for a way of doing that). So the pattern
1712 <pre>
1713 (sens|respons)e and \1ibility
1714 </pre>
1715 matches "sense and sensibility" and "response and responsibility", but not
1716 "sense and responsibility". If caseful matching is in force at the time of the
1717 back reference, the case of letters is relevant. For example,
1718 <pre>
1719 ((?i)rah)\s+\1
1720 </pre>
1721 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1722 capturing subpattern is matched caselessly.
1723 </P>
1724 <P>
1725 There are several different ways of writing back references to named
1726 subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1727 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1728 back reference syntax, in which \g can be used for both numeric and named
1729 references, is also supported. We could rewrite the above example in any of
1730 the following ways:
1731 <pre>
1732 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1733 (?'p1'(?i)rah)\s+\k{p1}
1734 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1735 (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1736 </pre>
1737 A subpattern that is referenced by name may appear in the pattern before or
1738 after the reference.
1739 </P>
1740 <P>
1741 There may be more than one back reference to the same subpattern. If a
1742 subpattern has not actually been used in a particular match, any back
1743 references to it always fail by default. For example, the pattern
1744 <pre>
1745 (a|(bc))\2
1746 </pre>
1747 always fails if it starts to match "a" rather than "bc". However, if the
1748 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1749 unset value matches an empty string.
1750 </P>
1751 <P>
1752 Because there may be many capturing parentheses in a pattern, all digits
1753 following a backslash are taken as part of a potential back reference number.
1754 If the pattern continues with a digit character, some delimiter must be used to
1755 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1756 whitespace. Otherwise, the \g{ syntax or an empty comment (see
1757 <a href="#comments">"Comments"</a>
1758 below) can be used.
1759 </P>
1760 <br><b>
1761 Recursive back references
1762 </b><br>
1763 <P>
1764 A back reference that occurs inside the parentheses to which it refers fails
1765 when the subpattern is first used, so, for example, (a\1) never matches.
1766 However, such references can be useful inside repeated subpatterns. For
1767 example, the pattern
1768 <pre>
1769 (a|b\1)+
1770 </pre>
1771 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1772 the subpattern, the back reference matches the character string corresponding
1773 to the previous iteration. In order for this to work, the pattern must be such
1774 that the first iteration does not need to match the back reference. This can be
1775 done using alternation, as in the example above, or by a quantifier with a
1776 minimum of zero.
1777 </P>
1778 <P>
1779 Back references of this type cause the group that they reference to be treated
1780 as an
1781 <a href="#atomicgroup">atomic group.</a>
1782 Once the whole group has been matched, a subsequent matching failure cannot
1783 cause backtracking into the middle of the group.
1784 <a name="bigassertions"></a></P>
1785 <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1786 <P>
1787 An assertion is a test on the characters following or preceding the current
1788 matching point that does not actually consume any characters. The simple
1789 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1790 <a href="#smallassertions">above.</a>
1791 </P>
1792 <P>
1793 More complicated assertions are coded as subpatterns. There are two kinds:
1794 those that look ahead of the current position in the subject string, and those
1795 that look behind it. An assertion subpattern is matched in the normal way,
1796 except that it does not cause the current matching position to be changed.
1797 </P>
1798 <P>
1799 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1800 because it makes no sense to assert the same thing several times. If any kind
1801 of assertion contains capturing subpatterns within it, these are counted for
1802 the purposes of numbering the capturing subpatterns in the whole pattern.
1803 However, substring capturing is carried out only for positive assertions,
1804 because it does not make sense for negative assertions.
1805 </P>
1806 <br><b>
1807 Lookahead assertions
1808 </b><br>
1809 <P>
1810 Lookahead assertions start with (?= for positive assertions and (?! for
1811 negative assertions. For example,
1812 <pre>
1813 \w+(?=;)
1814 </pre>
1815 matches a word followed by a semicolon, but does not include the semicolon in
1816 the match, and
1817 <pre>
1818 foo(?!bar)
1819 </pre>
1820 matches any occurrence of "foo" that is not followed by "bar". Note that the
1821 apparently similar pattern
1822 <pre>
1823 (?!foo)bar
1824 </pre>
1825 does not find an occurrence of "bar" that is preceded by something other than
1826 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1827 (?!foo) is always true when the next three characters are "bar". A
1828 lookbehind assertion is needed to achieve the other effect.
1829 </P>
1830 <P>
1831 If you want to force a matching failure at some point in a pattern, the most
1832 convenient way to do it is with (?!) because an empty string always matches, so
1833 an assertion that requires there not to be an empty string must always fail.
1834 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1835 <a name="lookbehind"></a></P>
1836 <br><b>
1837 Lookbehind assertions
1838 </b><br>
1839 <P>
1840 Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1841 negative assertions. For example,
1842 <pre>
1843 (?&#60;!foo)bar
1844 </pre>
1845 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1846 a lookbehind assertion are restricted such that all the strings it matches must
1847 have a fixed length. However, if there are several top-level alternatives, they
1848 do not all have to have the same fixed length. Thus
1849 <pre>
1850 (?&#60;=bullock|donkey)
1851 </pre>
1852 is permitted, but
1853 <pre>
1854 (?&#60;!dogs?|cats?)
