Annotation of embedaddon/pcre/doc/pcrepattern.3, revision 1.1.1.3
1.1.1.3 ! misho 1: .TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31"
1.1 misho 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
1.1.1.3 ! misho 201: If a pattern is compiled with the PCRE_EXTENDED option, white space in the
1.1 misho 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
1.1.1.3 ! misho 204: be used to include a white space or # character as part of the pattern.
1.1 misho 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)
1.1.1.3 ! misho 240: \ef form feed (hex 0C)
1.1 misho 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.
1.1.1.3 ! misho 280: Character codes specified by \eu in JavaScript mode are constrained in the same
! 281: was as those specified by \ex in non-JavaScript mode.
1.1 misho 282: .P
283: Characters whose value is less than 256 can be defined by either of the two
284: syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
285: way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
286: \eu00dc in JavaScript mode).
287: .P
288: After \e0 up to two further octal digits are read. If there are fewer than two
289: digits, just those that are present are used. Thus the sequence \e0\ex\e07
290: specifies two binary zeros followed by a BEL character (code value 7). Make
291: sure you supply two digits after the initial zero if the pattern character that
292: follows is itself an octal digit.
293: .P
294: The handling of a backslash followed by a digit other than 0 is complicated.
295: Outside a character class, PCRE reads it and any following digits as a decimal
296: number. If the number is less than 10, or if there have been at least that many
297: previous capturing left parentheses in the expression, the entire sequence is
298: taken as a \fIback reference\fP. A description of how this works is given
299: .\" HTML <a href="#backreferences">
300: .\" </a>
301: later,
302: .\"
303: following the discussion of
304: .\" HTML <a href="#subpattern">
305: .\" </a>
306: parenthesized subpatterns.
307: .\"
308: .P
309: Inside a character class, or if the decimal number is greater than 9 and there
310: have not been that many capturing subpatterns, PCRE re-reads up to three octal
311: digits following the backslash, and uses them to generate a data character. Any
1.1.1.2 misho 312: subsequent digits stand for themselves. The value of the character is
313: constrained in the same way as characters specified in hexadecimal.
314: For example:
1.1 misho 315: .sp
316: \e040 is another way of writing a space
317: .\" JOIN
318: \e40 is the same, provided there are fewer than 40
319: previous capturing subpatterns
320: \e7 is always a back reference
321: .\" JOIN
322: \e11 might be a back reference, or another way of
323: writing a tab
324: \e011 is always a tab
325: \e0113 is a tab followed by the character "3"
326: .\" JOIN
327: \e113 might be a back reference, otherwise the
328: character with octal code 113
329: .\" JOIN
330: \e377 might be a back reference, otherwise
1.1.1.2 misho 331: the value 255 (decimal)
1.1 misho 332: .\" JOIN
333: \e81 is either a back reference, or a binary zero
334: followed by the two characters "8" and "1"
335: .sp
336: Note that octal values of 100 or greater must not be introduced by a leading
337: zero, because no more than three octal digits are ever read.
338: .P
339: All the sequences that define a single character value can be used both inside
340: and outside character classes. In addition, inside a character class, \eb is
341: interpreted as the backspace character (hex 08).
342: .P
343: \eN is not allowed in a character class. \eB, \eR, and \eX are not special
344: inside a character class. Like other unrecognized escape sequences, they are
345: treated as the literal characters "B", "R", and "X" by default, but cause an
346: error if the PCRE_EXTRA option is set. Outside a character class, these
347: sequences have different meanings.
348: .
349: .
350: .SS "Unsupported escape sequences"
351: .rs
352: .sp
353: In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
354: handler and used to modify the case of following characters. By default, PCRE
355: does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
356: option is set, \eU matches a "U" character, and \eu can be used to define a
357: character by code point, as described in the previous section.
358: .
359: .
360: .SS "Absolute and relative back references"
361: .rs
362: .sp
363: The sequence \eg followed by an unsigned or a negative number, optionally
364: enclosed in braces, is an absolute or relative back reference. A named back
365: reference can be coded as \eg{name}. Back references are discussed
366: .\" HTML <a href="#backreferences">
367: .\" </a>
368: later,
369: .\"
370: following the discussion of
371: .\" HTML <a href="#subpattern">
372: .\" </a>
373: parenthesized subpatterns.
374: .\"
375: .
376: .
377: .SS "Absolute and relative subroutine calls"
378: .rs
379: .sp
380: For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
381: a number enclosed either in angle brackets or single quotes, is an alternative
382: syntax for referencing a subpattern as a "subroutine". Details are discussed
383: .\" HTML <a href="#onigurumasubroutines">
384: .\" </a>
385: later.
386: .\"
387: Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
388: synonymous. The former is a back reference; the latter is a
389: .\" HTML <a href="#subpatternsassubroutines">
390: .\" </a>
391: subroutine
392: .\"
393: call.
394: .
395: .
396: .\" HTML <a name="genericchartypes"></a>
397: .SS "Generic character types"
398: .rs
399: .sp
400: Another use of backslash is for specifying generic character types:
401: .sp
402: \ed any decimal digit
403: \eD any character that is not a decimal digit
1.1.1.3 ! misho 404: \eh any horizontal white space character
! 405: \eH any character that is not a horizontal white space character
! 406: \es any white space character
! 407: \eS any character that is not a white space character
! 408: \ev any vertical white space character
! 409: \eV any character that is not a vertical white space character
1.1 misho 410: \ew any "word" character
411: \eW any "non-word" character
412: .sp
413: There is also the single sequence \eN, which matches a non-newline character.
414: This is the same as
415: .\" HTML <a href="#fullstopdot">
416: .\" </a>
417: the "." metacharacter
418: .\"
419: when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
420: PCRE does not support this.
421: .P
422: Each pair of lower and upper case escape sequences partitions the complete set
423: of characters into two disjoint sets. Any given character matches one, and only
424: one, of each pair. The sequences can appear both inside and outside character
425: classes. They each match one character of the appropriate type. If the current
426: matching point is at the end of the subject string, all of them fail, because
427: there is no character to match.
428: .P
429: For compatibility with Perl, \es does not match the VT character (code 11).
430: This makes it different from the the POSIX "space" class. The \es characters
431: are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
432: included in a Perl script, \es may match the VT character. In PCRE, it never
433: does.
434: .P
435: A "word" character is an underscore or any character that is a letter or digit.
436: By default, the definition of letters and digits is controlled by PCRE's
437: low-valued character tables, and may vary if locale-specific matching is taking
438: place (see
439: .\" HTML <a href="pcreapi.html#localesupport">
440: .\" </a>
441: "Locale support"
442: .\"
443: in the
444: .\" HREF
445: \fBpcreapi\fP
446: .\"
447: page). For example, in a French locale such as "fr_FR" in Unix-like systems,
448: or "french" in Windows, some character codes greater than 128 are used for
449: accented letters, and these are then matched by \ew. The use of locales with
450: Unicode is discouraged.
451: .P
1.1.1.2 misho 452: By default, in a UTF mode, characters with values greater than 128 never match
1.1 misho 453: \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
1.1.1.2 misho 454: their original meanings from before UTF support was available, mainly for
1.1 misho 455: efficiency reasons. However, if PCRE is compiled with Unicode property support,
456: and the PCRE_UCP option is set, the behaviour is changed so that Unicode
457: properties are used to determine character types, as follows:
458: .sp
459: \ed any character that \ep{Nd} matches (decimal digit)
460: \es any character that \ep{Z} matches, plus HT, LF, FF, CR
461: \ew any character that \ep{L} or \ep{N} matches, plus underscore
462: .sp
463: The upper case escapes match the inverse sets of characters. Note that \ed
464: matches only decimal digits, whereas \ew matches any Unicode digit, as well as
465: any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
466: \eB because they are defined in terms of \ew and \eW. Matching these sequences
467: is noticeably slower when PCRE_UCP is set.
468: .P
469: The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
470: release 5.10. In contrast to the other sequences, which match only ASCII
1.1.1.2 misho 471: characters by default, these always match certain high-valued codepoints,
472: whether or not PCRE_UCP is set. The horizontal space characters are:
1.1 misho 473: .sp
474: U+0009 Horizontal tab
475: U+0020 Space
476: U+00A0 Non-break space
477: U+1680 Ogham space mark
478: U+180E Mongolian vowel separator
479: U+2000 En quad
480: U+2001 Em quad
481: U+2002 En space
482: U+2003 Em space
483: U+2004 Three-per-em space
484: U+2005 Four-per-em space
485: U+2006 Six-per-em space
486: U+2007 Figure space
487: U+2008 Punctuation space
488: U+2009 Thin space
489: U+200A Hair space
490: U+202F Narrow no-break space
491: U+205F Medium mathematical space
492: U+3000 Ideographic space
493: .sp
494: The vertical space characters are:
495: .sp
496: U+000A Linefeed
497: U+000B Vertical tab
1.1.1.3 ! misho 498: U+000C Form feed
1.1 misho 499: U+000D Carriage return
500: U+0085 Next line
501: U+2028 Line separator
502: U+2029 Paragraph separator
1.1.1.2 misho 503: .sp
504: In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
505: relevant.
1.1 misho 506: .
507: .
508: .\" HTML <a name="newlineseq"></a>
509: .SS "Newline sequences"
510: .rs
511: .sp
512: Outside a character class, by default, the escape sequence \eR matches any
1.1.1.2 misho 513: Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
514: following:
1.1 misho 515: .sp
516: (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
517: .sp
518: This is an example of an "atomic group", details of which are given
519: .\" HTML <a href="#atomicgroup">
520: .\" </a>
521: below.
522: .\"
523: This particular group matches either the two-character sequence CR followed by
524: LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
1.1.1.3 ! misho 525: U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
1.1 misho 526: line, U+0085). The two-character sequence is treated as a single unit that
527: cannot be split.
528: .P
1.1.1.2 misho 529: In other modes, two additional characters whose codepoints are greater than 255
1.1 misho 530: are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
531: Unicode character property support is not needed for these characters to be
532: recognized.
533: .P
534: It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
535: complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
536: either at compile time or when the pattern is matched. (BSR is an abbrevation
537: for "backslash R".) This can be made the default when PCRE is built; if this is
538: the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
539: It is also possible to specify these settings by starting a pattern string with
540: one of the following sequences:
541: .sp
542: (*BSR_ANYCRLF) CR, LF, or CRLF only
543: (*BSR_UNICODE) any Unicode newline sequence
544: .sp
1.1.1.2 misho 545: These override the default and the options given to the compiling function, but
546: they can themselves be overridden by options given to a matching function. Note
547: that these special settings, which are not Perl-compatible, are recognized only
548: at the very start of a pattern, and that they must be in upper case. If more
549: than one of them is present, the last one is used. They can be combined with a
550: change of newline convention; for example, a pattern can start with:
1.1 misho 551: .sp
552: (*ANY)(*BSR_ANYCRLF)
553: .sp
1.1.1.2 misho 554: They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
555: sequences. Inside a character class, \eR is treated as an unrecognized escape
556: sequence, and so matches the letter "R" by default, but causes an error if
557: PCRE_EXTRA is set.
1.1 misho 558: .
559: .
560: .\" HTML <a name="uniextseq"></a>
561: .SS Unicode character properties
562: .rs
563: .sp
564: When PCRE is built with Unicode character property support, three additional
565: escape sequences that match characters with specific properties are available.
1.1.1.2 misho 566: When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
1.1 misho 567: characters whose codepoints are less than 256, but they do work in this mode.
568: The extra escape sequences are:
569: .sp
570: \ep{\fIxx\fP} a character with the \fIxx\fP property
571: \eP{\fIxx\fP} a character without the \fIxx\fP property
572: \eX an extended Unicode sequence
573: .sp
574: The property names represented by \fIxx\fP above are limited to the Unicode
575: script names, the general category properties, "Any", which matches any
576: character (including newline), and some special PCRE properties (described
577: in the
578: .\" HTML <a href="#extraprops">
579: .\" </a>
580: next section).
581: .\"
582: Other Perl properties such as "InMusicalSymbols" are not currently supported by
583: PCRE. Note that \eP{Any} does not match any characters, so always causes a
584: match failure.
