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3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 The syntax and semantics of the regular expressions that are supported by PCRE
8 are described in detail below. There is a quick-reference syntax summary in the
9 .\" HREF
10 \fBpcresyntax\fP
11 .\"
12 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13 also supports some alternative regular expression syntax (which does not
14 conflict with the Perl syntax) in order to provide some compatibility with
15 regular expressions in Python, .NET, and Oniguruma.
16 .P
17 Perl's regular expressions are described in its own documentation, and
18 regular expressions in general are covered in a number of books, some of which
19 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20 published by O'Reilly, covers regular expressions in great detail. This
21 description of PCRE's regular expressions is intended as reference material.
22 .P
23 The original operation of PCRE was on strings of one-byte characters. However,
24 there is now also support for UTF-8 character strings. To use this,
25 PCRE must be built to include UTF-8 support, and you must call
26 \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27 is also a special sequence that can be given at the start of a pattern:
28 .sp
29 (*UTF8)
30 .sp
31 Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32 option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33 pattern matching is mentioned in several places below. There is also a summary
34 of UTF-8 features in the
35 .\" HREF
36 \fBpcreunicode\fP
37 .\"
38 page.
39 .P
40 Another special sequence that may appear at the start of a pattern or in
41 combination with (*UTF8) is:
42 .sp
43 (*UCP)
44 .sp
45 This has the same effect as setting the PCRE_UCP option: it causes sequences
46 such as \ed and \ew to use Unicode properties to determine character types,
47 instead of recognizing only characters with codes less than 128 via a lookup
48 table.
49 .P
50 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
51 PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
52 also some more of these special sequences that are concerned with the handling
53 of newlines; they are described below.
54 .P
55 The remainder of this document discusses the patterns that are supported by
56 PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57 From release 6.0, PCRE offers a second matching function,
58 \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
59 Perl-compatible. Some of the features discussed below are not available when
60 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
61 alternative function, and how it differs from the normal function, are
62 discussed in the
63 .\" HREF
64 \fBpcrematching\fP
65 .\"
66 page.
67 .
68 .
69 .\" HTML <a name="newlines"></a>
71 .rs
72 .sp
73 PCRE supports five different conventions for indicating line breaks in
74 strings: a single CR (carriage return) character, a single LF (linefeed)
75 character, the two-character sequence CRLF, any of the three preceding, or any
76 Unicode newline sequence. The
77 .\" HREF
78 \fBpcreapi\fP
79 .\"
80 page has
81 .\" HTML <a href="pcreapi.html#newlines">
82 .\" </a>
83 further discussion
84 .\"
85 about newlines, and shows how to set the newline convention in the
86 \fIoptions\fP arguments for the compiling and matching functions.
87 .P
88 It is also possible to specify a newline convention by starting a pattern
89 string with one of the following five sequences:
90 .sp
91 (*CR) carriage return
92 (*LF) linefeed
93 (*CRLF) carriage return, followed by linefeed
94 (*ANYCRLF) any of the three above
95 (*ANY) all Unicode newline sequences
96 .sp
97 These override the default and the options given to \fBpcre_compile()\fP or
98 \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
99 newline sequence, the pattern
100 .sp
101 (*CR)a.b
102 .sp
103 changes the convention to CR. That pattern matches "a\enb" because LF is no
104 longer a newline. Note that these special settings, which are not
105 Perl-compatible, are recognized only at the very start of a pattern, and that
106 they must be in upper case. If more than one of them is present, the last one
107 is used.
108 .P
109 The newline convention affects the interpretation of the dot metacharacter when
110 PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
111 affect what the \eR escape sequence matches. By default, this is any Unicode
112 newline sequence, for Perl compatibility. However, this can be changed; see the
113 description of \eR in the section entitled
114 .\" HTML <a href="#newlineseq">
115 .\" </a>
116 "Newline sequences"
117 .\"
118 below. A change of \eR setting can be combined with a change of newline
119 convention.
120 .
121 .
123 .rs
124 .sp
125 A regular expression is a pattern that is matched against a subject string from
126 left to right. Most characters stand for themselves in a pattern, and match the
127 corresponding characters in the subject. As a trivial example, the pattern
128 .sp
129 The quick brown fox
130 .sp
131 matches a portion of a subject string that is identical to itself. When
132 caseless matching is specified (the PCRE_CASELESS option), letters are matched
133 independently of case. In UTF-8 mode, PCRE always understands the concept of
134 case for characters whose values are less than 128, so caseless matching is
135 always possible. For characters with higher values, the concept of case is
136 supported if PCRE is compiled with Unicode property support, but not otherwise.
137 If you want to use caseless matching for characters 128 and above, you must
138 ensure that PCRE is compiled with Unicode property support as well as with
139 UTF-8 support.
140 .P
141 The power of regular expressions comes from the ability to include alternatives
142 and repetitions in the pattern. These are encoded in the pattern by the use of
143 \fImetacharacters\fP, which do not stand for themselves but instead are
144 interpreted in some special way.
145 .P
146 There are two different sets of metacharacters: those that are recognized
147 anywhere in the pattern except within square brackets, and those that are
148 recognized within square brackets. Outside square brackets, the metacharacters
149 are as follows:
150 .sp
151 \e general escape character with several uses
152 ^ assert start of string (or line, in multiline mode)
153 $ assert end of string (or line, in multiline mode)
154 . match any character except newline (by default)
155 [ start character class definition
156 | start of alternative branch
157 ( start subpattern
158 ) end subpattern
159 ? extends the meaning of (
160 also 0 or 1 quantifier
161 also quantifier minimizer
162 * 0 or more quantifier
163 + 1 or more quantifier
164 also "possessive quantifier"
165 { start min/max quantifier
166 .sp
167 Part of a pattern that is in square brackets is called a "character class". In
168 a character class the only metacharacters are:
169 .sp
170 \e general escape character
171 ^ negate the class, but only if the first character
172 - indicates character range
173 .\" JOIN
174 [ POSIX character class (only if followed by POSIX
175 syntax)
176 ] terminates the character class
177 .sp
178 The following sections describe the use of each of the metacharacters.
179 .
180 .
182 .rs
183 .sp
184 The backslash character has several uses. Firstly, if it is followed by a
185 character that is not a number or a letter, it takes away any special meaning
186 that character may have. This use of backslash as an escape character applies
187 both inside and outside character classes.
188 .P
189 For example, if you want to match a * character, you write \e* in the pattern.
190 This escaping action applies whether or not the following character would
191 otherwise be interpreted as a metacharacter, so it is always safe to precede a
192 non-alphanumeric with backslash to specify that it stands for itself. In
193 particular, if you want to match a backslash, you write \e\e.
194 .P
195 In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196 backslash. All other characters (in particular, those whose codepoints are
197 greater than 127) are treated as literals.
198 .P
199 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
200 pattern (other than in a character class) and characters between a # outside
201 a character class and the next newline are ignored. An escaping backslash can
202 be used to include a whitespace or # character as part of the pattern.
203 .P
204 If you want to remove the special meaning from a sequence of characters, you
205 can do so by putting them between \eQ and \eE. This is different from Perl in
206 that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
207 Perl, $ and @ cause variable interpolation. Note the following examples:
208 .sp
209 Pattern PCRE matches Perl matches
210 .sp
211 .\" JOIN
212 \eQabc$xyz\eE abc$xyz abc followed by the
213 contents of $xyz
214 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
215 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
216 .sp
217 The \eQ...\eE sequence is recognized both inside and outside character classes.
218 An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
219 by \eE later in the pattern, the literal interpretation continues to the end of
220 the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
221 a character class, this causes an error, because the character class is not
222 terminated.
223 .
224 .
225 .\" HTML <a name="digitsafterbackslash"></a>
226 .SS "Non-printing characters"
227 .rs
228 .sp
229 A second use of backslash provides a way of encoding non-printing characters
230 in patterns in a visible manner. There is no restriction on the appearance of
231 non-printing characters, apart from the binary zero that terminates a pattern,
232 but when a pattern is being prepared by text editing, it is often easier to use
233 one of the following escape sequences than the binary character it represents:
234 .sp
235 \ea alarm, that is, the BEL character (hex 07)
236 \ecx "control-x", where x is any ASCII character
237 \ee escape (hex 1B)
238 \ef formfeed (hex 0C)
239 \en linefeed (hex 0A)
240 \er carriage return (hex 0D)
241 \et tab (hex 09)
242 \eddd character with octal code ddd, or back reference
243 \exhh character with hex code hh
244 \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
245 \euhhhh character with hex code hhhh (JavaScript mode only)
246 .sp
247 The precise effect of \ecx is as follows: if x is a lower case letter, it
248 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
249 Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
250 \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
251 than 127, a compile-time error occurs. This locks out non-ASCII characters in
252 both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
253 values are valid. A lower case letter is converted to upper case, and then the
254 0xc0 bits are flipped.)
255 .P
256 By default, after \ex, from zero to two hexadecimal digits are read (letters
257 can be in upper or lower case). Any number of hexadecimal digits may appear
258 between \ex{ and }, but the value of the character code must be less than 256
259 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum
260 value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest
261 Unicode code point, which is 10FFFF.
262 .P
263 If characters other than hexadecimal digits appear between \ex{ and }, or if
264 there is no terminating }, this form of escape is not recognized. Instead, the
265 initial \ex will be interpreted as a basic hexadecimal escape, with no
266 following digits, giving a character whose value is zero.
267 .P
268 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
269 as just described only when it is followed by two hexadecimal digits.
270 Otherwise, it matches a literal "x" character. In JavaScript mode, support for
271 code points greater than 256 is provided by \eu, which must be followed by
272 four hexadecimal digits; otherwise it matches a literal "u" character.
273 .P
274 Characters whose value is less than 256 can be defined by either of the two
275 syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
276 way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
277 \eu00dc in JavaScript mode).
278 .P
279 After \e0 up to two further octal digits are read. If there are fewer than two
280 digits, just those that are present are used. Thus the sequence \e0\ex\e07
281 specifies two binary zeros followed by a BEL character (code value 7). Make
282 sure you supply two digits after the initial zero if the pattern character that
283 follows is itself an octal digit.
284 .P
285 The handling of a backslash followed by a digit other than 0 is complicated.
286 Outside a character class, PCRE reads it and any following digits as a decimal
287 number. If the number is less than 10, or if there have been at least that many
288 previous capturing left parentheses in the expression, the entire sequence is
289 taken as a \fIback reference\fP. A description of how this works is given
290 .\" HTML <a href="#backreferences">
291 .\" </a>
292 later,
293 .\"
294 following the discussion of
295 .\" HTML <a href="#subpattern">
296 .\" </a>
297 parenthesized subpatterns.
298 .\"
299 .P
300 Inside a character class, or if the decimal number is greater than 9 and there
301 have not been that many capturing subpatterns, PCRE re-reads up to three octal
302 digits following the backslash, and uses them to generate a data character. Any
303 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
304 character specified in octal must be less than \e400. In UTF-8 mode, values up
305 to \e777 are permitted. For example:
306 .sp
307 \e040 is another way of writing a space
308 .\" JOIN
309 \e40 is the same, provided there are fewer than 40
310 previous capturing subpatterns
311 \e7 is always a back reference
312 .\" JOIN
313 \e11 might be a back reference, or another way of
314 writing a tab
315 \e011 is always a tab
316 \e0113 is a tab followed by the character "3"
317 .\" JOIN
318 \e113 might be a back reference, otherwise the
319 character with octal code 113
320 .\" JOIN
321 \e377 might be a back reference, otherwise
322 the byte consisting entirely of 1 bits
323 .\" JOIN
324 \e81 is either a back reference, or a binary zero
325 followed by the two characters "8" and "1"
326 .sp
327 Note that octal values of 100 or greater must not be introduced by a leading
328 zero, because no more than three octal digits are ever read.
329 .P
330 All the sequences that define a single character value can be used both inside
331 and outside character classes. In addition, inside a character class, \eb is
332 interpreted as the backspace character (hex 08).
333 .P
334 \eN is not allowed in a character class. \eB, \eR, and \eX are not special
335 inside a character class. Like other unrecognized escape sequences, they are
336 treated as the literal characters "B", "R", and "X" by default, but cause an
337 error if the PCRE_EXTRA option is set. Outside a character class, these
338 sequences have different meanings.
339 .
340 .
341 .SS "Unsupported escape sequences"
342 .rs
343 .sp
344 In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
345 handler and used to modify the case of following characters. By default, PCRE
346 does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
347 option is set, \eU matches a "U" character, and \eu can be used to define a
348 character by code point, as described in the previous section.
349 .
350 .
351 .SS "Absolute and relative back references"
352 .rs
353 .sp
354 The sequence \eg followed by an unsigned or a negative number, optionally
355 enclosed in braces, is an absolute or relative back reference. A named back
356 reference can be coded as \eg{name}. Back references are discussed
357 .\" HTML <a href="#backreferences">
358 .\" </a>
359 later,
360 .\"
361 following the discussion of
362 .\" HTML <a href="#subpattern">
363 .\" </a>
364 parenthesized subpatterns.
365 .\"
366 .
367 .
368 .SS "Absolute and relative subroutine calls"
369 .rs
370 .sp
371 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
372 a number enclosed either in angle brackets or single quotes, is an alternative
373 syntax for referencing a subpattern as a "subroutine". Details are discussed
374 .\" HTML <a href="#onigurumasubroutines">
375 .\" </a>
376 later.
