1 <html> 2 <head> 3 <title>pcrepattern specification</title> 4 </head> 5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"> 6 <h1>pcrepattern man page</h1> 7 <p> 8 Return to the <a href="index.html">PCRE index page</a>. 9 </p> 10 <p> 11 This page is part of the PCRE HTML documentation. It was generated automatically 12 from the original man page. If there is any nonsense in it, please consult the 13 man page, in case the conversion went wrong. 14 <br> 15 <ul> 16 <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a> 17 <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a> 18 <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a> 19 <li><a name="TOC4" href="#SEC4">BACKSLASH</a> 20 <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a> 21 <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT) AND \N</a> 22 <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a> 23 <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a> 24 <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a> 25 <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a> 26 <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a> 27 <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a> 28 <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a> 29 <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a> 30 <li><a name="TOC15" href="#SEC15">REPETITION</a> 31 <li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a> 32 <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a> 33 <li><a name="TOC18" href="#SEC18">ASSERTIONS</a> 34 <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a> 35 <li><a name="TOC20" href="#SEC20">COMMENTS</a> 36 <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a> 37 <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a> 38 <li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a> 39 <li><a name="TOC24" href="#SEC24">CALLOUTS</a> 40 <li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a> 41 <li><a name="TOC26" href="#SEC26">SEE ALSO</a> 42 <li><a name="TOC27" href="#SEC27">AUTHOR</a> 43 <li><a name="TOC28" href="#SEC28">REVISION</a> 44 </ul> 45 <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br> 46 <P> 47 The syntax and semantics of the regular expressions that are supported by PCRE 48 are described in detail below. There is a quick-reference syntax summary in the 49 <a href="pcresyntax.html"><b>pcresyntax</b></a> 50 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE 51 also supports some alternative regular expression syntax (which does not 52 conflict with the Perl syntax) in order to provide some compatibility with 53 regular expressions in Python, .NET, and Oniguruma. 54 </P> 55 <P> 56 Perl's regular expressions are described in its own documentation, and 57 regular expressions in general are covered in a number of books, some of which 58 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", 59 published by O'Reilly, covers regular expressions in great detail. This 60 description of PCRE's regular expressions is intended as reference material. 61 </P> 62 <P> 63 The original operation of PCRE was on strings of one-byte characters. However, 64 there is now also support for UTF-8 character strings. To use this, 65 PCRE must be built to include UTF-8 support, and you must call 66 <b>pcre_compile()</b> or <b>pcre_compile2()</b> with the PCRE_UTF8 option. There 67 is also a special sequence that can be given at the start of a pattern: 68 <pre> 69 (*UTF8) 70 </pre> 71 Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8 72 option. This feature is not Perl-compatible. How setting UTF-8 mode affects 73 pattern matching is mentioned in several places below. There is also a summary 74 of UTF-8 features in the 75 <a href="pcre.html#utf8support">section on UTF-8 support</a> 76 in the main 77 <a href="pcre.html"><b>pcre</b></a> 78 page. 79 </P> 80 <P> 81 Another special sequence that may appear at the start of a pattern or in 82 combination with (*UTF8) is: 83 <pre> 84 (*UCP) 85 </pre> 86 This has the same effect as setting the PCRE_UCP option: it causes sequences 87 such as \d and \w to use Unicode properties to determine character types, 88 instead of recognizing only characters with codes less than 128 via a lookup 89 table. 