Annotation of embedaddon/pcre/doc/pcrepattern.3, revision 1.1.1.2
1.1 misho 1: .TH PCREPATTERN 3
2: .SH NAME
3: PCRE - Perl-compatible regular expressions
4: .SH "PCRE REGULAR EXPRESSION DETAILS"
5: .rs
6: .sp
7: The syntax and semantics of the regular expressions that are supported by PCRE
8: are described in detail below. There is a quick-reference syntax summary in the
9: .\" HREF
10: \fBpcresyntax\fP
11: .\"
12: page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13: also supports some alternative regular expression syntax (which does not
14: conflict with the Perl syntax) in order to provide some compatibility with
15: regular expressions in Python, .NET, and Oniguruma.
16: .P
17: Perl's regular expressions are described in its own documentation, and
18: regular expressions in general are covered in a number of books, some of which
19: have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20: published by O'Reilly, covers regular expressions in great detail. This
21: description of PCRE's regular expressions is intended as reference material.
22: .P
23: The original operation of PCRE was on strings of one-byte characters. However,
1.1.1.2 ! misho 24: there is now also support for UTF-8 strings in the original library, and a
! 25: second library that supports 16-bit and UTF-16 character strings. To use these
! 26: features, PCRE must be built to include appropriate support. When using UTF
! 27: strings you must either call the compiling function with the PCRE_UTF8 or
! 28: PCRE_UTF16 option, or the pattern must start with one of these special
! 29: sequences:
1.1 misho 30: .sp
31: (*UTF8)
1.1.1.2 ! misho 32: (*UTF16)
1.1 misho 33: .sp
1.1.1.2 ! misho 34: Starting a pattern with such a sequence is equivalent to setting the relevant
! 35: option. This feature is not Perl-compatible. How setting a UTF mode affects
1.1 misho 36: pattern matching is mentioned in several places below. There is also a summary
1.1.1.2 ! misho 37: of features in the
1.1 misho 38: .\" HREF
39: \fBpcreunicode\fP
40: .\"
41: page.
42: .P
43: Another special sequence that may appear at the start of a pattern or in
1.1.1.2 ! misho 44: combination with (*UTF8) or (*UTF16) is:
1.1 misho 45: .sp
46: (*UCP)
47: .sp
48: This has the same effect as setting the PCRE_UCP option: it causes sequences
49: such as \ed and \ew to use Unicode properties to determine character types,
50: instead of recognizing only characters with codes less than 128 via a lookup
51: table.
52: .P
53: If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
54: PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
55: also some more of these special sequences that are concerned with the handling
56: of newlines; they are described below.
57: .P
58: The remainder of this document discusses the patterns that are supported by
1.1.1.2 ! misho 59: PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
! 60: \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching
! 61: functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using
! 62: a different algorithm that is not Perl-compatible. Some of the features
! 63: discussed below are not available when DFA matching is used. The advantages and
! 64: disadvantages of the alternative functions, and how they differ from the normal
! 65: functions, are discussed in the
1.1 misho 66: .\" HREF
67: \fBpcrematching\fP
68: .\"
69: page.
70: .
71: .
72: .\" HTML <a name="newlines"></a>
73: .SH "NEWLINE CONVENTIONS"
74: .rs
75: .sp
76: PCRE supports five different conventions for indicating line breaks in
77: strings: a single CR (carriage return) character, a single LF (linefeed)
78: character, the two-character sequence CRLF, any of the three preceding, or any
79: Unicode newline sequence. The
80: .\" HREF
81: \fBpcreapi\fP
82: .\"
83: page has
84: .\" HTML <a href="pcreapi.html#newlines">
85: .\" </a>
86: further discussion
87: .\"
88: about newlines, and shows how to set the newline convention in the
89: \fIoptions\fP arguments for the compiling and matching functions.
90: .P
91: It is also possible to specify a newline convention by starting a pattern
92: string with one of the following five sequences:
93: .sp
94: (*CR) carriage return
95: (*LF) linefeed
96: (*CRLF) carriage return, followed by linefeed
97: (*ANYCRLF) any of the three above
98: (*ANY) all Unicode newline sequences
99: .sp
1.1.1.2 ! misho 100: These override the default and the options given to the compiling function. For
! 101: example, on a Unix system where LF is the default newline sequence, the pattern
1.1 misho 102: .sp
103: (*CR)a.b
104: .sp
105: changes the convention to CR. That pattern matches "a\enb" because LF is no
106: longer a newline. Note that these special settings, which are not
107: Perl-compatible, are recognized only at the very start of a pattern, and that
108: they must be in upper case. If more than one of them is present, the last one
109: is used.
110: .P
111: The newline convention affects the interpretation of the dot metacharacter when
112: PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
113: affect what the \eR escape sequence matches. By default, this is any Unicode
114: newline sequence, for Perl compatibility. However, this can be changed; see the
115: description of \eR in the section entitled
116: .\" HTML <a href="#newlineseq">
117: .\" </a>
118: "Newline sequences"
119: .\"
120: below. A change of \eR setting can be combined with a change of newline
121: convention.
122: .
123: .
124: .SH "CHARACTERS AND METACHARACTERS"
125: .rs
126: .sp
127: A regular expression is a pattern that is matched against a subject string from
128: left to right. Most characters stand for themselves in a pattern, and match the
129: corresponding characters in the subject. As a trivial example, the pattern
130: .sp
131: The quick brown fox
132: .sp
133: matches a portion of a subject string that is identical to itself. When
134: caseless matching is specified (the PCRE_CASELESS option), letters are matched
1.1.1.2 ! misho 135: independently of case. In a UTF mode, PCRE always understands the concept of
1.1 misho 136: case for characters whose values are less than 128, so caseless matching is
137: always possible. For characters with higher values, the concept of case is
138: supported if PCRE is compiled with Unicode property support, but not otherwise.
139: If you want to use caseless matching for characters 128 and above, you must
140: ensure that PCRE is compiled with Unicode property support as well as with
1.1.1.2 ! misho 141: UTF support.
1.1 misho 142: .P
143: The power of regular expressions comes from the ability to include alternatives
144: and repetitions in the pattern. These are encoded in the pattern by the use of
145: \fImetacharacters\fP, which do not stand for themselves but instead are
146: interpreted in some special way.
147: .P
148: There are two different sets of metacharacters: those that are recognized
149: anywhere in the pattern except within square brackets, and those that are
150: recognized within square brackets. Outside square brackets, the metacharacters
151: are as follows:
152: .sp
153: \e general escape character with several uses
154: ^ assert start of string (or line, in multiline mode)
155: $ assert end of string (or line, in multiline mode)
156: . match any character except newline (by default)
157: [ start character class definition
158: | start of alternative branch
159: ( start subpattern
160: ) end subpattern
161: ? extends the meaning of (
162: also 0 or 1 quantifier
163: also quantifier minimizer
164: * 0 or more quantifier
165: + 1 or more quantifier
166: also "possessive quantifier"
167: { start min/max quantifier
168: .sp
169: Part of a pattern that is in square brackets is called a "character class". In
170: a character class the only metacharacters are:
171: .sp
172: \e general escape character
173: ^ negate the class, but only if the first character
174: - indicates character range
175: .\" JOIN
176: [ POSIX character class (only if followed by POSIX
177: syntax)
178: ] terminates the character class
179: .sp
180: The following sections describe the use of each of the metacharacters.
181: .
182: .
183: .SH BACKSLASH
184: .rs
185: .sp
186: The backslash character has several uses. Firstly, if it is followed by a
187: character that is not a number or a letter, it takes away any special meaning
188: that character may have. This use of backslash as an escape character applies
189: both inside and outside character classes.
190: .P
191: For example, if you want to match a * character, you write \e* in the pattern.
192: This escaping action applies whether or not the following character would
193: otherwise be interpreted as a metacharacter, so it is always safe to precede a
194: non-alphanumeric with backslash to specify that it stands for itself. In
195: particular, if you want to match a backslash, you write \e\e.
196: .P
1.1.1.2 ! misho 197: In a UTF mode, only ASCII numbers and letters have any special meaning after a
1.1 misho 198: backslash. All other characters (in particular, those whose codepoints are
199: greater than 127) are treated as literals.
200: .P
201: If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
202: pattern (other than in a character class) and characters between a # outside
203: a character class and the next newline are ignored. An escaping backslash can
204: be used to include a whitespace or # character as part of the pattern.
205: .P
206: If you want to remove the special meaning from a sequence of characters, you
207: can do so by putting them between \eQ and \eE. This is different from Perl in
208: that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
209: Perl, $ and @ cause variable interpolation. Note the following examples:
210: .sp
211: Pattern PCRE matches Perl matches
212: .sp
213: .\" JOIN
214: \eQabc$xyz\eE abc$xyz abc followed by the
215: contents of $xyz
216: \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
217: \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
218: .sp
219: The \eQ...\eE sequence is recognized both inside and outside character classes.
220: An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
221: by \eE later in the pattern, the literal interpretation continues to the end of
222: the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
223: a character class, this causes an error, because the character class is not
224: terminated.
225: .
226: .
227: .\" HTML <a name="digitsafterbackslash"></a>
228: .SS "Non-printing characters"
229: .rs
230: .sp
231: A second use of backslash provides a way of encoding non-printing characters
232: in patterns in a visible manner. There is no restriction on the appearance of
233: non-printing characters, apart from the binary zero that terminates a pattern,
234: but when a pattern is being prepared by text editing, it is often easier to use
235: one of the following escape sequences than the binary character it represents:
236: .sp
237: \ea alarm, that is, the BEL character (hex 07)
238: \ecx "control-x", where x is any ASCII character
239: \ee escape (hex 1B)
240: \ef formfeed (hex 0C)
241: \en linefeed (hex 0A)
242: \er carriage return (hex 0D)
243: \et tab (hex 09)
244: \eddd character with octal code ddd, or back reference
245: \exhh character with hex code hh
246: \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
247: \euhhhh character with hex code hhhh (JavaScript mode only)
248: .sp
249: The precise effect of \ecx is as follows: if x is a lower case letter, it
250: is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
251: Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
252: \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
253: than 127, a compile-time error occurs. This locks out non-ASCII characters in
1.1.1.2 ! misho 254: all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A
! 255: lower case letter is converted to upper case, and then the 0xc0 bits are
! 256: flipped.)
1.1 misho 257: .P
258: By default, after \ex, from zero to two hexadecimal digits are read (letters
259: can be in upper or lower case). Any number of hexadecimal digits may appear
1.1.1.2 ! misho 260: between \ex{ and }, but the character code is constrained as follows:
! 261: .sp
! 262: 8-bit non-UTF mode less than 0x100
! 263: 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
! 264: 16-bit non-UTF mode less than 0x10000
! 265: 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
! 266: .sp
! 267: Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
! 268: "surrogate" codepoints).
1.1 misho 269: .P
270: If characters other than hexadecimal digits appear between \ex{ and }, or if
271: there is no terminating }, this form of escape is not recognized. Instead, the
272: initial \ex will be interpreted as a basic hexadecimal escape, with no
273: following digits, giving a character whose value is zero.
274: .P
275: If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
276: as just described only when it is followed by two hexadecimal digits.
277: Otherwise, it matches a literal "x" character. In JavaScript mode, support for
278: code points greater than 256 is provided by \eu, which must be followed by
279: four hexadecimal digits; otherwise it matches a literal "u" character.
280: .P
281: Characters whose value is less than 256 can be defined by either of the two
282: syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
283: way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
284: \eu00dc in JavaScript mode).
285: .P
286: After \e0 up to two further octal digits are read. If there are fewer than two
287: digits, just those that are present are used. Thus the sequence \e0\ex\e07
288: specifies two binary zeros followed by a BEL character (code value 7). Make
289: sure you supply two digits after the initial zero if the pattern character that
290: follows is itself an octal digit.
291: .P
292: The handling of a backslash followed by a digit other than 0 is complicated.
293: Outside a character class, PCRE reads it and any following digits as a decimal
294: number. If the number is less than 10, or if there have been at least that many
295: previous capturing left parentheses in the expression, the entire sequence is
296: taken as a \fIback reference\fP. A description of how this works is given
297: .\" HTML <a href="#backreferences">
298: .\" </a>
299: later,
300: .\"
301: following the discussion of
302: .\" HTML <a href="#subpattern">
303: .\" </a>
304: parenthesized subpatterns.
305: .\"
306: .P
307: Inside a character class, or if the decimal number is greater than 9 and there
308: have not been that many capturing subpatterns, PCRE re-reads up to three octal
309: digits following the backslash, and uses them to generate a data character. Any
1.1.1.2 ! misho 310: subsequent digits stand for themselves. The value of the character is
! 311: constrained in the same way as characters specified in hexadecimal.
! 312: For example:
1.1 misho 313: .sp
314: \e040 is another way of writing a space
315: .\" JOIN
316: \e40 is the same, provided there are fewer than 40
317: previous capturing subpatterns
318: \e7 is always a back reference
319: .\" JOIN
320: \e11 might be a back reference, or another way of
321: writing a tab
322: \e011 is always a tab
323: \e0113 is a tab followed by the character "3"
324: .\" JOIN
325: \e113 might be a back reference, otherwise the
326: character with octal code 113
327: .\" JOIN
328: \e377 might be a back reference, otherwise
1.1.1.2 ! misho 329: the value 255 (decimal)
1.1 misho 330: .\" JOIN
331: \e81 is either a back reference, or a binary zero
332: followed by the two characters "8" and "1"
333: .sp
334: Note that octal values of 100 or greater must not be introduced by a leading
335: zero, because no more than three octal digits are ever read.
336: .P
337: All the sequences that define a single character value can be used both inside
338: and outside character classes. In addition, inside a character class, \eb is
339: interpreted as the backspace character (hex 08).
340: .P
341: \eN is not allowed in a character class. \eB, \eR, and \eX are not special
342: inside a character class. Like other unrecognized escape sequences, they are
343: treated as the literal characters "B", "R", and "X" by default, but cause an
344: error if the PCRE_EXTRA option is set. Outside a character class, these
345: sequences have different meanings.
346: .
347: .
348: .SS "Unsupported escape sequences"
349: .rs
350: .sp
351: In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
352: handler and used to modify the case of following characters. By default, PCRE
353: does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
354: option is set, \eU matches a "U" character, and \eu can be used to define a
355: character by code point, as described in the previous section.