1855 </pre>
1856 causes an error at compile time. Branches that match different length strings
1857 are permitted only at the top level of a lookbehind assertion. This is an
1858 extension compared with Perl, which requires all branches to match the same
1859 length of string. An assertion such as
1860 <pre>
1861 (?&#60;=ab(c|de))
1862 </pre>
1863 is not permitted, because its single top-level branch can match two different
1864 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1865 branches:
1866 <pre>
1867 (?&#60;=abc|abde)
1868 </pre>
1869 In some cases, the escape sequence \K
1870 <a href="#resetmatchstart">(see above)</a>
1871 can be used instead of a lookbehind assertion to get round the fixed-length
1872 restriction.
1873 </P>
1874 <P>
1875 The implementation of lookbehind assertions is, for each alternative, to
1876 temporarily move the current position back by the fixed length and then try to
1877 match. If there are insufficient characters before the current position, the
1878 assertion fails.
1879 </P>
1880 <P>
1881 PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1882 to appear in lookbehind assertions, because it makes it impossible to calculate
1883 the length of the lookbehind. The \X and \R escapes, which can match
1884 different numbers of bytes, are also not permitted.
1885 </P>
1886 <P>
1887 <a href="#subpatternsassubroutines">"Subroutine"</a>
1888 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1889 as the subpattern matches a fixed-length string.
1890 <a href="#recursion">Recursion,</a>
1891 however, is not supported.
1892 </P>
1893 <P>
1894 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1895 specify efficient matching of fixed-length strings at the end of subject
1896 strings. Consider a simple pattern such as
1897 <pre>
1898 abcd$
1899 </pre>
1900 when applied to a long string that does not match. Because matching proceeds
1901 from left to right, PCRE will look for each "a" in the subject and then see if
1902 what follows matches the rest of the pattern. If the pattern is specified as
1903 <pre>
1904 ^.*abcd$
1905 </pre>
1906 the initial .* matches the entire string at first, but when this fails (because
1907 there is no following "a"), it backtracks to match all but the last character,
1908 then all but the last two characters, and so on. Once again the search for "a"
1909 covers the entire string, from right to left, so we are no better off. However,
1910 if the pattern is written as
1911 <pre>
1912 ^.*+(?&#60;=abcd)
1913 </pre>
1914 there can be no backtracking for the .*+ item; it can match only the entire
1915 string. The subsequent lookbehind assertion does a single test on the last four
1916 characters. If it fails, the match fails immediately. For long strings, this
1917 approach makes a significant difference to the processing time.
1918 </P>
1919 <br><b>
1920 Using multiple assertions
1921 </b><br>
1922 <P>
1923 Several assertions (of any sort) may occur in succession. For example,
1924 <pre>
1925 (?&#60;=\d{3})(?&#60;!999)foo
1926 </pre>
1927 matches "foo" preceded by three digits that are not "999". Notice that each of
1928 the assertions is applied independently at the same point in the subject
1929 string. First there is a check that the previous three characters are all
1930 digits, and then there is a check that the same three characters are not "999".
1931 This pattern does <i>not</i> match "foo" preceded by six characters, the first
1932 of which are digits and the last three of which are not "999". For example, it
1933 doesn't match "123abcfoo". A pattern to do that is
1934 <pre>
1935 (?&#60;=\d{3}...)(?&#60;!999)foo
1936 </pre>
1937 This time the first assertion looks at the preceding six characters, checking
1938 that the first three are digits, and then the second assertion checks that the
1939 preceding three characters are not "999".
1940 </P>
1941 <P>
1942 Assertions can be nested in any combination. For example,
1943 <pre>
1944 (?&#60;=(?&#60;!foo)bar)baz
1945 </pre>
1946 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1947 preceded by "foo", while
1948 <pre>
1949 (?&#60;=\d{3}(?!999)...)foo
1950 </pre>
1951 is another pattern that matches "foo" preceded by three digits and any three
1952 characters that are not "999".
1953 <a name="conditions"></a></P>
1954 <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1955 <P>
1956 It is possible to cause the matching process to obey a subpattern
1957 conditionally or to choose between two alternative subpatterns, depending on
1958 the result of an assertion, or whether a specific capturing subpattern has
1959 already been matched. The two possible forms of conditional subpattern are:
1960 <pre>
1961 (?(condition)yes-pattern)
1962 (?(condition)yes-pattern|no-pattern)
1963 </pre>
1964 If the condition is satisfied, the yes-pattern is used; otherwise the
1965 no-pattern (if present) is used. If there are more than two alternatives in the
1966 subpattern, a compile-time error occurs. Each of the two alternatives may
1967 itself contain nested subpatterns of any form, including conditional
1968 subpatterns; the restriction to two alternatives applies only at the level of
1969 the condition. This pattern fragment is an example where the alternatives are
1970 complex:
1971 <pre>
1972 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
1974 </PRE>
1975 </P>
1976 <P>
1977 There are four kinds of condition: references to subpatterns, references to
1978 recursion, a pseudo-condition called DEFINE, and assertions.
