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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:  8: Return to the <a href="index.html">PCRE index page</a>. 9:  10:  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:   15: <ul> 16: <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a> 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> 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> 46: </ul> 47:  <a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a>  48:  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">pcresyntax</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:  57:  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:  64:  65: This document discusses the patterns that are supported by PCRE when one its 66: main matching functions, pcre_exec() (8-bit) or pcre[16|32]_exec() 67: (16- or 32-bit), is used. PCRE also has alternative matching functions, 68: pcre_dfa_exec() and pcre[16|32_dfa_exec(), 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">pcrematching</a> 74: page. 75:  76:  <a name="SEC2" href="#TOC1">SPECIAL START-OF-PATTERN ITEMS</a>  77:  78: A number of options that can be passed to pcre_compile() 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:  85:   86: UTF support 87:   88:  89: The original operation of PCRE was on strings of one-byte characters. However, 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 93: features, PCRE must be built to include appropriate support. When using UTF 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: 97: <pre> 98: (*UTF8) 99: (*UTF16) 100: (*UTF32) 101: (*UTF) 102: </pre> 103: (*UTF) is a generic sequence that can be used with any of the libraries. 104: Starting a pattern with such a sequence is equivalent to setting the relevant 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 107: <a href="pcreunicode.html">pcreunicode</a> 108: page. 109:  110:  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:  116:   117: Unicode property support 118:   119:  120: Another special sequence that may appear at the start of a pattern is (*UCP). 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:  126:   127: Disabling auto-possessification 128:   129:  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">pcreapi</a> 135: documentation. 136:  137:   138: Disabling start-up optimizations 139:   140:  141: If a pattern starts with (*NO_START_OPT), it has the same effect as setting the 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">pcreapi</a> 146: documentation. 147: <a name="newlines"></a> 148:   149: Newline conventions 150:   151:  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">pcreapi</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: options arguments for the compiling and matching functions. 161:  162:  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> 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 174: <pre> 175: (*CR)a.b 176: </pre> 177: changes the convention to CR. That pattern matches "a\nb" because LF is no 178: longer a newline. If more than one of these settings is present, the last one 179: is used. 180:  181:  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 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:  192:   193: Setting match and recursion limits 194:   195:  196: The caller of pcre_exec() can set a limit on the number of times the 197: internal match() 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, pcre_exec() 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 209: be less than the value set (or defaulted) by the caller of pcre_exec() 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. 213:  214:  <a name="SEC3" href="#TOC1">EBCDIC CHARACTER CODES</a>  215:  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:  222:  <a name="SEC4" href="#TOC1">CHARACTERS AND METACHARACTERS</a>  223:  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 232: independently of case. In a UTF mode, PCRE always understands the concept of 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 238: UTF support. 239:  240:  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: metacharacters, which do not stand for themselves but instead are 244: interpreted in some special way. 245:  246:  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:  279:  <a name="SEC5" href="#TOC1">BACKSLASH</a>  280:  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:  286:  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:  293:  294: In a UTF mode, only ASCII numbers and letters have any special meaning after a 295: backslash. All other characters (in particular, those whose codepoints are 296: greater than 127) are treated as literals. 297:  298:  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. 304:  305:  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> 324:   325: Non-printing characters 326:   327:  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) 337: \f form feed (hex 0C) 338: \n linefeed (hex 0A) 339: \r carriage return (hex 0D) 340: \t tab (hex 09) 341: \0dd character with octal code 0dd 342: \ddd character with octal code ddd, or back reference 343: \o{ddd..} character with octal code ddd.. 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> 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:  355:  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. 364:  365:  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:  372:  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:  379:  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:  385:  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 390: previous capturing left parentheses in the expression, the entire sequence is 391: taken as a back reference. 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:  396:  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: 402: <pre> 403: \040 is another way of writing an ASCII space 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 113 410: \377 might be a back reference, otherwise the value 255 (decimal) 411: \81 is either a back reference, or the two characters "8" and "1" 412: </pre> 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:  417:  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. 423:  424:  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:  431:  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:  437:   438: Constraints on character values 439:   440:  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:  454:   455: Escape sequences in character classes 456:   457:  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:  462:  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:  469:   470: Unsupported escape sequences 471:   472:  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:  479:   480: Absolute and relative back references 481:   482:  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:  490:   491: Absolute and relative subroutine calls 492:   493:  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 not 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> 503:   504: Generic character types 505:   506:  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 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 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:  526:  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:  534:  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. 