377 .\"
378 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
379 synonymous. The former is a back reference; the latter is a
380 .\" HTML <a href="#subpatternsassubroutines">
381 .\" </a>
382 subroutine
383 .\"
384 call.
385 .
386 .
387 .\" HTML <a name="genericchartypes"></a>
388 .SS "Generic character types"
389 .rs
390 .sp
391 Another use of backslash is for specifying generic character types:
392 .sp
393 \ed any decimal digit
394 \eD any character that is not a decimal digit
395 \eh any horizontal whitespace character
396 \eH any character that is not a horizontal whitespace character
397 \es any whitespace character
398 \eS any character that is not a whitespace character
399 \ev any vertical whitespace character
400 \eV any character that is not a vertical whitespace character
401 \ew any "word" character
402 \eW any "non-word" character
403 .sp
404 There is also the single sequence \eN, which matches a non-newline character.
405 This is the same as
406 .\" HTML <a href="#fullstopdot">
407 .\" </a>
408 the "." metacharacter
409 .\"
410 when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
411 PCRE does not support this.
412 .P
413 Each pair of lower and upper case escape sequences partitions the complete set
414 of characters into two disjoint sets. Any given character matches one, and only
415 one, of each pair. The sequences can appear both inside and outside character
416 classes. They each match one character of the appropriate type. If the current
417 matching point is at the end of the subject string, all of them fail, because
418 there is no character to match.
419 .P
420 For compatibility with Perl, \es does not match the VT character (code 11).
421 This makes it different from the the POSIX "space" class. The \es characters
422 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
423 included in a Perl script, \es may match the VT character. In PCRE, it never
424 does.
425 .P
426 A "word" character is an underscore or any character that is a letter or digit.
427 By default, the definition of letters and digits is controlled by PCRE's
428 low-valued character tables, and may vary if locale-specific matching is taking
429 place (see
430 .\" HTML <a href="pcreapi.html#localesupport">
431 .\" </a>
432 "Locale support"
433 .\"
434 in the
435 .\" HREF
436 \fBpcreapi\fP
437 .\"
438 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
439 or "french" in Windows, some character codes greater than 128 are used for
440 accented letters, and these are then matched by \ew. The use of locales with
441 Unicode is discouraged.
442 .P
443 By default, in UTF-8 mode, characters with values greater than 128 never match
444 \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
445 their original meanings from before UTF-8 support was available, mainly for
446 efficiency reasons. However, if PCRE is compiled with Unicode property support,
447 and the PCRE_UCP option is set, the behaviour is changed so that Unicode
448 properties are used to determine character types, as follows:
449 .sp
450 \ed any character that \ep{Nd} matches (decimal digit)
451 \es any character that \ep{Z} matches, plus HT, LF, FF, CR
452 \ew any character that \ep{L} or \ep{N} matches, plus underscore
453 .sp
454 The upper case escapes match the inverse sets of characters. Note that \ed
455 matches only decimal digits, whereas \ew matches any Unicode digit, as well as
456 any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
457 \eB because they are defined in terms of \ew and \eW. Matching these sequences
458 is noticeably slower when PCRE_UCP is set.
459 .P
460 The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
461 release 5.10. In contrast to the other sequences, which match only ASCII
462 characters by default, these always match certain high-valued codepoints in
463 UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
464 are:
465 .sp
466 U+0009 Horizontal tab
467 U+0020 Space
468 U+00A0 Non-break space
469 U+1680 Ogham space mark
470 U+180E Mongolian vowel separator
471 U+2000 En quad
472 U+2001 Em quad
473 U+2002 En space
474 U+2003 Em space
475 U+2004 Three-per-em space
476 U+2005 Four-per-em space
477 U+2006 Six-per-em space
478 U+2007 Figure space
479 U+2008 Punctuation space
480 U+2009 Thin space
481 U+200A Hair space
482 U+202F Narrow no-break space
483 U+205F Medium mathematical space
484 U+3000 Ideographic space
485 .sp
486 The vertical space characters are:
487 .sp
488 U+000A Linefeed
489 U+000B Vertical tab
490 U+000C Formfeed
491 U+000D Carriage return
492 U+0085 Next line
493 U+2028 Line separator
494 U+2029 Paragraph separator
495 .
496 .
497 .\" HTML <a name="newlineseq"></a>
498 .SS "Newline sequences"
499 .rs
500 .sp
501 Outside a character class, by default, the escape sequence \eR matches any
502 Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
503 .sp
504 (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
505 .sp
506 This is an example of an "atomic group", details of which are given
507 .\" HTML <a href="#atomicgroup">
508 .\" </a>
509 below.
510 .\"
511 This particular group matches either the two-character sequence CR followed by
512 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
513 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
514 line, U+0085). The two-character sequence is treated as a single unit that
515 cannot be split.
516 .P
517 In UTF-8 mode, two additional characters whose codepoints are greater than 255
518 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
519 Unicode character property support is not needed for these characters to be
520 recognized.
521 .P
522 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
523 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
524 either at compile time or when the pattern is matched. (BSR is an abbrevation
525 for "backslash R".) This can be made the default when PCRE is built; if this is
526 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
527 It is also possible to specify these settings by starting a pattern string with
528 one of the following sequences:
529 .sp
530 (*BSR_ANYCRLF) CR, LF, or CRLF only
531 (*BSR_UNICODE) any Unicode newline sequence
532 .sp
533 These override the default and the options given to \fBpcre_compile()\fP or
534 \fBpcre_compile2()\fP, but they can be overridden by options given to
535 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
536 which are not Perl-compatible, are recognized only at the very start of a
537 pattern, and that they must be in upper case. If more than one of them is
538 present, the last one is used. They can be combined with a change of newline
539 convention; for example, a pattern can start with:
540 .sp
542 .sp
543 They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
544 a character class, \eR is treated as an unrecognized escape sequence, and so
545 matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
546 .
547 .
548 .\" HTML <a name="uniextseq"></a>
549 .SS Unicode character properties
550 .rs
551 .sp
552 When PCRE is built with Unicode character property support, three additional
553 escape sequences that match characters with specific properties are available.
554 When not in UTF-8 mode, these sequences are of course limited to testing
555 characters whose codepoints are less than 256, but they do work in this mode.
556 The extra escape sequences are:
557 .sp
558 \ep{\fIxx\fP} a character with the \fIxx\fP property
559 \eP{\fIxx\fP} a character without the \fIxx\fP property
560 \eX an extended Unicode sequence
561 .sp
562 The property names represented by \fIxx\fP above are limited to the Unicode
563 script names, the general category properties, "Any", which matches any
564 character (including newline), and some special PCRE properties (described
565 in the
566 .\" HTML <a href="#extraprops">
567 .\" </a>
568 next section).
569 .\"
570 Other Perl properties such as "InMusicalSymbols" are not currently supported by
571 PCRE. Note that \eP{Any} does not match any characters, so always causes a
572 match failure.
573 .P
574 Sets of Unicode characters are defined as belonging to certain scripts. A
575 character from one of these sets can be matched using a script name. For
576 example:
577 .sp
578 \ep{Greek}
579 \eP{Han}
580 .sp
581 Those that are not part of an identified script are lumped together as
582 "Common". The current list of scripts is:
583 .P
584 Arabic,
585 Armenian,
586 Avestan,
587 Balinese,
588 Bamum,
589 Bengali,
590 Bopomofo,
591 Braille,
592 Buginese,
593 Buhid,
594 Canadian_Aboriginal,
595 Carian,
596 Cham,
597 Cherokee,
598 Common,
599 Coptic,
600 Cuneiform,
601 Cypriot,
602 Cyrillic,
603 Deseret,
604 Devanagari,
605 Egyptian_Hieroglyphs,
606 Ethiopic,
607 Georgian,
608 Glagolitic,
609 Gothic,
610 Greek,
611 Gujarati,
612 Gurmukhi,
613 Han,
614 Hangul,
615 Hanunoo,
616 Hebrew,
617 Hiragana,
618 Imperial_Aramaic,
619 Inherited,
620 Inscriptional_Pahlavi,
621 Inscriptional_Parthian,
622 Javanese,
623 Kaithi,
624 Kannada,
625 Katakana,
626 Kayah_Li,
627 Kharoshthi,
628 Khmer,
629 Lao,
630 Latin,
631 Lepcha,
632 Limbu,
633 Linear_B,
634 Lisu,
635 Lycian,
636 Lydian,
637 Malayalam,
638 Meetei_Mayek,
639 Mongolian,
640 Myanmar,
641 New_Tai_Lue,
642 Nko,
643 Ogham,
644 Old_Italic,
645 Old_Persian,
646 Old_South_Arabian,
647 Old_Turkic,
648 Ol_Chiki,
649 Oriya,
650 Osmanya,
651 Phags_Pa,
652 Phoenician,
653 Rejang,
654 Runic,
655 Samaritan,
656 Saurashtra,
657 Shavian,
658 Sinhala,
659 Sundanese,
660 Syloti_Nagri,
661 Syriac,
662 Tagalog,
663 Tagbanwa,
664 Tai_Le,
665 Tai_Tham,
666 Tai_Viet,
667 Tamil,
668 Telugu,
669 Thaana,
670 Thai,
671 Tibetan,
672 Tifinagh,
673 Ugaritic,
674 Vai,
675 Yi.
676 .P
677 Each character has exactly one Unicode general category property, specified by
678 a two-letter abbreviation. For compatibility with Perl, negation can be
679 specified by including a circumflex between the opening brace and the property
680 name. For example, \ep{^Lu} is the same as \eP{Lu}.
681 .P
682 If only one letter is specified with \ep or \eP, it includes all the general
683 category properties that start with that letter. In this case, in the absence
684 of negation, the curly brackets in the escape sequence are optional; these two
685 examples have the same effect:
686 .sp
687 \ep{L}
688 \epL
689 .sp
690 The following general category property codes are supported:
691 .sp
692 C Other
693 Cc Control
694 Cf Format
695 Cn Unassigned
696 Co Private use
697 Cs Surrogate
698 .sp
699 L Letter
700 Ll Lower case letter
701 Lm Modifier letter
702 Lo Other letter
703 Lt Title case letter
704 Lu Upper case letter
705 .sp
706 M Mark
707 Mc Spacing mark
708 Me Enclosing mark
709 Mn Non-spacing mark
710 .sp
711 N Number
712 Nd Decimal number
713 Nl Letter number
714 No Other number
715 .sp
716 P Punctuation
717 Pc Connector punctuation
718 Pd Dash punctuation
719 Pe Close punctuation
720 Pf Final punctuation
721 Pi Initial punctuation
722 Po Other punctuation
723 Ps Open punctuation
724 .sp
725 S Symbol
726 Sc Currency symbol
727 Sk Modifier symbol
728 Sm Mathematical symbol
729 So Other symbol
730 .sp
731 Z Separator
732 Zl Line separator
733 Zp Paragraph separator
734 Zs Space separator
735 .sp
736 The special property L& is also supported: it matches a character that has
737 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
738 a modifier or "other".
739 .P
740 The Cs (Surrogate) property applies only to characters in the range U+D800 to
741 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
742 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
743 (see the discussion of PCRE_NO_UTF8_CHECK in the
744 .\" HREF
745 \fBpcreapi\fP
746 .\"
747 page). Perl does not support the Cs property.
748 .P
749 The long synonyms for property names that Perl supports (such as \ep{Letter})
750 are not supported by PCRE, nor is it permitted to prefix any of these
751 properties with "Is".
752 .P
753 No character that is in the Unicode table has the Cn (unassigned) property.
754 Instead, this property is assumed for any code point that is not in the
755 Unicode table.
756 .P
757 Specifying caseless matching does not affect these escape sequences. For
758 example, \ep{Lu} always matches only upper case letters.
759 .P
760 The \eX escape matches any number of Unicode characters that form an extended
761 Unicode sequence. \eX is equivalent to
762 .sp
763 (?>\ePM\epM*)
764 .sp
765 That is, it matches a character without the "mark" property, followed by zero
766 or more characters with the "mark" property, and treats the sequence as an
767 atomic group
768 .\" HTML <a href="#atomicgroup">
769 .\" </a>
770 (see below).
771 .\"
772 Characters with the "mark" property are typically accents that affect the
773 preceding character. None of them have codepoints less than 256, so in
774 non-UTF-8 mode \eX matches any one character.
775 .P
776 Note that recent versions of Perl have changed \eX to match what Unicode calls
777 an "extended grapheme cluster", which has a more complicated definition.
778 .P
779 Matching characters by Unicode property is not fast, because PCRE has to search
780 a structure that contains data for over fifteen thousand characters. That is
781 why the traditional escape sequences such as \ed and \ew do not use Unicode
782 properties in PCRE by default, though you can make them do so by setting the
783 PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
784 (*UCP).
785 .
786 .
787 .\" HTML <a name="extraprops"></a>
788 .SS PCRE's additional properties
789 .rs
790 .sp
791 As well as the standard Unicode properties described in the previous
792 section, PCRE supports four more that make it possible to convert traditional
793 escape sequences such as \ew and \es and POSIX character classes to use Unicode
794 properties. PCRE uses these non-standard, non-Perl properties internally when
795 PCRE_UCP is set. They are:
796 .sp
797 Xan Any alphanumeric character
798 Xps Any POSIX space character
799 Xsp Any Perl space character
800 Xwd Any Perl "word" character
801 .sp
802 Xan matches characters that have either the L (letter) or the N (number)
803 property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
804 carriage return, and any other character that has the Z (separator) property.