1979 </P>
1980 <br><b>
1981 Checking for a used subpattern by number
1982 </b><br>
1983 <P>
1984 If the text between the parentheses consists of a sequence of digits, the
1985 condition is true if a capturing subpattern of that number has previously
1986 matched. If there is more than one capturing subpattern with the same number
1987 (see the earlier
1988 <a href="#recursion">section about duplicate subpattern numbers),</a>
1989 the condition is true if any of them have matched. An alternative notation is
1990 to precede the digits with a plus or minus sign. In this case, the subpattern
1991 number is relative rather than absolute. The most recently opened parentheses
1992 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
1993 loops it can also make sense to refer to subsequent groups. The next
1994 parentheses to be opened can be referenced as (?(+1), and so on. (The value
1995 zero in any of these forms is not used; it provokes a compile-time error.)
1996 </P>
1997 <P>
1998 Consider the following pattern, which contains non-significant white space to
1999 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2000 three parts for ease of discussion:
2001 <pre>
2002 ( \( )? [^()]+ (?(1) \) )
2003 </pre>
2004 The first part matches an optional opening parenthesis, and if that
2005 character is present, sets it as the first captured substring. The second part
2006 matches one or more characters that are not parentheses. The third part is a
2007 conditional subpattern that tests whether or not the first set of parentheses
2008 matched. If they did, that is, if subject started with an opening parenthesis,
2009 the condition is true, and so the yes-pattern is executed and a closing
2010 parenthesis is required. Otherwise, since no-pattern is not present, the
2011 subpattern matches nothing. In other words, this pattern matches a sequence of
2012 non-parentheses, optionally enclosed in parentheses.
2013 </P>
2014 <P>
2015 If you were embedding this pattern in a larger one, you could use a relative
2016 reference:
2017 <pre>
2018 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
2019 </pre>
2020 This makes the fragment independent of the parentheses in the larger pattern.
2021 </P>
2022 <br><b>
2023 Checking for a used subpattern by name
2024 </b><br>
2025 <P>
2026 Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
2027 subpattern by name. For compatibility with earlier versions of PCRE, which had
2028 this facility before Perl, the syntax (?(name)...) is also recognized. However,
2029 there is a possible ambiguity with this syntax, because subpattern names may
2030 consist entirely of digits. PCRE looks first for a named subpattern; if it
2031 cannot find one and the name consists entirely of digits, PCRE looks for a
2032 subpattern of that number, which must be greater than zero. Using subpattern
2033 names that consist entirely of digits is not recommended.
2034 </P>
2035 <P>
2036 Rewriting the above example to use a named subpattern gives this:
2037 <pre>
2038 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
2039 </pre>
2040 If the name used in a condition of this kind is a duplicate, the test is
2041 applied to all subpatterns of the same name, and is true if any one of them has
2042 matched.
2043 </P>
2044 <br><b>
2045 Checking for pattern recursion
2046 </b><br>
2047 <P>
2048 If the condition is the string (R), and there is no subpattern with the name R,
2049 the condition is true if a recursive call to the whole pattern or any
2050 subpattern has been made. If digits or a name preceded by ampersand follow the
2051 letter R, for example:
2052 <pre>
2053 (?(R3)...) or (?(R&name)...)
2054 </pre>
2055 the condition is true if the most recent recursion is into a subpattern whose
2056 number or name is given. This condition does not check the entire recursion
2057 stack. If the name used in a condition of this kind is a duplicate, the test is
2058 applied to all subpatterns of the same name, and is true if any one of them is
2059 the most recent recursion.
2060 </P>
2061 <P>
2062 At "top level", all these recursion test conditions are false.
2063 <a href="#recursion">The syntax for recursive patterns</a>
2064 is described below.
2065 <a name="subdefine"></a></P>
2066 <br><b>
2067 Defining subpatterns for use by reference only
2068 </b><br>
2069 <P>
2070 If the condition is the string (DEFINE), and there is no subpattern with the
2071 name DEFINE, the condition is always false. In this case, there may be only one
2072 alternative in the subpattern. It is always skipped if control reaches this
2073 point in the pattern; the idea of DEFINE is that it can be used to define
2074 "subroutines" that can be referenced from elsewhere. (The use of
2075 <a href="#subpatternsassubroutines">"subroutines"</a>
2076 is described below.) For example, a pattern to match an IPv4 address such as
2077 "" could be written like this (ignore whitespace and line
2078 breaks):
2079 <pre>
2080 (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2081 \b (?&byte) (\.(?&byte)){3} \b
2082 </pre>
2083 The first part of the pattern is a DEFINE group inside which a another group
2084 named "byte" is defined. This matches an individual component of an IPv4
2085 address (a number less than 256). When matching takes place, this part of the
2086 pattern is skipped because DEFINE acts like a false condition. The rest of the
2087 pattern uses references to the named group to match the four dot-separated
2088 components of an IPv4 address, insisting on a word boundary at each end.