90 </P> 91 <P> 92 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the 93 PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are 94 also some more of these special sequences that are concerned with the handling 95 of newlines; they are described below. 96 </P> 97 <P> 98 The remainder of this document discusses the patterns that are supported by 99 PCRE when its main matching function, <b>pcre_exec()</b>, is used. 100 From release 6.0, PCRE offers a second matching function, 101 <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not 102 Perl-compatible. Some of the features discussed below are not available when 103 <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the 104 alternative function, and how it differs from the normal function, are 105 discussed in the 106 <a href="pcrematching.html"><b>pcrematching</b></a> 107 page. 108 <a name="newlines"></a></P> 109 <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br> 110 <P> 111 PCRE supports five different conventions for indicating line breaks in 112 strings: a single CR (carriage return) character, a single LF (linefeed) 113 character, the two-character sequence CRLF, any of the three preceding, or any 114 Unicode newline sequence. The 115 <a href="pcreapi.html"><b>pcreapi</b></a> 116 page has 117 <a href="pcreapi.html#newlines">further discussion</a> 118 about newlines, and shows how to set the newline convention in the 119 <i>options</i> arguments for the compiling and matching functions. 120 </P> 121 <P> 122 It is also possible to specify a newline convention by starting a pattern 123 string with one of the following five sequences: 124 <pre> 125 (*CR) carriage return 126 (*LF) linefeed 127 (*CRLF) carriage return, followed by linefeed 128 (*ANYCRLF) any of the three above 129 (*ANY) all Unicode newline sequences 130 </pre> 131 These override the default and the options given to <b>pcre_compile()</b> or 132 <b>pcre_compile2()</b>. For example, on a Unix system where LF is the default 133 newline sequence, the pattern 134 <pre> 135 (*CR)a.b 136 </pre> 137 changes the convention to CR. That pattern matches "a\nb" because LF is no 138 longer a newline. Note that these special settings, which are not 139 Perl-compatible, are recognized only at the very start of a pattern, and that 140 they must be in upper case. If more than one of them is present, the last one 141 is used. 142 </P> 143 <P> 144 The newline convention affects the interpretation of the dot metacharacter when 145 PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not 146 affect what the \R escape sequence matches. By default, this is any Unicode 147 newline sequence, for Perl compatibility. However, this can be changed; see the 148 description of \R in the section entitled 149 <a href="#newlineseq">"Newline sequences"</a> 150 below. A change of \R setting can be combined with a change of newline 151 convention. 152 </P> 153 <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br> 154 <P> 155 A regular expression is a pattern that is matched against a subject string from 156 left to right. Most characters stand for themselves in a pattern, and match the 157 corresponding characters in the subject. As a trivial example, the pattern 158 <pre> 159 The quick brown fox 160 </pre> 161 matches a portion of a subject string that is identical to itself. When 162 caseless matching is specified (the PCRE_CASELESS option), letters are matched 163 independently of case. In UTF-8 mode, PCRE always understands the concept of 164 case for characters whose values are less than 128, so caseless matching is 165 always possible. For characters with higher values, the concept of case is 166 supported if PCRE is compiled with Unicode property support, but not otherwise. 167 If you want to use caseless matching for characters 128 and above, you must 168 ensure that PCRE is compiled with Unicode property support as well as with 169 UTF-8 support. 