Annotation of embedaddon/pcre/doc/html/pcrepattern.html, revision 1.1.1.5
1.1 misho 1: <html>
2: <head>
3: <title>pcrepattern specification</title>
4: </head>
5: <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6: <h1>pcrepattern man page</h1>
7: <p>
8: Return to the <a href="index.html">PCRE index page</a>.
9: </p>
10: <p>
11: This page is part of the PCRE HTML documentation. It was generated automatically
12: from the original man page. If there is any nonsense in it, please consult the
13: man page, in case the conversion went wrong.
14: <br>
15: <ul>
16: <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
1.1.1.4 misho 17: <li><a name="TOC2" href="#SEC2">SPECIAL START-OF-PATTERN ITEMS</a>
18: <li><a name="TOC3" href="#SEC3">EBCDIC CHARACTER CODES</a>
19: <li><a name="TOC4" href="#SEC4">CHARACTERS AND METACHARACTERS</a>
20: <li><a name="TOC5" href="#SEC5">BACKSLASH</a>
21: <li><a name="TOC6" href="#SEC6">CIRCUMFLEX AND DOLLAR</a>
22: <li><a name="TOC7" href="#SEC7">FULL STOP (PERIOD, DOT) AND \N</a>
23: <li><a name="TOC8" href="#SEC8">MATCHING A SINGLE DATA UNIT</a>
24: <li><a name="TOC9" href="#SEC9">SQUARE BRACKETS AND CHARACTER CLASSES</a>
25: <li><a name="TOC10" href="#SEC10">POSIX CHARACTER CLASSES</a>
1.1.1.5 ! misho 26: <li><a name="TOC11" href="#SEC11">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a>
! 27: <li><a name="TOC12" href="#SEC12">VERTICAL BAR</a>
! 28: <li><a name="TOC13" href="#SEC13">INTERNAL OPTION SETTING</a>
! 29: <li><a name="TOC14" href="#SEC14">SUBPATTERNS</a>
! 30: <li><a name="TOC15" href="#SEC15">DUPLICATE SUBPATTERN NUMBERS</a>
! 31: <li><a name="TOC16" href="#SEC16">NAMED SUBPATTERNS</a>
! 32: <li><a name="TOC17" href="#SEC17">REPETITION</a>
! 33: <li><a name="TOC18" href="#SEC18">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
! 34: <li><a name="TOC19" href="#SEC19">BACK REFERENCES</a>
! 35: <li><a name="TOC20" href="#SEC20">ASSERTIONS</a>
! 36: <li><a name="TOC21" href="#SEC21">CONDITIONAL SUBPATTERNS</a>
! 37: <li><a name="TOC22" href="#SEC22">COMMENTS</a>
! 38: <li><a name="TOC23" href="#SEC23">RECURSIVE PATTERNS</a>
! 39: <li><a name="TOC24" href="#SEC24">SUBPATTERNS AS SUBROUTINES</a>
! 40: <li><a name="TOC25" href="#SEC25">ONIGURUMA SUBROUTINE SYNTAX</a>
! 41: <li><a name="TOC26" href="#SEC26">CALLOUTS</a>
! 42: <li><a name="TOC27" href="#SEC27">BACKTRACKING CONTROL</a>
! 43: <li><a name="TOC28" href="#SEC28">SEE ALSO</a>
! 44: <li><a name="TOC29" href="#SEC29">AUTHOR</a>
! 45: <li><a name="TOC30" href="#SEC30">REVISION</a>
1.1 misho 46: </ul>
47: <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
48: <P>
49: The syntax and semantics of the regular expressions that are supported by PCRE
50: are described in detail below. There is a quick-reference syntax summary in the
51: <a href="pcresyntax.html"><b>pcresyntax</b></a>
52: page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
53: also supports some alternative regular expression syntax (which does not
54: conflict with the Perl syntax) in order to provide some compatibility with
55: regular expressions in Python, .NET, and Oniguruma.
56: </P>
57: <P>
58: Perl's regular expressions are described in its own documentation, and
59: regular expressions in general are covered in a number of books, some of which
60: have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
61: published by O'Reilly, covers regular expressions in great detail. This
62: description of PCRE's regular expressions is intended as reference material.
63: </P>
64: <P>
1.1.1.4 misho 65: This document discusses the patterns that are supported by PCRE when one its
66: main matching functions, <b>pcre_exec()</b> (8-bit) or <b>pcre[16|32]_exec()</b>
67: (16- or 32-bit), is used. PCRE also has alternative matching functions,
68: <b>pcre_dfa_exec()</b> and <b>pcre[16|32_dfa_exec()</b>, which match using a
69: different algorithm that is not Perl-compatible. Some of the features discussed
70: below are not available when DFA matching is used. The advantages and
71: disadvantages of the alternative functions, and how they differ from the normal
72: functions, are discussed in the
73: <a href="pcrematching.html"><b>pcrematching</b></a>
74: page.
75: </P>
76: <br><a name="SEC2" href="#TOC1">SPECIAL START-OF-PATTERN ITEMS</a><br>
77: <P>
78: A number of options that can be passed to <b>pcre_compile()</b> can also be set
79: by special items at the start of a pattern. These are not Perl-compatible, but
80: are provided to make these options accessible to pattern writers who are not
81: able to change the program that processes the pattern. Any number of these
82: items may appear, but they must all be together right at the start of the
83: pattern string, and the letters must be in upper case.
84: </P>
85: <br><b>
86: UTF support
87: </b><br>
88: <P>
1.1 misho 89: The original operation of PCRE was on strings of one-byte characters. However,
1.1.1.4 misho 90: there is now also support for UTF-8 strings in the original library, an
91: extra library that supports 16-bit and UTF-16 character strings, and a
92: third library that supports 32-bit and UTF-32 character strings. To use these
1.1.1.2 misho 93: features, PCRE must be built to include appropriate support. When using UTF
1.1.1.4 misho 94: strings you must either call the compiling function with the PCRE_UTF8,
95: PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
96: these special sequences:
1.1 misho 97: <pre>
98: (*UTF8)
1.1.1.2 misho 99: (*UTF16)
1.1.1.4 misho 100: (*UTF32)
101: (*UTF)
1.1 misho 102: </pre>
1.1.1.4 misho 103: (*UTF) is a generic sequence that can be used with any of the libraries.
1.1.1.2 misho 104: Starting a pattern with such a sequence is equivalent to setting the relevant
1.1.1.4 misho 105: option. How setting a UTF mode affects pattern matching is mentioned in several
106: places below. There is also a summary of features in the
1.1 misho 107: <a href="pcreunicode.html"><b>pcreunicode</b></a>
108: page.
109: </P>
110: <P>
1.1.1.4 misho 111: Some applications that allow their users to supply patterns may wish to
112: restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
113: option is set at compile time, (*UTF) etc. are not allowed, and their
114: appearance causes an error.
115: </P>
116: <br><b>
117: Unicode property support
118: </b><br>
119: <P>
1.1.1.5 ! misho 120: Another special sequence that may appear at the start of a pattern is (*UCP).
1.1 misho 121: This has the same effect as setting the PCRE_UCP option: it causes sequences
122: such as \d and \w to use Unicode properties to determine character types,
123: instead of recognizing only characters with codes less than 128 via a lookup
124: table.
125: </P>
1.1.1.4 misho 126: <br><b>
1.1.1.5 ! misho 127: Disabling auto-possessification
! 128: </b><br>
! 129: <P>
! 130: If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
! 131: the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making
! 132: quantifiers possessive when what follows cannot match the repeated item. For
! 133: example, by default a+b is treated as a++b. For more details, see the
! 134: <a href="pcreapi.html"><b>pcreapi</b></a>
! 135: documentation.
! 136: </P>
! 137: <br><b>
1.1.1.4 misho 138: Disabling start-up optimizations
139: </b><br>
1.1 misho 140: <P>
141: If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
1.1.1.5 ! misho 142: PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables
! 143: several optimizations for quickly reaching "no match" results. For more
! 144: details, see the
! 145: <a href="pcreapi.html"><b>pcreapi</b></a>
! 146: documentation.
1.1 misho 147: <a name="newlines"></a></P>
1.1.1.4 misho 148: <br><b>
149: Newline conventions
150: </b><br>
1.1 misho 151: <P>
152: PCRE supports five different conventions for indicating line breaks in
153: strings: a single CR (carriage return) character, a single LF (linefeed)
154: character, the two-character sequence CRLF, any of the three preceding, or any
155: Unicode newline sequence. The
156: <a href="pcreapi.html"><b>pcreapi</b></a>
157: page has
158: <a href="pcreapi.html#newlines">further discussion</a>
159: about newlines, and shows how to set the newline convention in the
160: <i>options</i> arguments for the compiling and matching functions.
161: </P>
162: <P>
163: It is also possible to specify a newline convention by starting a pattern
164: string with one of the following five sequences:
165: <pre>
166: (*CR) carriage return
167: (*LF) linefeed
168: (*CRLF) carriage return, followed by linefeed
169: (*ANYCRLF) any of the three above
170: (*ANY) all Unicode newline sequences
171: </pre>
1.1.1.2 misho 172: These override the default and the options given to the compiling function. For
173: example, on a Unix system where LF is the default newline sequence, the pattern
1.1 misho 174: <pre>
175: (*CR)a.b
176: </pre>
177: changes the convention to CR. That pattern matches "a\nb" because LF is no
1.1.1.4 misho 178: longer a newline. If more than one of these settings is present, the last one
1.1 misho 179: is used.
180: </P>
181: <P>
1.1.1.4 misho 182: The newline convention affects where the circumflex and dollar assertions are
183: true. It also affects the interpretation of the dot metacharacter when
184: PCRE_DOTALL is not set, and the behaviour of \N. However, it does not affect
185: what the \R escape sequence matches. By default, this is any Unicode newline
186: sequence, for Perl compatibility. However, this can be changed; see the
1.1 misho 187: description of \R in the section entitled
188: <a href="#newlineseq">"Newline sequences"</a>
189: below. A change of \R setting can be combined with a change of newline
190: convention.
191: </P>
1.1.1.4 misho 192: <br><b>
193: Setting match and recursion limits
194: </b><br>
195: <P>
196: The caller of <b>pcre_exec()</b> can set a limit on the number of times the
197: internal <b>match()</b> function is called and on the maximum depth of
198: recursive calls. These facilities are provided to catch runaway matches that
199: are provoked by patterns with huge matching trees (a typical example is a
200: pattern with nested unlimited repeats) and to avoid running out of system stack
201: by too much recursion. When one of these limits is reached, <b>pcre_exec()</b>
202: gives an error return. The limits can also be set by items at the start of the
203: pattern of the form
204: <pre>
205: (*LIMIT_MATCH=d)
206: (*LIMIT_RECURSION=d)
207: </pre>
208: where d is any number of decimal digits. However, the value of the setting must
1.1.1.5 ! misho 209: be less than the value set (or defaulted) by the caller of <b>pcre_exec()</b>
! 210: for it to have any effect. In other words, the pattern writer can lower the
! 211: limits set by the programmer, but not raise them. If there is more than one
! 212: setting of one of these limits, the lower value is used.
1.1.1.4 misho 213: </P>
214: <br><a name="SEC3" href="#TOC1">EBCDIC CHARACTER CODES</a><br>
215: <P>
216: PCRE can be compiled to run in an environment that uses EBCDIC as its character
217: code rather than ASCII or Unicode (typically a mainframe system). In the
218: sections below, character code values are ASCII or Unicode; in an EBCDIC
219: environment these characters may have different code values, and there are no
220: code points greater than 255.
221: </P>
222: <br><a name="SEC4" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
1.1 misho 223: <P>
224: A regular expression is a pattern that is matched against a subject string from
225: left to right. Most characters stand for themselves in a pattern, and match the
226: corresponding characters in the subject. As a trivial example, the pattern
227: <pre>
228: The quick brown fox
229: </pre>
230: matches a portion of a subject string that is identical to itself. When
231: caseless matching is specified (the PCRE_CASELESS option), letters are matched
1.1.1.2 misho 232: independently of case. In a UTF mode, PCRE always understands the concept of
1.1 misho 233: case for characters whose values are less than 128, so caseless matching is
234: always possible. For characters with higher values, the concept of case is
235: supported if PCRE is compiled with Unicode property support, but not otherwise.
236: If you want to use caseless matching for characters 128 and above, you must
237: ensure that PCRE is compiled with Unicode property support as well as with
1.1.1.2 misho 238: UTF support.
1.1 misho 239: </P>
240: <P>
241: The power of regular expressions comes from the ability to include alternatives
242: and repetitions in the pattern. These are encoded in the pattern by the use of
243: <i>metacharacters</i>, which do not stand for themselves but instead are
244: interpreted in some special way.
245: </P>
246: <P>
247: There are two different sets of metacharacters: those that are recognized
248: anywhere in the pattern except within square brackets, and those that are
249: recognized within square brackets. Outside square brackets, the metacharacters
250: are as follows:
251: <pre>
252: \ general escape character with several uses
253: ^ assert start of string (or line, in multiline mode)
254: $ assert end of string (or line, in multiline mode)
255: . match any character except newline (by default)
256: [ start character class definition
257: | start of alternative branch
258: ( start subpattern
259: ) end subpattern
260: ? extends the meaning of (
261: also 0 or 1 quantifier
262: also quantifier minimizer
263: * 0 or more quantifier
264: + 1 or more quantifier
265: also "possessive quantifier"
266: { start min/max quantifier
267: </pre>
268: Part of a pattern that is in square brackets is called a "character class". In
269: a character class the only metacharacters are:
270: <pre>
271: \ general escape character
272: ^ negate the class, but only if the first character
273: - indicates character range
274: [ POSIX character class (only if followed by POSIX syntax)
275: ] terminates the character class
276: </pre>
277: The following sections describe the use of each of the metacharacters.
278: </P>
1.1.1.4 misho 279: <br><a name="SEC5" href="#TOC1">BACKSLASH</a><br>
1.1 misho 280: <P>
281: The backslash character has several uses. Firstly, if it is followed by a
282: character that is not a number or a letter, it takes away any special meaning
283: that character may have. This use of backslash as an escape character applies
284: both inside and outside character classes.
285: </P>
286: <P>
287: For example, if you want to match a * character, you write \* in the pattern.
288: This escaping action applies whether or not the following character would
289: otherwise be interpreted as a metacharacter, so it is always safe to precede a
290: non-alphanumeric with backslash to specify that it stands for itself. In
291: particular, if you want to match a backslash, you write \\.
292: </P>
293: <P>
1.1.1.2 misho 294: In a UTF mode, only ASCII numbers and letters have any special meaning after a
1.1 misho 295: backslash. All other characters (in particular, those whose codepoints are
296: greater than 127) are treated as literals.
297: </P>
298: <P>
1.1.1.5 ! misho 299: If a pattern is compiled with the PCRE_EXTENDED option, most white space in the
! 300: pattern (other than in a character class), and characters between a # outside a
! 301: character class and the next newline, inclusive, are ignored. An escaping
! 302: backslash can be used to include a white space or # character as part of the
! 303: pattern.
1.1 misho 304: </P>
305: <P>
306: If you want to remove the special meaning from a sequence of characters, you
307: can do so by putting them between \Q and \E. This is different from Perl in
308: that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
309: Perl, $ and @ cause variable interpolation. Note the following examples:
310: <pre>
311: Pattern PCRE matches Perl matches
312:
313: \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
314: \Qabc\$xyz\E abc\$xyz abc\$xyz
315: \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
316: </pre>
317: The \Q...\E sequence is recognized both inside and outside character classes.
318: An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
319: by \E later in the pattern, the literal interpretation continues to the end of
320: the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
321: a character class, this causes an error, because the character class is not
322: terminated.
323: <a name="digitsafterbackslash"></a></P>
324: <br><b>
325: Non-printing characters
326: </b><br>
327: <P>
328: A second use of backslash provides a way of encoding non-printing characters
329: in patterns in a visible manner. There is no restriction on the appearance of
330: non-printing characters, apart from the binary zero that terminates a pattern,
331: but when a pattern is being prepared by text editing, it is often easier to use
332: one of the following escape sequences than the binary character it represents:
333: <pre>
334: \a alarm, that is, the BEL character (hex 07)
335: \cx "control-x", where x is any ASCII character
336: \e escape (hex 1B)
1.1.1.3 misho 337: \f form feed (hex 0C)
1.1 misho 338: \n linefeed (hex 0A)
339: \r carriage return (hex 0D)
340: \t tab (hex 09)
1.1.1.5 ! misho 341: \0dd character with octal code 0dd
1.1 misho 342: \ddd character with octal code ddd, or back reference
1.1.1.5 ! misho 343: \o{ddd..} character with octal code ddd..
1.1 misho 344: \xhh character with hex code hh
345: \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
346: \uhhhh character with hex code hhhh (JavaScript mode only)
347: </pre>
1.1.1.4 misho 348: The precise effect of \cx on ASCII characters is as follows: if x is a lower
349: case letter, it is converted to upper case. Then bit 6 of the character (hex
350: 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A),
351: but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the
352: data item (byte or 16-bit value) following \c has a value greater than 127, a
353: compile-time error occurs. This locks out non-ASCII characters in all modes.