2089 </P>
2090 <br><b>
2091 Assertion conditions
2092 </b><br>
2093 <P>
2094 If the condition is not in any of the above formats, it must be an assertion.
2095 This may be a positive or negative lookahead or lookbehind assertion. Consider
2096 this pattern, again containing non-significant white space, and with the two
2097 alternatives on the second line:
2098 <pre>
2099 (?(?=[^a-z]*[a-z])
2100 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2101 </pre>
2102 The condition is a positive lookahead assertion that matches an optional
2103 sequence of non-letters followed by a letter. In other words, it tests for the
2104 presence of at least one letter in the subject. If a letter is found, the
2105 subject is matched against the first alternative; otherwise it is matched
2106 against the second. This pattern matches strings in one of the two forms
2107 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2108 <a name="comments"></a></P>
2109 <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
2110 <P>
2111 There are two ways of including comments in patterns that are processed by
2112 PCRE. In both cases, the start of the comment must not be in a character class,
2113 nor in the middle of any other sequence of related characters such as (?: or a
2114 subpattern name or number. The characters that make up a comment play no part
2115 in the pattern matching.
2116 </P>
2117 <P>
2118 The sequence (?# marks the start of a comment that continues up to the next
2119 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2120 option is set, an unescaped # character also introduces a comment, which in
2121 this case continues to immediately after the next newline character or
2122 character sequence in the pattern. Which characters are interpreted as newlines
2123 is controlled by the options passed to <b>pcre_compile()</b> or by a special
2124 sequence at the start of the pattern, as described in the section entitled
2125 <a href="#newlines">"Newline conventions"</a>
2126 above. Note that the end of this type of comment is a literal newline sequence
2127 in the pattern; escape sequences that happen to represent a newline do not
2128 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2129 default newline convention is in force:
2130 <pre>
2131 abc #comment \n still comment
2132 </pre>
2133 On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2134 a newline in the pattern. The sequence \n is still literal at this stage, so
2135 it does not terminate the comment. Only an actual character with the code value
2136 0x0a (the default newline) does so.
2137 <a name="recursion"></a></P>
2138 <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
2139 <P>
2140 Consider the problem of matching a string in parentheses, allowing for
2141 unlimited nested parentheses. Without the use of recursion, the best that can
2142 be done is to use a pattern that matches up to some fixed depth of nesting. It
2143 is not possible to handle an arbitrary nesting depth.
2144 </P>
2145 <P>
2146 For some time, Perl has provided a facility that allows regular expressions to
2147 recurse (amongst other things). It does this by interpolating Perl code in the
2148 expression at run time, and the code can refer to the expression itself. A Perl
2149 pattern using code interpolation to solve the parentheses problem can be
2150 created like this:
2151 <pre>
2152 $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
2153 </pre>
2154 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2155 recursively to the pattern in which it appears.
2156 </P>
2157 <P>
2158 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2159 supports special syntax for recursion of the entire pattern, and also for
2160 individual subpattern recursion. After its introduction in PCRE and Python,
2161 this kind of recursion was subsequently introduced into Perl at release 5.10.
2162 </P>
2163 <P>
2164 A special item that consists of (? followed by a number greater than zero and a
2165 closing parenthesis is a recursive call of the subpattern of the given number,
2166 provided that it occurs inside that subpattern. (If not, it is a
2167 <a href="#subpatternsassubroutines">"subroutine"</a>
2168 call, which is described in the next section.) The special item (?R) or (?0) is
2169 a recursive call of the entire regular expression.
2170 </P>
2171 <P>
2172 This PCRE pattern solves the nested parentheses problem (assume the
2173 PCRE_EXTENDED option is set so that white space is ignored):
2174 <pre>
2175 \( ( [^()]++ | (?R) )* \)
2176 </pre>
2177 First it matches an opening parenthesis. Then it matches any number of
2178 substrings which can either be a sequence of non-parentheses, or a recursive
2179 match of the pattern itself (that is, a correctly parenthesized substring).
2180 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2181 to avoid backtracking into sequences of non-parentheses.
2182 </P>
2183 <P>
2184 If this were part of a larger pattern, you would not want to recurse the entire
2185 pattern, so instead you could use this:
2186 <pre>
2187 ( \( ( [^()]++ | (?1) )* \) )
2188 </pre>
2189 We have put the pattern into parentheses, and caused the recursion to refer to
2190 them instead of the whole pattern.
2191 </P>
2192 <P>
2193 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2194 is made easier by the use of relative references. Instead of (?1) in the
2195 pattern above you can write (?-2) to refer to the second most recently opened
2196 parentheses preceding the recursion. In other words, a negative number counts
2197 capturing parentheses leftwards from the point at which it is encountered.
2198 </P>
2199 <P>
2200 It is also possible to refer to subsequently opened parentheses, by writing
2201 references such as (?+2). However, these cannot be recursive because the
2202 reference is not inside the parentheses that are referenced. They are always
2203 <a href="#subpatternsassubroutines">"subroutine"</a>
2204 calls, as described in the next section.
