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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">SPECIAL START-OF-PATTERN ITEMS</a>
18 <li><a name="TOC3" href="#SEC3">EBCDIC CHARACTER CODES</a>
19 <li><a name="TOC4" href="#SEC4">CHARACTERS AND METACHARACTERS</a>
20 <li><a name="TOC5" href="#SEC5">BACKSLASH</a>
21 <li><a name="TOC6" href="#SEC6">CIRCUMFLEX AND DOLLAR</a>
22 <li><a name="TOC7" href="#SEC7">FULL STOP (PERIOD, DOT) AND \N</a>
23 <li><a name="TOC8" href="#SEC8">MATCHING A SINGLE DATA UNIT</a>
25 <li><a name="TOC10" href="#SEC10">POSIX CHARACTER CLASSES</a>
27 <li><a name="TOC12" href="#SEC12">VERTICAL BAR</a>
28 <li><a name="TOC13" href="#SEC13">INTERNAL OPTION SETTING</a>
29 <li><a name="TOC14" href="#SEC14">SUBPATTERNS</a>
30 <li><a name="TOC15" href="#SEC15">DUPLICATE SUBPATTERN NUMBERS</a>
31 <li><a name="TOC16" href="#SEC16">NAMED SUBPATTERNS</a>
32 <li><a name="TOC17" href="#SEC17">REPETITION</a>
34 <li><a name="TOC19" href="#SEC19">BACK REFERENCES</a>
35 <li><a name="TOC20" href="#SEC20">ASSERTIONS</a>
36 <li><a name="TOC21" href="#SEC21">CONDITIONAL SUBPATTERNS</a>
37 <li><a name="TOC22" href="#SEC22">COMMENTS</a>
38 <li><a name="TOC23" href="#SEC23">RECURSIVE PATTERNS</a>
39 <li><a name="TOC24" href="#SEC24">SUBPATTERNS AS SUBROUTINES</a>
40 <li><a name="TOC25" href="#SEC25">ONIGURUMA SUBROUTINE SYNTAX</a>
41 <li><a name="TOC26" href="#SEC26">CALLOUTS</a>
42 <li><a name="TOC27" href="#SEC27">BACKTRACKING CONTROL</a>
43 <li><a name="TOC28" href="#SEC28">SEE ALSO</a>
44 <li><a name="TOC29" href="#SEC29">AUTHOR</a>
45 <li><a name="TOC30" href="#SEC30">REVISION</a>
46 </ul>
47 <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
48 <P>
49 The syntax and semantics of the regular expressions that are supported by PCRE
50 are described in detail below. There is a quick-reference syntax summary in the
51 <a href="pcresyntax.html"><b>pcresyntax</b></a>
52 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
53 also supports some alternative regular expression syntax (which does not
54 conflict with the Perl syntax) in order to provide some compatibility with
55 regular expressions in Python, .NET, and Oniguruma.
56 </P>
57 <P>
58 Perl's regular expressions are described in its own documentation, and
59 regular expressions in general are covered in a number of books, some of which
60 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
61 published by O'Reilly, covers regular expressions in great detail. This
62 description of PCRE's regular expressions is intended as reference material.
63 </P>
64 <P>
65 This document discusses the patterns that are supported by PCRE when one its
66 main matching functions, <b>pcre_exec()</b> (8-bit) or <b>pcre[16|32]_exec()</b>
67 (16- or 32-bit), is used. PCRE also has alternative matching functions,
68 <b>pcre_dfa_exec()</b> and <b>pcre[16|32_dfa_exec()</b>, which match using a
69 different algorithm that is not Perl-compatible. Some of the features discussed
70 below are not available when DFA matching is used. The advantages and
71 disadvantages of the alternative functions, and how they differ from the normal
72 functions, are discussed in the
73 <a href="pcrematching.html"><b>pcrematching</b></a>
74 page.
75 </P>
76 <br><a name="SEC2" href="#TOC1">SPECIAL START-OF-PATTERN ITEMS</a><br>
77 <P>
78 A number of options that can be passed to <b>pcre_compile()</b> can also be set
79 by special items at the start of a pattern. These are not Perl-compatible, but
80 are provided to make these options accessible to pattern writers who are not
81 able to change the program that processes the pattern. Any number of these
82 items may appear, but they must all be together right at the start of the
83 pattern string, and the letters must be in upper case.
84 </P>
85 <br><b>
86 UTF support
87 </b><br>
88 <P>
89 The original operation of PCRE was on strings of one-byte characters. However,
90 there is now also support for UTF-8 strings in the original library, an
91 extra library that supports 16-bit and UTF-16 character strings, and a
92 third library that supports 32-bit and UTF-32 character strings. To use these
93 features, PCRE must be built to include appropriate support. When using UTF
94 strings you must either call the compiling function with the PCRE_UTF8,
95 PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
96 these special sequences:
97 <pre>
98 (*UTF8)
99 (*UTF16)
100 (*UTF32)
101 (*UTF)
102 </pre>
103 (*UTF) is a generic sequence that can be used with any of the libraries.
104 Starting a pattern with such a sequence is equivalent to setting the relevant
105 option. How setting a UTF mode affects pattern matching is mentioned in several
106 places below. There is also a summary of features in the
107 <a href="pcreunicode.html"><b>pcreunicode</b></a>
108 page.
109 </P>
110 <P>
111 Some applications that allow their users to supply patterns may wish to
112 restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
113 option is set at compile time, (*UTF) etc. are not allowed, and their
114 appearance causes an error.
115 </P>
116 <br><b>
117 Unicode property support
118 </b><br>
119 <P>
120 Another special sequence that may appear at the start of a pattern is (*UCP).
121 This has the same effect as setting the PCRE_UCP option: it causes sequences
122 such as \d and \w to use Unicode properties to determine character types,
123 instead of recognizing only characters with codes less than 128 via a lookup
124 table.
125 </P>
126 <br><b>
127 Disabling auto-possessification
128 </b><br>
129 <P>
130 If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
131 the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making
132 quantifiers possessive when what follows cannot match the repeated item. For
133 example, by default a+b is treated as a++b. For more details, see the
134 <a href="pcreapi.html"><b>pcreapi</b></a>
135 documentation.
136 </P>
137 <br><b>
138 Disabling start-up optimizations
139 </b><br>
140 <P>
141 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
142 PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables
143 several optimizations for quickly reaching "no match" results. For more
144 details, see the
145 <a href="pcreapi.html"><b>pcreapi</b></a>
146 documentation.
147 <a name="newlines"></a></P>
148 <br><b>
149 Newline conventions
150 </b><br>
151 <P>
152 PCRE supports five different conventions for indicating line breaks in
153 strings: a single CR (carriage return) character, a single LF (linefeed)
154 character, the two-character sequence CRLF, any of the three preceding, or any
155 Unicode newline sequence. The
156 <a href="pcreapi.html"><b>pcreapi</b></a>
157 page has
158 <a href="pcreapi.html#newlines">further discussion</a>
159 about newlines, and shows how to set the newline convention in the
160 <i>options</i> arguments for the compiling and matching functions.
161 </P>
162 <P>
163 It is also possible to specify a newline convention by starting a pattern
164 string with one of the following five sequences:
165 <pre>
166 (*CR) carriage return
167 (*LF) linefeed
168 (*CRLF) carriage return, followed by linefeed
169 (*ANYCRLF) any of the three above
170 (*ANY) all Unicode newline sequences
171 </pre>
172 These override the default and the options given to the compiling function. For
173 example, on a Unix system where LF is the default newline sequence, the pattern
174 <pre>
175 (*CR)a.b
176 </pre>
177 changes the convention to CR. That pattern matches "a\nb" because LF is no
178 longer a newline. If more than one of these settings is present, the last one
179 is used.
180 </P>
181 <P>
182 The newline convention affects where the circumflex and dollar assertions are
183 true. It also affects the interpretation of the dot metacharacter when
184 PCRE_DOTALL is not set, and the behaviour of \N. However, it does not affect
185 what the \R escape sequence matches. By default, this is any Unicode newline
186 sequence, for Perl compatibility. However, this can be changed; see the
187 description of \R in the section entitled
188 <a href="#newlineseq">"Newline sequences"</a>
189 below. A change of \R setting can be combined with a change of newline
190 convention.
191 </P>
192 <br><b>
193 Setting match and recursion limits
194 </b><br>
195 <P>
196 The caller of <b>pcre_exec()</b> can set a limit on the number of times the
197 internal <b>match()</b> function is called and on the maximum depth of
198 recursive calls. These facilities are provided to catch runaway matches that
199 are provoked by patterns with huge matching trees (a typical example is a
200 pattern with nested unlimited repeats) and to avoid running out of system stack
201 by too much recursion. When one of these limits is reached, <b>pcre_exec()</b>
202 gives an error return. The limits can also be set by items at the start of the
203 pattern of the form
204 <pre>
205 (*LIMIT_MATCH=d)
207 </pre>
208 where d is any number of decimal digits. However, the value of the setting must
209 be less than the value set (or defaulted) by the caller of <b>pcre_exec()</b>
210 for it to have any effect. In other words, the pattern writer can lower the
211 limits set by the programmer, but not raise them. If there is more than one
212 setting of one of these limits, the lower value is used.
213 </P>
214 <br><a name="SEC3" href="#TOC1">EBCDIC CHARACTER CODES</a><br>
215 <P>
216 PCRE can be compiled to run in an environment that uses EBCDIC as its character
217 code rather than ASCII or Unicode (typically a mainframe system). In the
218 sections below, character code values are ASCII or Unicode; in an EBCDIC
219 environment these characters may have different code values, and there are no
220 code points greater than 255.
221 </P>
222 <br><a name="SEC4" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
223 <P>
224 A regular expression is a pattern that is matched against a subject string from
225 left to right. Most characters stand for themselves in a pattern, and match the
226 corresponding characters in the subject. As a trivial example, the pattern
227 <pre>
228 The quick brown fox
229 </pre>
230 matches a portion of a subject string that is identical to itself. When
231 caseless matching is specified (the PCRE_CASELESS option), letters are matched
232 independently of case. In a UTF mode, PCRE always understands the concept of
233 case for characters whose values are less than 128, so caseless matching is
234 always possible. For characters with higher values, the concept of case is
235 supported if PCRE is compiled with Unicode property support, but not otherwise.
236 If you want to use caseless matching for characters 128 and above, you must
237 ensure that PCRE is compiled with Unicode property support as well as with
238 UTF support.
239 </P>
240 <P>
241 The power of regular expressions comes from the ability to include alternatives
242 and repetitions in the pattern. These are encoded in the pattern by the use of
243 <i>metacharacters</i>, which do not stand for themselves but instead are
244 interpreted in some special way.
245 </P>
246 <P>
247 There are two different sets of metacharacters: those that are recognized
248 anywhere in the pattern except within square brackets, and those that are
249 recognized within square brackets. Outside square brackets, the metacharacters
250 are as follows:
251 <pre>
252 \ general escape character with several uses
253 ^ assert start of string (or line, in multiline mode)
254 $ assert end of string (or line, in multiline mode)
255 . match any character except newline (by default)
256 [ start character class definition
257 | start of alternative branch
258 ( start subpattern
259 ) end subpattern
260 ? extends the meaning of (
261 also 0 or 1 quantifier
262 also quantifier minimizer
263 * 0 or more quantifier
264 + 1 or more quantifier
265 also "possessive quantifier"
266 { start min/max quantifier
267 </pre>
268 Part of a pattern that is in square brackets is called a "character class". In
269 a character class the only metacharacters are:
270 <pre>
271 \ general escape character
272 ^ negate the class, but only if the first character
273 - indicates character range
274 [ POSIX character class (only if followed by POSIX syntax)
275 ] terminates the character class
276 </pre>
277 The following sections describe the use of each of the metacharacters.
278 </P>
279 <br><a name="SEC5" href="#TOC1">BACKSLASH</a><br>
280 <P>
281 The backslash character has several uses. Firstly, if it is followed by a
282 character that is not a number or a letter, it takes away any special meaning
283 that character may have. This use of backslash as an escape character applies
284 both inside and outside character classes.
285 </P>
286 <P>
287 For example, if you want to match a * character, you write \* in the pattern.
288 This escaping action applies whether or not the following character would
289 otherwise be interpreted as a metacharacter, so it is always safe to precede a
290 non-alphanumeric with backslash to specify that it stands for itself. In
291 particular, if you want to match a backslash, you write \\.
292 </P>
293 <P>
294 In a UTF mode, only ASCII numbers and letters have any special meaning after a
295 backslash. All other characters (in particular, those whose codepoints are
296 greater than 127) are treated as literals.
297 </P>
298 <P>
299 If a pattern is compiled with the PCRE_EXTENDED option, most white space in the
300 pattern (other than in a character class), and characters between a # outside a
301 character class and the next newline, inclusive, are ignored. An escaping
302 backslash can be used to include a white space or # character as part of the
303 pattern.
304 </P>
305 <P>
306 If you want to remove the special meaning from a sequence of characters, you
307 can do so by putting them between \Q and \E. This is different from Perl in
308 that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
309 Perl, $ and @ cause variable interpolation. Note the following examples:
310 <pre>
311 Pattern PCRE matches Perl matches
313 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
314 \Qabc\$xyz\E abc\$xyz abc\$xyz
315 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
316 </pre>
317 The \Q...\E sequence is recognized both inside and outside character classes.
318 An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
319 by \E later in the pattern, the literal interpretation continues to the end of
320 the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
321 a character class, this causes an error, because the character class is not
322 terminated.
323 <a name="digitsafterbackslash"></a></P>
324 <br><b>
325 Non-printing characters
326 </b><br>
327 <P>
328 A second use of backslash provides a way of encoding non-printing characters
329 in patterns in a visible manner. There is no restriction on the appearance of
330 non-printing characters, apart from the binary zero that terminates a pattern,
331 but when a pattern is being prepared by text editing, it is often easier to use
332 one of the following escape sequences than the binary character it represents.
333 In an ASCII or Unicode environment, these escapes are as follows:
334 <pre>
335 \a alarm, that is, the BEL character (hex 07)
336 \cx "control-x", where x is any ASCII character
337 \e escape (hex 1B)
338 \f form feed (hex 0C)
339 \n linefeed (hex 0A)
340 \r carriage return (hex 0D)
341 \t tab (hex 09)
342 \0dd character with octal code 0dd
343 \ddd character with octal code ddd, or back reference
344 \o{ddd..} character with octal code ddd..
345 \xhh character with hex code hh
346 \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
347 \uhhhh character with hex code hhhh (JavaScript mode only)
348 </pre>
349 The precise effect of \cx on ASCII characters is as follows: if x is a lower
350 case letter, it is converted to upper case. Then bit 6 of the character (hex
351 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A),
352 but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the
353 data item (byte or 16-bit value) following \c has a value greater than 127, a
354 compile-time error occurs. This locks out non-ASCII characters in all modes.
355 </P>
356 <P>
357 When PCRE is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t
358 generate the appropriate EBCDIC code values. The \c escape is processed
359 as specified for Perl in the <b>perlebcdic</b> document. The only characters
360 that are allowed after \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?. Any
361 other character provokes a compile-time error. The sequence \c@ encodes
362 character code 0; after \c the letters (in either case) encode characters 1-26
363 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31 (hex 1B to hex
364 1F), and \c? becomes either 255 (hex FF) or 95 (hex 5F).
365 </P>
366 <P>
367 Thus, apart from \c?, these escapes generate the same character code values as
368 they do in an ASCII environment, though the meanings of the values mostly
369 differ. For example, \cG always generates code value 7, which is BEL in ASCII
370 but DEL in EBCDIC.
371 </P>
372 <P>
373 The sequence \c? generates DEL (127, hex 7F) in an ASCII environment, but
374 because 127 is not a control character in EBCDIC, Perl makes it generate the
375 APC character. Unfortunately, there are several variants of EBCDIC. In most of
376 them the APC character has the value 255 (hex FF), but in the one Perl calls
377 POSIX-BC its value is 95 (hex 5F). If certain other characters have POSIX-BC
378 values, PCRE makes \c? generate 95; otherwise it generates 255.