354: </P>
355: <P>
356: The \c facility was designed for use with ASCII characters, but with the
357: extension to Unicode it is even less useful than it once was. It is, however,
358: recognized when PCRE is compiled in EBCDIC mode, where data items are always
359: bytes. In this mode, all values are valid after \c. If the next character is a
360: lower case letter, it is converted to upper case. Then the 0xc0 bits of the
361: byte are inverted. Thus \cA becomes hex 01, as in ASCII (A is C1), but because
362: the EBCDIC letters are disjoint, \cZ becomes hex 29 (Z is E9), and other
363: characters also generate different values.
1.1 misho 364: </P>
365: <P>
366: After \0 up to two further octal digits are read. If there are fewer than two
367: digits, just those that are present are used. Thus the sequence \0\x\07
368: specifies two binary zeros followed by a BEL character (code value 7). Make
369: sure you supply two digits after the initial zero if the pattern character that
370: follows is itself an octal digit.
371: </P>
372: <P>
1.1.1.5 ! misho 373: The escape \o must be followed by a sequence of octal digits, enclosed in
! 374: braces. An error occurs if this is not the case. This escape is a recent
! 375: addition to Perl; it provides way of specifying character code points as octal
! 376: numbers greater than 0777, and it also allows octal numbers and back references
! 377: to be unambiguously specified.
! 378: </P>
! 379: <P>
! 380: For greater clarity and unambiguity, it is best to avoid following \ by a
! 381: digit greater than zero. Instead, use \o{} or \x{} to specify character
! 382: numbers, and \g{} to specify back references. The following paragraphs
! 383: describe the old, ambiguous syntax.
! 384: </P>
! 385: <P>
! 386: The handling of a backslash followed by a digit other than 0 is complicated,
! 387: and Perl has changed in recent releases, causing PCRE also to change. Outside a
! 388: character class, PCRE reads the digit and any following digits as a decimal
! 389: number. If the number is less than 8, or if there have been at least that many
1.1 misho 390: previous capturing left parentheses in the expression, the entire sequence is
391: taken as a <i>back reference</i>. A description of how this works is given
392: <a href="#backreferences">later,</a>
393: following the discussion of
394: <a href="#subpattern">parenthesized subpatterns.</a>
395: </P>
396: <P>
1.1.1.5 ! misho 397: Inside a character class, or if the decimal number following \ is greater than
! 398: 7 and there have not been that many capturing subpatterns, PCRE handles \8 and
! 399: \9 as the literal characters "8" and "9", and otherwise re-reads up to three
! 400: octal digits following the backslash, using them to generate a data character.
! 401: Any subsequent digits stand for themselves. For example:
1.1 misho 402: <pre>
1.1.1.4 misho 403: \040 is another way of writing an ASCII space
1.1 misho 404: \40 is the same, provided there are fewer than 40 previous capturing subpatterns
405: \7 is always a back reference
406: \11 might be a back reference, or another way of writing a tab
407: \011 is always a tab
408: \0113 is a tab followed by the character "3"
409: \113 might be a back reference, otherwise the character with octal code 113
1.1.1.2 misho 410: \377 might be a back reference, otherwise the value 255 (decimal)
1.1.1.5 ! misho 411: \81 is either a back reference, or the two characters "8" and "1"
1.1 misho 412: </pre>
1.1.1.5 ! misho 413: Note that octal values of 100 or greater that are specified using this syntax
! 414: must not be introduced by a leading zero, because no more than three octal
! 415: digits are ever read.
! 416: </P>
! 417: <P>
! 418: By default, after \x that is not followed by {, from zero to two hexadecimal
! 419: digits are read (letters can be in upper or lower case). Any number of
! 420: hexadecimal digits may appear between \x{ and }. If a character other than
! 421: a hexadecimal digit appears between \x{ and }, or if there is no terminating
! 422: }, an error occurs.
1.1 misho 423: </P>
424: <P>
1.1.1.5 ! misho 425: If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is
! 426: as just described only when it is followed by two hexadecimal digits.
! 427: Otherwise, it matches a literal "x" character. In JavaScript mode, support for
! 428: code points greater than 256 is provided by \u, which must be followed by
! 429: four hexadecimal digits; otherwise it matches a literal "u" character.
! 430: </P>
! 431: <P>
! 432: Characters whose value is less than 256 can be defined by either of the two
! 433: syntaxes for \x (or by \u in JavaScript mode). There is no difference in the
! 434: way they are handled. For example, \xdc is exactly the same as \x{dc} (or
! 435: \u00dc in JavaScript mode).
! 436: </P>
! 437: <br><b>
! 438: Constraints on character values
! 439: </b><br>
! 440: <P>
! 441: Characters that are specified using octal or hexadecimal numbers are
! 442: limited to certain values, as follows:
! 443: <pre>
! 444: 8-bit non-UTF mode less than 0x100
! 445: 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
! 446: 16-bit non-UTF mode less than 0x10000
! 447: 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
! 448: 32-bit non-UTF mode less than 0x100000000
! 449: 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint
! 450: </pre>
! 451: Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
! 452: "surrogate" codepoints), and 0xffef.
! 453: </P>
! 454: <br><b>
! 455: Escape sequences in character classes
! 456: </b><br>
! 457: <P>
1.1 misho 458: All the sequences that define a single character value can be used both inside
459: and outside character classes. In addition, inside a character class, \b is
460: interpreted as the backspace character (hex 08).
461: </P>
462: <P>
463: \N is not allowed in a character class. \B, \R, and \X are not special
464: inside a character class. Like other unrecognized escape sequences, they are
465: treated as the literal characters "B", "R", and "X" by default, but cause an
466: error if the PCRE_EXTRA option is set. Outside a character class, these
467: sequences have different meanings.
468: </P>
469: <br><b>
470: Unsupported escape sequences
471: </b><br>
472: <P>
473: In Perl, the sequences \l, \L, \u, and \U are recognized by its string
474: handler and used to modify the case of following characters. By default, PCRE
475: does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
476: option is set, \U matches a "U" character, and \u can be used to define a
477: character by code point, as described in the previous section.
478: </P>
479: <br><b>
480: Absolute and relative back references
481: </b><br>
482: <P>
483: The sequence \g followed by an unsigned or a negative number, optionally
484: enclosed in braces, is an absolute or relative back reference. A named back
485: reference can be coded as \g{name}. Back references are discussed
486: <a href="#backreferences">later,</a>
487: following the discussion of
488: <a href="#subpattern">parenthesized subpatterns.</a>
489: </P>
490: <br><b>
491: Absolute and relative subroutine calls
492: </b><br>
493: <P>
494: For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
495: a number enclosed either in angle brackets or single quotes, is an alternative
496: syntax for referencing a subpattern as a "subroutine". Details are discussed
497: <a href="#onigurumasubroutines">later.</a>
498: Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
499: synonymous. The former is a back reference; the latter is a
500: <a href="#subpatternsassubroutines">subroutine</a>
501: call.
502: <a name="genericchartypes"></a></P>
503: <br><b>
504: Generic character types
505: </b><br>
506: <P>
507: Another use of backslash is for specifying generic character types:
508: <pre>
509: \d any decimal digit
510: \D any character that is not a decimal digit
1.1.1.3 misho 511: \h any horizontal white space character
512: \H any character that is not a horizontal white space character
513: \s any white space character
514: \S any character that is not a white space character
515: \v any vertical white space character
516: \V any character that is not a vertical white space character
1.1 misho 517: \w any "word" character
518: \W any "non-word" character
519: </pre>
520: There is also the single sequence \N, which matches a non-newline character.
521: This is the same as
522: <a href="#fullstopdot">the "." metacharacter</a>
523: when PCRE_DOTALL is not set. Perl also uses \N to match characters by name;
524: PCRE does not support this.
525: </P>
526: <P>
527: Each pair of lower and upper case escape sequences partitions the complete set
528: of characters into two disjoint sets. Any given character matches one, and only
529: one, of each pair. The sequences can appear both inside and outside character
530: classes. They each match one character of the appropriate type. If the current
531: matching point is at the end of the subject string, all of them fail, because
532: there is no character to match.
533: </P>
534: <P>
1.1.1.5 ! misho 535: For compatibility with Perl, \s did not used to match the VT character (code
! 536: 11), which made it different from the the POSIX "space" class. However, Perl
! 537: added VT at release 5.18, and PCRE followed suit at release 8.34. The default
! 538: \s characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space
! 539: (32), which are defined as white space in the "C" locale. This list may vary if
! 540: locale-specific matching is taking place. For example, in some locales the
! 541: "non-breaking space" character (\xA0) is recognized as white space, and in
! 542: others the VT character is not.
1.1 misho 543: </P>
544: <P>
545: A "word" character is an underscore or any character that is a letter or digit.
546: By default, the definition of letters and digits is controlled by PCRE's
547: low-valued character tables, and may vary if locale-specific matching is taking
548: place (see
549: <a href="pcreapi.html#localesupport">"Locale support"</a>
550: in the
551: <a href="pcreapi.html"><b>pcreapi</b></a>
552: page). For example, in a French locale such as "fr_FR" in Unix-like systems,
1.1.1.5 ! misho 553: or "french" in Windows, some character codes greater than 127 are used for
1.1 misho 554: accented letters, and these are then matched by \w. The use of locales with
555: Unicode is discouraged.
556: </P>
557: <P>
1.1.1.5 ! misho 558: By default, characters whose code points are greater than 127 never match \d,
! 559: \s, or \w, and always match \D, \S, and \W, although this may vary for
! 560: characters in the range 128-255 when locale-specific matching is happening.
! 561: These escape sequences retain their original meanings from before Unicode
! 562: support was available, mainly for efficiency reasons. If PCRE is compiled with
! 563: Unicode property support, and the PCRE_UCP option is set, the behaviour is
! 564: changed so that Unicode properties are used to determine character types, as
! 565: follows:
! 566: <pre>
! 567: \d any character that matches \p{Nd} (decimal digit)
! 568: \s any character that matches \p{Z} or \h or \v
! 569: \w any character that matches \p{L} or \p{N}, plus underscore
1.1 misho 570: </pre>
571: The upper case escapes match the inverse sets of characters. Note that \d
572: matches only decimal digits, whereas \w matches any Unicode digit, as well as
573: any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and
574: \B because they are defined in terms of \w and \W. Matching these sequences
575: is noticeably slower when PCRE_UCP is set.
576: </P>
577: <P>
578: The sequences \h, \H, \v, and \V are features that were added to Perl at
579: release 5.10. In contrast to the other sequences, which match only ASCII
1.1.1.5 ! misho 580: characters by default, these always match certain high-valued code points,
1.1.1.2 misho 581: whether or not PCRE_UCP is set. The horizontal space characters are:
1.1 misho 582: <pre>
1.1.1.4 misho 583: U+0009 Horizontal tab (HT)
1.1 misho 584: U+0020 Space
585: U+00A0 Non-break space
586: U+1680 Ogham space mark
587: U+180E Mongolian vowel separator
588: U+2000 En quad
589: U+2001 Em quad
590: U+2002 En space
591: U+2003 Em space
592: U+2004 Three-per-em space
593: U+2005 Four-per-em space
594: U+2006 Six-per-em space
595: U+2007 Figure space
596: U+2008 Punctuation space
597: U+2009 Thin space
598: U+200A Hair space
599: U+202F Narrow no-break space
600: U+205F Medium mathematical space
601: U+3000 Ideographic space
602: </pre>
603: The vertical space characters are:
604: <pre>
1.1.1.4 misho 605: U+000A Linefeed (LF)
606: U+000B Vertical tab (VT)
607: U+000C Form feed (FF)
608: U+000D Carriage return (CR)
609: U+0085 Next line (NEL)
1.1 misho 610: U+2028 Line separator
611: U+2029 Paragraph separator
1.1.1.2 misho 612: </pre>
613: In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
614: relevant.
615: <a name="newlineseq"></a></P>
1.1 misho 616: <br><b>
617: Newline sequences
618: </b><br>
619: <P>
620: Outside a character class, by default, the escape sequence \R matches any
1.1.1.2 misho 621: Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
622: following:
1.1 misho 623: <pre>
624: (?>\r\n|\n|\x0b|\f|\r|\x85)
625: </pre>
626: This is an example of an "atomic group", details of which are given
627: <a href="#atomicgroup">below.</a>
628: This particular group matches either the two-character sequence CR followed by
629: LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
1.1.1.3 misho 630: U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
1.1 misho 631: line, U+0085). The two-character sequence is treated as a single unit that
632: cannot be split.
633: </P>
634: <P>
1.1.1.2 misho 635: In other modes, two additional characters whose codepoints are greater than 255
1.1 misho 636: are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
637: Unicode character property support is not needed for these characters to be
638: recognized.
639: </P>
640: <P>
641: It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
642: complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
643: either at compile time or when the pattern is matched. (BSR is an abbrevation
644: for "backslash R".) This can be made the default when PCRE is built; if this is
645: the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
646: It is also possible to specify these settings by starting a pattern string with
647: one of the following sequences:
648: <pre>
649: (*BSR_ANYCRLF) CR, LF, or CRLF only
650: (*BSR_UNICODE) any Unicode newline sequence
651: </pre>
1.1.1.2 misho 652: These override the default and the options given to the compiling function, but
653: they can themselves be overridden by options given to a matching function. Note
654: that these special settings, which are not Perl-compatible, are recognized only
655: at the very start of a pattern, and that they must be in upper case. If more
656: than one of them is present, the last one is used. They can be combined with a
657: change of newline convention; for example, a pattern can start with:
1.1 misho 658: <pre>
659: (*ANY)(*BSR_ANYCRLF)
660: </pre>
1.1.1.4 misho 661: They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
662: (*UCP) special sequences. Inside a character class, \R is treated as an
663: unrecognized escape sequence, and so matches the letter "R" by default, but
664: causes an error if PCRE_EXTRA is set.
1.1 misho 665: <a name="uniextseq"></a></P>
666: <br><b>
667: Unicode character properties
668: </b><br>
669: <P>
670: When PCRE is built with Unicode character property support, three additional
671: escape sequences that match characters with specific properties are available.
1.1.1.2 misho 672: When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
1.1 misho 673: characters whose codepoints are less than 256, but they do work in this mode.
674: The extra escape sequences are:
675: <pre>
676: \p{<i>xx</i>} a character with the <i>xx</i> property
677: \P{<i>xx</i>} a character without the <i>xx</i> property
1.1.1.4 misho 678: \X a Unicode extended grapheme cluster
1.1 misho 679: </pre>
680: The property names represented by <i>xx</i> above are limited to the Unicode
681: script names, the general category properties, "Any", which matches any
682: character (including newline), and some special PCRE properties (described
683: in the
684: <a href="#extraprops">next section).</a>
685: Other Perl properties such as "InMusicalSymbols" are not currently supported by
686: PCRE. Note that \P{Any} does not match any characters, so always causes a
687: match failure.
688: </P>
689: <P>
690: Sets of Unicode characters are defined as belonging to certain scripts. A
691: character from one of these sets can be matched using a script name. For
692: example:
693: <pre>
694: \p{Greek}
695: \P{Han}
696: </pre>
697: Those that are not part of an identified script are lumped together as
698: "Common". The current list of scripts is:
699: </P>
700: <P>
701: Arabic,
702: Armenian,
703: Avestan,
704: Balinese,
705: Bamum,
1.1.1.3 misho 706: Batak,
1.1 misho 707: Bengali,
708: Bopomofo,
1.1.1.3 misho 709: Brahmi,
1.1 misho 710: Braille,
711: Buginese,
712: Buhid,
713: Canadian_Aboriginal,
714: Carian,
1.1.1.3 misho 715: Chakma,
1.1 misho 716: Cham,
717: Cherokee,
718: Common,
719: Coptic,
720: Cuneiform,
721: Cypriot,
722: Cyrillic,
723: Deseret,
724: Devanagari,
725: Egyptian_Hieroglyphs,
726: Ethiopic,
727: Georgian,
728: Glagolitic,
729: Gothic,
730: Greek,
731: Gujarati,
732: Gurmukhi,
733: Han,
734: Hangul,
735: Hanunoo,
736: Hebrew,
737: Hiragana,
738: Imperial_Aramaic,
739: Inherited,
740: Inscriptional_Pahlavi,
741: Inscriptional_Parthian,
742: Javanese,
743: Kaithi,
744: Kannada,
745: Katakana,
746: Kayah_Li,
747: Kharoshthi,
748: Khmer,
749: Lao,
750: Latin,
751: Lepcha,
752: Limbu,
753: Linear_B,
754: Lisu,
755: Lycian,
756: Lydian,
757: Malayalam,
1.1.1.3 misho 758: Mandaic,
1.1 misho 759: Meetei_Mayek,
1.1.1.3 misho 760: Meroitic_Cursive,
761: Meroitic_Hieroglyphs,
762: Miao,
1.1 misho 763: Mongolian,
764: Myanmar,
765: New_Tai_Lue,
766: Nko,
767: Ogham,
768: Old_Italic,
769: Old_Persian,
770: Old_South_Arabian,
771: Old_Turkic,
772: Ol_Chiki,
773: Oriya,
774: Osmanya,
775: Phags_Pa,
776: Phoenician,
777: Rejang,
778: Runic,
779: Samaritan,
780: Saurashtra,
1.1.1.3 misho 781: Sharada,
1.1 misho 782: Shavian,
783: Sinhala,
1.1.1.3 misho 784: Sora_Sompeng,
1.1 misho 785: Sundanese,
786: Syloti_Nagri,
787: Syriac,
788: Tagalog,
789: Tagbanwa,
790: Tai_Le,
791: Tai_Tham,
792: Tai_Viet,
1.1.1.3 misho 793: Takri,
1.1 misho 794: Tamil,
795: Telugu,
796: Thaana,
797: Thai,
798: Tibetan,
799: Tifinagh,
800: Ugaritic,
801: Vai,
802: Yi.
