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1 .TH PCREPATTERN 3 "03 December 2013" "PCRE 8.34"
2 .SH NAME
3 PCRE - Perl-compatible regular expressions
4 .SH "PCRE REGULAR EXPRESSION DETAILS"
5 .rs
6 .sp
7 The syntax and semantics of the regular expressions that are supported by PCRE
8 are described in detail below. There is a quick-reference syntax summary in the
9 .\" HREF
10 \fBpcresyntax\fP
11 .\"
12 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13 also supports some alternative regular expression syntax (which does not
14 conflict with the Perl syntax) in order to provide some compatibility with
15 regular expressions in Python, .NET, and Oniguruma.
16 .P
17 Perl's regular expressions are described in its own documentation, and
18 regular expressions in general are covered in a number of books, some of which
19 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20 published by O'Reilly, covers regular expressions in great detail. This
21 description of PCRE's regular expressions is intended as reference material.
22 .P
23 This document discusses the patterns that are supported by PCRE when one its
24 main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16|32]_exec()\fP
25 (16- or 32-bit), is used. PCRE also has alternative matching functions,
26 \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP, which match using a
27 different algorithm that is not Perl-compatible. Some of the features discussed
28 below are not available when DFA matching is used. The advantages and
29 disadvantages of the alternative functions, and how they differ from the normal
30 functions, are discussed in the
31 .\" HREF
32 \fBpcrematching\fP
33 .\"
34 page.
35 .
36 .
37 .SH "SPECIAL START-OF-PATTERN ITEMS"
38 .rs
39 .sp
40 A number of options that can be passed to \fBpcre_compile()\fP can also be set
41 by special items at the start of a pattern. These are not Perl-compatible, but
42 are provided to make these options accessible to pattern writers who are not
43 able to change the program that processes the pattern. Any number of these
44 items may appear, but they must all be together right at the start of the
45 pattern string, and the letters must be in upper case.
46 .
47 .
48 .SS "UTF support"
49 .rs
50 .sp
51 The original operation of PCRE was on strings of one-byte characters. However,
52 there is now also support for UTF-8 strings in the original library, an
53 extra library that supports 16-bit and UTF-16 character strings, and a
54 third library that supports 32-bit and UTF-32 character strings. To use these
55 features, PCRE must be built to include appropriate support. When using UTF
56 strings you must either call the compiling function with the PCRE_UTF8,
57 PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
58 these special sequences:
59 .sp
60 (*UTF8)
61 (*UTF16)
62 (*UTF32)
63 (*UTF)
64 .sp
65 (*UTF) is a generic sequence that can be used with any of the libraries.
66 Starting a pattern with such a sequence is equivalent to setting the relevant
67 option. How setting a UTF mode affects pattern matching is mentioned in several
68 places below. There is also a summary of features in the
69 .\" HREF
70 \fBpcreunicode\fP
71 .\"
72 page.
73 .P
74 Some applications that allow their users to supply patterns may wish to
75 restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
76 option is set at compile time, (*UTF) etc. are not allowed, and their
77 appearance causes an error.
78 .
79 .
80 .SS "Unicode property support"
81 .rs
82 .sp
83 Another special sequence that may appear at the start of a pattern is (*UCP).
84 This has the same effect as setting the PCRE_UCP option: it causes sequences
85 such as \ed and \ew to use Unicode properties to determine character types,
86 instead of recognizing only characters with codes less than 128 via a lookup
87 table.
88 .
89 .
90 .SS "Disabling auto-possessification"
91 .rs
92 .sp
93 If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
94 the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making
95 quantifiers possessive when what follows cannot match the repeated item. For
96 example, by default a+b is treated as a++b. For more details, see the
97 .\" HREF
98 \fBpcreapi\fP
99 .\"
100 documentation.
101 .
102 .
103 .SS "Disabling start-up optimizations"
104 .rs
105 .sp
106 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
107 PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables
108 several optimizations for quickly reaching "no match" results. For more
109 details, see the
110 .\" HREF
111 \fBpcreapi\fP
112 .\"
113 documentation.
114 .
115 .
116 .\" HTML <a name="newlines"></a>
117 .SS "Newline conventions"
118 .rs
119 .sp
120 PCRE supports five different conventions for indicating line breaks in
121 strings: a single CR (carriage return) character, a single LF (linefeed)
122 character, the two-character sequence CRLF, any of the three preceding, or any
123 Unicode newline sequence. The
124 .\" HREF
125 \fBpcreapi\fP
126 .\"
127 page has
128 .\" HTML <a href="pcreapi.html#newlines">
129 .\" </a>
130 further discussion
131 .\"
132 about newlines, and shows how to set the newline convention in the
133 \fIoptions\fP arguments for the compiling and matching functions.
134 .P
135 It is also possible to specify a newline convention by starting a pattern
136 string with one of the following five sequences:
137 .sp
138 (*CR) carriage return
139 (*LF) linefeed
140 (*CRLF) carriage return, followed by linefeed
141 (*ANYCRLF) any of the three above
142 (*ANY) all Unicode newline sequences
143 .sp
144 These override the default and the options given to the compiling function. For
145 example, on a Unix system where LF is the default newline sequence, the pattern
146 .sp
147 (*CR)a.b
148 .sp
149 changes the convention to CR. That pattern matches "a\enb" because LF is no
150 longer a newline. If more than one of these settings is present, the last one
151 is used.
152 .P
153 The newline convention affects where the circumflex and dollar assertions are
154 true. It also affects the interpretation of the dot metacharacter when
155 PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
156 what the \eR escape sequence matches. By default, this is any Unicode newline
157 sequence, for Perl compatibility. However, this can be changed; see the
158 description of \eR in the section entitled
159 .\" HTML <a href="#newlineseq">
160 .\" </a>
161 "Newline sequences"
162 .\"
163 below. A change of \eR setting can be combined with a change of newline
164 convention.
165 .
166 .
167 .SS "Setting match and recursion limits"
168 .rs
169 .sp
170 The caller of \fBpcre_exec()\fP can set a limit on the number of times the
171 internal \fBmatch()\fP function is called and on the maximum depth of
172 recursive calls. These facilities are provided to catch runaway matches that
173 are provoked by patterns with huge matching trees (a typical example is a
174 pattern with nested unlimited repeats) and to avoid running out of system stack
175 by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
176 gives an error return. The limits can also be set by items at the start of the
177 pattern of the form
178 .sp
179 (*LIMIT_MATCH=d)
180 (*LIMIT_RECURSION=d)
181 .sp
182 where d is any number of decimal digits. However, the value of the setting must
183 be less than the value set (or defaulted) by the caller of \fBpcre_exec()\fP
184 for it to have any effect. In other words, the pattern writer can lower the
185 limits set by the programmer, but not raise them. If there is more than one
186 setting of one of these limits, the lower value is used.
187 .
188 .
189 .SH "EBCDIC CHARACTER CODES"
190 .rs
191 .sp
192 PCRE can be compiled to run in an environment that uses EBCDIC as its character
193 code rather than ASCII or Unicode (typically a mainframe system). In the
194 sections below, character code values are ASCII or Unicode; in an EBCDIC
195 environment these characters may have different code values, and there are no
196 code points greater than 255.
197 .
198 .
199 .SH "CHARACTERS AND METACHARACTERS"
200 .rs
201 .sp
202 A regular expression is a pattern that is matched against a subject string from
203 left to right. Most characters stand for themselves in a pattern, and match the
204 corresponding characters in the subject. As a trivial example, the pattern
205 .sp
206 The quick brown fox
207 .sp
208 matches a portion of a subject string that is identical to itself. When
209 caseless matching is specified (the PCRE_CASELESS option), letters are matched
210 independently of case. In a UTF mode, PCRE always understands the concept of
211 case for characters whose values are less than 128, so caseless matching is
212 always possible. For characters with higher values, the concept of case is
213 supported if PCRE is compiled with Unicode property support, but not otherwise.
214 If you want to use caseless matching for characters 128 and above, you must
215 ensure that PCRE is compiled with Unicode property support as well as with
216 UTF support.
217 .P
218 The power of regular expressions comes from the ability to include alternatives
219 and repetitions in the pattern. These are encoded in the pattern by the use of
220 \fImetacharacters\fP, which do not stand for themselves but instead are
221 interpreted in some special way.
222 .P
223 There are two different sets of metacharacters: those that are recognized
224 anywhere in the pattern except within square brackets, and those that are
225 recognized within square brackets. Outside square brackets, the metacharacters
226 are as follows:
227 .sp
228 \e general escape character with several uses
229 ^ assert start of string (or line, in multiline mode)
230 $ assert end of string (or line, in multiline mode)
231 . match any character except newline (by default)
232 [ start character class definition
233 | start of alternative branch
234 ( start subpattern
235 ) end subpattern
236 ? extends the meaning of (
237 also 0 or 1 quantifier
238 also quantifier minimizer
239 * 0 or more quantifier
240 + 1 or more quantifier
241 also "possessive quantifier"
242 { start min/max quantifier
243 .sp
244 Part of a pattern that is in square brackets is called a "character class". In
245 a character class the only metacharacters are:
246 .sp
247 \e general escape character
248 ^ negate the class, but only if the first character
249 - indicates character range
250 .\" JOIN
251 [ POSIX character class (only if followed by POSIX
252 syntax)
253 ] terminates the character class
254 .sp
255 The following sections describe the use of each of the metacharacters.
256 .
257 .
258 .SH BACKSLASH
259 .rs
260 .sp
261 The backslash character has several uses. Firstly, if it is followed by a
262 character that is not a number or a letter, it takes away any special meaning
263 that character may have. This use of backslash as an escape character applies
264 both inside and outside character classes.
265 .P
266 For example, if you want to match a * character, you write \e* in the pattern.
267 This escaping action applies whether or not the following character would
268 otherwise be interpreted as a metacharacter, so it is always safe to precede a
269 non-alphanumeric with backslash to specify that it stands for itself. In
270 particular, if you want to match a backslash, you write \e\e.
271 .P
272 In a UTF mode, only ASCII numbers and letters have any special meaning after a
273 backslash. All other characters (in particular, those whose codepoints are
274 greater than 127) are treated as literals.
275 .P
276 If a pattern is compiled with the PCRE_EXTENDED option, most white space in the
277 pattern (other than in a character class), and characters between a # outside a
278 character class and the next newline, inclusive, are ignored. An escaping
279 backslash can be used to include a white space or # character as part of the
280 pattern.
281 .P
282 If you want to remove the special meaning from a sequence of characters, you
283 can do so by putting them between \eQ and \eE. This is different from Perl in
284 that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
285 Perl, $ and @ cause variable interpolation. Note the following examples:
286 .sp
287 Pattern PCRE matches Perl matches
288 .sp
289 .\" JOIN
290 \eQabc$xyz\eE abc$xyz abc followed by the
291 contents of $xyz
292 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
293 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
294 .sp
295 The \eQ...\eE sequence is recognized both inside and outside character classes.
296 An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
297 by \eE later in the pattern, the literal interpretation continues to the end of
298 the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
299 a character class, this causes an error, because the character class is not
300 terminated.
301 .
302 .
303 .\" HTML <a name="digitsafterbackslash"></a>
304 .SS "Non-printing characters"
305 .rs
306 .sp
307 A second use of backslash provides a way of encoding non-printing characters
308 in patterns in a visible manner. There is no restriction on the appearance of
309 non-printing characters, apart from the binary zero that terminates a pattern,
310 but when a pattern is being prepared by text editing, it is often easier to use
311 one of the following escape sequences than the binary character it represents:
312 .sp
313 \ea alarm, that is, the BEL character (hex 07)
314 \ecx "control-x", where x is any ASCII character
315 \ee escape (hex 1B)
316 \ef form feed (hex 0C)
317 \en linefeed (hex 0A)
318 \er carriage return (hex 0D)
319 \et tab (hex 09)
320 \e0dd character with octal code 0dd
321 \eddd character with octal code ddd, or back reference
322 \eo{ddd..} character with octal code ddd..
323 \exhh character with hex code hh
324 \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
325 \euhhhh character with hex code hhhh (JavaScript mode only)
326 .sp
327 The precise effect of \ecx on ASCII characters is as follows: if x is a lower
328 case letter, it is converted to upper case. Then bit 6 of the character (hex
329 40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
330 but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
331 data item (byte or 16-bit value) following \ec has a value greater than 127, a
332 compile-time error occurs. This locks out non-ASCII characters in all modes.
333 .P
334 The \ec facility was designed for use with ASCII characters, but with the
335 extension to Unicode it is even less useful than it once was. It is, however,
336 recognized when PCRE is compiled in EBCDIC mode, where data items are always
337 bytes. In this mode, all values are valid after \ec. If the next character is a
338 lower case letter, it is converted to upper case. Then the 0xc0 bits of the
339 byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
340 the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
341 characters also generate different values.
342 .P
343 After \e0 up to two further octal digits are read. If there are fewer than two
344 digits, just those that are present are used. Thus the sequence \e0\ex\e07
345 specifies two binary zeros followed by a BEL character (code value 7). Make
346 sure you supply two digits after the initial zero if the pattern character that
347 follows is itself an octal digit.
348 .P
349 The escape \eo must be followed by a sequence of octal digits, enclosed in
350 braces. An error occurs if this is not the case. This escape is a recent
351 addition to Perl; it provides way of specifying character code points as octal
352 numbers greater than 0777, and it also allows octal numbers and back references
353 to be unambiguously specified.
354 .P
355 For greater clarity and unambiguity, it is best to avoid following \e by a
356 digit greater than zero. Instead, use \eo{} or \ex{} to specify character
357 numbers, and \eg{} to specify back references. The following paragraphs
358 describe the old, ambiguous syntax.
359 .P
360 The handling of a backslash followed by a digit other than 0 is complicated,
361 and Perl has changed in recent releases, causing PCRE also to change. Outside a
362 character class, PCRE reads the digit and any following digits as a decimal
363 number. If the number is less than 8, or if there have been at least that many
364 previous capturing left parentheses in the expression, the entire sequence is
365 taken as a \fIback reference\fP. A description of how this works is given
366 .\" HTML <a href="#backreferences">
367 .\" </a>
368 later,
369 .\"
370 following the discussion of
371 .\" HTML <a href="#subpattern">
372 .\" </a>
373 parenthesized subpatterns.
374 .\"
375 .P
376 Inside a character class, or if the decimal number following \e is greater than
377 7 and there have not been that many capturing subpatterns, PCRE handles \e8 and
378 \e9 as the literal characters "8" and "9", and otherwise re-reads up to three
379 octal digits following the backslash, using them to generate a data character.
380 Any subsequent digits stand for themselves. For example:
381 .sp
382 \e040 is another way of writing an ASCII space
383 .\" JOIN
384 \e40 is the same, provided there are fewer than 40
385 previous capturing subpatterns
386 \e7 is always a back reference
387 .\" JOIN
388 \e11 might be a back reference, or another way of
389 writing a tab
390 \e011 is always a tab
391 \e0113 is a tab followed by the character "3"
392 .\" JOIN
393 \e113 might be a back reference, otherwise the
394 character with octal code 113
395 .\" JOIN
396 \e377 might be a back reference, otherwise
397 the value 255 (decimal)
398 .\" JOIN
399 \e81 is either a back reference, or the two
400 characters "8" and "1"
401 .sp
402 Note that octal values of 100 or greater that are specified using this syntax
403 must not be introduced by a leading zero, because no more than three octal
404 digits are ever read.