379 </P>
380 <P>
381 After \0 up to two further octal digits are read. If there are fewer than two
382 digits, just those that are present are used. Thus the sequence \0\x\015
383 specifies two binary zeros followed by a CR character (code value 13). Make
384 sure you supply two digits after the initial zero if the pattern character that
385 follows is itself an octal digit.
386 </P>
387 <P>
388 The escape \o must be followed by a sequence of octal digits, enclosed in
389 braces. An error occurs if this is not the case. This escape is a recent
390 addition to Perl; it provides way of specifying character code points as octal
391 numbers greater than 0777, and it also allows octal numbers and back references
392 to be unambiguously specified.
393 </P>
394 <P>
395 For greater clarity and unambiguity, it is best to avoid following \ by a
396 digit greater than zero. Instead, use \o{} or \x{} to specify character
397 numbers, and \g{} to specify back references. The following paragraphs
398 describe the old, ambiguous syntax.
399 </P>
400 <P>
401 The handling of a backslash followed by a digit other than 0 is complicated,
402 and Perl has changed in recent releases, causing PCRE also to change. Outside a
403 character class, PCRE reads the digit and any following digits as a decimal
404 number. If the number is less than 8, or if there have been at least that many
405 previous capturing left parentheses in the expression, the entire sequence is
406 taken as a <i>back reference</i>. A description of how this works is given
407 <a href="#backreferences">later,</a>
408 following the discussion of
409 <a href="#subpattern">parenthesized subpatterns.</a>
410 </P>
411 <P>
412 Inside a character class, or if the decimal number following \ is greater than
413 7 and there have not been that many capturing subpatterns, PCRE handles \8 and
414 \9 as the literal characters "8" and "9", and otherwise re-reads up to three
415 octal digits following the backslash, using them to generate a data character.
416 Any subsequent digits stand for themselves. For example:
417 <pre>
418 \040 is another way of writing an ASCII space
419 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
420 \7 is always a back reference
421 \11 might be a back reference, or another way of writing a tab
422 \011 is always a tab
423 \0113 is a tab followed by the character "3"
424 \113 might be a back reference, otherwise the character with octal code 113
425 \377 might be a back reference, otherwise the value 255 (decimal)
426 \81 is either a back reference, or the two characters "8" and "1"
427 </pre>
428 Note that octal values of 100 or greater that are specified using this syntax
429 must not be introduced by a leading zero, because no more than three octal
430 digits are ever read.
431 </P>
432 <P>
433 By default, after \x that is not followed by {, from zero to two hexadecimal
434 digits are read (letters can be in upper or lower case). Any number of
435 hexadecimal digits may appear between \x{ and }. If a character other than
436 a hexadecimal digit appears between \x{ and }, or if there is no terminating
437 }, an error occurs.
438 </P>
439 <P>
440 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is
441 as just described only when it is followed by two hexadecimal digits.
442 Otherwise, it matches a literal "x" character. In JavaScript mode, support for
443 code points greater than 256 is provided by \u, which must be followed by
444 four hexadecimal digits; otherwise it matches a literal "u" character.
445 </P>
446 <P>
447 Characters whose value is less than 256 can be defined by either of the two
448 syntaxes for \x (or by \u in JavaScript mode). There is no difference in the
449 way they are handled. For example, \xdc is exactly the same as \x{dc} (or
450 \u00dc in JavaScript mode).
451 </P>
452 <br><b>
453 Constraints on character values
454 </b><br>
455 <P>
456 Characters that are specified using octal or hexadecimal numbers are
457 limited to certain values, as follows:
458 <pre>
459 8-bit non-UTF mode less than 0x100
460 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
461 16-bit non-UTF mode less than 0x10000
462 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
463 32-bit non-UTF mode less than 0x100000000
464 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
465 </pre>
466 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
467 "surrogate" codepoints), and 0xffef.
468 </P>
469 <br><b>
470 Escape sequences in character classes
471 </b><br>
472 <P>
473 All the sequences that define a single character value can be used both inside
474 and outside character classes. In addition, inside a character class, \b is
475 interpreted as the backspace character (hex 08).
476 </P>
477 <P>
478 \N is not allowed in a character class. \B, \R, and \X are not special
479 inside a character class. Like other unrecognized escape sequences, they are
480 treated as the literal characters "B", "R", and "X" by default, but cause an
481 error if the PCRE_EXTRA option is set. Outside a character class, these
482 sequences have different meanings.
483 </P>
484 <br><b>
485 Unsupported escape sequences
486 </b><br>
487 <P>
488 In Perl, the sequences \l, \L, \u, and \U are recognized by its string
489 handler and used to modify the case of following characters. By default, PCRE
490 does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
491 option is set, \U matches a "U" character, and \u can be used to define a
492 character by code point, as described in the previous section.
493 </P>
494 <br><b>
495 Absolute and relative back references
496 </b><br>
497 <P>
498 The sequence \g followed by an unsigned or a negative number, optionally
499 enclosed in braces, is an absolute or relative back reference. A named back
500 reference can be coded as \g{name}. Back references are discussed
501 <a href="#backreferences">later,</a>
502 following the discussion of
503 <a href="#subpattern">parenthesized subpatterns.</a>
504 </P>
505 <br><b>
506 Absolute and relative subroutine calls
507 </b><br>
508 <P>
509 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
510 a number enclosed either in angle brackets or single quotes, is an alternative
511 syntax for referencing a subpattern as a "subroutine". Details are discussed
512 <a href="#onigurumasubroutines">later.</a>
513 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
514 synonymous. The former is a back reference; the latter is a
515 <a href="#subpatternsassubroutines">subroutine</a>
516 call.
517 <a name="genericchartypes"></a></P>
518 <br><b>
519 Generic character types
520 </b><br>
521 <P>
522 Another use of backslash is for specifying generic character types:
523 <pre>
524 \d any decimal digit
525 \D any character that is not a decimal digit
526 \h any horizontal white space character
527 \H any character that is not a horizontal white space character
528 \s any white space character
529 \S any character that is not a white space character
530 \v any vertical white space character
531 \V any character that is not a vertical white space character
532 \w any "word" character
533 \W any "non-word" character
534 </pre>
535 There is also the single sequence \N, which matches a non-newline character.
536 This is the same as
537 <a href="#fullstopdot">the "." metacharacter</a>
538 when PCRE_DOTALL is not set. Perl also uses \N to match characters by name;
539 PCRE does not support this.
540 </P>
541 <P>
542 Each pair of lower and upper case escape sequences partitions the complete set
543 of characters into two disjoint sets. Any given character matches one, and only
544 one, of each pair. The sequences can appear both inside and outside character
545 classes. They each match one character of the appropriate type. If the current
546 matching point is at the end of the subject string, all of them fail, because
547 there is no character to match.
548 </P>
549 <P>
550 For compatibility with Perl, \s did not used to match the VT character (code
551 11), which made it different from the the POSIX "space" class. However, Perl
552 added VT at release 5.18, and PCRE followed suit at release 8.34. The default
553 \s characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space
554 (32), which are defined as white space in the "C" locale. This list may vary if
555 locale-specific matching is taking place. For example, in some locales the
556 "non-breaking space" character (\xA0) is recognized as white space, and in
557 others the VT character is not.
558 </P>
559 <P>
560 A "word" character is an underscore or any character that is a letter or digit.
561 By default, the definition of letters and digits is controlled by PCRE's
562 low-valued character tables, and may vary if locale-specific matching is taking
563 place (see
564 <a href="pcreapi.html#localesupport">"Locale support"</a>
565 in the
566 <a href="pcreapi.html"><b>pcreapi</b></a>
567 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
568 or "french" in Windows, some character codes greater than 127 are used for
569 accented letters, and these are then matched by \w. The use of locales with
570 Unicode is discouraged.
571 </P>
572 <P>
573 By default, characters whose code points are greater than 127 never match \d,
574 \s, or \w, and always match \D, \S, and \W, although this may vary for
575 characters in the range 128-255 when locale-specific matching is happening.
576 These escape sequences retain their original meanings from before Unicode
577 support was available, mainly for efficiency reasons. If PCRE is compiled with
578 Unicode property support, and the PCRE_UCP option is set, the behaviour is
579 changed so that Unicode properties are used to determine character types, as
580 follows:
581 <pre>
582 \d any character that matches \p{Nd} (decimal digit)
583 \s any character that matches \p{Z} or \h or \v
584 \w any character that matches \p{L} or \p{N}, plus underscore
585 </pre>
586 The upper case escapes match the inverse sets of characters. Note that \d
587 matches only decimal digits, whereas \w matches any Unicode digit, as well as
588 any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
589 \B because they are defined in terms of \w and \W. Matching these sequences
590 is noticeably slower when PCRE_UCP is set.
591 </P>
592 <P>
593 The sequences \h, \H, \v, and \V are features that were added to Perl at
594 release 5.10. In contrast to the other sequences, which match only ASCII
595 characters by default, these always match certain high-valued code points,
596 whether or not PCRE_UCP is set. The horizontal space characters are:
597 <pre>
598 U+0009 Horizontal tab (HT)
599 U+0020 Space
600 U+00A0 Non-break space
601 U+1680 Ogham space mark
602 U+180E Mongolian vowel separator
603 U+2000 En quad
604 U+2001 Em quad
605 U+2002 En space
606 U+2003 Em space
607 U+2004 Three-per-em space
608 U+2005 Four-per-em space
609 U+2006 Six-per-em space
610 U+2007 Figure space
611 U+2008 Punctuation space
612 U+2009 Thin space
613 U+200A Hair space
614 U+202F Narrow no-break space
615 U+205F Medium mathematical space
616 U+3000 Ideographic space
617 </pre>
618 The vertical space characters are:
619 <pre>
620 U+000A Linefeed (LF)
621 U+000B Vertical tab (VT)
622 U+000C Form feed (FF)
623 U+000D Carriage return (CR)
624 U+0085 Next line (NEL)
625 U+2028 Line separator
626 U+2029 Paragraph separator
627 </pre>
628 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
629 relevant.
630 <a name="newlineseq"></a></P>
631 <br><b>
632 Newline sequences
633 </b><br>
634 <P>
635 Outside a character class, by default, the escape sequence \R matches any
636 Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
637 following:
638 <pre>
639 (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
640 </pre>
641 This is an example of an "atomic group", details of which are given
642 <a href="#atomicgroup">below.</a>
643 This particular group matches either the two-character sequence CR followed by
644 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
645 U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
646 line, U+0085). The two-character sequence is treated as a single unit that
647 cannot be split.
648 </P>
649 <P>
650 In other modes, two additional characters whose codepoints are greater than 255
651 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
652 Unicode character property support is not needed for these characters to be
653 recognized.
654 </P>
655 <P>
656 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
657 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
658 either at compile time or when the pattern is matched. (BSR is an abbreviation
659 for "backslash R".) This can be made the default when PCRE is built; if this is
660 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
661 It is also possible to specify these settings by starting a pattern string with
662 one of the following sequences:
663 <pre>
664 (*BSR_ANYCRLF) CR, LF, or CRLF only
665 (*BSR_UNICODE) any Unicode newline sequence
666 </pre>
667 These override the default and the options given to the compiling function, but
668 they can themselves be overridden by options given to a matching function. Note
669 that these special settings, which are not Perl-compatible, are recognized only
670 at the very start of a pattern, and that they must be in upper case. If more
671 than one of them is present, the last one is used. They can be combined with a
672 change of newline convention; for example, a pattern can start with:
673 <pre>
675 </pre>
676 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
677 (*UCP) special sequences. Inside a character class, \R is treated as an
678 unrecognized escape sequence, and so matches the letter "R" by default, but
679 causes an error if PCRE_EXTRA is set.
680 <a name="uniextseq"></a></P>
681 <br><b>
682 Unicode character properties
683 </b><br>
684 <P>
685 When PCRE is built with Unicode character property support, three additional
686 escape sequences that match characters with specific properties are available.
687 When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
688 characters whose codepoints are less than 256, but they do work in this mode.
689 The extra escape sequences are:
690 <pre>
691 \p{<i>xx</i>} a character with the <i>xx</i> property
692 \P{<i>xx</i>} a character without the <i>xx</i> property
693 \X a Unicode extended grapheme cluster
694 </pre>
695 The property names represented by <i>xx</i> above are limited to the Unicode
696 script names, the general category properties, "Any", which matches any
697 character (including newline), and some special PCRE properties (described
698 in the
699 <a href="#extraprops">next section).</a>
700 Other Perl properties such as "InMusicalSymbols" are not currently supported by
701 PCRE. Note that \P{Any} does not match any characters, so always causes a
702 match failure.
703 </P>
704 <P>
705 Sets of Unicode characters are defined as belonging to certain scripts. A
706 character from one of these sets can be matched using a script name. For
707 example:
708 <pre>
709 \p{Greek}
710 \P{Han}
711 </pre>
712 Those that are not part of an identified script are lumped together as
713 "Common". The current list of scripts is:
714 </P>
715 <P>
716 Arabic,
717 Armenian,
718 Avestan,
719 Balinese,
720 Bamum,
721 Bassa_Vah,
722 Batak,
723 Bengali,
724 Bopomofo,
725 Brahmi,
726 Braille,
727 Buginese,
728 Buhid,
729 Canadian_Aboriginal,
730 Carian,
731 Caucasian_Albanian,
732 Chakma,
733 Cham,
734 Cherokee,
735 Common,
736 Coptic,
737 Cuneiform,
738 Cypriot,
739 Cyrillic,
740 Deseret,
741 Devanagari,
742 Duployan,
743 Egyptian_Hieroglyphs,
744 Elbasan,
745 Ethiopic,
746 Georgian,
747 Glagolitic,
748 Gothic,
749 Grantha,
750 Greek,
751 Gujarati,
752 Gurmukhi,
753 Han,
754 Hangul,
755 Hanunoo,
756 Hebrew,
757 Hiragana,
758 Imperial_Aramaic,
759 Inherited,
760 Inscriptional_Pahlavi,
761 Inscriptional_Parthian,
762 Javanese,
763 Kaithi,
764 Kannada,
765 Katakana,
766 Kayah_Li,
767 Kharoshthi,
768 Khmer,
769 Khojki,
770 Khudawadi,
771 Lao,
772 Latin,
773 Lepcha,
774 Limbu,
775 Linear_A,
776 Linear_B,
777 Lisu,
778 Lycian,
779 Lydian,
780 Mahajani,
781 Malayalam,
782 Mandaic,
783 Manichaean,
784 Meetei_Mayek,
785 Mende_Kikakui,
786 Meroitic_Cursive,
787 Meroitic_Hieroglyphs,
788 Miao,
789 Modi,
790 Mongolian,
791 Mro,
792 Myanmar,
793 Nabataean,
794 New_Tai_Lue,
795 Nko,
796 Ogham,
797 Ol_Chiki,
798 Old_Italic,
799 Old_North_Arabian,
800 Old_Permic,
801 Old_Persian,
802 Old_South_Arabian,
803 Old_Turkic,
804 Oriya,
805 Osmanya,
806 Pahawh_Hmong,
807 Palmyrene,
808 Pau_Cin_Hau,
809 Phags_Pa,
810 Phoenician,
811 Psalter_Pahlavi,
812 Rejang,
813 Runic,
814 Samaritan,
815 Saurashtra,
816 Sharada,
817 Shavian,
818 Siddham,
819 Sinhala,
820 Sora_Sompeng,
821 Sundanese,
822 Syloti_Nagri,
823 Syriac,
824 Tagalog,
825 Tagbanwa,
826 Tai_Le,
827 Tai_Tham,
828 Tai_Viet,
829 Takri,
830 Tamil,
831 Telugu,
832 Thaana,
833 Thai,
834 Tibetan,
835 Tifinagh,
836 Tirhuta,
837 Ugaritic,
838 Vai,
839 Warang_Citi,
840 Yi.
841 </P>
842 <P>
843 Each character has exactly one Unicode general category property, specified by
844 a two-letter abbreviation. For compatibility with Perl, negation can be
845 specified by including a circumflex between the opening brace and the property
846 name. For example, \p{^Lu} is the same as \P{Lu}.