543:  544:  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">pcreapi</a> 552: page). For example, in a French locale such as "fr_FR" in Unix-like systems, 553: or "french" in Windows, some character codes greater than 127 are used for 554: accented letters, and these are then matched by \w. The use of locales with 555: Unicode is discouraged. 556:  557:  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 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:  577:  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 580: characters by default, these always match certain high-valued code points, 581: whether or not PCRE_UCP is set. The horizontal space characters are: 582: <pre> 583: U+0009 Horizontal tab (HT) 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> 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) 610: U+2028 Line separator 611: U+2029 Paragraph separator 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> 616:   617: Newline sequences 618:   619:  620: Outside a character class, by default, the escape sequence \R matches any 621: Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the 622: following: 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, 630: U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next 631: line, U+0085). The two-character sequence is treated as a single unit that 632: cannot be split. 633:  634:  635: In other modes, two additional characters whose codepoints are greater than 255 636: 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:  640:  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> 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: 658: <pre> 659: (*ANY)(*BSR_ANYCRLF) 660: </pre> 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. 665: <a name="uniextseq"></a> 666:   667: Unicode character properties 668:   669:  670: When PCRE is built with Unicode character property support, three additional 671: escape sequences that match characters with specific properties are available. 672: When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing 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{xx} a character with the xx property 677: \P{xx} a character without the xx property 678: \X a Unicode extended grapheme cluster 679: </pre> 680: The property names represented by xx 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:  689:  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:  700:  701: Arabic, 702: Armenian, 703: Avestan, 704: Balinese, 705: Bamum, 706: Batak, 707: Bengali, 708: Bopomofo, 709: Brahmi, 710: Braille, 711: Buginese, 712: Buhid, 713: Canadian_Aboriginal, 714: Carian, 715: Chakma, 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, 758: Mandaic, 759: Meetei_Mayek, 760: Meroitic_Cursive, 761: Meroitic_Hieroglyphs, 762: Miao, 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, 781: Sharada, 782: Shavian, 783: Sinhala, 784: Sora_Sompeng, 785: Sundanese, 786: Syloti_Nagri, 787: Syriac, 788: Tagalog, 789: Tagbanwa, 790: Tai_Le, 791: Tai_Tham, 792: Tai_Viet, 793: Takri, 794: Tamil, 795: Telugu, 796: Thaana, 797: Thai, 798: Tibetan, 799: Tifinagh, 800: Ugaritic, 801: Vai, 802: Yi. 803:  804:  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:  810:  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: 869: 870: The Cs (Surrogate) property applies only to characters in the range U+D800 to 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 873: (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and 874: PCRE_NO_UTF32_CHECK in the 875: <a href="pcreapi.html">pcreapi</a> 876: page). Perl does not support the Cs property. 877: 878: 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: 883: 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: 888: 889: Specifying caseless matching does not affect these escape sequences. For 890: example, \p{Lu} always matches only upper case letters. This is different from 891: the behaviour of current versions of Perl. 892: 893: 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: 900: 901: Extended grapheme clusters 902: 903: 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 909: <pre> 910: (?>\PM\pM*) 911: </pre> 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. 915:  916:  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. 922:  923:  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:  927:  928: 1. End at the end of the subject string. 929:  930:  931: 2. Do not end between CR and LF; otherwise end after any control character. 932:  933:  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:  939:  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:  943:  944: 5. Do not end after prepend characters. 945:  946:  947: 6. Otherwise, end the cluster. 948: <a name="extraprops"></a> 949:   950: PCRE's additional properties 951:   952:  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 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: 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) 965: property. Xps matches the characters tab, linefeed, vertical tab, form feed, or 966: carriage return, and any other character that has the Z (separator) property. 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. 970:  971:  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.) 980: <a name="resetmatchstart"></a> 981:   982: Resetting the match start 983:   984:  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:  1003:  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> 1008:   1009: Simple assertions 1010:   1011:  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:  1033:  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 1037: first or last character matches \w, respectively. In a UTF mode, the meanings 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:  1044:  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 startoffset 1051: argument of pcre_exec() 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:  1056:  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 startoffset argument of 1059: pcre_exec(). It differs from \A when the value of startoffset is 1060: non-zero. By calling pcre_exec() 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:  1064:  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:  1071:  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:  1076:  <a name="SEC6" href="#TOC1">CIRCUMFLEX AND DOLLAR</a>  1077:  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:  1082:  1083: Outside a character class, in the default matching mode, the circumflex 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 startoffset argument of 1086: pcre_exec() 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:  1091:  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:  1100:  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. 1107:  1108:  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:  1113:  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:  1122:  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 startoffset argument of pcre_exec() is non-zero. The 1128: PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. 