405 .P
406 By default, after \ex that is not followed by {, from zero to two hexadecimal
407 digits are read (letters can be in upper or lower case). Any number of
408 hexadecimal digits may appear between \ex{ and }. If a character other than
409 a hexadecimal digit appears between \ex{ and }, or if there is no terminating
410 }, an error occurs.
411 .P
412 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
413 as just described only when it is followed by two hexadecimal digits.
414 Otherwise, it matches a literal "x" character. In JavaScript mode, support for
415 code points greater than 256 is provided by \eu, which must be followed by
416 four hexadecimal digits; otherwise it matches a literal "u" character.
417 .P
418 Characters whose value is less than 256 can be defined by either of the two
419 syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
420 way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
421 \eu00dc in JavaScript mode).
422 .
423 .
424 .SS "Constraints on character values"
425 .rs
426 .sp
427 Characters that are specified using octal or hexadecimal numbers are
428 limited to certain values, as follows:
429 .sp
430 8-bit non-UTF mode less than 0x100
431 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
432 16-bit non-UTF mode less than 0x10000
433 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
434 32-bit non-UTF mode less than 0x100000000
435 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
436 .sp
437 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
438 "surrogate" codepoints), and 0xffef.
439 .
440 .
441 .SS "Escape sequences in character classes"
442 .rs
443 .sp
444 All the sequences that define a single character value can be used both inside
445 and outside character classes. In addition, inside a character class, \eb is
446 interpreted as the backspace character (hex 08).
447 .P
448 \eN is not allowed in a character class. \eB, \eR, and \eX are not special
449 inside a character class. Like other unrecognized escape sequences, they are
450 treated as the literal characters "B", "R", and "X" by default, but cause an
451 error if the PCRE_EXTRA option is set. Outside a character class, these
452 sequences have different meanings.
453 .
454 .
455 .SS "Unsupported escape sequences"
456 .rs
457 .sp
458 In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
459 handler and used to modify the case of following characters. By default, PCRE
460 does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
461 option is set, \eU matches a "U" character, and \eu can be used to define a
462 character by code point, as described in the previous section.
463 .
464 .
465 .SS "Absolute and relative back references"
466 .rs
467 .sp
468 The sequence \eg followed by an unsigned or a negative number, optionally
469 enclosed in braces, is an absolute or relative back reference. A named back
470 reference can be coded as \eg{name}. Back references are discussed
471 .\" HTML <a href="#backreferences">
472 .\" </a>
473 later,
474 .\"
475 following the discussion of
476 .\" HTML <a href="#subpattern">
477 .\" </a>
478 parenthesized subpatterns.
479 .\"
480 .
481 .
482 .SS "Absolute and relative subroutine calls"
483 .rs
484 .sp
485 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
486 a number enclosed either in angle brackets or single quotes, is an alternative
487 syntax for referencing a subpattern as a "subroutine". Details are discussed
488 .\" HTML <a href="#onigurumasubroutines">
489 .\" </a>
490 later.
491 .\"
492 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
493 synonymous. The former is a back reference; the latter is a
494 .\" HTML <a href="#subpatternsassubroutines">
495 .\" </a>
496 subroutine
497 .\"
498 call.
499 .
500 .
501 .\" HTML <a name="genericchartypes"></a>
502 .SS "Generic character types"
503 .rs
504 .sp
505 Another use of backslash is for specifying generic character types:
506 .sp
507 \ed any decimal digit
508 \eD any character that is not a decimal digit
509 \eh any horizontal white space character
510 \eH any character that is not a horizontal white space character
511 \es any white space character
512 \eS any character that is not a white space character
513 \ev any vertical white space character
514 \eV any character that is not a vertical white space character
515 \ew any "word" character
516 \eW any "non-word" character
517 .sp
518 There is also the single sequence \eN, which matches a non-newline character.
519 This is the same as
520 .\" HTML <a href="#fullstopdot">
521 .\" </a>
522 the "." metacharacter
523 .\"
524 when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
525 PCRE does not support this.
526 .P
527 Each pair of lower and upper case escape sequences partitions the complete set
528 of characters into two disjoint sets. Any given character matches one, and only
529 one, of each pair. The sequences can appear both inside and outside character
530 classes. They each match one character of the appropriate type. If the current
531 matching point is at the end of the subject string, all of them fail, because
532 there is no character to match.
533 .P
534 For compatibility with Perl, \es did not used to match the VT character (code
535 11), which made it different from the the POSIX "space" class. However, Perl
536 added VT at release 5.18, and PCRE followed suit at release 8.34. The default
537 \es characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space
538 (32), which are defined as white space in the "C" locale. This list may vary if
539 locale-specific matching is taking place. For example, in some locales the
540 "non-breaking space" character (\exA0) is recognized as white space, and in
541 others the VT character is not.
542 .P
543 A "word" character is an underscore or any character that is a letter or digit.
544 By default, the definition of letters and digits is controlled by PCRE's
545 low-valued character tables, and may vary if locale-specific matching is taking
546 place (see
547 .\" HTML <a href="pcreapi.html#localesupport">
548 .\" </a>
549 "Locale support"
550 .\"
551 in the
552 .\" HREF
553 \fBpcreapi\fP
554 .\"
555 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
556 or "french" in Windows, some character codes greater than 127 are used for
557 accented letters, and these are then matched by \ew. The use of locales with
558 Unicode is discouraged.
559 .P
560 By default, characters whose code points are greater than 127 never match \ed,
561 \es, or \ew, and always match \eD, \eS, and \eW, although this may vary for
562 characters in the range 128-255 when locale-specific matching is happening.
563 These escape sequences retain their original meanings from before Unicode
564 support was available, mainly for efficiency reasons. If PCRE is compiled with
565 Unicode property support, and the PCRE_UCP option is set, the behaviour is
566 changed so that Unicode properties are used to determine character types, as
567 follows:
568 .sp
569 \ed any character that matches \ep{Nd} (decimal digit)
570 \es any character that matches \ep{Z} or \eh or \ev
571 \ew any character that matches \ep{L} or \ep{N}, plus underscore
572 .sp
573 The upper case escapes match the inverse sets of characters. Note that \ed
574 matches only decimal digits, whereas \ew matches any Unicode digit, as well as
575 any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
576 \eB because they are defined in terms of \ew and \eW. Matching these sequences
577 is noticeably slower when PCRE_UCP is set.
578 .P
579 The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
580 release 5.10. In contrast to the other sequences, which match only ASCII
581 characters by default, these always match certain high-valued code points,
582 whether or not PCRE_UCP is set. The horizontal space characters are:
583 .sp
584 U+0009 Horizontal tab (HT)
585 U+0020 Space
586 U+00A0 Non-break space
587 U+1680 Ogham space mark
588 U+180E Mongolian vowel separator
589 U+2000 En quad
590 U+2001 Em quad
591 U+2002 En space
592 U+2003 Em space
593 U+2004 Three-per-em space
594 U+2005 Four-per-em space
595 U+2006 Six-per-em space
596 U+2007 Figure space
597 U+2008 Punctuation space
598 U+2009 Thin space
599 U+200A Hair space
600 U+202F Narrow no-break space
601 U+205F Medium mathematical space
602 U+3000 Ideographic space
603 .sp
604 The vertical space characters are:
605 .sp
606 U+000A Linefeed (LF)
607 U+000B Vertical tab (VT)
608 U+000C Form feed (FF)
609 U+000D Carriage return (CR)
610 U+0085 Next line (NEL)
611 U+2028 Line separator
612 U+2029 Paragraph separator
613 .sp
614 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
615 relevant.
616 .
617 .
618 .\" HTML <a name="newlineseq"></a>
619 .SS "Newline sequences"
620 .rs
621 .sp
622 Outside a character class, by default, the escape sequence \eR matches any
623 Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
624 following:
625 .sp
626 (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
627 .sp
628 This is an example of an "atomic group", details of which are given
629 .\" HTML <a href="#atomicgroup">
630 .\" </a>
631 below.
632 .\"
633 This particular group matches either the two-character sequence CR followed by
634 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
635 U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
636 line, U+0085). The two-character sequence is treated as a single unit that
637 cannot be split.
638 .P
639 In other modes, two additional characters whose codepoints are greater than 255
640 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
641 Unicode character property support is not needed for these characters to be
642 recognized.
643 .P
644 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
645 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
646 either at compile time or when the pattern is matched. (BSR is an abbrevation
647 for "backslash R".) This can be made the default when PCRE is built; if this is
648 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
649 It is also possible to specify these settings by starting a pattern string with
650 one of the following sequences:
651 .sp
652 (*BSR_ANYCRLF) CR, LF, or CRLF only
653 (*BSR_UNICODE) any Unicode newline sequence
654 .sp
655 These override the default and the options given to the compiling function, but
656 they can themselves be overridden by options given to a matching function. Note
657 that these special settings, which are not Perl-compatible, are recognized only
658 at the very start of a pattern, and that they must be in upper case. If more
659 than one of them is present, the last one is used. They can be combined with a
660 change of newline convention; for example, a pattern can start with:
661 .sp
662 (*ANY)(*BSR_ANYCRLF)
663 .sp
664 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
665 (*UCP) special sequences. Inside a character class, \eR is treated as an
666 unrecognized escape sequence, and so matches the letter "R" by default, but
667 causes an error if PCRE_EXTRA is set.
668 .
669 .
670 .\" HTML <a name="uniextseq"></a>
671 .SS Unicode character properties
672 .rs
673 .sp
674 When PCRE is built with Unicode character property support, three additional
675 escape sequences that match characters with specific properties are available.
676 When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
677 characters whose codepoints are less than 256, but they do work in this mode.
678 The extra escape sequences are:
679 .sp
680 \ep{\fIxx\fP} a character with the \fIxx\fP property
681 \eP{\fIxx\fP} a character without the \fIxx\fP property
682 \eX a Unicode extended grapheme cluster
683 .sp
684 The property names represented by \fIxx\fP above are limited to the Unicode
685 script names, the general category properties, "Any", which matches any
686 character (including newline), and some special PCRE properties (described
687 in the
688 .\" HTML <a href="#extraprops">
689 .\" </a>
690 next section).
691 .\"
692 Other Perl properties such as "InMusicalSymbols" are not currently supported by
693 PCRE. Note that \eP{Any} does not match any characters, so always causes a
694 match failure.
695 .P
696 Sets of Unicode characters are defined as belonging to certain scripts. A
697 character from one of these sets can be matched using a script name. For
698 example:
699 .sp
700 \ep{Greek}
701 \eP{Han}
702 .sp
703 Those that are not part of an identified script are lumped together as
704 "Common". The current list of scripts is:
705 .P
706 Arabic,
707 Armenian,
708 Avestan,
709 Balinese,
710 Bamum,
711 Batak,
712 Bengali,
713 Bopomofo,
714 Brahmi,
715 Braille,
716 Buginese,
717 Buhid,
718 Canadian_Aboriginal,
719 Carian,
720 Chakma,
721 Cham,
722 Cherokee,
723 Common,
724 Coptic,
725 Cuneiform,
726 Cypriot,
727 Cyrillic,
728 Deseret,
729 Devanagari,
730 Egyptian_Hieroglyphs,
731 Ethiopic,
732 Georgian,
733 Glagolitic,
734 Gothic,
735 Greek,
736 Gujarati,
737 Gurmukhi,
738 Han,
739 Hangul,
740 Hanunoo,
741 Hebrew,
742 Hiragana,
743 Imperial_Aramaic,
744 Inherited,
745 Inscriptional_Pahlavi,
746 Inscriptional_Parthian,
747 Javanese,
748 Kaithi,
749 Kannada,
750 Katakana,
751 Kayah_Li,
752 Kharoshthi,
753 Khmer,
754 Lao,
755 Latin,
756 Lepcha,
757 Limbu,
758 Linear_B,
759 Lisu,
760 Lycian,
761 Lydian,
762 Malayalam,
763 Mandaic,
764 Meetei_Mayek,
765 Meroitic_Cursive,
766 Meroitic_Hieroglyphs,
767 Miao,
768 Mongolian,
769 Myanmar,
770 New_Tai_Lue,
771 Nko,
772 Ogham,
773 Old_Italic,
774 Old_Persian,
775 Old_South_Arabian,
776 Old_Turkic,
777 Ol_Chiki,
778 Oriya,
779 Osmanya,
780 Phags_Pa,
781 Phoenician,
782 Rejang,
783 Runic,
784 Samaritan,
785 Saurashtra,
786 Sharada,
787 Shavian,
788 Sinhala,
789 Sora_Sompeng,
790 Sundanese,
791 Syloti_Nagri,
792 Syriac,
793 Tagalog,
794 Tagbanwa,
795 Tai_Le,
796 Tai_Tham,
797 Tai_Viet,
798 Takri,
799 Tamil,
800 Telugu,
801 Thaana,
802 Thai,
803 Tibetan,
804 Tifinagh,
805 Ugaritic,
806 Vai,
807 Yi.
808 .P
809 Each character has exactly one Unicode general category property, specified by
810 a two-letter abbreviation. For compatibility with Perl, negation can be
811 specified by including a circumflex between the opening brace and the property
812 name. For example, \ep{^Lu} is the same as \eP{Lu}.
813 .P
814 If only one letter is specified with \ep or \eP, it includes all the general
815 category properties that start with that letter. In this case, in the absence
816 of negation, the curly brackets in the escape sequence are optional; these two
817 examples have the same effect:
818 .sp
819 \ep{L}
820 \epL
821 .sp
822 The following general category property codes are supported:
823 .sp
824 C Other
825 Cc Control
826 Cf Format
827 Cn Unassigned
828 Co Private use
829 Cs Surrogate
830 .sp
831 L Letter
832 Ll Lower case letter
833 Lm Modifier letter
834 Lo Other letter
835 Lt Title case letter
836 Lu Upper case letter
837 .sp
838 M Mark
839 Mc Spacing mark
840 Me Enclosing mark
841 Mn Non-spacing mark
842 .sp
843 N Number
844 Nd Decimal number
845 Nl Letter number
846 No Other number
847 .sp
848 P Punctuation
849 Pc Connector punctuation
850 Pd Dash punctuation
851 Pe Close punctuation
852 Pf Final punctuation
853 Pi Initial punctuation
854 Po Other punctuation
855 Ps Open punctuation
856 .sp
857 S Symbol
858 Sc Currency symbol
859 Sk Modifier symbol
860 Sm Mathematical symbol
861 So Other symbol
862 .sp
863 Z Separator
864 Zl Line separator
865 Zp Paragraph separator
866 Zs Space separator
867 .sp
868 The special property L& is also supported: it matches a character that has
869 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
870 a modifier or "other".
871 .P
872 The Cs (Surrogate) property applies only to characters in the range U+D800 to
873 U+DFFF. Such characters are not valid in Unicode strings and so
874 cannot be tested by PCRE, unless UTF validity checking has been turned off
875 (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
876 PCRE_NO_UTF32_CHECK in the
877 .\" HREF
878 \fBpcreapi\fP
879 .\"
880 page). Perl does not support the Cs property.
881 .P
882 The long synonyms for property names that Perl supports (such as \ep{Letter})
883 are not supported by PCRE, nor is it permitted to prefix any of these
884 properties with "Is".
885 .P
886 No character that is in the Unicode table has the Cn (unassigned) property.
887 Instead, this property is assumed for any code point that is not in the
888 Unicode table.
889 .P
890 Specifying caseless matching does not affect these escape sequences. For
891 example, \ep{Lu} always matches only upper case letters. This is different from
892 the behaviour of current versions of Perl.