356: .
357: .
358: .SS "Absolute and relative back references"
359: .rs
360: .sp
361: The sequence \eg followed by an unsigned or a negative number, optionally
362: enclosed in braces, is an absolute or relative back reference. A named back
363: reference can be coded as \eg{name}. Back references are discussed
364: .\" HTML <a href="#backreferences">
365: .\" </a>
366: later,
367: .\"
368: following the discussion of
369: .\" HTML <a href="#subpattern">
370: .\" </a>
371: parenthesized subpatterns.
372: .\"
373: .
374: .
375: .SS "Absolute and relative subroutine calls"
376: .rs
377: .sp
378: For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
379: a number enclosed either in angle brackets or single quotes, is an alternative
380: syntax for referencing a subpattern as a "subroutine". Details are discussed
381: .\" HTML <a href="#onigurumasubroutines">
382: .\" </a>
383: later.
384: .\"
385: Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
386: synonymous. The former is a back reference; the latter is a
387: .\" HTML <a href="#subpatternsassubroutines">
388: .\" </a>
389: subroutine
390: .\"
391: call.
392: .
393: .
394: .\" HTML <a name="genericchartypes"></a>
395: .SS "Generic character types"
396: .rs
397: .sp
398: Another use of backslash is for specifying generic character types:
399: .sp
400: \ed any decimal digit
401: \eD any character that is not a decimal digit
402: \eh any horizontal whitespace character
403: \eH any character that is not a horizontal whitespace character
404: \es any whitespace character
405: \eS any character that is not a whitespace character
406: \ev any vertical whitespace character
407: \eV any character that is not a vertical whitespace character
408: \ew any "word" character
409: \eW any "non-word" character
410: .sp
411: There is also the single sequence \eN, which matches a non-newline character.
412: This is the same as
413: .\" HTML <a href="#fullstopdot">
414: .\" </a>
415: the "." metacharacter
416: .\"
417: when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
418: PCRE does not support this.
419: .P
420: Each pair of lower and upper case escape sequences partitions the complete set
421: of characters into two disjoint sets. Any given character matches one, and only
422: one, of each pair. The sequences can appear both inside and outside character
423: classes. They each match one character of the appropriate type. If the current
424: matching point is at the end of the subject string, all of them fail, because
425: there is no character to match.
426: .P
427: For compatibility with Perl, \es does not match the VT character (code 11).
428: This makes it different from the the POSIX "space" class. The \es characters
429: are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
430: included in a Perl script, \es may match the VT character. In PCRE, it never
431: does.
432: .P
433: A "word" character is an underscore or any character that is a letter or digit.
434: By default, the definition of letters and digits is controlled by PCRE's
435: low-valued character tables, and may vary if locale-specific matching is taking
436: place (see
437: .\" HTML <a href="pcreapi.html#localesupport">
438: .\" </a>
439: "Locale support"
440: .\"
441: in the
442: .\" HREF
443: \fBpcreapi\fP
444: .\"
445: page). For example, in a French locale such as "fr_FR" in Unix-like systems,
446: or "french" in Windows, some character codes greater than 128 are used for
447: accented letters, and these are then matched by \ew. The use of locales with
448: Unicode is discouraged.
449: .P
1.1.1.2 ! misho 450: By default, in a UTF mode, characters with values greater than 128 never match
1.1 misho 451: \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
1.1.1.2 ! misho 452: their original meanings from before UTF support was available, mainly for
1.1 misho 453: efficiency reasons. However, if PCRE is compiled with Unicode property support,
454: and the PCRE_UCP option is set, the behaviour is changed so that Unicode
455: properties are used to determine character types, as follows:
456: .sp
457: \ed any character that \ep{Nd} matches (decimal digit)
458: \es any character that \ep{Z} matches, plus HT, LF, FF, CR
459: \ew any character that \ep{L} or \ep{N} matches, plus underscore
460: .sp
461: The upper case escapes match the inverse sets of characters. Note that \ed
462: matches only decimal digits, whereas \ew matches any Unicode digit, as well as
463: any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
464: \eB because they are defined in terms of \ew and \eW. Matching these sequences
465: is noticeably slower when PCRE_UCP is set.
466: .P
467: The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
468: release 5.10. In contrast to the other sequences, which match only ASCII
1.1.1.2 ! misho 469: characters by default, these always match certain high-valued codepoints,
! 470: whether or not PCRE_UCP is set. The horizontal space characters are:
1.1 misho 471: .sp
472: U+0009 Horizontal tab
473: U+0020 Space
474: U+00A0 Non-break space
475: U+1680 Ogham space mark
476: U+180E Mongolian vowel separator
477: U+2000 En quad
478: U+2001 Em quad
479: U+2002 En space
480: U+2003 Em space
481: U+2004 Three-per-em space
482: U+2005 Four-per-em space
483: U+2006 Six-per-em space
484: U+2007 Figure space
485: U+2008 Punctuation space
486: U+2009 Thin space
487: U+200A Hair space
488: U+202F Narrow no-break space
489: U+205F Medium mathematical space
490: U+3000 Ideographic space
491: .sp
492: The vertical space characters are:
493: .sp
494: U+000A Linefeed
495: U+000B Vertical tab
496: U+000C Formfeed
497: U+000D Carriage return
498: U+0085 Next line
499: U+2028 Line separator
500: U+2029 Paragraph separator
1.1.1.2 ! misho 501: .sp
! 502: In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
! 503: relevant.
1.1 misho 504: .
505: .
506: .\" HTML <a name="newlineseq"></a>
507: .SS "Newline sequences"
508: .rs
509: .sp
510: Outside a character class, by default, the escape sequence \eR matches any
1.1.1.2 ! misho 511: Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
! 512: following:
1.1 misho 513: .sp
514: (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
515: .sp
516: This is an example of an "atomic group", details of which are given
517: .\" HTML <a href="#atomicgroup">
518: .\" </a>
519: below.
520: .\"
521: This particular group matches either the two-character sequence CR followed by
522: LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
523: U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
524: line, U+0085). The two-character sequence is treated as a single unit that
525: cannot be split.
526: .P
1.1.1.2 ! misho 527: In other modes, two additional characters whose codepoints are greater than 255
1.1 misho 528: are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
529: Unicode character property support is not needed for these characters to be
530: recognized.
531: .P
532: It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
533: complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
534: either at compile time or when the pattern is matched. (BSR is an abbrevation
535: for "backslash R".) This can be made the default when PCRE is built; if this is
536: the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
537: It is also possible to specify these settings by starting a pattern string with
538: one of the following sequences:
539: .sp
540: (*BSR_ANYCRLF) CR, LF, or CRLF only
541: (*BSR_UNICODE) any Unicode newline sequence
542: .sp
1.1.1.2 ! misho 543: These override the default and the options given to the compiling function, but
! 544: they can themselves be overridden by options given to a matching function. Note
! 545: that these special settings, which are not Perl-compatible, are recognized only
! 546: at the very start of a pattern, and that they must be in upper case. If more
! 547: than one of them is present, the last one is used. They can be combined with a
! 548: change of newline convention; for example, a pattern can start with:
1.1 misho 549: .sp
550: (*ANY)(*BSR_ANYCRLF)
551: .sp
1.1.1.2 ! misho 552: They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
! 553: sequences. Inside a character class, \eR is treated as an unrecognized escape
! 554: sequence, and so matches the letter "R" by default, but causes an error if
! 555: PCRE_EXTRA is set.
1.1 misho 556: .
557: .
558: .\" HTML <a name="uniextseq"></a>
559: .SS Unicode character properties
560: .rs
561: .sp
562: When PCRE is built with Unicode character property support, three additional
563: escape sequences that match characters with specific properties are available.
1.1.1.2 ! misho 564: When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
1.1 misho 565: characters whose codepoints are less than 256, but they do work in this mode.
566: The extra escape sequences are:
567: .sp
568: \ep{\fIxx\fP} a character with the \fIxx\fP property
569: \eP{\fIxx\fP} a character without the \fIxx\fP property
570: \eX an extended Unicode sequence
571: .sp
572: The property names represented by \fIxx\fP above are limited to the Unicode
573: script names, the general category properties, "Any", which matches any
574: character (including newline), and some special PCRE properties (described
575: in the
576: .\" HTML <a href="#extraprops">
577: .\" </a>
578: next section).
579: .\"
580: Other Perl properties such as "InMusicalSymbols" are not currently supported by
581: PCRE. Note that \eP{Any} does not match any characters, so always causes a
582: match failure.
583: .P
584: Sets of Unicode characters are defined as belonging to certain scripts. A
585: character from one of these sets can be matched using a script name. For
586: example:
587: .sp
588: \ep{Greek}
589: \eP{Han}
590: .sp
591: Those that are not part of an identified script are lumped together as
592: "Common". The current list of scripts is:
593: .P
594: Arabic,
595: Armenian,
596: Avestan,
597: Balinese,
598: Bamum,
599: Bengali,
600: Bopomofo,
601: Braille,
602: Buginese,
603: Buhid,
604: Canadian_Aboriginal,
605: Carian,
606: Cham,
607: Cherokee,
608: Common,
609: Coptic,
610: Cuneiform,
611: Cypriot,
612: Cyrillic,
613: Deseret,
614: Devanagari,
615: Egyptian_Hieroglyphs,
616: Ethiopic,
617: Georgian,
618: Glagolitic,
619: Gothic,
620: Greek,
621: Gujarati,
622: Gurmukhi,
623: Han,
624: Hangul,
625: Hanunoo,
626: Hebrew,
627: Hiragana,
628: Imperial_Aramaic,
629: Inherited,
630: Inscriptional_Pahlavi,
631: Inscriptional_Parthian,
632: Javanese,
633: Kaithi,
634: Kannada,
635: Katakana,
636: Kayah_Li,
637: Kharoshthi,
638: Khmer,
639: Lao,
640: Latin,
641: Lepcha,
642: Limbu,
643: Linear_B,
644: Lisu,
645: Lycian,
646: Lydian,
647: Malayalam,
648: Meetei_Mayek,
649: Mongolian,
650: Myanmar,
651: New_Tai_Lue,
652: Nko,
653: Ogham,
654: Old_Italic,
655: Old_Persian,
656: Old_South_Arabian,
657: Old_Turkic,
658: Ol_Chiki,
659: Oriya,
660: Osmanya,
661: Phags_Pa,
662: Phoenician,
663: Rejang,
664: Runic,
665: Samaritan,
666: Saurashtra,
667: Shavian,
668: Sinhala,
669: Sundanese,
670: Syloti_Nagri,
671: Syriac,
672: Tagalog,
673: Tagbanwa,
674: Tai_Le,
675: Tai_Tham,
676: Tai_Viet,
677: Tamil,
678: Telugu,
679: Thaana,
680: Thai,
681: Tibetan,
682: Tifinagh,
683: Ugaritic,
684: Vai,
685: Yi.
686: .P
687: Each character has exactly one Unicode general category property, specified by
688: a two-letter abbreviation. For compatibility with Perl, negation can be
689: specified by including a circumflex between the opening brace and the property
690: name. For example, \ep{^Lu} is the same as \eP{Lu}.
691: .P
692: If only one letter is specified with \ep or \eP, it includes all the general
693: category properties that start with that letter. In this case, in the absence
694: of negation, the curly brackets in the escape sequence are optional; these two
695: examples have the same effect:
696: .sp
697: \ep{L}
698: \epL
699: .sp
700: The following general category property codes are supported:
701: .sp
702: C Other
703: Cc Control
704: Cf Format
705: Cn Unassigned
706: Co Private use
707: Cs Surrogate
708: .sp
709: L Letter
710: Ll Lower case letter
711: Lm Modifier letter
712: Lo Other letter
713: Lt Title case letter
714: Lu Upper case letter
715: .sp
716: M Mark
717: Mc Spacing mark
718: Me Enclosing mark
719: Mn Non-spacing mark
720: .sp
721: N Number
722: Nd Decimal number
723: Nl Letter number
724: No Other number
725: .sp
726: P Punctuation
727: Pc Connector punctuation
728: Pd Dash punctuation
729: Pe Close punctuation
730: Pf Final punctuation
731: Pi Initial punctuation
732: Po Other punctuation
733: Ps Open punctuation
734: .sp
735: S Symbol
736: Sc Currency symbol
737: Sk Modifier symbol
738: Sm Mathematical symbol
739: So Other symbol
740: .sp
741: Z Separator
742: Zl Line separator
743: Zp Paragraph separator
744: Zs Space separator
745: .sp
746: The special property L& is also supported: it matches a character that has
747: the Lu, Ll, or Lt property, in other words, a letter that is not classified as
748: a modifier or "other".
749: .P
750: The Cs (Surrogate) property applies only to characters in the range U+D800 to
1.1.1.2 ! misho 751: U+DFFF. Such characters are not valid in Unicode strings and so
! 752: cannot be tested by PCRE, unless UTF validity checking has been turned off
! 753: (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
1.1 misho 754: .\" HREF
755: \fBpcreapi\fP
756: .\"
757: page). Perl does not support the Cs property.
758: .P
759: The long synonyms for property names that Perl supports (such as \ep{Letter})
760: are not supported by PCRE, nor is it permitted to prefix any of these
761: properties with "Is".
762: .P
763: No character that is in the Unicode table has the Cn (unassigned) property.
764: Instead, this property is assumed for any code point that is not in the
765: Unicode table.
766: .P
767: Specifying caseless matching does not affect these escape sequences. For
768: example, \ep{Lu} always matches only upper case letters.
769: .P
770: The \eX escape matches any number of Unicode characters that form an extended
771: Unicode sequence. \eX is equivalent to
772: .sp
773: (?>\ePM\epM*)
774: .sp
775: That is, it matches a character without the "mark" property, followed by zero
776: or more characters with the "mark" property, and treats the sequence as an
777: atomic group
778: .\" HTML <a href="#atomicgroup">
779: .\" </a>
780: (see below).
781: .\"
782: Characters with the "mark" property are typically accents that affect the
783: preceding character. None of them have codepoints less than 256, so in
1.1.1.2 ! misho 784: 8-bit non-UTF-8 mode \eX matches any one character.
1.1 misho 785: .P
786: Note that recent versions of Perl have changed \eX to match what Unicode calls
787: an "extended grapheme cluster", which has a more complicated definition.