1129:  1130:  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> 1135:  <a name="SEC7" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a>  1136:  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 1139: line. 1140:  1141:  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:  1149:  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:  1155:  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:  1160:  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:  1166:  <a name="SEC8" href="#TOC1">MATCHING A SINGLE DATA UNIT</a>  1167:  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 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 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 1178: start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or 1179: PCRE_NO_UTF32_CHECK option is used). 1180:  1181:  1182: PCRE does not allow \C to appear in lookbehind assertions 1183: <a href="#lookbehind">(described below)</a> 1184: in a UTF mode, because this would make it impossible to calculate the length of 1185: the lookbehind. 1186:  1187:  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): 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> 1206:  <a name="SEC9" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a>  1207:  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:  1215:  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 1222: backslash. 1223:  1224:  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:  1233:  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. 1237:  1238:  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 1242: caseful version would. In a UTF mode, PCRE always understands the concept of 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. 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. 1249:  1250:  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:  1256:  1257: The minus (hyphen) character can be used to specify a range of characters in a 1258: character class. For example, [d-m] matches any letter between d and m, 1259: inclusive. If a minus character is required in a class, it must be escaped with 1260: a backslash or appear in a position where it cannot be interpreted as 1261: indicating a range, typically as the first or last character in the class, or 1262: immediately after a range. For example, [b-d-z] matches letters in the range b 1263: to d, a hyphen character, or z. 1264:  1265:  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:  1274:  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:  1280:  1281: Ranges operate in the collating sequence of character values. They can also be 1282: used for characters specified numerically, for example [\000-\037]. Ranges 1283: can include any characters that are valid for the current mode. 1284:  1285:  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 1288: [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character 1289: tables for a French locale are in use, [\xc8-\xcb] matches accented E 1290: characters in both cases. In UTF modes, PCRE supports the concept of case for 1291: characters with values greater than 128 only when it is compiled with Unicode 1292: property support. 1293:  1294:  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 1298: digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w 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:  1308:  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:  1316:  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 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. 1323:  1324:  <a name="SEC10" href="#TOC1">POSIX CHARACTER CLASSES</a>  1325:  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 1345: space white space (the same as \s from PCRE 8.34) 1346: upper upper case letters 1347: word "word" characters (same as \w) 1348: xdigit hexadecimal digits 1349: </pre> 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. 1356:  1357:  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:  1368:  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: pcre_compile(), 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: 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> 1384: Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX 1385: classes are handled specially in UCP mode: 1386:  1387:  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:  1399:  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:  1404:  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:  1410:  1411: The other POSIX classes are unchanged, and match only characters with code 1412: points less than 128. 1413:  1414:  <a name="SEC11" href="#TOC1">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a>  1415:  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:  1433:  <a name="SEC12" href="#TOC1">VERTICAL BAR</a>  1434:  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:  1448:  <a name="SEC13" href="#TOC1">INTERNAL OPTION SETTING</a>  1449:  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:  1467:  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:  1472:  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 pcre_fullinfo() function). 1478:  1479:  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:  1497:  1498: Note: There are other PCRE-specific options that can be set by the 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 1503: <a href="#newlineseq">"Newline sequences"</a> 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. 1510: <a name="subpattern"></a> 1511:  <a name="SEC14" href="#TOC1">SUBPATTERNS</a>  1512:  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:   1516:   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:   1524:   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 1527: subpattern is passed back to the caller via the ovector argument of the 1528: matching function. (This applies only to the traditional matching functions; 1529: the DFA matching functions do not support capturing.) 1530:  1531:  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 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:  1541:  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:  1554:  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> 1567:  <a name="SEC15" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a>  1568:  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:  1607:  1608: An alternative approach to using this "branch reset" feature is to use 1609: duplicate named subpatterns, as described in the next section. 1610:  1611:  <a name="SEC16" href="#TOC1">NAMED SUBPATTERNS</a>  1612:  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:  1622:  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:  1632:  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. 1639:  1640:  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:  1660:  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:  1665:  1666: If you make a back reference to a non-unique named subpattern from elsewhere in 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:  1676:  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:  1682:  1683: If you use a named reference in a condition 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">pcreapi</a> 1692: documentation. 1693:  1694:  1695: Warning: 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 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. 