893 .P
894 Matching characters by Unicode property is not fast, because PCRE has to do a
895 multistage table lookup in order to find a character's property. That is why
896 the traditional escape sequences such as \ed and \ew do not use Unicode
897 properties in PCRE by default, though you can make them do so by setting the
898 PCRE_UCP option or by starting the pattern with (*UCP).
899 .
900 .
901 .SS Extended grapheme clusters
902 .rs
903 .sp
904 The \eX escape matches any number of Unicode characters that form an "extended
905 grapheme cluster", and treats the sequence as an atomic group
906 .\" HTML <a href="#atomicgroup">
907 .\" </a>
908 (see below).
909 .\"
910 Up to and including release 8.31, PCRE matched an earlier, simpler definition
911 that was equivalent to
912 .sp
913 (?>\ePM\epM*)
914 .sp
915 That is, it matched a character without the "mark" property, followed by zero
916 or more characters with the "mark" property. Characters with the "mark"
917 property are typically non-spacing accents that affect the preceding character.
918 .P
919 This simple definition was extended in Unicode to include more complicated
920 kinds of composite character by giving each character a grapheme breaking
921 property, and creating rules that use these properties to define the boundaries
922 of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
923 one of these clusters.
924 .P
925 \eX always matches at least one character. Then it decides whether to add
926 additional characters according to the following rules for ending a cluster:
927 .P
928 1. End at the end of the subject string.
929 .P
930 2. Do not end between CR and LF; otherwise end after any control character.
931 .P
932 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
933 are of five types: L, V, T, LV, and LVT. An L character may be followed by an
934 L, V, LV, or LVT character; an LV or V character may be followed by a V or T
935 character; an LVT or T character may be follwed only by a T character.
936 .P
937 4. Do not end before extending characters or spacing marks. Characters with
938 the "mark" property always have the "extend" grapheme breaking property.
939 .P
940 5. Do not end after prepend characters.
941 .P
942 6. Otherwise, end the cluster.
943 .
944 .
945 .\" HTML <a name="extraprops"></a>
946 .SS PCRE's additional properties
947 .rs
948 .sp
949 As well as the standard Unicode properties described above, PCRE supports four
950 more that make it possible to convert traditional escape sequences such as \ew
951 and \es to use Unicode properties. PCRE uses these non-standard, non-Perl
952 properties internally when PCRE_UCP is set. However, they may also be used
953 explicitly. These properties are:
954 .sp
955 Xan Any alphanumeric character
956 Xps Any POSIX space character
957 Xsp Any Perl space character
958 Xwd Any Perl "word" character
959 .sp
960 Xan matches characters that have either the L (letter) or the N (number)
961 property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
962 carriage return, and any other character that has the Z (separator) property.
963 Xsp is the same as Xps; it used to exclude vertical tab, for Perl
964 compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
965 matches the same characters as Xan, plus underscore.
966 .P
967 There is another non-standard property, Xuc, which matches any character that
968 can be represented by a Universal Character Name in C++ and other programming
969 languages. These are the characters $, @, ` (grave accent), and all characters
970 with Unicode code points greater than or equal to U+00A0, except for the
971 surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
972 excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
973 where H is a hexadecimal digit. Note that the Xuc property does not match these
974 sequences but the characters that they represent.)
975 .
976 .
977 .\" HTML <a name="resetmatchstart"></a>
978 .SS "Resetting the match start"
979 .rs
980 .sp
981 The escape sequence \eK causes any previously matched characters not to be
982 included in the final matched sequence. For example, the pattern:
983 .sp
984 foo\eKbar
985 .sp
986 matches "foobar", but reports that it has matched "bar". This feature is
987 similar to a lookbehind assertion
988 .\" HTML <a href="#lookbehind">
989 .\" </a>
990 (described below).
991 .\"
992 However, in this case, the part of the subject before the real match does not
993 have to be of fixed length, as lookbehind assertions do. The use of \eK does
994 not interfere with the setting of
995 .\" HTML <a href="#subpattern">
996 .\" </a>
997 captured substrings.
998 .\"
999 For example, when the pattern
1000 .sp
1001 (foo)\eKbar
1002 .sp
1003 matches "foobar", the first substring is still set to "foo".
1004 .P
1005 Perl documents that the use of \eK within assertions is "not well defined". In
1006 PCRE, \eK is acted upon when it occurs inside positive assertions, but is
1007 ignored in negative assertions.
1008 .
1009 .
1010 .\" HTML <a name="smallassertions"></a>
1011 .SS "Simple assertions"
1012 .rs
1013 .sp
1014 The final use of backslash is for certain simple assertions. An assertion
1015 specifies a condition that has to be met at a particular point in a match,
1016 without consuming any characters from the subject string. The use of
1017 subpatterns for more complicated assertions is described
1018 .\" HTML <a href="#bigassertions">
1019 .\" </a>
1020 below.
1021 .\"
1022 The backslashed assertions are:
1023 .sp
1024 \eb matches at a word boundary
1025 \eB matches when not at a word boundary
1026 \eA matches at the start of the subject
1027 \eZ matches at the end of the subject
1028 also matches before a newline at the end of the subject
1029 \ez matches only at the end of the subject
1030 \eG matches at the first matching position in the subject
1031 .sp
1032 Inside a character class, \eb has a different meaning; it matches the backspace
1033 character. If any other of these assertions appears in a character class, by
1034 default it matches the corresponding literal character (for example, \eB
1035 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
1036 escape sequence" error is generated instead.
1037 .P
1038 A word boundary is a position in the subject string where the current character
1039 and the previous character do not both match \ew or \eW (i.e. one matches
1040 \ew and the other matches \eW), or the start or end of the string if the
1041 first or last character matches \ew, respectively. In a UTF mode, the meanings
1042 of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1043 done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1044 of word" or "end of word" metasequence. However, whatever follows \eb normally
1045 determines which it is. For example, the fragment \eba matches "a" at the start
1046 of a word.
1047 .P
1048 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
1049 dollar (described in the next section) in that they only ever match at the very
1050 start and end of the subject string, whatever options are set. Thus, they are
1051 independent of multiline mode. These three assertions are not affected by the
1052 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
1053 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
1054 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
1055 at a point other than the beginning of the subject, \eA can never match. The
1056 difference between \eZ and \ez is that \eZ matches before a newline at the end
1057 of the string as well as at the very end, whereas \ez matches only at the end.
1058 .P
1059 The \eG assertion is true only when the current matching position is at the
1060 start point of the match, as specified by the \fIstartoffset\fP argument of
1061 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
1062 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
1063 arguments, you can mimic Perl's /g option, and it is in this kind of
1064 implementation where \eG can be useful.
1065 .P
1066 Note, however, that PCRE's interpretation of \eG, as the start of the current
1067 match, is subtly different from Perl's, which defines it as the end of the
1068 previous match. In Perl, these can be different when the previously matched
1069 string was empty. Because PCRE does just one match at a time, it cannot
1070 reproduce this behaviour.
1071 .P
1072 If all the alternatives of a pattern begin with \eG, the expression is anchored
1073 to the starting match position, and the "anchored" flag is set in the compiled
1074 regular expression.
1075 .
1076 .
1077 .SH "CIRCUMFLEX AND DOLLAR"
1078 .rs
1079 .sp
1080 The circumflex and dollar metacharacters are zero-width assertions. That is,
1081 they test for a particular condition being true without consuming any
1082 characters from the subject string.
1083 .P
1084 Outside a character class, in the default matching mode, the circumflex
1085 character is an assertion that is true only if the current matching point is at
1086 the start of the subject string. If the \fIstartoffset\fP argument of
1087 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
1088 option is unset. Inside a character class, circumflex has an entirely different
1089 meaning
1090 .\" HTML <a href="#characterclass">
1091 .\" </a>
1092 (see below).
1093 .\"
1094 .P
1095 Circumflex need not be the first character of the pattern if a number of
1096 alternatives are involved, but it should be the first thing in each alternative
1097 in which it appears if the pattern is ever to match that branch. If all
1098 possible alternatives start with a circumflex, that is, if the pattern is
1099 constrained to match only at the start of the subject, it is said to be an
1100 "anchored" pattern. (There are also other constructs that can cause a pattern
1101 to be anchored.)
1102 .P
1103 The dollar character is an assertion that is true only if the current matching
1104 point is at the end of the subject string, or immediately before a newline at
1105 the end of the string (by default). Note, however, that it does not actually
1106 match the newline. Dollar need not be the last character of the pattern if a
1107 number of alternatives are involved, but it should be the last item in any
1108 branch in which it appears. Dollar has no special meaning in a character class.
1109 .P
1110 The meaning of dollar can be changed so that it matches only at the very end of
1111 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
1112 does not affect the \eZ assertion.
1113 .P
1114 The meanings of the circumflex and dollar characters are changed if the
1115 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
1116 immediately after internal newlines as well as at the start of the subject
1117 string. It does not match after a newline that ends the string. A dollar
1118 matches before any newlines in the string, as well as at the very end, when
1119 PCRE_MULTILINE is set. When newline is specified as the two-character
1120 sequence CRLF, isolated CR and LF characters do not indicate newlines.
1121 .P
1122 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
1123 \en represents a newline) in multiline mode, but not otherwise. Consequently,
1124 patterns that are anchored in single line mode because all branches start with
1125 ^ are not anchored in multiline mode, and a match for circumflex is possible
1126 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
1127 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
1128 .P
1129 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
1130 end of the subject in both modes, and if all branches of a pattern start with
1131 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1132 .
1133 .
1134 .\" HTML <a name="fullstopdot"></a>
1135 .SH "FULL STOP (PERIOD, DOT) AND \eN"
1136 .rs
1137 .sp
1138 Outside a character class, a dot in the pattern matches any one character in
1139 the subject string except (by default) a character that signifies the end of a
1140 line.
1141 .P
1142 When a line ending is defined as a single character, dot never matches that
1143 character; when the two-character sequence CRLF is used, dot does not match CR
1144 if it is immediately followed by LF, but otherwise it matches all characters
1145 (including isolated CRs and LFs). When any Unicode line endings are being
1146 recognized, dot does not match CR or LF or any of the other line ending
1147 characters.
1148 .P
1149 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
1150 option is set, a dot matches any one character, without exception. If the
1151 two-character sequence CRLF is present in the subject string, it takes two dots
1152 to match it.
1153 .P
1154 The handling of dot is entirely independent of the handling of circumflex and
1155 dollar, the only relationship being that they both involve newlines. Dot has no
1156 special meaning in a character class.
1157 .P
1158 The escape sequence \eN behaves like a dot, except that it is not affected by
1159 the PCRE_DOTALL option. In other words, it matches any character except one
1160 that signifies the end of a line. Perl also uses \eN to match characters by
1161 name; PCRE does not support this.
1162 .
1163 .
1164 .SH "MATCHING A SINGLE DATA UNIT"
1165 .rs
1166 .sp
1167 Outside a character class, the escape sequence \eC matches any one data unit,
1168 whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1169 byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1170 a 32-bit unit. Unlike a dot, \eC always
1171 matches line-ending characters. The feature is provided in Perl in order to
1172 match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1173 used. Because \eC breaks up characters into individual data units, matching one
1174 unit with \eC in a UTF mode means that the rest of the string may start with a
1175 malformed UTF character. This has undefined results, because PCRE assumes that
1176 it is dealing with valid UTF strings (and by default it checks this at the
1177 start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1178 PCRE_NO_UTF32_CHECK option is used).
1179 .P
1180 PCRE does not allow \eC to appear in lookbehind assertions
1181 .\" HTML <a href="#lookbehind">
1182 .\" </a>
1183 (described below)
1184 .\"
1185 in a UTF mode, because this would make it impossible to calculate the length of
1186 the lookbehind.
1187 .P
1188 In general, the \eC escape sequence is best avoided. However, one
1189 way of using it that avoids the problem of malformed UTF characters is to use a
1190 lookahead to check the length of the next character, as in this pattern, which
1191 could be used with a UTF-8 string (ignore white space and line breaks):
1192 .sp
1193 (?| (?=[\ex00-\ex7f])(\eC) |
1194 (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1195 (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1196 (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1197 .sp
1198 A group that starts with (?| resets the capturing parentheses numbers in each
1199 alternative (see
1200 .\" HTML <a href="#dupsubpatternnumber">
1201 .\" </a>
1202 "Duplicate Subpattern Numbers"
1203 .\"
1204 below). The assertions at the start of each branch check the next UTF-8
1205 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1206 character's individual bytes are then captured by the appropriate number of
1207 groups.
1208 .
1209 .
1210 .\" HTML <a name="characterclass"></a>
1211 .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
1212 .rs
1213 .sp
1214 An opening square bracket introduces a character class, terminated by a closing
1215 square bracket. A closing square bracket on its own is not special by default.
1216 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1217 bracket causes a compile-time error. If a closing square bracket is required as
1218 a member of the class, it should be the first data character in the class
1219 (after an initial circumflex, if present) or escaped with a backslash.
1220 .P
1221 A character class matches a single character in the subject. In a UTF mode, the
1222 character may be more than one data unit long. A matched character must be in
1223 the set of characters defined by the class, unless the first character in the
1224 class definition is a circumflex, in which case the subject character must not
1225 be in the set defined by the class. If a circumflex is actually required as a
1226 member of the class, ensure it is not the first character, or escape it with a
1227 backslash.
1228 .P
1229 For example, the character class [aeiou] matches any lower case vowel, while
1230 [^aeiou] matches any character that is not a lower case vowel. Note that a
1231 circumflex is just a convenient notation for specifying the characters that
1232 are in the class by enumerating those that are not. A class that starts with a
1233 circumflex is not an assertion; it still consumes a character from the subject
1234 string, and therefore it fails if the current pointer is at the end of the
1235 string.
1236 .P
1237 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1238 can be included in a class as a literal string of data units, or by using the
1239 \ex{ escaping mechanism.
1240 .P
1241 When caseless matching is set, any letters in a class represent both their
1242 upper case and lower case versions, so for example, a caseless [aeiou] matches
1243 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1244 caseful version would. In a UTF mode, PCRE always understands the concept of
1245 case for characters whose values are less than 128, so caseless matching is
1246 always possible. For characters with higher values, the concept of case is
1247 supported if PCRE is compiled with Unicode property support, but not otherwise.
1248 If you want to use caseless matching in a UTF mode for characters 128 and
1249 above, you must ensure that PCRE is compiled with Unicode property support as
1250 well as with UTF support.
1251 .P
1252 Characters that might indicate line breaks are never treated in any special way
1253 when matching character classes, whatever line-ending sequence is in use, and
1254 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1255 such as [^a] always matches one of these characters.
1256 .P
1257 The minus (hyphen) character can be used to specify a range of characters in a
1258 character class. For example, [d-m] matches any letter between d and m,
1259 inclusive. If a minus character is required in a class, it must be escaped with
1260 a backslash or appear in a position where it cannot be interpreted as
1261 indicating a range, typically as the first or last character in the class, or
1262 immediately after a range. For example, [b-d-z] matches letters in the range b
1263 to d, a hyphen character, or z.
1264 .P
1265 It is not possible to have the literal character "]" as the end character of a
1266 range. A pattern such as [W-]46] is interpreted as a class of two characters
1267 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1268 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1269 the end of range, so [W-\e]46] is interpreted as a class containing a range
1270 followed by two other characters. The octal or hexadecimal representation of
1271 "]" can also be used to end a range.