170 </P> 171 <P> 172 The power of regular expressions comes from the ability to include alternatives 173 and repetitions in the pattern. These are encoded in the pattern by the use of 174 <i>metacharacters</i>, which do not stand for themselves but instead are 175 interpreted in some special way. 176 </P> 177 <P> 178 There are two different sets of metacharacters: those that are recognized 179 anywhere in the pattern except within square brackets, and those that are 180 recognized within square brackets. Outside square brackets, the metacharacters 181 are as follows: 182 <pre> 183 \ general escape character with several uses 184 ^ assert start of string (or line, in multiline mode) 185 $ assert end of string (or line, in multiline mode) 186 . match any character except newline (by default) 187 [ start character class definition 188 | start of alternative branch 189 ( start subpattern 190 ) end subpattern 191 ? extends the meaning of ( 192 also 0 or 1 quantifier 193 also quantifier minimizer 194 * 0 or more quantifier 195 + 1 or more quantifier 196 also "possessive quantifier" 197 { start min/max quantifier 198 </pre> 199 Part of a pattern that is in square brackets is called a "character class". In 200 a character class the only metacharacters are: 201 <pre> 202 \ general escape character 203 ^ negate the class, but only if the first character 204 - indicates character range 205 [ POSIX character class (only if followed by POSIX syntax) 206 ] terminates the character class 207 </pre> 208 The following sections describe the use of each of the metacharacters. 209 </P> 210 <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br> 211 <P> 212 The backslash character has several uses. Firstly, if it is followed by a 213 character that is not a number or a letter, it takes away any special meaning 214 that character may have. This use of backslash as an escape character applies 215 both inside and outside character classes. 216 </P> 217 <P> 218 For example, if you want to match a * character, you write \* in the pattern. 219 This escaping action applies whether or not the following character would 220 otherwise be interpreted as a metacharacter, so it is always safe to precede a 221 non-alphanumeric with backslash to specify that it stands for itself. In 222 particular, if you want to match a backslash, you write \\. 223 </P> 224 <P> 225 In UTF-8 mode, only ASCII numbers and letters have any special meaning after a 226 backslash. All other characters (in particular, those whose codepoints are 227 greater than 127) are treated as literals. 228 </P> 229 <P> 230 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the 231 pattern (other than in a character class) and characters between a # outside 232 a character class and the next newline are ignored. An escaping backslash can 233 be used to include a whitespace or # character as part of the pattern. 234 </P> 235 <P> 236 If you want to remove the special meaning from a sequence of characters, you 237 can do so by putting them between \Q and \E. This is different from Perl in 238 that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in 239 Perl, $ and @ cause variable interpolation. Note the following examples: 240 <pre> 241 Pattern PCRE matches Perl matches 242 243 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz 244 \Qabc\$xyz\E abc\$xyz abc\$xyz 245 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 246 </pre> 247 The \Q...\E sequence is recognized both inside and outside character classes. 248 An isolated \E that is not preceded by \Q is ignored. 249 <a name="digitsafterbackslash"></a></P> 250 <br><b> 251 Non-printing characters 252 </b><br> 253 <P> 254 A second use of backslash provides a way of encoding non-printing characters 255 in patterns in a visible manner. There is no restriction on the appearance of 256 non-printing characters, apart from the binary zero that terminates a pattern, 257 but when a pattern is being prepared by text editing, it is often easier to use 258 one of the following escape sequences than the binary character it represents: 259 <pre> 260 \a alarm, that is, the BEL character (hex 07) 261 \cx "control-x", where x is any ASCII character 262 \e escape (hex 1B) 263 \f formfeed (hex 0C) 264 \n linefeed (hex 0A) 265 \r carriage return (hex 0D) 266 \t tab (hex 09) 267 \ddd character with octal code ddd, or back reference 268 \xhh character with hex code hh 269 \x{hhh..} character with hex code hhh.. 270 </pre> 271 The precise effect of \cx is as follows: if x is a lower case letter, it 272 is converted to upper case. Then bit 6 of the character (hex 40) is inverted. 273 Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while 274 \c; becomes hex 7B (; is 3B). If the byte following \c has a value greater 275 than 127, a compile-time error occurs. This locks out non-ASCII characters in 276 both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte 277 values are valid. A lower case letter is converted to upper case, and then the 278 0xc0 bits are flipped.) 