803: </P>
804: <P>
805: Each character has exactly one Unicode general category property, specified by
806: a two-letter abbreviation. For compatibility with Perl, negation can be
807: specified by including a circumflex between the opening brace and the property
808: name. For example, \p{^Lu} is the same as \P{Lu}.
809: </P>
810: <P>
811: If only one letter is specified with \p or \P, it includes all the general
812: category properties that start with that letter. In this case, in the absence
813: of negation, the curly brackets in the escape sequence are optional; these two
814: examples have the same effect:
815: <pre>
816: \p{L}
817: \pL
818: </pre>
819: The following general category property codes are supported:
820: <pre>
821: C Other
822: Cc Control
823: Cf Format
824: Cn Unassigned
825: Co Private use
826: Cs Surrogate
827:
828: L Letter
829: Ll Lower case letter
830: Lm Modifier letter
831: Lo Other letter
832: Lt Title case letter
833: Lu Upper case letter
834:
835: M Mark
836: Mc Spacing mark
837: Me Enclosing mark
838: Mn Non-spacing mark
839:
840: N Number
841: Nd Decimal number
842: Nl Letter number
843: No Other number
844:
845: P Punctuation
846: Pc Connector punctuation
847: Pd Dash punctuation
848: Pe Close punctuation
849: Pf Final punctuation
850: Pi Initial punctuation
851: Po Other punctuation
852: Ps Open punctuation
853:
854: S Symbol
855: Sc Currency symbol
856: Sk Modifier symbol
857: Sm Mathematical symbol
858: So Other symbol
859:
860: Z Separator
861: Zl Line separator
862: Zp Paragraph separator
863: Zs Space separator
864: </pre>
865: The special property L& is also supported: it matches a character that has
866: the Lu, Ll, or Lt property, in other words, a letter that is not classified as
867: a modifier or "other".
868: </P>
869: <P>
870: The Cs (Surrogate) property applies only to characters in the range U+D800 to
1.1.1.2 misho 871: U+DFFF. Such characters are not valid in Unicode strings and so
872: cannot be tested by PCRE, unless UTF validity checking has been turned off
1.1.1.4 misho 873: (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
874: PCRE_NO_UTF32_CHECK in the
1.1 misho 875: <a href="pcreapi.html"><b>pcreapi</b></a>
876: page). Perl does not support the Cs property.
877: </P>
878: <P>
879: The long synonyms for property names that Perl supports (such as \p{Letter})
880: are not supported by PCRE, nor is it permitted to prefix any of these
881: properties with "Is".
882: </P>
883: <P>
884: No character that is in the Unicode table has the Cn (unassigned) property.
885: Instead, this property is assumed for any code point that is not in the
886: Unicode table.
887: </P>
888: <P>
889: Specifying caseless matching does not affect these escape sequences. For
1.1.1.4 misho 890: example, \p{Lu} always matches only upper case letters. This is different from
891: the behaviour of current versions of Perl.
1.1 misho 892: </P>
893: <P>
1.1.1.4 misho 894: Matching characters by Unicode property is not fast, because PCRE has to do a
895: multistage table lookup in order to find a character's property. That is why
896: the traditional escape sequences such as \d and \w do not use Unicode
897: properties in PCRE by default, though you can make them do so by setting the
898: PCRE_UCP option or by starting the pattern with (*UCP).
899: </P>
900: <br><b>
901: Extended grapheme clusters
902: </b><br>
903: <P>
904: The \X escape matches any number of Unicode characters that form an "extended
905: grapheme cluster", and treats the sequence as an atomic group
906: <a href="#atomicgroup">(see below).</a>
907: Up to and including release 8.31, PCRE matched an earlier, simpler definition
908: that was equivalent to
1.1 misho 909: <pre>
910: (?>\PM\pM*)
911: </pre>
1.1.1.4 misho 912: That is, it matched a character without the "mark" property, followed by zero
913: or more characters with the "mark" property. Characters with the "mark"
914: property are typically non-spacing accents that affect the preceding character.
1.1 misho 915: </P>
916: <P>
1.1.1.4 misho 917: This simple definition was extended in Unicode to include more complicated
918: kinds of composite character by giving each character a grapheme breaking
919: property, and creating rules that use these properties to define the boundaries
920: of extended grapheme clusters. In releases of PCRE later than 8.31, \X matches
921: one of these clusters.
1.1 misho 922: </P>
923: <P>
1.1.1.4 misho 924: \X always matches at least one character. Then it decides whether to add
925: additional characters according to the following rules for ending a cluster:
926: </P>
927: <P>
928: 1. End at the end of the subject string.
929: </P>
930: <P>
931: 2. Do not end between CR and LF; otherwise end after any control character.
932: </P>
933: <P>
934: 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
935: are of five types: L, V, T, LV, and LVT. An L character may be followed by an
936: L, V, LV, or LVT character; an LV or V character may be followed by a V or T
937: character; an LVT or T character may be follwed only by a T character.
938: </P>
939: <P>
940: 4. Do not end before extending characters or spacing marks. Characters with
941: the "mark" property always have the "extend" grapheme breaking property.
942: </P>
943: <P>
944: 5. Do not end after prepend characters.
945: </P>
946: <P>
947: 6. Otherwise, end the cluster.
1.1 misho 948: <a name="extraprops"></a></P>
949: <br><b>
950: PCRE's additional properties
951: </b><br>
952: <P>
1.1.1.4 misho 953: As well as the standard Unicode properties described above, PCRE supports four
954: more that make it possible to convert traditional escape sequences such as \w
1.1.1.5 ! misho 955: and \s to use Unicode properties. PCRE uses these non-standard, non-Perl
! 956: properties internally when PCRE_UCP is set. However, they may also be used
! 957: explicitly. These properties are:
1.1 misho 958: <pre>
959: Xan Any alphanumeric character
960: Xps Any POSIX space character
961: Xsp Any Perl space character
962: Xwd Any Perl "word" character
963: </pre>
964: Xan matches characters that have either the L (letter) or the N (number)
1.1.1.3 misho 965: property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
1.1 misho 966: carriage return, and any other character that has the Z (separator) property.
1.1.1.5 ! misho 967: Xsp is the same as Xps; it used to exclude vertical tab, for Perl
! 968: compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
! 969: matches the same characters as Xan, plus underscore.
1.1.1.4 misho 970: </P>
971: <P>
972: There is another non-standard property, Xuc, which matches any character that
973: can be represented by a Universal Character Name in C++ and other programming
974: languages. These are the characters $, @, ` (grave accent), and all characters
975: with Unicode code points greater than or equal to U+00A0, except for the
976: surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
977: excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH
978: where H is a hexadecimal digit. Note that the Xuc property does not match these
979: sequences but the characters that they represent.)
1.1 misho 980: <a name="resetmatchstart"></a></P>
981: <br><b>
982: Resetting the match start
983: </b><br>
984: <P>
985: The escape sequence \K causes any previously matched characters not to be
986: included in the final matched sequence. For example, the pattern:
987: <pre>
988: foo\Kbar
989: </pre>
990: matches "foobar", but reports that it has matched "bar". This feature is
991: similar to a lookbehind assertion
992: <a href="#lookbehind">(described below).</a>
993: However, in this case, the part of the subject before the real match does not
994: have to be of fixed length, as lookbehind assertions do. The use of \K does
995: not interfere with the setting of
996: <a href="#subpattern">captured substrings.</a>
997: For example, when the pattern
998: <pre>
999: (foo)\Kbar
1000: </pre>
1001: matches "foobar", the first substring is still set to "foo".
1002: </P>
1003: <P>
1004: Perl documents that the use of \K within assertions is "not well defined". In
1005: PCRE, \K is acted upon when it occurs inside positive assertions, but is
1006: ignored in negative assertions.
1007: <a name="smallassertions"></a></P>
1008: <br><b>
1009: Simple assertions
1010: </b><br>
1011: <P>
1012: The final use of backslash is for certain simple assertions. An assertion
1013: specifies a condition that has to be met at a particular point in a match,
1014: without consuming any characters from the subject string. The use of
1015: subpatterns for more complicated assertions is described
1016: <a href="#bigassertions">below.</a>
1017: The backslashed assertions are:
1018: <pre>
1019: \b matches at a word boundary
1020: \B matches when not at a word boundary
1021: \A matches at the start of the subject
1022: \Z matches at the end of the subject
1023: also matches before a newline at the end of the subject
1024: \z matches only at the end of the subject
1025: \G matches at the first matching position in the subject
1026: </pre>
1027: Inside a character class, \b has a different meaning; it matches the backspace
1028: character. If any other of these assertions appears in a character class, by
1029: default it matches the corresponding literal character (for example, \B
1030: matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
1031: escape sequence" error is generated instead.
1032: </P>
1033: <P>
1034: A word boundary is a position in the subject string where the current character
1035: and the previous character do not both match \w or \W (i.e. one matches
1036: \w and the other matches \W), or the start or end of the string if the
1.1.1.2 misho 1037: first or last character matches \w, respectively. In a UTF mode, the meanings
1.1 misho 1038: of \w and \W can be changed by setting the PCRE_UCP option. When this is
1039: done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start
1040: of word" or "end of word" metasequence. However, whatever follows \b normally
1041: determines which it is. For example, the fragment \ba matches "a" at the start
1042: of a word.
1043: </P>
1044: <P>
1045: The \A, \Z, and \z assertions differ from the traditional circumflex and
1046: dollar (described in the next section) in that they only ever match at the very
1047: start and end of the subject string, whatever options are set. Thus, they are
1048: independent of multiline mode. These three assertions are not affected by the
1049: PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
1050: circumflex and dollar metacharacters. However, if the <i>startoffset</i>
1051: argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
1052: at a point other than the beginning of the subject, \A can never match. The
1053: difference between \Z and \z is that \Z matches before a newline at the end
1054: of the string as well as at the very end, whereas \z matches only at the end.
1055: </P>
1056: <P>
1057: The \G assertion is true only when the current matching position is at the
1058: start point of the match, as specified by the <i>startoffset</i> argument of
1059: <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
1060: non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
1061: arguments, you can mimic Perl's /g option, and it is in this kind of
1062: implementation where \G can be useful.
1063: </P>
1064: <P>
1065: Note, however, that PCRE's interpretation of \G, as the start of the current
1066: match, is subtly different from Perl's, which defines it as the end of the
1067: previous match. In Perl, these can be different when the previously matched
1068: string was empty. Because PCRE does just one match at a time, it cannot
1069: reproduce this behaviour.
1070: </P>
1071: <P>
1072: If all the alternatives of a pattern begin with \G, the expression is anchored
1073: to the starting match position, and the "anchored" flag is set in the compiled
1074: regular expression.
1075: </P>
1.1.1.4 misho 1076: <br><a name="SEC6" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
1077: <P>
1078: The circumflex and dollar metacharacters are zero-width assertions. That is,
1079: they test for a particular condition being true without consuming any
1080: characters from the subject string.
1081: </P>
1.1 misho 1082: <P>
1083: Outside a character class, in the default matching mode, the circumflex
1.1.1.4 misho 1084: character is an assertion that is true only if the current matching point is at
1085: the start of the subject string. If the <i>startoffset</i> argument of
1.1 misho 1086: <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
1087: option is unset. Inside a character class, circumflex has an entirely different
1088: meaning
1089: <a href="#characterclass">(see below).</a>
1090: </P>
1091: <P>
1092: Circumflex need not be the first character of the pattern if a number of
1093: alternatives are involved, but it should be the first thing in each alternative
1094: in which it appears if the pattern is ever to match that branch. If all
1095: possible alternatives start with a circumflex, that is, if the pattern is
1096: constrained to match only at the start of the subject, it is said to be an
1097: "anchored" pattern. (There are also other constructs that can cause a pattern
1098: to be anchored.)
1099: </P>
1100: <P>
1.1.1.4 misho 1101: The dollar character is an assertion that is true only if the current matching
1102: point is at the end of the subject string, or immediately before a newline at
1103: the end of the string (by default). Note, however, that it does not actually
1104: match the newline. Dollar need not be the last character of the pattern if a
1105: number of alternatives are involved, but it should be the last item in any
1106: branch in which it appears. Dollar has no special meaning in a character class.
1.1 misho 1107: </P>
1108: <P>
1109: The meaning of dollar can be changed so that it matches only at the very end of
1110: the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
1111: does not affect the \Z assertion.
1112: </P>
1113: <P>
1114: The meanings of the circumflex and dollar characters are changed if the
1115: PCRE_MULTILINE option is set. When this is the case, a circumflex matches
1116: immediately after internal newlines as well as at the start of the subject
1117: string. It does not match after a newline that ends the string. A dollar
1118: matches before any newlines in the string, as well as at the very end, when
1119: PCRE_MULTILINE is set. When newline is specified as the two-character
1120: sequence CRLF, isolated CR and LF characters do not indicate newlines.
1121: </P>
1122: <P>
1123: For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
1124: \n represents a newline) in multiline mode, but not otherwise. Consequently,
1125: patterns that are anchored in single line mode because all branches start with
1126: ^ are not anchored in multiline mode, and a match for circumflex is possible
1127: when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
1128: PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
1129: </P>
1130: <P>
1131: Note that the sequences \A, \Z, and \z can be used to match the start and
1132: end of the subject in both modes, and if all branches of a pattern start with
1133: \A it is always anchored, whether or not PCRE_MULTILINE is set.
1134: <a name="fullstopdot"></a></P>
1.1.1.4 misho 1135: <br><a name="SEC7" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
1.1 misho 1136: <P>
1137: Outside a character class, a dot in the pattern matches any one character in
1138: the subject string except (by default) a character that signifies the end of a
1.1.1.2 misho 1139: line.
1.1 misho 1140: </P>
1141: <P>
1142: When a line ending is defined as a single character, dot never matches that
1143: character; when the two-character sequence CRLF is used, dot does not match CR
1144: if it is immediately followed by LF, but otherwise it matches all characters
1145: (including isolated CRs and LFs). When any Unicode line endings are being
1146: recognized, dot does not match CR or LF or any of the other line ending
1147: characters.
1148: </P>
1149: <P>
1150: The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
1151: option is set, a dot matches any one character, without exception. If the
1152: two-character sequence CRLF is present in the subject string, it takes two dots
1153: to match it.
1154: </P>
1155: <P>
1156: The handling of dot is entirely independent of the handling of circumflex and
1157: dollar, the only relationship being that they both involve newlines. Dot has no
1158: special meaning in a character class.
1159: </P>
1160: <P>
1161: The escape sequence \N behaves like a dot, except that it is not affected by
1162: the PCRE_DOTALL option. In other words, it matches any character except one
1163: that signifies the end of a line. Perl also uses \N to match characters by
1164: name; PCRE does not support this.
1165: </P>
1.1.1.4 misho 1166: <br><a name="SEC8" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br>
1.1 misho 1167: <P>
1.1.1.2 misho 1168: Outside a character class, the escape sequence \C matches any one data unit,
1169: whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1.1.1.4 misho 1170: byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1171: a 32-bit unit. Unlike a dot, \C always
1.1.1.2 misho 1172: matches line-ending characters. The feature is provided in Perl in order to
1173: match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1174: used. Because \C breaks up characters into individual data units, matching one
1175: unit with \C in a UTF mode means that the rest of the string may start with a
1176: malformed UTF character. This has undefined results, because PCRE assumes that
1177: it is dealing with valid UTF strings (and by default it checks this at the
1.1.1.4 misho 1178: start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1179: PCRE_NO_UTF32_CHECK option is used).
1.1 misho 1180: </P>
1181: <P>
1182: PCRE does not allow \C to appear in lookbehind assertions
1183: <a href="#lookbehind">(described below)</a>
1.1.1.2 misho 1184: in a UTF mode, because this would make it impossible to calculate the length of
1.1 misho 1185: the lookbehind.
1186: </P>
1187: <P>
1.1.1.2 misho 1188: In general, the \C escape sequence is best avoided. However, one
1189: way of using it that avoids the problem of malformed UTF characters is to use a
1190: lookahead to check the length of the next character, as in this pattern, which
1191: could be used with a UTF-8 string (ignore white space and line breaks):
1.1 misho 1192: <pre>
1193: (?| (?=[\x00-\x7f])(\C) |
1194: (?=[\x80-\x{7ff}])(\C)(\C) |
1195: (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
1196: (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
1197: </pre>
1198: A group that starts with (?| resets the capturing parentheses numbers in each
1199: alternative (see
1200: <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a>
1201: below). The assertions at the start of each branch check the next UTF-8
1202: character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1203: character's individual bytes are then captured by the appropriate number of
1204: groups.
1205: <a name="characterclass"></a></P>
1.1.1.4 misho 1206: <br><a name="SEC9" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
1.1 misho 1207: <P>
1208: An opening square bracket introduces a character class, terminated by a closing
1209: square bracket. A closing square bracket on its own is not special by default.
1210: However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1211: bracket causes a compile-time error. If a closing square bracket is required as
1212: a member of the class, it should be the first data character in the class
1213: (after an initial circumflex, if present) or escaped with a backslash.