788: .P
789: Matching characters by Unicode property is not fast, because PCRE has to search
790: a structure that contains data for over fifteen thousand characters. That is
791: why the traditional escape sequences such as \ed and \ew do not use Unicode
792: properties in PCRE by default, though you can make them do so by setting the
1.1.1.2 ! misho 793: PCRE_UCP option or by starting the pattern with (*UCP).
1.1 misho 794: .
795: .
796: .\" HTML <a name="extraprops"></a>
797: .SS PCRE's additional properties
798: .rs
799: .sp
800: As well as the standard Unicode properties described in the previous
801: section, PCRE supports four more that make it possible to convert traditional
802: escape sequences such as \ew and \es and POSIX character classes to use Unicode
803: properties. PCRE uses these non-standard, non-Perl properties internally when
804: PCRE_UCP is set. They are:
805: .sp
806: Xan Any alphanumeric character
807: Xps Any POSIX space character
808: Xsp Any Perl space character
809: Xwd Any Perl "word" character
810: .sp
811: Xan matches characters that have either the L (letter) or the N (number)
812: property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
813: carriage return, and any other character that has the Z (separator) property.
814: Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
815: same characters as Xan, plus underscore.
816: .
817: .
818: .\" HTML <a name="resetmatchstart"></a>
819: .SS "Resetting the match start"
820: .rs
821: .sp
822: The escape sequence \eK causes any previously matched characters not to be
823: included in the final matched sequence. For example, the pattern:
824: .sp
825: foo\eKbar
826: .sp
827: matches "foobar", but reports that it has matched "bar". This feature is
828: similar to a lookbehind assertion
829: .\" HTML <a href="#lookbehind">
830: .\" </a>
831: (described below).
832: .\"
833: However, in this case, the part of the subject before the real match does not
834: have to be of fixed length, as lookbehind assertions do. The use of \eK does
835: not interfere with the setting of
836: .\" HTML <a href="#subpattern">
837: .\" </a>
838: captured substrings.
839: .\"
840: For example, when the pattern
841: .sp
842: (foo)\eKbar
843: .sp
844: matches "foobar", the first substring is still set to "foo".
845: .P
846: Perl documents that the use of \eK within assertions is "not well defined". In
847: PCRE, \eK is acted upon when it occurs inside positive assertions, but is
848: ignored in negative assertions.
849: .
850: .
851: .\" HTML <a name="smallassertions"></a>
852: .SS "Simple assertions"
853: .rs
854: .sp
855: The final use of backslash is for certain simple assertions. An assertion
856: specifies a condition that has to be met at a particular point in a match,
857: without consuming any characters from the subject string. The use of
858: subpatterns for more complicated assertions is described
859: .\" HTML <a href="#bigassertions">
860: .\" </a>
861: below.
862: .\"
863: The backslashed assertions are:
864: .sp
865: \eb matches at a word boundary
866: \eB matches when not at a word boundary
867: \eA matches at the start of the subject
868: \eZ matches at the end of the subject
869: also matches before a newline at the end of the subject
870: \ez matches only at the end of the subject
871: \eG matches at the first matching position in the subject
872: .sp
873: Inside a character class, \eb has a different meaning; it matches the backspace
874: character. If any other of these assertions appears in a character class, by
875: default it matches the corresponding literal character (for example, \eB
876: matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
877: escape sequence" error is generated instead.
878: .P
879: A word boundary is a position in the subject string where the current character
880: and the previous character do not both match \ew or \eW (i.e. one matches
881: \ew and the other matches \eW), or the start or end of the string if the
1.1.1.2 ! misho 882: first or last character matches \ew, respectively. In a UTF mode, the meanings
1.1 misho 883: of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
884: done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
885: of word" or "end of word" metasequence. However, whatever follows \eb normally
886: determines which it is. For example, the fragment \eba matches "a" at the start
887: of a word.
888: .P
889: The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
890: dollar (described in the next section) in that they only ever match at the very
891: start and end of the subject string, whatever options are set. Thus, they are
892: independent of multiline mode. These three assertions are not affected by the
893: PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
894: circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
895: argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
896: at a point other than the beginning of the subject, \eA can never match. The
897: difference between \eZ and \ez is that \eZ matches before a newline at the end
898: of the string as well as at the very end, whereas \ez matches only at the end.
899: .P
900: The \eG assertion is true only when the current matching position is at the
901: start point of the match, as specified by the \fIstartoffset\fP argument of
902: \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
903: non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
904: arguments, you can mimic Perl's /g option, and it is in this kind of
905: implementation where \eG can be useful.
906: .P
907: Note, however, that PCRE's interpretation of \eG, as the start of the current
908: match, is subtly different from Perl's, which defines it as the end of the
909: previous match. In Perl, these can be different when the previously matched
910: string was empty. Because PCRE does just one match at a time, it cannot
911: reproduce this behaviour.
912: .P
913: If all the alternatives of a pattern begin with \eG, the expression is anchored
914: to the starting match position, and the "anchored" flag is set in the compiled
915: regular expression.
916: .
917: .
918: .SH "CIRCUMFLEX AND DOLLAR"
919: .rs
920: .sp
921: Outside a character class, in the default matching mode, the circumflex
922: character is an assertion that is true only if the current matching point is
923: at the start of the subject string. If the \fIstartoffset\fP argument of
924: \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
925: option is unset. Inside a character class, circumflex has an entirely different
926: meaning
927: .\" HTML <a href="#characterclass">
928: .\" </a>
929: (see below).
930: .\"
931: .P
932: Circumflex need not be the first character of the pattern if a number of
933: alternatives are involved, but it should be the first thing in each alternative
934: in which it appears if the pattern is ever to match that branch. If all
935: possible alternatives start with a circumflex, that is, if the pattern is
936: constrained to match only at the start of the subject, it is said to be an
937: "anchored" pattern. (There are also other constructs that can cause a pattern
938: to be anchored.)
939: .P
940: A dollar character is an assertion that is true only if the current matching
941: point is at the end of the subject string, or immediately before a newline
942: at the end of the string (by default). Dollar need not be the last character of
943: the pattern if a number of alternatives are involved, but it should be the last
944: item in any branch in which it appears. Dollar has no special meaning in a
945: character class.
946: .P
947: The meaning of dollar can be changed so that it matches only at the very end of
948: the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
949: does not affect the \eZ assertion.
950: .P
951: The meanings of the circumflex and dollar characters are changed if the
952: PCRE_MULTILINE option is set. When this is the case, a circumflex matches
953: immediately after internal newlines as well as at the start of the subject
954: string. It does not match after a newline that ends the string. A dollar
955: matches before any newlines in the string, as well as at the very end, when
956: PCRE_MULTILINE is set. When newline is specified as the two-character
957: sequence CRLF, isolated CR and LF characters do not indicate newlines.
958: .P
959: For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
960: \en represents a newline) in multiline mode, but not otherwise. Consequently,
961: patterns that are anchored in single line mode because all branches start with
962: ^ are not anchored in multiline mode, and a match for circumflex is possible
963: when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
964: PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
965: .P
966: Note that the sequences \eA, \eZ, and \ez can be used to match the start and
967: end of the subject in both modes, and if all branches of a pattern start with
968: \eA it is always anchored, whether or not PCRE_MULTILINE is set.
969: .
970: .
971: .\" HTML <a name="fullstopdot"></a>
972: .SH "FULL STOP (PERIOD, DOT) AND \eN"
973: .rs
974: .sp
975: Outside a character class, a dot in the pattern matches any one character in
976: the subject string except (by default) a character that signifies the end of a
1.1.1.2 ! misho 977: line.
1.1 misho 978: .P
979: When a line ending is defined as a single character, dot never matches that
980: character; when the two-character sequence CRLF is used, dot does not match CR
981: if it is immediately followed by LF, but otherwise it matches all characters
982: (including isolated CRs and LFs). When any Unicode line endings are being
983: recognized, dot does not match CR or LF or any of the other line ending
984: characters.
985: .P
986: The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
987: option is set, a dot matches any one character, without exception. If the
988: two-character sequence CRLF is present in the subject string, it takes two dots
989: to match it.
990: .P
991: The handling of dot is entirely independent of the handling of circumflex and
992: dollar, the only relationship being that they both involve newlines. Dot has no
993: special meaning in a character class.
994: .P
995: The escape sequence \eN behaves like a dot, except that it is not affected by
996: the PCRE_DOTALL option. In other words, it matches any character except one
997: that signifies the end of a line. Perl also uses \eN to match characters by
998: name; PCRE does not support this.
999: .
1000: .
1.1.1.2 ! misho 1001: .SH "MATCHING A SINGLE DATA UNIT"
1.1 misho 1002: .rs
1003: .sp
1.1.1.2 ! misho 1004: Outside a character class, the escape sequence \eC matches any one data unit,
! 1005: whether or not a UTF mode is set. In the 8-bit library, one data unit is one
! 1006: byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
! 1007: matches line-ending characters. The feature is provided in Perl in order to
! 1008: match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
! 1009: used. Because \eC breaks up characters into individual data units, matching one
! 1010: unit with \eC in a UTF mode means that the rest of the string may start with a
! 1011: malformed UTF character. This has undefined results, because PCRE assumes that
! 1012: it is dealing with valid UTF strings (and by default it checks this at the
! 1013: start of processing unless the PCRE_NO_UTF8_CHECK option is used).
1.1 misho 1014: .P
1015: PCRE does not allow \eC to appear in lookbehind assertions
1016: .\" HTML <a href="#lookbehind">
1017: .\" </a>
1018: (described below)
1019: .\"
1.1.1.2 ! misho 1020: in a UTF mode, because this would make it impossible to calculate the length of
1.1 misho 1021: the lookbehind.
1022: .P
1.1.1.2 ! misho 1023: In general, the \eC escape sequence is best avoided. However, one
! 1024: way of using it that avoids the problem of malformed UTF characters is to use a
! 1025: lookahead to check the length of the next character, as in this pattern, which
! 1026: could be used with a UTF-8 string (ignore white space and line breaks):
1.1 misho 1027: .sp
1028: (?| (?=[\ex00-\ex7f])(\eC) |
1029: (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1030: (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1031: (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1032: .sp
1033: A group that starts with (?| resets the capturing parentheses numbers in each
1034: alternative (see
1035: .\" HTML <a href="#dupsubpatternnumber">
1036: .\" </a>
1037: "Duplicate Subpattern Numbers"
1038: .\"
1039: below). The assertions at the start of each branch check the next UTF-8
1040: character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1041: character's individual bytes are then captured by the appropriate number of
1042: groups.
1043: .
1044: .
1045: .\" HTML <a name="characterclass"></a>
1046: .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
1047: .rs
1048: .sp
1049: An opening square bracket introduces a character class, terminated by a closing
1050: square bracket. A closing square bracket on its own is not special by default.
1051: However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1052: bracket causes a compile-time error. If a closing square bracket is required as
1053: a member of the class, it should be the first data character in the class
1054: (after an initial circumflex, if present) or escaped with a backslash.
1055: .P
1.1.1.2 ! misho 1056: A character class matches a single character in the subject. In a UTF mode, the
! 1057: character may be more than one data unit long. A matched character must be in
! 1058: the set of characters defined by the class, unless the first character in the
! 1059: class definition is a circumflex, in which case the subject character must not
! 1060: be in the set defined by the class. If a circumflex is actually required as a
! 1061: member of the class, ensure it is not the first character, or escape it with a
1.1 misho 1062: backslash.
1063: .P
1064: For example, the character class [aeiou] matches any lower case vowel, while
1065: [^aeiou] matches any character that is not a lower case vowel. Note that a
1066: circumflex is just a convenient notation for specifying the characters that
1067: are in the class by enumerating those that are not. A class that starts with a
1068: circumflex is not an assertion; it still consumes a character from the subject
1069: string, and therefore it fails if the current pointer is at the end of the
1070: string.
1071: .P
1.1.1.2 ! misho 1072: In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
! 1073: included in a class as a literal string of data units, or by using the \ex{
! 1074: escaping mechanism.
1.1 misho 1075: .P
1076: When caseless matching is set, any letters in a class represent both their
1077: upper case and lower case versions, so for example, a caseless [aeiou] matches
1078: "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1.1.1.2 ! misho 1079: caseful version would. In a UTF mode, PCRE always understands the concept of
1.1 misho 1080: case for characters whose values are less than 128, so caseless matching is
1081: always possible. For characters with higher values, the concept of case is
1082: supported if PCRE is compiled with Unicode property support, but not otherwise.
1.1.1.2 ! misho 1083: If you want to use caseless matching in a UTF mode for characters 128 and
! 1084: above, you must ensure that PCRE is compiled with Unicode property support as
! 1085: well as with UTF support.
1.1 misho 1086: .P
1087: Characters that might indicate line breaks are never treated in any special way
1088: when matching character classes, whatever line-ending sequence is in use, and
1089: whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1090: such as [^a] always matches one of these characters.
1091: .P
1092: The minus (hyphen) character can be used to specify a range of characters in a
1093: character class. For example, [d-m] matches any letter between d and m,
1094: inclusive. If a minus character is required in a class, it must be escaped with
1095: a backslash or appear in a position where it cannot be interpreted as
1096: indicating a range, typically as the first or last character in the class.
1097: .P
1098: It is not possible to have the literal character "]" as the end character of a
1099: range. A pattern such as [W-]46] is interpreted as a class of two characters
1100: ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1101: "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1102: the end of range, so [W-\e]46] is interpreted as a class containing a range
1103: followed by two other characters. The octal or hexadecimal representation of
1104: "]" can also be used to end a range.
1105: .P
1106: Ranges operate in the collating sequence of character values. They can also be
1.1.1.2 ! misho 1107: used for characters specified numerically, for example [\e000-\e037]. Ranges
! 1108: can include any characters that are valid for the current mode.
1.1 misho 1109: .P
1110: If a range that includes letters is used when caseless matching is set, it
1111: matches the letters in either case. For example, [W-c] is equivalent to
1.1.1.2 ! misho 1112: [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1.1 misho 1113: tables for a French locale are in use, [\exc8-\excb] matches accented E
1.1.1.2 ! misho 1114: characters in both cases. In UTF modes, PCRE supports the concept of case for
1.1 misho 1115: characters with values greater than 128 only when it is compiled with Unicode
1116: property support.