847 </P>
848 <P>
849 If only one letter is specified with \p or \P, it includes all the general
850 category properties that start with that letter. In this case, in the absence
851 of negation, the curly brackets in the escape sequence are optional; these two
852 examples have the same effect:
853 <pre>
854 \p{L}
855 \pL
856 </pre>
857 The following general category property codes are supported:
858 <pre>
859 C Other
860 Cc Control
861 Cf Format
862 Cn Unassigned
863 Co Private use
864 Cs Surrogate
866 L Letter
867 Ll Lower case letter
868 Lm Modifier letter
869 Lo Other letter
870 Lt Title case letter
871 Lu Upper case letter
873 M Mark
874 Mc Spacing mark
875 Me Enclosing mark
876 Mn Non-spacing mark
878 N Number
879 Nd Decimal number
880 Nl Letter number
881 No Other number
883 P Punctuation
884 Pc Connector punctuation
885 Pd Dash punctuation
886 Pe Close punctuation
887 Pf Final punctuation
888 Pi Initial punctuation
889 Po Other punctuation
890 Ps Open punctuation
892 S Symbol
893 Sc Currency symbol
894 Sk Modifier symbol
895 Sm Mathematical symbol
896 So Other symbol
898 Z Separator
899 Zl Line separator
900 Zp Paragraph separator
901 Zs Space separator
902 </pre>
903 The special property L& is also supported: it matches a character that has
904 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
905 a modifier or "other".
906 </P>
907 <P>
908 The Cs (Surrogate) property applies only to characters in the range U+D800 to
909 U+DFFF. Such characters are not valid in Unicode strings and so
910 cannot be tested by PCRE, unless UTF validity checking has been turned off
911 (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
912 PCRE_NO_UTF32_CHECK in the
913 <a href="pcreapi.html"><b>pcreapi</b></a>
914 page). Perl does not support the Cs property.
915 </P>
916 <P>
917 The long synonyms for property names that Perl supports (such as \p{Letter})
918 are not supported by PCRE, nor is it permitted to prefix any of these
919 properties with "Is".
920 </P>
921 <P>
922 No character that is in the Unicode table has the Cn (unassigned) property.
923 Instead, this property is assumed for any code point that is not in the
924 Unicode table.
925 </P>
926 <P>
927 Specifying caseless matching does not affect these escape sequences. For
928 example, \p{Lu} always matches only upper case letters. This is different from
929 the behaviour of current versions of Perl.
930 </P>
931 <P>
932 Matching characters by Unicode property is not fast, because PCRE has to do a
933 multistage table lookup in order to find a character's property. That is why
934 the traditional escape sequences such as \d and \w do not use Unicode
935 properties in PCRE by default, though you can make them do so by setting the
936 PCRE_UCP option or by starting the pattern with (*UCP).
937 </P>
938 <br><b>
939 Extended grapheme clusters
940 </b><br>
941 <P>
942 The \X escape matches any number of Unicode characters that form an "extended
943 grapheme cluster", and treats the sequence as an atomic group
944 <a href="#atomicgroup">(see below).</a>
945 Up to and including release 8.31, PCRE matched an earlier, simpler definition
946 that was equivalent to
947 <pre>
948 (?&#62;\PM\pM*)
949 </pre>
950 That is, it matched a character without the "mark" property, followed by zero
951 or more characters with the "mark" property. Characters with the "mark"
952 property are typically non-spacing accents that affect the preceding character.
953 </P>
954 <P>
955 This simple definition was extended in Unicode to include more complicated
956 kinds of composite character by giving each character a grapheme breaking
957 property, and creating rules that use these properties to define the boundaries
958 of extended grapheme clusters. In releases of PCRE later than 8.31, \X matches
959 one of these clusters.
960 </P>
961 <P>
962 \X always matches at least one character. Then it decides whether to add
963 additional characters according to the following rules for ending a cluster:
964 </P>
965 <P>
966 1. End at the end of the subject string.
967 </P>
968 <P>
969 2. Do not end between CR and LF; otherwise end after any control character.
970 </P>
971 <P>
972 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
973 are of five types: L, V, T, LV, and LVT. An L character may be followed by an
974 L, V, LV, or LVT character; an LV or V character may be followed by a V or T
975 character; an LVT or T character may be followed only by a T character.
976 </P>
977 <P>
978 4. Do not end before extending characters or spacing marks. Characters with
979 the "mark" property always have the "extend" grapheme breaking property.
980 </P>
981 <P>
982 5. Do not end after prepend characters.
983 </P>
984 <P>
985 6. Otherwise, end the cluster.
986 <a name="extraprops"></a></P>
987 <br><b>
988 PCRE's additional properties
989 </b><br>
990 <P>
991 As well as the standard Unicode properties described above, PCRE supports four
992 more that make it possible to convert traditional escape sequences such as \w
993 and \s to use Unicode properties. PCRE uses these non-standard, non-Perl
994 properties internally when PCRE_UCP is set. However, they may also be used
995 explicitly. These properties are:
996 <pre>
997 Xan Any alphanumeric character
998 Xps Any POSIX space character
999 Xsp Any Perl space character
1000 Xwd Any Perl "word" character
1001 </pre>
1002 Xan matches characters that have either the L (letter) or the N (number)
1003 property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
1004 carriage return, and any other character that has the Z (separator) property.
1005 Xsp is the same as Xps; it used to exclude vertical tab, for Perl
1006 compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
1007 matches the same characters as Xan, plus underscore.
1008 </P>
1009 <P>
1010 There is another non-standard property, Xuc, which matches any character that
1011 can be represented by a Universal Character Name in C++ and other programming
1012 languages. These are the characters $, @, ` (grave accent), and all characters
1013 with Unicode code points greater than or equal to U+00A0, except for the
1014 surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
1015 excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH
1016 where H is a hexadecimal digit. Note that the Xuc property does not match these
1017 sequences but the characters that they represent.)
1018 <a name="resetmatchstart"></a></P>
1019 <br><b>
1020 Resetting the match start
1021 </b><br>
1022 <P>
1023 The escape sequence \K causes any previously matched characters not to be
1024 included in the final matched sequence. For example, the pattern:
1025 <pre>
1026 foo\Kbar
1027 </pre>
1028 matches "foobar", but reports that it has matched "bar". This feature is
1029 similar to a lookbehind assertion
1030 <a href="#lookbehind">(described below).</a>
1031 However, in this case, the part of the subject before the real match does not
1032 have to be of fixed length, as lookbehind assertions do. The use of \K does
1033 not interfere with the setting of
1034 <a href="#subpattern">captured substrings.</a>
1035 For example, when the pattern
1036 <pre>
1037 (foo)\Kbar
1038 </pre>
1039 matches "foobar", the first substring is still set to "foo".
1040 </P>
1041 <P>
1042 Perl documents that the use of \K within assertions is "not well defined". In
1043 PCRE, \K is acted upon when it occurs inside positive assertions, but is
1044 ignored in negative assertions. Note that when a pattern such as (?=ab\K)
1045 matches, the reported start of the match can be greater than the end of the
1046 match.
1047 <a name="smallassertions"></a></P>
1048 <br><b>
1049 Simple assertions
1050 </b><br>
1051 <P>
1052 The final use of backslash is for certain simple assertions. An assertion
1053 specifies a condition that has to be met at a particular point in a match,
1054 without consuming any characters from the subject string. The use of
1055 subpatterns for more complicated assertions is described
1056 <a href="#bigassertions">below.</a>
1057 The backslashed assertions are:
1058 <pre>
1059 \b matches at a word boundary
1060 \B matches when not at a word boundary
1061 \A matches at the start of the subject
1062 \Z matches at the end of the subject
1063 also matches before a newline at the end of the subject
1064 \z matches only at the end of the subject
1065 \G matches at the first matching position in the subject
1066 </pre>
1067 Inside a character class, \b has a different meaning; it matches the backspace
1068 character. If any other of these assertions appears in a character class, by
1069 default it matches the corresponding literal character (for example, \B
1070 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
1071 escape sequence" error is generated instead.
1072 </P>
1073 <P>
1074 A word boundary is a position in the subject string where the current character
1075 and the previous character do not both match \w or \W (i.e. one matches
1076 \w and the other matches \W), or the start or end of the string if the
1077 first or last character matches \w, respectively. In a UTF mode, the meanings
1078 of \w and \W can be changed by setting the PCRE_UCP option. When this is
1079 done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
1080 of word" or "end of word" metasequence. However, whatever follows \b normally
1081 determines which it is. For example, the fragment \ba matches "a" at the start
1082 of a word.
1083 </P>
1084 <P>
1085 The \A, \Z, and \z assertions differ from the traditional circumflex and
1086 dollar (described in the next section) in that they only ever match at the very
1087 start and end of the subject string, whatever options are set. Thus, they are
1088 independent of multiline mode. These three assertions are not affected by the
1089 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
1090 circumflex and dollar metacharacters. However, if the <i>startoffset</i>
1091 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
1092 at a point other than the beginning of the subject, \A can never match. The
1093 difference between \Z and \z is that \Z matches before a newline at the end
1094 of the string as well as at the very end, whereas \z matches only at the end.
1095 </P>
1096 <P>
1097 The \G assertion is true only when the current matching position is at the
1098 start point of the match, as specified by the <i>startoffset</i> argument of
1099 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
1100 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
1101 arguments, you can mimic Perl's /g option, and it is in this kind of
1102 implementation where \G can be useful.
1103 </P>
1104 <P>
1105 Note, however, that PCRE's interpretation of \G, as the start of the current
1106 match, is subtly different from Perl's, which defines it as the end of the
1107 previous match. In Perl, these can be different when the previously matched
1108 string was empty. Because PCRE does just one match at a time, it cannot
1109 reproduce this behaviour.
1110 </P>
1111 <P>
1112 If all the alternatives of a pattern begin with \G, the expression is anchored
1113 to the starting match position, and the "anchored" flag is set in the compiled
1114 regular expression.
1115 </P>
1116 <br><a name="SEC6" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
1117 <P>
1118 The circumflex and dollar metacharacters are zero-width assertions. That is,
1119 they test for a particular condition being true without consuming any
1120 characters from the subject string.
1121 </P>
1122 <P>
1123 Outside a character class, in the default matching mode, the circumflex
1124 character is an assertion that is true only if the current matching point is at
1125 the start of the subject string. If the <i>startoffset</i> argument of
1126 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
1127 option is unset. Inside a character class, circumflex has an entirely different
1128 meaning
1129 <a href="#characterclass">(see below).</a>
1130 </P>
1131 <P>
1132 Circumflex need not be the first character of the pattern if a number of
1133 alternatives are involved, but it should be the first thing in each alternative
1134 in which it appears if the pattern is ever to match that branch. If all
1135 possible alternatives start with a circumflex, that is, if the pattern is
1136 constrained to match only at the start of the subject, it is said to be an
1137 "anchored" pattern. (There are also other constructs that can cause a pattern
1138 to be anchored.)
1139 </P>
1140 <P>
1141 The dollar character is an assertion that is true only if the current matching
1142 point is at the end of the subject string, or immediately before a newline at
1143 the end of the string (by default). Note, however, that it does not actually
1144 match the newline. Dollar need not be the last character of the pattern if a
1145 number of alternatives are involved, but it should be the last item in any
1146 branch in which it appears. Dollar has no special meaning in a character class.
1147 </P>
1148 <P>
1149 The meaning of dollar can be changed so that it matches only at the very end of
1150 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
1151 does not affect the \Z assertion.
1152 </P>
1153 <P>
1154 The meanings of the circumflex and dollar characters are changed if the
1155 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
1156 immediately after internal newlines as well as at the start of the subject
1157 string. It does not match after a newline that ends the string. A dollar
1158 matches before any newlines in the string, as well as at the very end, when
1159 PCRE_MULTILINE is set. When newline is specified as the two-character
1160 sequence CRLF, isolated CR and LF characters do not indicate newlines.
1161 </P>
1162 <P>
1163 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
1164 \n represents a newline) in multiline mode, but not otherwise. Consequently,
1165 patterns that are anchored in single line mode because all branches start with
1166 ^ are not anchored in multiline mode, and a match for circumflex is possible
1167 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
1168 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
1169 </P>
1170 <P>
1171 Note that the sequences \A, \Z, and \z can be used to match the start and
1172 end of the subject in both modes, and if all branches of a pattern start with
1173 \A it is always anchored, whether or not PCRE_MULTILINE is set.
1174 <a name="fullstopdot"></a></P>
1175 <br><a name="SEC7" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
1176 <P>
1177 Outside a character class, a dot in the pattern matches any one character in
1178 the subject string except (by default) a character that signifies the end of a
1179 line.
1180 </P>
1181 <P>
1182 When a line ending is defined as a single character, dot never matches that
1183 character; when the two-character sequence CRLF is used, dot does not match CR
1184 if it is immediately followed by LF, but otherwise it matches all characters
1185 (including isolated CRs and LFs). When any Unicode line endings are being
1186 recognized, dot does not match CR or LF or any of the other line ending
1187 characters.
1188 </P>
1189 <P>
1190 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
1191 option is set, a dot matches any one character, without exception. If the
1192 two-character sequence CRLF is present in the subject string, it takes two dots
1193 to match it.
1194 </P>
1195 <P>
1196 The handling of dot is entirely independent of the handling of circumflex and
1197 dollar, the only relationship being that they both involve newlines. Dot has no
1198 special meaning in a character class.
1199 </P>
1200 <P>
1201 The escape sequence \N behaves like a dot, except that it is not affected by
1202 the PCRE_DOTALL option. In other words, it matches any character except one
1203 that signifies the end of a line. Perl also uses \N to match characters by
1204 name; PCRE does not support this.
1205 </P>
1206 <br><a name="SEC8" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br>
1207 <P>
1208 Outside a character class, the escape sequence \C matches any one data unit,
1209 whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1210 byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1211 a 32-bit unit. Unlike a dot, \C always
1212 matches line-ending characters. The feature is provided in Perl in order to
1213 match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1214 used. Because \C breaks up characters into individual data units, matching one
1215 unit with \C in a UTF mode means that the rest of the string may start with a
1216 malformed UTF character. This has undefined results, because PCRE assumes that
1217 it is dealing with valid UTF strings (and by default it checks this at the
1218 start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1219 PCRE_NO_UTF32_CHECK option is used).
1220 </P>
1221 <P>
1222 PCRE does not allow \C to appear in lookbehind assertions
1223 <a href="#lookbehind">(described below)</a>
1224 in a UTF mode, because this would make it impossible to calculate the length of
1225 the lookbehind.
1226 </P>
1227 <P>
1228 In general, the \C escape sequence is best avoided. However, one
1229 way of using it that avoids the problem of malformed UTF characters is to use a
1230 lookahead to check the length of the next character, as in this pattern, which
1231 could be used with a UTF-8 string (ignore white space and line breaks):
1232 <pre>
1233 (?| (?=[\x00-\x7f])(\C) |
1234 (?=[\x80-\x{7ff}])(\C)(\C) |
1235 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
1236 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
1237 </pre>
1238 A group that starts with (?| resets the capturing parentheses numbers in each
1239 alternative (see
1240 <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
1241 below). The assertions at the start of each branch check the next UTF-8
1242 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1243 character's individual bytes are then captured by the appropriate number of
1244 groups.
1245 <a name="characterclass"></a></P>
1246 <br><a name="SEC9" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
1247 <P>
1248 An opening square bracket introduces a character class, terminated by a closing
1249 square bracket. A closing square bracket on its own is not special by default.
1250 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1251 bracket causes a compile-time error. If a closing square bracket is required as
1252 a member of the class, it should be the first data character in the class
1253 (after an initial circumflex, if present) or escaped with a backslash.
1254 </P>
1255 <P>
1256 A character class matches a single character in the subject. In a UTF mode, the
1257 character may be more than one data unit long. A matched character must be in
1258 the set of characters defined by the class, unless the first character in the
1259 class definition is a circumflex, in which case the subject character must not
1260 be in the set defined by the class. If a circumflex is actually required as a
1261 member of the class, ensure it is not the first character, or escape it with a
1262 backslash.