2205 </P>
2206 <P>
2207 An alternative approach is to use named parentheses instead. The Perl syntax
2208 for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
2209 could rewrite the above example as follows:
2210 <pre>
2211 (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
2212 </pre>
2213 If there is more than one subpattern with the same name, the earliest one is
2214 used.
2215 </P>
2216 <P>
2217 This particular example pattern that we have been looking at contains nested
2218 unlimited repeats, and so the use of a possessive quantifier for matching
2219 strings of non-parentheses is important when applying the pattern to strings
2220 that do not match. For example, when this pattern is applied to
2221 <pre>
2222 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2223 </pre>
2224 it yields "no match" quickly. However, if a possessive quantifier is not used,
2225 the match runs for a very long time indeed because there are so many different
2226 ways the + and * repeats can carve up the subject, and all have to be tested
2227 before failure can be reported.
2228 </P>
2229 <P>
2230 At the end of a match, the values of capturing parentheses are those from
2231 the outermost level. If you want to obtain intermediate values, a callout
2232 function can be used (see below and the
2233 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2234 documentation). If the pattern above is matched against
2235 <pre>
2236 (ab(cd)ef)
2237 </pre>
2238 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2239 the last value taken on at the top level. If a capturing subpattern is not
2240 matched at the top level, its final value is unset, even if it is (temporarily)
2241 set at a deeper level.
2242 </P>
2243 <P>
2244 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2245 obtain extra memory to store data during a recursion, which it does by using
2246 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2247 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2248 </P>
2249 <P>
2250 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2251 Consider this pattern, which matches text in angle brackets, allowing for
2252 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2253 recursing), whereas any characters are permitted at the outer level.
2254 <pre>
2255 &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
2256 </pre>
2257 In this pattern, (?(R) is the start of a conditional subpattern, with two
2258 different alternatives for the recursive and non-recursive cases. The (?R) item
2259 is the actual recursive call.
2260 <a name="recursiondifference"></a></P>
2261 <br><b>
2262 Recursion difference from Perl
2263 </b><br>
2264 <P>
2265 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2266 treated as an atomic group. That is, once it has matched some of the subject
2267 string, it is never re-entered, even if it contains untried alternatives and
2268 there is a subsequent matching failure. This can be illustrated by the
2269 following pattern, which purports to match a palindromic string that contains
2270 an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2271 <pre>
2272 ^(.|(.)(?1)\2)$
2273 </pre>
2274 The idea is that it either matches a single character, or two identical
2275 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2276 it does not if the pattern is longer than three characters. Consider the
2277 subject string "abcba":
2278 </P>
2279 <P>
2280 At the top level, the first character is matched, but as it is not at the end
2281 of the string, the first alternative fails; the second alternative is taken
2282 and the recursion kicks in. The recursive call to subpattern 1 successfully
2283 matches the next character ("b"). (Note that the beginning and end of line
2284 tests are not part of the recursion).
2285 </P>
2286 <P>
2287 Back at the top level, the next character ("c") is compared with what
2288 subpattern 2 matched, which was "a". This fails. Because the recursion is
2289 treated as an atomic group, there are now no backtracking points, and so the
2290 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2291 try the second alternative.) However, if the pattern is written with the
2292 alternatives in the other order, things are different:
2293 <pre>
2294 ^((.)(?1)\2|.)$
2295 </pre>
2296 This time, the recursing alternative is tried first, and continues to recurse
2297 until it runs out of characters, at which point the recursion fails. But this
2298 time we do have another alternative to try at the higher level. That is the big
2299 difference: in the previous case the remaining alternative is at a deeper
2300 recursion level, which PCRE cannot use.
2301 </P>
2302 <P>
2303 To change the pattern so that it matches all palindromic strings, not just
2304 those with an odd number of characters, it is tempting to change the pattern to
2305 this:
2306 <pre>
2307 ^((.)(?1)\2|.?)$
2308 </pre>
2309 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2310 deeper recursion has matched a single character, it cannot be entered again in
2311 order to match an empty string. The solution is to separate the two cases, and
2312 write out the odd and even cases as alternatives at the higher level:
2313 <pre>
2314 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2315 </pre>
2316 If you want to match typical palindromic phrases, the pattern has to ignore all
2317 non-word characters, which can be done like this:
2318 <pre>
2319 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2320 </pre>
2321 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2322 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2323 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2324 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2325 more) to match typical phrases, and Perl takes so long that you think it has
2326 gone into a loop.
2327 </P>
2328 <P>
2329 <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2330 string does not start with a palindrome that is shorter than the entire string.
2331 For example, although "abcba" is correctly matched, if the subject is "ababa",
2332 PCRE finds the palindrome "aba" at the start, then fails at top level because
2333 the end of the string does not follow. Once again, it cannot jump back into the
2334 recursion to try other alternatives, so the entire match fails.