1272 .P
1273 An error is generated if a POSIX character class (see below) or an escape
1274 sequence other than one that defines a single character appears at a point
1275 where a range ending character is expected. For example, [z-\exff] is valid,
1276 but [A-\ed] and [A-[:digit:]] are not.
1277 .P
1278 Ranges operate in the collating sequence of character values. They can also be
1279 used for characters specified numerically, for example [\e000-\e037]. Ranges
1280 can include any characters that are valid for the current mode.
1281 .P
1282 If a range that includes letters is used when caseless matching is set, it
1283 matches the letters in either case. For example, [W-c] is equivalent to
1284 [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1285 tables for a French locale are in use, [\exc8-\excb] matches accented E
1286 characters in both cases. In UTF modes, PCRE supports the concept of case for
1287 characters with values greater than 128 only when it is compiled with Unicode
1288 property support.
1289 .P
1290 The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1291 \eV, \ew, and \eW may appear in a character class, and add the characters that
1292 they match to the class. For example, [\edABCDEF] matches any hexadecimal
1293 digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1294 and their upper case partners, just as it does when they appear outside a
1295 character class, as described in the section entitled
1296 .\" HTML <a href="#genericchartypes">
1297 .\" </a>
1298 "Generic character types"
1299 .\"
1300 above. The escape sequence \eb has a different meaning inside a character
1301 class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1302 are not special inside a character class. Like any other unrecognized escape
1303 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1304 default, but cause an error if the PCRE_EXTRA option is set.
1305 .P
1306 A circumflex can conveniently be used with the upper case character types to
1307 specify a more restricted set of characters than the matching lower case type.
1308 For example, the class [^\eW_] matches any letter or digit, but not underscore,
1309 whereas [\ew] includes underscore. A positive character class should be read as
1310 "something OR something OR ..." and a negative class as "NOT something AND NOT
1311 something AND NOT ...".
1312 .P
1313 The only metacharacters that are recognized in character classes are backslash,
1314 hyphen (only where it can be interpreted as specifying a range), circumflex
1315 (only at the start), opening square bracket (only when it can be interpreted as
1316 introducing a POSIX class name, or for a special compatibility feature - see
1317 the next two sections), and the terminating closing square bracket. However,
1318 escaping other non-alphanumeric characters does no harm.
1319 .
1320 .
1321 .SH "POSIX CHARACTER CLASSES"
1322 .rs
1323 .sp
1324 Perl supports the POSIX notation for character classes. This uses names
1325 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1326 this notation. For example,
1327 .sp
1328 [01[:alpha:]%]
1329 .sp
1330 matches "0", "1", any alphabetic character, or "%". The supported class names
1331 are:
1332 .sp
1333 alnum letters and digits
1334 alpha letters
1335 ascii character codes 0 - 127
1336 blank space or tab only
1337 cntrl control characters
1338 digit decimal digits (same as \ed)
1339 graph printing characters, excluding space
1340 lower lower case letters
1341 print printing characters, including space
1342 punct printing characters, excluding letters and digits and space
1343 space white space (the same as \es from PCRE 8.34)
1344 upper upper case letters
1345 word "word" characters (same as \ew)
1346 xdigit hexadecimal digits
1347 .sp
1348 The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
1349 and space (32). If locale-specific matching is taking place, the list of space
1350 characters may be different; there may be fewer or more of them. "Space" used
1351 to be different to \es, which did not include VT, for Perl compatibility.
1352 However, Perl changed at release 5.18, and PCRE followed at release 8.34.
1353 "Space" and \es now match the same set of characters.
1354 .P
1355 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1356 5.8. Another Perl extension is negation, which is indicated by a ^ character
1357 after the colon. For example,
1358 .sp
1359 [12[:^digit:]]
1360 .sp
1361 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1362 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1363 supported, and an error is given if they are encountered.
1364 .P
1365 By default, characters with values greater than 128 do not match any of the
1366 POSIX character classes. However, if the PCRE_UCP option is passed to
1367 \fBpcre_compile()\fP, some of the classes are changed so that Unicode character
1368 properties are used. This is achieved by replacing certain POSIX classes by
1369 other sequences, as follows:
1370 .sp
1371 [:alnum:] becomes \ep{Xan}
1372 [:alpha:] becomes \ep{L}
1373 [:blank:] becomes \eh
1374 [:digit:] becomes \ep{Nd}
1375 [:lower:] becomes \ep{Ll}
1376 [:space:] becomes \ep{Xps}
1377 [:upper:] becomes \ep{Lu}
1378 [:word:] becomes \ep{Xwd}
1379 .sp
1380 Negated versions, such as [:^alpha:] use \eP instead of \ep. Three other POSIX
1381 classes are handled specially in UCP mode:
1382 .TP 10
1383 [:graph:]
1384 This matches characters that have glyphs that mark the page when printed. In
1385 Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
1386 properties, except for:
1387 .sp
1388 U+061C Arabic Letter Mark
1389 U+180E Mongolian Vowel Separator
1390 U+2066 - U+2069 Various "isolate"s
1391 .sp
1392 .TP 10
1393 [:print:]
1394 This matches the same characters as [:graph:] plus space characters that are
1395 not controls, that is, characters with the Zs property.
1396 .TP 10
1397 [:punct:]
1398 This matches all characters that have the Unicode P (punctuation) property,
1399 plus those characters whose code points are less than 128 that have the S
1400 (Symbol) property.
1401 .P
1402 The other POSIX classes are unchanged, and match only characters with code
1403 points less than 128.
1404 .
1405 .
1406 .SH "COMPATIBILITY FEATURE FOR WORD BOUNDARIES"
1407 .rs
1408 .sp
1409 In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly
1410 syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of
1411 word". PCRE treats these items as follows:
1412 .sp
1413 [[:<:]] is converted to \eb(?=\ew)
1414 [[:>:]] is converted to \eb(?<=\ew)
1415 .sp
1416 Only these exact character sequences are recognized. A sequence such as
1417 [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is
1418 not compatible with Perl. It is provided to help migrations from other
1419 environments, and is best not used in any new patterns. Note that \eb matches
1420 at the start and the end of a word (see
1421 .\" HTML <a href="#smallassertions">
1422 .\" </a>
1423 "Simple assertions"
1424 .\"
1425 above), and in a Perl-style pattern the preceding or following character
1426 normally shows which is wanted, without the need for the assertions that are
1427 used above in order to give exactly the POSIX behaviour.
1428 .
1429 .
1430 .SH "VERTICAL BAR"
1431 .rs
1432 .sp
1433 Vertical bar characters are used to separate alternative patterns. For example,
1434 the pattern
1435 .sp
1436 gilbert|sullivan
1437 .sp
1438 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1439 and an empty alternative is permitted (matching the empty string). The matching
1440 process tries each alternative in turn, from left to right, and the first one
1441 that succeeds is used. If the alternatives are within a subpattern
1442 .\" HTML <a href="#subpattern">
1443 .\" </a>
1444 (defined below),
1445 .\"
1446 "succeeds" means matching the rest of the main pattern as well as the
1447 alternative in the subpattern.
1448 .
1449 .
1450 .SH "INTERNAL OPTION SETTING"
1451 .rs
1452 .sp
1453 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1454 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1455 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1456 The option letters are
1457 .sp
1458 i for PCRE_CASELESS
1459 m for PCRE_MULTILINE
1460 s for PCRE_DOTALL
1461 x for PCRE_EXTENDED
1462 .sp
1463 For example, (?im) sets caseless, multiline matching. It is also possible to
1464 unset these options by preceding the letter with a hyphen, and a combined
1465 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1466 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1467 permitted. If a letter appears both before and after the hyphen, the option is
1468 unset.
1469 .P
1470 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1471 changed in the same way as the Perl-compatible options by using the characters
1472 J, U and X respectively.
1473 .P
1474 When one of these option changes occurs at top level (that is, not inside
1475 subpattern parentheses), the change applies to the remainder of the pattern
1476 that follows. If the change is placed right at the start of a pattern, PCRE
1477 extracts it into the global options (and it will therefore show up in data
1478 extracted by the \fBpcre_fullinfo()\fP function).
1479 .P
1480 An option change within a subpattern (see below for a description of
1481 subpatterns) affects only that part of the subpattern that follows it, so
1482 .sp
1483 (a(?i)b)c
1484 .sp
1485 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1486 By this means, options can be made to have different settings in different
1487 parts of the pattern. Any changes made in one alternative do carry on
1488 into subsequent branches within the same subpattern. For example,
1489 .sp
1490 (a(?i)b|c)
1491 .sp
1492 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1493 branch is abandoned before the option setting. This is because the effects of
1494 option settings happen at compile time. There would be some very weird
1495 behaviour otherwise.
1496 .P
1497 \fBNote:\fP There are other PCRE-specific options that can be set by the
1498 application when the compiling or matching functions are called. In some cases
1499 the pattern can contain special leading sequences such as (*CRLF) to override
1500 what the application has set or what has been defaulted. Details are given in
1501 the section entitled
1502 .\" HTML <a href="#newlineseq">
1503 .\" </a>
1504 "Newline sequences"
1505 .\"
1506 above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1507 sequences that can be used to set UTF and Unicode property modes; they are
1508 equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1509 options, respectively. The (*UTF) sequence is a generic version that can be
1510 used with any of the libraries. However, the application can set the
1511 PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1512 .
1513 .
1514 .\" HTML <a name="subpattern"></a>
1515 .SH SUBPATTERNS
1516 .rs
1517 .sp
1518 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1519 Turning part of a pattern into a subpattern does two things:
1520 .sp
1521 1. It localizes a set of alternatives. For example, the pattern
1522 .sp
1523 cat(aract|erpillar|)
1524 .sp
1525 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1526 match "cataract", "erpillar" or an empty string.
1527 .sp
1528 2. It sets up the subpattern as a capturing subpattern. This means that, when
1529 the whole pattern matches, that portion of the subject string that matched the
1530 subpattern is passed back to the caller via the \fIovector\fP argument of the
1531 matching function. (This applies only to the traditional matching functions;
1532 the DFA matching functions do not support capturing.)
1533 .P
1534 Opening parentheses are counted from left to right (starting from 1) to obtain
1535 numbers for the capturing subpatterns. For example, if the string "the red
1536 king" is matched against the pattern
1537 .sp
1538 the ((red|white) (king|queen))
1539 .sp
1540 the captured substrings are "red king", "red", and "king", and are numbered 1,
1541 2, and 3, respectively.
1542 .P
1543 The fact that plain parentheses fulfil two functions is not always helpful.
1544 There are often times when a grouping subpattern is required without a
1545 capturing requirement. If an opening parenthesis is followed by a question mark
1546 and a colon, the subpattern does not do any capturing, and is not counted when
1547 computing the number of any subsequent capturing subpatterns. For example, if
1548 the string "the white queen" is matched against the pattern
1549 .sp
1550 the ((?:red|white) (king|queen))
1551 .sp
1552 the captured substrings are "white queen" and "queen", and are numbered 1 and
1553 2. The maximum number of capturing subpatterns is 65535.
1554 .P
1555 As a convenient shorthand, if any option settings are required at the start of
1556 a non-capturing subpattern, the option letters may appear between the "?" and
1557 the ":". Thus the two patterns
1558 .sp
1559 (?i:saturday|sunday)
1560 (?:(?i)saturday|sunday)
1561 .sp
1562 match exactly the same set of strings. Because alternative branches are tried
1563 from left to right, and options are not reset until the end of the subpattern
1564 is reached, an option setting in one branch does affect subsequent branches, so
1565 the above patterns match "SUNDAY" as well as "Saturday".
1566 .
1567 .
1568 .\" HTML <a name="dupsubpatternnumber"></a>
1569 .SH "DUPLICATE SUBPATTERN NUMBERS"
1570 .rs
1571 .sp
1572 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1573 the same numbers for its capturing parentheses. Such a subpattern starts with
1574 (?| and is itself a non-capturing subpattern. For example, consider this
1575 pattern:
1576 .sp
1577 (?|(Sat)ur|(Sun))day
1578 .sp
1579 Because the two alternatives are inside a (?| group, both sets of capturing
1580 parentheses are numbered one. Thus, when the pattern matches, you can look
1581 at captured substring number one, whichever alternative matched. This construct
1582 is useful when you want to capture part, but not all, of one of a number of
1583 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1584 number is reset at the start of each branch. The numbers of any capturing
1585 parentheses that follow the subpattern start after the highest number used in
1586 any branch. The following example is taken from the Perl documentation. The
1587 numbers underneath show in which buffer the captured content will be stored.
1588 .sp
1589 # before ---------------branch-reset----------- after
1590 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1591 # 1 2 2 3 2 3 4
1592 .sp
1593 A back reference to a numbered subpattern uses the most recent value that is
1594 set for that number by any subpattern. The following pattern matches "abcabc"
1595 or "defdef":
1596 .sp
1597 /(?|(abc)|(def))\e1/
1598 .sp
1599 In contrast, a subroutine call to a numbered subpattern always refers to the
1600 first one in the pattern with the given number. The following pattern matches
1601 "abcabc" or "defabc":
1602 .sp
1603 /(?|(abc)|(def))(?1)/
1604 .sp
1605 If a
1606 .\" HTML <a href="#conditions">
1607 .\" </a>
1608 condition test
1609 .\"
1610 for a subpattern's having matched refers to a non-unique number, the test is
1611 true if any of the subpatterns of that number have matched.
1612 .P
1613 An alternative approach to using this "branch reset" feature is to use
1614 duplicate named subpatterns, as described in the next section.
1615 .
1616 .
1617 .SH "NAMED SUBPATTERNS"
1618 .rs
1619 .sp
1620 Identifying capturing parentheses by number is simple, but it can be very hard
1621 to keep track of the numbers in complicated regular expressions. Furthermore,
1622 if an expression is modified, the numbers may change. To help with this
1623 difficulty, PCRE supports the naming of subpatterns. This feature was not
1624 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1625 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1626 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1627 have different names, but PCRE does not.
1628 .P
1629 In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1630 (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1631 parentheses from other parts of the pattern, such as
1632 .\" HTML <a href="#backreferences">
1633 .\" </a>
1634 back references,
1635 .\"
1636 .\" HTML <a href="#recursion">
1637 .\" </a>
1638 recursion,
1639 .\"
1640 and
1641 .\" HTML <a href="#conditions">
1642 .\" </a>
1643 conditions,
1644 .\"
1645 can be made by name as well as by number.
1646 .P
1647 Names consist of up to 32 alphanumeric characters and underscores, but must
1648 start with a non-digit. Named capturing parentheses are still allocated numbers
1649 as well as names, exactly as if the names were not present. The PCRE API
1650 provides function calls for extracting the name-to-number translation table
1651 from a compiled pattern. There is also a convenience function for extracting a
1652 captured substring by name.
1653 .P
1654 By default, a name must be unique within a pattern, but it is possible to relax
1655 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1656 names are also always permitted for subpatterns with the same number, set up as
1657 described in the previous section.) Duplicate names can be useful for patterns
1658 where only one instance of the named parentheses can match. Suppose you want to
1659 match the name of a weekday, either as a 3-letter abbreviation or as the full
1660 name, and in both cases you want to extract the abbreviation. This pattern
1661 (ignoring the line breaks) does the job:
1662 .sp
1663 (?<DN>Mon|Fri|Sun)(?:day)?|
1664 (?<DN>Tue)(?:sday)?|
1665 (?<DN>Wed)(?:nesday)?|
1666 (?<DN>Thu)(?:rsday)?|
1667 (?<DN>Sat)(?:urday)?