585: .P
586: Sets of Unicode characters are defined as belonging to certain scripts. A
587: character from one of these sets can be matched using a script name. For
588: example:
589: .sp
590: \ep{Greek}
591: \eP{Han}
592: .sp
593: Those that are not part of an identified script are lumped together as
594: "Common". The current list of scripts is:
595: .P
596: Arabic,
597: Armenian,
598: Avestan,
599: Balinese,
600: Bamum,
1.1.1.3 ! misho 601: Batak,
1.1 misho 602: Bengali,
603: Bopomofo,
1.1.1.3 ! misho 604: Brahmi,
1.1 misho 605: Braille,
606: Buginese,
607: Buhid,
608: Canadian_Aboriginal,
609: Carian,
1.1.1.3 ! misho 610: Chakma,
1.1 misho 611: Cham,
612: Cherokee,
613: Common,
614: Coptic,
615: Cuneiform,
616: Cypriot,
617: Cyrillic,
618: Deseret,
619: Devanagari,
620: Egyptian_Hieroglyphs,
621: Ethiopic,
622: Georgian,
623: Glagolitic,
624: Gothic,
625: Greek,
626: Gujarati,
627: Gurmukhi,
628: Han,
629: Hangul,
630: Hanunoo,
631: Hebrew,
632: Hiragana,
633: Imperial_Aramaic,
634: Inherited,
635: Inscriptional_Pahlavi,
636: Inscriptional_Parthian,
637: Javanese,
638: Kaithi,
639: Kannada,
640: Katakana,
641: Kayah_Li,
642: Kharoshthi,
643: Khmer,
644: Lao,
645: Latin,
646: Lepcha,
647: Limbu,
648: Linear_B,
649: Lisu,
650: Lycian,
651: Lydian,
652: Malayalam,
1.1.1.3 ! misho 653: Mandaic,
1.1 misho 654: Meetei_Mayek,
1.1.1.3 ! misho 655: Meroitic_Cursive,
! 656: Meroitic_Hieroglyphs,
! 657: Miao,
1.1 misho 658: Mongolian,
659: Myanmar,
660: New_Tai_Lue,
661: Nko,
662: Ogham,
663: Old_Italic,
664: Old_Persian,
665: Old_South_Arabian,
666: Old_Turkic,
667: Ol_Chiki,
668: Oriya,
669: Osmanya,
670: Phags_Pa,
671: Phoenician,
672: Rejang,
673: Runic,
674: Samaritan,
675: Saurashtra,
1.1.1.3 ! misho 676: Sharada,
1.1 misho 677: Shavian,
678: Sinhala,
1.1.1.3 ! misho 679: Sora_Sompeng,
1.1 misho 680: Sundanese,
681: Syloti_Nagri,
682: Syriac,
683: Tagalog,
684: Tagbanwa,
685: Tai_Le,
686: Tai_Tham,
687: Tai_Viet,
1.1.1.3 ! misho 688: Takri,
1.1 misho 689: Tamil,
690: Telugu,
691: Thaana,
692: Thai,
693: Tibetan,
694: Tifinagh,
695: Ugaritic,
696: Vai,
697: Yi.
698: .P
699: Each character has exactly one Unicode general category property, specified by
700: a two-letter abbreviation. For compatibility with Perl, negation can be
701: specified by including a circumflex between the opening brace and the property
702: name. For example, \ep{^Lu} is the same as \eP{Lu}.
703: .P
704: If only one letter is specified with \ep or \eP, it includes all the general
705: category properties that start with that letter. In this case, in the absence
706: of negation, the curly brackets in the escape sequence are optional; these two
707: examples have the same effect:
708: .sp
709: \ep{L}
710: \epL
711: .sp
712: The following general category property codes are supported:
713: .sp
714: C Other
715: Cc Control
716: Cf Format
717: Cn Unassigned
718: Co Private use
719: Cs Surrogate
720: .sp
721: L Letter
722: Ll Lower case letter
723: Lm Modifier letter
724: Lo Other letter
725: Lt Title case letter
726: Lu Upper case letter
727: .sp
728: M Mark
729: Mc Spacing mark
730: Me Enclosing mark
731: Mn Non-spacing mark
732: .sp
733: N Number
734: Nd Decimal number
735: Nl Letter number
736: No Other number
737: .sp
738: P Punctuation
739: Pc Connector punctuation
740: Pd Dash punctuation
741: Pe Close punctuation
742: Pf Final punctuation
743: Pi Initial punctuation
744: Po Other punctuation
745: Ps Open punctuation
746: .sp
747: S Symbol
748: Sc Currency symbol
749: Sk Modifier symbol
750: Sm Mathematical symbol
751: So Other symbol
752: .sp
753: Z Separator
754: Zl Line separator
755: Zp Paragraph separator
756: Zs Space separator
757: .sp
758: The special property L& is also supported: it matches a character that has
759: the Lu, Ll, or Lt property, in other words, a letter that is not classified as
760: a modifier or "other".
761: .P
762: The Cs (Surrogate) property applies only to characters in the range U+D800 to
1.1.1.2 misho 763: U+DFFF. Such characters are not valid in Unicode strings and so
764: cannot be tested by PCRE, unless UTF validity checking has been turned off
765: (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
1.1 misho 766: .\" HREF
767: \fBpcreapi\fP
768: .\"
769: page). Perl does not support the Cs property.
770: .P
771: The long synonyms for property names that Perl supports (such as \ep{Letter})
772: are not supported by PCRE, nor is it permitted to prefix any of these
773: properties with "Is".
774: .P
775: No character that is in the Unicode table has the Cn (unassigned) property.
776: Instead, this property is assumed for any code point that is not in the
777: Unicode table.
778: .P
779: Specifying caseless matching does not affect these escape sequences. For
780: example, \ep{Lu} always matches only upper case letters.
781: .P
782: The \eX escape matches any number of Unicode characters that form an extended
783: Unicode sequence. \eX is equivalent to
784: .sp
785: (?>\ePM\epM*)
786: .sp
787: That is, it matches a character without the "mark" property, followed by zero
788: or more characters with the "mark" property, and treats the sequence as an
789: atomic group
790: .\" HTML <a href="#atomicgroup">
791: .\" </a>
792: (see below).
793: .\"
794: Characters with the "mark" property are typically accents that affect the
795: preceding character. None of them have codepoints less than 256, so in
1.1.1.2 misho 796: 8-bit non-UTF-8 mode \eX matches any one character.
1.1 misho 797: .P
798: Note that recent versions of Perl have changed \eX to match what Unicode calls
799: an "extended grapheme cluster", which has a more complicated definition.
800: .P
801: Matching characters by Unicode property is not fast, because PCRE has to search
802: a structure that contains data for over fifteen thousand characters. That is
803: why the traditional escape sequences such as \ed and \ew do not use Unicode
804: properties in PCRE by default, though you can make them do so by setting the
1.1.1.2 misho 805: PCRE_UCP option or by starting the pattern with (*UCP).
1.1 misho 806: .
807: .
808: .\" HTML <a name="extraprops"></a>
809: .SS PCRE's additional properties
810: .rs
811: .sp
812: As well as the standard Unicode properties described in the previous
813: section, PCRE supports four more that make it possible to convert traditional
814: escape sequences such as \ew and \es and POSIX character classes to use Unicode
815: properties. PCRE uses these non-standard, non-Perl properties internally when
816: PCRE_UCP is set. They are:
817: .sp
818: Xan Any alphanumeric character
819: Xps Any POSIX space character
820: Xsp Any Perl space character
821: Xwd Any Perl "word" character
822: .sp
823: Xan matches characters that have either the L (letter) or the N (number)
1.1.1.3 ! misho 824: property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
1.1 misho 825: carriage return, and any other character that has the Z (separator) property.
826: Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
827: same characters as Xan, plus underscore.
828: .
829: .
830: .\" HTML <a name="resetmatchstart"></a>
831: .SS "Resetting the match start"
832: .rs
833: .sp
834: The escape sequence \eK causes any previously matched characters not to be
835: included in the final matched sequence. For example, the pattern:
836: .sp
837: foo\eKbar
838: .sp
839: matches "foobar", but reports that it has matched "bar". This feature is
840: similar to a lookbehind assertion
841: .\" HTML <a href="#lookbehind">
842: .\" </a>
843: (described below).
844: .\"
845: However, in this case, the part of the subject before the real match does not
846: have to be of fixed length, as lookbehind assertions do. The use of \eK does
847: not interfere with the setting of
848: .\" HTML <a href="#subpattern">
849: .\" </a>
850: captured substrings.
851: .\"
852: For example, when the pattern
853: .sp
854: (foo)\eKbar
855: .sp
856: matches "foobar", the first substring is still set to "foo".
857: .P
858: Perl documents that the use of \eK within assertions is "not well defined". In
859: PCRE, \eK is acted upon when it occurs inside positive assertions, but is
860: ignored in negative assertions.
861: .
862: .
863: .\" HTML <a name="smallassertions"></a>
864: .SS "Simple assertions"
865: .rs
866: .sp
867: The final use of backslash is for certain simple assertions. An assertion
868: specifies a condition that has to be met at a particular point in a match,
869: without consuming any characters from the subject string. The use of
870: subpatterns for more complicated assertions is described
871: .\" HTML <a href="#bigassertions">
872: .\" </a>
873: below.
874: .\"
875: The backslashed assertions are:
876: .sp
877: \eb matches at a word boundary
878: \eB matches when not at a word boundary
879: \eA matches at the start of the subject
880: \eZ matches at the end of the subject
881: also matches before a newline at the end of the subject
882: \ez matches only at the end of the subject
883: \eG matches at the first matching position in the subject
884: .sp
885: Inside a character class, \eb has a different meaning; it matches the backspace
886: character. If any other of these assertions appears in a character class, by
887: default it matches the corresponding literal character (for example, \eB
888: matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
889: escape sequence" error is generated instead.
890: .P
891: A word boundary is a position in the subject string where the current character
892: and the previous character do not both match \ew or \eW (i.e. one matches
893: \ew and the other matches \eW), or the start or end of the string if the
1.1.1.2 misho 894: first or last character matches \ew, respectively. In a UTF mode, the meanings
1.1 misho 895: of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
896: done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
897: of word" or "end of word" metasequence. However, whatever follows \eb normally
898: determines which it is. For example, the fragment \eba matches "a" at the start
899: of a word.
900: .P
901: The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
902: dollar (described in the next section) in that they only ever match at the very
903: start and end of the subject string, whatever options are set. Thus, they are
904: independent of multiline mode. These three assertions are not affected by the
905: PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
906: circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
907: argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
908: at a point other than the beginning of the subject, \eA can never match. The
909: difference between \eZ and \ez is that \eZ matches before a newline at the end
910: of the string as well as at the very end, whereas \ez matches only at the end.
911: .P
912: The \eG assertion is true only when the current matching position is at the
913: start point of the match, as specified by the \fIstartoffset\fP argument of
914: \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
915: non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
916: arguments, you can mimic Perl's /g option, and it is in this kind of
917: implementation where \eG can be useful.
918: .P
919: Note, however, that PCRE's interpretation of \eG, as the start of the current
920: match, is subtly different from Perl's, which defines it as the end of the
921: previous match. In Perl, these can be different when the previously matched
922: string was empty. Because PCRE does just one match at a time, it cannot
923: reproduce this behaviour.
924: .P
925: If all the alternatives of a pattern begin with \eG, the expression is anchored
926: to the starting match position, and the "anchored" flag is set in the compiled
927: regular expression.
928: .
929: .
930: .SH "CIRCUMFLEX AND DOLLAR"
931: .rs
932: .sp
933: Outside a character class, in the default matching mode, the circumflex
934: character is an assertion that is true only if the current matching point is
935: at the start of the subject string. If the \fIstartoffset\fP argument of
936: \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
937: option is unset. Inside a character class, circumflex has an entirely different
938: meaning
939: .\" HTML <a href="#characterclass">
940: .\" </a>
941: (see below).
942: .\"
943: .P
944: Circumflex need not be the first character of the pattern if a number of
945: alternatives are involved, but it should be the first thing in each alternative
946: in which it appears if the pattern is ever to match that branch. If all
947: possible alternatives start with a circumflex, that is, if the pattern is
948: constrained to match only at the start of the subject, it is said to be an
949: "anchored" pattern. (There are also other constructs that can cause a pattern
950: to be anchored.)
951: .P
952: A dollar character is an assertion that is true only if the current matching
953: point is at the end of the subject string, or immediately before a newline
954: at the end of the string (by default). Dollar need not be the last character of
955: the pattern if a number of alternatives are involved, but it should be the last
956: item in any branch in which it appears. Dollar has no special meaning in a
957: character class.
958: .P
959: The meaning of dollar can be changed so that it matches only at the very end of
960: the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
961: does not affect the \eZ assertion.
962: .P
963: The meanings of the circumflex and dollar characters are changed if the
964: PCRE_MULTILINE option is set. When this is the case, a circumflex matches
965: immediately after internal newlines as well as at the start of the subject
966: string. It does not match after a newline that ends the string. A dollar
967: matches before any newlines in the string, as well as at the very end, when
968: PCRE_MULTILINE is set. When newline is specified as the two-character
969: sequence CRLF, isolated CR and LF characters do not indicate newlines.
970: .P
971: For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
972: \en represents a newline) in multiline mode, but not otherwise. Consequently,
973: patterns that are anchored in single line mode because all branches start with
974: ^ are not anchored in multiline mode, and a match for circumflex is possible
975: when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
976: PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
977: .P
978: Note that the sequences \eA, \eZ, and \ez can be used to match the start and
979: end of the subject in both modes, and if all branches of a pattern start with
980: \eA it is always anchored, whether or not PCRE_MULTILINE is set.
981: .
982: .
983: .\" HTML <a name="fullstopdot"></a>
984: .SH "FULL STOP (PERIOD, DOT) AND \eN"
985: .rs
986: .sp
987: Outside a character class, a dot in the pattern matches any one character in
988: the subject string except (by default) a character that signifies the end of a
1.1.1.2 misho 989: line.
1.1 misho 990: .P
991: When a line ending is defined as a single character, dot never matches that
992: character; when the two-character sequence CRLF is used, dot does not match CR
993: if it is immediately followed by LF, but otherwise it matches all characters
994: (including isolated CRs and LFs). When any Unicode line endings are being
995: recognized, dot does not match CR or LF or any of the other line ending
996: characters.