805 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
806 same characters as Xan, plus underscore.
807 .
808 .
809 .\" HTML <a name="resetmatchstart"></a>
810 .SS "Resetting the match start"
811 .rs
812 .sp
813 The escape sequence \eK causes any previously matched characters not to be
814 included in the final matched sequence. For example, the pattern:
815 .sp
816 foo\eKbar
817 .sp
818 matches "foobar", but reports that it has matched "bar". This feature is
819 similar to a lookbehind assertion
820 .\" HTML <a href="#lookbehind">
821 .\" </a>
822 (described below).
823 .\"
824 However, in this case, the part of the subject before the real match does not
825 have to be of fixed length, as lookbehind assertions do. The use of \eK does
826 not interfere with the setting of
827 .\" HTML <a href="#subpattern">
828 .\" </a>
829 captured substrings.
830 .\"
831 For example, when the pattern
832 .sp
833 (foo)\eKbar
834 .sp
835 matches "foobar", the first substring is still set to "foo".
836 .P
837 Perl documents that the use of \eK within assertions is "not well defined". In
838 PCRE, \eK is acted upon when it occurs inside positive assertions, but is
839 ignored in negative assertions.
840 .
841 .
842 .\" HTML <a name="smallassertions"></a>
843 .SS "Simple assertions"
844 .rs
845 .sp
846 The final use of backslash is for certain simple assertions. An assertion
847 specifies a condition that has to be met at a particular point in a match,
848 without consuming any characters from the subject string. The use of
849 subpatterns for more complicated assertions is described
850 .\" HTML <a href="#bigassertions">
851 .\" </a>
852 below.
853 .\"
854 The backslashed assertions are:
855 .sp
856 \eb matches at a word boundary
857 \eB matches when not at a word boundary
858 \eA matches at the start of the subject
859 \eZ matches at the end of the subject
860 also matches before a newline at the end of the subject
861 \ez matches only at the end of the subject
862 \eG matches at the first matching position in the subject
863 .sp
864 Inside a character class, \eb has a different meaning; it matches the backspace
865 character. If any other of these assertions appears in a character class, by
866 default it matches the corresponding literal character (for example, \eB
867 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
868 escape sequence" error is generated instead.
869 .P
870 A word boundary is a position in the subject string where the current character
871 and the previous character do not both match \ew or \eW (i.e. one matches
872 \ew and the other matches \eW), or the start or end of the string if the
873 first or last character matches \ew, respectively. In UTF-8 mode, the meanings
874 of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
875 done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
876 of word" or "end of word" metasequence. However, whatever follows \eb normally
877 determines which it is. For example, the fragment \eba matches "a" at the start
878 of a word.
879 .P
880 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
881 dollar (described in the next section) in that they only ever match at the very
882 start and end of the subject string, whatever options are set. Thus, they are
883 independent of multiline mode. These three assertions are not affected by the
884 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
885 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
886 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
887 at a point other than the beginning of the subject, \eA can never match. The
888 difference between \eZ and \ez is that \eZ matches before a newline at the end
889 of the string as well as at the very end, whereas \ez matches only at the end.
890 .P
891 The \eG assertion is true only when the current matching position is at the
892 start point of the match, as specified by the \fIstartoffset\fP argument of
893 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
894 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
895 arguments, you can mimic Perl's /g option, and it is in this kind of
896 implementation where \eG can be useful.
897 .P
898 Note, however, that PCRE's interpretation of \eG, as the start of the current
899 match, is subtly different from Perl's, which defines it as the end of the
900 previous match. In Perl, these can be different when the previously matched
901 string was empty. Because PCRE does just one match at a time, it cannot
902 reproduce this behaviour.
903 .P
904 If all the alternatives of a pattern begin with \eG, the expression is anchored
905 to the starting match position, and the "anchored" flag is set in the compiled
906 regular expression.
907 .
908 .
910 .rs
911 .sp
912 Outside a character class, in the default matching mode, the circumflex
913 character is an assertion that is true only if the current matching point is
914 at the start of the subject string. If the \fIstartoffset\fP argument of
915 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
916 option is unset. Inside a character class, circumflex has an entirely different
917 meaning
918 .\" HTML <a href="#characterclass">
919 .\" </a>
920 (see below).
921 .\"
922 .P
923 Circumflex need not be the first character of the pattern if a number of
924 alternatives are involved, but it should be the first thing in each alternative
925 in which it appears if the pattern is ever to match that branch. If all
926 possible alternatives start with a circumflex, that is, if the pattern is
927 constrained to match only at the start of the subject, it is said to be an
928 "anchored" pattern. (There are also other constructs that can cause a pattern
929 to be anchored.)
930 .P
931 A dollar character is an assertion that is true only if the current matching
932 point is at the end of the subject string, or immediately before a newline
933 at the end of the string (by default). Dollar need not be the last character of
934 the pattern if a number of alternatives are involved, but it should be the last
935 item in any branch in which it appears. Dollar has no special meaning in a
936 character class.
937 .P
938 The meaning of dollar can be changed so that it matches only at the very end of
939 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
940 does not affect the \eZ assertion.
941 .P
942 The meanings of the circumflex and dollar characters are changed if the
943 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
944 immediately after internal newlines as well as at the start of the subject
945 string. It does not match after a newline that ends the string. A dollar
946 matches before any newlines in the string, as well as at the very end, when
947 PCRE_MULTILINE is set. When newline is specified as the two-character
948 sequence CRLF, isolated CR and LF characters do not indicate newlines.
949 .P
950 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
951 \en represents a newline) in multiline mode, but not otherwise. Consequently,
952 patterns that are anchored in single line mode because all branches start with
953 ^ are not anchored in multiline mode, and a match for circumflex is possible
954 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
955 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
956 .P
957 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
958 end of the subject in both modes, and if all branches of a pattern start with
959 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
960 .
961 .
962 .\" HTML <a name="fullstopdot"></a>
964 .rs
965 .sp
966 Outside a character class, a dot in the pattern matches any one character in
967 the subject string except (by default) a character that signifies the end of a
968 line. In UTF-8 mode, the matched character may be more than one byte long.
969 .P
970 When a line ending is defined as a single character, dot never matches that
971 character; when the two-character sequence CRLF is used, dot does not match CR
972 if it is immediately followed by LF, but otherwise it matches all characters
973 (including isolated CRs and LFs). When any Unicode line endings are being
974 recognized, dot does not match CR or LF or any of the other line ending
975 characters.
976 .P
977 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
978 option is set, a dot matches any one character, without exception. If the
979 two-character sequence CRLF is present in the subject string, it takes two dots
980 to match it.
981 .P
982 The handling of dot is entirely independent of the handling of circumflex and
983 dollar, the only relationship being that they both involve newlines. Dot has no
984 special meaning in a character class.
985 .P
986 The escape sequence \eN behaves like a dot, except that it is not affected by
987 the PCRE_DOTALL option. In other words, it matches any character except one
988 that signifies the end of a line. Perl also uses \eN to match characters by
989 name; PCRE does not support this.
990 .
991 .
993 .rs
994 .sp
995 Outside a character class, the escape sequence \eC matches any one byte, both
996 in and out of UTF-8 mode. Unlike a dot, it always matches line-ending
997 characters. The feature is provided in Perl in order to match individual bytes
998 in UTF-8 mode, but it is unclear how it can usefully be used. Because \eC
999 breaks up characters into individual bytes, matching one byte with \eC in UTF-8
1000 mode means that the rest of the string may start with a malformed UTF-8
1001 character. This has undefined results, because PCRE assumes that it is dealing
1002 with valid UTF-8 strings (and by default it checks this at the start of
1003 processing unless the PCRE_NO_UTF8_CHECK option is used).
1004 .P
1005 PCRE does not allow \eC to appear in lookbehind assertions
1006 .\" HTML <a href="#lookbehind">
1007 .\" </a>
1008 (described below)
1009 .\"
1010 in UTF-8 mode, because this would make it impossible to calculate the length of
1011 the lookbehind.
1012 .P
1013 In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one
1014 way of using it that avoids the problem of malformed UTF-8 characters is to
1015 use a lookahead to check the length of the next character, as in this pattern
1016 (ignore white space and line breaks):
1017 .sp
1018 (?| (?=[\ex00-\ex7f])(\eC) |
1019 (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1020 (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1021 (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1022 .sp
1023 A group that starts with (?| resets the capturing parentheses numbers in each
1024 alternative (see
1025 .\" HTML <a href="#dupsubpatternnumber">
1026 .\" </a>
1027 "Duplicate Subpattern Numbers"
1028 .\"
1029 below). The assertions at the start of each branch check the next UTF-8
1030 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1031 character's individual bytes are then captured by the appropriate number of
1032 groups.
1033 .
1034 .
1035 .\" HTML <a name="characterclass"></a>
1037 .rs
1038 .sp
1039 An opening square bracket introduces a character class, terminated by a closing
1040 square bracket. A closing square bracket on its own is not special by default.
1041 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1042 bracket causes a compile-time error. If a closing square bracket is required as
1043 a member of the class, it should be the first data character in the class
1044 (after an initial circumflex, if present) or escaped with a backslash.
1045 .P
1046 A character class matches a single character in the subject. In UTF-8 mode, the
1047 character may be more than one byte long. A matched character must be in the
1048 set of characters defined by the class, unless the first character in the class
1049 definition is a circumflex, in which case the subject character must not be in
1050 the set defined by the class. If a circumflex is actually required as a member
1051 of the class, ensure it is not the first character, or escape it with a
1052 backslash.
1053 .P
1054 For example, the character class [aeiou] matches any lower case vowel, while
1055 [^aeiou] matches any character that is not a lower case vowel. Note that a
1056 circumflex is just a convenient notation for specifying the characters that
1057 are in the class by enumerating those that are not. A class that starts with a
1058 circumflex is not an assertion; it still consumes a character from the subject
1059 string, and therefore it fails if the current pointer is at the end of the
1060 string.
1061 .P
1062 In UTF-8 mode, characters with values greater than 255 can be included in a
1063 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
1064 .P
1065 When caseless matching is set, any letters in a class represent both their
1066 upper case and lower case versions, so for example, a caseless [aeiou] matches
1067 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1068 caseful version would. In UTF-8 mode, PCRE always understands the concept of
1069 case for characters whose values are less than 128, so caseless matching is
1070 always possible. For characters with higher values, the concept of case is
1071 supported if PCRE is compiled with Unicode property support, but not otherwise.
1072 If you want to use caseless matching in UTF8-mode for characters 128 and above,
1073 you must ensure that PCRE is compiled with Unicode property support as well as
1074 with UTF-8 support.
1075 .P
1076 Characters that might indicate line breaks are never treated in any special way
1077 when matching character classes, whatever line-ending sequence is in use, and
1078 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1079 such as [^a] always matches one of these characters.
1080 .P
1081 The minus (hyphen) character can be used to specify a range of characters in a
1082 character class. For example, [d-m] matches any letter between d and m,
1083 inclusive. If a minus character is required in a class, it must be escaped with
1084 a backslash or appear in a position where it cannot be interpreted as
1085 indicating a range, typically as the first or last character in the class.
1086 .P
1087 It is not possible to have the literal character "]" as the end character of a
1088 range. A pattern such as [W-]46] is interpreted as a class of two characters
1089 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1090 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1091 the end of range, so [W-\e]46] is interpreted as a class containing a range
1092 followed by two other characters. The octal or hexadecimal representation of
1093 "]" can also be used to end a range.
1094 .P
1095 Ranges operate in the collating sequence of character values. They can also be
1096 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
1097 mode, ranges can include characters whose values are greater than 255, for
1098 example [\ex{100}-\ex{2ff}].
1099 .P
1100 If a range that includes letters is used when caseless matching is set, it
1101 matches the letters in either case. For example, [W-c] is equivalent to
1102 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1103 tables for a French locale are in use, [\exc8-\excb] matches accented E
1104 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1105 characters with values greater than 128 only when it is compiled with Unicode
1106 property support.
1107 .P
1108 The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1109 \eV, \ew, and \eW may appear in a character class, and add the characters that
1110 they match to the class. For example, [\edABCDEF] matches any hexadecimal
1111 digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1112 and their upper case partners, just as it does when they appear outside a
1113 character class, as described in the section entitled
1114 .\" HTML <a href="#genericchartypes">
1115 .\" </a>
1116 "Generic character types"
1117 .\"
1118 above. The escape sequence \eb has a different meaning inside a character
1119 class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1120 are not special inside a character class. Like any other unrecognized escape
1121 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1122 default, but cause an error if the PCRE_EXTRA option is set.
1123 .P
1124 A circumflex can conveniently be used with the upper case character types to
1125 specify a more restricted set of characters than the matching lower case type.
1126 For example, the class [^\eW_] matches any letter or digit, but not underscore,
1127 whereas [\ew] includes underscore. A positive character class should be read as
1128 "something OR something OR ..." and a negative class as "NOT something AND NOT
1129 something AND NOT ...".
1130 .P
1131 The only metacharacters that are recognized in character classes are backslash,
1132 hyphen (only where it can be interpreted as specifying a range), circumflex
1133 (only at the start), opening square bracket (only when it can be interpreted as
1134 introducing a POSIX class name - see the next section), and the terminating
1135 closing square bracket. However, escaping other non-alphanumeric characters
1136 does no harm.