1214: </P>
1215: <P>
1.1.1.2 misho 1216: A character class matches a single character in the subject. In a UTF mode, the
1217: character may be more than one data unit long. A matched character must be in
1218: the set of characters defined by the class, unless the first character in the
1219: class definition is a circumflex, in which case the subject character must not
1220: be in the set defined by the class. If a circumflex is actually required as a
1221: member of the class, ensure it is not the first character, or escape it with a
1.1 misho 1222: backslash.
1223: </P>
1224: <P>
1225: For example, the character class [aeiou] matches any lower case vowel, while
1226: [^aeiou] matches any character that is not a lower case vowel. Note that a
1227: circumflex is just a convenient notation for specifying the characters that
1228: are in the class by enumerating those that are not. A class that starts with a
1229: circumflex is not an assertion; it still consumes a character from the subject
1230: string, and therefore it fails if the current pointer is at the end of the
1231: string.
1232: </P>
1233: <P>
1.1.1.4 misho 1234: In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1235: can be included in a class as a literal string of data units, or by using the
1236: \x{ escaping mechanism.
1.1 misho 1237: </P>
1238: <P>
1239: When caseless matching is set, any letters in a class represent both their
1240: upper case and lower case versions, so for example, a caseless [aeiou] matches
1241: "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1.1.1.2 misho 1242: caseful version would. In a UTF mode, PCRE always understands the concept of
1.1 misho 1243: case for characters whose values are less than 128, so caseless matching is
1244: always possible. For characters with higher values, the concept of case is
1245: supported if PCRE is compiled with Unicode property support, but not otherwise.
1.1.1.2 misho 1246: If you want to use caseless matching in a UTF mode for characters 128 and
1247: above, you must ensure that PCRE is compiled with Unicode property support as
1248: well as with UTF support.
1.1 misho 1249: </P>
1250: <P>
1251: Characters that might indicate line breaks are never treated in any special way
1252: when matching character classes, whatever line-ending sequence is in use, and
1253: whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1254: such as [^a] always matches one of these characters.
1255: </P>
1256: <P>
1257: The minus (hyphen) character can be used to specify a range of characters in a
1258: character class. For example, [d-m] matches any letter between d and m,
1259: inclusive. If a minus character is required in a class, it must be escaped with
1260: a backslash or appear in a position where it cannot be interpreted as
1.1.1.5 ! misho 1261: indicating a range, typically as the first or last character in the class, or
! 1262: immediately after a range. For example, [b-d-z] matches letters in the range b
! 1263: to d, a hyphen character, or z.
1.1 misho 1264: </P>
1265: <P>
1266: It is not possible to have the literal character "]" as the end character of a
1267: range. A pattern such as [W-]46] is interpreted as a class of two characters
1268: ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1269: "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1270: the end of range, so [W-\]46] is interpreted as a class containing a range
1271: followed by two other characters. The octal or hexadecimal representation of
1272: "]" can also be used to end a range.
1273: </P>
1274: <P>
1.1.1.5 ! misho 1275: An error is generated if a POSIX character class (see below) or an escape
! 1276: sequence other than one that defines a single character appears at a point
! 1277: where a range ending character is expected. For example, [z-\xff] is valid,
! 1278: but [A-\d] and [A-[:digit:]] are not.
! 1279: </P>
! 1280: <P>
1.1 misho 1281: Ranges operate in the collating sequence of character values. They can also be
1.1.1.2 misho 1282: used for characters specified numerically, for example [\000-\037]. Ranges
1283: can include any characters that are valid for the current mode.
1.1 misho 1284: </P>
1285: <P>
1286: If a range that includes letters is used when caseless matching is set, it
1287: matches the letters in either case. For example, [W-c] is equivalent to
1.1.1.2 misho 1288: [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1.1 misho 1289: tables for a French locale are in use, [\xc8-\xcb] matches accented E
1.1.1.2 misho 1290: characters in both cases. In UTF modes, PCRE supports the concept of case for
1.1 misho 1291: characters with values greater than 128 only when it is compiled with Unicode
1292: property support.
1293: </P>
1294: <P>
1295: The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
1296: \V, \w, and \W may appear in a character class, and add the characters that
1297: they match to the class. For example, [\dABCDEF] matches any hexadecimal
1.1.1.2 misho 1298: digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w
1.1 misho 1299: and their upper case partners, just as it does when they appear outside a
1300: character class, as described in the section entitled
1301: <a href="#genericchartypes">"Generic character types"</a>
1302: above. The escape sequence \b has a different meaning inside a character
1303: class; it matches the backspace character. The sequences \B, \N, \R, and \X
1304: are not special inside a character class. Like any other unrecognized escape
1305: sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1306: default, but cause an error if the PCRE_EXTRA option is set.
1307: </P>
1308: <P>
1309: A circumflex can conveniently be used with the upper case character types to
1310: specify a more restricted set of characters than the matching lower case type.
1311: For example, the class [^\W_] matches any letter or digit, but not underscore,
1312: whereas [\w] includes underscore. A positive character class should be read as
1313: "something OR something OR ..." and a negative class as "NOT something AND NOT
1314: something AND NOT ...".
1315: </P>
1316: <P>
1317: The only metacharacters that are recognized in character classes are backslash,
1318: hyphen (only where it can be interpreted as specifying a range), circumflex
1319: (only at the start), opening square bracket (only when it can be interpreted as
1.1.1.5 ! misho 1320: introducing a POSIX class name, or for a special compatibility feature - see
! 1321: the next two sections), and the terminating closing square bracket. However,
! 1322: escaping other non-alphanumeric characters does no harm.
1.1 misho 1323: </P>
1.1.1.4 misho 1324: <br><a name="SEC10" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
1.1 misho 1325: <P>
1326: Perl supports the POSIX notation for character classes. This uses names
1327: enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1328: this notation. For example,
1329: <pre>
1330: [01[:alpha:]%]
1331: </pre>
1332: matches "0", "1", any alphabetic character, or "%". The supported class names
1333: are:
1334: <pre>
1335: alnum letters and digits
1336: alpha letters
1337: ascii character codes 0 - 127
1338: blank space or tab only
1339: cntrl control characters
1340: digit decimal digits (same as \d)
1341: graph printing characters, excluding space
1342: lower lower case letters
1343: print printing characters, including space
1344: punct printing characters, excluding letters and digits and space
1.1.1.5 ! misho 1345: space white space (the same as \s from PCRE 8.34)
1.1 misho 1346: upper upper case letters
1347: word "word" characters (same as \w)
1348: xdigit hexadecimal digits
1349: </pre>
1.1.1.5 ! misho 1350: The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
! 1351: and space (32). If locale-specific matching is taking place, the list of space
! 1352: characters may be different; there may be fewer or more of them. "Space" used
! 1353: to be different to \s, which did not include VT, for Perl compatibility.
! 1354: However, Perl changed at release 5.18, and PCRE followed at release 8.34.
! 1355: "Space" and \s now match the same set of characters.
1.1 misho 1356: </P>
1357: <P>
1358: The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1359: 5.8. Another Perl extension is negation, which is indicated by a ^ character
1360: after the colon. For example,
1361: <pre>
1362: [12[:^digit:]]
1363: </pre>
1364: matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1365: syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1366: supported, and an error is given if they are encountered.
1367: </P>
1368: <P>
1.1.1.5 ! misho 1369: By default, characters with values greater than 128 do not match any of the
! 1370: POSIX character classes. However, if the PCRE_UCP option is passed to
! 1371: <b>pcre_compile()</b>, some of the classes are changed so that Unicode character
! 1372: properties are used. This is achieved by replacing certain POSIX classes by
! 1373: other sequences, as follows:
1.1 misho 1374: <pre>
1375: [:alnum:] becomes \p{Xan}
1376: [:alpha:] becomes \p{L}
1377: [:blank:] becomes \h
1378: [:digit:] becomes \p{Nd}
1379: [:lower:] becomes \p{Ll}
1380: [:space:] becomes \p{Xps}
1381: [:upper:] becomes \p{Lu}
1382: [:word:] becomes \p{Xwd}
1383: </pre>
1.1.1.5 ! misho 1384: Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX
! 1385: classes are handled specially in UCP mode:
1.1 misho 1386: </P>
1.1.1.5 ! misho 1387: <P>
! 1388: [:graph:]
! 1389: This matches characters that have glyphs that mark the page when printed. In
! 1390: Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
! 1391: properties, except for:
! 1392: <pre>
! 1393: U+061C Arabic Letter Mark
! 1394: U+180E Mongolian Vowel Separator
! 1395: U+2066 - U+2069 Various "isolate"s
! 1396:
! 1397: </PRE>
! 1398: </P>
! 1399: <P>
! 1400: [:print:]
! 1401: This matches the same characters as [:graph:] plus space characters that are
! 1402: not controls, that is, characters with the Zs property.
! 1403: </P>
! 1404: <P>
! 1405: [:punct:]
! 1406: This matches all characters that have the Unicode P (punctuation) property,
! 1407: plus those characters whose code points are less than 128 that have the S
! 1408: (Symbol) property.
! 1409: </P>
! 1410: <P>
! 1411: The other POSIX classes are unchanged, and match only characters with code
! 1412: points less than 128.
! 1413: </P>
! 1414: <br><a name="SEC11" href="#TOC1">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a><br>
! 1415: <P>
! 1416: In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly
! 1417: syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of
! 1418: word". PCRE treats these items as follows:
! 1419: <pre>
! 1420: [[:<:]] is converted to \b(?=\w)
! 1421: [[:>:]] is converted to \b(?<=\w)
! 1422: </pre>
! 1423: Only these exact character sequences are recognized. A sequence such as
! 1424: [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is
! 1425: not compatible with Perl. It is provided to help migrations from other
! 1426: environments, and is best not used in any new patterns. Note that \b matches
! 1427: at the start and the end of a word (see
! 1428: <a href="#smallassertions">"Simple assertions"</a>
! 1429: above), and in a Perl-style pattern the preceding or following character
! 1430: normally shows which is wanted, without the need for the assertions that are
! 1431: used above in order to give exactly the POSIX behaviour.
! 1432: </P>
! 1433: <br><a name="SEC12" href="#TOC1">VERTICAL BAR</a><br>
1.1 misho 1434: <P>
1435: Vertical bar characters are used to separate alternative patterns. For example,
1436: the pattern
1437: <pre>
1438: gilbert|sullivan
1439: </pre>
1440: matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1441: and an empty alternative is permitted (matching the empty string). The matching
1442: process tries each alternative in turn, from left to right, and the first one
1443: that succeeds is used. If the alternatives are within a subpattern
1444: <a href="#subpattern">(defined below),</a>
1445: "succeeds" means matching the rest of the main pattern as well as the
1446: alternative in the subpattern.
1447: </P>
1.1.1.5 ! misho 1448: <br><a name="SEC13" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1.1 misho 1449: <P>
1450: The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1451: PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1452: the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1453: The option letters are
1454: <pre>
1455: i for PCRE_CASELESS
1456: m for PCRE_MULTILINE
1457: s for PCRE_DOTALL
1458: x for PCRE_EXTENDED
1459: </pre>
1460: For example, (?im) sets caseless, multiline matching. It is also possible to
1461: unset these options by preceding the letter with a hyphen, and a combined
1462: setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1463: PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1464: permitted. If a letter appears both before and after the hyphen, the option is
1465: unset.
1466: </P>
1467: <P>
1468: The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1469: changed in the same way as the Perl-compatible options by using the characters
1470: J, U and X respectively.
1471: </P>
1472: <P>
1473: When one of these option changes occurs at top level (that is, not inside
1474: subpattern parentheses), the change applies to the remainder of the pattern
1475: that follows. If the change is placed right at the start of a pattern, PCRE
1476: extracts it into the global options (and it will therefore show up in data
1477: extracted by the <b>pcre_fullinfo()</b> function).
1478: </P>
1479: <P>
1480: An option change within a subpattern (see below for a description of
1481: subpatterns) affects only that part of the subpattern that follows it, so
1482: <pre>
1483: (a(?i)b)c
1484: </pre>
1485: matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1486: By this means, options can be made to have different settings in different
1487: parts of the pattern. Any changes made in one alternative do carry on
1488: into subsequent branches within the same subpattern. For example,
1489: <pre>
1490: (a(?i)b|c)
1491: </pre>
1492: matches "ab", "aB", "c", and "C", even though when matching "C" the first
1493: branch is abandoned before the option setting. This is because the effects of
1494: option settings happen at compile time. There would be some very weird
1495: behaviour otherwise.
1496: </P>
1497: <P>
1498: <b>Note:</b> There are other PCRE-specific options that can be set by the
1.1.1.2 misho 1499: application when the compiling or matching functions are called. In some cases
1500: the pattern can contain special leading sequences such as (*CRLF) to override
1501: what the application has set or what has been defaulted. Details are given in
1502: the section entitled
1.1 misho 1503: <a href="#newlineseq">"Newline sequences"</a>
1.1.1.4 misho 1504: above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1505: sequences that can be used to set UTF and Unicode property modes; they are
1506: equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1507: options, respectively. The (*UTF) sequence is a generic version that can be
1508: used with any of the libraries. However, the application can set the
1509: PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1.1 misho 1510: <a name="subpattern"></a></P>
1.1.1.5 ! misho 1511: <br><a name="SEC14" href="#TOC1">SUBPATTERNS</a><br>
1.1 misho 1512: <P>
1513: Subpatterns are delimited by parentheses (round brackets), which can be nested.
1514: Turning part of a pattern into a subpattern does two things:
1515: <br>
1516: <br>
1517: 1. It localizes a set of alternatives. For example, the pattern
1518: <pre>
1519: cat(aract|erpillar|)
1520: </pre>
1521: matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1522: match "cataract", "erpillar" or an empty string.
1523: <br>
1524: <br>
1525: 2. It sets up the subpattern as a capturing subpattern. This means that, when
1526: the whole pattern matches, that portion of the subject string that matched the
1.1.1.2 misho 1527: subpattern is passed back to the caller via the <i>ovector</i> argument of the
1528: matching function. (This applies only to the traditional matching functions;
1529: the DFA matching functions do not support capturing.)
1530: </P>
1531: <P>
1532: Opening parentheses are counted from left to right (starting from 1) to obtain
1533: numbers for the capturing subpatterns. For example, if the string "the red
1534: king" is matched against the pattern
1.1 misho 1535: <pre>
1536: the ((red|white) (king|queen))
1537: </pre>
1538: the captured substrings are "red king", "red", and "king", and are numbered 1,
1539: 2, and 3, respectively.
1540: </P>
1541: <P>
1542: The fact that plain parentheses fulfil two functions is not always helpful.
1543: There are often times when a grouping subpattern is required without a
1544: capturing requirement. If an opening parenthesis is followed by a question mark
1545: and a colon, the subpattern does not do any capturing, and is not counted when
1546: computing the number of any subsequent capturing subpatterns. For example, if
1547: the string "the white queen" is matched against the pattern
1548: <pre>
1549: the ((?:red|white) (king|queen))
1550: </pre>
1551: the captured substrings are "white queen" and "queen", and are numbered 1 and
1552: 2. The maximum number of capturing subpatterns is 65535.
1553: </P>
1554: <P>
1555: As a convenient shorthand, if any option settings are required at the start of
1556: a non-capturing subpattern, the option letters may appear between the "?" and
1557: the ":". Thus the two patterns
1558: <pre>
1559: (?i:saturday|sunday)
1560: (?:(?i)saturday|sunday)
1561: </pre>
1562: match exactly the same set of strings. Because alternative branches are tried
1563: from left to right, and options are not reset until the end of the subpattern
1564: is reached, an option setting in one branch does affect subsequent branches, so
1565: the above patterns match "SUNDAY" as well as "Saturday".
1566: <a name="dupsubpatternnumber"></a></P>
1.1.1.5 ! misho 1567: <br><a name="SEC15" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1.1 misho 1568: <P>
1569: Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1570: the same numbers for its capturing parentheses. Such a subpattern starts with
1571: (?| and is itself a non-capturing subpattern. For example, consider this
1572: pattern:
1573: <pre>
1574: (?|(Sat)ur|(Sun))day
1575: </pre>
1576: Because the two alternatives are inside a (?| group, both sets of capturing
1577: parentheses are numbered one. Thus, when the pattern matches, you can look
1578: at captured substring number one, whichever alternative matched. This construct
1579: is useful when you want to capture part, but not all, of one of a number of
1580: alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1581: number is reset at the start of each branch. The numbers of any capturing
1582: parentheses that follow the subpattern start after the highest number used in
1583: any branch. The following example is taken from the Perl documentation. The
1584: numbers underneath show in which buffer the captured content will be stored.
1585: <pre>
1586: # before ---------------branch-reset----------- after
1587: / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1588: # 1 2 2 3 2 3 4
1589: </pre>
1590: A back reference to a numbered subpattern uses the most recent value that is
1591: set for that number by any subpattern. The following pattern matches "abcabc"
1592: or "defdef":
1593: <pre>
1594: /(?|(abc)|(def))\1/
1595: </pre>
1596: In contrast, a subroutine call to a numbered subpattern always refers to the
1597: first one in the pattern with the given number. The following pattern matches
1598: "abcabc" or "defabc":
1599: <pre>
1600: /(?|(abc)|(def))(?1)/
1601: </pre>
1602: If a
1603: <a href="#conditions">condition test</a>
1604: for a subpattern's having matched refers to a non-unique number, the test is
1605: true if any of the subpatterns of that number have matched.