997: .P
998: The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
999: option is set, a dot matches any one character, without exception. If the
1000: two-character sequence CRLF is present in the subject string, it takes two dots
1001: to match it.
1002: .P
1003: The handling of dot is entirely independent of the handling of circumflex and
1004: dollar, the only relationship being that they both involve newlines. Dot has no
1005: special meaning in a character class.
1006: .P
1007: The escape sequence \eN behaves like a dot, except that it is not affected by
1008: the PCRE_DOTALL option. In other words, it matches any character except one
1009: that signifies the end of a line. Perl also uses \eN to match characters by
1010: name; PCRE does not support this.
1011: .
1012: .
1.1.1.2 misho 1013: .SH "MATCHING A SINGLE DATA UNIT"
1.1 misho 1014: .rs
1015: .sp
1.1.1.2 misho 1016: Outside a character class, the escape sequence \eC matches any one data unit,
1017: whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1018: byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
1019: matches line-ending characters. The feature is provided in Perl in order to
1020: match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1021: used. Because \eC breaks up characters into individual data units, matching one
1022: unit with \eC in a UTF mode means that the rest of the string may start with a
1023: malformed UTF character. This has undefined results, because PCRE assumes that
1024: it is dealing with valid UTF strings (and by default it checks this at the
1.1.1.3 ! misho 1025: start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option
! 1026: is used).
1.1 misho 1027: .P
1028: PCRE does not allow \eC to appear in lookbehind assertions
1029: .\" HTML <a href="#lookbehind">
1030: .\" </a>
1031: (described below)
1032: .\"
1.1.1.2 misho 1033: in a UTF mode, because this would make it impossible to calculate the length of
1.1 misho 1034: the lookbehind.
1035: .P
1.1.1.2 misho 1036: In general, the \eC escape sequence is best avoided. However, one
1037: way of using it that avoids the problem of malformed UTF characters is to use a
1038: lookahead to check the length of the next character, as in this pattern, which
1039: could be used with a UTF-8 string (ignore white space and line breaks):
1.1 misho 1040: .sp
1041: (?| (?=[\ex00-\ex7f])(\eC) |
1042: (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1043: (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1044: (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1045: .sp
1046: A group that starts with (?| resets the capturing parentheses numbers in each
1047: alternative (see
1048: .\" HTML <a href="#dupsubpatternnumber">
1049: .\" </a>
1050: "Duplicate Subpattern Numbers"
1051: .\"
1052: below). The assertions at the start of each branch check the next UTF-8
1053: character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1054: character's individual bytes are then captured by the appropriate number of
1055: groups.
1056: .
1057: .
1058: .\" HTML <a name="characterclass"></a>
1059: .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
1060: .rs
1061: .sp
1062: An opening square bracket introduces a character class, terminated by a closing
1063: square bracket. A closing square bracket on its own is not special by default.
1064: However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1065: bracket causes a compile-time error. If a closing square bracket is required as
1066: a member of the class, it should be the first data character in the class
1067: (after an initial circumflex, if present) or escaped with a backslash.
1068: .P
1.1.1.2 misho 1069: A character class matches a single character in the subject. In a UTF mode, the
1070: character may be more than one data unit long. A matched character must be in
1071: the set of characters defined by the class, unless the first character in the
1072: class definition is a circumflex, in which case the subject character must not
1073: be in the set defined by the class. If a circumflex is actually required as a
1074: member of the class, ensure it is not the first character, or escape it with a
1.1 misho 1075: backslash.
1076: .P
1077: For example, the character class [aeiou] matches any lower case vowel, while
1078: [^aeiou] matches any character that is not a lower case vowel. Note that a
1079: circumflex is just a convenient notation for specifying the characters that
1080: are in the class by enumerating those that are not. A class that starts with a
1081: circumflex is not an assertion; it still consumes a character from the subject
1082: string, and therefore it fails if the current pointer is at the end of the
1083: string.
1084: .P
1.1.1.2 misho 1085: In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
1086: included in a class as a literal string of data units, or by using the \ex{
1087: escaping mechanism.
1.1 misho 1088: .P
1089: When caseless matching is set, any letters in a class represent both their
1090: upper case and lower case versions, so for example, a caseless [aeiou] matches
1091: "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1.1.1.2 misho 1092: caseful version would. In a UTF mode, PCRE always understands the concept of
1.1 misho 1093: case for characters whose values are less than 128, so caseless matching is
1094: always possible. For characters with higher values, the concept of case is
1095: supported if PCRE is compiled with Unicode property support, but not otherwise.
1.1.1.2 misho 1096: If you want to use caseless matching in a UTF mode for characters 128 and
1097: above, you must ensure that PCRE is compiled with Unicode property support as
1098: well as with UTF support.
1.1 misho 1099: .P
1100: Characters that might indicate line breaks are never treated in any special way
1101: when matching character classes, whatever line-ending sequence is in use, and
1102: whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1103: such as [^a] always matches one of these characters.
1104: .P
1105: The minus (hyphen) character can be used to specify a range of characters in a
1106: character class. For example, [d-m] matches any letter between d and m,
1107: inclusive. If a minus character is required in a class, it must be escaped with
1108: a backslash or appear in a position where it cannot be interpreted as
1109: indicating a range, typically as the first or last character in the class.
1110: .P
1111: It is not possible to have the literal character "]" as the end character of a
1112: range. A pattern such as [W-]46] is interpreted as a class of two characters
1113: ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1114: "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1115: the end of range, so [W-\e]46] is interpreted as a class containing a range
1116: followed by two other characters. The octal or hexadecimal representation of
1117: "]" can also be used to end a range.
1118: .P
1119: Ranges operate in the collating sequence of character values. They can also be
1.1.1.2 misho 1120: used for characters specified numerically, for example [\e000-\e037]. Ranges
1121: can include any characters that are valid for the current mode.
1.1 misho 1122: .P
1123: If a range that includes letters is used when caseless matching is set, it
1124: matches the letters in either case. For example, [W-c] is equivalent to
1.1.1.2 misho 1125: [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1.1 misho 1126: tables for a French locale are in use, [\exc8-\excb] matches accented E
1.1.1.2 misho 1127: characters in both cases. In UTF modes, PCRE supports the concept of case for
1.1 misho 1128: characters with values greater than 128 only when it is compiled with Unicode
1129: property support.
1130: .P
1131: The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1132: \eV, \ew, and \eW may appear in a character class, and add the characters that
1133: they match to the class. For example, [\edABCDEF] matches any hexadecimal
1.1.1.2 misho 1134: digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1.1 misho 1135: and their upper case partners, just as it does when they appear outside a
1136: character class, as described in the section entitled
1137: .\" HTML <a href="#genericchartypes">
1138: .\" </a>
1139: "Generic character types"
1140: .\"
1141: above. The escape sequence \eb has a different meaning inside a character
1142: class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1143: are not special inside a character class. Like any other unrecognized escape
1144: sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1145: default, but cause an error if the PCRE_EXTRA option is set.
1146: .P
1147: A circumflex can conveniently be used with the upper case character types to
1148: specify a more restricted set of characters than the matching lower case type.
1149: For example, the class [^\eW_] matches any letter or digit, but not underscore,
1150: whereas [\ew] includes underscore. A positive character class should be read as
1151: "something OR something OR ..." and a negative class as "NOT something AND NOT
1152: something AND NOT ...".
1153: .P
1154: The only metacharacters that are recognized in character classes are backslash,
1155: hyphen (only where it can be interpreted as specifying a range), circumflex
1156: (only at the start), opening square bracket (only when it can be interpreted as
1157: introducing a POSIX class name - see the next section), and the terminating
1158: closing square bracket. However, escaping other non-alphanumeric characters
1159: does no harm.
1160: .
1161: .
1162: .SH "POSIX CHARACTER CLASSES"
1163: .rs
1164: .sp
1165: Perl supports the POSIX notation for character classes. This uses names
1166: enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1167: this notation. For example,
1168: .sp
1169: [01[:alpha:]%]
1170: .sp
1171: matches "0", "1", any alphabetic character, or "%". The supported class names
1172: are:
1173: .sp
1174: alnum letters and digits
1175: alpha letters
1176: ascii character codes 0 - 127
1177: blank space or tab only
1178: cntrl control characters
1179: digit decimal digits (same as \ed)
1180: graph printing characters, excluding space
1181: lower lower case letters
1182: print printing characters, including space
1183: punct printing characters, excluding letters and digits and space
1184: space white space (not quite the same as \es)
1185: upper upper case letters
1186: word "word" characters (same as \ew)
1187: xdigit hexadecimal digits
1188: .sp
1189: The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1190: space (32). Notice that this list includes the VT character (code 11). This
1191: makes "space" different to \es, which does not include VT (for Perl
1192: compatibility).
1193: .P
1194: The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1195: 5.8. Another Perl extension is negation, which is indicated by a ^ character
1196: after the colon. For example,
1197: .sp
1198: [12[:^digit:]]
1199: .sp
1200: matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1201: syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1202: supported, and an error is given if they are encountered.
1203: .P
1.1.1.2 misho 1204: By default, in UTF modes, characters with values greater than 128 do not match
1.1 misho 1205: any of the POSIX character classes. However, if the PCRE_UCP option is passed
1206: to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1207: character properties are used. This is achieved by replacing the POSIX classes
1208: by other sequences, as follows:
1209: .sp
1210: [:alnum:] becomes \ep{Xan}
1211: [:alpha:] becomes \ep{L}
1212: [:blank:] becomes \eh
1213: [:digit:] becomes \ep{Nd}
1214: [:lower:] becomes \ep{Ll}
1215: [:space:] becomes \ep{Xps}
1216: [:upper:] becomes \ep{Lu}
1217: [:word:] becomes \ep{Xwd}
1218: .sp
1219: Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1220: classes are unchanged, and match only characters with code points less than
1221: 128.
1222: .
1223: .
1224: .SH "VERTICAL BAR"
1225: .rs
1226: .sp
1227: Vertical bar characters are used to separate alternative patterns. For example,
1228: the pattern
1229: .sp
1230: gilbert|sullivan
1231: .sp
1232: matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1233: and an empty alternative is permitted (matching the empty string). The matching
1234: process tries each alternative in turn, from left to right, and the first one
1235: that succeeds is used. If the alternatives are within a subpattern
1236: .\" HTML <a href="#subpattern">
1237: .\" </a>
1238: (defined below),
1239: .\"
1240: "succeeds" means matching the rest of the main pattern as well as the
1241: alternative in the subpattern.
1242: .
1243: .
1244: .SH "INTERNAL OPTION SETTING"
1245: .rs
1246: .sp
1247: The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1248: PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1249: the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1250: The option letters are
1251: .sp
1252: i for PCRE_CASELESS
1253: m for PCRE_MULTILINE
1254: s for PCRE_DOTALL
1255: x for PCRE_EXTENDED
1256: .sp
1257: For example, (?im) sets caseless, multiline matching. It is also possible to
1258: unset these options by preceding the letter with a hyphen, and a combined
1259: setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1260: PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1261: permitted. If a letter appears both before and after the hyphen, the option is
1262: unset.
1263: .P
1264: The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1265: changed in the same way as the Perl-compatible options by using the characters
1266: J, U and X respectively.
1267: .P
1268: When one of these option changes occurs at top level (that is, not inside
1269: subpattern parentheses), the change applies to the remainder of the pattern
1270: that follows. If the change is placed right at the start of a pattern, PCRE
1271: extracts it into the global options (and it will therefore show up in data
1272: extracted by the \fBpcre_fullinfo()\fP function).
1273: .P
1274: An option change within a subpattern (see below for a description of
1275: subpatterns) affects only that part of the subpattern that follows it, so
1276: .sp
1277: (a(?i)b)c
1278: .sp
1279: matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1280: By this means, options can be made to have different settings in different
1281: parts of the pattern. Any changes made in one alternative do carry on
1282: into subsequent branches within the same subpattern. For example,
1283: .sp
1284: (a(?i)b|c)
1285: .sp
1286: matches "ab", "aB", "c", and "C", even though when matching "C" the first
1287: branch is abandoned before the option setting. This is because the effects of
1288: option settings happen at compile time. There would be some very weird
1289: behaviour otherwise.
1290: .P
1291: \fBNote:\fP There are other PCRE-specific options that can be set by the
1.1.1.2 misho 1292: application when the compiling or matching functions are called. In some cases
1293: the pattern can contain special leading sequences such as (*CRLF) to override
1294: what the application has set or what has been defaulted. Details are given in
1295: the section entitled
1.1 misho 1296: .\" HTML <a href="#newlineseq">
1297: .\" </a>
1298: "Newline sequences"
1299: .\"
1.1.1.2 misho 1300: above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1301: can be used to set UTF and Unicode property modes; they are equivalent to
1302: setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1.1 misho 1303: .
1304: .
1305: .\" HTML <a name="subpattern"></a>
1306: .SH SUBPATTERNS
1307: .rs
1308: .sp
1309: Subpatterns are delimited by parentheses (round brackets), which can be nested.