1137 .
1138 .
1140 .rs
1141 .sp
1142 Perl supports the POSIX notation for character classes. This uses names
1143 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1144 this notation. For example,
1145 .sp
1146 [01[:alpha:]%]
1147 .sp
1148 matches "0", "1", any alphabetic character, or "%". The supported class names
1149 are:
1150 .sp
1151 alnum letters and digits
1152 alpha letters
1153 ascii character codes 0 - 127
1154 blank space or tab only
1155 cntrl control characters
1156 digit decimal digits (same as \ed)
1157 graph printing characters, excluding space
1158 lower lower case letters
1159 print printing characters, including space
1160 punct printing characters, excluding letters and digits and space
1161 space white space (not quite the same as \es)
1162 upper upper case letters
1163 word "word" characters (same as \ew)
1164 xdigit hexadecimal digits
1165 .sp
1166 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1167 space (32). Notice that this list includes the VT character (code 11). This
1168 makes "space" different to \es, which does not include VT (for Perl
1169 compatibility).
1170 .P
1171 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1172 5.8. Another Perl extension is negation, which is indicated by a ^ character
1173 after the colon. For example,
1174 .sp
1175 [12[:^digit:]]
1176 .sp
1177 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1178 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1179 supported, and an error is given if they are encountered.
1180 .P
1181 By default, in UTF-8 mode, characters with values greater than 128 do not match
1182 any of the POSIX character classes. However, if the PCRE_UCP option is passed
1183 to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1184 character properties are used. This is achieved by replacing the POSIX classes
1185 by other sequences, as follows:
1186 .sp
1187 [:alnum:] becomes \ep{Xan}
1188 [:alpha:] becomes \ep{L}
1189 [:blank:] becomes \eh
1190 [:digit:] becomes \ep{Nd}
1191 [:lower:] becomes \ep{Ll}
1192 [:space:] becomes \ep{Xps}
1193 [:upper:] becomes \ep{Lu}
1194 [:word:] becomes \ep{Xwd}
1195 .sp
1196 Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1197 classes are unchanged, and match only characters with code points less than
1198 128.
1199 .
1200 .
1202 .rs
1203 .sp
1204 Vertical bar characters are used to separate alternative patterns. For example,
1205 the pattern
1206 .sp
1207 gilbert|sullivan
1208 .sp
1209 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1210 and an empty alternative is permitted (matching the empty string). The matching
1211 process tries each alternative in turn, from left to right, and the first one
1212 that succeeds is used. If the alternatives are within a subpattern
1213 .\" HTML <a href="#subpattern">
1214 .\" </a>
1215 (defined below),
1216 .\"
1217 "succeeds" means matching the rest of the main pattern as well as the
1218 alternative in the subpattern.
1219 .
1220 .
1222 .rs
1223 .sp
1224 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1225 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1226 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1227 The option letters are
1228 .sp
1229 i for PCRE_CASELESS
1231 s for PCRE_DOTALL
1232 x for PCRE_EXTENDED
1233 .sp
1234 For example, (?im) sets caseless, multiline matching. It is also possible to
1235 unset these options by preceding the letter with a hyphen, and a combined
1236 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1237 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1238 permitted. If a letter appears both before and after the hyphen, the option is
1239 unset.
1240 .P
1241 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1242 changed in the same way as the Perl-compatible options by using the characters
1243 J, U and X respectively.
1244 .P
1245 When one of these option changes occurs at top level (that is, not inside
1246 subpattern parentheses), the change applies to the remainder of the pattern
1247 that follows. If the change is placed right at the start of a pattern, PCRE
1248 extracts it into the global options (and it will therefore show up in data
1249 extracted by the \fBpcre_fullinfo()\fP function).
1250 .P
1251 An option change within a subpattern (see below for a description of
1252 subpatterns) affects only that part of the subpattern that follows it, so
1253 .sp
1254 (a(?i)b)c
1255 .sp
1256 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1257 By this means, options can be made to have different settings in different
1258 parts of the pattern. Any changes made in one alternative do carry on
1259 into subsequent branches within the same subpattern. For example,
1260 .sp
1261 (a(?i)b|c)
1262 .sp
1263 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1264 branch is abandoned before the option setting. This is because the effects of
1265 option settings happen at compile time. There would be some very weird
1266 behaviour otherwise.
1267 .P
1268 \fBNote:\fP There are other PCRE-specific options that can be set by the
1269 application when the compile or match functions are called. In some cases the
1270 pattern can contain special leading sequences such as (*CRLF) to override what
1271 the application has set or what has been defaulted. Details are given in the
1272 section entitled
1273 .\" HTML <a href="#newlineseq">
1274 .\" </a>
1275 "Newline sequences"
1276 .\"
1277 above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1278 to set UTF-8 and Unicode property modes; they are equivalent to setting the
1279 PCRE_UTF8 and the PCRE_UCP options, respectively.
1280 .
1281 .
1282 .\" HTML <a name="subpattern"></a>
1284 .rs
1285 .sp
1286 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1287 Turning part of a pattern into a subpattern does two things:
1288 .sp
1289 1. It localizes a set of alternatives. For example, the pattern
1290 .sp
1291 cat(aract|erpillar|)
1292 .sp
1293 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1294 match "cataract", "erpillar" or an empty string.
1295 .sp
1296 2. It sets up the subpattern as a capturing subpattern. This means that, when
1297 the whole pattern matches, that portion of the subject string that matched the
1298 subpattern is passed back to the caller via the \fIovector\fP argument of
1299 \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1300 from 1) to obtain numbers for the capturing subpatterns. For example, if the
1301 string "the red king" is matched against the pattern
1302 .sp
1303 the ((red|white) (king|queen))
1304 .sp
1305 the captured substrings are "red king", "red", and "king", and are numbered 1,
1306 2, and 3, respectively.
1307 .P
1308 The fact that plain parentheses fulfil two functions is not always helpful.
1309 There are often times when a grouping subpattern is required without a
1310 capturing requirement. If an opening parenthesis is followed by a question mark
1311 and a colon, the subpattern does not do any capturing, and is not counted when
1312 computing the number of any subsequent capturing subpatterns. For example, if
1313 the string "the white queen" is matched against the pattern
1314 .sp
1315 the ((?:red|white) (king|queen))
1316 .sp
1317 the captured substrings are "white queen" and "queen", and are numbered 1 and
1318 2. The maximum number of capturing subpatterns is 65535.
1319 .P
1320 As a convenient shorthand, if any option settings are required at the start of
1321 a non-capturing subpattern, the option letters may appear between the "?" and
1322 the ":". Thus the two patterns
1323 .sp
1324 (?i:saturday|sunday)
1325 (?:(?i)saturday|sunday)
1326 .sp
1327 match exactly the same set of strings. Because alternative branches are tried
1328 from left to right, and options are not reset until the end of the subpattern
1329 is reached, an option setting in one branch does affect subsequent branches, so
1330 the above patterns match "SUNDAY" as well as "Saturday".
1331 .
1332 .
1333 .\" HTML <a name="dupsubpatternnumber"></a>
1335 .rs
1336 .sp
1337 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1338 the same numbers for its capturing parentheses. Such a subpattern starts with
1339 (?| and is itself a non-capturing subpattern. For example, consider this
1340 pattern:
1341 .sp
1342 (?|(Sat)ur|(Sun))day
1343 .sp
1344 Because the two alternatives are inside a (?| group, both sets of capturing
1345 parentheses are numbered one. Thus, when the pattern matches, you can look
1346 at captured substring number one, whichever alternative matched. This construct
1347 is useful when you want to capture part, but not all, of one of a number of
1348 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1349 number is reset at the start of each branch. The numbers of any capturing
1350 parentheses that follow the subpattern start after the highest number used in
1351 any branch. The following example is taken from the Perl documentation. The
1352 numbers underneath show in which buffer the captured content will be stored.
1353 .sp
1354 # before ---------------branch-reset----------- after
1355 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1356 # 1 2 2 3 2 3 4
1357 .sp
1358 A back reference to a numbered subpattern uses the most recent value that is
1359 set for that number by any subpattern. The following pattern matches "abcabc"
1360 or "defdef":
1361 .sp
1362 /(?|(abc)|(def))\e1/
1363 .sp
1364 In contrast, a subroutine call to a numbered subpattern always refers to the
1365 first one in the pattern with the given number. The following pattern matches
1366 "abcabc" or "defabc":
1367 .sp
1368 /(?|(abc)|(def))(?1)/
1369 .sp
1370 If a
1371 .\" HTML <a href="#conditions">
1372 .\" </a>
1373 condition test
1374 .\"
1375 for a subpattern's having matched refers to a non-unique number, the test is
1376 true if any of the subpatterns of that number have matched.
1377 .P
1378 An alternative approach to using this "branch reset" feature is to use
1379 duplicate named subpatterns, as described in the next section.
1380 .
1381 .
1383 .rs
1384 .sp
1385 Identifying capturing parentheses by number is simple, but it can be very hard
1386 to keep track of the numbers in complicated regular expressions. Furthermore,
1387 if an expression is modified, the numbers may change. To help with this
1388 difficulty, PCRE supports the naming of subpatterns. This feature was not
1389 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1390 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1391 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1392 have different names, but PCRE does not.
1393 .P
1394 In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1395 (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1396 parentheses from other parts of the pattern, such as
1397 .\" HTML <a href="#backreferences">
1398 .\" </a>
1399 back references,
1400 .\"
1401 .\" HTML <a href="#recursion">
1402 .\" </a>
1403 recursion,
1404 .\"
1405 and
1406 .\" HTML <a href="#conditions">
1407 .\" </a>
1408 conditions,
1409 .\"
1410 can be made by name as well as by number.
1411 .P
1412 Names consist of up to 32 alphanumeric characters and underscores. Named
1413 capturing parentheses are still allocated numbers as well as names, exactly as
1414 if the names were not present. The PCRE API provides function calls for
1415 extracting the name-to-number translation table from a compiled pattern. There
1416 is also a convenience function for extracting a captured substring by name.
1417 .P
1418 By default, a name must be unique within a pattern, but it is possible to relax
1419 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1420 names are also always permitted for subpatterns with the same number, set up as
1421 described in the previous section.) Duplicate names can be useful for patterns
1422 where only one instance of the named parentheses can match. Suppose you want to
1423 match the name of a weekday, either as a 3-letter abbreviation or as the full
1424 name, and in both cases you want to extract the abbreviation. This pattern
1425 (ignoring the line breaks) does the job:
1426 .sp
1427 (?<DN>Mon|Fri|Sun)(?:day)?|
1428 (?<DN>Tue)(?:sday)?|
1429 (?<DN>Wed)(?:nesday)?|
1430 (?<DN>Thu)(?:rsday)?|
1431 (?<DN>Sat)(?:urday)?
1432 .sp
1433 There are five capturing substrings, but only one is ever set after a match.
1434 (An alternative way of solving this problem is to use a "branch reset"
1435 subpattern, as described in the previous section.)
1436 .P
1437 The convenience function for extracting the data by name returns the substring
1438 for the first (and in this example, the only) subpattern of that name that
1439 matched. This saves searching to find which numbered subpattern it was.
1440 .P
1441 If you make a back reference to a non-unique named subpattern from elsewhere in
1442 the pattern, the one that corresponds to the first occurrence of the name is
1443 used. In the absence of duplicate numbers (see the previous section) this is
1444 the one with the lowest number. If you use a named reference in a condition
1445 test (see the
1446 .\"
1447 .\" HTML <a href="#conditions">
1448 .\" </a>
1449 section about conditions
1450 .\"
1451 below), either to check whether a subpattern has matched, or to check for
1452 recursion, all subpatterns with the same name are tested. If the condition is
1453 true for any one of them, the overall condition is true. This is the same
1454 behaviour as testing by number. For further details of the interfaces for
1455 handling named subpatterns, see the
1456 .\" HREF
1457 \fBpcreapi\fP
1458 .\"
1459 documentation.
1460 .P
1461 \fBWarning:\fP You cannot use different names to distinguish between two
1462 subpatterns with the same number because PCRE uses only the numbers when
1463 matching. For this reason, an error is given at compile time if different names
1464 are given to subpatterns with the same number. However, you can give the same
1465 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1466 .
1467 .
1469 .rs
1470 .sp
1471 Repetition is specified by quantifiers, which can follow any of the following
1472 items:
1473 .sp
1474 a literal data character
1475 the dot metacharacter
1476 the \eC escape sequence
1477 the \eX escape sequence (in UTF-8 mode with Unicode properties)
1478 the \eR escape sequence
1479 an escape such as \ed or \epL that matches a single character
1480 a character class
1481 a back reference (see next section)
1482 a parenthesized subpattern (including assertions)
1483 a subroutine call to a subpattern (recursive or otherwise)
1484 .sp
1485 The general repetition quantifier specifies a minimum and maximum number of
1486 permitted matches, by giving the two numbers in curly brackets (braces),
1487 separated by a comma. The numbers must be less than 65536, and the first must
1488 be less than or equal to the second. For example:
1489 .sp
1490 z{2,4}
1491 .sp
1492 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1493 character. If the second number is omitted, but the comma is present, there is
1494 no upper limit; if the second number and the comma are both omitted, the
1495 quantifier specifies an exact number of required matches. Thus
1496 .sp
1497 [aeiou]{3,}
1498 .sp
1499 matches at least 3 successive vowels, but may match many more, while
1500 .sp
1501 \ed{8}
1502 .sp
1503 matches exactly 8 digits. An opening curly bracket that appears in a position
1504 where a quantifier is not allowed, or one that does not match the syntax of a
1505 quantifier, is taken as a literal character. For example, {,6} is not a
1506 quantifier, but a literal string of four characters.