279 </P> 280 <P> 281 After \x, from zero to two hexadecimal digits are read (letters can be in 282 upper or lower case). Any number of hexadecimal digits may appear between \x{ 283 and }, but the value of the character code must be less than 256 in non-UTF-8 284 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in 285 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code 286 point, which is 10FFFF. 287 </P> 288 <P> 289 If characters other than hexadecimal digits appear between \x{ and }, or if 290 there is no terminating }, this form of escape is not recognized. Instead, the 291 initial \x will be interpreted as a basic hexadecimal escape, with no 292 following digits, giving a character whose value is zero. 293 </P> 294 <P> 295 Characters whose value is less than 256 can be defined by either of the two 296 syntaxes for \x. There is no difference in the way they are handled. For 297 example, \xdc is exactly the same as \x{dc}. 298 </P> 299 <P> 300 After \0 up to two further octal digits are read. If there are fewer than two 301 digits, just those that are present are used. Thus the sequence \0\x\07 302 specifies two binary zeros followed by a BEL character (code value 7). Make 303 sure you supply two digits after the initial zero if the pattern character that 304 follows is itself an octal digit. 305 </P> 306 <P> 307 The handling of a backslash followed by a digit other than 0 is complicated. 308 Outside a character class, PCRE reads it and any following digits as a decimal 309 number. If the number is less than 10, or if there have been at least that many 310 previous capturing left parentheses in the expression, the entire sequence is 311 taken as a <i>back reference</i>. A description of how this works is given 312 <a href="#backreferences">later,</a> 313 following the discussion of 314 <a href="#subpattern">parenthesized subpatterns.</a> 315 </P> 316 <P> 317 Inside a character class, or if the decimal number is greater than 9 and there 318 have not been that many capturing subpatterns, PCRE re-reads up to three octal 319 digits following the backslash, and uses them to generate a data character. Any 320 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a 321 character specified in octal must be less than \400. In UTF-8 mode, values up 322 to \777 are permitted. For example: 323 <pre> 324 \040 is another way of writing a space 325 \40 is the same, provided there are fewer than 40 previous capturing subpatterns 326 \7 is always a back reference 327 \11 might be a back reference, or another way of writing a tab 328 \011 is always a tab 329 \0113 is a tab followed by the character "3" 330 \113 might be a back reference, otherwise the character with octal code 113 331 \377 might be a back reference, otherwise the byte consisting entirely of 1 bits 332 \81 is either a back reference, or a binary zero followed by the two characters "8" and "1" 333 </pre> 334 Note that octal values of 100 or greater must not be introduced by a leading 335 zero, because no more than three octal digits are ever read. 336 </P> 337 <P> 338 All the sequences that define a single character value can be used both inside 339 and outside character classes. In addition, inside a character class, the 340 sequence \b is interpreted as the backspace character (hex 08). The sequences 341 \B, \N, \R, and \X are not special inside a character class. Like any other 342 unrecognized escape sequences, they are treated as the literal characters "B", 343 "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is 344 set. Outside a character class, these sequences have different meanings. 345 </P> 346 <br><b> 347 Absolute and relative back references 348 </b><br> 349 <P> 350 The sequence \g followed by an unsigned or a negative number, optionally 351 enclosed in braces, is an absolute or relative back reference. A named back 352 reference can be coded as \g{name}. Back references are discussed 353 <a href="#backreferences">later,</a> 354 following the discussion of 355 <a href="#subpattern">parenthesized subpatterns.