1117: .P
1118: The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1119: \eV, \ew, and \eW may appear in a character class, and add the characters that
1120: they match to the class. For example, [\edABCDEF] matches any hexadecimal
1.1.1.2 ! misho 1121: digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1.1 misho 1122: and their upper case partners, just as it does when they appear outside a
1123: character class, as described in the section entitled
1124: .\" HTML <a href="#genericchartypes">
1125: .\" </a>
1126: "Generic character types"
1127: .\"
1128: above. The escape sequence \eb has a different meaning inside a character
1129: class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1130: are not special inside a character class. Like any other unrecognized escape
1131: sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1132: default, but cause an error if the PCRE_EXTRA option is set.
1133: .P
1134: A circumflex can conveniently be used with the upper case character types to
1135: specify a more restricted set of characters than the matching lower case type.
1136: For example, the class [^\eW_] matches any letter or digit, but not underscore,
1137: whereas [\ew] includes underscore. A positive character class should be read as
1138: "something OR something OR ..." and a negative class as "NOT something AND NOT
1139: something AND NOT ...".
1140: .P
1141: The only metacharacters that are recognized in character classes are backslash,
1142: hyphen (only where it can be interpreted as specifying a range), circumflex
1143: (only at the start), opening square bracket (only when it can be interpreted as
1144: introducing a POSIX class name - see the next section), and the terminating
1145: closing square bracket. However, escaping other non-alphanumeric characters
1146: does no harm.
1147: .
1148: .
1149: .SH "POSIX CHARACTER CLASSES"
1150: .rs
1151: .sp
1152: Perl supports the POSIX notation for character classes. This uses names
1153: enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1154: this notation. For example,
1155: .sp
1156: [01[:alpha:]%]
1157: .sp
1158: matches "0", "1", any alphabetic character, or "%". The supported class names
1159: are:
1160: .sp
1161: alnum letters and digits
1162: alpha letters
1163: ascii character codes 0 - 127
1164: blank space or tab only
1165: cntrl control characters
1166: digit decimal digits (same as \ed)
1167: graph printing characters, excluding space
1168: lower lower case letters
1169: print printing characters, including space
1170: punct printing characters, excluding letters and digits and space
1171: space white space (not quite the same as \es)
1172: upper upper case letters
1173: word "word" characters (same as \ew)
1174: xdigit hexadecimal digits
1175: .sp
1176: The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1177: space (32). Notice that this list includes the VT character (code 11). This
1178: makes "space" different to \es, which does not include VT (for Perl
1179: compatibility).
1180: .P
1181: The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1182: 5.8. Another Perl extension is negation, which is indicated by a ^ character
1183: after the colon. For example,
1184: .sp
1185: [12[:^digit:]]
1186: .sp
1187: matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1188: syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1189: supported, and an error is given if they are encountered.
1190: .P
1.1.1.2 ! misho 1191: By default, in UTF modes, characters with values greater than 128 do not match
1.1 misho 1192: any of the POSIX character classes. However, if the PCRE_UCP option is passed
1193: to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1194: character properties are used. This is achieved by replacing the POSIX classes
1195: by other sequences, as follows:
1196: .sp
1197: [:alnum:] becomes \ep{Xan}
1198: [:alpha:] becomes \ep{L}
1199: [:blank:] becomes \eh
1200: [:digit:] becomes \ep{Nd}
1201: [:lower:] becomes \ep{Ll}
1202: [:space:] becomes \ep{Xps}
1203: [:upper:] becomes \ep{Lu}
1204: [:word:] becomes \ep{Xwd}
1205: .sp
1206: Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1207: classes are unchanged, and match only characters with code points less than
1208: 128.
1209: .
1210: .
1211: .SH "VERTICAL BAR"
1212: .rs
1213: .sp
1214: Vertical bar characters are used to separate alternative patterns. For example,
1215: the pattern
1216: .sp
1217: gilbert|sullivan
1218: .sp
1219: matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1220: and an empty alternative is permitted (matching the empty string). The matching
1221: process tries each alternative in turn, from left to right, and the first one
1222: that succeeds is used. If the alternatives are within a subpattern
1223: .\" HTML <a href="#subpattern">
1224: .\" </a>
1225: (defined below),
1226: .\"
1227: "succeeds" means matching the rest of the main pattern as well as the
1228: alternative in the subpattern.
1229: .
1230: .
1231: .SH "INTERNAL OPTION SETTING"
1232: .rs
1233: .sp
1234: The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1235: PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1236: the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1237: The option letters are
1238: .sp
1239: i for PCRE_CASELESS
1240: m for PCRE_MULTILINE
1241: s for PCRE_DOTALL
1242: x for PCRE_EXTENDED
1243: .sp
1244: For example, (?im) sets caseless, multiline matching. It is also possible to
1245: unset these options by preceding the letter with a hyphen, and a combined
1246: setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1247: PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1248: permitted. If a letter appears both before and after the hyphen, the option is
1249: unset.
1250: .P
1251: The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1252: changed in the same way as the Perl-compatible options by using the characters
1253: J, U and X respectively.
1254: .P
1255: When one of these option changes occurs at top level (that is, not inside
1256: subpattern parentheses), the change applies to the remainder of the pattern
1257: that follows. If the change is placed right at the start of a pattern, PCRE
1258: extracts it into the global options (and it will therefore show up in data
1259: extracted by the \fBpcre_fullinfo()\fP function).
1260: .P
1261: An option change within a subpattern (see below for a description of
1262: subpatterns) affects only that part of the subpattern that follows it, so
1263: .sp
1264: (a(?i)b)c
1265: .sp
1266: matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1267: By this means, options can be made to have different settings in different
1268: parts of the pattern. Any changes made in one alternative do carry on
1269: into subsequent branches within the same subpattern. For example,
1270: .sp
1271: (a(?i)b|c)
1272: .sp
1273: matches "ab", "aB", "c", and "C", even though when matching "C" the first
1274: branch is abandoned before the option setting. This is because the effects of
1275: option settings happen at compile time. There would be some very weird
1276: behaviour otherwise.
1277: .P
1278: \fBNote:\fP There are other PCRE-specific options that can be set by the
1.1.1.2 ! misho 1279: application when the compiling or matching functions are called. In some cases
! 1280: the pattern can contain special leading sequences such as (*CRLF) to override
! 1281: what the application has set or what has been defaulted. Details are given in
! 1282: the section entitled
1.1 misho 1283: .\" HTML <a href="#newlineseq">
1284: .\" </a>
1285: "Newline sequences"
1286: .\"
1.1.1.2 ! misho 1287: above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
! 1288: can be used to set UTF and Unicode property modes; they are equivalent to
! 1289: setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1.1 misho 1290: .
1291: .
1292: .\" HTML <a name="subpattern"></a>
1293: .SH SUBPATTERNS
1294: .rs
1295: .sp
1296: Subpatterns are delimited by parentheses (round brackets), which can be nested.
1297: Turning part of a pattern into a subpattern does two things:
1298: .sp
1299: 1. It localizes a set of alternatives. For example, the pattern
1300: .sp
1301: cat(aract|erpillar|)
1302: .sp
1303: matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1304: match "cataract", "erpillar" or an empty string.
1305: .sp
1306: 2. It sets up the subpattern as a capturing subpattern. This means that, when
1307: the whole pattern matches, that portion of the subject string that matched the
1.1.1.2 ! misho 1308: subpattern is passed back to the caller via the \fIovector\fP argument of the
! 1309: matching function. (This applies only to the traditional matching functions;
! 1310: the DFA matching functions do not support capturing.)
! 1311: .P
! 1312: Opening parentheses are counted from left to right (starting from 1) to obtain
! 1313: numbers for the capturing subpatterns. For example, if the string "the red
! 1314: king" is matched against the pattern
1.1 misho 1315: .sp
1316: the ((red|white) (king|queen))
1317: .sp
1318: the captured substrings are "red king", "red", and "king", and are numbered 1,
1319: 2, and 3, respectively.
1320: .P
1321: The fact that plain parentheses fulfil two functions is not always helpful.
1322: There are often times when a grouping subpattern is required without a
1323: capturing requirement. If an opening parenthesis is followed by a question mark
1324: and a colon, the subpattern does not do any capturing, and is not counted when
1325: computing the number of any subsequent capturing subpatterns. For example, if
1326: the string "the white queen" is matched against the pattern
1327: .sp
1328: the ((?:red|white) (king|queen))
1329: .sp
1330: the captured substrings are "white queen" and "queen", and are numbered 1 and
1331: 2. The maximum number of capturing subpatterns is 65535.
1332: .P
1333: As a convenient shorthand, if any option settings are required at the start of
1334: a non-capturing subpattern, the option letters may appear between the "?" and
1335: the ":". Thus the two patterns
1336: .sp
1337: (?i:saturday|sunday)
1338: (?:(?i)saturday|sunday)
1339: .sp
1340: match exactly the same set of strings. Because alternative branches are tried
1341: from left to right, and options are not reset until the end of the subpattern
1342: is reached, an option setting in one branch does affect subsequent branches, so
1343: the above patterns match "SUNDAY" as well as "Saturday".
1344: .
1345: .
1346: .\" HTML <a name="dupsubpatternnumber"></a>
1347: .SH "DUPLICATE SUBPATTERN NUMBERS"
1348: .rs
1349: .sp
1350: Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1351: the same numbers for its capturing parentheses. Such a subpattern starts with
1352: (?| and is itself a non-capturing subpattern. For example, consider this
1353: pattern:
1354: .sp
1355: (?|(Sat)ur|(Sun))day
1356: .sp
1357: Because the two alternatives are inside a (?| group, both sets of capturing
1358: parentheses are numbered one. Thus, when the pattern matches, you can look
1359: at captured substring number one, whichever alternative matched. This construct
1360: is useful when you want to capture part, but not all, of one of a number of
1361: alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1362: number is reset at the start of each branch. The numbers of any capturing
1363: parentheses that follow the subpattern start after the highest number used in
1364: any branch. The following example is taken from the Perl documentation. The
1365: numbers underneath show in which buffer the captured content will be stored.
1366: .sp
1367: # before ---------------branch-reset----------- after
1368: / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1369: # 1 2 2 3 2 3 4
1370: .sp
1371: A back reference to a numbered subpattern uses the most recent value that is
1372: set for that number by any subpattern. The following pattern matches "abcabc"
1373: or "defdef":
1374: .sp
1375: /(?|(abc)|(def))\e1/
1376: .sp
1377: In contrast, a subroutine call to a numbered subpattern always refers to the
1378: first one in the pattern with the given number. The following pattern matches
1379: "abcabc" or "defabc":
1380: .sp
1381: /(?|(abc)|(def))(?1)/
1382: .sp
1383: If a
1384: .\" HTML <a href="#conditions">
1385: .\" </a>
1386: condition test
1387: .\"
1388: for a subpattern's having matched refers to a non-unique number, the test is
1389: true if any of the subpatterns of that number have matched.
1390: .P
1391: An alternative approach to using this "branch reset" feature is to use
1392: duplicate named subpatterns, as described in the next section.
1393: .
1394: .
1395: .SH "NAMED SUBPATTERNS"
1396: .rs
1397: .sp
1398: Identifying capturing parentheses by number is simple, but it can be very hard
1399: to keep track of the numbers in complicated regular expressions. Furthermore,
1400: if an expression is modified, the numbers may change. To help with this
1401: difficulty, PCRE supports the naming of subpatterns. This feature was not
1402: added to Perl until release 5.10. Python had the feature earlier, and PCRE
1403: introduced it at release 4.0, using the Python syntax. PCRE now supports both
1404: the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1405: have different names, but PCRE does not.
1406: .P
1407: In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1408: (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1409: parentheses from other parts of the pattern, such as
1410: .\" HTML <a href="#backreferences">
1411: .\" </a>
1412: back references,
1413: .\"
1414: .\" HTML <a href="#recursion">
1415: .\" </a>
1416: recursion,
1417: .\"
1418: and
1419: .\" HTML <a href="#conditions">
1420: .\" </a>
1421: conditions,
1422: .\"
1423: can be made by name as well as by number.
1424: .P
1425: Names consist of up to 32 alphanumeric characters and underscores. Named
1426: capturing parentheses are still allocated numbers as well as names, exactly as
1427: if the names were not present. The PCRE API provides function calls for
1428: extracting the name-to-number translation table from a compiled pattern. There
1429: is also a convenience function for extracting a captured substring by name.
1430: .P
1431: By default, a name must be unique within a pattern, but it is possible to relax
1432: this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1433: names are also always permitted for subpatterns with the same number, set up as
1434: described in the previous section.) Duplicate names can be useful for patterns
1435: where only one instance of the named parentheses can match. Suppose you want to
1436: match the name of a weekday, either as a 3-letter abbreviation or as the full
1437: name, and in both cases you want to extract the abbreviation. This pattern
1438: (ignoring the line breaks) does the job:
1439: .sp
1440: (?<DN>Mon|Fri|Sun)(?:day)?|
1441: (?<DN>Tue)(?:sday)?|
1442: (?<DN>Wed)(?:nesday)?|
1443: (?<DN>Thu)(?:rsday)?|
1444: (?<DN>Sat)(?:urday)?
1445: .sp
1446: There are five capturing substrings, but only one is ever set after a match.
1447: (An alternative way of solving this problem is to use a "branch reset"
1448: subpattern, as described in the previous section.)
1449: .P
1450: The convenience function for extracting the data by name returns the substring
1451: for the first (and in this example, the only) subpattern of that name that
1452: matched. This saves searching to find which numbered subpattern it was.
1453: .P
1454: If you make a back reference to a non-unique named subpattern from elsewhere in
1455: the pattern, the one that corresponds to the first occurrence of the name is
1456: used. In the absence of duplicate numbers (see the previous section) this is
1457: the one with the lowest number. If you use a named reference in a condition
1458: test (see the
1459: .\"
1460: .\" HTML <a href="#conditions">
1461: .\" </a>
1462: section about conditions
1463: .\"
1464: below), either to check whether a subpattern has matched, or to check for
1465: recursion, all subpatterns with the same name are tested. If the condition is
1466: true for any one of them, the overall condition is true. This is the same
1467: behaviour as testing by number. For further details of the interfaces for
1468: handling named subpatterns, see the
1469: .\" HREF
1470: \fBpcreapi\fP
1471: .\"
1472: documentation.