1668 .sp
1669 There are five capturing substrings, but only one is ever set after a match.
1670 (An alternative way of solving this problem is to use a "branch reset"
1671 subpattern, as described in the previous section.)
1672 .P
1673 The convenience function for extracting the data by name returns the substring
1674 for the first (and in this example, the only) subpattern of that name that
1675 matched. This saves searching to find which numbered subpattern it was.
1676 .P
1677 If you make a back reference to a non-unique named subpattern from elsewhere in
1678 the pattern, the subpatterns to which the name refers are checked in the order
1679 in which they appear in the overall pattern. The first one that is set is used
1680 for the reference. For example, this pattern matches both "foofoo" and
1681 "barbar" but not "foobar" or "barfoo":
1682 .sp
1683 (?:(?<n>foo)|(?<n>bar))\ek<n>
1684 .sp
1685 .P
1686 If you make a subroutine call to a non-unique named subpattern, the one that
1687 corresponds to the first occurrence of the name is used. In the absence of
1688 duplicate numbers (see the previous section) this is the one with the lowest
1689 number.
1690 .P
1691 If you use a named reference in a condition
1692 test (see the
1693 .\"
1694 .\" HTML <a href="#conditions">
1695 .\" </a>
1696 section about conditions
1697 .\"
1698 below), either to check whether a subpattern has matched, or to check for
1699 recursion, all subpatterns with the same name are tested. If the condition is
1700 true for any one of them, the overall condition is true. This is the same
1701 behaviour as testing by number. For further details of the interfaces for
1702 handling named subpatterns, see the
1703 .\" HREF
1704 \fBpcreapi\fP
1705 .\"
1706 documentation.
1707 .P
1708 \fBWarning:\fP You cannot use different names to distinguish between two
1709 subpatterns with the same number because PCRE uses only the numbers when
1710 matching. For this reason, an error is given at compile time if different names
1711 are given to subpatterns with the same number. However, you can always give the
1712 same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1713 set.
1714 .
1715 .
1716 .SH REPETITION
1717 .rs
1718 .sp
1719 Repetition is specified by quantifiers, which can follow any of the following
1720 items:
1721 .sp
1722 a literal data character
1723 the dot metacharacter
1724 the \eC escape sequence
1725 the \eX escape sequence
1726 the \eR escape sequence
1727 an escape such as \ed or \epL that matches a single character
1728 a character class
1729 a back reference (see next section)
1730 a parenthesized subpattern (including assertions)
1731 a subroutine call to a subpattern (recursive or otherwise)
1732 .sp
1733 The general repetition quantifier specifies a minimum and maximum number of
1734 permitted matches, by giving the two numbers in curly brackets (braces),
1735 separated by a comma. The numbers must be less than 65536, and the first must
1736 be less than or equal to the second. For example:
1737 .sp
1738 z{2,4}
1739 .sp
1740 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1741 character. If the second number is omitted, but the comma is present, there is
1742 no upper limit; if the second number and the comma are both omitted, the
1743 quantifier specifies an exact number of required matches. Thus
1744 .sp
1745 [aeiou]{3,}
1746 .sp
1747 matches at least 3 successive vowels, but may match many more, while
1748 .sp
1749 \ed{8}
1750 .sp
1751 matches exactly 8 digits. An opening curly bracket that appears in a position
1752 where a quantifier is not allowed, or one that does not match the syntax of a
1753 quantifier, is taken as a literal character. For example, {,6} is not a
1754 quantifier, but a literal string of four characters.
1755 .P
1756 In UTF modes, quantifiers apply to characters rather than to individual data
1757 units. Thus, for example, \ex{100}{2} matches two characters, each of
1758 which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1759 \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1760 several data units long (and they may be of different lengths).
1761 .P
1762 The quantifier {0} is permitted, causing the expression to behave as if the
1763 previous item and the quantifier were not present. This may be useful for
1764 subpatterns that are referenced as
1765 .\" HTML <a href="#subpatternsassubroutines">
1766 .\" </a>
1767 subroutines
1768 .\"
1769 from elsewhere in the pattern (but see also the section entitled
1770 .\" HTML <a href="#subdefine">
1771 .\" </a>
1772 "Defining subpatterns for use by reference only"
1773 .\"
1774 below). Items other than subpatterns that have a {0} quantifier are omitted
1775 from the compiled pattern.
1776 .P
1777 For convenience, the three most common quantifiers have single-character
1778 abbreviations:
1779 .sp
1780 * is equivalent to {0,}
1781 + is equivalent to {1,}
1782 ? is equivalent to {0,1}
1783 .sp
1784 It is possible to construct infinite loops by following a subpattern that can
1785 match no characters with a quantifier that has no upper limit, for example:
1786 .sp
1787 (a?)*
1788 .sp
1789 Earlier versions of Perl and PCRE used to give an error at compile time for
1790 such patterns. However, because there are cases where this can be useful, such
1791 patterns are now accepted, but if any repetition of the subpattern does in fact
1792 match no characters, the loop is forcibly broken.
1793 .P
1794 By default, the quantifiers are "greedy", that is, they match as much as
1795 possible (up to the maximum number of permitted times), without causing the
1796 rest of the pattern to fail. The classic example of where this gives problems
1797 is in trying to match comments in C programs. These appear between /* and */
1798 and within the comment, individual * and / characters may appear. An attempt to
1799 match C comments by applying the pattern
1800 .sp
1801 /\e*.*\e*/
1802 .sp
1803 to the string
1804 .sp
1805 /* first comment */ not comment /* second comment */
1806 .sp
1807 fails, because it matches the entire string owing to the greediness of the .*
1808 item.
1809 .P
1810 However, if a quantifier is followed by a question mark, it ceases to be
1811 greedy, and instead matches the minimum number of times possible, so the
1812 pattern
1813 .sp
1814 /\e*.*?\e*/
1815 .sp
1816 does the right thing with the C comments. The meaning of the various
1817 quantifiers is not otherwise changed, just the preferred number of matches.
1818 Do not confuse this use of question mark with its use as a quantifier in its
1819 own right. Because it has two uses, it can sometimes appear doubled, as in
1820 .sp
1821 \ed??\ed
1822 .sp
1823 which matches one digit by preference, but can match two if that is the only
1824 way the rest of the pattern matches.
1825 .P
1826 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1827 the quantifiers are not greedy by default, but individual ones can be made
1828 greedy by following them with a question mark. In other words, it inverts the
1829 default behaviour.
1830 .P
1831 When a parenthesized subpattern is quantified with a minimum repeat count that
1832 is greater than 1 or with a limited maximum, more memory is required for the
1833 compiled pattern, in proportion to the size of the minimum or maximum.
1834 .P
1835 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1836 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1837 implicitly anchored, because whatever follows will be tried against every
1838 character position in the subject string, so there is no point in retrying the
1839 overall match at any position after the first. PCRE normally treats such a
1840 pattern as though it were preceded by \eA.
1841 .P
1842 In cases where it is known that the subject string contains no newlines, it is
1843 worth setting PCRE_DOTALL in order to obtain this optimization, or
1844 alternatively using ^ to indicate anchoring explicitly.
1845 .P
1846 However, there are some cases where the optimization cannot be used. When .*
1847 is inside capturing parentheses that are the subject of a back reference
1848 elsewhere in the pattern, a match at the start may fail where a later one
1849 succeeds. Consider, for example:
1850 .sp
1851 (.*)abc\e1
1852 .sp
1853 If the subject is "xyz123abc123" the match point is the fourth character. For
1854 this reason, such a pattern is not implicitly anchored.
1855 .P
1856 Another case where implicit anchoring is not applied is when the leading .* is
1857 inside an atomic group. Once again, a match at the start may fail where a later
1858 one succeeds. Consider this pattern:
1859 .sp
1860 (?>.*?a)b
1861 .sp
1862 It matches "ab" in the subject "aab". The use of the backtracking control verbs
1863 (*PRUNE) and (*SKIP) also disable this optimization.
1864 .P
1865 When a capturing subpattern is repeated, the value captured is the substring
1866 that matched the final iteration. For example, after
1867 .sp
1868 (tweedle[dume]{3}\es*)+
1869 .sp
1870 has matched "tweedledum tweedledee" the value of the captured substring is
1871 "tweedledee". However, if there are nested capturing subpatterns, the
1872 corresponding captured values may have been set in previous iterations. For
1873 example, after
1874 .sp
1875 /(a|(b))+/
1876 .sp
1877 matches "aba" the value of the second captured substring is "b".
1878 .
1879 .
1880 .\" HTML <a name="atomicgroup"></a>
1881 .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1882 .rs
1883 .sp
1884 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1885 repetition, failure of what follows normally causes the repeated item to be
1886 re-evaluated to see if a different number of repeats allows the rest of the
1887 pattern to match. Sometimes it is useful to prevent this, either to change the
1888 nature of the match, or to cause it fail earlier than it otherwise might, when
1889 the author of the pattern knows there is no point in carrying on.
1890 .P
1891 Consider, for example, the pattern \ed+foo when applied to the subject line
1892 .sp
1893 123456bar
1894 .sp
1895 After matching all 6 digits and then failing to match "foo", the normal
1896 action of the matcher is to try again with only 5 digits matching the \ed+
1897 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1898 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1899 that once a subpattern has matched, it is not to be re-evaluated in this way.
1900 .P
1901 If we use atomic grouping for the previous example, the matcher gives up
1902 immediately on failing to match "foo" the first time. The notation is a kind of
1903 special parenthesis, starting with (?> as in this example:
1904 .sp
1905 (?>\ed+)foo
1906 .sp
1907 This kind of parenthesis "locks up" the part of the pattern it contains once
1908 it has matched, and a failure further into the pattern is prevented from
1909 backtracking into it. Backtracking past it to previous items, however, works as
1910 normal.
1911 .P
1912 An alternative description is that a subpattern of this type matches the string
1913 of characters that an identical standalone pattern would match, if anchored at
1914 the current point in the subject string.
1915 .P
1916 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1917 the above example can be thought of as a maximizing repeat that must swallow
1918 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1919 number of digits they match in order to make the rest of the pattern match,
1920 (?>\ed+) can only match an entire sequence of digits.
1921 .P
1922 Atomic groups in general can of course contain arbitrarily complicated
1923 subpatterns, and can be nested. However, when the subpattern for an atomic
1924 group is just a single repeated item, as in the example above, a simpler
1925 notation, called a "possessive quantifier" can be used. This consists of an
1926 additional + character following a quantifier. Using this notation, the
1927 previous example can be rewritten as
1928 .sp
1929 \ed++foo
1930 .sp
1931 Note that a possessive quantifier can be used with an entire group, for
1932 example:
1933 .sp
1934 (abc|xyz){2,3}+
1935 .sp
1936 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1937 option is ignored. They are a convenient notation for the simpler forms of
1938 atomic group. However, there is no difference in the meaning of a possessive
1939 quantifier and the equivalent atomic group, though there may be a performance
1940 difference; possessive quantifiers should be slightly faster.
1941 .P
1942 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1943 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1944 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1945 package, and PCRE copied it from there. It ultimately found its way into Perl
1946 at release 5.10.
1947 .P
1948 PCRE has an optimization that automatically "possessifies" certain simple
1949 pattern constructs. For example, the sequence A+B is treated as A++B because
1950 there is no point in backtracking into a sequence of A's when B must follow.
1951 .P
1952 When a pattern contains an unlimited repeat inside a subpattern that can itself
1953 be repeated an unlimited number of times, the use of an atomic group is the
1954 only way to avoid some failing matches taking a very long time indeed. The
1955 pattern
1956 .sp
1957 (\eD+|<\ed+>)*[!?]
1958 .sp
1959 matches an unlimited number of substrings that either consist of non-digits, or
1960 digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1961 quickly. However, if it is applied to
1962 .sp
1963 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1964 .sp
1965 it takes a long time before reporting failure. This is because the string can
1966 be divided between the internal \eD+ repeat and the external * repeat in a
1967 large number of ways, and all have to be tried. (The example uses [!?] rather
1968 than a single character at the end, because both PCRE and Perl have an
1969 optimization that allows for fast failure when a single character is used. They
1970 remember the last single character that is required for a match, and fail early
1971 if it is not present in the string.) If the pattern is changed so that it uses
1972 an atomic group, like this:
1973 .sp
1974 ((?>\eD+)|<\ed+>)*[!?]
1975 .sp
1976 sequences of non-digits cannot be broken, and failure happens quickly.
1977 .
1978 .
1979 .\" HTML <a name="backreferences"></a>
1980 .SH "BACK REFERENCES"
1981 .rs
1982 .sp
1983 Outside a character class, a backslash followed by a digit greater than 0 (and
1984 possibly further digits) is a back reference to a capturing subpattern earlier
1985 (that is, to its left) in the pattern, provided there have been that many
1986 previous capturing left parentheses.
1987 .P
1988 However, if the decimal number following the backslash is less than 10, it is
1989 always taken as a back reference, and causes an error only if there are not
1990 that many capturing left parentheses in the entire pattern. In other words, the
1991 parentheses that are referenced need not be to the left of the reference for
1992 numbers less than 10. A "forward back reference" of this type can make sense
1993 when a repetition is involved and the subpattern to the right has participated
1994 in an earlier iteration.
1995 .P
1996 It is not possible to have a numerical "forward back reference" to a subpattern
1997 whose number is 10 or more using this syntax because a sequence such as \e50 is
1998 interpreted as a character defined in octal. See the subsection entitled
1999 "Non-printing characters"
2000 .\" HTML <a href="#digitsafterbackslash">
2001 .\" </a>
2002 above
2003 .\"
2004 for further details of the handling of digits following a backslash. There is
2005 no such problem when named parentheses are used. A back reference to any
2006 subpattern is possible using named parentheses (see below).
2007 .P
2008 Another way of avoiding the ambiguity inherent in the use of digits following a
2009 backslash is to use the \eg escape sequence. This escape must be followed by an
2010 unsigned number or a negative number, optionally enclosed in braces. These
2011 examples are all identical:
2012 .sp
2013 (ring), \e1
2014 (ring), \eg1
2015 (ring), \eg{1}
2016 .sp
2017 An unsigned number specifies an absolute reference without the ambiguity that
2018 is present in the older syntax. It is also useful when literal digits follow
2019 the reference. A negative number is a relative reference. Consider this
2020 example:
2021 .sp
2022 (abc(def)ghi)\eg{-1}
2023 .sp
2024 The sequence \eg{-1} is a reference to the most recently started capturing
2025 subpattern before \eg, that is, is it equivalent to \e2 in this example.
2026 Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
2027 can be helpful in long patterns, and also in patterns that are created by
2028 joining together fragments that contain references within themselves.
2029 .P
2030 A back reference matches whatever actually matched the capturing subpattern in
2031 the current subject string, rather than anything matching the subpattern
2032 itself (see
2033 .\" HTML <a href="#subpatternsassubroutines">
2034 .\" </a>
2035 "Subpatterns as subroutines"
2036 .\"
2037 below for a way of doing that). So the pattern
2038 .sp
2039 (sens|respons)e and \e1ibility
2040 .sp
2041 matches "sense and sensibility" and "response and responsibility", but not
2042 "sense and responsibility". If caseful matching is in force at the time of the
2043 back reference, the case of letters is relevant. For example,
2044 .sp
2045 ((?i)rah)\es+\e1
2046 .sp
2047 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
2048 capturing subpattern is matched caselessly.