1263 </P>
1264 <P>
1265 For example, the character class [aeiou] matches any lower case vowel, while
1266 [^aeiou] matches any character that is not a lower case vowel. Note that a
1267 circumflex is just a convenient notation for specifying the characters that
1268 are in the class by enumerating those that are not. A class that starts with a
1269 circumflex is not an assertion; it still consumes a character from the subject
1270 string, and therefore it fails if the current pointer is at the end of the
1271 string.
1272 </P>
1273 <P>
1274 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1275 can be included in a class as a literal string of data units, or by using the
1276 \x{ escaping mechanism.
1277 </P>
1278 <P>
1279 When caseless matching is set, any letters in a class represent both their
1280 upper case and lower case versions, so for example, a caseless [aeiou] matches
1281 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1282 caseful version would. In a UTF mode, PCRE always understands the concept of
1283 case for characters whose values are less than 128, so caseless matching is
1284 always possible. For characters with higher values, the concept of case is
1285 supported if PCRE is compiled with Unicode property support, but not otherwise.
1286 If you want to use caseless matching in a UTF mode for characters 128 and
1287 above, you must ensure that PCRE is compiled with Unicode property support as
1288 well as with UTF support.
1289 </P>
1290 <P>
1291 Characters that might indicate line breaks are never treated in any special way
1292 when matching character classes, whatever line-ending sequence is in use, and
1293 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1294 such as [^a] always matches one of these characters.
1295 </P>
1296 <P>
1297 The minus (hyphen) character can be used to specify a range of characters in a
1298 character class. For example, [d-m] matches any letter between d and m,
1299 inclusive. If a minus character is required in a class, it must be escaped with
1300 a backslash or appear in a position where it cannot be interpreted as
1301 indicating a range, typically as the first or last character in the class, or
1302 immediately after a range. For example, [b-d-z] matches letters in the range b
1303 to d, a hyphen character, or z.
1304 </P>
1305 <P>
1306 It is not possible to have the literal character "]" as the end character of a
1307 range. A pattern such as [W-]46] is interpreted as a class of two characters
1308 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1309 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1310 the end of range, so [W-\]46] is interpreted as a class containing a range
1311 followed by two other characters. The octal or hexadecimal representation of
1312 "]" can also be used to end a range.
1313 </P>
1314 <P>
1315 An error is generated if a POSIX character class (see below) or an escape
1316 sequence other than one that defines a single character appears at a point
1317 where a range ending character is expected. For example, [z-\xff] is valid,
1318 but [A-\d] and [A-[:digit:]] are not.
1319 </P>
1320 <P>
1321 Ranges operate in the collating sequence of character values. They can also be
1322 used for characters specified numerically, for example [\000-\037]. Ranges
1323 can include any characters that are valid for the current mode.
1324 </P>
1325 <P>
1326 If a range that includes letters is used when caseless matching is set, it
1327 matches the letters in either case. For example, [W-c] is equivalent to
1328 [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1329 tables for a French locale are in use, [\xc8-\xcb] matches accented E
1330 characters in both cases. In UTF modes, PCRE supports the concept of case for
1331 characters with values greater than 128 only when it is compiled with Unicode
1332 property support.
1333 </P>
1334 <P>
1335 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
1336 \V, \w, and \W may appear in a character class, and add the characters that
1337 they match to the class. For example, [\dABCDEF] matches any hexadecimal
1338 digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w
1339 and their upper case partners, just as it does when they appear outside a
1340 character class, as described in the section entitled
1341 <a href="#genericchartypes">"Generic character types"</a>
1342 above. The escape sequence \b has a different meaning inside a character
1343 class; it matches the backspace character. The sequences \B, \N, \R, and \X
1344 are not special inside a character class. Like any other unrecognized escape
1345 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1346 default, but cause an error if the PCRE_EXTRA option is set.
1347 </P>
1348 <P>
1349 A circumflex can conveniently be used with the upper case character types to
1350 specify a more restricted set of characters than the matching lower case type.
1351 For example, the class [^\W_] matches any letter or digit, but not underscore,
1352 whereas [\w] includes underscore. A positive character class should be read as
1353 "something OR something OR ..." and a negative class as "NOT something AND NOT
1354 something AND NOT ...".
1355 </P>
1356 <P>
1357 The only metacharacters that are recognized in character classes are backslash,
1358 hyphen (only where it can be interpreted as specifying a range), circumflex
1359 (only at the start), opening square bracket (only when it can be interpreted as
1360 introducing a POSIX class name, or for a special compatibility feature - see
1361 the next two sections), and the terminating closing square bracket. However,
1362 escaping other non-alphanumeric characters does no harm.
1363 </P>
1364 <br><a name="SEC10" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
1365 <P>
1366 Perl supports the POSIX notation for character classes. This uses names
1367 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1368 this notation. For example,
1369 <pre>
1370 [01[:alpha:]%]
1371 </pre>
1372 matches "0", "1", any alphabetic character, or "%". The supported class names
1373 are:
1374 <pre>
1375 alnum letters and digits
1376 alpha letters
1377 ascii character codes 0 - 127
1378 blank space or tab only
1379 cntrl control characters
1380 digit decimal digits (same as \d)
1381 graph printing characters, excluding space
1382 lower lower case letters
1383 print printing characters, including space
1384 punct printing characters, excluding letters and digits and space
1385 space white space (the same as \s from PCRE 8.34)
1386 upper upper case letters
1387 word "word" characters (same as \w)
1388 xdigit hexadecimal digits
1389 </pre>
1390 The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
1391 and space (32). If locale-specific matching is taking place, the list of space
1392 characters may be different; there may be fewer or more of them. "Space" used
1393 to be different to \s, which did not include VT, for Perl compatibility.
1394 However, Perl changed at release 5.18, and PCRE followed at release 8.34.
1395 "Space" and \s now match the same set of characters.
1396 </P>
1397 <P>
1398 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1399 5.8. Another Perl extension is negation, which is indicated by a ^ character
1400 after the colon. For example,
1401 <pre>
1402 [12[:^digit:]]
1403 </pre>
1404 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1405 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1406 supported, and an error is given if they are encountered.
1407 </P>
1408 <P>
1409 By default, characters with values greater than 128 do not match any of the
1410 POSIX character classes. However, if the PCRE_UCP option is passed to
1411 <b>pcre_compile()</b>, some of the classes are changed so that Unicode character
1412 properties are used. This is achieved by replacing certain POSIX classes by
1413 other sequences, as follows:
1414 <pre>
1415 [:alnum:] becomes \p{Xan}
1416 [:alpha:] becomes \p{L}
1417 [:blank:] becomes \h
1418 [:digit:] becomes \p{Nd}
1419 [:lower:] becomes \p{Ll}
1420 [:space:] becomes \p{Xps}
1421 [:upper:] becomes \p{Lu}
1422 [:word:] becomes \p{Xwd}
1423 </pre>
1424 Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX
1425 classes are handled specially in UCP mode:
1426 </P>
1427 <P>
1428 [:graph:]
1429 This matches characters that have glyphs that mark the page when printed. In
1430 Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
1431 properties, except for:
1432 <pre>
1433 U+061C Arabic Letter Mark
1434 U+180E Mongolian Vowel Separator
1435 U+2066 - U+2069 Various "isolate"s
1437 </PRE>
1438 </P>
1439 <P>
1440 [:print:]
1441 This matches the same characters as [:graph:] plus space characters that are
1442 not controls, that is, characters with the Zs property.
1443 </P>
1444 <P>
1445 [:punct:]
1446 This matches all characters that have the Unicode P (punctuation) property,
1447 plus those characters whose code points are less than 128 that have the S
1448 (Symbol) property.
1449 </P>
1450 <P>
1451 The other POSIX classes are unchanged, and match only characters with code
1452 points less than 128.
1453 </P>
1454 <br><a name="SEC11" href="#TOC1">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a><br>
1455 <P>
1456 In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly
1457 syntax [[:&#60;:]] and [[:&#62;:]] is used for matching "start of word" and "end of
1458 word". PCRE treats these items as follows:
1459 <pre>
1460 [[:&#60;:]] is converted to \b(?=\w)
1461 [[:&#62;:]] is converted to \b(?&#60;=\w)
1462 </pre>
1463 Only these exact character sequences are recognized. A sequence such as
1464 [a[:&#60;:]b] provokes error for an unrecognized POSIX class name. This support is
1465 not compatible with Perl. It is provided to help migrations from other
1466 environments, and is best not used in any new patterns. Note that \b matches
1467 at the start and the end of a word (see
1468 <a href="#smallassertions">"Simple assertions"</a>
1469 above), and in a Perl-style pattern the preceding or following character
1470 normally shows which is wanted, without the need for the assertions that are
1471 used above in order to give exactly the POSIX behaviour.
1472 </P>
1473 <br><a name="SEC12" href="#TOC1">VERTICAL BAR</a><br>
1474 <P>
1475 Vertical bar characters are used to separate alternative patterns. For example,
1476 the pattern
1477 <pre>
1478 gilbert|sullivan
1479 </pre>
1480 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1481 and an empty alternative is permitted (matching the empty string). The matching
1482 process tries each alternative in turn, from left to right, and the first one
1483 that succeeds is used. If the alternatives are within a subpattern
1484 <a href="#subpattern">(defined below),</a>
1485 "succeeds" means matching the rest of the main pattern as well as the
1486 alternative in the subpattern.
1487 </P>
1488 <br><a name="SEC13" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1489 <P>
1490 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1491 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1492 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1493 The option letters are
1494 <pre>
1495 i for PCRE_CASELESS
1497 s for PCRE_DOTALL
1498 x for PCRE_EXTENDED
1499 </pre>
1500 For example, (?im) sets caseless, multiline matching. It is also possible to
1501 unset these options by preceding the letter with a hyphen, and a combined
1502 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1503 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1504 permitted. If a letter appears both before and after the hyphen, the option is
1505 unset.
1506 </P>
1507 <P>
1508 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1509 changed in the same way as the Perl-compatible options by using the characters
1510 J, U and X respectively.
1511 </P>
1512 <P>
1513 When one of these option changes occurs at top level (that is, not inside
1514 subpattern parentheses), the change applies to the remainder of the pattern
1515 that follows. An option change within a subpattern (see below for a description
1516 of subpatterns) affects only that part of the subpattern that follows it, so
1517 <pre>
1518 (a(?i)b)c
1519 </pre>
1520 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1521 By this means, options can be made to have different settings in different
1522 parts of the pattern. Any changes made in one alternative do carry on
1523 into subsequent branches within the same subpattern. For example,
1524 <pre>
1525 (a(?i)b|c)
1526 </pre>
1527 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1528 branch is abandoned before the option setting. This is because the effects of
1529 option settings happen at compile time. There would be some very weird
1530 behaviour otherwise.
1531 </P>
1532 <P>
1533 <b>Note:</b> There are other PCRE-specific options that can be set by the
1534 application when the compiling or matching functions are called. In some cases
1535 the pattern can contain special leading sequences such as (*CRLF) to override
1536 what the application has set or what has been defaulted. Details are given in
1537 the section entitled
1538 <a href="#newlineseq">"Newline sequences"</a>
1539 above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1540 sequences that can be used to set UTF and Unicode property modes; they are
1541 equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1542 options, respectively. The (*UTF) sequence is a generic version that can be
1543 used with any of the libraries. However, the application can set the
1544 PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1545 <a name="subpattern"></a></P>
1546 <br><a name="SEC14" href="#TOC1">SUBPATTERNS</a><br>
1547 <P>
1548 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1549 Turning part of a pattern into a subpattern does two things:
1550 <br>
1551 <br>
1552 1. It localizes a set of alternatives. For example, the pattern
1553 <pre>
1554 cat(aract|erpillar|)
1555 </pre>
1556 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1557 match "cataract", "erpillar" or an empty string.
1558 <br>
1559 <br>
1560 2. It sets up the subpattern as a capturing subpattern. This means that, when
1561 the whole pattern matches, that portion of the subject string that matched the
1562 subpattern is passed back to the caller via the <i>ovector</i> argument of the
1563 matching function. (This applies only to the traditional matching functions;
1564 the DFA matching functions do not support capturing.)
1565 </P>
1566 <P>
1567 Opening parentheses are counted from left to right (starting from 1) to obtain
1568 numbers for the capturing subpatterns. For example, if the string "the red
1569 king" is matched against the pattern
1570 <pre>
1571 the ((red|white) (king|queen))
1572 </pre>
1573 the captured substrings are "red king", "red", and "king", and are numbered 1,
1574 2, and 3, respectively.
1575 </P>
1576 <P>
1577 The fact that plain parentheses fulfil two functions is not always helpful.
1578 There are often times when a grouping subpattern is required without a
1579 capturing requirement. If an opening parenthesis is followed by a question mark
1580 and a colon, the subpattern does not do any capturing, and is not counted when
1581 computing the number of any subsequent capturing subpatterns. For example, if
1582 the string "the white queen" is matched against the pattern
1583 <pre>
1584 the ((?:red|white) (king|queen))
1585 </pre>
1586 the captured substrings are "white queen" and "queen", and are numbered 1 and
1587 2. The maximum number of capturing subpatterns is 65535.
1588 </P>
1589 <P>
1590 As a convenient shorthand, if any option settings are required at the start of
1591 a non-capturing subpattern, the option letters may appear between the "?" and
1592 the ":". Thus the two patterns
1593 <pre>
1594 (?i:saturday|sunday)
1595 (?:(?i)saturday|sunday)
1596 </pre>
1597 match exactly the same set of strings. Because alternative branches are tried
1598 from left to right, and options are not reset until the end of the subpattern
1599 is reached, an option setting in one branch does affect subsequent branches, so
1600 the above patterns match "SUNDAY" as well as "Saturday".
1601 <a name="dupsubpatternnumber"></a></P>
1602 <br><a name="SEC15" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1603 <P>
1604 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1605 the same numbers for its capturing parentheses. Such a subpattern starts with
1606 (?| and is itself a non-capturing subpattern. For example, consider this
1607 pattern:
1608 <pre>
1609 (?|(Sat)ur|(Sun))day
1610 </pre>
1611 Because the two alternatives are inside a (?| group, both sets of capturing
1612 parentheses are numbered one. Thus, when the pattern matches, you can look
1613 at captured substring number one, whichever alternative matched. This construct
1614 is useful when you want to capture part, but not all, of one of a number of
1615 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1616 number is reset at the start of each branch. The numbers of any capturing
1617 parentheses that follow the subpattern start after the highest number used in
1618 any branch. The following example is taken from the Perl documentation. The
1619 numbers underneath show in which buffer the captured content will be stored.
1620 <pre>
1621 # before ---------------branch-reset----------- after
1622 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1623 # 1 2 2 3 2 3 4
1624 </pre>
1625 A back reference to a numbered subpattern uses the most recent value that is
1626 set for that number by any subpattern. The following pattern matches "abcabc"
1627 or "defdef":
1628 <pre>
1629 /(?|(abc)|(def))\1/
1630 </pre>
1631 In contrast, a subroutine call to a numbered subpattern always refers to the
1632 first one in the pattern with the given number. The following pattern matches
1633 "abcabc" or "defabc":
1634 <pre>
1635 /(?|(abc)|(def))(?1)/
1636 </pre>
1637 If a
1638 <a href="#conditions">condition test</a>
1639 for a subpattern's having matched refers to a non-unique number, the test is
1640 true if any of the subpatterns of that number have matched.
1641 </P>
1642 <P>
1643 An alternative approach to using this "branch reset" feature is to use
1644 duplicate named subpatterns, as described in the next section.
1645 </P>
1646 <br><a name="SEC16" href="#TOC1">NAMED SUBPATTERNS</a><br>
1647 <P>
1648 Identifying capturing parentheses by number is simple, but it can be very hard
1649 to keep track of the numbers in complicated regular expressions. Furthermore,
1650 if an expression is modified, the numbers may change. To help with this
1651 difficulty, PCRE supports the naming of subpatterns. This feature was not
1652 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1653 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1654 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1655 have different names, but PCRE does not.
1656 </P>
1657 <P>
1658 In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1659 (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1660 parentheses from other parts of the pattern, such as
1661 <a href="#backreferences">back references,</a>
1662 <a href="#recursion">recursion,</a>
1663 and
1664 <a href="#conditions">conditions,</a>
1665 can be made by name as well as by number.