1473: .P
1474: \fBWarning:\fP You cannot use different names to distinguish between two
1475: subpatterns with the same number because PCRE uses only the numbers when
1476: matching. For this reason, an error is given at compile time if different names
1477: are given to subpatterns with the same number. However, you can give the same
1478: name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1479: .
1480: .
1481: .SH REPETITION
1482: .rs
1483: .sp
1484: Repetition is specified by quantifiers, which can follow any of the following
1485: items:
1486: .sp
1487: a literal data character
1488: the dot metacharacter
1489: the \eC escape sequence
1.1.1.2 ! misho 1490: the \eX escape sequence
1.1 misho 1491: the \eR escape sequence
1492: an escape such as \ed or \epL that matches a single character
1493: a character class
1494: a back reference (see next section)
1495: a parenthesized subpattern (including assertions)
1496: a subroutine call to a subpattern (recursive or otherwise)
1497: .sp
1498: The general repetition quantifier specifies a minimum and maximum number of
1499: permitted matches, by giving the two numbers in curly brackets (braces),
1500: separated by a comma. The numbers must be less than 65536, and the first must
1501: be less than or equal to the second. For example:
1502: .sp
1503: z{2,4}
1504: .sp
1505: matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1506: character. If the second number is omitted, but the comma is present, there is
1507: no upper limit; if the second number and the comma are both omitted, the
1508: quantifier specifies an exact number of required matches. Thus
1509: .sp
1510: [aeiou]{3,}
1511: .sp
1512: matches at least 3 successive vowels, but may match many more, while
1513: .sp
1514: \ed{8}
1515: .sp
1516: matches exactly 8 digits. An opening curly bracket that appears in a position
1517: where a quantifier is not allowed, or one that does not match the syntax of a
1518: quantifier, is taken as a literal character. For example, {,6} is not a
1519: quantifier, but a literal string of four characters.
1520: .P
1.1.1.2 ! misho 1521: In UTF modes, quantifiers apply to characters rather than to individual data
! 1522: units. Thus, for example, \ex{100}{2} matches two characters, each of
! 1523: which is represented by a two-byte sequence in a UTF-8 string. Similarly,
! 1524: \eX{3} matches three Unicode extended sequences, each of which may be several
! 1525: data units long (and they may be of different lengths).
1.1 misho 1526: .P
1527: The quantifier {0} is permitted, causing the expression to behave as if the
1528: previous item and the quantifier were not present. This may be useful for
1529: subpatterns that are referenced as
1530: .\" HTML <a href="#subpatternsassubroutines">
1531: .\" </a>
1532: subroutines
1533: .\"
1534: from elsewhere in the pattern (but see also the section entitled
1535: .\" HTML <a href="#subdefine">
1536: .\" </a>
1537: "Defining subpatterns for use by reference only"
1538: .\"
1539: below). Items other than subpatterns that have a {0} quantifier are omitted
1540: from the compiled pattern.
1541: .P
1542: For convenience, the three most common quantifiers have single-character
1543: abbreviations:
1544: .sp
1545: * is equivalent to {0,}
1546: + is equivalent to {1,}
1547: ? is equivalent to {0,1}
1548: .sp
1549: It is possible to construct infinite loops by following a subpattern that can
1550: match no characters with a quantifier that has no upper limit, for example:
1551: .sp
1552: (a?)*
1553: .sp
1554: Earlier versions of Perl and PCRE used to give an error at compile time for
1555: such patterns. However, because there are cases where this can be useful, such
1556: patterns are now accepted, but if any repetition of the subpattern does in fact
1557: match no characters, the loop is forcibly broken.
1558: .P
1559: By default, the quantifiers are "greedy", that is, they match as much as
1560: possible (up to the maximum number of permitted times), without causing the
1561: rest of the pattern to fail. The classic example of where this gives problems
1562: is in trying to match comments in C programs. These appear between /* and */
1563: and within the comment, individual * and / characters may appear. An attempt to
1564: match C comments by applying the pattern
1565: .sp
1566: /\e*.*\e*/
1567: .sp
1568: to the string
1569: .sp
1570: /* first comment */ not comment /* second comment */
1571: .sp
1572: fails, because it matches the entire string owing to the greediness of the .*
1573: item.
1574: .P
1575: However, if a quantifier is followed by a question mark, it ceases to be
1576: greedy, and instead matches the minimum number of times possible, so the
1577: pattern
1578: .sp
1579: /\e*.*?\e*/
1580: .sp
1581: does the right thing with the C comments. The meaning of the various
1582: quantifiers is not otherwise changed, just the preferred number of matches.
1583: Do not confuse this use of question mark with its use as a quantifier in its
1584: own right. Because it has two uses, it can sometimes appear doubled, as in
1585: .sp
1586: \ed??\ed
1587: .sp
1588: which matches one digit by preference, but can match two if that is the only
1589: way the rest of the pattern matches.
1590: .P
1591: If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1592: the quantifiers are not greedy by default, but individual ones can be made
1593: greedy by following them with a question mark. In other words, it inverts the
1594: default behaviour.
1595: .P
1596: When a parenthesized subpattern is quantified with a minimum repeat count that
1597: is greater than 1 or with a limited maximum, more memory is required for the
1598: compiled pattern, in proportion to the size of the minimum or maximum.
1599: .P
1600: If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1601: to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1602: implicitly anchored, because whatever follows will be tried against every
1603: character position in the subject string, so there is no point in retrying the
1604: overall match at any position after the first. PCRE normally treats such a
1605: pattern as though it were preceded by \eA.
1606: .P
1607: In cases where it is known that the subject string contains no newlines, it is
1608: worth setting PCRE_DOTALL in order to obtain this optimization, or
1609: alternatively using ^ to indicate anchoring explicitly.
1610: .P
1611: However, there is one situation where the optimization cannot be used. When .*
1612: is inside capturing parentheses that are the subject of a back reference
1613: elsewhere in the pattern, a match at the start may fail where a later one
1614: succeeds. Consider, for example:
1615: .sp
1616: (.*)abc\e1
1617: .sp
1618: If the subject is "xyz123abc123" the match point is the fourth character. For
1619: this reason, such a pattern is not implicitly anchored.
1620: .P
1621: When a capturing subpattern is repeated, the value captured is the substring
1622: that matched the final iteration. For example, after
1623: .sp
1624: (tweedle[dume]{3}\es*)+
1625: .sp
1626: has matched "tweedledum tweedledee" the value of the captured substring is
1627: "tweedledee". However, if there are nested capturing subpatterns, the
1628: corresponding captured values may have been set in previous iterations. For
1629: example, after
1630: .sp
1631: /(a|(b))+/
1632: .sp
1633: matches "aba" the value of the second captured substring is "b".
1634: .
1635: .
1636: .\" HTML <a name="atomicgroup"></a>
1637: .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1638: .rs
1639: .sp
1640: With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1641: repetition, failure of what follows normally causes the repeated item to be
1642: re-evaluated to see if a different number of repeats allows the rest of the
1643: pattern to match. Sometimes it is useful to prevent this, either to change the
1644: nature of the match, or to cause it fail earlier than it otherwise might, when
1645: the author of the pattern knows there is no point in carrying on.
1646: .P
1647: Consider, for example, the pattern \ed+foo when applied to the subject line
1648: .sp
1649: 123456bar
1650: .sp
1651: After matching all 6 digits and then failing to match "foo", the normal
1652: action of the matcher is to try again with only 5 digits matching the \ed+
1653: item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1654: (a term taken from Jeffrey Friedl's book) provides the means for specifying
1655: that once a subpattern has matched, it is not to be re-evaluated in this way.
1656: .P
1657: If we use atomic grouping for the previous example, the matcher gives up
1658: immediately on failing to match "foo" the first time. The notation is a kind of
1659: special parenthesis, starting with (?> as in this example:
1660: .sp
1661: (?>\ed+)foo
1662: .sp
1663: This kind of parenthesis "locks up" the part of the pattern it contains once
1664: it has matched, and a failure further into the pattern is prevented from
1665: backtracking into it. Backtracking past it to previous items, however, works as
1666: normal.
1667: .P
1668: An alternative description is that a subpattern of this type matches the string
1669: of characters that an identical standalone pattern would match, if anchored at
1670: the current point in the subject string.
1671: .P
1672: Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1673: the above example can be thought of as a maximizing repeat that must swallow
1674: everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1675: number of digits they match in order to make the rest of the pattern match,
1676: (?>\ed+) can only match an entire sequence of digits.
1677: .P
1678: Atomic groups in general can of course contain arbitrarily complicated
1679: subpatterns, and can be nested. However, when the subpattern for an atomic
1680: group is just a single repeated item, as in the example above, a simpler
1681: notation, called a "possessive quantifier" can be used. This consists of an
1682: additional + character following a quantifier. Using this notation, the
1683: previous example can be rewritten as
1684: .sp
1685: \ed++foo
1686: .sp
1687: Note that a possessive quantifier can be used with an entire group, for
1688: example:
1689: .sp
1690: (abc|xyz){2,3}+
1691: .sp
1692: Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1693: option is ignored. They are a convenient notation for the simpler forms of
1694: atomic group. However, there is no difference in the meaning of a possessive
1695: quantifier and the equivalent atomic group, though there may be a performance
1696: difference; possessive quantifiers should be slightly faster.
1697: .P
1698: The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1699: Jeffrey Friedl originated the idea (and the name) in the first edition of his
1700: book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1701: package, and PCRE copied it from there. It ultimately found its way into Perl
1702: at release 5.10.
1703: .P
1704: PCRE has an optimization that automatically "possessifies" certain simple
1705: pattern constructs. For example, the sequence A+B is treated as A++B because
1706: there is no point in backtracking into a sequence of A's when B must follow.
1707: .P
1708: When a pattern contains an unlimited repeat inside a subpattern that can itself
1709: be repeated an unlimited number of times, the use of an atomic group is the
1710: only way to avoid some failing matches taking a very long time indeed. The
1711: pattern
1712: .sp
1713: (\eD+|<\ed+>)*[!?]
1714: .sp
1715: matches an unlimited number of substrings that either consist of non-digits, or
1716: digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1717: quickly. However, if it is applied to
1718: .sp
1719: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1720: .sp
1721: it takes a long time before reporting failure. This is because the string can
1722: be divided between the internal \eD+ repeat and the external * repeat in a
1723: large number of ways, and all have to be tried. (The example uses [!?] rather
1724: than a single character at the end, because both PCRE and Perl have an
1725: optimization that allows for fast failure when a single character is used. They
1726: remember the last single character that is required for a match, and fail early
1727: if it is not present in the string.) If the pattern is changed so that it uses
1728: an atomic group, like this:
1729: .sp
1730: ((?>\eD+)|<\ed+>)*[!?]
1731: .sp
1732: sequences of non-digits cannot be broken, and failure happens quickly.
1733: .
1734: .
1735: .\" HTML <a name="backreferences"></a>
1736: .SH "BACK REFERENCES"
1737: .rs
1738: .sp
1739: Outside a character class, a backslash followed by a digit greater than 0 (and
1740: possibly further digits) is a back reference to a capturing subpattern earlier
1741: (that is, to its left) in the pattern, provided there have been that many
1742: previous capturing left parentheses.
1743: .P
1744: However, if the decimal number following the backslash is less than 10, it is
1745: always taken as a back reference, and causes an error only if there are not
1746: that many capturing left parentheses in the entire pattern. In other words, the
1747: parentheses that are referenced need not be to the left of the reference for
1748: numbers less than 10. A "forward back reference" of this type can make sense
1749: when a repetition is involved and the subpattern to the right has participated
1750: in an earlier iteration.
1751: .P
1752: It is not possible to have a numerical "forward back reference" to a subpattern
1753: whose number is 10 or more using this syntax because a sequence such as \e50 is
1754: interpreted as a character defined in octal. See the subsection entitled
1755: "Non-printing characters"
1756: .\" HTML <a href="#digitsafterbackslash">
1757: .\" </a>
1758: above
1759: .\"
1760: for further details of the handling of digits following a backslash. There is
1761: no such problem when named parentheses are used. A back reference to any
1762: subpattern is possible using named parentheses (see below).
1763: .P
1764: Another way of avoiding the ambiguity inherent in the use of digits following a
1765: backslash is to use the \eg escape sequence. This escape must be followed by an
1766: unsigned number or a negative number, optionally enclosed in braces. These
1767: examples are all identical:
1768: .sp
1769: (ring), \e1
1770: (ring), \eg1
1771: (ring), \eg{1}
1772: .sp
1773: An unsigned number specifies an absolute reference without the ambiguity that
1774: is present in the older syntax. It is also useful when literal digits follow
1775: the reference. A negative number is a relative reference. Consider this
1776: example:
1777: .sp
1778: (abc(def)ghi)\eg{-1}
1779: .sp
1780: The sequence \eg{-1} is a reference to the most recently started capturing
1781: subpattern before \eg, that is, is it equivalent to \e2 in this example.
1782: Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1783: can be helpful in long patterns, and also in patterns that are created by
1784: joining together fragments that contain references within themselves.
1785: .P
1786: A back reference matches whatever actually matched the capturing subpattern in
1787: the current subject string, rather than anything matching the subpattern
1788: itself (see
1789: .\" HTML <a href="#subpatternsassubroutines">
1790: .\" </a>
1791: "Subpatterns as subroutines"
1792: .\"
1793: below for a way of doing that). So the pattern
1794: .sp
1795: (sens|respons)e and \e1ibility
1796: .sp
1797: matches "sense and sensibility" and "response and responsibility", but not
1798: "sense and responsibility". If caseful matching is in force at the time of the
1799: back reference, the case of letters is relevant. For example,
1800: .sp
1801: ((?i)rah)\es+\e1
1802: .sp
1803: matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1804: capturing subpattern is matched caselessly.
1805: .P
1806: There are several different ways of writing back references to named
1807: subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1808: \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1809: back reference syntax, in which \eg can be used for both numeric and named
1810: references, is also supported. We could rewrite the above example in any of
1811: the following ways:
1812: .sp
1813: (?<p1>(?i)rah)\es+\ek<p1>
1814: (?'p1'(?i)rah)\es+\ek{p1}
1815: (?P<p1>(?i)rah)\es+(?P=p1)
1816: (?<p1>(?i)rah)\es+\eg{p1}
1817: .sp
1818: A subpattern that is referenced by name may appear in the pattern before or
1819: after the reference.