2335 <a name="subpatternsassubroutines"></a></P>
2336 <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2337 <P>
2338 If the syntax for a recursive subpattern reference (either by number or by
2339 name) is used outside the parentheses to which it refers, it operates like a
2340 subroutine in a programming language. The "called" subpattern may be defined
2341 before or after the reference. A numbered reference can be absolute or
2342 relative, as in these examples:
2343 <pre>
2344 (...(absolute)...)...(?2)...
2345 (...(relative)...)...(?-1)...
2346 (...(?+1)...(relative)...
2347 </pre>
2348 An earlier example pointed out that the pattern
2349 <pre>
2350 (sens|respons)e and \1ibility
2351 </pre>
2352 matches "sense and sensibility" and "response and responsibility", but not
2353 "sense and responsibility". If instead the pattern
2354 <pre>
2355 (sens|respons)e and (?1)ibility
2356 </pre>
2357 is used, it does match "sense and responsibility" as well as the other two
2358 strings. Another example is given in the discussion of DEFINE above.
2359 </P>
2360 <P>
2361 Like recursive subpatterns, a subroutine call is always treated as an atomic
2362 group. That is, once it has matched some of the subject string, it is never
2363 re-entered, even if it contains untried alternatives and there is a subsequent
2364 matching failure. Any capturing parentheses that are set during the subroutine
2365 call revert to their previous values afterwards.
2366 </P>
2367 <P>
2368 When a subpattern is used as a subroutine, processing options such as
2369 case-independence are fixed when the subpattern is defined. They cannot be
2370 changed for different calls. For example, consider this pattern:
2371 <pre>
2372 (abc)(?i:(?-1))
2373 </pre>
2374 It matches "abcabc". It does not match "abcABC" because the change of
2375 processing option does not affect the called subpattern.
2376 <a name="onigurumasubroutines"></a></P>
2377 <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2378 <P>
2379 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2380 a number enclosed either in angle brackets or single quotes, is an alternative
2381 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2382 are two of the examples used above, rewritten using this syntax:
2383 <pre>
2384 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
2385 (sens|respons)e and \g'1'ibility
2386 </pre>
2387 PCRE supports an extension to Oniguruma: if a number is preceded by a
2388 plus or a minus sign it is taken as a relative reference. For example:
2389 <pre>
2390 (abc)(?i:\g&#60;-1&#62;)
2391 </pre>
2392 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
2393 synonymous. The former is a back reference; the latter is a subroutine call.
2394 </P>
2395 <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
2396 <P>
2397 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2398 code to be obeyed in the middle of matching a regular expression. This makes it
2399 possible, amongst other things, to extract different substrings that match the
2400 same pair of parentheses when there is a repetition.
2401 </P>
2402 <P>
2403 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2404 code. The feature is called "callout". The caller of PCRE provides an external
2405 function by putting its entry point in the global variable <i>pcre_callout</i>.
2406 By default, this variable contains NULL, which disables all calling out.
2407 </P>
2408 <P>
2409 Within a regular expression, (?C) indicates the points at which the external
2410 function is to be called. If you want to identify different callout points, you
2411 can put a number less than 256 after the letter C. The default value is zero.
2412 For example, this pattern has two callout points:
2413 <pre>
2414 (?C1)abc(?C2)def
2415 </pre>
2416 If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
2417 automatically installed before each item in the pattern. They are all numbered
2418 255.
2419 </P>
2420 <P>
2421 During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
2422 set), the external function is called. It is provided with the number of the
2423 callout, the position in the pattern, and, optionally, one item of data
2424 originally supplied by the caller of <b>pcre_exec()</b>. The callout function
2425 may cause matching to proceed, to backtrack, or to fail altogether. A complete
2426 description of the interface to the callout function is given in the
2427 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2428 documentation.
2429 <a name="backtrackcontrol"></a></P>
2430 <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
2431 <P>
2432 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2433 are described in the Perl documentation as "experimental and subject to change
2434 or removal in a future version of Perl". It goes on to say: "Their usage in
2435 production code should be noted to avoid problems during upgrades." The same
2436 remarks apply to the PCRE features described in this section.
2437 </P>
2438 <P>
2439 Since these verbs are specifically related to backtracking, most of them can be
2440 used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses
2441 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2442 failing negative assertion, they cause an error if encountered by
2443 <b>pcre_dfa_exec()</b>.
2444 </P>
2445 <P>
2446 If any of these verbs are used in an assertion or subroutine subpattern
2447 (including recursive subpatterns), their effect is confined to that subpattern;
2448 it does not extend to the surrounding pattern. Note that such subpatterns are
2449 processed as anchored at the point where they are tested.
2450 </P>
2451 <P>
2452 The new verbs make use of what was previously invalid syntax: an opening
2453 parenthesis followed by an asterisk. They are generally of the form
2454 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2455 depending on whether or not an argument is present. An name is a sequence of
2456 letters, digits, and underscores. If the name is empty, that is, if the closing
2457 parenthesis immediately follows the colon, the effect is as if the colon were
2458 not there. Any number of these verbs may occur in a pattern.