1606: </P>
1607: <P>
1608: An alternative approach to using this "branch reset" feature is to use
1609: duplicate named subpatterns, as described in the next section.
1610: </P>
1.1.1.5 ! misho 1611: <br><a name="SEC16" href="#TOC1">NAMED SUBPATTERNS</a><br>
1.1 misho 1612: <P>
1613: Identifying capturing parentheses by number is simple, but it can be very hard
1614: to keep track of the numbers in complicated regular expressions. Furthermore,
1615: if an expression is modified, the numbers may change. To help with this
1616: difficulty, PCRE supports the naming of subpatterns. This feature was not
1617: added to Perl until release 5.10. Python had the feature earlier, and PCRE
1618: introduced it at release 4.0, using the Python syntax. PCRE now supports both
1619: the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1620: have different names, but PCRE does not.
1621: </P>
1622: <P>
1623: In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1624: (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1625: parentheses from other parts of the pattern, such as
1626: <a href="#backreferences">back references,</a>
1627: <a href="#recursion">recursion,</a>
1628: and
1629: <a href="#conditions">conditions,</a>
1630: can be made by name as well as by number.
1631: </P>
1632: <P>
1.1.1.5 ! misho 1633: Names consist of up to 32 alphanumeric characters and underscores, but must
! 1634: start with a non-digit. Named capturing parentheses are still allocated numbers
! 1635: as well as names, exactly as if the names were not present. The PCRE API
! 1636: provides function calls for extracting the name-to-number translation table
! 1637: from a compiled pattern. There is also a convenience function for extracting a
! 1638: captured substring by name.
1.1 misho 1639: </P>
1640: <P>
1641: By default, a name must be unique within a pattern, but it is possible to relax
1642: this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1643: names are also always permitted for subpatterns with the same number, set up as
1644: described in the previous section.) Duplicate names can be useful for patterns
1645: where only one instance of the named parentheses can match. Suppose you want to
1646: match the name of a weekday, either as a 3-letter abbreviation or as the full
1647: name, and in both cases you want to extract the abbreviation. This pattern
1648: (ignoring the line breaks) does the job:
1649: <pre>
1650: (?<DN>Mon|Fri|Sun)(?:day)?|
1651: (?<DN>Tue)(?:sday)?|
1652: (?<DN>Wed)(?:nesday)?|
1653: (?<DN>Thu)(?:rsday)?|
1654: (?<DN>Sat)(?:urday)?
1655: </pre>
1656: There are five capturing substrings, but only one is ever set after a match.
1657: (An alternative way of solving this problem is to use a "branch reset"
1658: subpattern, as described in the previous section.)
1659: </P>
1660: <P>
1661: The convenience function for extracting the data by name returns the substring
1662: for the first (and in this example, the only) subpattern of that name that
1663: matched. This saves searching to find which numbered subpattern it was.
1664: </P>
1665: <P>
1666: If you make a back reference to a non-unique named subpattern from elsewhere in
1.1.1.5 ! misho 1667: the pattern, the subpatterns to which the name refers are checked in the order
! 1668: in which they appear in the overall pattern. The first one that is set is used
! 1669: for the reference. For example, this pattern matches both "foofoo" and
! 1670: "barbar" but not "foobar" or "barfoo":
! 1671: <pre>
! 1672: (?:(?<n>foo)|(?<n>bar))\k<n>
! 1673:
! 1674: </PRE>
! 1675: </P>
! 1676: <P>
! 1677: If you make a subroutine call to a non-unique named subpattern, the one that
! 1678: corresponds to the first occurrence of the name is used. In the absence of
! 1679: duplicate numbers (see the previous section) this is the one with the lowest
! 1680: number.
! 1681: </P>
! 1682: <P>
! 1683: If you use a named reference in a condition
1.1 misho 1684: test (see the
1685: <a href="#conditions">section about conditions</a>
1686: below), either to check whether a subpattern has matched, or to check for
1687: recursion, all subpatterns with the same name are tested. If the condition is
1688: true for any one of them, the overall condition is true. This is the same
1689: behaviour as testing by number. For further details of the interfaces for
1690: handling named subpatterns, see the
1691: <a href="pcreapi.html"><b>pcreapi</b></a>
1692: documentation.
1693: </P>
1694: <P>
1695: <b>Warning:</b> You cannot use different names to distinguish between two
1696: subpatterns with the same number because PCRE uses only the numbers when
1697: matching. For this reason, an error is given at compile time if different names
1.1.1.5 ! misho 1698: are given to subpatterns with the same number. However, you can always give the
! 1699: same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
! 1700: set.
1.1 misho 1701: </P>
1.1.1.5 ! misho 1702: <br><a name="SEC17" href="#TOC1">REPETITION</a><br>
1.1 misho 1703: <P>
1704: Repetition is specified by quantifiers, which can follow any of the following
1705: items:
1706: <pre>
1707: a literal data character
1708: the dot metacharacter
1709: the \C escape sequence
1.1.1.2 misho 1710: the \X escape sequence
1.1 misho 1711: the \R escape sequence
1712: an escape such as \d or \pL that matches a single character
1713: a character class
1714: a back reference (see next section)
1715: a parenthesized subpattern (including assertions)
1716: a subroutine call to a subpattern (recursive or otherwise)
1717: </pre>
1718: The general repetition quantifier specifies a minimum and maximum number of
1719: permitted matches, by giving the two numbers in curly brackets (braces),
1720: separated by a comma. The numbers must be less than 65536, and the first must
1721: be less than or equal to the second. For example:
1722: <pre>
1723: z{2,4}
1724: </pre>
1725: matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1726: character. If the second number is omitted, but the comma is present, there is
1727: no upper limit; if the second number and the comma are both omitted, the
1728: quantifier specifies an exact number of required matches. Thus
1729: <pre>
1730: [aeiou]{3,}
1731: </pre>
1732: matches at least 3 successive vowels, but may match many more, while
1733: <pre>
1734: \d{8}
1735: </pre>
1736: matches exactly 8 digits. An opening curly bracket that appears in a position
1737: where a quantifier is not allowed, or one that does not match the syntax of a
1738: quantifier, is taken as a literal character. For example, {,6} is not a
1739: quantifier, but a literal string of four characters.
1740: </P>
1741: <P>
1.1.1.2 misho 1742: In UTF modes, quantifiers apply to characters rather than to individual data
1743: units. Thus, for example, \x{100}{2} matches two characters, each of
1744: which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1.1.1.4 misho 1745: \X{3} matches three Unicode extended grapheme clusters, each of which may be
1746: several data units long (and they may be of different lengths).
1.1 misho 1747: </P>
1748: <P>
1749: The quantifier {0} is permitted, causing the expression to behave as if the
1750: previous item and the quantifier were not present. This may be useful for
1751: subpatterns that are referenced as
1752: <a href="#subpatternsassubroutines">subroutines</a>
1753: from elsewhere in the pattern (but see also the section entitled
1754: <a href="#subdefine">"Defining subpatterns for use by reference only"</a>
1755: below). Items other than subpatterns that have a {0} quantifier are omitted
1756: from the compiled pattern.
1757: </P>
1758: <P>
1759: For convenience, the three most common quantifiers have single-character
1760: abbreviations:
1761: <pre>
1762: * is equivalent to {0,}
1763: + is equivalent to {1,}
1764: ? is equivalent to {0,1}
1765: </pre>
1766: It is possible to construct infinite loops by following a subpattern that can
1767: match no characters with a quantifier that has no upper limit, for example:
1768: <pre>
1769: (a?)*
1770: </pre>
1771: Earlier versions of Perl and PCRE used to give an error at compile time for
1772: such patterns. However, because there are cases where this can be useful, such
1773: patterns are now accepted, but if any repetition of the subpattern does in fact
1774: match no characters, the loop is forcibly broken.
1775: </P>
1776: <P>
1777: By default, the quantifiers are "greedy", that is, they match as much as
1778: possible (up to the maximum number of permitted times), without causing the
1779: rest of the pattern to fail. The classic example of where this gives problems
1780: is in trying to match comments in C programs. These appear between /* and */
1781: and within the comment, individual * and / characters may appear. An attempt to
1782: match C comments by applying the pattern
1783: <pre>
1784: /\*.*\*/
1785: </pre>
1786: to the string
1787: <pre>
1788: /* first comment */ not comment /* second comment */
1789: </pre>
1790: fails, because it matches the entire string owing to the greediness of the .*
1791: item.
1792: </P>
1793: <P>
1794: However, if a quantifier is followed by a question mark, it ceases to be
1795: greedy, and instead matches the minimum number of times possible, so the
1796: pattern
1797: <pre>
1798: /\*.*?\*/
1799: </pre>
1800: does the right thing with the C comments. The meaning of the various
1801: quantifiers is not otherwise changed, just the preferred number of matches.
1802: Do not confuse this use of question mark with its use as a quantifier in its
1803: own right. Because it has two uses, it can sometimes appear doubled, as in
1804: <pre>
1805: \d??\d
1806: </pre>
1807: which matches one digit by preference, but can match two if that is the only
1808: way the rest of the pattern matches.
1809: </P>
1810: <P>
1811: If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1812: the quantifiers are not greedy by default, but individual ones can be made
1813: greedy by following them with a question mark. In other words, it inverts the
1814: default behaviour.
1815: </P>
1816: <P>
1817: When a parenthesized subpattern is quantified with a minimum repeat count that
1818: is greater than 1 or with a limited maximum, more memory is required for the
1819: compiled pattern, in proportion to the size of the minimum or maximum.
1820: </P>
1821: <P>
1822: If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1823: to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1824: implicitly anchored, because whatever follows will be tried against every
1825: character position in the subject string, so there is no point in retrying the
1826: overall match at any position after the first. PCRE normally treats such a
1827: pattern as though it were preceded by \A.
1828: </P>
1829: <P>
1830: In cases where it is known that the subject string contains no newlines, it is
1831: worth setting PCRE_DOTALL in order to obtain this optimization, or
1832: alternatively using ^ to indicate anchoring explicitly.
1833: </P>
1834: <P>
1.1.1.4 misho 1835: However, there are some cases where the optimization cannot be used. When .*
1.1 misho 1836: is inside capturing parentheses that are the subject of a back reference
1837: elsewhere in the pattern, a match at the start may fail where a later one
1838: succeeds. Consider, for example:
1839: <pre>
1840: (.*)abc\1
1841: </pre>
1842: If the subject is "xyz123abc123" the match point is the fourth character. For
1843: this reason, such a pattern is not implicitly anchored.
1844: </P>
1845: <P>
1.1.1.4 misho 1846: Another case where implicit anchoring is not applied is when the leading .* is
1847: inside an atomic group. Once again, a match at the start may fail where a later
1848: one succeeds. Consider this pattern:
1849: <pre>
1850: (?>.*?a)b
1851: </pre>
1852: It matches "ab" in the subject "aab". The use of the backtracking control verbs
1853: (*PRUNE) and (*SKIP) also disable this optimization.
1854: </P>
1855: <P>
1.1 misho 1856: When a capturing subpattern is repeated, the value captured is the substring
1857: that matched the final iteration. For example, after
1858: <pre>
1859: (tweedle[dume]{3}\s*)+
1860: </pre>
1861: has matched "tweedledum tweedledee" the value of the captured substring is
1862: "tweedledee". However, if there are nested capturing subpatterns, the
1863: corresponding captured values may have been set in previous iterations. For
1864: example, after
1865: <pre>
1866: /(a|(b))+/
1867: </pre>
1868: matches "aba" the value of the second captured substring is "b".
1869: <a name="atomicgroup"></a></P>
1.1.1.5 ! misho 1870: <br><a name="SEC18" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1.1 misho 1871: <P>
1872: With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1873: repetition, failure of what follows normally causes the repeated item to be
1874: re-evaluated to see if a different number of repeats allows the rest of the
1875: pattern to match. Sometimes it is useful to prevent this, either to change the
1876: nature of the match, or to cause it fail earlier than it otherwise might, when
1877: the author of the pattern knows there is no point in carrying on.
1878: </P>
1879: <P>
1880: Consider, for example, the pattern \d+foo when applied to the subject line
1881: <pre>
1882: 123456bar
1883: </pre>
1884: After matching all 6 digits and then failing to match "foo", the normal
1885: action of the matcher is to try again with only 5 digits matching the \d+
1886: item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1887: (a term taken from Jeffrey Friedl's book) provides the means for specifying
1888: that once a subpattern has matched, it is not to be re-evaluated in this way.
1889: </P>
1890: <P>
1891: If we use atomic grouping for the previous example, the matcher gives up
1892: immediately on failing to match "foo" the first time. The notation is a kind of
1893: special parenthesis, starting with (?> as in this example:
1894: <pre>
1895: (?>\d+)foo
1896: </pre>
1897: This kind of parenthesis "locks up" the part of the pattern it contains once
1898: it has matched, and a failure further into the pattern is prevented from
1899: backtracking into it. Backtracking past it to previous items, however, works as
1900: normal.
1901: </P>
1902: <P>
1903: An alternative description is that a subpattern of this type matches the string
1904: of characters that an identical standalone pattern would match, if anchored at
1905: the current point in the subject string.
1906: </P>
1907: <P>
1908: Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1909: the above example can be thought of as a maximizing repeat that must swallow
1910: everything it can. So, while both \d+ and \d+? are prepared to adjust the
1911: number of digits they match in order to make the rest of the pattern match,
1912: (?>\d+) can only match an entire sequence of digits.
1913: </P>
1914: <P>
1915: Atomic groups in general can of course contain arbitrarily complicated
1916: subpatterns, and can be nested. However, when the subpattern for an atomic
1917: group is just a single repeated item, as in the example above, a simpler
1918: notation, called a "possessive quantifier" can be used. This consists of an
1919: additional + character following a quantifier. Using this notation, the
1920: previous example can be rewritten as
1921: <pre>
1922: \d++foo
1923: </pre>
1924: Note that a possessive quantifier can be used with an entire group, for
1925: example:
1926: <pre>
1927: (abc|xyz){2,3}+
1928: </pre>
1929: Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1930: option is ignored. They are a convenient notation for the simpler forms of
1931: atomic group. However, there is no difference in the meaning of a possessive
1932: quantifier and the equivalent atomic group, though there may be a performance
1933: difference; possessive quantifiers should be slightly faster.
1934: </P>
1935: <P>
1936: The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1937: Jeffrey Friedl originated the idea (and the name) in the first edition of his
1938: book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1939: package, and PCRE copied it from there. It ultimately found its way into Perl
1940: at release 5.10.
1941: </P>
1942: <P>
1943: PCRE has an optimization that automatically "possessifies" certain simple
1944: pattern constructs. For example, the sequence A+B is treated as A++B because
1945: there is no point in backtracking into a sequence of A's when B must follow.
1946: </P>
1947: <P>
1948: When a pattern contains an unlimited repeat inside a subpattern that can itself
1949: be repeated an unlimited number of times, the use of an atomic group is the
1950: only way to avoid some failing matches taking a very long time indeed. The
1951: pattern
1952: <pre>
1953: (\D+|<\d+>)*[!?]
1954: </pre>
1955: matches an unlimited number of substrings that either consist of non-digits, or
1956: digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1957: quickly. However, if it is applied to
1958: <pre>
1959: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1960: </pre>
1961: it takes a long time before reporting failure. This is because the string can
1962: be divided between the internal \D+ repeat and the external * repeat in a
1963: large number of ways, and all have to be tried. (The example uses [!?] rather
1964: than a single character at the end, because both PCRE and Perl have an
1965: optimization that allows for fast failure when a single character is used. They
1966: remember the last single character that is required for a match, and fail early
1967: if it is not present in the string.) If the pattern is changed so that it uses
1968: an atomic group, like this:
1969: <pre>
1970: ((?>\D+)|<\d+>)*[!?]
1971: </pre>
1972: sequences of non-digits cannot be broken, and failure happens quickly.
1973: <a name="backreferences"></a></P>
1.1.1.5 ! misho 1974: <br><a name="SEC19" href="#TOC1">BACK REFERENCES</a><br>
1.1 misho 1975: <P>
1976: Outside a character class, a backslash followed by a digit greater than 0 (and
1977: possibly further digits) is a back reference to a capturing subpattern earlier
1978: (that is, to its left) in the pattern, provided there have been that many
1979: previous capturing left parentheses.
1980: </P>
1981: <P>
1982: However, if the decimal number following the backslash is less than 10, it is
1983: always taken as a back reference, and causes an error only if there are not
1984: that many capturing left parentheses in the entire pattern. In other words, the
1985: parentheses that are referenced need not be to the left of the reference for
1986: numbers less than 10. A "forward back reference" of this type can make sense
1987: when a repetition is involved and the subpattern to the right has participated
1988: in an earlier iteration.
1989: </P>
1990: <P>
1991: It is not possible to have a numerical "forward back reference" to a subpattern
1992: whose number is 10 or more using this syntax because a sequence such as \50 is
1993: interpreted as a character defined in octal. See the subsection entitled
1994: "Non-printing characters"
1995: <a href="#digitsafterbackslash">above</a>
1996: for further details of the handling of digits following a backslash. There is
1997: no such problem when named parentheses are used. A back reference to any
1998: subpattern is possible using named parentheses (see below).