1507 .P
1508 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1509 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1510 which is represented by a two-byte sequence. Similarly, when Unicode property
1511 support is available, \eX{3} matches three Unicode extended sequences, each of
1512 which may be several bytes long (and they may be of different lengths).
1513 .P
1514 The quantifier {0} is permitted, causing the expression to behave as if the
1515 previous item and the quantifier were not present. This may be useful for
1516 subpatterns that are referenced as
1517 .\" HTML <a href="#subpatternsassubroutines">
1518 .\" </a>
1519 subroutines
1520 .\"
1521 from elsewhere in the pattern (but see also the section entitled
1522 .\" HTML <a href="#subdefine">
1523 .\" </a>
1524 "Defining subpatterns for use by reference only"
1525 .\"
1526 below). Items other than subpatterns that have a {0} quantifier are omitted
1527 from the compiled pattern.
1528 .P
1529 For convenience, the three most common quantifiers have single-character
1530 abbreviations:
1531 .sp
1532 * is equivalent to {0,}
1533 + is equivalent to {1,}
1534 ? is equivalent to {0,1}
1535 .sp
1536 It is possible to construct infinite loops by following a subpattern that can
1537 match no characters with a quantifier that has no upper limit, for example:
1538 .sp
1539 (a?)*
1540 .sp
1541 Earlier versions of Perl and PCRE used to give an error at compile time for
1542 such patterns. However, because there are cases where this can be useful, such
1543 patterns are now accepted, but if any repetition of the subpattern does in fact
1544 match no characters, the loop is forcibly broken.
1545 .P
1546 By default, the quantifiers are "greedy", that is, they match as much as
1547 possible (up to the maximum number of permitted times), without causing the
1548 rest of the pattern to fail. The classic example of where this gives problems
1549 is in trying to match comments in C programs. These appear between /* and */
1550 and within the comment, individual * and / characters may appear. An attempt to
1551 match C comments by applying the pattern
1552 .sp
1553 /\e*.*\e*/
1554 .sp
1555 to the string
1556 .sp
1557 /* first comment */ not comment /* second comment */
1558 .sp
1559 fails, because it matches the entire string owing to the greediness of the .*
1560 item.
1561 .P
1562 However, if a quantifier is followed by a question mark, it ceases to be
1563 greedy, and instead matches the minimum number of times possible, so the
1564 pattern
1565 .sp
1566 /\e*.*?\e*/
1567 .sp
1568 does the right thing with the C comments. The meaning of the various
1569 quantifiers is not otherwise changed, just the preferred number of matches.
1570 Do not confuse this use of question mark with its use as a quantifier in its
1571 own right. Because it has two uses, it can sometimes appear doubled, as in
1572 .sp
1573 \ed??\ed
1574 .sp
1575 which matches one digit by preference, but can match two if that is the only
1576 way the rest of the pattern matches.
1577 .P
1578 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1579 the quantifiers are not greedy by default, but individual ones can be made
1580 greedy by following them with a question mark. In other words, it inverts the
1581 default behaviour.
1582 .P
1583 When a parenthesized subpattern is quantified with a minimum repeat count that
1584 is greater than 1 or with a limited maximum, more memory is required for the
1585 compiled pattern, in proportion to the size of the minimum or maximum.
1586 .P
1587 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1588 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1589 implicitly anchored, because whatever follows will be tried against every
1590 character position in the subject string, so there is no point in retrying the
1591 overall match at any position after the first. PCRE normally treats such a
1592 pattern as though it were preceded by \eA.
1593 .P
1594 In cases where it is known that the subject string contains no newlines, it is
1595 worth setting PCRE_DOTALL in order to obtain this optimization, or
1596 alternatively using ^ to indicate anchoring explicitly.
1597 .P
1598 However, there is one situation where the optimization cannot be used. When .*
1599 is inside capturing parentheses that are the subject of a back reference
1600 elsewhere in the pattern, a match at the start may fail where a later one
1601 succeeds. Consider, for example:
1602 .sp
1603 (.*)abc\e1
1604 .sp
1605 If the subject is "xyz123abc123" the match point is the fourth character. For
1606 this reason, such a pattern is not implicitly anchored.
1607 .P
1608 When a capturing subpattern is repeated, the value captured is the substring
1609 that matched the final iteration. For example, after
1610 .sp
1611 (tweedle[dume]{3}\es*)+
1612 .sp
1613 has matched "tweedledum tweedledee" the value of the captured substring is
1614 "tweedledee". However, if there are nested capturing subpatterns, the
1615 corresponding captured values may have been set in previous iterations. For
1616 example, after
1617 .sp
1618 /(a|(b))+/
1619 .sp
1620 matches "aba" the value of the second captured substring is "b".
1621 .
1622 .
1623 .\" HTML <a name="atomicgroup"></a>
1625 .rs
1626 .sp
1627 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1628 repetition, failure of what follows normally causes the repeated item to be
1629 re-evaluated to see if a different number of repeats allows the rest of the
1630 pattern to match. Sometimes it is useful to prevent this, either to change the
1631 nature of the match, or to cause it fail earlier than it otherwise might, when
1632 the author of the pattern knows there is no point in carrying on.
1633 .P
1634 Consider, for example, the pattern \ed+foo when applied to the subject line
1635 .sp
1636 123456bar
1637 .sp
1638 After matching all 6 digits and then failing to match "foo", the normal
1639 action of the matcher is to try again with only 5 digits matching the \ed+
1640 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1641 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1642 that once a subpattern has matched, it is not to be re-evaluated in this way.
1643 .P
1644 If we use atomic grouping for the previous example, the matcher gives up
1645 immediately on failing to match "foo" the first time. The notation is a kind of
1646 special parenthesis, starting with (?> as in this example:
1647 .sp
1648 (?>\ed+)foo
1649 .sp
1650 This kind of parenthesis "locks up" the part of the pattern it contains once
1651 it has matched, and a failure further into the pattern is prevented from
1652 backtracking into it. Backtracking past it to previous items, however, works as
1653 normal.
1654 .P
1655 An alternative description is that a subpattern of this type matches the string
1656 of characters that an identical standalone pattern would match, if anchored at
1657 the current point in the subject string.
1658 .P
1659 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1660 the above example can be thought of as a maximizing repeat that must swallow
1661 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1662 number of digits they match in order to make the rest of the pattern match,
1663 (?>\ed+) can only match an entire sequence of digits.
1664 .P
1665 Atomic groups in general can of course contain arbitrarily complicated
1666 subpatterns, and can be nested. However, when the subpattern for an atomic
1667 group is just a single repeated item, as in the example above, a simpler
1668 notation, called a "possessive quantifier" can be used. This consists of an
1669 additional + character following a quantifier. Using this notation, the
1670 previous example can be rewritten as
1671 .sp
1672 \ed++foo
1673 .sp
1674 Note that a possessive quantifier can be used with an entire group, for
1675 example:
1676 .sp
1677 (abc|xyz){2,3}+
1678 .sp
1679 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1680 option is ignored. They are a convenient notation for the simpler forms of
1681 atomic group. However, there is no difference in the meaning of a possessive
1682 quantifier and the equivalent atomic group, though there may be a performance
1683 difference; possessive quantifiers should be slightly faster.
1684 .P
1685 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1686 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1687 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1688 package, and PCRE copied it from there. It ultimately found its way into Perl
1689 at release 5.10.
1690 .P
1691 PCRE has an optimization that automatically "possessifies" certain simple
1692 pattern constructs. For example, the sequence A+B is treated as A++B because
1693 there is no point in backtracking into a sequence of A's when B must follow.
1694 .P
1695 When a pattern contains an unlimited repeat inside a subpattern that can itself
1696 be repeated an unlimited number of times, the use of an atomic group is the
1697 only way to avoid some failing matches taking a very long time indeed. The
1698 pattern
1699 .sp
1700 (\eD+|<\ed+>)*[!?]
1701 .sp
1702 matches an unlimited number of substrings that either consist of non-digits, or
1703 digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1704 quickly. However, if it is applied to
1705 .sp
1706 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1707 .sp
1708 it takes a long time before reporting failure. This is because the string can
1709 be divided between the internal \eD+ repeat and the external * repeat in a
1710 large number of ways, and all have to be tried. (The example uses [!?] rather
1711 than a single character at the end, because both PCRE and Perl have an
1712 optimization that allows for fast failure when a single character is used. They
1713 remember the last single character that is required for a match, and fail early
1714 if it is not present in the string.) If the pattern is changed so that it uses
1715 an atomic group, like this:
1716 .sp
1717 ((?>\eD+)|<\ed+>)*[!?]
1718 .sp
1719 sequences of non-digits cannot be broken, and failure happens quickly.
1720 .
1721 .
1722 .\" HTML <a name="backreferences"></a>
1724 .rs
1725 .sp
1726 Outside a character class, a backslash followed by a digit greater than 0 (and
1727 possibly further digits) is a back reference to a capturing subpattern earlier
1728 (that is, to its left) in the pattern, provided there have been that many
1729 previous capturing left parentheses.
1730 .P
1731 However, if the decimal number following the backslash is less than 10, it is
1732 always taken as a back reference, and causes an error only if there are not
1733 that many capturing left parentheses in the entire pattern. In other words, the
1734 parentheses that are referenced need not be to the left of the reference for
1735 numbers less than 10. A "forward back reference" of this type can make sense
1736 when a repetition is involved and the subpattern to the right has participated
1737 in an earlier iteration.
1738 .P
1739 It is not possible to have a numerical "forward back reference" to a subpattern
1740 whose number is 10 or more using this syntax because a sequence such as \e50 is
1741 interpreted as a character defined in octal. See the subsection entitled
1742 "Non-printing characters"
1743 .\" HTML <a href="#digitsafterbackslash">
1744 .\" </a>
1745 above
1746 .\"
1747 for further details of the handling of digits following a backslash. There is
1748 no such problem when named parentheses are used. A back reference to any
1749 subpattern is possible using named parentheses (see below).
1750 .P
1751 Another way of avoiding the ambiguity inherent in the use of digits following a
1752 backslash is to use the \eg escape sequence. This escape must be followed by an
1753 unsigned number or a negative number, optionally enclosed in braces. These
1754 examples are all identical:
1755 .sp
1756 (ring), \e1
1757 (ring), \eg1
1758 (ring), \eg{1}
1759 .sp
1760 An unsigned number specifies an absolute reference without the ambiguity that
1761 is present in the older syntax. It is also useful when literal digits follow
1762 the reference. A negative number is a relative reference. Consider this
1763 example:
1764 .sp
1765 (abc(def)ghi)\eg{-1}
1766 .sp
1767 The sequence \eg{-1} is a reference to the most recently started capturing
1768 subpattern before \eg, that is, is it equivalent to \e2 in this example.
1769 Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1770 can be helpful in long patterns, and also in patterns that are created by
1771 joining together fragments that contain references within themselves.
1772 .P
1773 A back reference matches whatever actually matched the capturing subpattern in
1774 the current subject string, rather than anything matching the subpattern
1775 itself (see
1776 .\" HTML <a href="#subpatternsassubroutines">
1777 .\" </a>
1778 "Subpatterns as subroutines"
1779 .\"
1780 below for a way of doing that). So the pattern
1781 .sp
1782 (sens|respons)e and \e1ibility
1783 .sp
1784 matches "sense and sensibility" and "response and responsibility", but not
1785 "sense and responsibility". If caseful matching is in force at the time of the
1786 back reference, the case of letters is relevant. For example,
1787 .sp
1788 ((?i)rah)\es+\e1
1789 .sp
1790 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1791 capturing subpattern is matched caselessly.
1792 .P
1793 There are several different ways of writing back references to named
1794 subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1795 \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1796 back reference syntax, in which \eg can be used for both numeric and named
1797 references, is also supported. We could rewrite the above example in any of
1798 the following ways:
1799 .sp
1800 (?<p1>(?i)rah)\es+\ek<p1>
1801 (?'p1'(?i)rah)\es+\ek{p1}
1802 (?P<p1>(?i)rah)\es+(?P=p1)
1803 (?<p1>(?i)rah)\es+\eg{p1}
1804 .sp
1805 A subpattern that is referenced by name may appear in the pattern before or
1806 after the reference.
1807 .P
1808 There may be more than one back reference to the same subpattern. If a
1809 subpattern has not actually been used in a particular match, any back
1810 references to it always fail by default. For example, the pattern
1811 .sp
1812 (a|(bc))\e2
1813 .sp
1814 always fails if it starts to match "a" rather than "bc". However, if the
1815 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1816 unset value matches an empty string.
1817 .P
1818 Because there may be many capturing parentheses in a pattern, all digits
1819 following a backslash are taken as part of a potential back reference number.
1820 If the pattern continues with a digit character, some delimiter must be used to
1821 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1822 whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1823 .\" HTML <a href="#comments">
1824 .\" </a>
1825 "Comments"
1826 .\"
1827 below) can be used.
1828 .