2459 </P>
2460 <P>
2461 PCRE contains some optimizations that are used to speed up matching by running
2462 some checks at the start of each match attempt. For example, it may know the
2463 minimum length of matching subject, or that a particular character must be
2464 present. When one of these optimizations suppresses the running of a match, any
2465 included backtracking verbs will not, of course, be processed. You can suppress
2466 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2467 when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
2468 pattern with (*NO_START_OPT).
2469 </P>
2470 <br><b>
2471 Verbs that act immediately
2472 </b><br>
2473 <P>
2474 The following verbs act as soon as they are encountered. They may not be
2475 followed by a name.
2476 <pre>
2477 (*ACCEPT)
2478 </pre>
2479 This verb causes the match to end successfully, skipping the remainder of the
2480 pattern. When inside a recursion, only the innermost pattern is ended
2481 immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2482 captured. (This feature was added to PCRE at release 8.00.) For example:
2483 <pre>
2484 A((?:A|B(*ACCEPT)|C)D)
2485 </pre>
2486 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2487 the outer parentheses.
2488 <pre>
2489 (*FAIL) or (*F)
2490 </pre>
2491 This verb causes the match to fail, forcing backtracking to occur. It is
2492 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2493 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2494 Perl features that are not present in PCRE. The nearest equivalent is the
2495 callout feature, as for example in this pattern:
2496 <pre>
2497 a+(?C)(*FAIL)
2498 </pre>
2499 A match with the string "aaaa" always fails, but the callout is taken before
2500 each backtrack happens (in this example, 10 times).
2501 </P>
2502 <br><b>
2503 Recording which path was taken
2504 </b><br>
2505 <P>
2506 There is one verb whose main purpose is to track how a match was arrived at,
2507 though it also has a secondary use in conjunction with advancing the match
2508 starting point (see (*SKIP) below).
2509 <pre>
2510 (*MARK:NAME) or (*:NAME)
2511 </pre>
2512 A name is always required with this verb. There may be as many instances of
2513 (*MARK) as you like in a pattern, and their names do not have to be unique.
2514 </P>
2515 <P>
2516 When a match succeeds, the name of the last-encountered (*MARK) is passed back
2517 to the caller via the <i>pcre_extra</i> data structure, as described in the
2518 <a href="pcreapi.html#extradata">section on <i>pcre_extra</i></a>
2519 in the
2520 <a href="pcreapi.html"><b>pcreapi</b></a>
2521 documentation. No data is returned for a partial match. Here is an example of
2522 <b>pcretest</b> output, where the /K modifier requests the retrieval and
2523 outputting of (*MARK) data:
2524 <pre>
2525 /X(*MARK:A)Y|X(*MARK:B)Z/K
2526 XY
2527 0: XY
2528 MK: A
2529 XZ
2530 0: XZ
2531 MK: B
2532 </pre>
2533 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2534 indicates which of the two alternatives matched. This is a more efficient way
2535 of obtaining this information than putting each alternative in its own
2536 capturing parentheses.
2537 </P>
2538 <P>
2539 A name may also be returned after a failed match if the final path through the
2540 pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2541 (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2542 starting point for matching is advanced, the final check is often with an empty
2543 string, causing a failure before (*MARK) is reached. For example:
2544 <pre>
2545 /X(*MARK:A)Y|X(*MARK:B)Z/K
2546 XP
2547 No match
2548 </pre>
2549 There are three potential starting points for this match (starting with X,
2550 starting with P, and with an empty string). If the pattern is anchored, the
2551 result is different:
2552 <pre>
2553 /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2554 XP
2555 No match, mark = B
2556 </pre>
2557 PCRE's start-of-match optimizations can also interfere with this. For example,
2558 if, as a result of a call to <b>pcre_study()</b>, it knows the minimum
2559 subject length for a match, a shorter subject will not be scanned at all.
2560 </P>
2561 <P>
2562 Note that similar anomalies (though different in detail) exist in Perl, no
2563 doubt for the same reasons. The use of (*MARK) data after a failed match of an
2564 unanchored pattern is not recommended, unless (*COMMIT) is involved.
2565 </P>
2566 <br><b>
2567 Verbs that act after backtracking
2568 </b><br>
2569 <P>
2570 The following verbs do nothing when they are encountered. Matching continues
2571 with what follows, but if there is no subsequent match, causing a backtrack to
2572 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2573 the verb. However, when one of these verbs appears inside an atomic group, its
2574 effect is confined to that group, because once the group has been matched,
2575 there is never any backtracking into it. In this situation, backtracking can
2576 "jump back" to the left of the entire atomic group. (Remember also, as stated
2577 above, that this localization also applies in subroutine calls and assertions.)
2578 </P>
2579 <P>
2580 These verbs differ in exactly what kind of failure occurs when backtracking
2581 reaches them.