1666 </P>
1667 <P>
1668 Names consist of up to 32 alphanumeric characters and underscores, but must
1669 start with a non-digit. Named capturing parentheses are still allocated numbers
1670 as well as names, exactly as if the names were not present. The PCRE API
1671 provides function calls for extracting the name-to-number translation table
1672 from a compiled pattern. There is also a convenience function for extracting a
1673 captured substring by name.
1674 </P>
1675 <P>
1676 By default, a name must be unique within a pattern, but it is possible to relax
1677 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1678 names are also always permitted for subpatterns with the same number, set up as
1679 described in the previous section.) Duplicate names can be useful for patterns
1680 where only one instance of the named parentheses can match. Suppose you want to
1681 match the name of a weekday, either as a 3-letter abbreviation or as the full
1682 name, and in both cases you want to extract the abbreviation. This pattern
1683 (ignoring the line breaks) does the job:
1684 <pre>
1685 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1686 (?&#60;DN&#62;Tue)(?:sday)?|
1687 (?&#60;DN&#62;Wed)(?:nesday)?|
1688 (?&#60;DN&#62;Thu)(?:rsday)?|
1689 (?&#60;DN&#62;Sat)(?:urday)?
1690 </pre>
1691 There are five capturing substrings, but only one is ever set after a match.
1692 (An alternative way of solving this problem is to use a "branch reset"
1693 subpattern, as described in the previous section.)
1694 </P>
1695 <P>
1696 The convenience function for extracting the data by name returns the substring
1697 for the first (and in this example, the only) subpattern of that name that
1698 matched. This saves searching to find which numbered subpattern it was.
1699 </P>
1700 <P>
1701 If you make a back reference to a non-unique named subpattern from elsewhere in
1702 the pattern, the subpatterns to which the name refers are checked in the order
1703 in which they appear in the overall pattern. The first one that is set is used
1704 for the reference. For example, this pattern matches both "foofoo" and
1705 "barbar" but not "foobar" or "barfoo":
1706 <pre>
1707 (?:(?&#60;n&#62;foo)|(?&#60;n&#62;bar))\k&#60;n&#62;
1709 </PRE>
1710 </P>
1711 <P>
1712 If you make a subroutine call to a non-unique named subpattern, the one that
1713 corresponds to the first occurrence of the name is used. In the absence of
1714 duplicate numbers (see the previous section) this is the one with the lowest
1715 number.
1716 </P>
1717 <P>
1718 If you use a named reference in a condition
1719 test (see the
1720 <a href="#conditions">section about conditions</a>
1721 below), either to check whether a subpattern has matched, or to check for
1722 recursion, all subpatterns with the same name are tested. If the condition is
1723 true for any one of them, the overall condition is true. This is the same
1724 behaviour as testing by number. For further details of the interfaces for
1725 handling named subpatterns, see the
1726 <a href="pcreapi.html"><b>pcreapi</b></a>
1727 documentation.
1728 </P>
1729 <P>
1730 <b>Warning:</b> You cannot use different names to distinguish between two
1731 subpatterns with the same number because PCRE uses only the numbers when
1732 matching. For this reason, an error is given at compile time if different names
1733 are given to subpatterns with the same number. However, you can always give the
1734 same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1735 set.
1736 </P>
1737 <br><a name="SEC17" href="#TOC1">REPETITION</a><br>
1738 <P>
1739 Repetition is specified by quantifiers, which can follow any of the following
1740 items:
1741 <pre>
1742 a literal data character
1743 the dot metacharacter
1744 the \C escape sequence
1745 the \X escape sequence
1746 the \R escape sequence
1747 an escape such as \d or \pL that matches a single character
1748 a character class
1749 a back reference (see next section)
1750 a parenthesized subpattern (including assertions)
1751 a subroutine call to a subpattern (recursive or otherwise)
1752 </pre>
1753 The general repetition quantifier specifies a minimum and maximum number of
1754 permitted matches, by giving the two numbers in curly brackets (braces),
1755 separated by a comma. The numbers must be less than 65536, and the first must
1756 be less than or equal to the second. For example:
1757 <pre>
1758 z{2,4}
1759 </pre>
1760 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1761 character. If the second number is omitted, but the comma is present, there is
1762 no upper limit; if the second number and the comma are both omitted, the
1763 quantifier specifies an exact number of required matches. Thus
1764 <pre>
1765 [aeiou]{3,}
1766 </pre>
1767 matches at least 3 successive vowels, but may match many more, while
1768 <pre>
1769 \d{8}
1770 </pre>
1771 matches exactly 8 digits. An opening curly bracket that appears in a position
1772 where a quantifier is not allowed, or one that does not match the syntax of a
1773 quantifier, is taken as a literal character. For example, {,6} is not a
1774 quantifier, but a literal string of four characters.
1775 </P>
1776 <P>
1777 In UTF modes, quantifiers apply to characters rather than to individual data
1778 units. Thus, for example, \x{100}{2} matches two characters, each of
1779 which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1780 \X{3} matches three Unicode extended grapheme clusters, each of which may be
1781 several data units long (and they may be of different lengths).
1782 </P>
1783 <P>
1784 The quantifier {0} is permitted, causing the expression to behave as if the
1785 previous item and the quantifier were not present. This may be useful for
1786 subpatterns that are referenced as
1787 <a href="#subpatternsassubroutines">subroutines</a>
1788 from elsewhere in the pattern (but see also the section entitled
1789 <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1790 below). Items other than subpatterns that have a {0} quantifier are omitted
1791 from the compiled pattern.
1792 </P>
1793 <P>
1794 For convenience, the three most common quantifiers have single-character
1795 abbreviations:
1796 <pre>
1797 * is equivalent to {0,}
1798 + is equivalent to {1,}
1799 ? is equivalent to {0,1}
1800 </pre>
1801 It is possible to construct infinite loops by following a subpattern that can
1802 match no characters with a quantifier that has no upper limit, for example:
1803 <pre>
1804 (a?)*
1805 </pre>
1806 Earlier versions of Perl and PCRE used to give an error at compile time for
1807 such patterns. However, because there are cases where this can be useful, such
1808 patterns are now accepted, but if any repetition of the subpattern does in fact
1809 match no characters, the loop is forcibly broken.
1810 </P>
1811 <P>
1812 By default, the quantifiers are "greedy", that is, they match as much as
1813 possible (up to the maximum number of permitted times), without causing the
1814 rest of the pattern to fail. The classic example of where this gives problems
1815 is in trying to match comments in C programs. These appear between /* and */
1816 and within the comment, individual * and / characters may appear. An attempt to
1817 match C comments by applying the pattern
1818 <pre>
1819 /\*.*\*/
1820 </pre>
1821 to the string
1822 <pre>
1823 /* first comment */ not comment /* second comment */
1824 </pre>
1825 fails, because it matches the entire string owing to the greediness of the .*
1826 item.
1827 </P>
1828 <P>
1829 However, if a quantifier is followed by a question mark, it ceases to be
1830 greedy, and instead matches the minimum number of times possible, so the
1831 pattern
1832 <pre>
1833 /\*.*?\*/
1834 </pre>
1835 does the right thing with the C comments. The meaning of the various
1836 quantifiers is not otherwise changed, just the preferred number of matches.
1837 Do not confuse this use of question mark with its use as a quantifier in its
1838 own right. Because it has two uses, it can sometimes appear doubled, as in
1839 <pre>
1840 \d??\d
1841 </pre>
1842 which matches one digit by preference, but can match two if that is the only
1843 way the rest of the pattern matches.
1844 </P>
1845 <P>
1846 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1847 the quantifiers are not greedy by default, but individual ones can be made
1848 greedy by following them with a question mark. In other words, it inverts the
1849 default behaviour.
1850 </P>
1851 <P>
1852 When a parenthesized subpattern is quantified with a minimum repeat count that
1853 is greater than 1 or with a limited maximum, more memory is required for the
1854 compiled pattern, in proportion to the size of the minimum or maximum.
1855 </P>
1856 <P>
1857 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1858 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1859 implicitly anchored, because whatever follows will be tried against every
1860 character position in the subject string, so there is no point in retrying the
1861 overall match at any position after the first. PCRE normally treats such a
1862 pattern as though it were preceded by \A.
1863 </P>
1864 <P>
1865 In cases where it is known that the subject string contains no newlines, it is
1866 worth setting PCRE_DOTALL in order to obtain this optimization, or
1867 alternatively using ^ to indicate anchoring explicitly.
1868 </P>
1869 <P>
1870 However, there are some cases where the optimization cannot be used. When .*
1871 is inside capturing parentheses that are the subject of a back reference
1872 elsewhere in the pattern, a match at the start may fail where a later one
1873 succeeds. Consider, for example:
1874 <pre>
1875 (.*)abc\1
1876 </pre>
1877 If the subject is "xyz123abc123" the match point is the fourth character. For
1878 this reason, such a pattern is not implicitly anchored.
1879 </P>
1880 <P>
1881 Another case where implicit anchoring is not applied is when the leading .* is
1882 inside an atomic group. Once again, a match at the start may fail where a later
1883 one succeeds. Consider this pattern:
1884 <pre>
1885 (?&#62;.*?a)b
1886 </pre>
1887 It matches "ab" in the subject "aab". The use of the backtracking control verbs
1888 (*PRUNE) and (*SKIP) also disable this optimization.
1889 </P>
1890 <P>
1891 When a capturing subpattern is repeated, the value captured is the substring
1892 that matched the final iteration. For example, after
1893 <pre>
1894 (tweedle[dume]{3}\s*)+
1895 </pre>
1896 has matched "tweedledum tweedledee" the value of the captured substring is
1897 "tweedledee". However, if there are nested capturing subpatterns, the
1898 corresponding captured values may have been set in previous iterations. For
1899 example, after
1900 <pre>
1901 /(a|(b))+/
1902 </pre>
1903 matches "aba" the value of the second captured substring is "b".
1904 <a name="atomicgroup"></a></P>
1905 <br><a name="SEC18" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1906 <P>
1907 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1908 repetition, failure of what follows normally causes the repeated item to be
1909 re-evaluated to see if a different number of repeats allows the rest of the
1910 pattern to match. Sometimes it is useful to prevent this, either to change the
1911 nature of the match, or to cause it fail earlier than it otherwise might, when
1912 the author of the pattern knows there is no point in carrying on.
1913 </P>
1914 <P>
1915 Consider, for example, the pattern \d+foo when applied to the subject line
1916 <pre>
1917 123456bar
1918 </pre>
1919 After matching all 6 digits and then failing to match "foo", the normal
1920 action of the matcher is to try again with only 5 digits matching the \d+
1921 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1922 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1923 that once a subpattern has matched, it is not to be re-evaluated in this way.
1924 </P>
1925 <P>
1926 If we use atomic grouping for the previous example, the matcher gives up
1927 immediately on failing to match "foo" the first time. The notation is a kind of
1928 special parenthesis, starting with (?&#62; as in this example:
1929 <pre>
1930 (?&#62;\d+)foo
1931 </pre>
1932 This kind of parenthesis "locks up" the part of the pattern it contains once
1933 it has matched, and a failure further into the pattern is prevented from
1934 backtracking into it. Backtracking past it to previous items, however, works as
1935 normal.
1936 </P>
1937 <P>
1938 An alternative description is that a subpattern of this type matches the string
1939 of characters that an identical standalone pattern would match, if anchored at
1940 the current point in the subject string.
1941 </P>
1942 <P>
1943 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1944 the above example can be thought of as a maximizing repeat that must swallow
1945 everything it can. So, while both \d+ and \d+? are prepared to adjust the
1946 number of digits they match in order to make the rest of the pattern match,
1947 (?&#62;\d+) can only match an entire sequence of digits.
1948 </P>
1949 <P>
1950 Atomic groups in general can of course contain arbitrarily complicated
1951 subpatterns, and can be nested. However, when the subpattern for an atomic
1952 group is just a single repeated item, as in the example above, a simpler
1953 notation, called a "possessive quantifier" can be used. This consists of an
1954 additional + character following a quantifier. Using this notation, the
1955 previous example can be rewritten as
1956 <pre>
1957 \d++foo
1958 </pre>
1959 Note that a possessive quantifier can be used with an entire group, for
1960 example:
1961 <pre>
1962 (abc|xyz){2,3}+
1963 </pre>
1964 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1965 option is ignored. They are a convenient notation for the simpler forms of
1966 atomic group. However, there is no difference in the meaning of a possessive
1967 quantifier and the equivalent atomic group, though there may be a performance
1968 difference; possessive quantifiers should be slightly faster.
1969 </P>
1970 <P>
1971 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1972 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1973 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1974 package, and PCRE copied it from there. It ultimately found its way into Perl
1975 at release 5.10.
1976 </P>
1977 <P>
1978 PCRE has an optimization that automatically "possessifies" certain simple
1979 pattern constructs. For example, the sequence A+B is treated as A++B because
1980 there is no point in backtracking into a sequence of A's when B must follow.
1981 </P>
1982 <P>
1983 When a pattern contains an unlimited repeat inside a subpattern that can itself
1984 be repeated an unlimited number of times, the use of an atomic group is the
1985 only way to avoid some failing matches taking a very long time indeed. The
1986 pattern
1987 <pre>
1988 (\D+|&#60;\d+&#62;)*[!?]
1989 </pre>
1990 matches an unlimited number of substrings that either consist of non-digits, or
1991 digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1992 quickly. However, if it is applied to
1993 <pre>
1994 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1995 </pre>
1996 it takes a long time before reporting failure. This is because the string can
1997 be divided between the internal \D+ repeat and the external * repeat in a
1998 large number of ways, and all have to be tried. (The example uses [!?] rather
1999 than a single character at the end, because both PCRE and Perl have an
2000 optimization that allows for fast failure when a single character is used. They
2001 remember the last single character that is required for a match, and fail early
2002 if it is not present in the string.) If the pattern is changed so that it uses
2003 an atomic group, like this:
2004 <pre>
2005 ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
2006 </pre>
2007 sequences of non-digits cannot be broken, and failure happens quickly.
2008 <a name="backreferences"></a></P>
2009 <br><a name="SEC19" href="#TOC1">BACK REFERENCES</a><br>
2010 <P>
2011 Outside a character class, a backslash followed by a digit greater than 0 (and
2012 possibly further digits) is a back reference to a capturing subpattern earlier
2013 (that is, to its left) in the pattern, provided there have been that many
2014 previous capturing left parentheses.
2015 </P>
2016 <P>
2017 However, if the decimal number following the backslash is less than 10, it is
2018 always taken as a back reference, and causes an error only if there are not
2019 that many capturing left parentheses in the entire pattern. In other words, the
2020 parentheses that are referenced need not be to the left of the reference for
2021 numbers less than 10. A "forward back reference" of this type can make sense
2022 when a repetition is involved and the subpattern to the right has participated
2023 in an earlier iteration.
2024 </P>
2025 <P>
2026 It is not possible to have a numerical "forward back reference" to a subpattern
2027 whose number is 10 or more using this syntax because a sequence such as \50 is
2028 interpreted as a character defined in octal. See the subsection entitled
2029 "Non-printing characters"
2030 <a href="#digitsafterbackslash">above</a>
2031 for further details of the handling of digits following a backslash. There is
2032 no such problem when named parentheses are used. A back reference to any
2033 subpattern is possible using named parentheses (see below).
2034 </P>
2035 <P>
2036 Another way of avoiding the ambiguity inherent in the use of digits following a
2037 backslash is to use the \g escape sequence. This escape must be followed by an
2038 unsigned number or a negative number, optionally enclosed in braces. These
2039 examples are all identical:
2040 <pre>
2041 (ring), \1
2042 (ring), \g1
2043 (ring), \g{1}
2044 </pre>
2045 An unsigned number specifies an absolute reference without the ambiguity that
2046 is present in the older syntax. It is also useful when literal digits follow
2047 the reference. A negative number is a relative reference. Consider this
2048 example:
2049 <pre>
2050 (abc(def)ghi)\g{-1}
2051 </pre>
2052 The sequence \g{-1} is a reference to the most recently started capturing
2053 subpattern before \g, that is, is it equivalent to \2 in this example.