1999: </P>
2000: <P>
2001: Another way of avoiding the ambiguity inherent in the use of digits following a
2002: backslash is to use the \g escape sequence. This escape must be followed by an
2003: unsigned number or a negative number, optionally enclosed in braces. These
2004: examples are all identical:
2005: <pre>
2006: (ring), \1
2007: (ring), \g1
2008: (ring), \g{1}
2009: </pre>
2010: An unsigned number specifies an absolute reference without the ambiguity that
2011: is present in the older syntax. It is also useful when literal digits follow
2012: the reference. A negative number is a relative reference. Consider this
2013: example:
2014: <pre>
2015: (abc(def)ghi)\g{-1}
2016: </pre>
2017: The sequence \g{-1} is a reference to the most recently started capturing
2018: subpattern before \g, that is, is it equivalent to \2 in this example.
2019: Similarly, \g{-2} would be equivalent to \1. The use of relative references
2020: can be helpful in long patterns, and also in patterns that are created by
2021: joining together fragments that contain references within themselves.
2022: </P>
2023: <P>
2024: A back reference matches whatever actually matched the capturing subpattern in
2025: the current subject string, rather than anything matching the subpattern
2026: itself (see
2027: <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
2028: below for a way of doing that). So the pattern
2029: <pre>
2030: (sens|respons)e and \1ibility
2031: </pre>
2032: matches "sense and sensibility" and "response and responsibility", but not
2033: "sense and responsibility". If caseful matching is in force at the time of the
2034: back reference, the case of letters is relevant. For example,
2035: <pre>
2036: ((?i)rah)\s+\1
2037: </pre>
2038: matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
2039: capturing subpattern is matched caselessly.
2040: </P>
2041: <P>
2042: There are several different ways of writing back references to named
2043: subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
2044: \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
2045: back reference syntax, in which \g can be used for both numeric and named
2046: references, is also supported. We could rewrite the above example in any of
2047: the following ways:
2048: <pre>
2049: (?<p1>(?i)rah)\s+\k<p1>
2050: (?'p1'(?i)rah)\s+\k{p1}
2051: (?P<p1>(?i)rah)\s+(?P=p1)
2052: (?<p1>(?i)rah)\s+\g{p1}
2053: </pre>
2054: A subpattern that is referenced by name may appear in the pattern before or
2055: after the reference.
2056: </P>
2057: <P>
2058: There may be more than one back reference to the same subpattern. If a
2059: subpattern has not actually been used in a particular match, any back
2060: references to it always fail by default. For example, the pattern
2061: <pre>
2062: (a|(bc))\2
2063: </pre>
2064: always fails if it starts to match "a" rather than "bc". However, if the
2065: PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
2066: unset value matches an empty string.
2067: </P>
2068: <P>
2069: Because there may be many capturing parentheses in a pattern, all digits
2070: following a backslash are taken as part of a potential back reference number.
2071: If the pattern continues with a digit character, some delimiter must be used to
2072: terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1.1.1.3 misho 2073: white space. Otherwise, the \g{ syntax or an empty comment (see
1.1 misho 2074: <a href="#comments">"Comments"</a>
2075: below) can be used.
2076: </P>
2077: <br><b>
2078: Recursive back references
2079: </b><br>
2080: <P>
2081: A back reference that occurs inside the parentheses to which it refers fails
2082: when the subpattern is first used, so, for example, (a\1) never matches.
2083: However, such references can be useful inside repeated subpatterns. For
2084: example, the pattern
2085: <pre>
2086: (a|b\1)+
2087: </pre>
2088: matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
2089: the subpattern, the back reference matches the character string corresponding
2090: to the previous iteration. In order for this to work, the pattern must be such
2091: that the first iteration does not need to match the back reference. This can be
2092: done using alternation, as in the example above, or by a quantifier with a
2093: minimum of zero.
2094: </P>
2095: <P>
2096: Back references of this type cause the group that they reference to be treated
2097: as an
2098: <a href="#atomicgroup">atomic group.</a>
2099: Once the whole group has been matched, a subsequent matching failure cannot
2100: cause backtracking into the middle of the group.
2101: <a name="bigassertions"></a></P>
1.1.1.5 ! misho 2102: <br><a name="SEC20" href="#TOC1">ASSERTIONS</a><br>
1.1 misho 2103: <P>
2104: An assertion is a test on the characters following or preceding the current
2105: matching point that does not actually consume any characters. The simple
2106: assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
2107: <a href="#smallassertions">above.</a>
2108: </P>
2109: <P>
2110: More complicated assertions are coded as subpatterns. There are two kinds:
2111: those that look ahead of the current position in the subject string, and those
2112: that look behind it. An assertion subpattern is matched in the normal way,
2113: except that it does not cause the current matching position to be changed.
2114: </P>
2115: <P>
2116: Assertion subpatterns are not capturing subpatterns. If such an assertion
2117: contains capturing subpatterns within it, these are counted for the purposes of
2118: numbering the capturing subpatterns in the whole pattern. However, substring
1.1.1.4 misho 2119: capturing is carried out only for positive assertions. (Perl sometimes, but not
2120: always, does do capturing in negative assertions.)
1.1 misho 2121: </P>
2122: <P>
2123: For compatibility with Perl, assertion subpatterns may be repeated; though
2124: it makes no sense to assert the same thing several times, the side effect of
2125: capturing parentheses may occasionally be useful. In practice, there only three
2126: cases:
2127: <br>
2128: <br>
2129: (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2130: However, it may contain internal capturing parenthesized groups that are called
2131: from elsewhere via the
2132: <a href="#subpatternsassubroutines">subroutine mechanism.</a>
2133: <br>
2134: <br>
2135: (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2136: were {0,1}. At run time, the rest of the pattern match is tried with and
2137: without the assertion, the order depending on the greediness of the quantifier.
2138: <br>
2139: <br>
2140: (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2141: The assertion is obeyed just once when encountered during matching.
2142: </P>
2143: <br><b>
2144: Lookahead assertions
2145: </b><br>
2146: <P>
2147: Lookahead assertions start with (?= for positive assertions and (?! for
2148: negative assertions. For example,
2149: <pre>
2150: \w+(?=;)
2151: </pre>
2152: matches a word followed by a semicolon, but does not include the semicolon in
2153: the match, and
2154: <pre>
2155: foo(?!bar)
2156: </pre>
2157: matches any occurrence of "foo" that is not followed by "bar". Note that the
2158: apparently similar pattern
2159: <pre>
2160: (?!foo)bar
2161: </pre>
2162: does not find an occurrence of "bar" that is preceded by something other than
2163: "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
2164: (?!foo) is always true when the next three characters are "bar". A
2165: lookbehind assertion is needed to achieve the other effect.
2166: </P>
2167: <P>
2168: If you want to force a matching failure at some point in a pattern, the most
2169: convenient way to do it is with (?!) because an empty string always matches, so
2170: an assertion that requires there not to be an empty string must always fail.
2171: The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2172: <a name="lookbehind"></a></P>
2173: <br><b>
2174: Lookbehind assertions
2175: </b><br>
2176: <P>
2177: Lookbehind assertions start with (?<= for positive assertions and (?<! for
2178: negative assertions. For example,
2179: <pre>
2180: (?<!foo)bar
2181: </pre>
2182: does find an occurrence of "bar" that is not preceded by "foo". The contents of
2183: a lookbehind assertion are restricted such that all the strings it matches must
2184: have a fixed length. However, if there are several top-level alternatives, they
2185: do not all have to have the same fixed length. Thus
2186: <pre>
2187: (?<=bullock|donkey)
2188: </pre>
2189: is permitted, but
2190: <pre>
2191: (?<!dogs?|cats?)
2192: </pre>
2193: causes an error at compile time. Branches that match different length strings
2194: are permitted only at the top level of a lookbehind assertion. This is an
2195: extension compared with Perl, which requires all branches to match the same
2196: length of string. An assertion such as
2197: <pre>
2198: (?<=ab(c|de))
2199: </pre>
2200: is not permitted, because its single top-level branch can match two different
2201: lengths, but it is acceptable to PCRE if rewritten to use two top-level
2202: branches:
2203: <pre>
2204: (?<=abc|abde)
2205: </pre>
2206: In some cases, the escape sequence \K
2207: <a href="#resetmatchstart">(see above)</a>
2208: can be used instead of a lookbehind assertion to get round the fixed-length
2209: restriction.
2210: </P>
2211: <P>
2212: The implementation of lookbehind assertions is, for each alternative, to
2213: temporarily move the current position back by the fixed length and then try to
2214: match. If there are insufficient characters before the current position, the
2215: assertion fails.
2216: </P>
2217: <P>
1.1.1.2 misho 2218: In a UTF mode, PCRE does not allow the \C escape (which matches a single data
2219: unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2220: it impossible to calculate the length of the lookbehind. The \X and \R
2221: escapes, which can match different numbers of data units, are also not
2222: permitted.
1.1 misho 2223: </P>
2224: <P>
2225: <a href="#subpatternsassubroutines">"Subroutine"</a>
2226: calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2227: as the subpattern matches a fixed-length string.
2228: <a href="#recursion">Recursion,</a>
2229: however, is not supported.
2230: </P>
2231: <P>
2232: Possessive quantifiers can be used in conjunction with lookbehind assertions to
2233: specify efficient matching of fixed-length strings at the end of subject
2234: strings. Consider a simple pattern such as
2235: <pre>
2236: abcd$
2237: </pre>
2238: when applied to a long string that does not match. Because matching proceeds
2239: from left to right, PCRE will look for each "a" in the subject and then see if
2240: what follows matches the rest of the pattern. If the pattern is specified as
2241: <pre>
2242: ^.*abcd$
2243: </pre>
2244: the initial .* matches the entire string at first, but when this fails (because
2245: there is no following "a"), it backtracks to match all but the last character,
2246: then all but the last two characters, and so on. Once again the search for "a"
2247: covers the entire string, from right to left, so we are no better off. However,
2248: if the pattern is written as
2249: <pre>
2250: ^.*+(?<=abcd)
2251: </pre>
2252: there can be no backtracking for the .*+ item; it can match only the entire
2253: string. The subsequent lookbehind assertion does a single test on the last four
2254: characters. If it fails, the match fails immediately. For long strings, this
2255: approach makes a significant difference to the processing time.
2256: </P>
2257: <br><b>
2258: Using multiple assertions
2259: </b><br>
2260: <P>
2261: Several assertions (of any sort) may occur in succession. For example,
2262: <pre>
2263: (?<=\d{3})(?<!999)foo
2264: </pre>
2265: matches "foo" preceded by three digits that are not "999". Notice that each of
2266: the assertions is applied independently at the same point in the subject
2267: string. First there is a check that the previous three characters are all
2268: digits, and then there is a check that the same three characters are not "999".
2269: This pattern does <i>not</i> match "foo" preceded by six characters, the first
2270: of which are digits and the last three of which are not "999". For example, it
2271: doesn't match "123abcfoo". A pattern to do that is
2272: <pre>
2273: (?<=\d{3}...)(?<!999)foo
2274: </pre>
2275: This time the first assertion looks at the preceding six characters, checking
2276: that the first three are digits, and then the second assertion checks that the
2277: preceding three characters are not "999".
2278: </P>
2279: <P>
2280: Assertions can be nested in any combination. For example,
2281: <pre>
2282: (?<=(?<!foo)bar)baz
2283: </pre>
2284: matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2285: preceded by "foo", while
2286: <pre>
2287: (?<=\d{3}(?!999)...)foo
2288: </pre>
2289: is another pattern that matches "foo" preceded by three digits and any three
2290: characters that are not "999".
2291: <a name="conditions"></a></P>
1.1.1.5 ! misho 2292: <br><a name="SEC21" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1.1 misho 2293: <P>
2294: It is possible to cause the matching process to obey a subpattern
2295: conditionally or to choose between two alternative subpatterns, depending on
2296: the result of an assertion, or whether a specific capturing subpattern has
2297: already been matched. The two possible forms of conditional subpattern are:
2298: <pre>
2299: (?(condition)yes-pattern)
2300: (?(condition)yes-pattern|no-pattern)
2301: </pre>
2302: If the condition is satisfied, the yes-pattern is used; otherwise the
2303: no-pattern (if present) is used. If there are more than two alternatives in the
2304: subpattern, a compile-time error occurs. Each of the two alternatives may
2305: itself contain nested subpatterns of any form, including conditional
2306: subpatterns; the restriction to two alternatives applies only at the level of
2307: the condition. This pattern fragment is an example where the alternatives are
2308: complex:
2309: <pre>
2310: (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2311:
2312: </PRE>
2313: </P>
2314: <P>
2315: There are four kinds of condition: references to subpatterns, references to
2316: recursion, a pseudo-condition called DEFINE, and assertions.
2317: </P>
2318: <br><b>
2319: Checking for a used subpattern by number
2320: </b><br>
2321: <P>
2322: If the text between the parentheses consists of a sequence of digits, the
2323: condition is true if a capturing subpattern of that number has previously
2324: matched. If there is more than one capturing subpattern with the same number
2325: (see the earlier
2326: <a href="#recursion">section about duplicate subpattern numbers),</a>
2327: the condition is true if any of them have matched. An alternative notation is
2328: to precede the digits with a plus or minus sign. In this case, the subpattern
2329: number is relative rather than absolute. The most recently opened parentheses
2330: can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2331: loops it can also make sense to refer to subsequent groups. The next
2332: parentheses to be opened can be referenced as (?(+1), and so on. (The value
2333: zero in any of these forms is not used; it provokes a compile-time error.)
2334: </P>
2335: <P>
2336: Consider the following pattern, which contains non-significant white space to
2337: make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2338: three parts for ease of discussion:
2339: <pre>
2340: ( \( )? [^()]+ (?(1) \) )
2341: </pre>
2342: The first part matches an optional opening parenthesis, and if that
2343: character is present, sets it as the first captured substring. The second part
2344: matches one or more characters that are not parentheses. The third part is a
2345: conditional subpattern that tests whether or not the first set of parentheses
2346: matched. If they did, that is, if subject started with an opening parenthesis,
2347: the condition is true, and so the yes-pattern is executed and a closing
2348: parenthesis is required. Otherwise, since no-pattern is not present, the
2349: subpattern matches nothing. In other words, this pattern matches a sequence of
2350: non-parentheses, optionally enclosed in parentheses.
2351: </P>
2352: <P>
2353: If you were embedding this pattern in a larger one, you could use a relative
2354: reference:
2355: <pre>
2356: ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
2357: </pre>
2358: This makes the fragment independent of the parentheses in the larger pattern.
2359: </P>
2360: <br><b>
2361: Checking for a used subpattern by name
2362: </b><br>
2363: <P>
2364: Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2365: subpattern by name. For compatibility with earlier versions of PCRE, which had
1.1.1.5 ! misho 2366: this facility before Perl, the syntax (?(name)...) is also recognized.
1.1 misho 2367: </P>
2368: <P>
2369: Rewriting the above example to use a named subpattern gives this:
2370: <pre>
2371: (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
2372: </pre>
2373: If the name used in a condition of this kind is a duplicate, the test is
2374: applied to all subpatterns of the same name, and is true if any one of them has
2375: matched.
2376: </P>
2377: <br><b>
2378: Checking for pattern recursion
2379: </b><br>
2380: <P>
2381: If the condition is the string (R), and there is no subpattern with the name R,
2382: the condition is true if a recursive call to the whole pattern or any
2383: subpattern has been made. If digits or a name preceded by ampersand follow the
2384: letter R, for example:
2385: <pre>
2386: (?(R3)...) or (?(R&name)...)
2387: </pre>
2388: the condition is true if the most recent recursion is into a subpattern whose
2389: number or name is given. This condition does not check the entire recursion
2390: stack. If the name used in a condition of this kind is a duplicate, the test is
2391: applied to all subpatterns of the same name, and is true if any one of them is
2392: the most recent recursion.
2393: </P>
2394: <P>
2395: At "top level", all these recursion test conditions are false.
2396: <a href="#recursion">The syntax for recursive patterns</a>
2397: is described below.
2398: <a name="subdefine"></a></P>
2399: <br><b>
2400: Defining subpatterns for use by reference only
2401: </b><br>
2402: <P>
2403: If the condition is the string (DEFINE), and there is no subpattern with the
2404: name DEFINE, the condition is always false. In this case, there may be only one
2405: alternative in the subpattern. It is always skipped if control reaches this
2406: point in the pattern; the idea of DEFINE is that it can be used to define
2407: subroutines that can be referenced from elsewhere. (The use of
2408: <a href="#subpatternsassubroutines">subroutines</a>
2409: is described below.) For example, a pattern to match an IPv4 address such as
1.1.1.3 misho 2410: "192.168.23.245" could be written like this (ignore white space and line
1.1 misho 2411: breaks):
2412: <pre>
2413: (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
2414: \b (?&byte) (\.(?&byte)){3} \b
2415: </pre>
2416: The first part of the pattern is a DEFINE group inside which a another group
2417: named "byte" is defined. This matches an individual component of an IPv4
2418: address (a number less than 256). When matching takes place, this part of the
2419: pattern is skipped because DEFINE acts like a false condition. The rest of the
2420: pattern uses references to the named group to match the four dot-separated
2421: components of an IPv4 address, insisting on a word boundary at each end.
2422: </P>
2423: <br><b>
2424: Assertion conditions
2425: </b><br>
2426: <P>
2427: If the condition is not in any of the above formats, it must be an assertion.