2582 <pre>
2583 (*COMMIT)
2584 </pre>
2585 This verb, which may not be followed by a name, causes the whole match to fail
2586 outright if the rest of the pattern does not match. Even if the pattern is
2587 unanchored, no further attempts to find a match by advancing the starting point
2588 take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to
2589 finding a match at the current starting point, or not at all. For example:
2590 <pre>
2591 a+(*COMMIT)b
2592 </pre>
2593 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2594 dynamic anchor, or "I've started, so I must finish." The name of the most
2595 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2596 match failure.
2597 </P>
2598 <P>
2599 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2600 unless PCRE's start-of-match optimizations are turned off, as shown in this
2601 <b>pcretest</b> example:
2602 <pre>
2603 /(*COMMIT)abc/
2604 xyzabc
2605 0: abc
2606 xyzabc\Y
2607 No match
2608 </pre>
2609 PCRE knows that any match must start with "a", so the optimization skips along
2610 the subject to "a" before running the first match attempt, which succeeds. When
2611 the optimization is disabled by the \Y escape in the second subject, the match
2612 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2613 starting points.
2614 <pre>
2615 (*PRUNE) or (*PRUNE:NAME)
2616 </pre>
2617 This verb causes the match to fail at the current starting position in the
2618 subject if the rest of the pattern does not match. If the pattern is
2619 unanchored, the normal "bumpalong" advance to the next starting character then
2620 happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2621 reached, or when matching to the right of (*PRUNE), but if there is no match to
2622 the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2623 (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2624 but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2625 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2626 match fails completely; the name is passed back if this is the final attempt.
2627 (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2628 pattern (*PRUNE) has the same effect as (*COMMIT).
2629 <pre>
2630 (*SKIP)
2631 </pre>
2632 This verb, when given without a name, is like (*PRUNE), except that if the
2633 pattern is unanchored, the "bumpalong" advance is not to the next character,
2634 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2635 signifies that whatever text was matched leading up to it cannot be part of a
2636 successful match. Consider:
2637 <pre>
2638 a+(*SKIP)b
2639 </pre>
2640 If the subject is "aaaac...", after the first match attempt fails (starting at
2641 the first character in the string), the starting point skips on to start the
2642 next attempt at "c". Note that a possessive quantifer does not have the same
2643 effect as this example; although it would suppress backtracking during the
2644 first match attempt, the second attempt would start at the second character
2645 instead of skipping on to "c".
2646 <pre>
2647 (*SKIP:NAME)
2648 </pre>
2649 When (*SKIP) has an associated name, its behaviour is modified. If the
2650 following pattern fails to match, the previous path through the pattern is
2651 searched for the most recent (*MARK) that has the same name. If one is found,
2652 the "bumpalong" advance is to the subject position that corresponds to that
2653 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2654 matching name is found, normal "bumpalong" of one character happens (the
2655 (*SKIP) is ignored).
2656 <pre>
2657 (*THEN) or (*THEN:NAME)
2658 </pre>
2659 This verb causes a skip to the next alternation in the innermost enclosing
2660 group if the rest of the pattern does not match. That is, it cancels pending
2661 backtracking, but only within the current alternation. Its name comes from the
2662 observation that it can be used for a pattern-based if-then-else block:
2663 <pre>
2664 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2665 </pre>
2666 If the COND1 pattern matches, FOO is tried (and possibly further items after
2667 the end of the group if FOO succeeds); on failure the matcher skips to the
2668 second alternative and tries COND2, without backtracking into COND1. The
2669 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2670 overall match fails. If (*THEN) is not directly inside an alternation, it acts
2671 like (*PRUNE).
2672 </P>
2673 <P>
2674 The above verbs provide four different "strengths" of control when subsequent
2675 matching fails. (*THEN) is the weakest, carrying on the match at the next
2676 alternation. (*PRUNE) comes next, failing the match at the current starting
2677 position, but allowing an advance to the next character (for an unanchored
2678 pattern). (*SKIP) is similar, except that the advance may be more than one
2679 character. (*COMMIT) is the strongest, causing the entire match to fail.
2680 </P>
2681 <P>
2682 If more than one is present in a pattern, the "stongest" one wins. For example,
2683 consider this pattern, where A, B, etc. are complex pattern fragments:
2684 <pre>
2685 (A(*COMMIT)B(*THEN)C|D)
2686 </pre>
2687 Once A has matched, PCRE is committed to this match, at the current starting
2688 position. If subsequently B matches, but C does not, the normal (*THEN) action
2689 of trying the next alternation (that is, D) does not happen because (*COMMIT)
2690 overrides.
2691 </P>
2692 <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
2693 <P>
2694 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
2695 <b>pcresyntax</b>(3), <b>pcre</b>(3).
2696 </P>
2697 <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
2698 <P>
2699 Philip Hazel
2700 <br>
2701 University Computing Service
2702 <br>
2703 Cambridge CB2 3QH, England.
2704 <br>
2705 </P>
2706 <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2707 <P>
2708 Last updated: 21 November 2010
2709 <br>
2710 Copyright &copy; 1997-2010 University of Cambridge.
2711 <br>
2712 <p>
2713 Return to the <a href="index.html">PCRE index page</a>.
2714 </p>


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