1310: Turning part of a pattern into a subpattern does two things:
1311: .sp
1312: 1. It localizes a set of alternatives. For example, the pattern
1313: .sp
1314: cat(aract|erpillar|)
1315: .sp
1316: matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1317: match "cataract", "erpillar" or an empty string.
1318: .sp
1319: 2. It sets up the subpattern as a capturing subpattern. This means that, when
1320: the whole pattern matches, that portion of the subject string that matched the
1.1.1.2 misho 1321: subpattern is passed back to the caller via the \fIovector\fP argument of the
1322: matching function. (This applies only to the traditional matching functions;
1323: the DFA matching functions do not support capturing.)
1324: .P
1325: Opening parentheses are counted from left to right (starting from 1) to obtain
1326: numbers for the capturing subpatterns. For example, if the string "the red
1327: king" is matched against the pattern
1.1 misho 1328: .sp
1329: the ((red|white) (king|queen))
1330: .sp
1331: the captured substrings are "red king", "red", and "king", and are numbered 1,
1332: 2, and 3, respectively.
1333: .P
1334: The fact that plain parentheses fulfil two functions is not always helpful.
1335: There are often times when a grouping subpattern is required without a
1336: capturing requirement. If an opening parenthesis is followed by a question mark
1337: and a colon, the subpattern does not do any capturing, and is not counted when
1338: computing the number of any subsequent capturing subpatterns. For example, if
1339: the string "the white queen" is matched against the pattern
1340: .sp
1341: the ((?:red|white) (king|queen))
1342: .sp
1343: the captured substrings are "white queen" and "queen", and are numbered 1 and
1344: 2. The maximum number of capturing subpatterns is 65535.
1345: .P
1346: As a convenient shorthand, if any option settings are required at the start of
1347: a non-capturing subpattern, the option letters may appear between the "?" and
1348: the ":". Thus the two patterns
1349: .sp
1350: (?i:saturday|sunday)
1351: (?:(?i)saturday|sunday)
1352: .sp
1353: match exactly the same set of strings. Because alternative branches are tried
1354: from left to right, and options are not reset until the end of the subpattern
1355: is reached, an option setting in one branch does affect subsequent branches, so
1356: the above patterns match "SUNDAY" as well as "Saturday".
1357: .
1358: .
1359: .\" HTML <a name="dupsubpatternnumber"></a>
1360: .SH "DUPLICATE SUBPATTERN NUMBERS"
1361: .rs
1362: .sp
1363: Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1364: the same numbers for its capturing parentheses. Such a subpattern starts with
1365: (?| and is itself a non-capturing subpattern. For example, consider this
1366: pattern:
1367: .sp
1368: (?|(Sat)ur|(Sun))day
1369: .sp
1370: Because the two alternatives are inside a (?| group, both sets of capturing
1371: parentheses are numbered one. Thus, when the pattern matches, you can look
1372: at captured substring number one, whichever alternative matched. This construct
1373: is useful when you want to capture part, but not all, of one of a number of
1374: alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1375: number is reset at the start of each branch. The numbers of any capturing
1376: parentheses that follow the subpattern start after the highest number used in
1377: any branch. The following example is taken from the Perl documentation. The
1378: numbers underneath show in which buffer the captured content will be stored.
1379: .sp
1380: # before ---------------branch-reset----------- after
1381: / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1382: # 1 2 2 3 2 3 4
1383: .sp
1384: A back reference to a numbered subpattern uses the most recent value that is
1385: set for that number by any subpattern. The following pattern matches "abcabc"
1386: or "defdef":
1387: .sp
1388: /(?|(abc)|(def))\e1/
1389: .sp
1390: In contrast, a subroutine call to a numbered subpattern always refers to the
1391: first one in the pattern with the given number. The following pattern matches
1392: "abcabc" or "defabc":
1393: .sp
1394: /(?|(abc)|(def))(?1)/
1395: .sp
1396: If a
1397: .\" HTML <a href="#conditions">
1398: .\" </a>
1399: condition test
1400: .\"
1401: for a subpattern's having matched refers to a non-unique number, the test is
1402: true if any of the subpatterns of that number have matched.
1403: .P
1404: An alternative approach to using this "branch reset" feature is to use
1405: duplicate named subpatterns, as described in the next section.
1406: .
1407: .
1408: .SH "NAMED SUBPATTERNS"
1409: .rs
1410: .sp
1411: Identifying capturing parentheses by number is simple, but it can be very hard
1412: to keep track of the numbers in complicated regular expressions. Furthermore,
1413: if an expression is modified, the numbers may change. To help with this
1414: difficulty, PCRE supports the naming of subpatterns. This feature was not
1415: added to Perl until release 5.10. Python had the feature earlier, and PCRE
1416: introduced it at release 4.0, using the Python syntax. PCRE now supports both
1417: the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1418: have different names, but PCRE does not.
1419: .P
1420: In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1421: (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1422: parentheses from other parts of the pattern, such as
1423: .\" HTML <a href="#backreferences">
1424: .\" </a>
1425: back references,
1426: .\"
1427: .\" HTML <a href="#recursion">
1428: .\" </a>
1429: recursion,
1430: .\"
1431: and
1432: .\" HTML <a href="#conditions">
1433: .\" </a>
1434: conditions,
1435: .\"
1436: can be made by name as well as by number.
1437: .P
1438: Names consist of up to 32 alphanumeric characters and underscores. Named
1439: capturing parentheses are still allocated numbers as well as names, exactly as
1440: if the names were not present. The PCRE API provides function calls for
1441: extracting the name-to-number translation table from a compiled pattern. There
1442: is also a convenience function for extracting a captured substring by name.
1443: .P
1444: By default, a name must be unique within a pattern, but it is possible to relax
1445: this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1446: names are also always permitted for subpatterns with the same number, set up as
1447: described in the previous section.) Duplicate names can be useful for patterns
1448: where only one instance of the named parentheses can match. Suppose you want to
1449: match the name of a weekday, either as a 3-letter abbreviation or as the full
1450: name, and in both cases you want to extract the abbreviation. This pattern
1451: (ignoring the line breaks) does the job:
1452: .sp
1453: (?<DN>Mon|Fri|Sun)(?:day)?|
1454: (?<DN>Tue)(?:sday)?|
1455: (?<DN>Wed)(?:nesday)?|
1456: (?<DN>Thu)(?:rsday)?|
1457: (?<DN>Sat)(?:urday)?
1458: .sp
1459: There are five capturing substrings, but only one is ever set after a match.
1460: (An alternative way of solving this problem is to use a "branch reset"
1461: subpattern, as described in the previous section.)
1462: .P
1463: The convenience function for extracting the data by name returns the substring
1464: for the first (and in this example, the only) subpattern of that name that
1465: matched. This saves searching to find which numbered subpattern it was.
1466: .P
1467: If you make a back reference to a non-unique named subpattern from elsewhere in
1468: the pattern, the one that corresponds to the first occurrence of the name is
1469: used. In the absence of duplicate numbers (see the previous section) this is
1470: the one with the lowest number. If you use a named reference in a condition
1471: test (see the
1472: .\"
1473: .\" HTML <a href="#conditions">
1474: .\" </a>
1475: section about conditions
1476: .\"
1477: below), either to check whether a subpattern has matched, or to check for
1478: recursion, all subpatterns with the same name are tested. If the condition is
1479: true for any one of them, the overall condition is true. This is the same
1480: behaviour as testing by number. For further details of the interfaces for
1481: handling named subpatterns, see the
1482: .\" HREF
1483: \fBpcreapi\fP
1484: .\"
1485: documentation.
1486: .P
1487: \fBWarning:\fP You cannot use different names to distinguish between two
1488: subpatterns with the same number because PCRE uses only the numbers when
1489: matching. For this reason, an error is given at compile time if different names
1490: are given to subpatterns with the same number. However, you can give the same
1491: name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1492: .
1493: .
1494: .SH REPETITION
1495: .rs
1496: .sp
1497: Repetition is specified by quantifiers, which can follow any of the following
1498: items:
1499: .sp
1500: a literal data character
1501: the dot metacharacter
1502: the \eC escape sequence
1.1.1.2 misho 1503: the \eX escape sequence
1.1 misho 1504: the \eR escape sequence
1505: an escape such as \ed or \epL that matches a single character
1506: a character class
1507: a back reference (see next section)
1508: a parenthesized subpattern (including assertions)
1509: a subroutine call to a subpattern (recursive or otherwise)
1510: .sp
1511: The general repetition quantifier specifies a minimum and maximum number of
1512: permitted matches, by giving the two numbers in curly brackets (braces),
1513: separated by a comma. The numbers must be less than 65536, and the first must
1514: be less than or equal to the second. For example:
1515: .sp
1516: z{2,4}
1517: .sp
1518: matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1519: character. If the second number is omitted, but the comma is present, there is
1520: no upper limit; if the second number and the comma are both omitted, the
1521: quantifier specifies an exact number of required matches. Thus
1522: .sp
1523: [aeiou]{3,}
1524: .sp
1525: matches at least 3 successive vowels, but may match many more, while
1526: .sp
1527: \ed{8}
1528: .sp
1529: matches exactly 8 digits. An opening curly bracket that appears in a position
1530: where a quantifier is not allowed, or one that does not match the syntax of a
1531: quantifier, is taken as a literal character. For example, {,6} is not a
1532: quantifier, but a literal string of four characters.
1533: .P
1.1.1.2 misho 1534: In UTF modes, quantifiers apply to characters rather than to individual data
1535: units. Thus, for example, \ex{100}{2} matches two characters, each of
1536: which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1537: \eX{3} matches three Unicode extended sequences, each of which may be several
1538: data units long (and they may be of different lengths).
1.1 misho 1539: .P
1540: The quantifier {0} is permitted, causing the expression to behave as if the
1541: previous item and the quantifier were not present. This may be useful for
1542: subpatterns that are referenced as
1543: .\" HTML <a href="#subpatternsassubroutines">
1544: .\" </a>
1545: subroutines
1546: .\"
1547: from elsewhere in the pattern (but see also the section entitled
1548: .\" HTML <a href="#subdefine">
1549: .\" </a>
1550: "Defining subpatterns for use by reference only"
1551: .\"
1552: below). Items other than subpatterns that have a {0} quantifier are omitted
1553: from the compiled pattern.
1554: .P
1555: For convenience, the three most common quantifiers have single-character
1556: abbreviations:
1557: .sp
1558: * is equivalent to {0,}
1559: + is equivalent to {1,}
1560: ? is equivalent to {0,1}
1561: .sp
1562: It is possible to construct infinite loops by following a subpattern that can
1563: match no characters with a quantifier that has no upper limit, for example:
1564: .sp
1565: (a?)*
1566: .sp
1567: Earlier versions of Perl and PCRE used to give an error at compile time for
1568: such patterns. However, because there are cases where this can be useful, such
1569: patterns are now accepted, but if any repetition of the subpattern does in fact
1570: match no characters, the loop is forcibly broken.
1571: .P
1572: By default, the quantifiers are "greedy", that is, they match as much as
1573: possible (up to the maximum number of permitted times), without causing the
1574: rest of the pattern to fail. The classic example of where this gives problems
1575: is in trying to match comments in C programs. These appear between /* and */
1576: and within the comment, individual * and / characters may appear. An attempt to
1577: match C comments by applying the pattern
1578: .sp
1579: /\e*.*\e*/
1580: .sp
1581: to the string
1582: .sp
1583: /* first comment */ not comment /* second comment */
1584: .sp
1585: fails, because it matches the entire string owing to the greediness of the .*
1586: item.
1587: .P
1588: However, if a quantifier is followed by a question mark, it ceases to be
1589: greedy, and instead matches the minimum number of times possible, so the
1590: pattern
1591: .sp
1592: /\e*.*?\e*/
1593: .sp
1594: does the right thing with the C comments. The meaning of the various
1595: quantifiers is not otherwise changed, just the preferred number of matches.
1596: Do not confuse this use of question mark with its use as a quantifier in its
1597: own right. Because it has two uses, it can sometimes appear doubled, as in
1598: .sp
1599: \ed??\ed
1600: .sp
1601: which matches one digit by preference, but can match two if that is the only
1602: way the rest of the pattern matches.
1603: .P
1604: If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1605: the quantifiers are not greedy by default, but individual ones can be made
1606: greedy by following them with a question mark. In other words, it inverts the
1607: default behaviour.
1608: .P
1609: When a parenthesized subpattern is quantified with a minimum repeat count that
1610: is greater than 1 or with a limited maximum, more memory is required for the
1611: compiled pattern, in proportion to the size of the minimum or maximum.
1612: .P
1613: If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1614: to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1615: implicitly anchored, because whatever follows will be tried against every
1616: character position in the subject string, so there is no point in retrying the
1617: overall match at any position after the first. PCRE normally treats such a
1618: pattern as though it were preceded by \eA.
1619: .P
1620: In cases where it is known that the subject string contains no newlines, it is
1621: worth setting PCRE_DOTALL in order to obtain this optimization, or
1622: alternatively using ^ to indicate anchoring explicitly.
1623: .P
1624: However, there is one situation where the optimization cannot be used. When .*
1625: is inside capturing parentheses that are the subject of a back reference
1626: elsewhere in the pattern, a match at the start may fail where a later one
1627: succeeds. Consider, for example:
1628: .sp
1629: (.*)abc\e1
1630: .sp
1631: If the subject is "xyz123abc123" the match point is the fourth character. For
1632: this reason, such a pattern is not implicitly anchored.