1701:  1702:  <a name="SEC17" href="#TOC1">REPETITION</a>  1703:  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 1710: the \X escape sequence 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:  1741:  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, 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). 1747:  1748:  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:  1758:  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:  1776:  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:  1793:  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:  1810:  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:  1816:  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:  1821:  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:  1829:  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:  1834:  1835: However, there are some cases where the optimization cannot be used. When .* 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:  1845:  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:  1855:  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> 1870:  <a name="SEC18" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>  1871:  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:  1879:  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:  1890:  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:  1902:  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:  1907:  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:  1914:  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:  1935:  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:  1942:  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:  1947:  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> 1974:  <a name="SEC19" href="#TOC1">BACK REFERENCES</a>  1975:  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:  1981:  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:  1990:  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:  2000:  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:  2023:  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:  2041:  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:  2057:  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:  2068:  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 2073: white space. Otherwise, the \g{ syntax or an empty comment (see 2074: <a href="#comments">"Comments"</a> 2075: below) can be used. 2076:  2077:   2078: Recursive back references 2079:   2080:  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:  2095:  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> 2102:  <a name="SEC20" href="#TOC1">ASSERTIONS</a>  2103:  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:  2109:  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:  2115:  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 2119: capturing is carried out only for positive assertions. (Perl sometimes, but not 2120: always, does do capturing in negative assertions.) 2121:  2122:  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:   2128:   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:   2134:   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:   2139:   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:  2143:   2144: Lookahead assertions 2145:   2146:  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:  2167:  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> 2173:   2174: Lookbehind assertions 2175:   2176:  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:  2211:  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:  2217:  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. 2223:  2224:  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: 2231: 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:  2257:   2258: Using multiple assertions 2259:   2260:  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 not 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:  2279:  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> 2292:  <a name="SEC21" href="#TOC1">CONDITIONAL SUBPATTERNS</a>  2293:  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:  2314:  2315: There are four kinds of condition: references to subpatterns, references to 2316: recursion, a pseudo-condition called DEFINE, and assertions. 2317:  2318:   2319: Checking for a used subpattern by number 2320:   2321:  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:  2335:  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:  2352:  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:  2360:   2361: Checking for a used subpattern by name 2362:   2363:  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 2366: this facility before Perl, the syntax (?(name)...) is also recognized. 2367:  2368:  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:  2377:   2378: Checking for pattern recursion 2379:   2380:  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: 2394: 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> 2399: 2400: Defining subpatterns for use by reference only 2401: 2402: 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 2410: "192.168.23.245" could be written like this (ignore white space and line 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: 2423: 2424: Assertion conditions 2425: 2426: 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> 2442:  <a name="SEC22" href="#TOC1">COMMENTS</a>  2443:  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:  2450:  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 2456: is controlled by the options passed to a compiling function or by a special 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, pcre_compile() 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> 2471:  <a name="SEC23" href="#TOC1">RECURSIVE PATTERNS</a>  2472:  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:  2478:  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:  2490:  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:  2496:  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:  2504:  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:  2516:  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:  2525:  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:  2532:  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:  2539:  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: 2549: 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: 2562: 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">pcrecallout</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: 2576: 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: pcre_malloc, freeing it via pcre_free afterwards. If no memory can 2580: be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. 