1820: .P
1821: There may be more than one back reference to the same subpattern. If a
1822: subpattern has not actually been used in a particular match, any back
1823: references to it always fail by default. For example, the pattern
1824: .sp
1825: (a|(bc))\e2
1826: .sp
1827: always fails if it starts to match "a" rather than "bc". However, if the
1828: PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1829: unset value matches an empty string.
1830: .P
1831: Because there may be many capturing parentheses in a pattern, all digits
1832: following a backslash are taken as part of a potential back reference number.
1833: If the pattern continues with a digit character, some delimiter must be used to
1834: terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1835: whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1836: .\" HTML <a href="#comments">
1837: .\" </a>
1838: "Comments"
1839: .\"
1840: below) can be used.
1841: .
1842: .SS "Recursive back references"
1843: .rs
1844: .sp
1845: A back reference that occurs inside the parentheses to which it refers fails
1846: when the subpattern is first used, so, for example, (a\e1) never matches.
1847: However, such references can be useful inside repeated subpatterns. For
1848: example, the pattern
1849: .sp
1850: (a|b\e1)+
1851: .sp
1852: matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1853: the subpattern, the back reference matches the character string corresponding
1854: to the previous iteration. In order for this to work, the pattern must be such
1855: that the first iteration does not need to match the back reference. This can be
1856: done using alternation, as in the example above, or by a quantifier with a
1857: minimum of zero.
1858: .P
1859: Back references of this type cause the group that they reference to be treated
1860: as an
1861: .\" HTML <a href="#atomicgroup">
1862: .\" </a>
1863: atomic group.
1864: .\"
1865: Once the whole group has been matched, a subsequent matching failure cannot
1866: cause backtracking into the middle of the group.
1867: .
1868: .
1869: .\" HTML <a name="bigassertions"></a>
1870: .SH ASSERTIONS
1871: .rs
1872: .sp
1873: An assertion is a test on the characters following or preceding the current
1874: matching point that does not actually consume any characters. The simple
1875: assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1876: .\" HTML <a href="#smallassertions">
1877: .\" </a>
1878: above.
1879: .\"
1880: .P
1881: More complicated assertions are coded as subpatterns. There are two kinds:
1882: those that look ahead of the current position in the subject string, and those
1883: that look behind it. An assertion subpattern is matched in the normal way,
1884: except that it does not cause the current matching position to be changed.
1885: .P
1886: Assertion subpatterns are not capturing subpatterns. If such an assertion
1887: contains capturing subpatterns within it, these are counted for the purposes of
1888: numbering the capturing subpatterns in the whole pattern. However, substring
1889: capturing is carried out only for positive assertions, because it does not make
1890: sense for negative assertions.
1891: .P
1892: For compatibility with Perl, assertion subpatterns may be repeated; though
1893: it makes no sense to assert the same thing several times, the side effect of
1894: capturing parentheses may occasionally be useful. In practice, there only three
1895: cases:
1896: .sp
1897: (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1898: However, it may contain internal capturing parenthesized groups that are called
1899: from elsewhere via the
1900: .\" HTML <a href="#subpatternsassubroutines">
1901: .\" </a>
1902: subroutine mechanism.
1903: .\"
1904: .sp
1905: (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1906: were {0,1}. At run time, the rest of the pattern match is tried with and
1907: without the assertion, the order depending on the greediness of the quantifier.
1908: .sp
1909: (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1910: The assertion is obeyed just once when encountered during matching.
1911: .
1912: .
1913: .SS "Lookahead assertions"
1914: .rs
1915: .sp
1916: Lookahead assertions start with (?= for positive assertions and (?! for
1917: negative assertions. For example,
1918: .sp
1919: \ew+(?=;)
1920: .sp
1921: matches a word followed by a semicolon, but does not include the semicolon in
1922: the match, and
1923: .sp
1924: foo(?!bar)
1925: .sp
1926: matches any occurrence of "foo" that is not followed by "bar". Note that the
1927: apparently similar pattern
1928: .sp
1929: (?!foo)bar
1930: .sp
1931: does not find an occurrence of "bar" that is preceded by something other than
1932: "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1933: (?!foo) is always true when the next three characters are "bar". A
1934: lookbehind assertion is needed to achieve the other effect.
1935: .P
1936: If you want to force a matching failure at some point in a pattern, the most
1937: convenient way to do it is with (?!) because an empty string always matches, so
1938: an assertion that requires there not to be an empty string must always fail.
1939: The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1940: .
1941: .
1942: .\" HTML <a name="lookbehind"></a>
1943: .SS "Lookbehind assertions"
1944: .rs
1945: .sp
1946: Lookbehind assertions start with (?<= for positive assertions and (?<! for
1947: negative assertions. For example,
1948: .sp
1949: (?<!foo)bar
1950: .sp
1951: does find an occurrence of "bar" that is not preceded by "foo". The contents of
1952: a lookbehind assertion are restricted such that all the strings it matches must
1953: have a fixed length. However, if there are several top-level alternatives, they
1954: do not all have to have the same fixed length. Thus
1955: .sp
1956: (?<=bullock|donkey)
1957: .sp
1958: is permitted, but
1959: .sp
1960: (?<!dogs?|cats?)
1961: .sp
1962: causes an error at compile time. Branches that match different length strings
1963: are permitted only at the top level of a lookbehind assertion. This is an
1964: extension compared with Perl, which requires all branches to match the same
1965: length of string. An assertion such as
1966: .sp
1967: (?<=ab(c|de))
1968: .sp
1969: is not permitted, because its single top-level branch can match two different
1970: lengths, but it is acceptable to PCRE if rewritten to use two top-level
1971: branches:
1972: .sp
1973: (?<=abc|abde)
1974: .sp
1975: In some cases, the escape sequence \eK
1976: .\" HTML <a href="#resetmatchstart">
1977: .\" </a>
1978: (see above)
1979: .\"
1980: can be used instead of a lookbehind assertion to get round the fixed-length
1981: restriction.
1982: .P
1983: The implementation of lookbehind assertions is, for each alternative, to
1984: temporarily move the current position back by the fixed length and then try to
1985: match. If there are insufficient characters before the current position, the
1986: assertion fails.
1987: .P
1.1.1.2 ! misho 1988: In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
! 1989: unit even in a UTF mode) to appear in lookbehind assertions, because it makes
! 1990: it impossible to calculate the length of the lookbehind. The \eX and \eR
! 1991: escapes, which can match different numbers of data units, are also not
! 1992: permitted.
1.1 misho 1993: .P
1994: .\" HTML <a href="#subpatternsassubroutines">
1995: .\" </a>
1996: "Subroutine"
1997: .\"
1998: calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1999: as the subpattern matches a fixed-length string.
2000: .\" HTML <a href="#recursion">
2001: .\" </a>
2002: Recursion,
2003: .\"
2004: however, is not supported.
2005: .P
2006: Possessive quantifiers can be used in conjunction with lookbehind assertions to
2007: specify efficient matching of fixed-length strings at the end of subject
2008: strings. Consider a simple pattern such as
2009: .sp
2010: abcd$
2011: .sp
2012: when applied to a long string that does not match. Because matching proceeds
2013: from left to right, PCRE will look for each "a" in the subject and then see if
2014: what follows matches the rest of the pattern. If the pattern is specified as
2015: .sp
2016: ^.*abcd$
2017: .sp
2018: the initial .* matches the entire string at first, but when this fails (because
2019: there is no following "a"), it backtracks to match all but the last character,
2020: then all but the last two characters, and so on. Once again the search for "a"
2021: covers the entire string, from right to left, so we are no better off. However,
2022: if the pattern is written as
2023: .sp
2024: ^.*+(?<=abcd)
2025: .sp
2026: there can be no backtracking for the .*+ item; it can match only the entire
2027: string. The subsequent lookbehind assertion does a single test on the last four
2028: characters. If it fails, the match fails immediately. For long strings, this
2029: approach makes a significant difference to the processing time.
2030: .
2031: .
2032: .SS "Using multiple assertions"
2033: .rs
2034: .sp
2035: Several assertions (of any sort) may occur in succession. For example,
2036: .sp
2037: (?<=\ed{3})(?<!999)foo
2038: .sp
2039: matches "foo" preceded by three digits that are not "999". Notice that each of
2040: the assertions is applied independently at the same point in the subject
2041: string. First there is a check that the previous three characters are all
2042: digits, and then there is a check that the same three characters are not "999".
2043: This pattern does \fInot\fP match "foo" preceded by six characters, the first
2044: of which are digits and the last three of which are not "999". For example, it
2045: doesn't match "123abcfoo". A pattern to do that is
2046: .sp
2047: (?<=\ed{3}...)(?<!999)foo
2048: .sp
2049: This time the first assertion looks at the preceding six characters, checking
2050: that the first three are digits, and then the second assertion checks that the
2051: preceding three characters are not "999".
2052: .P
2053: Assertions can be nested in any combination. For example,
2054: .sp
2055: (?<=(?<!foo)bar)baz
2056: .sp
2057: matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2058: preceded by "foo", while
2059: .sp
2060: (?<=\ed{3}(?!999)...)foo
2061: .sp
2062: is another pattern that matches "foo" preceded by three digits and any three
2063: characters that are not "999".
2064: .
2065: .
2066: .\" HTML <a name="conditions"></a>
2067: .SH "CONDITIONAL SUBPATTERNS"
2068: .rs
2069: .sp
2070: It is possible to cause the matching process to obey a subpattern
2071: conditionally or to choose between two alternative subpatterns, depending on
2072: the result of an assertion, or whether a specific capturing subpattern has
2073: already been matched. The two possible forms of conditional subpattern are:
2074: .sp
2075: (?(condition)yes-pattern)
2076: (?(condition)yes-pattern|no-pattern)
2077: .sp
2078: If the condition is satisfied, the yes-pattern is used; otherwise the
2079: no-pattern (if present) is used. If there are more than two alternatives in the
2080: subpattern, a compile-time error occurs. Each of the two alternatives may
2081: itself contain nested subpatterns of any form, including conditional
2082: subpatterns; the restriction to two alternatives applies only at the level of
2083: the condition. This pattern fragment is an example where the alternatives are
2084: complex:
2085: .sp
2086: (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2087: .sp
2088: .P
2089: There are four kinds of condition: references to subpatterns, references to
2090: recursion, a pseudo-condition called DEFINE, and assertions.
2091: .
2092: .SS "Checking for a used subpattern by number"
2093: .rs
2094: .sp
2095: If the text between the parentheses consists of a sequence of digits, the
2096: condition is true if a capturing subpattern of that number has previously
2097: matched. If there is more than one capturing subpattern with the same number
2098: (see the earlier
2099: .\"
2100: .\" HTML <a href="#recursion">
2101: .\" </a>
2102: section about duplicate subpattern numbers),
2103: .\"
2104: the condition is true if any of them have matched. An alternative notation is
2105: to precede the digits with a plus or minus sign. In this case, the subpattern
2106: number is relative rather than absolute. The most recently opened parentheses
2107: can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2108: loops it can also make sense to refer to subsequent groups. The next
2109: parentheses to be opened can be referenced as (?(+1), and so on. (The value
2110: zero in any of these forms is not used; it provokes a compile-time error.)
2111: .P
2112: Consider the following pattern, which contains non-significant white space to
2113: make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2114: three parts for ease of discussion:
2115: .sp
2116: ( \e( )? [^()]+ (?(1) \e) )
2117: .sp
2118: The first part matches an optional opening parenthesis, and if that
2119: character is present, sets it as the first captured substring. The second part
2120: matches one or more characters that are not parentheses. The third part is a
2121: conditional subpattern that tests whether or not the first set of parentheses
2122: matched. If they did, that is, if subject started with an opening parenthesis,
2123: the condition is true, and so the yes-pattern is executed and a closing
2124: parenthesis is required. Otherwise, since no-pattern is not present, the
2125: subpattern matches nothing. In other words, this pattern matches a sequence of
2126: non-parentheses, optionally enclosed in parentheses.
2127: .P
2128: If you were embedding this pattern in a larger one, you could use a relative
2129: reference:
2130: .sp
2131: ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
2132: .sp
2133: This makes the fragment independent of the parentheses in the larger pattern.
2134: .
2135: .SS "Checking for a used subpattern by name"
2136: .rs
2137: .sp
2138: Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2139: subpattern by name. For compatibility with earlier versions of PCRE, which had
2140: this facility before Perl, the syntax (?(name)...) is also recognized. However,
2141: there is a possible ambiguity with this syntax, because subpattern names may
2142: consist entirely of digits. PCRE looks first for a named subpattern; if it
2143: cannot find one and the name consists entirely of digits, PCRE looks for a
2144: subpattern of that number, which must be greater than zero. Using subpattern
2145: names that consist entirely of digits is not recommended.
2146: .P
2147: Rewriting the above example to use a named subpattern gives this:
2148: .sp
2149: (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
2150: .sp
2151: If the name used in a condition of this kind is a duplicate, the test is
2152: applied to all subpatterns of the same name, and is true if any one of them has
2153: matched.
2154: .
2155: .SS "Checking for pattern recursion"
2156: .rs
2157: .sp
2158: If the condition is the string (R), and there is no subpattern with the name R,
2159: the condition is true if a recursive call to the whole pattern or any
2160: subpattern has been made. If digits or a name preceded by ampersand follow the
2161: letter R, for example:
2162: .sp
2163: (?(R3)...) or (?(R&name)...)
2164: .sp
2165: the condition is true if the most recent recursion is into a subpattern whose
2166: number or name is given. This condition does not check the entire recursion
2167: stack. If the name used in a condition of this kind is a duplicate, the test is
2168: applied to all subpatterns of the same name, and is true if any one of them is
2169: the most recent recursion.
2170: .P
2171: At "top level", all these recursion test conditions are false.
2172: .\" HTML <a href="#recursion">
2173: .\" </a>
2174: The syntax for recursive patterns
2175: .\"
2176: is described below.
2177: .