1633: .P
1634: When a capturing subpattern is repeated, the value captured is the substring
1635: that matched the final iteration. For example, after
1636: .sp
1637: (tweedle[dume]{3}\es*)+
1638: .sp
1639: has matched "tweedledum tweedledee" the value of the captured substring is
1640: "tweedledee". However, if there are nested capturing subpatterns, the
1641: corresponding captured values may have been set in previous iterations. For
1642: example, after
1643: .sp
1644: /(a|(b))+/
1645: .sp
1646: matches "aba" the value of the second captured substring is "b".
1647: .
1648: .
1649: .\" HTML <a name="atomicgroup"></a>
1650: .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1651: .rs
1652: .sp
1653: With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1654: repetition, failure of what follows normally causes the repeated item to be
1655: re-evaluated to see if a different number of repeats allows the rest of the
1656: pattern to match. Sometimes it is useful to prevent this, either to change the
1657: nature of the match, or to cause it fail earlier than it otherwise might, when
1658: the author of the pattern knows there is no point in carrying on.
1659: .P
1660: Consider, for example, the pattern \ed+foo when applied to the subject line
1661: .sp
1662: 123456bar
1663: .sp
1664: After matching all 6 digits and then failing to match "foo", the normal
1665: action of the matcher is to try again with only 5 digits matching the \ed+
1666: item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1667: (a term taken from Jeffrey Friedl's book) provides the means for specifying
1668: that once a subpattern has matched, it is not to be re-evaluated in this way.
1669: .P
1670: If we use atomic grouping for the previous example, the matcher gives up
1671: immediately on failing to match "foo" the first time. The notation is a kind of
1672: special parenthesis, starting with (?> as in this example:
1673: .sp
1674: (?>\ed+)foo
1675: .sp
1676: This kind of parenthesis "locks up" the part of the pattern it contains once
1677: it has matched, and a failure further into the pattern is prevented from
1678: backtracking into it. Backtracking past it to previous items, however, works as
1679: normal.
1680: .P
1681: An alternative description is that a subpattern of this type matches the string
1682: of characters that an identical standalone pattern would match, if anchored at
1683: the current point in the subject string.
1684: .P
1685: Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1686: the above example can be thought of as a maximizing repeat that must swallow
1687: everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1688: number of digits they match in order to make the rest of the pattern match,
1689: (?>\ed+) can only match an entire sequence of digits.
1690: .P
1691: Atomic groups in general can of course contain arbitrarily complicated
1692: subpatterns, and can be nested. However, when the subpattern for an atomic
1693: group is just a single repeated item, as in the example above, a simpler
1694: notation, called a "possessive quantifier" can be used. This consists of an
1695: additional + character following a quantifier. Using this notation, the
1696: previous example can be rewritten as
1697: .sp
1698: \ed++foo
1699: .sp
1700: Note that a possessive quantifier can be used with an entire group, for
1701: example:
1702: .sp
1703: (abc|xyz){2,3}+
1704: .sp
1705: Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1706: option is ignored. They are a convenient notation for the simpler forms of
1707: atomic group. However, there is no difference in the meaning of a possessive
1708: quantifier and the equivalent atomic group, though there may be a performance
1709: difference; possessive quantifiers should be slightly faster.
1710: .P
1711: The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1712: Jeffrey Friedl originated the idea (and the name) in the first edition of his
1713: book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1714: package, and PCRE copied it from there. It ultimately found its way into Perl
1715: at release 5.10.
1716: .P
1717: PCRE has an optimization that automatically "possessifies" certain simple
1718: pattern constructs. For example, the sequence A+B is treated as A++B because
1719: there is no point in backtracking into a sequence of A's when B must follow.
1720: .P
1721: When a pattern contains an unlimited repeat inside a subpattern that can itself
1722: be repeated an unlimited number of times, the use of an atomic group is the
1723: only way to avoid some failing matches taking a very long time indeed. The
1724: pattern
1725: .sp
1726: (\eD+|<\ed+>)*[!?]
1727: .sp
1728: matches an unlimited number of substrings that either consist of non-digits, or
1729: digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1730: quickly. However, if it is applied to
1731: .sp
1732: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1733: .sp
1734: it takes a long time before reporting failure. This is because the string can
1735: be divided between the internal \eD+ repeat and the external * repeat in a
1736: large number of ways, and all have to be tried. (The example uses [!?] rather
1737: than a single character at the end, because both PCRE and Perl have an
1738: optimization that allows for fast failure when a single character is used. They
1739: remember the last single character that is required for a match, and fail early
1740: if it is not present in the string.) If the pattern is changed so that it uses
1741: an atomic group, like this:
1742: .sp
1743: ((?>\eD+)|<\ed+>)*[!?]
1744: .sp
1745: sequences of non-digits cannot be broken, and failure happens quickly.
1746: .
1747: .
1748: .\" HTML <a name="backreferences"></a>
1749: .SH "BACK REFERENCES"
1750: .rs
1751: .sp
1752: Outside a character class, a backslash followed by a digit greater than 0 (and
1753: possibly further digits) is a back reference to a capturing subpattern earlier
1754: (that is, to its left) in the pattern, provided there have been that many
1755: previous capturing left parentheses.
1756: .P
1757: However, if the decimal number following the backslash is less than 10, it is
1758: always taken as a back reference, and causes an error only if there are not
1759: that many capturing left parentheses in the entire pattern. In other words, the
1760: parentheses that are referenced need not be to the left of the reference for
1761: numbers less than 10. A "forward back reference" of this type can make sense
1762: when a repetition is involved and the subpattern to the right has participated
1763: in an earlier iteration.
1764: .P
1765: It is not possible to have a numerical "forward back reference" to a subpattern
1766: whose number is 10 or more using this syntax because a sequence such as \e50 is
1767: interpreted as a character defined in octal. See the subsection entitled
1768: "Non-printing characters"
1769: .\" HTML <a href="#digitsafterbackslash">
1770: .\" </a>
1771: above
1772: .\"
1773: for further details of the handling of digits following a backslash. There is
1774: no such problem when named parentheses are used. A back reference to any
1775: subpattern is possible using named parentheses (see below).
1776: .P
1777: Another way of avoiding the ambiguity inherent in the use of digits following a
1778: backslash is to use the \eg escape sequence. This escape must be followed by an
1779: unsigned number or a negative number, optionally enclosed in braces. These
1780: examples are all identical:
1781: .sp
1782: (ring), \e1
1783: (ring), \eg1
1784: (ring), \eg{1}
1785: .sp
1786: An unsigned number specifies an absolute reference without the ambiguity that
1787: is present in the older syntax. It is also useful when literal digits follow
1788: the reference. A negative number is a relative reference. Consider this
1789: example:
1790: .sp
1791: (abc(def)ghi)\eg{-1}
1792: .sp
1793: The sequence \eg{-1} is a reference to the most recently started capturing
1794: subpattern before \eg, that is, is it equivalent to \e2 in this example.
1795: Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1796: can be helpful in long patterns, and also in patterns that are created by
1797: joining together fragments that contain references within themselves.
1798: .P
1799: A back reference matches whatever actually matched the capturing subpattern in
1800: the current subject string, rather than anything matching the subpattern
1801: itself (see
1802: .\" HTML <a href="#subpatternsassubroutines">
1803: .\" </a>
1804: "Subpatterns as subroutines"
1805: .\"
1806: below for a way of doing that). So the pattern
1807: .sp
1808: (sens|respons)e and \e1ibility
1809: .sp
1810: matches "sense and sensibility" and "response and responsibility", but not
1811: "sense and responsibility". If caseful matching is in force at the time of the
1812: back reference, the case of letters is relevant. For example,
1813: .sp
1814: ((?i)rah)\es+\e1
1815: .sp
1816: matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1817: capturing subpattern is matched caselessly.
1818: .P
1819: There are several different ways of writing back references to named
1820: subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1821: \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1822: back reference syntax, in which \eg can be used for both numeric and named
1823: references, is also supported. We could rewrite the above example in any of
1824: the following ways:
1825: .sp
1826: (?<p1>(?i)rah)\es+\ek<p1>
1827: (?'p1'(?i)rah)\es+\ek{p1}
1828: (?P<p1>(?i)rah)\es+(?P=p1)
1829: (?<p1>(?i)rah)\es+\eg{p1}
1830: .sp
1831: A subpattern that is referenced by name may appear in the pattern before or
1832: after the reference.
1833: .P
1834: There may be more than one back reference to the same subpattern. If a
1835: subpattern has not actually been used in a particular match, any back
1836: references to it always fail by default. For example, the pattern
1837: .sp
1838: (a|(bc))\e2
1839: .sp
1840: always fails if it starts to match "a" rather than "bc". However, if the
1841: PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1842: unset value matches an empty string.
1843: .P
1844: Because there may be many capturing parentheses in a pattern, all digits
1845: following a backslash are taken as part of a potential back reference number.
1846: If the pattern continues with a digit character, some delimiter must be used to
1847: terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1.1.1.3 ! misho 1848: white space. Otherwise, the \eg{ syntax or an empty comment (see
1.1 misho 1849: .\" HTML <a href="#comments">
1850: .\" </a>
1851: "Comments"
1852: .\"
1853: below) can be used.
1854: .
1855: .SS "Recursive back references"
1856: .rs
1857: .sp
1858: A back reference that occurs inside the parentheses to which it refers fails
1859: when the subpattern is first used, so, for example, (a\e1) never matches.
1860: However, such references can be useful inside repeated subpatterns. For
1861: example, the pattern
1862: .sp
1863: (a|b\e1)+
1864: .sp
1865: matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1866: the subpattern, the back reference matches the character string corresponding
1867: to the previous iteration. In order for this to work, the pattern must be such
1868: that the first iteration does not need to match the back reference. This can be
1869: done using alternation, as in the example above, or by a quantifier with a
1870: minimum of zero.
1871: .P
1872: Back references of this type cause the group that they reference to be treated
1873: as an
1874: .\" HTML <a href="#atomicgroup">
1875: .\" </a>
1876: atomic group.
1877: .\"
1878: Once the whole group has been matched, a subsequent matching failure cannot
1879: cause backtracking into the middle of the group.
1880: .
1881: .
1882: .\" HTML <a name="bigassertions"></a>
1883: .SH ASSERTIONS
1884: .rs
1885: .sp
1886: An assertion is a test on the characters following or preceding the current
1887: matching point that does not actually consume any characters. The simple
1888: assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1889: .\" HTML <a href="#smallassertions">
1890: .\" </a>
1891: above.
1892: .\"
1893: .P
1894: More complicated assertions are coded as subpatterns. There are two kinds:
1895: those that look ahead of the current position in the subject string, and those
1896: that look behind it. An assertion subpattern is matched in the normal way,
1897: except that it does not cause the current matching position to be changed.
1898: .P
1899: Assertion subpatterns are not capturing subpatterns. If such an assertion
1900: contains capturing subpatterns within it, these are counted for the purposes of
1901: numbering the capturing subpatterns in the whole pattern. However, substring
1902: capturing is carried out only for positive assertions, because it does not make
1903: sense for negative assertions.
1904: .P
1905: For compatibility with Perl, assertion subpatterns may be repeated; though
1906: it makes no sense to assert the same thing several times, the side effect of
1907: capturing parentheses may occasionally be useful. In practice, there only three
1908: cases:
1909: .sp
1910: (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1911: However, it may contain internal capturing parenthesized groups that are called
1912: from elsewhere via the
1913: .\" HTML <a href="#subpatternsassubroutines">
1914: .\" </a>
1915: subroutine mechanism.
1916: .\"
1917: .sp
1918: (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1919: were {0,1}. At run time, the rest of the pattern match is tried with and
1920: without the assertion, the order depending on the greediness of the quantifier.
1921: .sp
1922: (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1923: The assertion is obeyed just once when encountered during matching.
1924: .
1925: .
1926: .SS "Lookahead assertions"
1927: .rs
1928: .sp
1929: Lookahead assertions start with (?= for positive assertions and (?! for
1930: negative assertions. For example,
1931: .sp
1932: \ew+(?=;)
1933: .sp
1934: matches a word followed by a semicolon, but does not include the semicolon in
1935: the match, and
1936: .sp
1937: foo(?!bar)
1938: .sp
1939: matches any occurrence of "foo" that is not followed by "bar". Note that the
1940: apparently similar pattern
1941: .sp
1942: (?!foo)bar
1943: .sp
1944: does not find an occurrence of "bar" that is preceded by something other than
1945: "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1946: (?!foo) is always true when the next three characters are "bar". A
1947: lookbehind assertion is needed to achieve the other effect.
1948: .P
1949: If you want to force a matching failure at some point in a pattern, the most
1950: convenient way to do it is with (?!) because an empty string always matches, so
1951: an assertion that requires there not to be an empty string must always fail.
1952: The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1953: .
1954: .