2054 Similarly, \g{-2} would be equivalent to \1. The use of relative references
2055 can be helpful in long patterns, and also in patterns that are created by
2056 joining together fragments that contain references within themselves.
2057 </P>
2058 <P>
2059 A back reference matches whatever actually matched the capturing subpattern in
2060 the current subject string, rather than anything matching the subpattern
2061 itself (see
2062 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
2063 below for a way of doing that). So the pattern
2064 <pre>
2065 (sens|respons)e and \1ibility
2066 </pre>
2067 matches "sense and sensibility" and "response and responsibility", but not
2068 "sense and responsibility". If caseful matching is in force at the time of the
2069 back reference, the case of letters is relevant. For example,
2070 <pre>
2071 ((?i)rah)\s+\1
2072 </pre>
2073 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
2074 capturing subpattern is matched caselessly.
2075 </P>
2076 <P>
2077 There are several different ways of writing back references to named
2078 subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
2079 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
2080 back reference syntax, in which \g can be used for both numeric and named
2081 references, is also supported. We could rewrite the above example in any of
2082 the following ways:
2083 <pre>
2084 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
2085 (?'p1'(?i)rah)\s+\k{p1}
2086 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
2087 (?&#60;p1&#62;(?i)rah)\s+\g{p1}
2088 </pre>
2089 A subpattern that is referenced by name may appear in the pattern before or
2090 after the reference.
2091 </P>
2092 <P>
2093 There may be more than one back reference to the same subpattern. If a
2094 subpattern has not actually been used in a particular match, any back
2095 references to it always fail by default. For example, the pattern
2096 <pre>
2097 (a|(bc))\2
2098 </pre>
2099 always fails if it starts to match "a" rather than "bc". However, if the
2100 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
2101 unset value matches an empty string.
2102 </P>
2103 <P>
2104 Because there may be many capturing parentheses in a pattern, all digits
2105 following a backslash are taken as part of a potential back reference number.
2106 If the pattern continues with a digit character, some delimiter must be used to
2107 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
2108 white space. Otherwise, the \g{ syntax or an empty comment (see
2109 <a href="#comments">"Comments"</a>
2110 below) can be used.
2111 </P>
2112 <br><b>
2113 Recursive back references
2114 </b><br>
2115 <P>
2116 A back reference that occurs inside the parentheses to which it refers fails
2117 when the subpattern is first used, so, for example, (a\1) never matches.
2118 However, such references can be useful inside repeated subpatterns. For
2119 example, the pattern
2120 <pre>
2121 (a|b\1)+
2122 </pre>
2123 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
2124 the subpattern, the back reference matches the character string corresponding
2125 to the previous iteration. In order for this to work, the pattern must be such
2126 that the first iteration does not need to match the back reference. This can be
2127 done using alternation, as in the example above, or by a quantifier with a
2128 minimum of zero.
2129 </P>
2130 <P>
2131 Back references of this type cause the group that they reference to be treated
2132 as an
2133 <a href="#atomicgroup">atomic group.</a>
2134 Once the whole group has been matched, a subsequent matching failure cannot
2135 cause backtracking into the middle of the group.
2136 <a name="bigassertions"></a></P>
2137 <br><a name="SEC20" href="#TOC1">ASSERTIONS</a><br>
2138 <P>
2139 An assertion is a test on the characters following or preceding the current
2140 matching point that does not actually consume any characters. The simple
2141 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
2142 <a href="#smallassertions">above.</a>
2143 </P>
2144 <P>
2145 More complicated assertions are coded as subpatterns. There are two kinds:
2146 those that look ahead of the current position in the subject string, and those
2147 that look behind it. An assertion subpattern is matched in the normal way,
2148 except that it does not cause the current matching position to be changed.
2149 </P>
2150 <P>
2151 Assertion subpatterns are not capturing subpatterns. If such an assertion
2152 contains capturing subpatterns within it, these are counted for the purposes of
2153 numbering the capturing subpatterns in the whole pattern. However, substring
2154 capturing is carried out only for positive assertions. (Perl sometimes, but not
2155 always, does do capturing in negative assertions.)
2156 </P>
2157 <P>
2158 WARNING: If a positive assertion containing one or more capturing subpatterns
2159 succeeds, but failure to match later in the pattern causes backtracking over
2160 this assertion, the captures within the assertion are reset only if no higher
2161 numbered captures are already set. This is, unfortunately, a fundamental
2162 limitation of the current implementation, and as PCRE1 is now in
2163 maintenance-only status, it is unlikely ever to change.
2164 </P>
2165 <P>
2166 For compatibility with Perl, assertion subpatterns may be repeated; though
2167 it makes no sense to assert the same thing several times, the side effect of
2168 capturing parentheses may occasionally be useful. In practice, there only three
2169 cases:
2170 <br>
2171 <br>
2172 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2173 However, it may contain internal capturing parenthesized groups that are called
2174 from elsewhere via the
2175 <a href="#subpatternsassubroutines">subroutine mechanism.</a>
2176 <br>
2177 <br>
2178 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2179 were {0,1}. At run time, the rest of the pattern match is tried with and
2180 without the assertion, the order depending on the greediness of the quantifier.
2181 <br>
2182 <br>
2183 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2184 The assertion is obeyed just once when encountered during matching.
2185 </P>
2186 <br><b>
2187 Lookahead assertions
2188 </b><br>
2189 <P>
2190 Lookahead assertions start with (?= for positive assertions and (?! for
2191 negative assertions. For example,
2192 <pre>
2193 \w+(?=;)
2194 </pre>
2195 matches a word followed by a semicolon, but does not include the semicolon in
2196 the match, and
2197 <pre>
2198 foo(?!bar)
2199 </pre>
2200 matches any occurrence of "foo" that is not followed by "bar". Note that the
2201 apparently similar pattern
2202 <pre>
2203 (?!foo)bar
2204 </pre>
2205 does not find an occurrence of "bar" that is preceded by something other than
2206 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
2207 (?!foo) is always true when the next three characters are "bar". A
2208 lookbehind assertion is needed to achieve the other effect.
2209 </P>
2210 <P>
2211 If you want to force a matching failure at some point in a pattern, the most
2212 convenient way to do it is with (?!) because an empty string always matches, so
2213 an assertion that requires there not to be an empty string must always fail.
2214 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2215 <a name="lookbehind"></a></P>
2216 <br><b>
2217 Lookbehind assertions
2218 </b><br>
2219 <P>
2220 Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
2221 negative assertions. For example,
2222 <pre>
2223 (?&#60;!foo)bar
2224 </pre>
2225 does find an occurrence of "bar" that is not preceded by "foo". The contents of
2226 a lookbehind assertion are restricted such that all the strings it matches must
2227 have a fixed length. However, if there are several top-level alternatives, they
2228 do not all have to have the same fixed length. Thus
2229 <pre>
2230 (?&#60;=bullock|donkey)
2231 </pre>
2232 is permitted, but
2233 <pre>
2234 (?&#60;!dogs?|cats?)
2235 </pre>
2236 causes an error at compile time. Branches that match different length strings
2237 are permitted only at the top level of a lookbehind assertion. This is an
2238 extension compared with Perl, which requires all branches to match the same
2239 length of string. An assertion such as
2240 <pre>
2241 (?&#60;=ab(c|de))
2242 </pre>
2243 is not permitted, because its single top-level branch can match two different
2244 lengths, but it is acceptable to PCRE if rewritten to use two top-level
2245 branches:
2246 <pre>
2247 (?&#60;=abc|abde)
2248 </pre>
2249 In some cases, the escape sequence \K
2250 <a href="#resetmatchstart">(see above)</a>
2251 can be used instead of a lookbehind assertion to get round the fixed-length
2252 restriction.
2253 </P>
2254 <P>
2255 The implementation of lookbehind assertions is, for each alternative, to
2256 temporarily move the current position back by the fixed length and then try to
2257 match. If there are insufficient characters before the current position, the
2258 assertion fails.
2259 </P>
2260 <P>
2261 In a UTF mode, PCRE does not allow the \C escape (which matches a single data
2262 unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2263 it impossible to calculate the length of the lookbehind. The \X and \R
2264 escapes, which can match different numbers of data units, are also not
2265 permitted.
2266 </P>
2267 <P>
2268 <a href="#subpatternsassubroutines">"Subroutine"</a>
2269 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2270 as the subpattern matches a fixed-length string.
2271 <a href="#recursion">Recursion,</a>
2272 however, is not supported.
2273 </P>
2274 <P>
2275 Possessive quantifiers can be used in conjunction with lookbehind assertions to
2276 specify efficient matching of fixed-length strings at the end of subject
2277 strings. Consider a simple pattern such as
2278 <pre>
2279 abcd$
2280 </pre>
2281 when applied to a long string that does not match. Because matching proceeds
2282 from left to right, PCRE will look for each "a" in the subject and then see if
2283 what follows matches the rest of the pattern. If the pattern is specified as
2284 <pre>
2285 ^.*abcd$
2286 </pre>
2287 the initial .* matches the entire string at first, but when this fails (because
2288 there is no following "a"), it backtracks to match all but the last character,
2289 then all but the last two characters, and so on. Once again the search for "a"
2290 covers the entire string, from right to left, so we are no better off. However,
2291 if the pattern is written as
2292 <pre>
2293 ^.*+(?&#60;=abcd)
2294 </pre>
2295 there can be no backtracking for the .*+ item; it can match only the entire
2296 string. The subsequent lookbehind assertion does a single test on the last four
2297 characters. If it fails, the match fails immediately. For long strings, this
2298 approach makes a significant difference to the processing time.
2299 </P>
2300 <br><b>
2301 Using multiple assertions
2302 </b><br>
2303 <P>
2304 Several assertions (of any sort) may occur in succession. For example,
2305 <pre>
2306 (?&#60;=\d{3})(?&#60;!999)foo
2307 </pre>
2308 matches "foo" preceded by three digits that are not "999". Notice that each of
2309 the assertions is applied independently at the same point in the subject
2310 string. First there is a check that the previous three characters are all
2311 digits, and then there is a check that the same three characters are not "999".
2312 This pattern does <i>not</i> match "foo" preceded by six characters, the first
2313 of which are digits and the last three of which are not "999". For example, it
2314 doesn't match "123abcfoo". A pattern to do that is
2315 <pre>
2316 (?&#60;=\d{3}...)(?&#60;!999)foo
2317 </pre>
2318 This time the first assertion looks at the preceding six characters, checking
2319 that the first three are digits, and then the second assertion checks that the
2320 preceding three characters are not "999".
2321 </P>
2322 <P>
2323 Assertions can be nested in any combination. For example,
2324 <pre>
2325 (?&#60;=(?&#60;!foo)bar)baz
2326 </pre>
2327 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2328 preceded by "foo", while
2329 <pre>
2330 (?&#60;=\d{3}(?!999)...)foo
2331 </pre>
2332 is another pattern that matches "foo" preceded by three digits and any three
2333 characters that are not "999".
2334 <a name="conditions"></a></P>
2335 <br><a name="SEC21" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
2336 <P>
2337 It is possible to cause the matching process to obey a subpattern
2338 conditionally or to choose between two alternative subpatterns, depending on
2339 the result of an assertion, or whether a specific capturing subpattern has
2340 already been matched. The two possible forms of conditional subpattern are:
2341 <pre>
2342 (?(condition)yes-pattern)
2343 (?(condition)yes-pattern|no-pattern)
2344 </pre>
2345 If the condition is satisfied, the yes-pattern is used; otherwise the
2346 no-pattern (if present) is used. If there are more than two alternatives in the
2347 subpattern, a compile-time error occurs. Each of the two alternatives may
2348 itself contain nested subpatterns of any form, including conditional
2349 subpatterns; the restriction to two alternatives applies only at the level of
2350 the condition. This pattern fragment is an example where the alternatives are
2351 complex:
2352 <pre>
2353 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2355 </PRE>
2356 </P>
2357 <P>
2358 There are four kinds of condition: references to subpatterns, references to
2359 recursion, a pseudo-condition called DEFINE, and assertions.
2360 </P>
2361 <br><b>
2362 Checking for a used subpattern by number
2363 </b><br>
2364 <P>
2365 If the text between the parentheses consists of a sequence of digits, the
2366 condition is true if a capturing subpattern of that number has previously
2367 matched. If there is more than one capturing subpattern with the same number
2368 (see the earlier
2369 <a href="#recursion">section about duplicate subpattern numbers),</a>
2370 the condition is true if any of them have matched. An alternative notation is
2371 to precede the digits with a plus or minus sign. In this case, the subpattern
2372 number is relative rather than absolute. The most recently opened parentheses
2373 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2374 loops it can also make sense to refer to subsequent groups. The next
2375 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2376 zero in any of these forms is not used; it provokes a compile-time error.)
2377 </P>
2378 <P>
2379 Consider the following pattern, which contains non-significant white space to
2380 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2381 three parts for ease of discussion:
2382 <pre>
2383 ( \( )? [^()]+ (?(1) \) )
2384 </pre>
2385 The first part matches an optional opening parenthesis, and if that
2386 character is present, sets it as the first captured substring. The second part
2387 matches one or more characters that are not parentheses. The third part is a
2388 conditional subpattern that tests whether or not the first set of parentheses
2389 matched. If they did, that is, if subject started with an opening parenthesis,
2390 the condition is true, and so the yes-pattern is executed and a closing
2391 parenthesis is required. Otherwise, since no-pattern is not present, the
2392 subpattern matches nothing. In other words, this pattern matches a sequence of
2393 non-parentheses, optionally enclosed in parentheses.
2394 </P>
2395 <P>
2396 If you were embedding this pattern in a larger one, you could use a relative
2397 reference:
2398 <pre>
2399 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
2400 </pre>
2401 This makes the fragment independent of the parentheses in the larger pattern.
2402 </P>
2403 <br><b>
2404 Checking for a used subpattern by name
2405 </b><br>
2406 <P>
2407 Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
2408 subpattern by name. For compatibility with earlier versions of PCRE, which had
2409 this facility before Perl, the syntax (?(name)...) is also recognized.
2410 </P>
2411 <P>
2412 Rewriting the above example to use a named subpattern gives this:
2413 <pre>
2414 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
2415 </pre>
2416 If the name used in a condition of this kind is a duplicate, the test is
2417 applied to all subpatterns of the same name, and is true if any one of them has
2418 matched.
2419 </P>
2420 <br><b>
2421 Checking for pattern recursion
2422 </b><br>
2423 <P>
2424 If the condition is the string (R), and there is no subpattern with the name R,
2425 the condition is true if a recursive call to the whole pattern or any
2426 subpattern has been made. If digits or a name preceded by ampersand follow the
2427 letter R, for example:
2428 <pre>
2429 (?(R3)...) or (?(R&name)...)
2430 </pre>
2431 the condition is true if the most recent recursion is into a subpattern whose
2432 number or name is given. This condition does not check the entire recursion
2433 stack. If the name used in a condition of this kind is a duplicate, the test is
2434 applied to all subpatterns of the same name, and is true if any one of them is
2435 the most recent recursion.
2436 </P>
2437 <P>
2438 At "top level", all these recursion test conditions are false.
2439 <a href="#recursion">The syntax for recursive patterns</a>
2440 is described below.
2441 <a name="subdefine"></a></P>
2442 <br><b>
2443 Defining subpatterns for use by reference only
2444 </b><br>
2445 <P>
2446 If the condition is the string (DEFINE), and there is no subpattern with the
2447 name DEFINE, the condition is always false. In this case, there may be only one
2448 alternative in the subpattern. It is always skipped if control reaches this
2449 point in the pattern; the idea of DEFINE is that it can be used to define
2450 subroutines that can be referenced from elsewhere. (The use of
2451 <a href="#subpatternsassubroutines">subroutines</a>
2452 is described below.) For example, a pattern to match an IPv4 address such as
2453 "" could be written like this (ignore white space and line
2454 breaks):
2455 <pre>
2456 (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2457 \b (?&byte) (\.(?&byte)){3} \b
2458 </pre>
2459 The first part of the pattern is a DEFINE group inside which a another group
2460 named "byte" is defined. This matches an individual component of an IPv4
2461 address (a number less than 256). When matching takes place, this part of the
2462 pattern is skipped because DEFINE acts like a false condition. The rest of the
2463 pattern uses references to the named group to match the four dot-separated
2464 components of an IPv4 address, insisting on a word boundary at each end.