2178: .\" HTML <a name="subdefine"></a>
2179: .SS "Defining subpatterns for use by reference only"
2180: .rs
2181: .sp
2182: If the condition is the string (DEFINE), and there is no subpattern with the
2183: name DEFINE, the condition is always false. In this case, there may be only one
2184: alternative in the subpattern. It is always skipped if control reaches this
2185: point in the pattern; the idea of DEFINE is that it can be used to define
2186: subroutines that can be referenced from elsewhere. (The use of
2187: .\" HTML <a href="#subpatternsassubroutines">
2188: .\" </a>
2189: subroutines
2190: .\"
2191: is described below.) For example, a pattern to match an IPv4 address such as
2192: "192.168.23.245" could be written like this (ignore whitespace and line
2193: breaks):
2194: .sp
2195: (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2196: \eb (?&byte) (\e.(?&byte)){3} \eb
2197: .sp
2198: The first part of the pattern is a DEFINE group inside which a another group
2199: named "byte" is defined. This matches an individual component of an IPv4
2200: address (a number less than 256). When matching takes place, this part of the
2201: pattern is skipped because DEFINE acts like a false condition. The rest of the
2202: pattern uses references to the named group to match the four dot-separated
2203: components of an IPv4 address, insisting on a word boundary at each end.
2204: .
2205: .SS "Assertion conditions"
2206: .rs
2207: .sp
2208: If the condition is not in any of the above formats, it must be an assertion.
2209: This may be a positive or negative lookahead or lookbehind assertion. Consider
2210: this pattern, again containing non-significant white space, and with the two
2211: alternatives on the second line:
2212: .sp
2213: (?(?=[^a-z]*[a-z])
2214: \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2215: .sp
2216: The condition is a positive lookahead assertion that matches an optional
2217: sequence of non-letters followed by a letter. In other words, it tests for the
2218: presence of at least one letter in the subject. If a letter is found, the
2219: subject is matched against the first alternative; otherwise it is matched
2220: against the second. This pattern matches strings in one of the two forms
2221: dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2222: .
2223: .
2224: .\" HTML <a name="comments"></a>
2225: .SH COMMENTS
2226: .rs
2227: .sp
2228: There are two ways of including comments in patterns that are processed by
2229: PCRE. In both cases, the start of the comment must not be in a character class,
2230: nor in the middle of any other sequence of related characters such as (?: or a
2231: subpattern name or number. The characters that make up a comment play no part
2232: in the pattern matching.
2233: .P
2234: The sequence (?# marks the start of a comment that continues up to the next
2235: closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2236: option is set, an unescaped # character also introduces a comment, which in
2237: this case continues to immediately after the next newline character or
2238: character sequence in the pattern. Which characters are interpreted as newlines
1.1.1.2 ! misho 2239: is controlled by the options passed to a compiling function or by a special
1.1 misho 2240: sequence at the start of the pattern, as described in the section entitled
2241: .\" HTML <a href="#newlines">
2242: .\" </a>
2243: "Newline conventions"
2244: .\"
2245: above. Note that the end of this type of comment is a literal newline sequence
2246: in the pattern; escape sequences that happen to represent a newline do not
2247: count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2248: default newline convention is in force:
2249: .sp
2250: abc #comment \en still comment
2251: .sp
2252: On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2253: a newline in the pattern. The sequence \en is still literal at this stage, so
2254: it does not terminate the comment. Only an actual character with the code value
2255: 0x0a (the default newline) does so.
2256: .
2257: .
2258: .\" HTML <a name="recursion"></a>
2259: .SH "RECURSIVE PATTERNS"
2260: .rs
2261: .sp
2262: Consider the problem of matching a string in parentheses, allowing for
2263: unlimited nested parentheses. Without the use of recursion, the best that can
2264: be done is to use a pattern that matches up to some fixed depth of nesting. It
2265: is not possible to handle an arbitrary nesting depth.
2266: .P
2267: For some time, Perl has provided a facility that allows regular expressions to
2268: recurse (amongst other things). It does this by interpolating Perl code in the
2269: expression at run time, and the code can refer to the expression itself. A Perl
2270: pattern using code interpolation to solve the parentheses problem can be
2271: created like this:
2272: .sp
2273: $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2274: .sp
2275: The (?p{...}) item interpolates Perl code at run time, and in this case refers
2276: recursively to the pattern in which it appears.
2277: .P
2278: Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2279: supports special syntax for recursion of the entire pattern, and also for
2280: individual subpattern recursion. After its introduction in PCRE and Python,
2281: this kind of recursion was subsequently introduced into Perl at release 5.10.
2282: .P
2283: A special item that consists of (? followed by a number greater than zero and a
2284: closing parenthesis is a recursive subroutine call of the subpattern of the
2285: given number, provided that it occurs inside that subpattern. (If not, it is a
2286: .\" HTML <a href="#subpatternsassubroutines">
2287: .\" </a>
2288: non-recursive subroutine
2289: .\"
2290: call, which is described in the next section.) The special item (?R) or (?0) is
2291: a recursive call of the entire regular expression.
2292: .P
2293: This PCRE pattern solves the nested parentheses problem (assume the
2294: PCRE_EXTENDED option is set so that white space is ignored):
2295: .sp
2296: \e( ( [^()]++ | (?R) )* \e)
2297: .sp
2298: First it matches an opening parenthesis. Then it matches any number of
2299: substrings which can either be a sequence of non-parentheses, or a recursive
2300: match of the pattern itself (that is, a correctly parenthesized substring).
2301: Finally there is a closing parenthesis. Note the use of a possessive quantifier
2302: to avoid backtracking into sequences of non-parentheses.
2303: .P
2304: If this were part of a larger pattern, you would not want to recurse the entire
2305: pattern, so instead you could use this:
2306: .sp
2307: ( \e( ( [^()]++ | (?1) )* \e) )
2308: .sp
2309: We have put the pattern into parentheses, and caused the recursion to refer to
2310: them instead of the whole pattern.
2311: .P
2312: In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2313: is made easier by the use of relative references. Instead of (?1) in the
2314: pattern above you can write (?-2) to refer to the second most recently opened
2315: parentheses preceding the recursion. In other words, a negative number counts
2316: capturing parentheses leftwards from the point at which it is encountered.
2317: .P
2318: It is also possible to refer to subsequently opened parentheses, by writing
2319: references such as (?+2). However, these cannot be recursive because the
2320: reference is not inside the parentheses that are referenced. They are always
2321: .\" HTML <a href="#subpatternsassubroutines">
2322: .\" </a>
2323: non-recursive subroutine
2324: .\"
2325: calls, as described in the next section.
2326: .P
2327: An alternative approach is to use named parentheses instead. The Perl syntax
2328: for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2329: could rewrite the above example as follows:
2330: .sp
2331: (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2332: .sp
2333: If there is more than one subpattern with the same name, the earliest one is
2334: used.
2335: .P
2336: This particular example pattern that we have been looking at contains nested
2337: unlimited repeats, and so the use of a possessive quantifier for matching
2338: strings of non-parentheses is important when applying the pattern to strings
2339: that do not match. For example, when this pattern is applied to
2340: .sp
2341: (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2342: .sp
2343: it yields "no match" quickly. However, if a possessive quantifier is not used,
2344: the match runs for a very long time indeed because there are so many different
2345: ways the + and * repeats can carve up the subject, and all have to be tested
2346: before failure can be reported.
2347: .P
2348: At the end of a match, the values of capturing parentheses are those from
2349: the outermost level. If you want to obtain intermediate values, a callout
2350: function can be used (see below and the
2351: .\" HREF
2352: \fBpcrecallout\fP
2353: .\"
2354: documentation). If the pattern above is matched against
2355: .sp
2356: (ab(cd)ef)
2357: .sp
2358: the value for the inner capturing parentheses (numbered 2) is "ef", which is
2359: the last value taken on at the top level. If a capturing subpattern is not
2360: matched at the top level, its final captured value is unset, even if it was
2361: (temporarily) set at a deeper level during the matching process.
2362: .P
2363: If there are more than 15 capturing parentheses in a pattern, PCRE has to
2364: obtain extra memory to store data during a recursion, which it does by using
2365: \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2366: be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2367: .P
2368: Do not confuse the (?R) item with the condition (R), which tests for recursion.
2369: Consider this pattern, which matches text in angle brackets, allowing for
2370: arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2371: recursing), whereas any characters are permitted at the outer level.
2372: .sp
2373: < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2374: .sp
2375: In this pattern, (?(R) is the start of a conditional subpattern, with two
2376: different alternatives for the recursive and non-recursive cases. The (?R) item
2377: is the actual recursive call.
2378: .
2379: .
2380: .\" HTML <a name="recursiondifference"></a>
2381: .SS "Differences in recursion processing between PCRE and Perl"
2382: .rs
2383: .sp
2384: Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2385: (like Python, but unlike Perl), a recursive subpattern call is always treated
2386: as an atomic group. That is, once it has matched some of the subject string, it
2387: is never re-entered, even if it contains untried alternatives and there is a
2388: subsequent matching failure. This can be illustrated by the following pattern,
2389: which purports to match a palindromic string that contains an odd number of
2390: characters (for example, "a", "aba", "abcba", "abcdcba"):
2391: .sp
2392: ^(.|(.)(?1)\e2)$
2393: .sp
2394: The idea is that it either matches a single character, or two identical
2395: characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2396: it does not if the pattern is longer than three characters. Consider the
2397: subject string "abcba":
2398: .P
2399: At the top level, the first character is matched, but as it is not at the end
2400: of the string, the first alternative fails; the second alternative is taken
2401: and the recursion kicks in. The recursive call to subpattern 1 successfully
2402: matches the next character ("b"). (Note that the beginning and end of line
2403: tests are not part of the recursion).
2404: .P
2405: Back at the top level, the next character ("c") is compared with what
2406: subpattern 2 matched, which was "a". This fails. Because the recursion is
2407: treated as an atomic group, there are now no backtracking points, and so the
2408: entire match fails. (Perl is able, at this point, to re-enter the recursion and
2409: try the second alternative.) However, if the pattern is written with the
2410: alternatives in the other order, things are different:
2411: .sp
2412: ^((.)(?1)\e2|.)$
2413: .sp
2414: This time, the recursing alternative is tried first, and continues to recurse
2415: until it runs out of characters, at which point the recursion fails. But this
2416: time we do have another alternative to try at the higher level. That is the big
2417: difference: in the previous case the remaining alternative is at a deeper
2418: recursion level, which PCRE cannot use.
2419: .P
2420: To change the pattern so that it matches all palindromic strings, not just
2421: those with an odd number of characters, it is tempting to change the pattern to
2422: this:
2423: .sp
2424: ^((.)(?1)\e2|.?)$
2425: .sp
2426: Again, this works in Perl, but not in PCRE, and for the same reason. When a
2427: deeper recursion has matched a single character, it cannot be entered again in
2428: order to match an empty string. The solution is to separate the two cases, and
2429: write out the odd and even cases as alternatives at the higher level:
2430: .sp
2431: ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2432: .sp
2433: If you want to match typical palindromic phrases, the pattern has to ignore all
2434: non-word characters, which can be done like this:
2435: .sp
2436: ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2437: .sp
2438: If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2439: man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2440: the use of the possessive quantifier *+ to avoid backtracking into sequences of
2441: non-word characters. Without this, PCRE takes a great deal longer (ten times or
2442: more) to match typical phrases, and Perl takes so long that you think it has
2443: gone into a loop.
2444: .P
2445: \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2446: string does not start with a palindrome that is shorter than the entire string.
2447: For example, although "abcba" is correctly matched, if the subject is "ababa",
2448: PCRE finds the palindrome "aba" at the start, then fails at top level because
2449: the end of the string does not follow. Once again, it cannot jump back into the
2450: recursion to try other alternatives, so the entire match fails.
2451: .P
2452: The second way in which PCRE and Perl differ in their recursion processing is
2453: in the handling of captured values. In Perl, when a subpattern is called
2454: recursively or as a subpattern (see the next section), it has no access to any
2455: values that were captured outside the recursion, whereas in PCRE these values
2456: can be referenced. Consider this pattern:
2457: .sp
2458: ^(.)(\e1|a(?2))
2459: .sp
2460: In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2461: then in the second group, when the back reference \e1 fails to match "b", the
2462: second alternative matches "a" and then recurses. In the recursion, \e1 does
2463: now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2464: match because inside the recursive call \e1 cannot access the externally set
2465: value.
2466: .
2467: .
2468: .\" HTML <a name="subpatternsassubroutines"></a>
2469: .SH "SUBPATTERNS AS SUBROUTINES"
2470: .rs
2471: .sp
2472: If the syntax for a recursive subpattern call (either by number or by
2473: name) is used outside the parentheses to which it refers, it operates like a
2474: subroutine in a programming language. The called subpattern may be defined
2475: before or after the reference. A numbered reference can be absolute or
2476: relative, as in these examples:
2477: .sp
2478: (...(absolute)...)...(?2)...
2479: (...(relative)...)...(?-1)...
2480: (...(?+1)...(relative)...
2481: .sp
2482: An earlier example pointed out that the pattern
2483: .sp
2484: (sens|respons)e and \e1ibility
2485: .sp
2486: matches "sense and sensibility" and "response and responsibility", but not
2487: "sense and responsibility". If instead the pattern
2488: .sp
2489: (sens|respons)e and (?1)ibility
2490: .sp
2491: is used, it does match "sense and responsibility" as well as the other two
2492: strings. Another example is given in the discussion of DEFINE above.
2493: .P
2494: All subroutine calls, whether recursive or not, are always treated as atomic
2495: groups. That is, once a subroutine has matched some of the subject string, it
2496: is never re-entered, even if it contains untried alternatives and there is a
2497: subsequent matching failure. Any capturing parentheses that are set during the
2498: subroutine call revert to their previous values afterwards.
2499: .P
2500: Processing options such as case-independence are fixed when a subpattern is
2501: defined, so if it is used as a subroutine, such options cannot be changed for
2502: different calls. For example, consider this pattern:
2503: .sp
2504: (abc)(?i:(?-1))
2505: .sp
2506: It matches "abcabc". It does not match "abcABC" because the change of
2507: processing option does not affect the called subpattern.
2508: .
2509: .
2510: .\" HTML <a name="onigurumasubroutines"></a>
2511: .SH "ONIGURUMA SUBROUTINE SYNTAX"
2512: .rs
2513: .sp
2514: For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2515: a number enclosed either in angle brackets or single quotes, is an alternative
2516: syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2517: are two of the examples used above, rewritten using this syntax:
2518: .sp
2519: (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2520: (sens|respons)e and \eg'1'ibility
2521: .sp
2522: PCRE supports an extension to Oniguruma: if a number is preceded by a
2523: plus or a minus sign it is taken as a relative reference. For example:
2524: .sp
2525: (abc)(?i:\eg<-1>)
2526: .sp
2527: Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2528: synonymous. The former is a back reference; the latter is a subroutine call.