1955: .\" HTML <a name="lookbehind"></a>
1956: .SS "Lookbehind assertions"
1957: .rs
1958: .sp
1959: Lookbehind assertions start with (?<= for positive assertions and (?<! for
1960: negative assertions. For example,
1961: .sp
1962: (?<!foo)bar
1963: .sp
1964: does find an occurrence of "bar" that is not preceded by "foo". The contents of
1965: a lookbehind assertion are restricted such that all the strings it matches must
1966: have a fixed length. However, if there are several top-level alternatives, they
1967: do not all have to have the same fixed length. Thus
1968: .sp
1969: (?<=bullock|donkey)
1970: .sp
1971: is permitted, but
1972: .sp
1973: (?<!dogs?|cats?)
1974: .sp
1975: causes an error at compile time. Branches that match different length strings
1976: are permitted only at the top level of a lookbehind assertion. This is an
1977: extension compared with Perl, which requires all branches to match the same
1978: length of string. An assertion such as
1979: .sp
1980: (?<=ab(c|de))
1981: .sp
1982: is not permitted, because its single top-level branch can match two different
1983: lengths, but it is acceptable to PCRE if rewritten to use two top-level
1984: branches:
1985: .sp
1986: (?<=abc|abde)
1987: .sp
1988: In some cases, the escape sequence \eK
1989: .\" HTML <a href="#resetmatchstart">
1990: .\" </a>
1991: (see above)
1992: .\"
1993: can be used instead of a lookbehind assertion to get round the fixed-length
1994: restriction.
1995: .P
1996: The implementation of lookbehind assertions is, for each alternative, to
1997: temporarily move the current position back by the fixed length and then try to
1998: match. If there are insufficient characters before the current position, the
1999: assertion fails.
2000: .P
1.1.1.2 misho 2001: In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
2002: unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2003: it impossible to calculate the length of the lookbehind. The \eX and \eR
2004: escapes, which can match different numbers of data units, are also not
2005: permitted.
1.1 misho 2006: .P
2007: .\" HTML <a href="#subpatternsassubroutines">
2008: .\" </a>
2009: "Subroutine"
2010: .\"
2011: calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2012: as the subpattern matches a fixed-length string.
2013: .\" HTML <a href="#recursion">
2014: .\" </a>
2015: Recursion,
2016: .\"
2017: however, is not supported.
2018: .P
2019: Possessive quantifiers can be used in conjunction with lookbehind assertions to
2020: specify efficient matching of fixed-length strings at the end of subject
2021: strings. Consider a simple pattern such as
2022: .sp
2023: abcd$
2024: .sp
2025: when applied to a long string that does not match. Because matching proceeds
2026: from left to right, PCRE will look for each "a" in the subject and then see if
2027: what follows matches the rest of the pattern. If the pattern is specified as
2028: .sp
2029: ^.*abcd$
2030: .sp
2031: the initial .* matches the entire string at first, but when this fails (because
2032: there is no following "a"), it backtracks to match all but the last character,
2033: then all but the last two characters, and so on. Once again the search for "a"
2034: covers the entire string, from right to left, so we are no better off. However,
2035: if the pattern is written as
2036: .sp
2037: ^.*+(?<=abcd)
2038: .sp
2039: there can be no backtracking for the .*+ item; it can match only the entire
2040: string. The subsequent lookbehind assertion does a single test on the last four
2041: characters. If it fails, the match fails immediately. For long strings, this
2042: approach makes a significant difference to the processing time.
2043: .
2044: .
2045: .SS "Using multiple assertions"
2046: .rs
2047: .sp
2048: Several assertions (of any sort) may occur in succession. For example,
2049: .sp
2050: (?<=\ed{3})(?<!999)foo
2051: .sp
2052: matches "foo" preceded by three digits that are not "999". Notice that each of
2053: the assertions is applied independently at the same point in the subject
2054: string. First there is a check that the previous three characters are all
2055: digits, and then there is a check that the same three characters are not "999".
2056: This pattern does \fInot\fP match "foo" preceded by six characters, the first
2057: of which are digits and the last three of which are not "999". For example, it
2058: doesn't match "123abcfoo". A pattern to do that is
2059: .sp
2060: (?<=\ed{3}...)(?<!999)foo
2061: .sp
2062: This time the first assertion looks at the preceding six characters, checking
2063: that the first three are digits, and then the second assertion checks that the
2064: preceding three characters are not "999".
2065: .P
2066: Assertions can be nested in any combination. For example,
2067: .sp
2068: (?<=(?<!foo)bar)baz
2069: .sp
2070: matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2071: preceded by "foo", while
2072: .sp
2073: (?<=\ed{3}(?!999)...)foo
2074: .sp
2075: is another pattern that matches "foo" preceded by three digits and any three
2076: characters that are not "999".
2077: .
2078: .
2079: .\" HTML <a name="conditions"></a>
2080: .SH "CONDITIONAL SUBPATTERNS"
2081: .rs
2082: .sp
2083: It is possible to cause the matching process to obey a subpattern
2084: conditionally or to choose between two alternative subpatterns, depending on
2085: the result of an assertion, or whether a specific capturing subpattern has
2086: already been matched. The two possible forms of conditional subpattern are:
2087: .sp
2088: (?(condition)yes-pattern)
2089: (?(condition)yes-pattern|no-pattern)
2090: .sp
2091: If the condition is satisfied, the yes-pattern is used; otherwise the
2092: no-pattern (if present) is used. If there are more than two alternatives in the
2093: subpattern, a compile-time error occurs. Each of the two alternatives may
2094: itself contain nested subpatterns of any form, including conditional
2095: subpatterns; the restriction to two alternatives applies only at the level of
2096: the condition. This pattern fragment is an example where the alternatives are
2097: complex:
2098: .sp
2099: (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2100: .sp
2101: .P
2102: There are four kinds of condition: references to subpatterns, references to
2103: recursion, a pseudo-condition called DEFINE, and assertions.
2104: .
2105: .SS "Checking for a used subpattern by number"
2106: .rs
2107: .sp
2108: If the text between the parentheses consists of a sequence of digits, the
2109: condition is true if a capturing subpattern of that number has previously
2110: matched. If there is more than one capturing subpattern with the same number
2111: (see the earlier
2112: .\"
2113: .\" HTML <a href="#recursion">
2114: .\" </a>
2115: section about duplicate subpattern numbers),
2116: .\"
2117: the condition is true if any of them have matched. An alternative notation is
2118: to precede the digits with a plus or minus sign. In this case, the subpattern
2119: number is relative rather than absolute. The most recently opened parentheses
2120: can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2121: loops it can also make sense to refer to subsequent groups. The next
2122: parentheses to be opened can be referenced as (?(+1), and so on. (The value
2123: zero in any of these forms is not used; it provokes a compile-time error.)
2124: .P
2125: Consider the following pattern, which contains non-significant white space to
2126: make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2127: three parts for ease of discussion:
2128: .sp
2129: ( \e( )? [^()]+ (?(1) \e) )
2130: .sp
2131: The first part matches an optional opening parenthesis, and if that
2132: character is present, sets it as the first captured substring. The second part
2133: matches one or more characters that are not parentheses. The third part is a
2134: conditional subpattern that tests whether or not the first set of parentheses
2135: matched. If they did, that is, if subject started with an opening parenthesis,
2136: the condition is true, and so the yes-pattern is executed and a closing
2137: parenthesis is required. Otherwise, since no-pattern is not present, the
2138: subpattern matches nothing. In other words, this pattern matches a sequence of
2139: non-parentheses, optionally enclosed in parentheses.
2140: .P
2141: If you were embedding this pattern in a larger one, you could use a relative
2142: reference:
2143: .sp
2144: ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
2145: .sp
2146: This makes the fragment independent of the parentheses in the larger pattern.
2147: .
2148: .SS "Checking for a used subpattern by name"
2149: .rs
2150: .sp
2151: Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2152: subpattern by name. For compatibility with earlier versions of PCRE, which had
2153: this facility before Perl, the syntax (?(name)...) is also recognized. However,
2154: there is a possible ambiguity with this syntax, because subpattern names may
2155: consist entirely of digits. PCRE looks first for a named subpattern; if it
2156: cannot find one and the name consists entirely of digits, PCRE looks for a
2157: subpattern of that number, which must be greater than zero. Using subpattern
2158: names that consist entirely of digits is not recommended.
2159: .P
2160: Rewriting the above example to use a named subpattern gives this:
2161: .sp
2162: (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
2163: .sp
2164: If the name used in a condition of this kind is a duplicate, the test is
2165: applied to all subpatterns of the same name, and is true if any one of them has
2166: matched.
2167: .
2168: .SS "Checking for pattern recursion"
2169: .rs
2170: .sp
2171: If the condition is the string (R), and there is no subpattern with the name R,
2172: the condition is true if a recursive call to the whole pattern or any
2173: subpattern has been made. If digits or a name preceded by ampersand follow the
2174: letter R, for example:
2175: .sp
2176: (?(R3)...) or (?(R&name)...)
2177: .sp
2178: the condition is true if the most recent recursion is into a subpattern whose
2179: number or name is given. This condition does not check the entire recursion
2180: stack. If the name used in a condition of this kind is a duplicate, the test is
2181: applied to all subpatterns of the same name, and is true if any one of them is
2182: the most recent recursion.
2183: .P
2184: At "top level", all these recursion test conditions are false.
2185: .\" HTML <a href="#recursion">
2186: .\" </a>
2187: The syntax for recursive patterns
2188: .\"
2189: is described below.
2190: .
2191: .\" HTML <a name="subdefine"></a>
2192: .SS "Defining subpatterns for use by reference only"
2193: .rs
2194: .sp
2195: If the condition is the string (DEFINE), and there is no subpattern with the
2196: name DEFINE, the condition is always false. In this case, there may be only one
2197: alternative in the subpattern. It is always skipped if control reaches this
2198: point in the pattern; the idea of DEFINE is that it can be used to define
2199: subroutines that can be referenced from elsewhere. (The use of
2200: .\" HTML <a href="#subpatternsassubroutines">
2201: .\" </a>
2202: subroutines
2203: .\"
2204: is described below.) For example, a pattern to match an IPv4 address such as
1.1.1.3 ! misho 2205: "192.168.23.245" could be written like this (ignore white space and line
1.1 misho 2206: breaks):
2207: .sp
2208: (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2209: \eb (?&byte) (\e.(?&byte)){3} \eb
2210: .sp
2211: The first part of the pattern is a DEFINE group inside which a another group
2212: named "byte" is defined. This matches an individual component of an IPv4
2213: address (a number less than 256). When matching takes place, this part of the
2214: pattern is skipped because DEFINE acts like a false condition. The rest of the
2215: pattern uses references to the named group to match the four dot-separated
2216: components of an IPv4 address, insisting on a word boundary at each end.
2217: .
2218: .SS "Assertion conditions"
2219: .rs
2220: .sp
2221: If the condition is not in any of the above formats, it must be an assertion.
2222: This may be a positive or negative lookahead or lookbehind assertion. Consider
2223: this pattern, again containing non-significant white space, and with the two
2224: alternatives on the second line:
2225: .sp
2226: (?(?=[^a-z]*[a-z])
2227: \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2228: .sp
2229: The condition is a positive lookahead assertion that matches an optional
2230: sequence of non-letters followed by a letter. In other words, it tests for the
2231: presence of at least one letter in the subject. If a letter is found, the
2232: subject is matched against the first alternative; otherwise it is matched
2233: against the second. This pattern matches strings in one of the two forms
2234: dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2235: .
2236: .
2237: .\" HTML <a name="comments"></a>
2238: .SH COMMENTS
2239: .rs
2240: .sp
2241: There are two ways of including comments in patterns that are processed by
2242: PCRE. In both cases, the start of the comment must not be in a character class,
2243: nor in the middle of any other sequence of related characters such as (?: or a
2244: subpattern name or number. The characters that make up a comment play no part
2245: in the pattern matching.
2246: .P
2247: The sequence (?# marks the start of a comment that continues up to the next
2248: closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2249: option is set, an unescaped # character also introduces a comment, which in
2250: this case continues to immediately after the next newline character or
2251: character sequence in the pattern. Which characters are interpreted as newlines
1.1.1.2 misho 2252: is controlled by the options passed to a compiling function or by a special
1.1 misho 2253: sequence at the start of the pattern, as described in the section entitled
2254: .\" HTML <a href="#newlines">
2255: .\" </a>
2256: "Newline conventions"
2257: .\"
2258: above. Note that the end of this type of comment is a literal newline sequence
2259: in the pattern; escape sequences that happen to represent a newline do not
2260: count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2261: default newline convention is in force:
2262: .sp
2263: abc #comment \en still comment
2264: .sp
2265: On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2266: a newline in the pattern. The sequence \en is still literal at this stage, so
2267: it does not terminate the comment. Only an actual character with the code value
2268: 0x0a (the default newline) does so.
2269: .
2270: .
2271: .\" HTML <a name="recursion"></a>
2272: .SH "RECURSIVE PATTERNS"
2273: .rs
2274: .sp
2275: Consider the problem of matching a string in parentheses, allowing for
2276: unlimited nested parentheses. Without the use of recursion, the best that can
2277: be done is to use a pattern that matches up to some fixed depth of nesting. It
2278: is not possible to handle an arbitrary nesting depth.