2049 .P
2050 There are several different ways of writing back references to named
2051 subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
2052 \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
2053 back reference syntax, in which \eg can be used for both numeric and named
2054 references, is also supported. We could rewrite the above example in any of
2055 the following ways:
2056 .sp
2057 (?<p1>(?i)rah)\es+\ek<p1>
2058 (?'p1'(?i)rah)\es+\ek{p1}
2059 (?P<p1>(?i)rah)\es+(?P=p1)
2060 (?<p1>(?i)rah)\es+\eg{p1}
2061 .sp
2062 A subpattern that is referenced by name may appear in the pattern before or
2063 after the reference.
2064 .P
2065 There may be more than one back reference to the same subpattern. If a
2066 subpattern has not actually been used in a particular match, any back
2067 references to it always fail by default. For example, the pattern
2068 .sp
2069 (a|(bc))\e2
2070 .sp
2071 always fails if it starts to match "a" rather than "bc". However, if the
2072 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
2073 unset value matches an empty string.
2074 .P
2075 Because there may be many capturing parentheses in a pattern, all digits
2076 following a backslash are taken as part of a potential back reference number.
2077 If the pattern continues with a digit character, some delimiter must be used to
2078 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
2079 white space. Otherwise, the \eg{ syntax or an empty comment (see
2080 .\" HTML <a href="#comments">
2081 .\" </a>
2082 "Comments"
2083 .\"
2084 below) can be used.
2085 .
2086 .SS "Recursive back references"
2087 .rs
2088 .sp
2089 A back reference that occurs inside the parentheses to which it refers fails
2090 when the subpattern is first used, so, for example, (a\e1) never matches.
2091 However, such references can be useful inside repeated subpatterns. For
2092 example, the pattern
2093 .sp
2094 (a|b\e1)+
2095 .sp
2096 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
2097 the subpattern, the back reference matches the character string corresponding
2098 to the previous iteration. In order for this to work, the pattern must be such
2099 that the first iteration does not need to match the back reference. This can be
2100 done using alternation, as in the example above, or by a quantifier with a
2101 minimum of zero.
2102 .P
2103 Back references of this type cause the group that they reference to be treated
2104 as an
2105 .\" HTML <a href="#atomicgroup">
2106 .\" </a>
2107 atomic group.
2108 .\"
2109 Once the whole group has been matched, a subsequent matching failure cannot
2110 cause backtracking into the middle of the group.
2111 .
2112 .
2113 .\" HTML <a name="bigassertions"></a>
2114 .SH ASSERTIONS
2115 .rs
2116 .sp
2117 An assertion is a test on the characters following or preceding the current
2118 matching point that does not actually consume any characters. The simple
2119 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
2120 .\" HTML <a href="#smallassertions">
2121 .\" </a>
2122 above.
2123 .\"
2124 .P
2125 More complicated assertions are coded as subpatterns. There are two kinds:
2126 those that look ahead of the current position in the subject string, and those
2127 that look behind it. An assertion subpattern is matched in the normal way,
2128 except that it does not cause the current matching position to be changed.
2129 .P
2130 Assertion subpatterns are not capturing subpatterns. If such an assertion
2131 contains capturing subpatterns within it, these are counted for the purposes of
2132 numbering the capturing subpatterns in the whole pattern. However, substring
2133 capturing is carried out only for positive assertions. (Perl sometimes, but not
2134 always, does do capturing in negative assertions.)
2135 .P
2136 For compatibility with Perl, assertion subpatterns may be repeated; though
2137 it makes no sense to assert the same thing several times, the side effect of
2138 capturing parentheses may occasionally be useful. In practice, there only three
2139 cases:
2140 .sp
2141 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2142 However, it may contain internal capturing parenthesized groups that are called
2143 from elsewhere via the
2144 .\" HTML <a href="#subpatternsassubroutines">
2145 .\" </a>
2146 subroutine mechanism.
2147 .\"
2148 .sp
2149 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2150 were {0,1}. At run time, the rest of the pattern match is tried with and
2151 without the assertion, the order depending on the greediness of the quantifier.
2152 .sp
2153 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2154 The assertion is obeyed just once when encountered during matching.
2155 .
2156 .
2157 .SS "Lookahead assertions"
2158 .rs
2159 .sp
2160 Lookahead assertions start with (?= for positive assertions and (?! for
2161 negative assertions. For example,
2162 .sp
2163 \ew+(?=;)
2164 .sp
2165 matches a word followed by a semicolon, but does not include the semicolon in
2166 the match, and
2167 .sp
2168 foo(?!bar)
2169 .sp
2170 matches any occurrence of "foo" that is not followed by "bar". Note that the
2171 apparently similar pattern
2172 .sp
2173 (?!foo)bar
2174 .sp
2175 does not find an occurrence of "bar" that is preceded by something other than
2176 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
2177 (?!foo) is always true when the next three characters are "bar". A
2178 lookbehind assertion is needed to achieve the other effect.
2179 .P
2180 If you want to force a matching failure at some point in a pattern, the most
2181 convenient way to do it is with (?!) because an empty string always matches, so
2182 an assertion that requires there not to be an empty string must always fail.
2183 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2184 .
2185 .
2186 .\" HTML <a name="lookbehind"></a>
2187 .SS "Lookbehind assertions"
2188 .rs
2189 .sp
2190 Lookbehind assertions start with (?<= for positive assertions and (?<! for
2191 negative assertions. For example,
2192 .sp
2193 (?<!foo)bar
2194 .sp
2195 does find an occurrence of "bar" that is not preceded by "foo". The contents of
2196 a lookbehind assertion are restricted such that all the strings it matches must
2197 have a fixed length. However, if there are several top-level alternatives, they
2198 do not all have to have the same fixed length. Thus
2199 .sp
2200 (?<=bullock|donkey)
2201 .sp
2202 is permitted, but
2203 .sp
2204 (?<!dogs?|cats?)
2205 .sp
2206 causes an error at compile time. Branches that match different length strings
2207 are permitted only at the top level of a lookbehind assertion. This is an
2208 extension compared with Perl, which requires all branches to match the same
2209 length of string. An assertion such as
2210 .sp
2211 (?<=ab(c|de))
2212 .sp
2213 is not permitted, because its single top-level branch can match two different
2214 lengths, but it is acceptable to PCRE if rewritten to use two top-level
2215 branches:
2216 .sp
2217 (?<=abc|abde)
2218 .sp
2219 In some cases, the escape sequence \eK
2220 .\" HTML <a href="#resetmatchstart">
2221 .\" </a>
2222 (see above)
2223 .\"
2224 can be used instead of a lookbehind assertion to get round the fixed-length
2225 restriction.
2226 .P
2227 The implementation of lookbehind assertions is, for each alternative, to
2228 temporarily move the current position back by the fixed length and then try to
2229 match. If there are insufficient characters before the current position, the
2230 assertion fails.
2231 .P
2232 In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
2233 unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2234 it impossible to calculate the length of the lookbehind. The \eX and \eR
2235 escapes, which can match different numbers of data units, are also not
2236 permitted.
2237 .P
2238 .\" HTML <a href="#subpatternsassubroutines">
2239 .\" </a>
2240 "Subroutine"
2241 .\"
2242 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2243 as the subpattern matches a fixed-length string.
2244 .\" HTML <a href="#recursion">
2245 .\" </a>
2246 Recursion,
2247 .\"
2248 however, is not supported.
2249 .P
2250 Possessive quantifiers can be used in conjunction with lookbehind assertions to
2251 specify efficient matching of fixed-length strings at the end of subject
2252 strings. Consider a simple pattern such as
2253 .sp
2254 abcd$
2255 .sp
2256 when applied to a long string that does not match. Because matching proceeds
2257 from left to right, PCRE will look for each "a" in the subject and then see if
2258 what follows matches the rest of the pattern. If the pattern is specified as
2259 .sp
2260 ^.*abcd$
2261 .sp
2262 the initial .* matches the entire string at first, but when this fails (because
2263 there is no following "a"), it backtracks to match all but the last character,
2264 then all but the last two characters, and so on. Once again the search for "a"
2265 covers the entire string, from right to left, so we are no better off. However,
2266 if the pattern is written as
2267 .sp
2268 ^.*+(?<=abcd)
2269 .sp
2270 there can be no backtracking for the .*+ item; it can match only the entire
2271 string. The subsequent lookbehind assertion does a single test on the last four
2272 characters. If it fails, the match fails immediately. For long strings, this
2273 approach makes a significant difference to the processing time.
2274 .
2275 .
2276 .SS "Using multiple assertions"
2277 .rs
2278 .sp
2279 Several assertions (of any sort) may occur in succession. For example,
2280 .sp
2281 (?<=\ed{3})(?<!999)foo
2282 .sp
2283 matches "foo" preceded by three digits that are not "999". Notice that each of
2284 the assertions is applied independently at the same point in the subject
2285 string. First there is a check that the previous three characters are all
2286 digits, and then there is a check that the same three characters are not "999".
2287 This pattern does \fInot\fP match "foo" preceded by six characters, the first
2288 of which are digits and the last three of which are not "999". For example, it
2289 doesn't match "123abcfoo". A pattern to do that is
2290 .sp
2291 (?<=\ed{3}...)(?<!999)foo
2292 .sp
2293 This time the first assertion looks at the preceding six characters, checking
2294 that the first three are digits, and then the second assertion checks that the
2295 preceding three characters are not "999".
2296 .P
2297 Assertions can be nested in any combination. For example,
2298 .sp
2299 (?<=(?<!foo)bar)baz
2300 .sp
2301 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2302 preceded by "foo", while
2303 .sp
2304 (?<=\ed{3}(?!999)...)foo
2305 .sp
2306 is another pattern that matches "foo" preceded by three digits and any three
2307 characters that are not "999".
2308 .
2309 .
2310 .\" HTML <a name="conditions"></a>
2311 .SH "CONDITIONAL SUBPATTERNS"
2312 .rs
2313 .sp
2314 It is possible to cause the matching process to obey a subpattern
2315 conditionally or to choose between two alternative subpatterns, depending on
2316 the result of an assertion, or whether a specific capturing subpattern has
2317 already been matched. The two possible forms of conditional subpattern are:
2318 .sp
2319 (?(condition)yes-pattern)
2320 (?(condition)yes-pattern|no-pattern)
2321 .sp
2322 If the condition is satisfied, the yes-pattern is used; otherwise the
2323 no-pattern (if present) is used. If there are more than two alternatives in the
2324 subpattern, a compile-time error occurs. Each of the two alternatives may
2325 itself contain nested subpatterns of any form, including conditional
2326 subpatterns; the restriction to two alternatives applies only at the level of
2327 the condition. This pattern fragment is an example where the alternatives are
2328 complex:
2329 .sp
2330 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2331 .sp
2332 .P
2333 There are four kinds of condition: references to subpatterns, references to
2334 recursion, a pseudo-condition called DEFINE, and assertions.
2335 .
2336 .SS "Checking for a used subpattern by number"
2337 .rs
2338 .sp
2339 If the text between the parentheses consists of a sequence of digits, the
2340 condition is true if a capturing subpattern of that number has previously
2341 matched. If there is more than one capturing subpattern with the same number
2342 (see the earlier
2343 .\"
2344 .\" HTML <a href="#recursion">
2345 .\" </a>
2346 section about duplicate subpattern numbers),
2347 .\"
2348 the condition is true if any of them have matched. An alternative notation is
2349 to precede the digits with a plus or minus sign. In this case, the subpattern
2350 number is relative rather than absolute. The most recently opened parentheses
2351 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2352 loops it can also make sense to refer to subsequent groups. The next
2353 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2354 zero in any of these forms is not used; it provokes a compile-time error.)
2355 .P
2356 Consider the following pattern, which contains non-significant white space to
2357 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2358 three parts for ease of discussion:
2359 .sp
2360 ( \e( )? [^()]+ (?(1) \e) )
2361 .sp
2362 The first part matches an optional opening parenthesis, and if that
2363 character is present, sets it as the first captured substring. The second part
2364 matches one or more characters that are not parentheses. The third part is a
2365 conditional subpattern that tests whether or not the first set of parentheses
2366 matched. If they did, that is, if subject started with an opening parenthesis,
2367 the condition is true, and so the yes-pattern is executed and a closing
2368 parenthesis is required. Otherwise, since no-pattern is not present, the
2369 subpattern matches nothing. In other words, this pattern matches a sequence of
2370 non-parentheses, optionally enclosed in parentheses.
2371 .P
2372 If you were embedding this pattern in a larger one, you could use a relative
2373 reference:
2374 .sp
2375 ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
2376 .sp
2377 This makes the fragment independent of the parentheses in the larger pattern.
2378 .
2379 .SS "Checking for a used subpattern by name"
2380 .rs
2381 .sp
2382 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2383 subpattern by name. For compatibility with earlier versions of PCRE, which had
2384 this facility before Perl, the syntax (?(name)...) is also recognized.
2385 .P
2386 Rewriting the above example to use a named subpattern gives this:
2387 .sp
2388 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
2389 .sp
2390 If the name used in a condition of this kind is a duplicate, the test is
2391 applied to all subpatterns of the same name, and is true if any one of them has
2392 matched.
2393 .
2394 .SS "Checking for pattern recursion"
2395 .rs
2396 .sp
2397 If the condition is the string (R), and there is no subpattern with the name R,
2398 the condition is true if a recursive call to the whole pattern or any
2399 subpattern has been made. If digits or a name preceded by ampersand follow the
2400 letter R, for example:
2401 .sp
2402 (?(R3)...) or (?(R&name)...)
2403 .sp
2404 the condition is true if the most recent recursion is into a subpattern whose
2405 number or name is given. This condition does not check the entire recursion
2406 stack. If the name used in a condition of this kind is a duplicate, the test is
2407 applied to all subpatterns of the same name, and is true if any one of them is
2408 the most recent recursion.
2409 .P
2410 At "top level", all these recursion test conditions are false.
2411 .\" HTML <a href="#recursion">
2412 .\" </a>
2413 The syntax for recursive patterns
2414 .\"
2415 is described below.
2416 .
2417 .\" HTML <a name="subdefine"></a>
2418 .SS "Defining subpatterns for use by reference only"
2419 .rs
2420 .sp
2421 If the condition is the string (DEFINE), and there is no subpattern with the
2422 name DEFINE, the condition is always false. In this case, there may be only one
2423 alternative in the subpattern. It is always skipped if control reaches this
2424 point in the pattern; the idea of DEFINE is that it can be used to define
2425 subroutines that can be referenced from elsewhere. (The use of
2426 .\" HTML <a href="#subpatternsassubroutines">
2427 .\" </a>
2428 subroutines
2429 .\"
2430 is described below.) For example, a pattern to match an IPv4 address such as
2431 "192.168.23.245" could be written like this (ignore white space and line
2432 breaks):
2433 .sp
2434 (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2435 \eb (?&byte) (\e.(?&byte)){3} \eb
2436 .sp
2437 The first part of the pattern is a DEFINE group inside which a another group
2438 named "byte" is defined. This matches an individual component of an IPv4
2439 address (a number less than 256). When matching takes place, this part of the
2440 pattern is skipped because DEFINE acts like a false condition. The rest of the
2441 pattern uses references to the named group to match the four dot-separated
2442 components of an IPv4 address, insisting on a word boundary at each end.
2443 .