</a> 356 </P> 357 <br><b> 358 Absolute and relative subroutine calls 359 </b><br> 360 <P> 361 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or 362 a number enclosed either in angle brackets or single quotes, is an alternative 363 syntax for referencing a subpattern as a "subroutine". Details are discussed 364 <a href="#onigurumasubroutines">later.</a> 365 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i> 366 synonymous. The former is a back reference; the latter is a 367 <a href="#subpatternsassubroutines">subroutine</a> 368 call. 369 <a name="genericchartypes"></a></P> 370 <br><b> 371 Generic character types 372 </b><br> 373 <P> 374 Another use of backslash is for specifying generic character types: 375 <pre> 376 \d any decimal digit 377 \D any character that is not a decimal digit 378 \h any horizontal whitespace character 379 \H any character that is not a horizontal whitespace character 380 \s any whitespace character 381 \S any character that is not a whitespace character 382 \v any vertical whitespace character 383 \V any character that is not a vertical whitespace character 384 \w any "word" character 385 \W any "non-word" character 386 </pre> 387 There is also the single sequence \N, which matches a non-newline character. 388 This is the same as 389 <a href="#fullstopdot">the "." metacharacter</a> 390 when PCRE_DOTALL is not set. 391 </P> 392 <P> 393 Each pair of lower and upper case escape sequences partitions the complete set 394 of characters into two disjoint sets. Any given character matches one, and only 395 one, of each pair. The sequences can appear both inside and outside character 396 classes. They each match one character of the appropriate type. If the current 397 matching point is at the end of the subject string, all of them fail, because 398 there is no character to match. 399 </P> 400 <P> 401 For compatibility with Perl, \s does not match the VT character (code 11). 402 This makes it different from the the POSIX "space" class. The \s characters 403 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is 404 included in a Perl script, \s may match the VT character. In PCRE, it never 405 does. 406 </P> 407 <P> 408 A "word" character is an underscore or any character that is a letter or digit. 409 By default, the definition of letters and digits is controlled by PCRE's 410 low-valued character tables, and may vary if locale-specific matching is taking 411 place (see 412 <a href="pcreapi.html#localesupport">"Locale support"</a> 413 in the 414 <a href="pcreapi.html"><b>pcreapi</b></a> 415 page). For example, in a French locale such as "fr_FR" in Unix-like systems, 416 or "french" in Windows, some character codes greater than 128 are used for 417 accented letters, and these are then matched by \w. The use of locales with 418 Unicode is discouraged. 419 </P> 420 <P> 421 By default, in UTF-8 mode, characters with values greater than 128 never match 422 \d, \s, or \w, and always match \D, \S, and \W. These sequences retain 423 their original meanings from before UTF-8 support was available, mainly for 424 efficiency reasons. However, if PCRE is compiled with Unicode property support, 425 and the PCRE_UCP option is set, the behaviour is changed so that Unicode 426 properties are used to determine character types, as follows: 427 <pre> 428 \d any character that \p{Nd} matches (decimal digit) 429 \s any character that \p{Z} matches, plus HT, LF, FF, CR 430 \w any character that \p{L} or \p{N} matches, plus underscore 431 </pre> 432 The upper case escapes match the inverse sets of characters. Note that \d 433 matches only decimal digits, whereas \w matches any Unicode digit, as well as 434 any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and 435 \B because they are defined in terms of \w and \W. Matching these sequences 436 is noticeably slower when PCRE_UCP is set. 437 </P> 438 <P> 439 The sequences \h, \H, \v, and \V are features that were added to Perl at 440 release 5.10. In contrast to the other sequences, which match only ASCII 441 characters by default, these always match certain high-valued codepoints in 442 UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters 443 are: 444 <pre> 445 U+0009 Horizontal tab 446 U+0020 Space 447 U+00A0 Non-break space 448 U+1680 Ogham space mark 449 U+180E Mongolian vowel separator 450 U+2000 En quad 451 U+2001 Em quad 452 U+2002 En space 453 U+2003 Em space 454 U+2004 Three-per-em space 455 U+2005 Four-per-em space 456 U+2006 