1829 .SS "Recursive back references"
1830 .rs
1831 .sp
1832 A back reference that occurs inside the parentheses to which it refers fails
1833 when the subpattern is first used, so, for example, (a\e1) never matches.
1834 However, such references can be useful inside repeated subpatterns. For
1835 example, the pattern
1836 .sp
1837 (a|b\e1)+
1838 .sp
1839 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1840 the subpattern, the back reference matches the character string corresponding
1841 to the previous iteration. In order for this to work, the pattern must be such
1842 that the first iteration does not need to match the back reference. This can be
1843 done using alternation, as in the example above, or by a quantifier with a
1844 minimum of zero.
1845 .P
1846 Back references of this type cause the group that they reference to be treated
1847 as an
1848 .\" HTML <a href="#atomicgroup">
1849 .\" </a>
1850 atomic group.
1851 .\"
1852 Once the whole group has been matched, a subsequent matching failure cannot
1853 cause backtracking into the middle of the group.
1854 .
1855 .
1856 .\" HTML <a name="bigassertions"></a>
1858 .rs
1859 .sp
1860 An assertion is a test on the characters following or preceding the current
1861 matching point that does not actually consume any characters. The simple
1862 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1863 .\" HTML <a href="#smallassertions">
1864 .\" </a>
1865 above.
1866 .\"
1867 .P
1868 More complicated assertions are coded as subpatterns. There are two kinds:
1869 those that look ahead of the current position in the subject string, and those
1870 that look behind it. An assertion subpattern is matched in the normal way,
1871 except that it does not cause the current matching position to be changed.
1872 .P
1873 Assertion subpatterns are not capturing subpatterns. If such an assertion
1874 contains capturing subpatterns within it, these are counted for the purposes of
1875 numbering the capturing subpatterns in the whole pattern. However, substring
1876 capturing is carried out only for positive assertions, because it does not make
1877 sense for negative assertions.
1878 .P
1879 For compatibility with Perl, assertion subpatterns may be repeated; though
1880 it makes no sense to assert the same thing several times, the side effect of
1881 capturing parentheses may occasionally be useful. In practice, there only three
1882 cases:
1883 .sp
1884 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1885 However, it may contain internal capturing parenthesized groups that are called
1886 from elsewhere via the
1887 .\" HTML <a href="#subpatternsassubroutines">
1888 .\" </a>
1889 subroutine mechanism.
1890 .\"
1891 .sp
1892 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1893 were {0,1}. At run time, the rest of the pattern match is tried with and
1894 without the assertion, the order depending on the greediness of the quantifier.
1895 .sp
1896 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1897 The assertion is obeyed just once when encountered during matching.
1898 .
1899 .
1900 .SS "Lookahead assertions"
1901 .rs
1902 .sp
1903 Lookahead assertions start with (?= for positive assertions and (?! for
1904 negative assertions. For example,
1905 .sp
1906 \ew+(?=;)
1907 .sp
1908 matches a word followed by a semicolon, but does not include the semicolon in
1909 the match, and
1910 .sp
1911 foo(?!bar)
1912 .sp
1913 matches any occurrence of "foo" that is not followed by "bar". Note that the
1914 apparently similar pattern
1915 .sp
1916 (?!foo)bar
1917 .sp
1918 does not find an occurrence of "bar" that is preceded by something other than
1919 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1920 (?!foo) is always true when the next three characters are "bar". A
1921 lookbehind assertion is needed to achieve the other effect.
1922 .P
1923 If you want to force a matching failure at some point in a pattern, the most
1924 convenient way to do it is with (?!) because an empty string always matches, so
1925 an assertion that requires there not to be an empty string must always fail.
1926 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1927 .
1928 .
1929 .\" HTML <a name="lookbehind"></a>
1930 .SS "Lookbehind assertions"
1931 .rs
1932 .sp
1933 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1934 negative assertions. For example,
1935 .sp
1936 (?<!foo)bar
1937 .sp
1938 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1939 a lookbehind assertion are restricted such that all the strings it matches must
1940 have a fixed length. However, if there are several top-level alternatives, they
1941 do not all have to have the same fixed length. Thus
1942 .sp
1943 (?<=bullock|donkey)
1944 .sp
1945 is permitted, but
1946 .sp
1947 (?<!dogs?|cats?)
1948 .sp
1949 causes an error at compile time. Branches that match different length strings
1950 are permitted only at the top level of a lookbehind assertion. This is an
1951 extension compared with Perl, which requires all branches to match the same
1952 length of string. An assertion such as
1953 .sp
1954 (?<=ab(c|de))
1955 .sp
1956 is not permitted, because its single top-level branch can match two different
1957 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1958 branches:
1959 .sp
1960 (?<=abc|abde)
1961 .sp
1962 In some cases, the escape sequence \eK
1963 .\" HTML <a href="#resetmatchstart">
1964 .\" </a>
1965 (see above)
1966 .\"
1967 can be used instead of a lookbehind assertion to get round the fixed-length
1968 restriction.
1969 .P
1970 The implementation of lookbehind assertions is, for each alternative, to
1971 temporarily move the current position back by the fixed length and then try to
1972 match. If there are insufficient characters before the current position, the
1973 assertion fails.
1974 .P
1975 In UTF-8 mode, PCRE does not allow the \eC escape (which matches a single byte,
1976 even in UTF-8 mode) to appear in lookbehind assertions, because it makes it
1977 impossible to calculate the length of the lookbehind. The \eX and \eR escapes,
1978 which can match different numbers of bytes, are also not permitted.
1979 .P
1980 .\" HTML <a href="#subpatternsassubroutines">
1981 .\" </a>
1982 "Subroutine"
1983 .\"
1984 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1985 as the subpattern matches a fixed-length string.
1986 .\" HTML <a href="#recursion">
1987 .\" </a>
1988 Recursion,
1989 .\"
1990 however, is not supported.
1991 .P
1992 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1993 specify efficient matching of fixed-length strings at the end of subject
1994 strings. Consider a simple pattern such as
1995 .sp
1996 abcd$
1997 .sp
1998 when applied to a long string that does not match. Because matching proceeds
1999 from left to right, PCRE will look for each "a" in the subject and then see if
2000 what follows matches the rest of the pattern. If the pattern is specified as
2001 .sp
2002 ^.*abcd$
2003 .sp
2004 the initial .* matches the entire string at first, but when this fails (because
2005 there is no following "a"), it backtracks to match all but the last character,
2006 then all but the last two characters, and so on. Once again the search for "a"
2007 covers the entire string, from right to left, so we are no better off. However,
2008 if the pattern is written as
2009 .sp
2010 ^.*+(?<=abcd)
2011 .sp
2012 there can be no backtracking for the .*+ item; it can match only the entire
2013 string. The subsequent lookbehind assertion does a single test on the last four
2014 characters. If it fails, the match fails immediately. For long strings, this
2015 approach makes a significant difference to the processing time.
2016 .
2017 .
2018 .SS "Using multiple assertions"
2019 .rs
2020 .sp
2021 Several assertions (of any sort) may occur in succession. For example,
2022 .sp
2023 (?<=\ed{3})(?<!999)foo
2024 .sp
2025 matches "foo" preceded by three digits that are not "999". Notice that each of
2026 the assertions is applied independently at the same point in the subject
2027 string. First there is a check that the previous three characters are all
2028 digits, and then there is a check that the same three characters are not "999".
2029 This pattern does \fInot\fP match "foo" preceded by six characters, the first
2030 of which are digits and the last three of which are not "999". For example, it
2031 doesn't match "123abcfoo". A pattern to do that is
2032 .sp
2033 (?<=\ed{3}...)(?<!999)foo
2034 .sp
2035 This time the first assertion looks at the preceding six characters, checking
2036 that the first three are digits, and then the second assertion checks that the
2037 preceding three characters are not "999".
2038 .P
2039 Assertions can be nested in any combination. For example,
2040 .sp
2041 (?<=(?<!foo)bar)baz
2042 .sp
2043 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2044 preceded by "foo", while
2045 .sp
2046 (?<=\ed{3}(?!999)...)foo
2047 .sp
2048 is another pattern that matches "foo" preceded by three digits and any three
2049 characters that are not "999".
2050 .
2051 .
2052 .\" HTML <a name="conditions"></a>
2054 .rs
2055 .sp
2056 It is possible to cause the matching process to obey a subpattern
2057 conditionally or to choose between two alternative subpatterns, depending on
2058 the result of an assertion, or whether a specific capturing subpattern has
2059 already been matched. The two possible forms of conditional subpattern are:
2060 .sp
2061 (?(condition)yes-pattern)
2062 (?(condition)yes-pattern|no-pattern)
2063 .sp
2064 If the condition is satisfied, the yes-pattern is used; otherwise the
2065 no-pattern (if present) is used. If there are more than two alternatives in the
2066 subpattern, a compile-time error occurs. Each of the two alternatives may
2067 itself contain nested subpatterns of any form, including conditional
2068 subpatterns; the restriction to two alternatives applies only at the level of
2069 the condition. This pattern fragment is an example where the alternatives are
2070 complex:
2071 .sp
2072 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2073 .sp
2074 .P
2075 There are four kinds of condition: references to subpatterns, references to
2076 recursion, a pseudo-condition called DEFINE, and assertions.
2077 .
2078 .SS "Checking for a used subpattern by number"
2079 .rs
2080 .sp
2081 If the text between the parentheses consists of a sequence of digits, the
2082 condition is true if a capturing subpattern of that number has previously
2083 matched. If there is more than one capturing subpattern with the same number
2084 (see the earlier
2085 .\"
2086 .\" HTML <a href="#recursion">
2087 .\" </a>
2088 section about duplicate subpattern numbers),
2089 .\"
2090 the condition is true if any of them have matched. An alternative notation is
2091 to precede the digits with a plus or minus sign. In this case, the subpattern
2092 number is relative rather than absolute. The most recently opened parentheses
2093 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2094 loops it can also make sense to refer to subsequent groups. The next
2095 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2096 zero in any of these forms is not used; it provokes a compile-time error.)
2097 .P
2098 Consider the following pattern, which contains non-significant white space to
2099 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2100 three parts for ease of discussion:
2101 .sp
2102 ( \e( )? [^()]+ (?(1) \e) )
2103 .sp
2104 The first part matches an optional opening parenthesis, and if that
2105 character is present, sets it as the first captured substring. The second part
2106 matches one or more characters that are not parentheses. The third part is a
2107 conditional subpattern that tests whether or not the first set of parentheses
2108 matched. If they did, that is, if subject started with an opening parenthesis,
2109 the condition is true, and so the yes-pattern is executed and a closing
2110 parenthesis is required. Otherwise, since no-pattern is not present, the
2111 subpattern matches nothing. In other words, this pattern matches a sequence of
2112 non-parentheses, optionally enclosed in parentheses.
2113 .P
2114 If you were embedding this pattern in a larger one, you could use a relative
2115 reference:
2116 .sp
2117 ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
2118 .sp
2119 This makes the fragment independent of the parentheses in the larger pattern.
2120 .
2121 .SS "Checking for a used subpattern by name"
2122 .rs
2123 .sp
2124 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2125 subpattern by name. For compatibility with earlier versions of PCRE, which had
2126 this facility before Perl, the syntax (?(name)...) is also recognized. However,
2127 there is a possible ambiguity with this syntax, because subpattern names may
2128 consist entirely of digits. PCRE looks first for a named subpattern; if it
2129 cannot find one and the name consists entirely of digits, PCRE looks for a
2130 subpattern of that number, which must be greater than zero. Using subpattern
2131 names that consist entirely of digits is not recommended.
2132 .P
2133 Rewriting the above example to use a named subpattern gives this:
2134 .sp
2135 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
2136 .sp
2137 If the name used in a condition of this kind is a duplicate, the test is
2138 applied to all subpatterns of the same name, and is true if any one of them has
2139 matched.
2140 .
2141 .SS "Checking for pattern recursion"
2142 .rs
2143 .sp
2144 If the condition is the string (R), and there is no subpattern with the name R,
2145 the condition is true if a recursive call to the whole pattern or any
2146 subpattern has been made. If digits or a name preceded by ampersand follow the
2147 letter R, for example:
2148 .sp
2149 (?(R3)...) or (?(R&name)...)
2150 .sp
2151 the condition is true if the most recent recursion is into a subpattern whose
2152 number or name is given. This condition does not check the entire recursion
2153 stack. If the name used in a condition of this kind is a duplicate, the test is
2154 applied to all subpatterns of the same name, and is true if any one of them is
2155 the most recent recursion.
2156 .P
2157 At "top level", all these recursion test conditions are false.
2158 .\" HTML <a href="#recursion">
2159 .\" </a>
2160 The syntax for recursive patterns
2161 .\"
2162 is described below.
2163 .
2164 .\" HTML <a name="subdefine"></a>
2165 .SS "Defining subpatterns for use by reference only"
2166 .rs
2167 .sp
2168 If the condition is the string (DEFINE), and there is no subpattern with the
2169 name DEFINE, the condition is always false. In this case, there may be only one
2170 alternative in the subpattern. It is always skipped if control reaches this
2171 point in the pattern; the idea of DEFINE is that it can be used to define
2172 subroutines that can be referenced from elsewhere. (The use of
2173 .\" HTML <a href="#subpatternsassubroutines">
2174 .\" </a>
2175 subroutines
2176 .\"
2177 is described below.) For example, a pattern to match an IPv4 address such as
2178 "" could be written like this (ignore whitespace and line
2179 breaks):
2180 .sp
2181 (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2182 \eb (?&byte) (\e.(?&byte)){3} \eb
2183 .sp
2184 The first part of the pattern is a DEFINE group inside which a another group
2185 named "byte" is defined. This matches an individual component of an IPv4
2186 address (a number less than 256). When matching takes place, this part of the
2187 pattern is skipped because DEFINE acts like a false condition. The rest of the
2188 pattern uses references to the named group to match the four dot-separated
2189 components of an IPv4 address, insisting on a word boundary at each end.