2279: .P
2280: For some time, Perl has provided a facility that allows regular expressions to
2281: recurse (amongst other things). It does this by interpolating Perl code in the
2282: expression at run time, and the code can refer to the expression itself. A Perl
2283: pattern using code interpolation to solve the parentheses problem can be
2284: created like this:
2285: .sp
2286: $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2287: .sp
2288: The (?p{...}) item interpolates Perl code at run time, and in this case refers
2289: recursively to the pattern in which it appears.
2290: .P
2291: Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2292: supports special syntax for recursion of the entire pattern, and also for
2293: individual subpattern recursion. After its introduction in PCRE and Python,
2294: this kind of recursion was subsequently introduced into Perl at release 5.10.
2295: .P
2296: A special item that consists of (? followed by a number greater than zero and a
2297: closing parenthesis is a recursive subroutine call of the subpattern of the
2298: given number, provided that it occurs inside that subpattern. (If not, it is a
2299: .\" HTML <a href="#subpatternsassubroutines">
2300: .\" </a>
2301: non-recursive subroutine
2302: .\"
2303: call, which is described in the next section.) The special item (?R) or (?0) is
2304: a recursive call of the entire regular expression.
2305: .P
2306: This PCRE pattern solves the nested parentheses problem (assume the
2307: PCRE_EXTENDED option is set so that white space is ignored):
2308: .sp
2309: \e( ( [^()]++ | (?R) )* \e)
2310: .sp
2311: First it matches an opening parenthesis. Then it matches any number of
2312: substrings which can either be a sequence of non-parentheses, or a recursive
2313: match of the pattern itself (that is, a correctly parenthesized substring).
2314: Finally there is a closing parenthesis. Note the use of a possessive quantifier
2315: to avoid backtracking into sequences of non-parentheses.
2316: .P
2317: If this were part of a larger pattern, you would not want to recurse the entire
2318: pattern, so instead you could use this:
2319: .sp
2320: ( \e( ( [^()]++ | (?1) )* \e) )
2321: .sp
2322: We have put the pattern into parentheses, and caused the recursion to refer to
2323: them instead of the whole pattern.
2324: .P
2325: In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2326: is made easier by the use of relative references. Instead of (?1) in the
2327: pattern above you can write (?-2) to refer to the second most recently opened
2328: parentheses preceding the recursion. In other words, a negative number counts
2329: capturing parentheses leftwards from the point at which it is encountered.
2330: .P
2331: It is also possible to refer to subsequently opened parentheses, by writing
2332: references such as (?+2). However, these cannot be recursive because the
2333: reference is not inside the parentheses that are referenced. They are always
2334: .\" HTML <a href="#subpatternsassubroutines">
2335: .\" </a>
2336: non-recursive subroutine
2337: .\"
2338: calls, as described in the next section.
2339: .P
2340: An alternative approach is to use named parentheses instead. The Perl syntax
2341: for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2342: could rewrite the above example as follows:
2343: .sp
2344: (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2345: .sp
2346: If there is more than one subpattern with the same name, the earliest one is
2347: used.
2348: .P
2349: This particular example pattern that we have been looking at contains nested
2350: unlimited repeats, and so the use of a possessive quantifier for matching
2351: strings of non-parentheses is important when applying the pattern to strings
2352: that do not match. For example, when this pattern is applied to
2353: .sp
2354: (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2355: .sp
2356: it yields "no match" quickly. However, if a possessive quantifier is not used,
2357: the match runs for a very long time indeed because there are so many different
2358: ways the + and * repeats can carve up the subject, and all have to be tested
2359: before failure can be reported.
2360: .P
2361: At the end of a match, the values of capturing parentheses are those from
2362: the outermost level. If you want to obtain intermediate values, a callout
2363: function can be used (see below and the
2364: .\" HREF
2365: \fBpcrecallout\fP
2366: .\"
2367: documentation). If the pattern above is matched against
2368: .sp
2369: (ab(cd)ef)
2370: .sp
2371: the value for the inner capturing parentheses (numbered 2) is "ef", which is
2372: the last value taken on at the top level. If a capturing subpattern is not
2373: matched at the top level, its final captured value is unset, even if it was
2374: (temporarily) set at a deeper level during the matching process.
2375: .P
2376: If there are more than 15 capturing parentheses in a pattern, PCRE has to
2377: obtain extra memory to store data during a recursion, which it does by using
2378: \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2379: be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2380: .P
2381: Do not confuse the (?R) item with the condition (R), which tests for recursion.
2382: Consider this pattern, which matches text in angle brackets, allowing for
2383: arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2384: recursing), whereas any characters are permitted at the outer level.
2385: .sp
2386: < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2387: .sp
2388: In this pattern, (?(R) is the start of a conditional subpattern, with two
2389: different alternatives for the recursive and non-recursive cases. The (?R) item
2390: is the actual recursive call.
2391: .
2392: .
2393: .\" HTML <a name="recursiondifference"></a>
2394: .SS "Differences in recursion processing between PCRE and Perl"
2395: .rs
2396: .sp
2397: Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2398: (like Python, but unlike Perl), a recursive subpattern call is always treated
2399: as an atomic group. That is, once it has matched some of the subject string, it
2400: is never re-entered, even if it contains untried alternatives and there is a
2401: subsequent matching failure. This can be illustrated by the following pattern,
2402: which purports to match a palindromic string that contains an odd number of
2403: characters (for example, "a", "aba", "abcba", "abcdcba"):
2404: .sp
2405: ^(.|(.)(?1)\e2)$
2406: .sp
2407: The idea is that it either matches a single character, or two identical
2408: characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2409: it does not if the pattern is longer than three characters. Consider the
2410: subject string "abcba":
2411: .P
2412: At the top level, the first character is matched, but as it is not at the end
2413: of the string, the first alternative fails; the second alternative is taken
2414: and the recursion kicks in. The recursive call to subpattern 1 successfully
2415: matches the next character ("b"). (Note that the beginning and end of line
2416: tests are not part of the recursion).
2417: .P
2418: Back at the top level, the next character ("c") is compared with what
2419: subpattern 2 matched, which was "a". This fails. Because the recursion is
2420: treated as an atomic group, there are now no backtracking points, and so the
2421: entire match fails. (Perl is able, at this point, to re-enter the recursion and
2422: try the second alternative.) However, if the pattern is written with the
2423: alternatives in the other order, things are different:
2424: .sp
2425: ^((.)(?1)\e2|.)$
2426: .sp
2427: This time, the recursing alternative is tried first, and continues to recurse
2428: until it runs out of characters, at which point the recursion fails. But this
2429: time we do have another alternative to try at the higher level. That is the big
2430: difference: in the previous case the remaining alternative is at a deeper
2431: recursion level, which PCRE cannot use.
2432: .P
2433: To change the pattern so that it matches all palindromic strings, not just
2434: those with an odd number of characters, it is tempting to change the pattern to
2435: this:
2436: .sp
2437: ^((.)(?1)\e2|.?)$
2438: .sp
2439: Again, this works in Perl, but not in PCRE, and for the same reason. When a
2440: deeper recursion has matched a single character, it cannot be entered again in
2441: order to match an empty string. The solution is to separate the two cases, and
2442: write out the odd and even cases as alternatives at the higher level:
2443: .sp
2444: ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2445: .sp
2446: If you want to match typical palindromic phrases, the pattern has to ignore all
2447: non-word characters, which can be done like this:
2448: .sp
2449: ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2450: .sp
2451: If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2452: man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2453: the use of the possessive quantifier *+ to avoid backtracking into sequences of
2454: non-word characters. Without this, PCRE takes a great deal longer (ten times or
2455: more) to match typical phrases, and Perl takes so long that you think it has
2456: gone into a loop.
2457: .P
2458: \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2459: string does not start with a palindrome that is shorter than the entire string.
2460: For example, although "abcba" is correctly matched, if the subject is "ababa",
2461: PCRE finds the palindrome "aba" at the start, then fails at top level because
2462: the end of the string does not follow. Once again, it cannot jump back into the
2463: recursion to try other alternatives, so the entire match fails.
2464: .P
2465: The second way in which PCRE and Perl differ in their recursion processing is
2466: in the handling of captured values. In Perl, when a subpattern is called
2467: recursively or as a subpattern (see the next section), it has no access to any
2468: values that were captured outside the recursion, whereas in PCRE these values
2469: can be referenced. Consider this pattern:
2470: .sp
2471: ^(.)(\e1|a(?2))
2472: .sp
2473: In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2474: then in the second group, when the back reference \e1 fails to match "b", the
2475: second alternative matches "a" and then recurses. In the recursion, \e1 does
2476: now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2477: match because inside the recursive call \e1 cannot access the externally set
2478: value.
2479: .
2480: .
2481: .\" HTML <a name="subpatternsassubroutines"></a>
2482: .SH "SUBPATTERNS AS SUBROUTINES"
2483: .rs
2484: .sp
2485: If the syntax for a recursive subpattern call (either by number or by
2486: name) is used outside the parentheses to which it refers, it operates like a
2487: subroutine in a programming language. The called subpattern may be defined
2488: before or after the reference. A numbered reference can be absolute or
2489: relative, as in these examples:
2490: .sp
2491: (...(absolute)...)...(?2)...
2492: (...(relative)...)...(?-1)...
2493: (...(?+1)...(relative)...
2494: .sp
2495: An earlier example pointed out that the pattern
2496: .sp
2497: (sens|respons)e and \e1ibility
2498: .sp
2499: matches "sense and sensibility" and "response and responsibility", but not
2500: "sense and responsibility". If instead the pattern
2501: .sp
2502: (sens|respons)e and (?1)ibility
2503: .sp
2504: is used, it does match "sense and responsibility" as well as the other two
2505: strings. Another example is given in the discussion of DEFINE above.
2506: .P
2507: All subroutine calls, whether recursive or not, are always treated as atomic
2508: groups. That is, once a subroutine has matched some of the subject string, it
2509: is never re-entered, even if it contains untried alternatives and there is a
2510: subsequent matching failure. Any capturing parentheses that are set during the
2511: subroutine call revert to their previous values afterwards.
2512: .P
2513: Processing options such as case-independence are fixed when a subpattern is
2514: defined, so if it is used as a subroutine, such options cannot be changed for
2515: different calls. For example, consider this pattern:
2516: .sp
2517: (abc)(?i:(?-1))
2518: .sp
2519: It matches "abcabc". It does not match "abcABC" because the change of
2520: processing option does not affect the called subpattern.
2521: .
2522: .
2523: .\" HTML <a name="onigurumasubroutines"></a>
2524: .SH "ONIGURUMA SUBROUTINE SYNTAX"
2525: .rs
2526: .sp
2527: For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2528: a number enclosed either in angle brackets or single quotes, is an alternative
2529: syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2530: are two of the examples used above, rewritten using this syntax:
2531: .sp
2532: (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2533: (sens|respons)e and \eg'1'ibility
2534: .sp
2535: PCRE supports an extension to Oniguruma: if a number is preceded by a
2536: plus or a minus sign it is taken as a relative reference. For example:
2537: .sp
2538: (abc)(?i:\eg<-1>)
2539: .sp
2540: Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2541: synonymous. The former is a back reference; the latter is a subroutine call.
2542: .
2543: .
2544: .SH CALLOUTS
2545: .rs
2546: .sp
2547: Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2548: code to be obeyed in the middle of matching a regular expression. This makes it
2549: possible, amongst other things, to extract different substrings that match the
2550: same pair of parentheses when there is a repetition.
2551: .P
2552: PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2553: code. The feature is called "callout". The caller of PCRE provides an external
1.1.1.2 misho 2554: function by putting its entry point in the global variable \fIpcre_callout\fP
2555: (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2556: variable contains NULL, which disables all calling out.
1.1 misho 2557: .P
2558: Within a regular expression, (?C) indicates the points at which the external
2559: function is to be called. If you want to identify different callout points, you
2560: can put a number less than 256 after the letter C. The default value is zero.
2561: For example, this pattern has two callout points:
2562: .sp
2563: (?C1)abc(?C2)def
2564: .sp
1.1.1.2 misho 2565: If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
1.1 misho 2566: automatically installed before each item in the pattern. They are all numbered
2567: 255.
2568: .P
1.1.1.2 misho 2569: During matching, when PCRE reaches a callout point, the external function is
2570: called. It is provided with the number of the callout, the position in the
2571: pattern, and, optionally, one item of data originally supplied by the caller of
2572: the matching function. The callout function may cause matching to proceed, to
2573: backtrack, or to fail altogether. A complete description of the interface to
2574: the callout function is given in the
1.1 misho 2575: .\" HREF
2576: \fBpcrecallout\fP
2577: .\"
2578: documentation.
2579: .
2580: .
2581: .\" HTML <a name="backtrackcontrol"></a>
2582: .SH "BACKTRACKING CONTROL"
2583: .rs
2584: .sp
2585: Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2586: are described in the Perl documentation as "experimental and subject to change
2587: or removal in a future version of Perl". It goes on to say: "Their usage in
2588: production code should be noted to avoid problems during upgrades." The same
2589: remarks apply to the PCRE features described in this section.
2590: .P
2591: Since these verbs are specifically related to backtracking, most of them can be
1.1.1.2 misho 2592: used only when the pattern is to be matched using one of the traditional
2593: matching functions, which use a backtracking algorithm. With the exception of
2594: (*FAIL), which behaves like a failing negative assertion, they cause an error
2595: if encountered by a DFA matching function.