2529: .
2530: .
2531: .SH CALLOUTS
2532: .rs
2533: .sp
2534: Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2535: code to be obeyed in the middle of matching a regular expression. This makes it
2536: possible, amongst other things, to extract different substrings that match the
2537: same pair of parentheses when there is a repetition.
2538: .P
2539: PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2540: code. The feature is called "callout". The caller of PCRE provides an external
1.1.1.2 ! misho 2541: function by putting its entry point in the global variable \fIpcre_callout\fP
! 2542: (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
! 2543: variable contains NULL, which disables all calling out.
1.1 misho 2544: .P
2545: Within a regular expression, (?C) indicates the points at which the external
2546: function is to be called. If you want to identify different callout points, you
2547: can put a number less than 256 after the letter C. The default value is zero.
2548: For example, this pattern has two callout points:
2549: .sp
2550: (?C1)abc(?C2)def
2551: .sp
1.1.1.2 ! misho 2552: If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
1.1 misho 2553: automatically installed before each item in the pattern. They are all numbered
2554: 255.
2555: .P
1.1.1.2 ! misho 2556: During matching, when PCRE reaches a callout point, the external function is
! 2557: called. It is provided with the number of the callout, the position in the
! 2558: pattern, and, optionally, one item of data originally supplied by the caller of
! 2559: the matching function. The callout function may cause matching to proceed, to
! 2560: backtrack, or to fail altogether. A complete description of the interface to
! 2561: the callout function is given in the
1.1 misho 2562: .\" HREF
2563: \fBpcrecallout\fP
2564: .\"
2565: documentation.
2566: .
2567: .
2568: .\" HTML <a name="backtrackcontrol"></a>
2569: .SH "BACKTRACKING CONTROL"
2570: .rs
2571: .sp
2572: Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2573: are described in the Perl documentation as "experimental and subject to change
2574: or removal in a future version of Perl". It goes on to say: "Their usage in
2575: production code should be noted to avoid problems during upgrades." The same
2576: remarks apply to the PCRE features described in this section.
2577: .P
2578: Since these verbs are specifically related to backtracking, most of them can be
1.1.1.2 ! misho 2579: used only when the pattern is to be matched using one of the traditional
! 2580: matching functions, which use a backtracking algorithm. With the exception of
! 2581: (*FAIL), which behaves like a failing negative assertion, they cause an error
! 2582: if encountered by a DFA matching function.
1.1 misho 2583: .P
2584: If any of these verbs are used in an assertion or in a subpattern that is
2585: called as a subroutine (whether or not recursively), their effect is confined
2586: to that subpattern; it does not extend to the surrounding pattern, with one
2587: exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2588: a successful positive assertion \fIis\fP passed back when a match succeeds
2589: (compare capturing parentheses in assertions). Note that such subpatterns are
2590: processed as anchored at the point where they are tested. Note also that Perl's
2591: treatment of subroutines is different in some cases.
2592: .P
2593: The new verbs make use of what was previously invalid syntax: an opening
2594: parenthesis followed by an asterisk. They are generally of the form
2595: (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2596: depending on whether or not an argument is present. A name is any sequence of
2597: characters that does not include a closing parenthesis. If the name is empty,
2598: that is, if the closing parenthesis immediately follows the colon, the effect
2599: is as if the colon were not there. Any number of these verbs may occur in a
2600: pattern.
2601: .P
2602: PCRE contains some optimizations that are used to speed up matching by running
2603: some checks at the start of each match attempt. For example, it may know the
2604: minimum length of matching subject, or that a particular character must be
2605: present. When one of these optimizations suppresses the running of a match, any
2606: included backtracking verbs will not, of course, be processed. You can suppress
2607: the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2608: when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2609: pattern with (*NO_START_OPT).
2610: .P
2611: Experiments with Perl suggest that it too has similar optimizations, sometimes
2612: leading to anomalous results.
2613: .
2614: .
2615: .SS "Verbs that act immediately"
2616: .rs
2617: .sp
2618: The following verbs act as soon as they are encountered. They may not be
2619: followed by a name.
2620: .sp
2621: (*ACCEPT)
2622: .sp
2623: This verb causes the match to end successfully, skipping the remainder of the
2624: pattern. However, when it is inside a subpattern that is called as a
2625: subroutine, only that subpattern is ended successfully. Matching then continues
2626: at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2627: far is captured. For example:
2628: .sp
2629: A((?:A|B(*ACCEPT)|C)D)
2630: .sp
2631: This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2632: the outer parentheses.
2633: .sp
2634: (*FAIL) or (*F)
2635: .sp
2636: This verb causes a matching failure, forcing backtracking to occur. It is
2637: equivalent to (?!) but easier to read. The Perl documentation notes that it is
2638: probably useful only when combined with (?{}) or (??{}). Those are, of course,
2639: Perl features that are not present in PCRE. The nearest equivalent is the
2640: callout feature, as for example in this pattern:
2641: .sp
2642: a+(?C)(*FAIL)
2643: .sp
2644: A match with the string "aaaa" always fails, but the callout is taken before
2645: each backtrack happens (in this example, 10 times).
2646: .
2647: .
2648: .SS "Recording which path was taken"
2649: .rs
2650: .sp
2651: There is one verb whose main purpose is to track how a match was arrived at,
2652: though it also has a secondary use in conjunction with advancing the match
2653: starting point (see (*SKIP) below).
2654: .sp
2655: (*MARK:NAME) or (*:NAME)
2656: .sp
2657: A name is always required with this verb. There may be as many instances of
2658: (*MARK) as you like in a pattern, and their names do not have to be unique.
2659: .P
2660: When a match succeeds, the name of the last-encountered (*MARK) on the matching
1.1.1.2 ! misho 2661: path is passed back to the caller as described in the section entitled
1.1 misho 2662: .\" HTML <a href="pcreapi.html#extradata">
2663: .\" </a>
1.1.1.2 ! misho 2664: "Extra data for \fBpcre_exec()\fP"
1.1 misho 2665: .\"
2666: in the
2667: .\" HREF
2668: \fBpcreapi\fP
2669: .\"
2670: documentation. Here is an example of \fBpcretest\fP output, where the /K
2671: modifier requests the retrieval and outputting of (*MARK) data:
2672: .sp
2673: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2674: data> XY
2675: 0: XY
2676: MK: A
2677: XZ
2678: 0: XZ
2679: MK: B
2680: .sp
2681: The (*MARK) name is tagged with "MK:" in this output, and in this example it
2682: indicates which of the two alternatives matched. This is a more efficient way
2683: of obtaining this information than putting each alternative in its own
2684: capturing parentheses.
2685: .P
2686: If (*MARK) is encountered in a positive assertion, its name is recorded and
2687: passed back if it is the last-encountered. This does not happen for negative
2688: assertions.
2689: .P
2690: After a partial match or a failed match, the name of the last encountered
2691: (*MARK) in the entire match process is returned. For example:
2692: .sp
2693: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2694: data> XP
2695: No match, mark = B
2696: .sp
2697: Note that in this unanchored example the mark is retained from the match
2698: attempt that started at the letter "X". Subsequent match attempts starting at
2699: "P" and then with an empty string do not get as far as the (*MARK) item, but
2700: nevertheless do not reset it.
2701: .
2702: .
2703: .SS "Verbs that act after backtracking"
2704: .rs
2705: .sp
2706: The following verbs do nothing when they are encountered. Matching continues
2707: with what follows, but if there is no subsequent match, causing a backtrack to
2708: the verb, a failure is forced. That is, backtracking cannot pass to the left of
2709: the verb. However, when one of these verbs appears inside an atomic group, its
2710: effect is confined to that group, because once the group has been matched,
2711: there is never any backtracking into it. In this situation, backtracking can
2712: "jump back" to the left of the entire atomic group. (Remember also, as stated
2713: above, that this localization also applies in subroutine calls and assertions.)
2714: .P
2715: These verbs differ in exactly what kind of failure occurs when backtracking
2716: reaches them.
2717: .sp
2718: (*COMMIT)
2719: .sp
2720: This verb, which may not be followed by a name, causes the whole match to fail
2721: outright if the rest of the pattern does not match. Even if the pattern is
2722: unanchored, no further attempts to find a match by advancing the starting point
2723: take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2724: finding a match at the current starting point, or not at all. For example:
2725: .sp
2726: a+(*COMMIT)b
2727: .sp
2728: This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2729: dynamic anchor, or "I've started, so I must finish." The name of the most
2730: recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2731: match failure.
2732: .P
2733: Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2734: unless PCRE's start-of-match optimizations are turned off, as shown in this
2735: \fBpcretest\fP example:
2736: .sp
2737: re> /(*COMMIT)abc/
2738: data> xyzabc
2739: 0: abc
2740: xyzabc\eY
2741: No match
2742: .sp
2743: PCRE knows that any match must start with "a", so the optimization skips along
2744: the subject to "a" before running the first match attempt, which succeeds. When
2745: the optimization is disabled by the \eY escape in the second subject, the match
2746: starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2747: starting points.
2748: .sp
2749: (*PRUNE) or (*PRUNE:NAME)
2750: .sp
2751: This verb causes the match to fail at the current starting position in the
2752: subject if the rest of the pattern does not match. If the pattern is
2753: unanchored, the normal "bumpalong" advance to the next starting character then
2754: happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2755: reached, or when matching to the right of (*PRUNE), but if there is no match to
2756: the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2757: (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2758: but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2759: The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2760: anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2761: .sp
2762: (*SKIP)
2763: .sp
2764: This verb, when given without a name, is like (*PRUNE), except that if the
2765: pattern is unanchored, the "bumpalong" advance is not to the next character,
2766: but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2767: signifies that whatever text was matched leading up to it cannot be part of a
2768: successful match. Consider:
2769: .sp
2770: a+(*SKIP)b
2771: .sp
2772: If the subject is "aaaac...", after the first match attempt fails (starting at
2773: the first character in the string), the starting point skips on to start the
2774: next attempt at "c". Note that a possessive quantifer does not have the same
2775: effect as this example; although it would suppress backtracking during the
2776: first match attempt, the second attempt would start at the second character
2777: instead of skipping on to "c".
2778: .sp
2779: (*SKIP:NAME)
2780: .sp
2781: When (*SKIP) has an associated name, its behaviour is modified. If the
2782: following pattern fails to match, the previous path through the pattern is
2783: searched for the most recent (*MARK) that has the same name. If one is found,
2784: the "bumpalong" advance is to the subject position that corresponds to that
2785: (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2786: matching name is found, the (*SKIP) is ignored.
2787: .sp
2788: (*THEN) or (*THEN:NAME)
2789: .sp
2790: This verb causes a skip to the next innermost alternative if the rest of the
2791: pattern does not match. That is, it cancels pending backtracking, but only
2792: within the current alternative. Its name comes from the observation that it can
2793: be used for a pattern-based if-then-else block:
2794: .sp
2795: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2796: .sp
2797: If the COND1 pattern matches, FOO is tried (and possibly further items after
2798: the end of the group if FOO succeeds); on failure, the matcher skips to the
2799: second alternative and tries COND2, without backtracking into COND1. The
2800: behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2801: If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2802: .P
2803: Note that a subpattern that does not contain a | character is just a part of
2804: the enclosing alternative; it is not a nested alternation with only one
2805: alternative. The effect of (*THEN) extends beyond such a subpattern to the
2806: enclosing alternative. Consider this pattern, where A, B, etc. are complex
2807: pattern fragments that do not contain any | characters at this level:
2808: .sp
2809: A (B(*THEN)C) | D
2810: .sp
2811: If A and B are matched, but there is a failure in C, matching does not
2812: backtrack into A; instead it moves to the next alternative, that is, D.
2813: However, if the subpattern containing (*THEN) is given an alternative, it
2814: behaves differently:
2815: .sp
2816: A (B(*THEN)C | (*FAIL)) | D
2817: .sp
2818: The effect of (*THEN) is now confined to the inner subpattern. After a failure
2819: in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2820: because there are no more alternatives to try. In this case, matching does now
2821: backtrack into A.
2822: .P
2823: Note also that a conditional subpattern is not considered as having two
2824: alternatives, because only one is ever used. In other words, the | character in
2825: a conditional subpattern has a different meaning. Ignoring white space,
2826: consider:
2827: .sp
2828: ^.*? (?(?=a) a | b(*THEN)c )
2829: .sp
2830: If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2831: it initially matches zero characters. The condition (?=a) then fails, the
2832: character "b" is matched, but "c" is not. At this point, matching does not
2833: backtrack to .*? as might perhaps be expected from the presence of the |
2834: character. The conditional subpattern is part of the single alternative that
2835: comprises the whole pattern, and so the match fails. (If there was a backtrack
2836: into .*?, allowing it to match "b", the match would succeed.)
2837: .P
2838: The verbs just described provide four different "strengths" of control when
2839: subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2840: next alternative. (*PRUNE) comes next, failing the match at the current
2841: starting position, but allowing an advance to the next character (for an
2842: unanchored pattern). (*SKIP) is similar, except that the advance may be more
2843: than one character. (*COMMIT) is the strongest, causing the entire match to
2844: fail.
2845: .P
2846: If more than one such verb is present in a pattern, the "strongest" one wins.
2847: For example, consider this pattern, where A, B, etc. are complex pattern
2848: fragments:
2849: .sp
2850: (A(*COMMIT)B(*THEN)C|D)
2851: .sp
2852: Once A has matched, PCRE is committed to this match, at the current starting
2853: position. If subsequently B matches, but C does not, the normal (*THEN) action
2854: of trying the next alternative (that is, D) does not happen because (*COMMIT)
2855: overrides.
2856: .
2857: .
2858: .SH "SEE ALSO"
2859: .rs
2860: .sp
2861: \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
1.1.1.2 ! misho 2862: \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
1.1 misho 2863: .
2864: .
2865: .SH AUTHOR
2866: .rs
2867: .sp
2868: .nf
2869: Philip Hazel
2870: University Computing Service
2871: Cambridge CB2 3QH, England.
2872: .fi
2873: .
2874: .
2875: .SH REVISION
2876: .rs
2877: .sp
2878: .nf
1.1.1.2 ! misho 2879: Last updated: 09 January 2012
! 2880: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 2881: .fi
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