2190 .
2191 .SS "Assertion conditions"
2192 .rs
2193 .sp
2194 If the condition is not in any of the above formats, it must be an assertion.
2195 This may be a positive or negative lookahead or lookbehind assertion. Consider
2196 this pattern, again containing non-significant white space, and with the two
2197 alternatives on the second line:
2198 .sp
2199 (?(?=[^a-z]*[a-z])
2200 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2201 .sp
2202 The condition is a positive lookahead assertion that matches an optional
2203 sequence of non-letters followed by a letter. In other words, it tests for the
2204 presence of at least one letter in the subject. If a letter is found, the
2205 subject is matched against the first alternative; otherwise it is matched
2206 against the second. This pattern matches strings in one of the two forms
2207 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2208 .
2209 .
2210 .\" HTML <a name="comments"></a>
2212 .rs
2213 .sp
2214 There are two ways of including comments in patterns that are processed by
2215 PCRE. In both cases, the start of the comment must not be in a character class,
2216 nor in the middle of any other sequence of related characters such as (?: or a
2217 subpattern name or number. The characters that make up a comment play no part
2218 in the pattern matching.
2219 .P
2220 The sequence (?# marks the start of a comment that continues up to the next
2221 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2222 option is set, an unescaped # character also introduces a comment, which in
2223 this case continues to immediately after the next newline character or
2224 character sequence in the pattern. Which characters are interpreted as newlines
2225 is controlled by the options passed to \fBpcre_compile()\fP or by a special
2226 sequence at the start of the pattern, as described in the section entitled
2227 .\" HTML <a href="#newlines">
2228 .\" </a>
2229 "Newline conventions"
2230 .\"
2231 above. Note that the end of this type of comment is a literal newline sequence
2232 in the pattern; escape sequences that happen to represent a newline do not
2233 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2234 default newline convention is in force:
2235 .sp
2236 abc #comment \en still comment
2237 .sp
2238 On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2239 a newline in the pattern. The sequence \en is still literal at this stage, so
2240 it does not terminate the comment. Only an actual character with the code value
2241 0x0a (the default newline) does so.
2242 .
2243 .
2244 .\" HTML <a name="recursion"></a>
2246 .rs
2247 .sp
2248 Consider the problem of matching a string in parentheses, allowing for
2249 unlimited nested parentheses. Without the use of recursion, the best that can
2250 be done is to use a pattern that matches up to some fixed depth of nesting. It
2251 is not possible to handle an arbitrary nesting depth.
2252 .P
2253 For some time, Perl has provided a facility that allows regular expressions to
2254 recurse (amongst other things). It does this by interpolating Perl code in the
2255 expression at run time, and the code can refer to the expression itself. A Perl
2256 pattern using code interpolation to solve the parentheses problem can be
2257 created like this:
2258 .sp
2259 $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2260 .sp
2261 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2262 recursively to the pattern in which it appears.
2263 .P
2264 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2265 supports special syntax for recursion of the entire pattern, and also for
2266 individual subpattern recursion. After its introduction in PCRE and Python,
2267 this kind of recursion was subsequently introduced into Perl at release 5.10.
2268 .P
2269 A special item that consists of (? followed by a number greater than zero and a
2270 closing parenthesis is a recursive subroutine call of the subpattern of the
2271 given number, provided that it occurs inside that subpattern. (If not, it is a
2272 .\" HTML <a href="#subpatternsassubroutines">
2273 .\" </a>
2274 non-recursive subroutine
2275 .\"
2276 call, which is described in the next section.) The special item (?R) or (?0) is
2277 a recursive call of the entire regular expression.
2278 .P
2279 This PCRE pattern solves the nested parentheses problem (assume the
2280 PCRE_EXTENDED option is set so that white space is ignored):
2281 .sp
2282 \e( ( [^()]++ | (?R) )* \e)
2283 .sp
2284 First it matches an opening parenthesis. Then it matches any number of
2285 substrings which can either be a sequence of non-parentheses, or a recursive
2286 match of the pattern itself (that is, a correctly parenthesized substring).
2287 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2288 to avoid backtracking into sequences of non-parentheses.
2289 .P
2290 If this were part of a larger pattern, you would not want to recurse the entire
2291 pattern, so instead you could use this:
2292 .sp
2293 ( \e( ( [^()]++ | (?1) )* \e) )
2294 .sp
2295 We have put the pattern into parentheses, and caused the recursion to refer to
2296 them instead of the whole pattern.
2297 .P
2298 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2299 is made easier by the use of relative references. Instead of (?1) in the
2300 pattern above you can write (?-2) to refer to the second most recently opened
2301 parentheses preceding the recursion. In other words, a negative number counts
2302 capturing parentheses leftwards from the point at which it is encountered.
2303 .P
2304 It is also possible to refer to subsequently opened parentheses, by writing
2305 references such as (?+2). However, these cannot be recursive because the
2306 reference is not inside the parentheses that are referenced. They are always
2307 .\" HTML <a href="#subpatternsassubroutines">
2308 .\" </a>
2309 non-recursive subroutine
2310 .\"
2311 calls, as described in the next section.
2312 .P
2313 An alternative approach is to use named parentheses instead. The Perl syntax
2314 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2315 could rewrite the above example as follows:
2316 .sp
2317 (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2318 .sp
2319 If there is more than one subpattern with the same name, the earliest one is
2320 used.
2321 .P
2322 This particular example pattern that we have been looking at contains nested
2323 unlimited repeats, and so the use of a possessive quantifier for matching
2324 strings of non-parentheses is important when applying the pattern to strings
2325 that do not match. For example, when this pattern is applied to
2326 .sp
2327 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2328 .sp
2329 it yields "no match" quickly. However, if a possessive quantifier is not used,
2330 the match runs for a very long time indeed because there are so many different
2331 ways the + and * repeats can carve up the subject, and all have to be tested
2332 before failure can be reported.
2333 .P
2334 At the end of a match, the values of capturing parentheses are those from
2335 the outermost level. If you want to obtain intermediate values, a callout
2336 function can be used (see below and the
2337 .\" HREF
2338 \fBpcrecallout\fP
2339 .\"
2340 documentation). If the pattern above is matched against
2341 .sp
2342 (ab(cd)ef)
2343 .sp
2344 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2345 the last value taken on at the top level. If a capturing subpattern is not
2346 matched at the top level, its final captured value is unset, even if it was
2347 (temporarily) set at a deeper level during the matching process.
2348 .P
2349 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2350 obtain extra memory to store data during a recursion, which it does by using
2351 \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2352 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2353 .P
2354 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2355 Consider this pattern, which matches text in angle brackets, allowing for
2356 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2357 recursing), whereas any characters are permitted at the outer level.
2358 .sp
2359 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2360 .sp
2361 In this pattern, (?(R) is the start of a conditional subpattern, with two
2362 different alternatives for the recursive and non-recursive cases. The (?R) item
2363 is the actual recursive call.
2364 .
2365 .
2366 .\" HTML <a name="recursiondifference"></a>
2367 .SS "Differences in recursion processing between PCRE and Perl"
2368 .rs
2369 .sp
2370 Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2371 (like Python, but unlike Perl), a recursive subpattern call is always treated
2372 as an atomic group. That is, once it has matched some of the subject string, it
2373 is never re-entered, even if it contains untried alternatives and there is a
2374 subsequent matching failure. This can be illustrated by the following pattern,
2375 which purports to match a palindromic string that contains an odd number of
2376 characters (for example, "a", "aba", "abcba", "abcdcba"):
2377 .sp
2378 ^(.|(.)(?1)\e2)$
2379 .sp
2380 The idea is that it either matches a single character, or two identical
2381 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2382 it does not if the pattern is longer than three characters. Consider the
2383 subject string "abcba":
2384 .P
2385 At the top level, the first character is matched, but as it is not at the end
2386 of the string, the first alternative fails; the second alternative is taken
2387 and the recursion kicks in. The recursive call to subpattern 1 successfully
2388 matches the next character ("b"). (Note that the beginning and end of line
2389 tests are not part of the recursion).
2390 .P
2391 Back at the top level, the next character ("c") is compared with what
2392 subpattern 2 matched, which was "a". This fails. Because the recursion is
2393 treated as an atomic group, there are now no backtracking points, and so the
2394 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2395 try the second alternative.) However, if the pattern is written with the
2396 alternatives in the other order, things are different:
2397 .sp
2398 ^((.)(?1)\e2|.)$
2399 .sp
2400 This time, the recursing alternative is tried first, and continues to recurse
2401 until it runs out of characters, at which point the recursion fails. But this
2402 time we do have another alternative to try at the higher level. That is the big
2403 difference: in the previous case the remaining alternative is at a deeper
2404 recursion level, which PCRE cannot use.
2405 .P
2406 To change the pattern so that it matches all palindromic strings, not just
2407 those with an odd number of characters, it is tempting to change the pattern to
2408 this:
2409 .sp
2410 ^((.)(?1)\e2|.?)$
2411 .sp
2412 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2413 deeper recursion has matched a single character, it cannot be entered again in
2414 order to match an empty string. The solution is to separate the two cases, and
2415 write out the odd and even cases as alternatives at the higher level:
2416 .sp
2417 ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2418 .sp
2419 If you want to match typical palindromic phrases, the pattern has to ignore all
2420 non-word characters, which can be done like this:
2421 .sp
2422 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2423 .sp
2424 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2425 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2426 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2427 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2428 more) to match typical phrases, and Perl takes so long that you think it has
2429 gone into a loop.
2430 .P
2431 \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2432 string does not start with a palindrome that is shorter than the entire string.
2433 For example, although "abcba" is correctly matched, if the subject is "ababa",
2434 PCRE finds the palindrome "aba" at the start, then fails at top level because
2435 the end of the string does not follow. Once again, it cannot jump back into the
2436 recursion to try other alternatives, so the entire match fails.
2437 .P
2438 The second way in which PCRE and Perl differ in their recursion processing is
2439 in the handling of captured values. In Perl, when a subpattern is called
2440 recursively or as a subpattern (see the next section), it has no access to any
2441 values that were captured outside the recursion, whereas in PCRE these values
2442 can be referenced. Consider this pattern:
2443 .sp
2444 ^(.)(\e1|a(?2))
2445 .sp
2446 In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2447 then in the second group, when the back reference \e1 fails to match "b", the
2448 second alternative matches "a" and then recurses. In the recursion, \e1 does
2449 now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2450 match because inside the recursive call \e1 cannot access the externally set
2451 value.
2452 .
2453 .
2454 .\" HTML <a name="subpatternsassubroutines"></a>
2456 .rs
2457 .sp
2458 If the syntax for a recursive subpattern call (either by number or by
2459 name) is used outside the parentheses to which it refers, it operates like a
2460 subroutine in a programming language. The called subpattern may be defined
2461 before or after the reference. A numbered reference can be absolute or
2462 relative, as in these examples:
2463 .sp
2464 (...(absolute)...)...(?2)...
2465 (...(relative)...)...(?-1)...
2466 (...(?+1)...(relative)...
2467 .sp
2468 An earlier example pointed out that the pattern
2469 .sp
2470 (sens|respons)e and \e1ibility
2471 .sp
2472 matches "sense and sensibility" and "response and responsibility", but not
2473 "sense and responsibility". If instead the pattern
2474 .sp
2475 (sens|respons)e and (?1)ibility
2476 .sp
2477 is used, it does match "sense and responsibility" as well as the other two
2478 strings. Another example is given in the discussion of DEFINE above.
2479 .P
2480 All subroutine calls, whether recursive or not, are always treated as atomic
2481 groups. That is, once a subroutine has matched some of the subject string, it
2482 is never re-entered, even if it contains untried alternatives and there is a
2483 subsequent matching failure. Any capturing parentheses that are set during the
2484 subroutine call revert to their previous values afterwards.
2485 .P
2486 Processing options such as case-independence are fixed when a subpattern is
2487 defined, so if it is used as a subroutine, such options cannot be changed for
2488 different calls. For example, consider this pattern:
2489 .sp
2490 (abc)(?i:(?-1))
2491 .sp
2492 It matches "abcabc". It does not match "abcABC" because the change of
2493 processing option does not affect the called subpattern.
2494 .
2495 .
2496 .\" HTML <a name="onigurumasubroutines"></a>
2498 .rs
2499 .sp
2500 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2501 a number enclosed either in angle brackets or single quotes, is an alternative
2502 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2503 are two of the examples used above, rewritten using this syntax:
2504 .sp
2505 (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2506 (sens|respons)e and \eg'1'ibility
2507 .sp
2508 PCRE supports an extension to Oniguruma: if a number is preceded by a
2509 plus or a minus sign it is taken as a relative reference. For example:
2510 .sp
2511 (abc)(?i:\eg<-1>)
2512 .sp
2513 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2514 synonymous. The former is a back reference; the latter is a subroutine call.