2444 .SS "Assertion conditions"
2445 .rs
2446 .sp
2447 If the condition is not in any of the above formats, it must be an assertion.
2448 This may be a positive or negative lookahead or lookbehind assertion. Consider
2449 this pattern, again containing non-significant white space, and with the two
2450 alternatives on the second line:
2451 .sp
2452 (?(?=[^a-z]*[a-z])
2453 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2454 .sp
2455 The condition is a positive lookahead assertion that matches an optional
2456 sequence of non-letters followed by a letter. In other words, it tests for the
2457 presence of at least one letter in the subject. If a letter is found, the
2458 subject is matched against the first alternative; otherwise it is matched
2459 against the second. This pattern matches strings in one of the two forms
2460 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2461 .
2462 .
2463 .\" HTML <a name="comments"></a>
2464 .SH COMMENTS
2465 .rs
2466 .sp
2467 There are two ways of including comments in patterns that are processed by
2468 PCRE. In both cases, the start of the comment must not be in a character class,
2469 nor in the middle of any other sequence of related characters such as (?: or a
2470 subpattern name or number. The characters that make up a comment play no part
2471 in the pattern matching.
2472 .P
2473 The sequence (?# marks the start of a comment that continues up to the next
2474 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2475 option is set, an unescaped # character also introduces a comment, which in
2476 this case continues to immediately after the next newline character or
2477 character sequence in the pattern. Which characters are interpreted as newlines
2478 is controlled by the options passed to a compiling function or by a special
2479 sequence at the start of the pattern, as described in the section entitled
2480 .\" HTML <a href="#newlines">
2481 .\" </a>
2482 "Newline conventions"
2483 .\"
2484 above. Note that the end of this type of comment is a literal newline sequence
2485 in the pattern; escape sequences that happen to represent a newline do not
2486 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2487 default newline convention is in force:
2488 .sp
2489 abc #comment \en still comment
2490 .sp
2491 On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2492 a newline in the pattern. The sequence \en is still literal at this stage, so
2493 it does not terminate the comment. Only an actual character with the code value
2494 0x0a (the default newline) does so.
2495 .
2496 .
2497 .\" HTML <a name="recursion"></a>
2498 .SH "RECURSIVE PATTERNS"
2499 .rs
2500 .sp
2501 Consider the problem of matching a string in parentheses, allowing for
2502 unlimited nested parentheses. Without the use of recursion, the best that can
2503 be done is to use a pattern that matches up to some fixed depth of nesting. It
2504 is not possible to handle an arbitrary nesting depth.
2505 .P
2506 For some time, Perl has provided a facility that allows regular expressions to
2507 recurse (amongst other things). It does this by interpolating Perl code in the
2508 expression at run time, and the code can refer to the expression itself. A Perl
2509 pattern using code interpolation to solve the parentheses problem can be
2510 created like this:
2511 .sp
2512 $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2513 .sp
2514 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2515 recursively to the pattern in which it appears.
2516 .P
2517 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2518 supports special syntax for recursion of the entire pattern, and also for
2519 individual subpattern recursion. After its introduction in PCRE and Python,
2520 this kind of recursion was subsequently introduced into Perl at release 5.10.
2521 .P
2522 A special item that consists of (? followed by a number greater than zero and a
2523 closing parenthesis is a recursive subroutine call of the subpattern of the
2524 given number, provided that it occurs inside that subpattern. (If not, it is a
2525 .\" HTML <a href="#subpatternsassubroutines">
2526 .\" </a>
2527 non-recursive subroutine
2528 .\"
2529 call, which is described in the next section.) The special item (?R) or (?0) is
2530 a recursive call of the entire regular expression.
2531 .P
2532 This PCRE pattern solves the nested parentheses problem (assume the
2533 PCRE_EXTENDED option is set so that white space is ignored):
2534 .sp
2535 \e( ( [^()]++ | (?R) )* \e)
2536 .sp
2537 First it matches an opening parenthesis. Then it matches any number of
2538 substrings which can either be a sequence of non-parentheses, or a recursive
2539 match of the pattern itself (that is, a correctly parenthesized substring).
2540 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2541 to avoid backtracking into sequences of non-parentheses.
2542 .P
2543 If this were part of a larger pattern, you would not want to recurse the entire
2544 pattern, so instead you could use this:
2545 .sp
2546 ( \e( ( [^()]++ | (?1) )* \e) )
2547 .sp
2548 We have put the pattern into parentheses, and caused the recursion to refer to
2549 them instead of the whole pattern.
2550 .P
2551 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2552 is made easier by the use of relative references. Instead of (?1) in the
2553 pattern above you can write (?-2) to refer to the second most recently opened
2554 parentheses preceding the recursion. In other words, a negative number counts
2555 capturing parentheses leftwards from the point at which it is encountered.
2556 .P
2557 It is also possible to refer to subsequently opened parentheses, by writing
2558 references such as (?+2). However, these cannot be recursive because the
2559 reference is not inside the parentheses that are referenced. They are always
2560 .\" HTML <a href="#subpatternsassubroutines">
2561 .\" </a>
2562 non-recursive subroutine
2563 .\"
2564 calls, as described in the next section.
2565 .P
2566 An alternative approach is to use named parentheses instead. The Perl syntax
2567 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2568 could rewrite the above example as follows:
2569 .sp
2570 (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2571 .sp
2572 If there is more than one subpattern with the same name, the earliest one is
2573 used.
2574 .P
2575 This particular example pattern that we have been looking at contains nested
2576 unlimited repeats, and so the use of a possessive quantifier for matching
2577 strings of non-parentheses is important when applying the pattern to strings
2578 that do not match. For example, when this pattern is applied to
2579 .sp
2580 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2581 .sp
2582 it yields "no match" quickly. However, if a possessive quantifier is not used,
2583 the match runs for a very long time indeed because there are so many different
2584 ways the + and * repeats can carve up the subject, and all have to be tested
2585 before failure can be reported.
2586 .P
2587 At the end of a match, the values of capturing parentheses are those from
2588 the outermost level. If you want to obtain intermediate values, a callout
2589 function can be used (see below and the
2590 .\" HREF
2591 \fBpcrecallout\fP
2592 .\"
2593 documentation). If the pattern above is matched against
2594 .sp
2595 (ab(cd)ef)
2596 .sp
2597 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2598 the last value taken on at the top level. If a capturing subpattern is not
2599 matched at the top level, its final captured value is unset, even if it was
2600 (temporarily) set at a deeper level during the matching process.
2601 .P
2602 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2603 obtain extra memory to store data during a recursion, which it does by using
2604 \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2605 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2606 .P
2607 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2608 Consider this pattern, which matches text in angle brackets, allowing for
2609 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2610 recursing), whereas any characters are permitted at the outer level.
2611 .sp
2612 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2613 .sp
2614 In this pattern, (?(R) is the start of a conditional subpattern, with two
2615 different alternatives for the recursive and non-recursive cases. The (?R) item
2616 is the actual recursive call.
2617 .
2618 .
2619 .\" HTML <a name="recursiondifference"></a>
2620 .SS "Differences in recursion processing between PCRE and Perl"
2621 .rs
2622 .sp
2623 Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2624 (like Python, but unlike Perl), a recursive subpattern call is always treated
2625 as an atomic group. That is, once it has matched some of the subject string, it
2626 is never re-entered, even if it contains untried alternatives and there is a
2627 subsequent matching failure. This can be illustrated by the following pattern,
2628 which purports to match a palindromic string that contains an odd number of
2629 characters (for example, "a", "aba", "abcba", "abcdcba"):
2630 .sp
2631 ^(.|(.)(?1)\e2)$
2632 .sp
2633 The idea is that it either matches a single character, or two identical
2634 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2635 it does not if the pattern is longer than three characters. Consider the
2636 subject string "abcba":
2637 .P
2638 At the top level, the first character is matched, but as it is not at the end
2639 of the string, the first alternative fails; the second alternative is taken
2640 and the recursion kicks in. The recursive call to subpattern 1 successfully
2641 matches the next character ("b"). (Note that the beginning and end of line
2642 tests are not part of the recursion).
2643 .P
2644 Back at the top level, the next character ("c") is compared with what
2645 subpattern 2 matched, which was "a". This fails. Because the recursion is
2646 treated as an atomic group, there are now no backtracking points, and so the
2647 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2648 try the second alternative.) However, if the pattern is written with the
2649 alternatives in the other order, things are different:
2650 .sp
2651 ^((.)(?1)\e2|.)$
2652 .sp
2653 This time, the recursing alternative is tried first, and continues to recurse
2654 until it runs out of characters, at which point the recursion fails. But this
2655 time we do have another alternative to try at the higher level. That is the big
2656 difference: in the previous case the remaining alternative is at a deeper
2657 recursion level, which PCRE cannot use.
2658 .P
2659 To change the pattern so that it matches all palindromic strings, not just
2660 those with an odd number of characters, it is tempting to change the pattern to
2661 this:
2662 .sp
2663 ^((.)(?1)\e2|.?)$
2664 .sp
2665 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2666 deeper recursion has matched a single character, it cannot be entered again in
2667 order to match an empty string. The solution is to separate the two cases, and
2668 write out the odd and even cases as alternatives at the higher level:
2669 .sp
2670 ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2671 .sp
2672 If you want to match typical palindromic phrases, the pattern has to ignore all
2673 non-word characters, which can be done like this:
2674 .sp
2675 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2676 .sp
2677 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2678 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2679 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2680 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2681 more) to match typical phrases, and Perl takes so long that you think it has
2682 gone into a loop.
2683 .P
2684 \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2685 string does not start with a palindrome that is shorter than the entire string.
2686 For example, although "abcba" is correctly matched, if the subject is "ababa",
2687 PCRE finds the palindrome "aba" at the start, then fails at top level because
2688 the end of the string does not follow. Once again, it cannot jump back into the
2689 recursion to try other alternatives, so the entire match fails.
2690 .P
2691 The second way in which PCRE and Perl differ in their recursion processing is
2692 in the handling of captured values. In Perl, when a subpattern is called
2693 recursively or as a subpattern (see the next section), it has no access to any
2694 values that were captured outside the recursion, whereas in PCRE these values
2695 can be referenced. Consider this pattern:
2696 .sp
2697 ^(.)(\e1|a(?2))
2698 .sp
2699 In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2700 then in the second group, when the back reference \e1 fails to match "b", the
2701 second alternative matches "a" and then recurses. In the recursion, \e1 does
2702 now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2703 match because inside the recursive call \e1 cannot access the externally set
2704 value.
2705 .
2706 .
2707 .\" HTML <a name="subpatternsassubroutines"></a>
2708 .SH "SUBPATTERNS AS SUBROUTINES"
2709 .rs
2710 .sp
2711 If the syntax for a recursive subpattern call (either by number or by
2712 name) is used outside the parentheses to which it refers, it operates like a
2713 subroutine in a programming language. The called subpattern may be defined
2714 before or after the reference. A numbered reference can be absolute or
2715 relative, as in these examples:
2716 .sp
2717 (...(absolute)...)...(?2)...
2718 (...(relative)...)...(?-1)...
2719 (...(?+1)...(relative)...
2720 .sp
2721 An earlier example pointed out that the pattern
2722 .sp
2723 (sens|respons)e and \e1ibility
2724 .sp
2725 matches "sense and sensibility" and "response and responsibility", but not
2726 "sense and responsibility". If instead the pattern
2727 .sp
2728 (sens|respons)e and (?1)ibility
2729 .sp
2730 is used, it does match "sense and responsibility" as well as the other two
2731 strings. Another example is given in the discussion of DEFINE above.
2732 .P
2733 All subroutine calls, whether recursive or not, are always treated as atomic
2734 groups. That is, once a subroutine has matched some of the subject string, it
2735 is never re-entered, even if it contains untried alternatives and there is a
2736 subsequent matching failure. Any capturing parentheses that are set during the
2737 subroutine call revert to their previous values afterwards.
2738 .P
2739 Processing options such as case-independence are fixed when a subpattern is
2740 defined, so if it is used as a subroutine, such options cannot be changed for
2741 different calls. For example, consider this pattern:
2742 .sp
2743 (abc)(?i:(?-1))
2744 .sp
2745 It matches "abcabc". It does not match "abcABC" because the change of
2746 processing option does not affect the called subpattern.
2747 .
2748 .
2749 .\" HTML <a name="onigurumasubroutines"></a>
2750 .SH "ONIGURUMA SUBROUTINE SYNTAX"
2751 .rs
2752 .sp
2753 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2754 a number enclosed either in angle brackets or single quotes, is an alternative
2755 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2756 are two of the examples used above, rewritten using this syntax:
2757 .sp
2758 (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2759 (sens|respons)e and \eg'1'ibility
2760 .sp
2761 PCRE supports an extension to Oniguruma: if a number is preceded by a
2762 plus or a minus sign it is taken as a relative reference. For example:
2763 .sp
2764 (abc)(?i:\eg<-1>)
2765 .sp
2766 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2767 synonymous. The former is a back reference; the latter is a subroutine call.
2768 .
2769 .
2770 .SH CALLOUTS
2771 .rs
2772 .sp
2773 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2774 code to be obeyed in the middle of matching a regular expression. This makes it
2775 possible, amongst other things, to extract different substrings that match the
2776 same pair of parentheses when there is a repetition.
2777 .P
2778 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2779 code. The feature is called "callout". The caller of PCRE provides an external
2780 function by putting its entry point in the global variable \fIpcre_callout\fP
2781 (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2782 By default, this variable contains NULL, which disables all calling out.
2783 .P
2784 Within a regular expression, (?C) indicates the points at which the external
2785 function is to be called. If you want to identify different callout points, you
2786 can put a number less than 256 after the letter C. The default value is zero.
2787 For example, this pattern has two callout points:
2788 .sp
2789 (?C1)abc(?C2)def
2790 .sp
2791 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2792 automatically installed before each item in the pattern. They are all numbered
2793 255. If there is a conditional group in the pattern whose condition is an
2794 assertion, an additional callout is inserted just before the condition. An
2795 explicit callout may also be set at this position, as in this example:
2796 .sp
2797 (?(?C9)(?=a)abc|def)
2798 .sp
2799 Note that this applies only to assertion conditions, not to other types of
2800 condition.
2801 .P
2802 During matching, when PCRE reaches a callout point, the external function is
2803 called. It is provided with the number of the callout, the position in the
2804 pattern, and, optionally, one item of data originally supplied by the caller of
2805 the matching function. The callout function may cause matching to proceed, to
2806 backtrack, or to fail altogether.
2807 .P
2808 By default, PCRE implements a number of optimizations at compile time and
2809 matching time, and one side-effect is that sometimes callouts are skipped. If
2810 you need all possible callouts to happen, you need to set options that disable
2811 the relevant optimizations. More details, and a complete description of the
2812 interface to the callout function, are given in the
2813 .\" HREF
2814 \fBpcrecallout\fP
2815 .\"
2816 documentation.
2817 .
2818 .
2819 .\" HTML <a name="backtrackcontrol"></a>
2820 .SH "BACKTRACKING CONTROL"
2821 .rs
2822 .sp
2823 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2824 are still described in the Perl documentation as "experimental and subject to
2825 change or removal in a future version of Perl". It goes on to say: "Their usage
2826 in production code should be noted to avoid problems during upgrades." The same
2827 remarks apply to the PCRE features described in this section.
2828 .P
2829 The new verbs make use of what was previously invalid syntax: an opening
2830 parenthesis followed by an asterisk. They are generally of the form
2831 (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2832 differently depending on whether or not a name is present. A name is any
2833 sequence of characters that does not include a closing parenthesis. The maximum
2834 length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2835 libraries. If the name is empty, that is, if the closing parenthesis
2836 immediately follows the colon, the effect is as if the colon were not there.