2581: 2582: 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> 2594:   2595: Differences in recursion processing between PCRE and Perl 2596:   2597:  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:  2613:  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:  2620:  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:  2636:  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:  2662:  2663: WARNING: 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:  2670:  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> 2686:  <a name="SEC24" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a>  2687:  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:  2710:  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:  2717:  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> 2727:  <a name="SEC25" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a>  2728:  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 not 2743: synonymous. The former is a back reference; the latter is a subroutine call. 2744:  2745:  <a name="SEC26" href="#TOC1">CALLOUTS</a>  2746:  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:  2752:  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 2755: function by putting its entry point in the global variable pcre_callout 2756: (8-bit library) or pcre[16|32]_callout (16-bit or 32-bit library). 2757: By default, this variable contains NULL, which disables all calling out. 2758:  2759:  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> 2767: If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are 2768: automatically installed before each item in the pattern. They are all numbered 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. 2777:  2778:  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 2783: backtrack, or to fail altogether. 2784:  2785:  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 2791: <a href="pcrecallout.html">pcrecallout</a> 2792: documentation. 2793: <a name="backtrackcontrol"></a> 2794:  <a name="SEC27" href="#TOC1">BACKTRACKING CONTROL</a>  2795:  2796: Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which 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 2800: remarks apply to the PCRE features described in this section. 2801:  2802:  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. 2812:  2813:  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. 2820:  2821:  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. 2828: <a name="nooptimize"></a> 2829:   2830: Optimizations that affect backtracking verbs 2831:   2832:  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 2836: present. When one of these optimizations bypasses the running of a match, any 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 pcre_compile() or pcre_exec(), or by starting the 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 pcre_exec()"</a> 2843: in the 2844: <a href="pcreapi.html">pcreapi</a> 2845: documentation. 2846:  2847:  2848: Experiments with Perl suggest that it too has similar optimizations, sometimes 2849: leading to anomalous results. 2850:  2851:   2852: Verbs that act immediately 2853:   2854:  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 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:  2866:  2867: If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For 2868: example: 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:  2888:   2889: Recording which path was taken 2890:   2891:  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:  2901:  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 2905: <a href="pcreapi.html#extradata">"Extra data for pcre_exec()"</a> 2906: in the 2907: <a href="pcreapi.html">pcreapi</a> 2908: documentation. Here is an example of pcretest 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:  2924:  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. 2928:  2929:  2930: After a partial match or a failed match, the last encountered name in the 2931: entire match process is returned. For example: 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 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:  2942:  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. 2947:  2948:   2949: Verbs that act after backtracking 2950:   2951:  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 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.) 2961:  2962:  2963: These verbs differ in exactly what kind of failure occurs when backtracking 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. 2967: <pre> 2968: (*COMMIT) 2969: </pre> 2970: This verb, which may not be followed by a name, causes the whole match to fail 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 pcre_exec() is 2975: committed to finding a match at the current starting point, or not at all. For 2976: example: 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:  2985:  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:  2990:  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: pcretest 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 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:  3020:  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). 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> 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:  3051:  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). 3054: <pre> 3055: (*THEN) or (*THEN:NAME) 3056: </pre> 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: 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 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:  3071:  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). 3075:  3076:  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 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:  3097:  3098: Note that a conditional subpattern is not considered as having two 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:  3113:  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:  3122:   3123: More than one backtracking verb 3124:   3125:  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> 3141: If there is a matching failure to the right, backtracking onto (*PRUNE) causes 3142: it to be triggered, and its action is taken. There can never be a backtrack 3143: onto (*COMMIT). 3144: <a name="btrepeat"></a> 3145:   3146: Backtracking verbs in repeated groups 3147:   3148:  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> 3157:   3158: Backtracking verbs in assertions 3159:   3160:  3161: (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack. 3162:  3163:  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:  3168:  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:  3174:  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> 3183:   3184: Backtracking verbs in subroutines 3185:   3186:  3187: These behaviours occur whether or not the subpattern is called recursively. 3188: Perl's treatment of subroutines is different in some cases. 3189:  3190:  3191: (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces 3192: an immediate backtrack. 3193:  3194:  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:  3199:  3200: (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause 3201: the subroutine match to fail. 3202:  3203:  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. 3207:  3208:  <a name="SEC28" href="#TOC1">SEE ALSO</a>  3209:  3210: pcreapi(3), pcrecallout(3), pcrematching(3), 3211: pcresyntax(3), pcre(3), pcre16(3), pcre32(3). 3212:  3213:  <a name="SEC29" href="#TOC1">AUTHOR</a>  3214:  3215: Philip Hazel 3216:   3217: University Computing Service 3218:   3219: Cambridge CB2 3QH, England. 3220:   3221:  3222:  <a name="SEC30" href="#TOC1">REVISION</a>  3223:  3224: Last updated: 03 December 2013 3225:   3226: Copyright © 1997-2013 University of Cambridge. 3227:   3228:  3229: Return to the <a href="index.html">PCRE index page</a>. 3230: 