1.1 misho 2596: .P
2597: If any of these verbs are used in an assertion or in a subpattern that is
2598: called as a subroutine (whether or not recursively), their effect is confined
2599: to that subpattern; it does not extend to the surrounding pattern, with one
2600: exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2601: a successful positive assertion \fIis\fP passed back when a match succeeds
2602: (compare capturing parentheses in assertions). Note that such subpatterns are
2603: processed as anchored at the point where they are tested. Note also that Perl's
1.1.1.3 ! misho 2604: treatment of subroutines and assertions is different in some cases.
1.1 misho 2605: .P
2606: The new verbs make use of what was previously invalid syntax: an opening
2607: parenthesis followed by an asterisk. They are generally of the form
2608: (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2609: depending on whether or not an argument is present. A name is any sequence of
1.1.1.3 ! misho 2610: characters that does not include a closing parenthesis. The maximum length of
! 2611: name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name
! 2612: is empty, that is, if the closing parenthesis immediately follows the colon,
! 2613: the effect is as if the colon were not there. Any number of these verbs may
! 2614: occur in a pattern.
! 2615: .
! 2616: .
! 2617: .\" HTML <a name="nooptimize"></a>
! 2618: .SS "Optimizations that affect backtracking verbs"
! 2619: .rs
! 2620: .sp
1.1 misho 2621: PCRE contains some optimizations that are used to speed up matching by running
2622: some checks at the start of each match attempt. For example, it may know the
2623: minimum length of matching subject, or that a particular character must be
2624: present. When one of these optimizations suppresses the running of a match, any
2625: included backtracking verbs will not, of course, be processed. You can suppress
2626: the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2627: when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
1.1.1.3 ! misho 2628: pattern with (*NO_START_OPT). There is more discussion of this option in the
! 2629: section entitled
! 2630: .\" HTML <a href="pcreapi.html#execoptions">
! 2631: .\" </a>
! 2632: "Option bits for \fBpcre_exec()\fP"
! 2633: .\"
! 2634: in the
! 2635: .\" HREF
! 2636: \fBpcreapi\fP
! 2637: .\"
! 2638: documentation.
1.1 misho 2639: .P
2640: Experiments with Perl suggest that it too has similar optimizations, sometimes
2641: leading to anomalous results.
2642: .
2643: .
2644: .SS "Verbs that act immediately"
2645: .rs
2646: .sp
2647: The following verbs act as soon as they are encountered. They may not be
2648: followed by a name.
2649: .sp
2650: (*ACCEPT)
2651: .sp
2652: This verb causes the match to end successfully, skipping the remainder of the
2653: pattern. However, when it is inside a subpattern that is called as a
2654: subroutine, only that subpattern is ended successfully. Matching then continues
2655: at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2656: far is captured. For example:
2657: .sp
2658: A((?:A|B(*ACCEPT)|C)D)
2659: .sp
2660: This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2661: the outer parentheses.
2662: .sp
2663: (*FAIL) or (*F)
2664: .sp
2665: This verb causes a matching failure, forcing backtracking to occur. It is
2666: equivalent to (?!) but easier to read. The Perl documentation notes that it is
2667: probably useful only when combined with (?{}) or (??{}). Those are, of course,
2668: Perl features that are not present in PCRE. The nearest equivalent is the
2669: callout feature, as for example in this pattern:
2670: .sp
2671: a+(?C)(*FAIL)
2672: .sp
2673: A match with the string "aaaa" always fails, but the callout is taken before
2674: each backtrack happens (in this example, 10 times).
2675: .
2676: .
2677: .SS "Recording which path was taken"
2678: .rs
2679: .sp
2680: There is one verb whose main purpose is to track how a match was arrived at,
2681: though it also has a secondary use in conjunction with advancing the match
2682: starting point (see (*SKIP) below).
2683: .sp
2684: (*MARK:NAME) or (*:NAME)
2685: .sp
2686: A name is always required with this verb. There may be as many instances of
2687: (*MARK) as you like in a pattern, and their names do not have to be unique.
2688: .P
2689: When a match succeeds, the name of the last-encountered (*MARK) on the matching
1.1.1.2 misho 2690: path is passed back to the caller as described in the section entitled
1.1 misho 2691: .\" HTML <a href="pcreapi.html#extradata">
2692: .\" </a>
1.1.1.2 misho 2693: "Extra data for \fBpcre_exec()\fP"
1.1 misho 2694: .\"
2695: in the
2696: .\" HREF
2697: \fBpcreapi\fP
2698: .\"
2699: documentation. Here is an example of \fBpcretest\fP output, where the /K
2700: modifier requests the retrieval and outputting of (*MARK) data:
2701: .sp
2702: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2703: data> XY
2704: 0: XY
2705: MK: A
2706: XZ
2707: 0: XZ
2708: MK: B
2709: .sp
2710: The (*MARK) name is tagged with "MK:" in this output, and in this example it
2711: indicates which of the two alternatives matched. This is a more efficient way
2712: of obtaining this information than putting each alternative in its own
2713: capturing parentheses.
2714: .P
2715: If (*MARK) is encountered in a positive assertion, its name is recorded and
2716: passed back if it is the last-encountered. This does not happen for negative
2717: assertions.
2718: .P
2719: After a partial match or a failed match, the name of the last encountered
2720: (*MARK) in the entire match process is returned. For example:
2721: .sp
2722: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2723: data> XP
2724: No match, mark = B
2725: .sp
2726: Note that in this unanchored example the mark is retained from the match
1.1.1.3 ! misho 2727: attempt that started at the letter "X" in the subject. Subsequent match
! 2728: attempts starting at "P" and then with an empty string do not get as far as the
! 2729: (*MARK) item, but nevertheless do not reset it.
! 2730: .P
! 2731: If you are interested in (*MARK) values after failed matches, you should
! 2732: probably set the PCRE_NO_START_OPTIMIZE option
! 2733: .\" HTML <a href="#nooptimize">
! 2734: .\" </a>
! 2735: (see above)
! 2736: .\"
! 2737: to ensure that the match is always attempted.
1.1 misho 2738: .
2739: .
2740: .SS "Verbs that act after backtracking"
2741: .rs
2742: .sp
2743: The following verbs do nothing when they are encountered. Matching continues
2744: with what follows, but if there is no subsequent match, causing a backtrack to
2745: the verb, a failure is forced. That is, backtracking cannot pass to the left of
2746: the verb. However, when one of these verbs appears inside an atomic group, its
2747: effect is confined to that group, because once the group has been matched,
2748: there is never any backtracking into it. In this situation, backtracking can
2749: "jump back" to the left of the entire atomic group. (Remember also, as stated
2750: above, that this localization also applies in subroutine calls and assertions.)
2751: .P
2752: These verbs differ in exactly what kind of failure occurs when backtracking
2753: reaches them.
2754: .sp
2755: (*COMMIT)
2756: .sp
2757: This verb, which may not be followed by a name, causes the whole match to fail
2758: outright if the rest of the pattern does not match. Even if the pattern is
2759: unanchored, no further attempts to find a match by advancing the starting point
2760: take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2761: finding a match at the current starting point, or not at all. For example:
2762: .sp
2763: a+(*COMMIT)b
2764: .sp
2765: This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2766: dynamic anchor, or "I've started, so I must finish." The name of the most
2767: recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2768: match failure.
2769: .P
2770: Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2771: unless PCRE's start-of-match optimizations are turned off, as shown in this
2772: \fBpcretest\fP example:
2773: .sp
2774: re> /(*COMMIT)abc/
2775: data> xyzabc
2776: 0: abc
2777: xyzabc\eY
2778: No match
2779: .sp
2780: PCRE knows that any match must start with "a", so the optimization skips along
2781: the subject to "a" before running the first match attempt, which succeeds. When
2782: the optimization is disabled by the \eY escape in the second subject, the match
2783: starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2784: starting points.
2785: .sp
2786: (*PRUNE) or (*PRUNE:NAME)
2787: .sp
2788: This verb causes the match to fail at the current starting position in the
2789: subject if the rest of the pattern does not match. If the pattern is
2790: unanchored, the normal "bumpalong" advance to the next starting character then
2791: happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2792: reached, or when matching to the right of (*PRUNE), but if there is no match to
2793: the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2794: (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2795: but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2796: The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2797: anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2798: .sp
2799: (*SKIP)
2800: .sp
2801: This verb, when given without a name, is like (*PRUNE), except that if the
2802: pattern is unanchored, the "bumpalong" advance is not to the next character,
2803: but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2804: signifies that whatever text was matched leading up to it cannot be part of a
2805: successful match. Consider:
2806: .sp
2807: a+(*SKIP)b
2808: .sp
2809: If the subject is "aaaac...", after the first match attempt fails (starting at
2810: the first character in the string), the starting point skips on to start the
2811: next attempt at "c". Note that a possessive quantifer does not have the same
2812: effect as this example; although it would suppress backtracking during the
2813: first match attempt, the second attempt would start at the second character
2814: instead of skipping on to "c".
2815: .sp
2816: (*SKIP:NAME)
2817: .sp
2818: When (*SKIP) has an associated name, its behaviour is modified. If the
2819: following pattern fails to match, the previous path through the pattern is
2820: searched for the most recent (*MARK) that has the same name. If one is found,
2821: the "bumpalong" advance is to the subject position that corresponds to that
2822: (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2823: matching name is found, the (*SKIP) is ignored.
2824: .sp
2825: (*THEN) or (*THEN:NAME)
2826: .sp
2827: This verb causes a skip to the next innermost alternative if the rest of the
2828: pattern does not match. That is, it cancels pending backtracking, but only
2829: within the current alternative. Its name comes from the observation that it can
2830: be used for a pattern-based if-then-else block:
2831: .sp
2832: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2833: .sp
2834: If the COND1 pattern matches, FOO is tried (and possibly further items after
2835: the end of the group if FOO succeeds); on failure, the matcher skips to the
2836: second alternative and tries COND2, without backtracking into COND1. The
2837: behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2838: If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2839: .P
2840: Note that a subpattern that does not contain a | character is just a part of
2841: the enclosing alternative; it is not a nested alternation with only one
2842: alternative. The effect of (*THEN) extends beyond such a subpattern to the
2843: enclosing alternative. Consider this pattern, where A, B, etc. are complex
2844: pattern fragments that do not contain any | characters at this level:
2845: .sp
2846: A (B(*THEN)C) | D
2847: .sp
2848: If A and B are matched, but there is a failure in C, matching does not
2849: backtrack into A; instead it moves to the next alternative, that is, D.
2850: However, if the subpattern containing (*THEN) is given an alternative, it
2851: behaves differently:
2852: .sp
2853: A (B(*THEN)C | (*FAIL)) | D
2854: .sp
2855: The effect of (*THEN) is now confined to the inner subpattern. After a failure
2856: in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2857: because there are no more alternatives to try. In this case, matching does now
2858: backtrack into A.
2859: .P
2860: Note also that a conditional subpattern is not considered as having two
2861: alternatives, because only one is ever used. In other words, the | character in
2862: a conditional subpattern has a different meaning. Ignoring white space,
2863: consider:
2864: .sp
2865: ^.*? (?(?=a) a | b(*THEN)c )
2866: .sp
2867: If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2868: it initially matches zero characters. The condition (?=a) then fails, the
2869: character "b" is matched, but "c" is not. At this point, matching does not
2870: backtrack to .*? as might perhaps be expected from the presence of the |
2871: character. The conditional subpattern is part of the single alternative that
2872: comprises the whole pattern, and so the match fails. (If there was a backtrack
2873: into .*?, allowing it to match "b", the match would succeed.)
2874: .P
2875: The verbs just described provide four different "strengths" of control when
2876: subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2877: next alternative. (*PRUNE) comes next, failing the match at the current
2878: starting position, but allowing an advance to the next character (for an
2879: unanchored pattern). (*SKIP) is similar, except that the advance may be more
2880: than one character. (*COMMIT) is the strongest, causing the entire match to
2881: fail.
2882: .P
2883: If more than one such verb is present in a pattern, the "strongest" one wins.
2884: For example, consider this pattern, where A, B, etc. are complex pattern
2885: fragments:
2886: .sp
2887: (A(*COMMIT)B(*THEN)C|D)
2888: .sp
2889: Once A has matched, PCRE is committed to this match, at the current starting
2890: position. If subsequently B matches, but C does not, the normal (*THEN) action
2891: of trying the next alternative (that is, D) does not happen because (*COMMIT)
2892: overrides.
2893: .
2894: .
2895: .SH "SEE ALSO"
2896: .rs
2897: .sp
2898: \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
1.1.1.2 misho 2899: \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
1.1 misho 2900: .
2901: .
2902: .SH AUTHOR
2903: .rs
2904: .sp
2905: .nf
2906: Philip Hazel
2907: University Computing Service
2908: Cambridge CB2 3QH, England.
2909: .fi
2910: .
2911: .
2912: .SH REVISION
2913: .rs
2914: .sp
2915: .nf
1.1.1.3 ! misho 2916: Last updated: 17 June 2012
1.1.1.2 misho 2917: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 2918: .fi
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