2837 Any number of these verbs may occur in a pattern.
2838 .P
2839 Since these verbs are specifically related to backtracking, most of them can be
2840 used only when the pattern is to be matched using one of the traditional
2841 matching functions, because these use a backtracking algorithm. With the
2842 exception of (*FAIL), which behaves like a failing negative assertion, the
2843 backtracking control verbs cause an error if encountered by a DFA matching
2844 function.
2845 .P
2846 The behaviour of these verbs in
2847 .\" HTML <a href="#btrepeat">
2848 .\" </a>
2849 repeated groups,
2850 .\"
2851 .\" HTML <a href="#btassert">
2852 .\" </a>
2853 assertions,
2854 .\"
2855 and in
2856 .\" HTML <a href="#btsub">
2857 .\" </a>
2858 subpatterns called as subroutines
2859 .\"
2860 (whether or not recursively) is documented below.
2861 .
2862 .
2863 .\" HTML <a name="nooptimize"></a>
2864 .SS "Optimizations that affect backtracking verbs"
2865 .rs
2866 .sp
2867 PCRE contains some optimizations that are used to speed up matching by running
2868 some checks at the start of each match attempt. For example, it may know the
2869 minimum length of matching subject, or that a particular character must be
2870 present. When one of these optimizations bypasses the running of a match, any
2871 included backtracking verbs will not, of course, be processed. You can suppress
2872 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2873 when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2874 pattern with (*NO_START_OPT). There is more discussion of this option in the
2875 section entitled
2876 .\" HTML <a href="pcreapi.html#execoptions">
2877 .\" </a>
2878 "Option bits for \fBpcre_exec()\fP"
2879 .\"
2880 in the
2881 .\" HREF
2882 \fBpcreapi\fP
2883 .\"
2884 documentation.
2885 .P
2886 Experiments with Perl suggest that it too has similar optimizations, sometimes
2887 leading to anomalous results.
2888 .
2889 .
2890 .SS "Verbs that act immediately"
2891 .rs
2892 .sp
2893 The following verbs act as soon as they are encountered. They may not be
2894 followed by a name.
2895 .sp
2896 (*ACCEPT)
2897 .sp
2898 This verb causes the match to end successfully, skipping the remainder of the
2899 pattern. However, when it is inside a subpattern that is called as a
2900 subroutine, only that subpattern is ended successfully. Matching then continues
2901 at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2902 assertion succeeds; in a negative assertion, the assertion fails.
2903 .P
2904 If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2905 example:
2906 .sp
2907 A((?:A|B(*ACCEPT)|C)D)
2908 .sp
2909 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2910 the outer parentheses.
2911 .sp
2912 (*FAIL) or (*F)
2913 .sp
2914 This verb causes a matching failure, forcing backtracking to occur. It is
2915 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2916 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2917 Perl features that are not present in PCRE. The nearest equivalent is the
2918 callout feature, as for example in this pattern:
2919 .sp
2920 a+(?C)(*FAIL)
2921 .sp
2922 A match with the string "aaaa" always fails, but the callout is taken before
2923 each backtrack happens (in this example, 10 times).
2924 .
2925 .
2926 .SS "Recording which path was taken"
2927 .rs
2928 .sp
2929 There is one verb whose main purpose is to track how a match was arrived at,
2930 though it also has a secondary use in conjunction with advancing the match
2931 starting point (see (*SKIP) below).
2932 .sp
2933 (*MARK:NAME) or (*:NAME)
2934 .sp
2935 A name is always required with this verb. There may be as many instances of
2936 (*MARK) as you like in a pattern, and their names do not have to be unique.
2937 .P
2938 When a match succeeds, the name of the last-encountered (*MARK:NAME),
2939 (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2940 caller as described in the section entitled
2941 .\" HTML <a href="pcreapi.html#extradata">
2942 .\" </a>
2943 "Extra data for \fBpcre_exec()\fP"
2944 .\"
2945 in the
2946 .\" HREF
2947 \fBpcreapi\fP
2948 .\"
2949 documentation. Here is an example of \fBpcretest\fP output, where the /K
2950 modifier requests the retrieval and outputting of (*MARK) data:
2951 .sp
2952 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2953 data> XY
2954 0: XY
2955 MK: A
2956 XZ
2957 0: XZ
2958 MK: B
2959 .sp
2960 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2961 indicates which of the two alternatives matched. This is a more efficient way
2962 of obtaining this information than putting each alternative in its own
2963 capturing parentheses.
2964 .P
2965 If a verb with a name is encountered in a positive assertion that is true, the
2966 name is recorded and passed back if it is the last-encountered. This does not
2967 happen for negative assertions or failing positive assertions.
2968 .P
2969 After a partial match or a failed match, the last encountered name in the
2970 entire match process is returned. For example:
2971 .sp
2972 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2973 data> XP
2974 No match, mark = B
2975 .sp
2976 Note that in this unanchored example the mark is retained from the match
2977 attempt that started at the letter "X" in the subject. Subsequent match
2978 attempts starting at "P" and then with an empty string do not get as far as the
2979 (*MARK) item, but nevertheless do not reset it.
2980 .P
2981 If you are interested in (*MARK) values after failed matches, you should
2982 probably set the PCRE_NO_START_OPTIMIZE option
2983 .\" HTML <a href="#nooptimize">
2984 .\" </a>
2985 (see above)
2986 .\"
2987 to ensure that the match is always attempted.
2988 .
2989 .
2990 .SS "Verbs that act after backtracking"
2991 .rs
2992 .sp
2993 The following verbs do nothing when they are encountered. Matching continues
2994 with what follows, but if there is no subsequent match, causing a backtrack to
2995 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2996 the verb. However, when one of these verbs appears inside an atomic group or an
2997 assertion that is true, its effect is confined to that group, because once the
2998 group has been matched, there is never any backtracking into it. In this
2999 situation, backtracking can "jump back" to the left of the entire atomic group
3000 or assertion. (Remember also, as stated above, that this localization also
3001 applies in subroutine calls.)
3002 .P
3003 These verbs differ in exactly what kind of failure occurs when backtracking
3004 reaches them. The behaviour described below is what happens when the verb is
3005 not in a subroutine or an assertion. Subsequent sections cover these special
3006 cases.
3007 .sp
3008 (*COMMIT)
3009 .sp
3010 This verb, which may not be followed by a name, causes the whole match to fail
3011 outright if there is a later matching failure that causes backtracking to reach
3012 it. Even if the pattern is unanchored, no further attempts to find a match by
3013 advancing the starting point take place. If (*COMMIT) is the only backtracking
3014 verb that is encountered, once it has been passed \fBpcre_exec()\fP is
3015 committed to finding a match at the current starting point, or not at all. For
3016 example:
3017 .sp
3018 a+(*COMMIT)b
3019 .sp
3020 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
3021 dynamic anchor, or "I've started, so I must finish." The name of the most
3022 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
3023 match failure.
3024 .P
3025 If there is more than one backtracking verb in a pattern, a different one that
3026 follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
3027 match does not always guarantee that a match must be at this starting point.
3028 .P
3029 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
3030 unless PCRE's start-of-match optimizations are turned off, as shown in this
3031 \fBpcretest\fP example:
3032 .sp
3033 re> /(*COMMIT)abc/
3034 data> xyzabc
3035 0: abc
3036 xyzabc\eY
3037 No match
3038 .sp
3039 PCRE knows that any match must start with "a", so the optimization skips along
3040 the subject to "a" before running the first match attempt, which succeeds. When
3041 the optimization is disabled by the \eY escape in the second subject, the match
3042 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
3043 starting points.
3044 .sp
3045 (*PRUNE) or (*PRUNE:NAME)
3046 .sp
3047 This verb causes the match to fail at the current starting position in the
3048 subject if there is a later matching failure that causes backtracking to reach
3049 it. If the pattern is unanchored, the normal "bumpalong" advance to the next
3050 starting character then happens. Backtracking can occur as usual to the left of
3051 (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
3052 if there is no match to the right, backtracking cannot cross (*PRUNE). In
3053 simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
3054 possessive quantifier, but there are some uses of (*PRUNE) that cannot be
3055 expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
3056 as (*COMMIT).
3057 .P
3058 The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3059 It is like (*MARK:NAME) in that the name is remembered for passing back to the
3060 caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3061 .sp
3062 (*SKIP)
3063 .sp
3064 This verb, when given without a name, is like (*PRUNE), except that if the
3065 pattern is unanchored, the "bumpalong" advance is not to the next character,
3066 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
3067 signifies that whatever text was matched leading up to it cannot be part of a
3068 successful match. Consider:
3069 .sp
3070 a+(*SKIP)b
3071 .sp
3072 If the subject is "aaaac...", after the first match attempt fails (starting at
3073 the first character in the string), the starting point skips on to start the
3074 next attempt at "c". Note that a possessive quantifer does not have the same
3075 effect as this example; although it would suppress backtracking during the
3076 first match attempt, the second attempt would start at the second character
3077 instead of skipping on to "c".
3078 .sp
3079 (*SKIP:NAME)
3080 .sp
3081 When (*SKIP) has an associated name, its behaviour is modified. When it is
3082 triggered, the previous path through the pattern is searched for the most
3083 recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
3084 is to the subject position that corresponds to that (*MARK) instead of to where
3085 (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
3086 (*SKIP) is ignored.
3087 .P
3088 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3089 names that are set by (*PRUNE:NAME) or (*THEN:NAME).
3090 .sp
3091 (*THEN) or (*THEN:NAME)
3092 .sp
3093 This verb causes a skip to the next innermost alternative when backtracking
3094 reaches it. That is, it cancels any further backtracking within the current
3095 alternative. Its name comes from the observation that it can be used for a
3096 pattern-based if-then-else block:
3097 .sp
3098 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3099 .sp
3100 If the COND1 pattern matches, FOO is tried (and possibly further items after
3101 the end of the group if FOO succeeds); on failure, the matcher skips to the
3102 second alternative and tries COND2, without backtracking into COND1. If that
3103 succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3104 more alternatives, so there is a backtrack to whatever came before the entire
3105 group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3106 .P
3107 The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3108 It is like (*MARK:NAME) in that the name is remembered for passing back to the
3109 caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3110 .P
3111 A subpattern that does not contain a | character is just a part of the
3112 enclosing alternative; it is not a nested alternation with only one
3113 alternative. The effect of (*THEN) extends beyond such a subpattern to the
3114 enclosing alternative. Consider this pattern, where A, B, etc. are complex
3115 pattern fragments that do not contain any | characters at this level:
3116 .sp
3117 A (B(*THEN)C) | D
3118 .sp
3119 If A and B are matched, but there is a failure in C, matching does not
3120 backtrack into A; instead it moves to the next alternative, that is, D.
3121 However, if the subpattern containing (*THEN) is given an alternative, it
3122 behaves differently:
3123 .sp
3124 A (B(*THEN)C | (*FAIL)) | D
3125 .sp
3126 The effect of (*THEN) is now confined to the inner subpattern. After a failure
3127 in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3128 because there are no more alternatives to try. In this case, matching does now
3129 backtrack into A.
3130 .P
3131 Note that a conditional subpattern is not considered as having two
3132 alternatives, because only one is ever used. In other words, the | character in
3133 a conditional subpattern has a different meaning. Ignoring white space,
3134 consider:
3135 .sp
3136 ^.*? (?(?=a) a | b(*THEN)c )
3137 .sp
3138 If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3139 it initially matches zero characters. The condition (?=a) then fails, the
3140 character "b" is matched, but "c" is not. At this point, matching does not
3141 backtrack to .*? as might perhaps be expected from the presence of the |
3142 character. The conditional subpattern is part of the single alternative that
3143 comprises the whole pattern, and so the match fails. (If there was a backtrack
3144 into .*?, allowing it to match "b", the match would succeed.)
3145 .P
3146 The verbs just described provide four different "strengths" of control when
3147 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3148 next alternative. (*PRUNE) comes next, failing the match at the current
3149 starting position, but allowing an advance to the next character (for an
3150 unanchored pattern). (*SKIP) is similar, except that the advance may be more
3151 than one character. (*COMMIT) is the strongest, causing the entire match to
3152 fail.
3153 .
3154 .
3155 .SS "More than one backtracking verb"
3156 .rs
3157 .sp
3158 If more than one backtracking verb is present in a pattern, the one that is
3159 backtracked onto first acts. For example, consider this pattern, where A, B,
3160 etc. are complex pattern fragments:
3161 .sp
3162 (A(*COMMIT)B(*THEN)C|ABD)
3163 .sp
3164 If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3165 fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3166 the next alternative (ABD) to be tried. This behaviour is consistent, but is
3167 not always the same as Perl's. It means that if two or more backtracking verbs
3168 appear in succession, all the the last of them has no effect. Consider this
3169 example:
3170 .sp
3171 ...(*COMMIT)(*PRUNE)...
3172 .sp
3173 If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3174 it to be triggered, and its action is taken. There can never be a backtrack
3175 onto (*COMMIT).
3176 .
3177 .
3178 .\" HTML <a name="btrepeat"></a>
3179 .SS "Backtracking verbs in repeated groups"
3180 .rs
3181 .sp
3182 PCRE differs from Perl in its handling of backtracking verbs in repeated
3183 groups. For example, consider:
3184 .sp
3185 /(a(*COMMIT)b)+ac/
3186 .sp
3187 If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3188 the second repeat of the group acts.
3189 .
3190 .
3191 .\" HTML <a name="btassert"></a>
3192 .SS "Backtracking verbs in assertions"
3193 .rs
3194 .sp
3195 (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3196 .P
3197 (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3198 further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3199 fail without any further processing.
3200 .P
3201 The other backtracking verbs are not treated specially if they appear in a
3202 positive assertion. In particular, (*THEN) skips to the next alternative in the
3203 innermost enclosing group that has alternations, whether or not this is within
3204 the assertion.
3205 .P
3206 Negative assertions are, however, different, in order to ensure that changing a
3207 positive assertion into a negative assertion changes its result. Backtracking
3208 into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3209 without considering any further alternative branches in the assertion.
3210 Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3211 within the assertion (the normal behaviour), but if the assertion does not have
3212 such an alternative, (*THEN) behaves like (*PRUNE).
3213 .
3214 .
3215 .\" HTML <a name="btsub"></a>
3216 .SS "Backtracking verbs in subroutines"
3217 .rs
3218 .sp
3219 These behaviours occur whether or not the subpattern is called recursively.
3220 Perl's treatment of subroutines is different in some cases.
3221 .P
3222 (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3223 an immediate backtrack.
3224 .P
3225 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3226 succeed without any further processing. Matching then continues after the
3227 subroutine call.
3228 .P
3229 (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3230 the subroutine match to fail.
3231 .P
3232 (*THEN) skips to the next alternative in the innermost enclosing group within
3233 the subpattern that has alternatives. If there is no such group within the
3234 subpattern, (*THEN) causes the subroutine match to fail.
3235 .
3236 .
3237 .SH "SEE ALSO"
3238 .rs
3239 .sp
3240 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3241 \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3242 .
3243 .
3244 .SH AUTHOR
3245 .rs
3246 .sp
3247 .nf
3248 Philip Hazel
3249 University Computing Service
3250 Cambridge CB2 3QH, England.
3251 .fi
3252 .
3253 .
3254 .SH REVISION
3255 .rs
3256 .sp
3257 .nf
3258 Last updated: 03 December 2013
3259 Copyright (c) 1997-2013 University of Cambridge.
3260 .fi

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