Six-per-em space 457 U+2007 Figure space 458 U+2008 Punctuation space 459 U+2009 Thin space 460 U+200A Hair space 461 U+202F Narrow no-break space 462 U+205F Medium mathematical space 463 U+3000 Ideographic space 464 </pre> 465 The vertical space characters are: 466 <pre> 467 U+000A Linefeed 468 U+000B Vertical tab 469 U+000C Formfeed 470 U+000D Carriage return 471 U+0085 Next line 472 U+2028 Line separator 473 U+2029 Paragraph separator 474 <a name="newlineseq"></a></PRE> 475 </P> 476 <br><b> 477 Newline sequences 478 </b><br> 479 <P> 480 Outside a character class, by default, the escape sequence \R matches any 481 Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the following: 482 <pre> 483 (?>\r\n|\n|\x0b|\f|\r|\x85) 484 </pre> 485 This is an example of an "atomic group", details of which are given 486 <a href="#atomicgroup">below.</a> 487 This particular group matches either the two-character sequence CR followed by 488 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, 489 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next 490 line, U+0085). The two-character sequence is treated as a single unit that 491 cannot be split. 492 </P> 493 <P> 494 In UTF-8 mode, two additional characters whose codepoints are greater than 255 495 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). 496 Unicode character property support is not needed for these characters to be 497 recognized. 498 </P> 499 <P> 500 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the 501 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF 502 either at compile time or when the pattern is matched. (BSR is an abbrevation 503 for "backslash R".) This can be made the default when PCRE is built; if this is 504 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option. 505 It is also possible to specify these settings by starting a pattern string with 506 one of the following sequences: 507 <pre> 508 (*BSR_ANYCRLF) CR, LF, or CRLF only 509 (*BSR_UNICODE) any Unicode newline sequence 510 </pre> 511 These override the default and the options given to <b>pcre_compile()</b> or 512 <b>pcre_compile2()</b>, but they can be overridden by options given to 513 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. Note that these special settings, 514 which are not Perl-compatible, are recognized only at the very start of a 515 pattern, and that they must be in upper case. If more than one of them is 516 present, the last one is used. They can be combined with a change of newline 517 convention; for example, a pattern can start with: 518 <pre> 519 (*ANY)(*BSR_ANYCRLF) 520 </pre> 521 They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside 522 a character class, \R is treated as an unrecognized escape sequence, and so 523 matches the letter "R" by default, but causes an error if PCRE_EXTRA is set. 524 <a name="uniextseq"></a></P> 525 <br><b> 526 Unicode character properties 527 </b><br> 528 <P> 529 When PCRE is built with Unicode character property support, three additional 530 escape sequences that match characters with specific properties are available. 531 When not in UTF-8 mode, these sequences are of course limited to testing 532 characters whose codepoints are less than 256, but they do work in this mode. 533 The extra escape sequences are: 534 <pre> 535 \p{<i>xx</i>} a character with the <i>xx</i> property 536 \P{<i>xx</i>} a character without the <i>xx</i> property 537 \X an extended Unicode sequence 538 </pre> 539 The property names represented by <i>xx</i> above are limited to the Unicode 540 script names, the general category properties, "Any", which matches any 541 character (including newline), and some special PCRE properties (described 542 in the 543 <a href="#extraprops">next section).</a> 544 Other Perl properties such as "InMusicalSymbols" are not currently supported by 545 PCRE. Note that \P{Any} does not match any characters, so always causes a 546 match failure. 