2465 </P>
2466 <br><b>
2467 Assertion conditions
2468 </b><br>
2469 <P>
2470 If the condition is not in any of the above formats, it must be an assertion.
2471 This may be a positive or negative lookahead or lookbehind assertion. Consider
2472 this pattern, again containing non-significant white space, and with the two
2473 alternatives on the second line:
2474 <pre>
2475 (?(?=[^a-z]*[a-z])
2476 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2477 </pre>
2478 The condition is a positive lookahead assertion that matches an optional
2479 sequence of non-letters followed by a letter. In other words, it tests for the
2480 presence of at least one letter in the subject. If a letter is found, the
2481 subject is matched against the first alternative; otherwise it is matched
2482 against the second. This pattern matches strings in one of the two forms
2483 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2484 <a name="comments"></a></P>
2485 <br><a name="SEC22" href="#TOC1">COMMENTS</a><br>
2486 <P>
2487 There are two ways of including comments in patterns that are processed by
2488 PCRE. In both cases, the start of the comment must not be in a character class,
2489 nor in the middle of any other sequence of related characters such as (?: or a
2490 subpattern name or number. The characters that make up a comment play no part
2491 in the pattern matching.
2492 </P>
2493 <P>
2494 The sequence (?# marks the start of a comment that continues up to the next
2495 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2496 option is set, an unescaped # character also introduces a comment, which in
2497 this case continues to immediately after the next newline character or
2498 character sequence in the pattern. Which characters are interpreted as newlines
2499 is controlled by the options passed to a compiling function or by a special
2500 sequence at the start of the pattern, as described in the section entitled
2501 <a href="#newlines">"Newline conventions"</a>
2502 above. Note that the end of this type of comment is a literal newline sequence
2503 in the pattern; escape sequences that happen to represent a newline do not
2504 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2505 default newline convention is in force:
2506 <pre>
2507 abc #comment \n still comment
2508 </pre>
2509 On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2510 a newline in the pattern. The sequence \n is still literal at this stage, so
2511 it does not terminate the comment. Only an actual character with the code value
2512 0x0a (the default newline) does so.
2513 <a name="recursion"></a></P>
2514 <br><a name="SEC23" href="#TOC1">RECURSIVE PATTERNS</a><br>
2515 <P>
2516 Consider the problem of matching a string in parentheses, allowing for
2517 unlimited nested parentheses. Without the use of recursion, the best that can
2518 be done is to use a pattern that matches up to some fixed depth of nesting. It
2519 is not possible to handle an arbitrary nesting depth.
2520 </P>
2521 <P>
2522 For some time, Perl has provided a facility that allows regular expressions to
2523 recurse (amongst other things). It does this by interpolating Perl code in the
2524 expression at run time, and the code can refer to the expression itself. A Perl
2525 pattern using code interpolation to solve the parentheses problem can be
2526 created like this:
2527 <pre>
2528 $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
2529 </pre>
2530 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2531 recursively to the pattern in which it appears.
2532 </P>
2533 <P>
2534 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2535 supports special syntax for recursion of the entire pattern, and also for
2536 individual subpattern recursion. After its introduction in PCRE and Python,
2537 this kind of recursion was subsequently introduced into Perl at release 5.10.
2538 </P>
2539 <P>
2540 A special item that consists of (? followed by a number greater than zero and a
2541 closing parenthesis is a recursive subroutine call of the subpattern of the
2542 given number, provided that it occurs inside that subpattern. (If not, it is a
2543 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2544 call, which is described in the next section.) The special item (?R) or (?0) is
2545 a recursive call of the entire regular expression.
2546 </P>
2547 <P>
2548 This PCRE pattern solves the nested parentheses problem (assume the
2549 PCRE_EXTENDED option is set so that white space is ignored):
2550 <pre>
2551 \( ( [^()]++ | (?R) )* \)
2552 </pre>
2553 First it matches an opening parenthesis. Then it matches any number of
2554 substrings which can either be a sequence of non-parentheses, or a recursive
2555 match of the pattern itself (that is, a correctly parenthesized substring).
2556 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2557 to avoid backtracking into sequences of non-parentheses.
2558 </P>
2559 <P>
2560 If this were part of a larger pattern, you would not want to recurse the entire
2561 pattern, so instead you could use this:
2562 <pre>
2563 ( \( ( [^()]++ | (?1) )* \) )
2564 </pre>
2565 We have put the pattern into parentheses, and caused the recursion to refer to
2566 them instead of the whole pattern.
2567 </P>
2568 <P>
2569 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2570 is made easier by the use of relative references. Instead of (?1) in the
2571 pattern above you can write (?-2) to refer to the second most recently opened
2572 parentheses preceding the recursion. In other words, a negative number counts
2573 capturing parentheses leftwards from the point at which it is encountered.
2574 </P>
2575 <P>
2576 It is also possible to refer to subsequently opened parentheses, by writing
2577 references such as (?+2). However, these cannot be recursive because the
2578 reference is not inside the parentheses that are referenced. They are always
2579 <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2580 calls, as described in the next section.
2581 </P>
2582 <P>
2583 An alternative approach is to use named parentheses instead. The Perl syntax
2584 for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
2585 could rewrite the above example as follows:
2586 <pre>
2587 (?&#60;pn&#62; \( ( [^()]++ | (?&pn) )* \) )
2588 </pre>
2589 If there is more than one subpattern with the same name, the earliest one is
2590 used.
2591 </P>
2592 <P>
2593 This particular example pattern that we have been looking at contains nested
2594 unlimited repeats, and so the use of a possessive quantifier for matching
2595 strings of non-parentheses is important when applying the pattern to strings
2596 that do not match. For example, when this pattern is applied to
2597 <pre>
2598 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2599 </pre>
2600 it yields "no match" quickly. However, if a possessive quantifier is not used,
2601 the match runs for a very long time indeed because there are so many different
2602 ways the + and * repeats can carve up the subject, and all have to be tested
2603 before failure can be reported.
2604 </P>
2605 <P>
2606 At the end of a match, the values of capturing parentheses are those from
2607 the outermost level. If you want to obtain intermediate values, a callout
2608 function can be used (see below and the
2609 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2610 documentation). If the pattern above is matched against
2611 <pre>
2612 (ab(cd)ef)
2613 </pre>
2614 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2615 the last value taken on at the top level. If a capturing subpattern is not
2616 matched at the top level, its final captured value is unset, even if it was
2617 (temporarily) set at a deeper level during the matching process.
2618 </P>
2619 <P>
2620 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2621 obtain extra memory to store data during a recursion, which it does by using
2622 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2623 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2624 </P>
2625 <P>
2626 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2627 Consider this pattern, which matches text in angle brackets, allowing for
2628 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2629 recursing), whereas any characters are permitted at the outer level.
2630 <pre>
2631 &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
2632 </pre>
2633 In this pattern, (?(R) is the start of a conditional subpattern, with two
2634 different alternatives for the recursive and non-recursive cases. The (?R) item
2635 is the actual recursive call.
2636 <a name="recursiondifference"></a></P>
2637 <br><b>
2638 Differences in recursion processing between PCRE and Perl
2639 </b><br>
2640 <P>
2641 Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2642 (like Python, but unlike Perl), a recursive subpattern call is always treated
2643 as an atomic group. That is, once it has matched some of the subject string, it
2644 is never re-entered, even if it contains untried alternatives and there is a
2645 subsequent matching failure. This can be illustrated by the following pattern,
2646 which purports to match a palindromic string that contains an odd number of
2647 characters (for example, "a", "aba", "abcba", "abcdcba"):
2648 <pre>
2649 ^(.|(.)(?1)\2)$
2650 </pre>
2651 The idea is that it either matches a single character, or two identical
2652 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2653 it does not if the pattern is longer than three characters. Consider the
2654 subject string "abcba":
2655 </P>
2656 <P>
2657 At the top level, the first character is matched, but as it is not at the end
2658 of the string, the first alternative fails; the second alternative is taken
2659 and the recursion kicks in. The recursive call to subpattern 1 successfully
2660 matches the next character ("b"). (Note that the beginning and end of line
2661 tests are not part of the recursion).
2662 </P>
2663 <P>
2664 Back at the top level, the next character ("c") is compared with what
2665 subpattern 2 matched, which was "a". This fails. Because the recursion is
2666 treated as an atomic group, there are now no backtracking points, and so the
2667 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2668 try the second alternative.) However, if the pattern is written with the
2669 alternatives in the other order, things are different:
2670 <pre>
2671 ^((.)(?1)\2|.)$
2672 </pre>
2673 This time, the recursing alternative is tried first, and continues to recurse
2674 until it runs out of characters, at which point the recursion fails. But this
2675 time we do have another alternative to try at the higher level. That is the big
2676 difference: in the previous case the remaining alternative is at a deeper
2677 recursion level, which PCRE cannot use.
2678 </P>
2679 <P>
2680 To change the pattern so that it matches all palindromic strings, not just
2681 those with an odd number of characters, it is tempting to change the pattern to
2682 this:
2683 <pre>
2684 ^((.)(?1)\2|.?)$
2685 </pre>
2686 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2687 deeper recursion has matched a single character, it cannot be entered again in
2688 order to match an empty string. The solution is to separate the two cases, and
2689 write out the odd and even cases as alternatives at the higher level:
2690 <pre>
2691 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2692 </pre>
2693 If you want to match typical palindromic phrases, the pattern has to ignore all
2694 non-word characters, which can be done like this:
2695 <pre>
2696 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2697 </pre>
2698 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2699 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2700 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2701 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2702 more) to match typical phrases, and Perl takes so long that you think it has
2703 gone into a loop.
2704 </P>
2705 <P>
2706 <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2707 string does not start with a palindrome that is shorter than the entire string.
2708 For example, although "abcba" is correctly matched, if the subject is "ababa",
2709 PCRE finds the palindrome "aba" at the start, then fails at top level because
2710 the end of the string does not follow. Once again, it cannot jump back into the
2711 recursion to try other alternatives, so the entire match fails.
2712 </P>
2713 <P>
2714 The second way in which PCRE and Perl differ in their recursion processing is
2715 in the handling of captured values. In Perl, when a subpattern is called
2716 recursively or as a subpattern (see the next section), it has no access to any
2717 values that were captured outside the recursion, whereas in PCRE these values
2718 can be referenced. Consider this pattern:
2719 <pre>
2720 ^(.)(\1|a(?2))
2721 </pre>
2722 In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2723 then in the second group, when the back reference \1 fails to match "b", the
2724 second alternative matches "a" and then recurses. In the recursion, \1 does
2725 now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2726 match because inside the recursive call \1 cannot access the externally set
2727 value.
2728 <a name="subpatternsassubroutines"></a></P>
2729 <br><a name="SEC24" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2730 <P>
2731 If the syntax for a recursive subpattern call (either by number or by
2732 name) is used outside the parentheses to which it refers, it operates like a
2733 subroutine in a programming language. The called subpattern may be defined
2734 before or after the reference. A numbered reference can be absolute or
2735 relative, as in these examples:
2736 <pre>
2737 (...(absolute)...)...(?2)...
2738 (...(relative)...)...(?-1)...
2739 (...(?+1)...(relative)...
2740 </pre>
2741 An earlier example pointed out that the pattern
2742 <pre>
2743 (sens|respons)e and \1ibility
2744 </pre>
2745 matches "sense and sensibility" and "response and responsibility", but not
2746 "sense and responsibility". If instead the pattern
2747 <pre>
2748 (sens|respons)e and (?1)ibility
2749 </pre>
2750 is used, it does match "sense and responsibility" as well as the other two
2751 strings. Another example is given in the discussion of DEFINE above.
2752 </P>
2753 <P>
2754 All subroutine calls, whether recursive or not, are always treated as atomic
2755 groups. That is, once a subroutine has matched some of the subject string, it
2756 is never re-entered, even if it contains untried alternatives and there is a
2757 subsequent matching failure. Any capturing parentheses that are set during the
2758 subroutine call revert to their previous values afterwards.
2759 </P>
2760 <P>
2761 Processing options such as case-independence are fixed when a subpattern is
2762 defined, so if it is used as a subroutine, such options cannot be changed for
2763 different calls. For example, consider this pattern:
2764 <pre>
2765 (abc)(?i:(?-1))
2766 </pre>
2767 It matches "abcabc". It does not match "abcABC" because the change of
2768 processing option does not affect the called subpattern.
2769 <a name="onigurumasubroutines"></a></P>
2770 <br><a name="SEC25" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2771 <P>
2772 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2773 a number enclosed either in angle brackets or single quotes, is an alternative
2774 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2775 are two of the examples used above, rewritten using this syntax:
2776 <pre>
2777 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
2778 (sens|respons)e and \g'1'ibility
2779 </pre>
2780 PCRE supports an extension to Oniguruma: if a number is preceded by a
2781 plus or a minus sign it is taken as a relative reference. For example:
2782 <pre>
2783 (abc)(?i:\g&#60;-1&#62;)
2784 </pre>
2785 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
2786 synonymous. The former is a back reference; the latter is a subroutine call.
2787 </P>
2788 <br><a name="SEC26" href="#TOC1">CALLOUTS</a><br>
2789 <P>
2790 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2791 code to be obeyed in the middle of matching a regular expression. This makes it
2792 possible, amongst other things, to extract different substrings that match the
2793 same pair of parentheses when there is a repetition.
2794 </P>
2795 <P>
2796 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2797 code. The feature is called "callout". The caller of PCRE provides an external
2798 function by putting its entry point in the global variable <i>pcre_callout</i>
2799 (8-bit library) or <i>pcre[16|32]_callout</i> (16-bit or 32-bit library).
2800 By default, this variable contains NULL, which disables all calling out.
2801 </P>
2802 <P>
2803 Within a regular expression, (?C) indicates the points at which the external
2804 function is to be called. If you want to identify different callout points, you
2805 can put a number less than 256 after the letter C. The default value is zero.
2806 For example, this pattern has two callout points:
2807 <pre>
2808 (?C1)abc(?C2)def
2809 </pre>
2810 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2811 automatically installed before each item in the pattern. They are all numbered
2812 255. If there is a conditional group in the pattern whose condition is an
2813 assertion, an additional callout is inserted just before the condition. An
2814 explicit callout may also be set at this position, as in this example:
2815 <pre>
2816 (?(?C9)(?=a)abc|def)
2817 </pre>
2818 Note that this applies only to assertion conditions, not to other types of
2819 condition.
2820 </P>
2821 <P>
2822 During matching, when PCRE reaches a callout point, the external function is
2823 called. It is provided with the number of the callout, the position in the
2824 pattern, and, optionally, one item of data originally supplied by the caller of
2825 the matching function. The callout function may cause matching to proceed, to
2826 backtrack, or to fail altogether.
2827 </P>
2828 <P>
2829 By default, PCRE implements a number of optimizations at compile time and
2830 matching time, and one side-effect is that sometimes callouts are skipped. If
2831 you need all possible callouts to happen, you need to set options that disable
2832 the relevant optimizations. More details, and a complete description of the
2833 interface to the callout function, are given in the
2834 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2835 documentation.
2836 <a name="backtrackcontrol"></a></P>
2837 <br><a name="SEC27" href="#TOC1">BACKTRACKING CONTROL</a><br>
2838 <P>
2839 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2840 are still described in the Perl documentation as "experimental and subject to
2841 change or removal in a future version of Perl". It goes on to say: "Their usage
2842 in production code should be noted to avoid problems during upgrades." The same
2843 remarks apply to the PCRE features described in this section.
2844 </P>
2845 <P>
2846 The new verbs make use of what was previously invalid syntax: an opening
2847 parenthesis followed by an asterisk. They are generally of the form
2848 (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2849 differently depending on whether or not a name is present. A name is any
2850 sequence of characters that does not include a closing parenthesis. The maximum
2851 length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2852 libraries. If the name is empty, that is, if the closing parenthesis
2853 immediately follows the colon, the effect is as if the colon were not there.