2428: This may be a positive or negative lookahead or lookbehind assertion. Consider
2429: this pattern, again containing non-significant white space, and with the two
2430: alternatives on the second line:
2431: <pre>
2432: (?(?=[^a-z]*[a-z])
2433: \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
2434: </pre>
2435: The condition is a positive lookahead assertion that matches an optional
2436: sequence of non-letters followed by a letter. In other words, it tests for the
2437: presence of at least one letter in the subject. If a letter is found, the
2438: subject is matched against the first alternative; otherwise it is matched
2439: against the second. This pattern matches strings in one of the two forms
2440: dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2441: <a name="comments"></a></P>
1.1.1.5 ! misho 2442: <br><a name="SEC22" href="#TOC1">COMMENTS</a><br>
1.1 misho 2443: <P>
2444: There are two ways of including comments in patterns that are processed by
2445: PCRE. In both cases, the start of the comment must not be in a character class,
2446: nor in the middle of any other sequence of related characters such as (?: or a
2447: subpattern name or number. The characters that make up a comment play no part
2448: in the pattern matching.
2449: </P>
2450: <P>
2451: The sequence (?# marks the start of a comment that continues up to the next
2452: closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2453: option is set, an unescaped # character also introduces a comment, which in
2454: this case continues to immediately after the next newline character or
2455: character sequence in the pattern. Which characters are interpreted as newlines
1.1.1.2 misho 2456: is controlled by the options passed to a compiling function or by a special
1.1 misho 2457: sequence at the start of the pattern, as described in the section entitled
2458: <a href="#newlines">"Newline conventions"</a>
2459: above. Note that the end of this type of comment is a literal newline sequence
2460: in the pattern; escape sequences that happen to represent a newline do not
2461: count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2462: default newline convention is in force:
2463: <pre>
2464: abc #comment \n still comment
2465: </pre>
2466: On encountering the # character, <b>pcre_compile()</b> skips along, looking for
2467: a newline in the pattern. The sequence \n is still literal at this stage, so
2468: it does not terminate the comment. Only an actual character with the code value
2469: 0x0a (the default newline) does so.
2470: <a name="recursion"></a></P>
1.1.1.5 ! misho 2471: <br><a name="SEC23" href="#TOC1">RECURSIVE PATTERNS</a><br>
1.1 misho 2472: <P>
2473: Consider the problem of matching a string in parentheses, allowing for
2474: unlimited nested parentheses. Without the use of recursion, the best that can
2475: be done is to use a pattern that matches up to some fixed depth of nesting. It
2476: is not possible to handle an arbitrary nesting depth.
2477: </P>
2478: <P>
2479: For some time, Perl has provided a facility that allows regular expressions to
2480: recurse (amongst other things). It does this by interpolating Perl code in the
2481: expression at run time, and the code can refer to the expression itself. A Perl
2482: pattern using code interpolation to solve the parentheses problem can be
2483: created like this:
2484: <pre>
2485: $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
2486: </pre>
2487: The (?p{...}) item interpolates Perl code at run time, and in this case refers
2488: recursively to the pattern in which it appears.
2489: </P>
2490: <P>
2491: Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2492: supports special syntax for recursion of the entire pattern, and also for
2493: individual subpattern recursion. After its introduction in PCRE and Python,
2494: this kind of recursion was subsequently introduced into Perl at release 5.10.
2495: </P>
2496: <P>
2497: A special item that consists of (? followed by a number greater than zero and a
2498: closing parenthesis is a recursive subroutine call of the subpattern of the
2499: given number, provided that it occurs inside that subpattern. (If not, it is a
2500: <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2501: call, which is described in the next section.) The special item (?R) or (?0) is
2502: a recursive call of the entire regular expression.
2503: </P>
2504: <P>
2505: This PCRE pattern solves the nested parentheses problem (assume the
2506: PCRE_EXTENDED option is set so that white space is ignored):
2507: <pre>
2508: \( ( [^()]++ | (?R) )* \)
2509: </pre>
2510: First it matches an opening parenthesis. Then it matches any number of
2511: substrings which can either be a sequence of non-parentheses, or a recursive
2512: match of the pattern itself (that is, a correctly parenthesized substring).
2513: Finally there is a closing parenthesis. Note the use of a possessive quantifier
2514: to avoid backtracking into sequences of non-parentheses.
2515: </P>
2516: <P>
2517: If this were part of a larger pattern, you would not want to recurse the entire
2518: pattern, so instead you could use this:
2519: <pre>
2520: ( \( ( [^()]++ | (?1) )* \) )
2521: </pre>
2522: We have put the pattern into parentheses, and caused the recursion to refer to
2523: them instead of the whole pattern.
2524: </P>
2525: <P>
2526: In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2527: is made easier by the use of relative references. Instead of (?1) in the
2528: pattern above you can write (?-2) to refer to the second most recently opened
2529: parentheses preceding the recursion. In other words, a negative number counts
2530: capturing parentheses leftwards from the point at which it is encountered.
2531: </P>
2532: <P>
2533: It is also possible to refer to subsequently opened parentheses, by writing
2534: references such as (?+2). However, these cannot be recursive because the
2535: reference is not inside the parentheses that are referenced. They are always
2536: <a href="#subpatternsassubroutines">non-recursive subroutine</a>
2537: calls, as described in the next section.
2538: </P>
2539: <P>
2540: An alternative approach is to use named parentheses instead. The Perl syntax
2541: for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2542: could rewrite the above example as follows:
2543: <pre>
2544: (?<pn> \( ( [^()]++ | (?&pn) )* \) )
2545: </pre>
2546: If there is more than one subpattern with the same name, the earliest one is
2547: used.
2548: </P>
2549: <P>
2550: This particular example pattern that we have been looking at contains nested
2551: unlimited repeats, and so the use of a possessive quantifier for matching
2552: strings of non-parentheses is important when applying the pattern to strings
2553: that do not match. For example, when this pattern is applied to
2554: <pre>
2555: (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2556: </pre>
2557: it yields "no match" quickly. However, if a possessive quantifier is not used,
2558: the match runs for a very long time indeed because there are so many different
2559: ways the + and * repeats can carve up the subject, and all have to be tested
2560: before failure can be reported.
2561: </P>
2562: <P>
2563: At the end of a match, the values of capturing parentheses are those from
2564: the outermost level. If you want to obtain intermediate values, a callout
2565: function can be used (see below and the
2566: <a href="pcrecallout.html"><b>pcrecallout</b></a>
2567: documentation). If the pattern above is matched against
2568: <pre>
2569: (ab(cd)ef)
2570: </pre>
2571: the value for the inner capturing parentheses (numbered 2) is "ef", which is
2572: the last value taken on at the top level. If a capturing subpattern is not
2573: matched at the top level, its final captured value is unset, even if it was
2574: (temporarily) set at a deeper level during the matching process.
2575: </P>
2576: <P>
2577: If there are more than 15 capturing parentheses in a pattern, PCRE has to
2578: obtain extra memory to store data during a recursion, which it does by using
2579: <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can
2580: be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2581: </P>
2582: <P>
2583: Do not confuse the (?R) item with the condition (R), which tests for recursion.
2584: Consider this pattern, which matches text in angle brackets, allowing for
2585: arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2586: recursing), whereas any characters are permitted at the outer level.
2587: <pre>
2588: < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
2589: </pre>
2590: In this pattern, (?(R) is the start of a conditional subpattern, with two
2591: different alternatives for the recursive and non-recursive cases. The (?R) item
2592: is the actual recursive call.
2593: <a name="recursiondifference"></a></P>
2594: <br><b>
2595: Differences in recursion processing between PCRE and Perl
2596: </b><br>
2597: <P>
2598: Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2599: (like Python, but unlike Perl), a recursive subpattern call is always treated
2600: as an atomic group. That is, once it has matched some of the subject string, it
2601: is never re-entered, even if it contains untried alternatives and there is a
2602: subsequent matching failure. This can be illustrated by the following pattern,
2603: which purports to match a palindromic string that contains an odd number of
2604: characters (for example, "a", "aba", "abcba", "abcdcba"):
2605: <pre>
2606: ^(.|(.)(?1)\2)$
2607: </pre>
2608: The idea is that it either matches a single character, or two identical
2609: characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2610: it does not if the pattern is longer than three characters. Consider the
2611: subject string "abcba":
2612: </P>
2613: <P>
2614: At the top level, the first character is matched, but as it is not at the end
2615: of the string, the first alternative fails; the second alternative is taken
2616: and the recursion kicks in. The recursive call to subpattern 1 successfully
2617: matches the next character ("b"). (Note that the beginning and end of line
2618: tests are not part of the recursion).
2619: </P>
2620: <P>
2621: Back at the top level, the next character ("c") is compared with what
2622: subpattern 2 matched, which was "a". This fails. Because the recursion is
2623: treated as an atomic group, there are now no backtracking points, and so the
2624: entire match fails. (Perl is able, at this point, to re-enter the recursion and
2625: try the second alternative.) However, if the pattern is written with the
2626: alternatives in the other order, things are different:
2627: <pre>
2628: ^((.)(?1)\2|.)$
2629: </pre>
2630: This time, the recursing alternative is tried first, and continues to recurse
2631: until it runs out of characters, at which point the recursion fails. But this
2632: time we do have another alternative to try at the higher level. That is the big
2633: difference: in the previous case the remaining alternative is at a deeper
2634: recursion level, which PCRE cannot use.
2635: </P>
2636: <P>
2637: To change the pattern so that it matches all palindromic strings, not just
2638: those with an odd number of characters, it is tempting to change the pattern to
2639: this:
2640: <pre>
2641: ^((.)(?1)\2|.?)$
2642: </pre>
2643: Again, this works in Perl, but not in PCRE, and for the same reason. When a
2644: deeper recursion has matched a single character, it cannot be entered again in
2645: order to match an empty string. The solution is to separate the two cases, and
2646: write out the odd and even cases as alternatives at the higher level:
2647: <pre>
2648: ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2649: </pre>
2650: If you want to match typical palindromic phrases, the pattern has to ignore all
2651: non-word characters, which can be done like this:
2652: <pre>
2653: ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2654: </pre>
2655: If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2656: man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2657: the use of the possessive quantifier *+ to avoid backtracking into sequences of
2658: non-word characters. Without this, PCRE takes a great deal longer (ten times or
2659: more) to match typical phrases, and Perl takes so long that you think it has
2660: gone into a loop.
2661: </P>
2662: <P>
2663: <b>WARNING</b>: The palindrome-matching patterns above work only if the subject
2664: string does not start with a palindrome that is shorter than the entire string.
2665: For example, although "abcba" is correctly matched, if the subject is "ababa",
2666: PCRE finds the palindrome "aba" at the start, then fails at top level because
2667: the end of the string does not follow. Once again, it cannot jump back into the
2668: recursion to try other alternatives, so the entire match fails.
2669: </P>
2670: <P>
2671: The second way in which PCRE and Perl differ in their recursion processing is
2672: in the handling of captured values. In Perl, when a subpattern is called
2673: recursively or as a subpattern (see the next section), it has no access to any
2674: values that were captured outside the recursion, whereas in PCRE these values
2675: can be referenced. Consider this pattern:
2676: <pre>
2677: ^(.)(\1|a(?2))
2678: </pre>
2679: In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2680: then in the second group, when the back reference \1 fails to match "b", the
2681: second alternative matches "a" and then recurses. In the recursion, \1 does
2682: now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2683: match because inside the recursive call \1 cannot access the externally set
2684: value.
2685: <a name="subpatternsassubroutines"></a></P>
1.1.1.5 ! misho 2686: <br><a name="SEC24" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
1.1 misho 2687: <P>
2688: If the syntax for a recursive subpattern call (either by number or by
2689: name) is used outside the parentheses to which it refers, it operates like a
2690: subroutine in a programming language. The called subpattern may be defined
2691: before or after the reference. A numbered reference can be absolute or
2692: relative, as in these examples:
2693: <pre>
2694: (...(absolute)...)...(?2)...
2695: (...(relative)...)...(?-1)...
2696: (...(?+1)...(relative)...
2697: </pre>
2698: An earlier example pointed out that the pattern
2699: <pre>
2700: (sens|respons)e and \1ibility
2701: </pre>
2702: matches "sense and sensibility" and "response and responsibility", but not
2703: "sense and responsibility". If instead the pattern
2704: <pre>
2705: (sens|respons)e and (?1)ibility
2706: </pre>
2707: is used, it does match "sense and responsibility" as well as the other two
2708: strings. Another example is given in the discussion of DEFINE above.
2709: </P>
2710: <P>
2711: All subroutine calls, whether recursive or not, are always treated as atomic
2712: groups. That is, once a subroutine has matched some of the subject string, it
2713: is never re-entered, even if it contains untried alternatives and there is a
2714: subsequent matching failure. Any capturing parentheses that are set during the
2715: subroutine call revert to their previous values afterwards.
2716: </P>
2717: <P>
2718: Processing options such as case-independence are fixed when a subpattern is
2719: defined, so if it is used as a subroutine, such options cannot be changed for
2720: different calls. For example, consider this pattern:
2721: <pre>
2722: (abc)(?i:(?-1))
2723: </pre>
2724: It matches "abcabc". It does not match "abcABC" because the change of
2725: processing option does not affect the called subpattern.
2726: <a name="onigurumasubroutines"></a></P>
1.1.1.5 ! misho 2727: <br><a name="SEC25" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
1.1 misho 2728: <P>
2729: For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2730: a number enclosed either in angle brackets or single quotes, is an alternative
2731: syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2732: are two of the examples used above, rewritten using this syntax:
2733: <pre>
2734: (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
2735: (sens|respons)e and \g'1'ibility
2736: </pre>
2737: PCRE supports an extension to Oniguruma: if a number is preceded by a
2738: plus or a minus sign it is taken as a relative reference. For example:
2739: <pre>
2740: (abc)(?i:\g<-1>)
2741: </pre>
2742: Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
2743: synonymous. The former is a back reference; the latter is a subroutine call.
2744: </P>
1.1.1.5 ! misho 2745: <br><a name="SEC26" href="#TOC1">CALLOUTS</a><br>
1.1 misho 2746: <P>
2747: Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2748: code to be obeyed in the middle of matching a regular expression. This makes it
2749: possible, amongst other things, to extract different substrings that match the
2750: same pair of parentheses when there is a repetition.
2751: </P>
2752: <P>
2753: PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2754: code. The feature is called "callout". The caller of PCRE provides an external
1.1.1.2 misho 2755: function by putting its entry point in the global variable <i>pcre_callout</i>
1.1.1.4 misho 2756: (8-bit library) or <i>pcre[16|32]_callout</i> (16-bit or 32-bit library).
2757: By default, this variable contains NULL, which disables all calling out.
1.1 misho 2758: </P>
2759: <P>
2760: Within a regular expression, (?C) indicates the points at which the external
2761: function is to be called. If you want to identify different callout points, you
2762: can put a number less than 256 after the letter C. The default value is zero.
2763: For example, this pattern has two callout points:
2764: <pre>
2765: (?C1)abc(?C2)def
2766: </pre>
1.1.1.2 misho 2767: If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
1.1 misho 2768: automatically installed before each item in the pattern. They are all numbered
1.1.1.4 misho 2769: 255. If there is a conditional group in the pattern whose condition is an
2770: assertion, an additional callout is inserted just before the condition. An
2771: explicit callout may also be set at this position, as in this example:
2772: <pre>
2773: (?(?C9)(?=a)abc|def)
2774: </pre>
2775: Note that this applies only to assertion conditions, not to other types of
2776: condition.
1.1 misho 2777: </P>
2778: <P>
1.1.1.2 misho 2779: During matching, when PCRE reaches a callout point, the external function is
2780: called. It is provided with the number of the callout, the position in the
2781: pattern, and, optionally, one item of data originally supplied by the caller of
2782: the matching function. The callout function may cause matching to proceed, to
1.1.1.5 ! misho 2783: backtrack, or to fail altogether.
! 2784: </P>
! 2785: <P>
! 2786: By default, PCRE implements a number of optimizations at compile time and
! 2787: matching time, and one side-effect is that sometimes callouts are skipped. If
! 2788: you need all possible callouts to happen, you need to set options that disable
! 2789: the relevant optimizations. More details, and a complete description of the
! 2790: interface to the callout function, are given in the
1.1 misho 2791: <a href="pcrecallout.html"><b>pcrecallout</b></a>
2792: documentation.
2793: <a name="backtrackcontrol"></a></P>
1.1.1.5 ! misho 2794: <br><a name="SEC27" href="#TOC1">BACKTRACKING CONTROL</a><br>
1.1 misho 2795: <P>
2796: Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
1.1.1.4 misho 2797: are still described in the Perl documentation as "experimental and subject to
2798: change or removal in a future version of Perl". It goes on to say: "Their usage
2799: in production code should be noted to avoid problems during upgrades." The same
1.1 misho 2800: remarks apply to the PCRE features described in this section.
2801: </P>
2802: <P>
1.1.1.4 misho 2803: The new verbs make use of what was previously invalid syntax: an opening
2804: parenthesis followed by an asterisk. They are generally of the form
2805: (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2806: differently depending on whether or not a name is present. A name is any
2807: sequence of characters that does not include a closing parenthesis. The maximum
2808: length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2809: libraries. If the name is empty, that is, if the closing parenthesis
2810: immediately follows the colon, the effect is as if the colon were not there.
2811: Any number of these verbs may occur in a pattern.