547 </P> 548 <P> 549 Sets of Unicode characters are defined as belonging to certain scripts. A 550 character from one of these sets can be matched using a script name. For 551 example: 552 <pre> 553 \p{Greek} 554 \P{Han} 555 </pre> 556 Those that are not part of an identified script are lumped together as 557 "Common". The current list of scripts is: 558 </P> 559 <P> 560 Arabic, 561 Armenian, 562 Avestan, 563 Balinese, 564 Bamum, 565 Bengali, 566 Bopomofo, 567 Braille, 568 Buginese, 569 Buhid, 570 Canadian_Aboriginal, 571 Carian, 572 Cham, 573 Cherokee, 574 Common, 575 Coptic, 576 Cuneiform, 577 Cypriot, 578 Cyrillic, 579 Deseret, 580 Devanagari, 581 Egyptian_Hieroglyphs, 582 Ethiopic, 583 Georgian, 584 Glagolitic, 585 Gothic, 586 Greek, 587 Gujarati, 588 Gurmukhi, 589 Han, 590 Hangul, 591 Hanunoo, 592 Hebrew, 593 Hiragana, 594 Imperial_Aramaic, 595 Inherited, 596 Inscriptional_Pahlavi, 597 Inscriptional_Parthian, 598 Javanese, 599 Kaithi, 600 Kannada, 601 Katakana, 602 Kayah_Li, 603 Kharoshthi, 604 Khmer, 605 Lao, 606 Latin, 607 Lepcha, 608 Limbu, 609 Linear_B, 610 Lisu, 611 Lycian, 612 Lydian, 613 Malayalam, 614 Meetei_Mayek, 615 Mongolian, 616 Myanmar, 617 New_Tai_Lue, 618 Nko, 619 Ogham, 620 Old_Italic, 621 Old_Persian, 622 Old_South_Arabian, 623 Old_Turkic, 624 Ol_Chiki, 625 Oriya, 626 Osmanya, 627 Phags_Pa, 628 Phoenician, 629 Rejang, 630 Runic, 631 Samaritan, 632 Saurashtra, 633 Shavian, 634 Sinhala, 635 Sundanese, 636 Syloti_Nagri, 637 Syriac, 638 Tagalog, 639 Tagbanwa, 640 Tai_Le, 641 Tai_Tham, 642 Tai_Viet, 643 Tamil, 644 Telugu, 645 Thaana, 646 Thai, 647 Tibetan, 648 Tifinagh, 649 Ugaritic, 650 Vai, 651 Yi. 652 </P> 653 <P> 654 Each character has exactly one Unicode general category property, specified by 655 a two-letter abbreviation. For compatibility with Perl, negation can be 656 specified by including a circumflex between the opening brace and the property 657 name. For example, \p{^Lu} is the same as \P{Lu}. 658 </P> 659 <P> 660 If only one letter is specified with \p or \P, it includes all the general 661 category properties that start with that letter. In this case, in the absence 662 of negation, the curly brackets in the escape sequence are optional; these two 663 examples have the same effect: 664 <pre> 665 \p{L} 666 \pL 667 </pre> 668 The following general category property codes are supported: 669 <pre> 670 C Other 671 Cc Control 672 Cf Format 673 Cn Unassigned 674 Co Private use 675 Cs Surrogate 676 677 L Letter 678 Ll Lower case letter 679 Lm Modifier letter 680 Lo Other letter 681 Lt Title case letter 682 Lu Upper case letter 683 684 M Mark 685 Mc Spacing mark 686 Me Enclosing mark 687 Mn Non-spacing mark 688 689 N Number 690 Nd Decimal number 691 Nl Letter number 692 No Other number 693 694 P Punctuation 695 Pc Connector punctuation 696 Pd Dash punctuation 697 Pe Close punctuation 698 Pf Final punctuation 699 Pi Initial punctuation 700 Po Other punctuation 701 Ps Open punctuation 702 703 S Symbol 704 Sc Currency symbol 705 Sk Modifier symbol 706 Sm Mathematical symbol 707 So Other symbol 708 709 Z Separator 710 Zl Line separator 711 Zp Paragraph separator 712 Zs Space separator 713 </pre> 714 The special property L& is also supported: it matches a character that has 715 the Lu, Ll, or Lt property, in other words, a letter that is not classified as 716 a modifier or "other". 717 </P> 718 <P> 719 The Cs (Surrogate) property applies only to characters in the range U+D800 to 720 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so 721 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off 722 (see the discussion of PCRE_NO_UTF8_CHECK in the 723 <a href="pcreapi.html"><b>pcreapi</b></a> 724 page). Perl does not support the Cs property. 725 </P> 726 <P> 727 The long synonyms for property names that Perl supports (such as \p{Letter}) 728 are not supported by PCRE, nor is it permitted to prefix any of these 729 properties with "Is". 730 </P> 731 <P> 732 No character that is in the Unicode table has the Cn (unassigned) property. 733 Instead, this property is assumed for any code point that is not in the 734 Unicode table. 735 </P> 736 <P> 737 Specifying caseless matching does not affect these escape sequences. For 738 example, \p{Lu} always matches only upper case letters. 739 </P> 740 <P> 741 The \X escape matches any number of Unicode characters that form an extended 742 Unicode sequence. \X is equivalent to 743 <pre> 744 (?>\PM\pM*) 745 </pre> 746 That is, it matches a character without the "mark" property, followed by zero 747 or more characters with the "mark" property, and treats the sequence as an 748 atomic group 749 <a href="#atomicgroup">(see below).