2854 Any number of these verbs may occur in a pattern.
2855 </P>
2856 <P>
2857 Since these verbs are specifically related to backtracking, most of them can be
2858 used only when the pattern is to be matched using one of the traditional
2859 matching functions, because these use a backtracking algorithm. With the
2860 exception of (*FAIL), which behaves like a failing negative assertion, the
2861 backtracking control verbs cause an error if encountered by a DFA matching
2862 function.
2863 </P>
2864 <P>
2865 The behaviour of these verbs in
2866 <a href="#btrepeat">repeated groups,</a>
2867 <a href="#btassert">assertions,</a>
2868 and in
2869 <a href="#btsub">subpatterns called as subroutines</a>
2870 (whether or not recursively) is documented below.
2871 <a name="nooptimize"></a></P>
2872 <br><b>
2873 Optimizations that affect backtracking verbs
2874 </b><br>
2875 <P>
2876 PCRE contains some optimizations that are used to speed up matching by running
2877 some checks at the start of each match attempt. For example, it may know the
2878 minimum length of matching subject, or that a particular character must be
2879 present. When one of these optimizations bypasses the running of a match, any
2880 included backtracking verbs will not, of course, be processed. You can suppress
2881 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2882 when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
2883 pattern with (*NO_START_OPT). There is more discussion of this option in the
2884 section entitled
2885 <a href="pcreapi.html#execoptions">"Option bits for <b>pcre_exec()</b>"</a>
2886 in the
2887 <a href="pcreapi.html"><b>pcreapi</b></a>
2888 documentation.
2889 </P>
2890 <P>
2891 Experiments with Perl suggest that it too has similar optimizations, sometimes
2892 leading to anomalous results.
2893 </P>
2894 <br><b>
2895 Verbs that act immediately
2896 </b><br>
2897 <P>
2898 The following verbs act as soon as they are encountered. They may not be
2899 followed by a name.
2900 <pre>
2901 (*ACCEPT)
2902 </pre>
2903 This verb causes the match to end successfully, skipping the remainder of the
2904 pattern. However, when it is inside a subpattern that is called as a
2905 subroutine, only that subpattern is ended successfully. Matching then continues
2906 at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2907 assertion succeeds; in a negative assertion, the assertion fails.
2908 </P>
2909 <P>
2910 If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2911 example:
2912 <pre>
2913 A((?:A|B(*ACCEPT)|C)D)
2914 </pre>
2915 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2916 the outer parentheses.
2917 <pre>
2918 (*FAIL) or (*F)
2919 </pre>
2920 This verb causes a matching failure, forcing backtracking to occur. It is
2921 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2922 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2923 Perl features that are not present in PCRE. The nearest equivalent is the
2924 callout feature, as for example in this pattern:
2925 <pre>
2926 a+(?C)(*FAIL)
2927 </pre>
2928 A match with the string "aaaa" always fails, but the callout is taken before
2929 each backtrack happens (in this example, 10 times).
2930 </P>
2931 <br><b>
2932 Recording which path was taken
2933 </b><br>
2934 <P>
2935 There is one verb whose main purpose is to track how a match was arrived at,
2936 though it also has a secondary use in conjunction with advancing the match
2937 starting point (see (*SKIP) below).
2938 <pre>
2939 (*MARK:NAME) or (*:NAME)
2940 </pre>
2941 A name is always required with this verb. There may be as many instances of
2942 (*MARK) as you like in a pattern, and their names do not have to be unique.
2943 </P>
2944 <P>
2945 When a match succeeds, the name of the last-encountered (*MARK:NAME),
2946 (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2947 caller as described in the section entitled
2948 <a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a>
2949 in the
2950 <a href="pcreapi.html"><b>pcreapi</b></a>
2951 documentation. Here is an example of <b>pcretest</b> output, where the /K
2952 modifier requests the retrieval and outputting of (*MARK) data:
2953 <pre>
2954 re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
2955 data&#62; XY
2956 0: XY
2957 MK: A
2958 XZ
2959 0: XZ
2960 MK: B
2961 </pre>
2962 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2963 indicates which of the two alternatives matched. This is a more efficient way
2964 of obtaining this information than putting each alternative in its own
2965 capturing parentheses.
2966 </P>
2967 <P>
2968 If a verb with a name is encountered in a positive assertion that is true, the
2969 name is recorded and passed back if it is the last-encountered. This does not
2970 happen for negative assertions or failing positive assertions.
2971 </P>
2972 <P>
2973 After a partial match or a failed match, the last encountered name in the
2974 entire match process is returned. For example:
2975 <pre>
2976 re&#62; /X(*MARK:A)Y|X(*MARK:B)Z/K
2977 data&#62; XP
2978 No match, mark = B
2979 </pre>
2980 Note that in this unanchored example the mark is retained from the match
2981 attempt that started at the letter "X" in the subject. Subsequent match
2982 attempts starting at "P" and then with an empty string do not get as far as the
2983 (*MARK) item, but nevertheless do not reset it.
2984 </P>
2985 <P>
2986 If you are interested in (*MARK) values after failed matches, you should
2987 probably set the PCRE_NO_START_OPTIMIZE option
2988 <a href="#nooptimize">(see above)</a>
2989 to ensure that the match is always attempted.
2990 </P>
2991 <br><b>
2992 Verbs that act after backtracking
2993 </b><br>
2994 <P>
2995 The following verbs do nothing when they are encountered. Matching continues
2996 with what follows, but if there is no subsequent match, causing a backtrack to
2997 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2998 the verb. However, when one of these verbs appears inside an atomic group or an
2999 assertion that is true, its effect is confined to that group, because once the
3000 group has been matched, there is never any backtracking into it. In this
3001 situation, backtracking can "jump back" to the left of the entire atomic group
3002 or assertion. (Remember also, as stated above, that this localization also
3003 applies in subroutine calls.)
3004 </P>
3005 <P>
3006 These verbs differ in exactly what kind of failure occurs when backtracking
3007 reaches them. The behaviour described below is what happens when the verb is
3008 not in a subroutine or an assertion. Subsequent sections cover these special
3009 cases.
3010 <pre>
3011 (*COMMIT)
3012 </pre>
3013 This verb, which may not be followed by a name, causes the whole match to fail
3014 outright if there is a later matching failure that causes backtracking to reach
3015 it. Even if the pattern is unanchored, no further attempts to find a match by
3016 advancing the starting point take place. If (*COMMIT) is the only backtracking
3017 verb that is encountered, once it has been passed <b>pcre_exec()</b> is
3018 committed to finding a match at the current starting point, or not at all. For
3019 example:
3020 <pre>
3021 a+(*COMMIT)b
3022 </pre>
3023 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
3024 dynamic anchor, or "I've started, so I must finish." The name of the most
3025 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
3026 match failure.
3027 </P>
3028 <P>
3029 If there is more than one backtracking verb in a pattern, a different one that
3030 follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
3031 match does not always guarantee that a match must be at this starting point.
3032 </P>
3033 <P>
3034 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
3035 unless PCRE's start-of-match optimizations are turned off, as shown in this
3036 output from <b>pcretest</b>:
3037 <pre>
3038 re&#62; /(*COMMIT)abc/
3039 data&#62; xyzabc
3040 0: abc
3041 data&#62; xyzabc\Y
3042 No match
3043 </pre>
3044 For this pattern, PCRE knows that any match must start with "a", so the
3045 optimization skips along the subject to "a" before applying the pattern to the
3046 first set of data. The match attempt then succeeds. In the second set of data,
3047 the escape sequence \Y is interpreted by the <b>pcretest</b> program. It causes
3048 the PCRE_NO_START_OPTIMIZE option to be set when <b>pcre_exec()</b> is called.
3049 This disables the optimization that skips along to the first character. The
3050 pattern is now applied starting at "x", and so the (*COMMIT) causes the match
3051 to fail without trying any other starting points.
3052 <pre>
3053 (*PRUNE) or (*PRUNE:NAME)
3054 </pre>
3055 This verb causes the match to fail at the current starting position in the
3056 subject if there is a later matching failure that causes backtracking to reach
3057 it. If the pattern is unanchored, the normal "bumpalong" advance to the next
3058 starting character then happens. Backtracking can occur as usual to the left of
3059 (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
3060 if there is no match to the right, backtracking cannot cross (*PRUNE). In
3061 simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
3062 possessive quantifier, but there are some uses of (*PRUNE) that cannot be
3063 expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
3064 as (*COMMIT).
3065 </P>
3066 <P>
3067 The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3068 It is like (*MARK:NAME) in that the name is remembered for passing back to the
3069 caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3070 <pre>
3071 (*SKIP)
3072 </pre>
3073 This verb, when given without a name, is like (*PRUNE), except that if the
3074 pattern is unanchored, the "bumpalong" advance is not to the next character,
3075 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
3076 signifies that whatever text was matched leading up to it cannot be part of a
3077 successful match. Consider:
3078 <pre>
3079 a+(*SKIP)b
3080 </pre>
3081 If the subject is "aaaac...", after the first match attempt fails (starting at
3082 the first character in the string), the starting point skips on to start the
3083 next attempt at "c". Note that a possessive quantifier does not have the same
3084 effect as this example; although it would suppress backtracking during the
3085 first match attempt, the second attempt would start at the second character
3086 instead of skipping on to "c".
3087 <pre>
3088 (*SKIP:NAME)
3089 </pre>
3090 When (*SKIP) has an associated name, its behaviour is modified. When it is
3091 triggered, the previous path through the pattern is searched for the most
3092 recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
3093 is to the subject position that corresponds to that (*MARK) instead of to where
3094 (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
3095 (*SKIP) is ignored.
3096 </P>
3097 <P>
3098 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3099 names that are set by (*PRUNE:NAME) or (*THEN:NAME).
3100 <pre>
3101 (*THEN) or (*THEN:NAME)
3102 </pre>
3103 This verb causes a skip to the next innermost alternative when backtracking
3104 reaches it. That is, it cancels any further backtracking within the current
3105 alternative. Its name comes from the observation that it can be used for a
3106 pattern-based if-then-else block:
3107 <pre>
3108 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3109 </pre>
3110 If the COND1 pattern matches, FOO is tried (and possibly further items after
3111 the end of the group if FOO succeeds); on failure, the matcher skips to the
3112 second alternative and tries COND2, without backtracking into COND1. If that
3113 succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3114 more alternatives, so there is a backtrack to whatever came before the entire
3115 group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3116 </P>
3117 <P>
3118 The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3119 It is like (*MARK:NAME) in that the name is remembered for passing back to the
3120 caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3121 </P>
3122 <P>
3123 A subpattern that does not contain a | character is just a part of the
3124 enclosing alternative; it is not a nested alternation with only one
3125 alternative. The effect of (*THEN) extends beyond such a subpattern to the
3126 enclosing alternative. Consider this pattern, where A, B, etc. are complex
3127 pattern fragments that do not contain any | characters at this level:
3128 <pre>
3129 A (B(*THEN)C) | D
3130 </pre>
3131 If A and B are matched, but there is a failure in C, matching does not
3132 backtrack into A; instead it moves to the next alternative, that is, D.
3133 However, if the subpattern containing (*THEN) is given an alternative, it
3134 behaves differently:
3135 <pre>
3136 A (B(*THEN)C | (*FAIL)) | D
3137 </pre>
3138 The effect of (*THEN) is now confined to the inner subpattern. After a failure
3139 in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3140 because there are no more alternatives to try. In this case, matching does now
3141 backtrack into A.
3142 </P>
3143 <P>
3144 Note that a conditional subpattern is not considered as having two
3145 alternatives, because only one is ever used. In other words, the | character in
3146 a conditional subpattern has a different meaning. Ignoring white space,
3147 consider:
3148 <pre>
3149 ^.*? (?(?=a) a | b(*THEN)c )
3150 </pre>
3151 If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3152 it initially matches zero characters. The condition (?=a) then fails, the
3153 character "b" is matched, but "c" is not. At this point, matching does not
3154 backtrack to .*? as might perhaps be expected from the presence of the |
3155 character. The conditional subpattern is part of the single alternative that
3156 comprises the whole pattern, and so the match fails. (If there was a backtrack
3157 into .*?, allowing it to match "b", the match would succeed.)
3158 </P>
3159 <P>
3160 The verbs just described provide four different "strengths" of control when
3161 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3162 next alternative. (*PRUNE) comes next, failing the match at the current
3163 starting position, but allowing an advance to the next character (for an
3164 unanchored pattern). (*SKIP) is similar, except that the advance may be more
3165 than one character. (*COMMIT) is the strongest, causing the entire match to
3166 fail.
3167 </P>
3168 <br><b>
3169 More than one backtracking verb
3170 </b><br>
3171 <P>
3172 If more than one backtracking verb is present in a pattern, the one that is
3173 backtracked onto first acts. For example, consider this pattern, where A, B,
3174 etc. are complex pattern fragments:
3175 <pre>
3177 </pre>
3178 If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3179 fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3180 the next alternative (ABD) to be tried. This behaviour is consistent, but is
3181 not always the same as Perl's. It means that if two or more backtracking verbs
3182 appear in succession, all the the last of them has no effect. Consider this
3183 example:
3184 <pre>
3185 ...(*COMMIT)(*PRUNE)...
3186 </pre>
3187 If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3188 it to be triggered, and its action is taken. There can never be a backtrack
3189 onto (*COMMIT).
3190 <a name="btrepeat"></a></P>
3191 <br><b>
3192 Backtracking verbs in repeated groups
3193 </b><br>
3194 <P>
3195 PCRE differs from Perl in its handling of backtracking verbs in repeated
3196 groups. For example, consider:
3197 <pre>
3198 /(a(*COMMIT)b)+ac/
3199 </pre>
3200 If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3201 the second repeat of the group acts.
3202 <a name="btassert"></a></P>
3203 <br><b>
3204 Backtracking verbs in assertions
3205 </b><br>
3206 <P>
3207 (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3208 </P>
3209 <P>
3210 (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3211 further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3212 fail without any further processing.
3213 </P>
3214 <P>
3215 The other backtracking verbs are not treated specially if they appear in a
3216 positive assertion. In particular, (*THEN) skips to the next alternative in the
3217 innermost enclosing group that has alternations, whether or not this is within
3218 the assertion.
3219 </P>
3220 <P>
3221 Negative assertions are, however, different, in order to ensure that changing a
3222 positive assertion into a negative assertion changes its result. Backtracking
3223 into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3224 without considering any further alternative branches in the assertion.
3225 Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3226 within the assertion (the normal behaviour), but if the assertion does not have
3227 such an alternative, (*THEN) behaves like (*PRUNE).
3228 <a name="btsub"></a></P>
3229 <br><b>
3230 Backtracking verbs in subroutines
3231 </b><br>
3232 <P>
3233 These behaviours occur whether or not the subpattern is called recursively.
3234 Perl's treatment of subroutines is different in some cases.
3235 </P>
3236 <P>
3237 (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3238 an immediate backtrack.
3239 </P>
3240 <P>
3241 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3242 succeed without any further processing. Matching then continues after the
3243 subroutine call.
3244 </P>
3245 <P>
3246 (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3247 the subroutine match to fail.
3248 </P>
3249 <P>
3250 (*THEN) skips to the next alternative in the innermost enclosing group within
3251 the subpattern that has alternatives. If there is no such group within the
3252 subpattern, (*THEN) causes the subroutine match to fail.
3253 </P>
3254 <br><a name="SEC28" href="#TOC1">SEE ALSO</a><br>
3255 <P>
3256 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
3257 <b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>, <b>pcre32(3)</b>.
3258 </P>
3259 <br><a name="SEC29" href="#TOC1">AUTHOR</a><br>
3260 <P>
3261 Philip Hazel
3262 <br>
3263 University Computing Service
3264 <br>
3265 Cambridge CB2 3QH, England.
3266 <br>
3267 </P>
3268 <br><a name="SEC30" href="#TOC1">REVISION</a><br>
3269 <P>
3270 Last updated: 23 October 2016
3271 <br>
3272 Copyright &copy; 1997-2016 University of Cambridge.
3273 <br>
3274 <p>
3275 Return to the <a href="index.html">PCRE index page</a>.
3276 </p>


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