1.1 misho 2812: </P>
2813: <P>
1.1.1.4 misho 2814: Since these verbs are specifically related to backtracking, most of them can be
2815: used only when the pattern is to be matched using one of the traditional
2816: matching functions, because these use a backtracking algorithm. With the
2817: exception of (*FAIL), which behaves like a failing negative assertion, the
2818: backtracking control verbs cause an error if encountered by a DFA matching
2819: function.
1.1 misho 2820: </P>
2821: <P>
1.1.1.4 misho 2822: The behaviour of these verbs in
2823: <a href="#btrepeat">repeated groups,</a>
2824: <a href="#btassert">assertions,</a>
2825: and in
2826: <a href="#btsub">subpatterns called as subroutines</a>
2827: (whether or not recursively) is documented below.
1.1.1.3 misho 2828: <a name="nooptimize"></a></P>
2829: <br><b>
2830: Optimizations that affect backtracking verbs
2831: </b><br>
1.1 misho 2832: <P>
2833: PCRE contains some optimizations that are used to speed up matching by running
2834: some checks at the start of each match attempt. For example, it may know the
2835: minimum length of matching subject, or that a particular character must be
1.1.1.4 misho 2836: present. When one of these optimizations bypasses the running of a match, any
1.1 misho 2837: included backtracking verbs will not, of course, be processed. You can suppress
2838: the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2839: when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the
1.1.1.3 misho 2840: pattern with (*NO_START_OPT). There is more discussion of this option in the
2841: section entitled
2842: <a href="pcreapi.html#execoptions">"Option bits for <b>pcre_exec()</b>"</a>
2843: in the
2844: <a href="pcreapi.html"><b>pcreapi</b></a>
2845: documentation.
1.1 misho 2846: </P>
2847: <P>
2848: Experiments with Perl suggest that it too has similar optimizations, sometimes
2849: leading to anomalous results.
2850: </P>
2851: <br><b>
2852: Verbs that act immediately
2853: </b><br>
2854: <P>
2855: The following verbs act as soon as they are encountered. They may not be
2856: followed by a name.
2857: <pre>
2858: (*ACCEPT)
2859: </pre>
2860: This verb causes the match to end successfully, skipping the remainder of the
2861: pattern. However, when it is inside a subpattern that is called as a
2862: subroutine, only that subpattern is ended successfully. Matching then continues
1.1.1.4 misho 2863: at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2864: assertion succeeds; in a negative assertion, the assertion fails.
2865: </P>
2866: <P>
2867: If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2868: example:
1.1 misho 2869: <pre>
2870: A((?:A|B(*ACCEPT)|C)D)
2871: </pre>
2872: This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2873: the outer parentheses.
2874: <pre>
2875: (*FAIL) or (*F)
2876: </pre>
2877: This verb causes a matching failure, forcing backtracking to occur. It is
2878: equivalent to (?!) but easier to read. The Perl documentation notes that it is
2879: probably useful only when combined with (?{}) or (??{}). Those are, of course,
2880: Perl features that are not present in PCRE. The nearest equivalent is the
2881: callout feature, as for example in this pattern:
2882: <pre>
2883: a+(?C)(*FAIL)
2884: </pre>
2885: A match with the string "aaaa" always fails, but the callout is taken before
2886: each backtrack happens (in this example, 10 times).
2887: </P>
2888: <br><b>
2889: Recording which path was taken
2890: </b><br>
2891: <P>
2892: There is one verb whose main purpose is to track how a match was arrived at,
2893: though it also has a secondary use in conjunction with advancing the match
2894: starting point (see (*SKIP) below).
2895: <pre>
2896: (*MARK:NAME) or (*:NAME)
2897: </pre>
2898: A name is always required with this verb. There may be as many instances of
2899: (*MARK) as you like in a pattern, and their names do not have to be unique.
2900: </P>
2901: <P>
1.1.1.4 misho 2902: When a match succeeds, the name of the last-encountered (*MARK:NAME),
2903: (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2904: caller as described in the section entitled
1.1.1.2 misho 2905: <a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a>
1.1 misho 2906: in the
2907: <a href="pcreapi.html"><b>pcreapi</b></a>
2908: documentation. Here is an example of <b>pcretest</b> output, where the /K
2909: modifier requests the retrieval and outputting of (*MARK) data:
2910: <pre>
2911: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2912: data> XY
2913: 0: XY
2914: MK: A
2915: XZ
2916: 0: XZ
2917: MK: B
2918: </pre>
2919: The (*MARK) name is tagged with "MK:" in this output, and in this example it
2920: indicates which of the two alternatives matched. This is a more efficient way
2921: of obtaining this information than putting each alternative in its own
2922: capturing parentheses.
2923: </P>
2924: <P>
1.1.1.4 misho 2925: If a verb with a name is encountered in a positive assertion that is true, the
2926: name is recorded and passed back if it is the last-encountered. This does not
2927: happen for negative assertions or failing positive assertions.
1.1 misho 2928: </P>
2929: <P>
1.1.1.4 misho 2930: After a partial match or a failed match, the last encountered name in the
2931: entire match process is returned. For example:
1.1 misho 2932: <pre>
2933: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2934: data> XP
2935: No match, mark = B
2936: </pre>
2937: Note that in this unanchored example the mark is retained from the match
1.1.1.3 misho 2938: attempt that started at the letter "X" in the subject. Subsequent match
2939: attempts starting at "P" and then with an empty string do not get as far as the
2940: (*MARK) item, but nevertheless do not reset it.
2941: </P>
2942: <P>
2943: If you are interested in (*MARK) values after failed matches, you should
2944: probably set the PCRE_NO_START_OPTIMIZE option
2945: <a href="#nooptimize">(see above)</a>
2946: to ensure that the match is always attempted.
1.1 misho 2947: </P>
2948: <br><b>
2949: Verbs that act after backtracking
2950: </b><br>
2951: <P>
2952: The following verbs do nothing when they are encountered. Matching continues
2953: with what follows, but if there is no subsequent match, causing a backtrack to
2954: the verb, a failure is forced. That is, backtracking cannot pass to the left of
1.1.1.4 misho 2955: the verb. However, when one of these verbs appears inside an atomic group or an
2956: assertion that is true, its effect is confined to that group, because once the
2957: group has been matched, there is never any backtracking into it. In this
2958: situation, backtracking can "jump back" to the left of the entire atomic group
2959: or assertion. (Remember also, as stated above, that this localization also
2960: applies in subroutine calls.)
1.1 misho 2961: </P>
2962: <P>
2963: These verbs differ in exactly what kind of failure occurs when backtracking
1.1.1.4 misho 2964: reaches them. The behaviour described below is what happens when the verb is
2965: not in a subroutine or an assertion. Subsequent sections cover these special
2966: cases.
1.1 misho 2967: <pre>
2968: (*COMMIT)
2969: </pre>
2970: This verb, which may not be followed by a name, causes the whole match to fail
1.1.1.4 misho 2971: outright if there is a later matching failure that causes backtracking to reach
2972: it. Even if the pattern is unanchored, no further attempts to find a match by
2973: advancing the starting point take place. If (*COMMIT) is the only backtracking
2974: verb that is encountered, once it has been passed <b>pcre_exec()</b> is
2975: committed to finding a match at the current starting point, or not at all. For
2976: example:
1.1 misho 2977: <pre>
2978: a+(*COMMIT)b
2979: </pre>
2980: This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2981: dynamic anchor, or "I've started, so I must finish." The name of the most
2982: recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2983: match failure.
2984: </P>
2985: <P>
1.1.1.4 misho 2986: If there is more than one backtracking verb in a pattern, a different one that
2987: follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2988: match does not always guarantee that a match must be at this starting point.
2989: </P>
2990: <P>
1.1 misho 2991: Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2992: unless PCRE's start-of-match optimizations are turned off, as shown in this
2993: <b>pcretest</b> example:
2994: <pre>
2995: re> /(*COMMIT)abc/
2996: data> xyzabc
2997: 0: abc
2998: xyzabc\Y
2999: No match
3000: </pre>
3001: PCRE knows that any match must start with "a", so the optimization skips along
3002: the subject to "a" before running the first match attempt, which succeeds. When
3003: the optimization is disabled by the \Y escape in the second subject, the match
3004: starts at "x" and so the (*COMMIT) causes it to fail without trying any other
3005: starting points.
3006: <pre>
3007: (*PRUNE) or (*PRUNE:NAME)
3008: </pre>
3009: This verb causes the match to fail at the current starting position in the
1.1.1.4 misho 3010: subject if there is a later matching failure that causes backtracking to reach
3011: it. If the pattern is unanchored, the normal "bumpalong" advance to the next
3012: starting character then happens. Backtracking can occur as usual to the left of
3013: (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
3014: if there is no match to the right, backtracking cannot cross (*PRUNE). In
3015: simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
3016: possessive quantifier, but there are some uses of (*PRUNE) that cannot be
3017: expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
3018: as (*COMMIT).
3019: </P>
3020: <P>
3021: The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3022: It is like (*MARK:NAME) in that the name is remembered for passing back to the
3023: caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
1.1 misho 3024: <pre>
3025: (*SKIP)
3026: </pre>
3027: This verb, when given without a name, is like (*PRUNE), except that if the
3028: pattern is unanchored, the "bumpalong" advance is not to the next character,
3029: but to the position in the subject where (*SKIP) was encountered. (*SKIP)
3030: signifies that whatever text was matched leading up to it cannot be part of a
3031: successful match. Consider:
3032: <pre>
3033: a+(*SKIP)b
3034: </pre>
3035: If the subject is "aaaac...", after the first match attempt fails (starting at
3036: the first character in the string), the starting point skips on to start the
3037: next attempt at "c". Note that a possessive quantifer does not have the same
3038: effect as this example; although it would suppress backtracking during the
3039: first match attempt, the second attempt would start at the second character
3040: instead of skipping on to "c".
3041: <pre>
3042: (*SKIP:NAME)
3043: </pre>
1.1.1.4 misho 3044: When (*SKIP) has an associated name, its behaviour is modified. When it is
3045: triggered, the previous path through the pattern is searched for the most
3046: recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
3047: is to the subject position that corresponds to that (*MARK) instead of to where
3048: (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
3049: (*SKIP) is ignored.
3050: </P>
3051: <P>
3052: Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3053: names that are set by (*PRUNE:NAME) or (*THEN:NAME).
1.1 misho 3054: <pre>
3055: (*THEN) or (*THEN:NAME)
3056: </pre>
1.1.1.4 misho 3057: This verb causes a skip to the next innermost alternative when backtracking
3058: reaches it. That is, it cancels any further backtracking within the current
3059: alternative. Its name comes from the observation that it can be used for a
3060: pattern-based if-then-else block:
1.1 misho 3061: <pre>
3062: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3063: </pre>
3064: If the COND1 pattern matches, FOO is tried (and possibly further items after
3065: the end of the group if FOO succeeds); on failure, the matcher skips to the
1.1.1.4 misho 3066: second alternative and tries COND2, without backtracking into COND1. If that
3067: succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3068: more alternatives, so there is a backtrack to whatever came before the entire
3069: group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3070: </P>
3071: <P>
3072: The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3073: It is like (*MARK:NAME) in that the name is remembered for passing back to the
3074: caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
1.1 misho 3075: </P>
3076: <P>
1.1.1.4 misho 3077: A subpattern that does not contain a | character is just a part of the
3078: enclosing alternative; it is not a nested alternation with only one
1.1 misho 3079: alternative. The effect of (*THEN) extends beyond such a subpattern to the
3080: enclosing alternative. Consider this pattern, where A, B, etc. are complex
3081: pattern fragments that do not contain any | characters at this level:
3082: <pre>
3083: A (B(*THEN)C) | D
3084: </pre>
3085: If A and B are matched, but there is a failure in C, matching does not
3086: backtrack into A; instead it moves to the next alternative, that is, D.
3087: However, if the subpattern containing (*THEN) is given an alternative, it
3088: behaves differently:
3089: <pre>
3090: A (B(*THEN)C | (*FAIL)) | D
3091: </pre>
3092: The effect of (*THEN) is now confined to the inner subpattern. After a failure
3093: in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3094: because there are no more alternatives to try. In this case, matching does now
3095: backtrack into A.
3096: </P>
3097: <P>
1.1.1.4 misho 3098: Note that a conditional subpattern is not considered as having two
1.1 misho 3099: alternatives, because only one is ever used. In other words, the | character in
3100: a conditional subpattern has a different meaning. Ignoring white space,
3101: consider:
3102: <pre>
3103: ^.*? (?(?=a) a | b(*THEN)c )
3104: </pre>
3105: If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3106: it initially matches zero characters. The condition (?=a) then fails, the
3107: character "b" is matched, but "c" is not. At this point, matching does not
3108: backtrack to .*? as might perhaps be expected from the presence of the |
3109: character. The conditional subpattern is part of the single alternative that
3110: comprises the whole pattern, and so the match fails. (If there was a backtrack
3111: into .*?, allowing it to match "b", the match would succeed.)
3112: </P>
3113: <P>
3114: The verbs just described provide four different "strengths" of control when
3115: subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3116: next alternative. (*PRUNE) comes next, failing the match at the current
3117: starting position, but allowing an advance to the next character (for an
3118: unanchored pattern). (*SKIP) is similar, except that the advance may be more
3119: than one character. (*COMMIT) is the strongest, causing the entire match to
3120: fail.
3121: </P>
1.1.1.4 misho 3122: <br><b>
3123: More than one backtracking verb
3124: </b><br>
3125: <P>
3126: If more than one backtracking verb is present in a pattern, the one that is
3127: backtracked onto first acts. For example, consider this pattern, where A, B,
3128: etc. are complex pattern fragments:
3129: <pre>
3130: (A(*COMMIT)B(*THEN)C|ABD)
3131: </pre>
3132: If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3133: fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3134: the next alternative (ABD) to be tried. This behaviour is consistent, but is
3135: not always the same as Perl's. It means that if two or more backtracking verbs
3136: appear in succession, all the the last of them has no effect. Consider this
3137: example:
3138: <pre>
3139: ...(*COMMIT)(*PRUNE)...
3140: </pre>
1.1.1.5 ! misho 3141: If there is a matching failure to the right, backtracking onto (*PRUNE) causes
1.1.1.4 misho 3142: it to be triggered, and its action is taken. There can never be a backtrack
3143: onto (*COMMIT).
3144: <a name="btrepeat"></a></P>
3145: <br><b>
3146: Backtracking verbs in repeated groups
3147: </b><br>
3148: <P>
3149: PCRE differs from Perl in its handling of backtracking verbs in repeated
3150: groups. For example, consider:
3151: <pre>
3152: /(a(*COMMIT)b)+ac/
3153: </pre>
3154: If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3155: the second repeat of the group acts.
3156: <a name="btassert"></a></P>
3157: <br><b>
3158: Backtracking verbs in assertions
3159: </b><br>
3160: <P>
3161: (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3162: </P>
3163: <P>
3164: (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3165: further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3166: fail without any further processing.
3167: </P>
3168: <P>
3169: The other backtracking verbs are not treated specially if they appear in a
3170: positive assertion. In particular, (*THEN) skips to the next alternative in the
3171: innermost enclosing group that has alternations, whether or not this is within
3172: the assertion.
3173: </P>
3174: <P>
3175: Negative assertions are, however, different, in order to ensure that changing a
3176: positive assertion into a negative assertion changes its result. Backtracking
3177: into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3178: without considering any further alternative branches in the assertion.
3179: Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3180: within the assertion (the normal behaviour), but if the assertion does not have
3181: such an alternative, (*THEN) behaves like (*PRUNE).
3182: <a name="btsub"></a></P>
3183: <br><b>
3184: Backtracking verbs in subroutines
3185: </b><br>
3186: <P>
3187: These behaviours occur whether or not the subpattern is called recursively.
3188: Perl's treatment of subroutines is different in some cases.
3189: </P>
3190: <P>
3191: (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3192: an immediate backtrack.
3193: </P>
3194: <P>
3195: (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3196: succeed without any further processing. Matching then continues after the
3197: subroutine call.
3198: </P>
3199: <P>
3200: (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3201: the subroutine match to fail.
3202: </P>
1.1 misho 3203: <P>
1.1.1.4 misho 3204: (*THEN) skips to the next alternative in the innermost enclosing group within
3205: the subpattern that has alternatives. If there is no such group within the
3206: subpattern, (*THEN) causes the subroutine match to fail.
1.1 misho 3207: </P>
1.1.1.5 ! misho 3208: <br><a name="SEC28" href="#TOC1">SEE ALSO</a><br>
1.1 misho 3209: <P>
3210: <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3),
1.1.1.4 misho 3211: <b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>, <b>pcre32(3)</b>.
1.1 misho 3212: </P>
1.1.1.5 ! misho 3213: <br><a name="SEC29" href="#TOC1">AUTHOR</a><br>
1.1 misho 3214: <P>
3215: Philip Hazel
3216: <br>
3217: University Computing Service
3218: <br>
3219: Cambridge CB2 3QH, England.
3220: <br>
3221: </P>
1.1.1.5 ! misho 3222: <br><a name="SEC30" href="#TOC1">REVISION</a><br>
1.1 misho 3223: <P>
1.1.1.5 ! misho 3224: Last updated: 03 December 2013
1.1 misho 3225: <br>
1.1.1.4 misho 3226: Copyright © 1997-2013 University of Cambridge.
1.1 misho 3227: <br>
3228: <p>
3229: Return to the <a href="index.html">PCRE index page</a>.
3230: </p>
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