2515 .
2516 .
2518 .rs
2519 .sp
2520 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2521 code to be obeyed in the middle of matching a regular expression. This makes it
2522 possible, amongst other things, to extract different substrings that match the
2523 same pair of parentheses when there is a repetition.
2524 .P
2525 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2526 code. The feature is called "callout". The caller of PCRE provides an external
2527 function by putting its entry point in the global variable \fIpcre_callout\fP.
2528 By default, this variable contains NULL, which disables all calling out.
2529 .P
2530 Within a regular expression, (?C) indicates the points at which the external
2531 function is to be called. If you want to identify different callout points, you
2532 can put a number less than 256 after the letter C. The default value is zero.
2533 For example, this pattern has two callout points:
2534 .sp
2535 (?C1)abc(?C2)def
2536 .sp
2537 If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2538 automatically installed before each item in the pattern. They are all numbered
2539 255.
2540 .P
2541 During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2542 set), the external function is called. It is provided with the number of the
2543 callout, the position in the pattern, and, optionally, one item of data
2544 originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2545 may cause matching to proceed, to backtrack, or to fail altogether. A complete
2546 description of the interface to the callout function is given in the
2547 .\" HREF
2548 \fBpcrecallout\fP
2549 .\"
2550 documentation.
2551 .
2552 .
2553 .\" HTML <a name="backtrackcontrol"></a>
2555 .rs
2556 .sp
2557 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2558 are described in the Perl documentation as "experimental and subject to change
2559 or removal in a future version of Perl". It goes on to say: "Their usage in
2560 production code should be noted to avoid problems during upgrades." The same
2561 remarks apply to the PCRE features described in this section.
2562 .P
2563 Since these verbs are specifically related to backtracking, most of them can be
2564 used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2565 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2566 failing negative assertion, they cause an error if encountered by
2567 \fBpcre_dfa_exec()\fP.
2568 .P
2569 If any of these verbs are used in an assertion or in a subpattern that is
2570 called as a subroutine (whether or not recursively), their effect is confined
2571 to that subpattern; it does not extend to the surrounding pattern, with one
2572 exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2573 a successful positive assertion \fIis\fP passed back when a match succeeds
2574 (compare capturing parentheses in assertions). Note that such subpatterns are
2575 processed as anchored at the point where they are tested. Note also that Perl's
2576 treatment of subroutines is different in some cases.
2577 .P
2578 The new verbs make use of what was previously invalid syntax: an opening
2579 parenthesis followed by an asterisk. They are generally of the form
2580 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2581 depending on whether or not an argument is present. A name is any sequence of
2582 characters that does not include a closing parenthesis. If the name is empty,
2583 that is, if the closing parenthesis immediately follows the colon, the effect
2584 is as if the colon were not there. Any number of these verbs may occur in a
2585 pattern.
2586 .P
2587 PCRE contains some optimizations that are used to speed up matching by running
2588 some checks at the start of each match attempt. For example, it may know the
2589 minimum length of matching subject, or that a particular character must be
2590 present. When one of these optimizations suppresses the running of a match, any
2591 included backtracking verbs will not, of course, be processed. You can suppress
2592 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2593 when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2594 pattern with (*NO_START_OPT).
2595 .P
2596 Experiments with Perl suggest that it too has similar optimizations, sometimes
2597 leading to anomalous results.
2598 .
2599 .
2600 .SS "Verbs that act immediately"
2601 .rs
2602 .sp
2603 The following verbs act as soon as they are encountered. They may not be
2604 followed by a name.
2605 .sp
2606 (*ACCEPT)
2607 .sp
2608 This verb causes the match to end successfully, skipping the remainder of the
2609 pattern. However, when it is inside a subpattern that is called as a
2610 subroutine, only that subpattern is ended successfully. Matching then continues
2611 at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2612 far is captured. For example:
2613 .sp
2614 A((?:A|B(*ACCEPT)|C)D)
2615 .sp
2616 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2617 the outer parentheses.
2618 .sp
2619 (*FAIL) or (*F)
2620 .sp
2621 This verb causes a matching failure, forcing backtracking to occur. It is
2622 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2623 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2624 Perl features that are not present in PCRE. The nearest equivalent is the
2625 callout feature, as for example in this pattern:
2626 .sp
2627 a+(?C)(*FAIL)
2628 .sp
2629 A match with the string "aaaa" always fails, but the callout is taken before
2630 each backtrack happens (in this example, 10 times).
2631 .
2632 .
2633 .SS "Recording which path was taken"
2634 .rs
2635 .sp
2636 There is one verb whose main purpose is to track how a match was arrived at,
2637 though it also has a secondary use in conjunction with advancing the match
2638 starting point (see (*SKIP) below).
2639 .sp
2640 (*MARK:NAME) or (*:NAME)
2641 .sp
2642 A name is always required with this verb. There may be as many instances of
2643 (*MARK) as you like in a pattern, and their names do not have to be unique.
2644 .P
2645 When a match succeeds, the name of the last-encountered (*MARK) on the matching
2646 path is passed back to the caller via the \fIpcre_extra\fP data structure, as
2647 described in the
2648 .\" HTML <a href="pcreapi.html#extradata">
2649 .\" </a>
2650 section on \fIpcre_extra\fP
2651 .\"
2652 in the
2653 .\" HREF
2654 \fBpcreapi\fP
2655 .\"
2656 documentation. Here is an example of \fBpcretest\fP output, where the /K
2657 modifier requests the retrieval and outputting of (*MARK) data:
2658 .sp
2659 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2660 data> XY
2661 0: XY
2662 MK: A
2663 XZ
2664 0: XZ
2665 MK: B
2666 .sp
2667 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2668 indicates which of the two alternatives matched. This is a more efficient way
2669 of obtaining this information than putting each alternative in its own
2670 capturing parentheses.
2671 .P
2672 If (*MARK) is encountered in a positive assertion, its name is recorded and
2673 passed back if it is the last-encountered. This does not happen for negative
2674 assertions.
2675 .P
2676 After a partial match or a failed match, the name of the last encountered
2677 (*MARK) in the entire match process is returned. For example:
2678 .sp
2679 re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2680 data> XP
2681 No match, mark = B
2682 .sp
2683 Note that in this unanchored example the mark is retained from the match
2684 attempt that started at the letter "X". Subsequent match attempts starting at
2685 "P" and then with an empty string do not get as far as the (*MARK) item, but
2686 nevertheless do not reset it.
2687 .
2688 .
2689 .SS "Verbs that act after backtracking"
2690 .rs
2691 .sp
2692 The following verbs do nothing when they are encountered. Matching continues
2693 with what follows, but if there is no subsequent match, causing a backtrack to
2694 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2695 the verb. However, when one of these verbs appears inside an atomic group, its
2696 effect is confined to that group, because once the group has been matched,
2697 there is never any backtracking into it. In this situation, backtracking can
2698 "jump back" to the left of the entire atomic group. (Remember also, as stated
2699 above, that this localization also applies in subroutine calls and assertions.)
2700 .P
2701 These verbs differ in exactly what kind of failure occurs when backtracking
2702 reaches them.
2703 .sp
2704 (*COMMIT)
2705 .sp
2706 This verb, which may not be followed by a name, causes the whole match to fail
2707 outright if the rest of the pattern does not match. Even if the pattern is
2708 unanchored, no further attempts to find a match by advancing the starting point
2709 take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2710 finding a match at the current starting point, or not at all. For example:
2711 .sp
2712 a+(*COMMIT)b
2713 .sp
2714 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2715 dynamic anchor, or "I've started, so I must finish." The name of the most
2716 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2717 match failure.
2718 .P
2719 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2720 unless PCRE's start-of-match optimizations are turned off, as shown in this
2721 \fBpcretest\fP example:
2722 .sp
2723 re> /(*COMMIT)abc/
2724 data> xyzabc
2725 0: abc
2726 xyzabc\eY
2727 No match
2728 .sp
2729 PCRE knows that any match must start with "a", so the optimization skips along
2730 the subject to "a" before running the first match attempt, which succeeds. When
2731 the optimization is disabled by the \eY escape in the second subject, the match
2732 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2733 starting points.
2734 .sp
2735 (*PRUNE) or (*PRUNE:NAME)
2736 .sp
2737 This verb causes the match to fail at the current starting position in the
2738 subject if the rest of the pattern does not match. If the pattern is
2739 unanchored, the normal "bumpalong" advance to the next starting character then
2740 happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2741 reached, or when matching to the right of (*PRUNE), but if there is no match to
2742 the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2743 (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2744 but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2745 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2746 anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2747 .sp
2748 (*SKIP)
2749 .sp
2750 This verb, when given without a name, is like (*PRUNE), except that if the
2751 pattern is unanchored, the "bumpalong" advance is not to the next character,
2752 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2753 signifies that whatever text was matched leading up to it cannot be part of a
2754 successful match. Consider:
2755 .sp
2756 a+(*SKIP)b
2757 .sp
2758 If the subject is "aaaac...", after the first match attempt fails (starting at
2759 the first character in the string), the starting point skips on to start the
2760 next attempt at "c". Note that a possessive quantifer does not have the same
2761 effect as this example; although it would suppress backtracking during the
2762 first match attempt, the second attempt would start at the second character
2763 instead of skipping on to "c".
2764 .sp
2765 (*SKIP:NAME)
2766 .sp
2767 When (*SKIP) has an associated name, its behaviour is modified. If the
2768 following pattern fails to match, the previous path through the pattern is
2769 searched for the most recent (*MARK) that has the same name. If one is found,
2770 the "bumpalong" advance is to the subject position that corresponds to that
2771 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2772 matching name is found, the (*SKIP) is ignored.
2773 .sp
2774 (*THEN) or (*THEN:NAME)
2775 .sp
2776 This verb causes a skip to the next innermost alternative if the rest of the
2777 pattern does not match. That is, it cancels pending backtracking, but only
2778 within the current alternative. Its name comes from the observation that it can
2779 be used for a pattern-based if-then-else block:
2780 .sp
2781 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2782 .sp
2783 If the COND1 pattern matches, FOO is tried (and possibly further items after
2784 the end of the group if FOO succeeds); on failure, the matcher skips to the
2785 second alternative and tries COND2, without backtracking into COND1. The
2786 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2787 If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2788 .P
2789 Note that a subpattern that does not contain a | character is just a part of
2790 the enclosing alternative; it is not a nested alternation with only one
2791 alternative. The effect of (*THEN) extends beyond such a subpattern to the
2792 enclosing alternative. Consider this pattern, where A, B, etc. are complex
2793 pattern fragments that do not contain any | characters at this level:
2794 .sp
2795 A (B(*THEN)C) | D
2796 .sp
2797 If A and B are matched, but there is a failure in C, matching does not
2798 backtrack into A; instead it moves to the next alternative, that is, D.
2799 However, if the subpattern containing (*THEN) is given an alternative, it
2800 behaves differently:
2801 .sp
2802 A (B(*THEN)C | (*FAIL)) | D
2803 .sp
2804 The effect of (*THEN) is now confined to the inner subpattern. After a failure
2805 in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2806 because there are no more alternatives to try. In this case, matching does now
2807 backtrack into A.
2808 .P
2809 Note also that a conditional subpattern is not considered as having two
2810 alternatives, because only one is ever used. In other words, the | character in
2811 a conditional subpattern has a different meaning. Ignoring white space,
2812 consider:
2813 .sp
2814 ^.*? (?(?=a) a | b(*THEN)c )
2815 .sp
2816 If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2817 it initially matches zero characters. The condition (?=a) then fails, the
2818 character "b" is matched, but "c" is not. At this point, matching does not
2819 backtrack to .*? as might perhaps be expected from the presence of the |
2820 character. The conditional subpattern is part of the single alternative that
2821 comprises the whole pattern, and so the match fails. (If there was a backtrack
2822 into .*?, allowing it to match "b", the match would succeed.)
2823 .P
2824 The verbs just described provide four different "strengths" of control when
2825 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2826 next alternative. (*PRUNE) comes next, failing the match at the current
2827 starting position, but allowing an advance to the next character (for an
2828 unanchored pattern). (*SKIP) is similar, except that the advance may be more
2829 than one character. (*COMMIT) is the strongest, causing the entire match to
2830 fail.
2831 .P
2832 If more than one such verb is present in a pattern, the "strongest" one wins.
2833 For example, consider this pattern, where A, B, etc. are complex pattern
2834 fragments:
2835 .sp
2836 (A(*COMMIT)B(*THEN)C|D)
2837 .sp
2838 Once A has matched, PCRE is committed to this match, at the current starting
2839 position. If subsequently B matches, but C does not, the normal (*THEN) action
2840 of trying the next alternative (that is, D) does not happen because (*COMMIT)
2841 overrides.
2842 .
2843 .
2844 .SH "SEE ALSO"
2845 .rs
2846 .sp
2847 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2848 \fBpcresyntax\fP(3), \fBpcre\fP(3).
2849 .
2850 .
2852 .rs
2853 .sp
2854 .nf
2855 Philip Hazel
2856 University Computing Service
2857 Cambridge CB2 3QH, England.
2858 .fi
2859 .
2860 .
2862 .rs
2863 .sp
2864 .nf
2865 Last updated: 29 November 2011
2866 Copyright (c) 1997-2011 University of Cambridge.
2867 .fi


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