</a> 750 Characters with the "mark" property are typically accents that affect the 751 preceding character. None of them have codepoints less than 256, so in 752 non-UTF-8 mode \X matches any one character. 753 </P> 754 <P> 755 Matching characters by Unicode property is not fast, because PCRE has to search 756 a structure that contains data for over fifteen thousand characters. That is 757 why the traditional escape sequences such as \d and \w do not use Unicode 758 properties in PCRE by default, though you can make them do so by setting the 759 PCRE_UCP option for <b>pcre_compile()</b> or by starting the pattern with 760 (*UCP). 761 <a name="extraprops"></a></P> 762 <br><b> 763 PCRE's additional properties 764 </b><br> 765 <P> 766 As well as the standard Unicode properties described in the previous 767 section, PCRE supports four more that make it possible to convert traditional 768 escape sequences such as \w and \s and POSIX character classes to use Unicode 769 properties. PCRE uses these non-standard, non-Perl properties internally when 770 PCRE_UCP is set. They are: 771 <pre> 772 Xan Any alphanumeric character 773 Xps Any POSIX space character 774 Xsp Any Perl space character 775 Xwd Any Perl "word" character 776 </pre> 777 Xan matches characters that have either the L (letter) or the N (number) 778 property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or 779 carriage return, and any other character that has the Z (separator) property. 780 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the 781 same characters as Xan, plus underscore. 782 <a name="resetmatchstart"></a></P> 783 <br><b> 784 Resetting the match start 785 </b><br> 786 <P> 787 The escape sequence \K causes any previously matched characters not to be 788 included in the final matched sequence. For example, the pattern: 789 <pre> 790 foo\Kbar 791 </pre> 792 matches "foobar", but reports that it has matched "bar". This feature is 793 similar to a lookbehind assertion 794 <a href="#lookbehind">(described below).</a> 795 However, in this case, the part of the subject before the real match does not 796 have to be of fixed length, as lookbehind assertions do. The use of \K does 797 not interfere with the setting of 798 <a href="#subpattern">captured substrings.</a> 799 For example, when the pattern 800 <pre> 801 (foo)\Kbar 802 </pre> 803 matches "foobar", the first substring is still set to "foo". 804 </P> 805 <P> 806 Perl documents that the use of \K within assertions is "not well defined". In 807 PCRE, \K is acted upon when it occurs inside positive assertions, but is 808 ignored in negative assertions. 809 <a name="smallassertions"></a></P> 810 <br><b> 811 Simple assertions 812 </b><br> 813 <P> 814 The final use of backslash is for certain simple assertions. An assertion 815 specifies a condition that has to be met at a particular point in a match, 816 without consuming any characters from the subject string. The use of 817 subpatterns for more complicated assertions is described 818 <a href="#bigassertions">below.</a> 819 The backslashed assertions are: 820 <pre> 821 \b matches at a word boundary 822 \B matches when not at a word boundary 823 \A matches at the start of the subject 824 \Z matches at the end of the subject 825 also matches before a newline at the end of the subject 826 \z matches only at the end of the subject 827 \G matches at the first matching position in the subject 828 </pre> 829 Inside a character class, \b has a different meaning; it matches the backspace 830 character. If any other of these assertions appears in a character class, by 831 default it matches the corresponding literal character (for example, \B 832 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid 833 escape sequence" error is generated instead. 834 </P> 835 <P> 836 A word boundary is a position in the subject string where the current character 837 and the previous character do not both match \w or \W (i.e. one matches 838 \w and the other matches \W), or the start or end of the string if the 839 first or last character matches \w, respectively. In UTF-8 mode, the meanings 840 of \w and \W can be changed by setting the PCRE_UCP option. When this is 841 done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start 842 of word" or "end of word" metasequence. However, whatever follows \b normally 843 determines which it is. For example, the fragment \ba matches "a" at the start 844 of a word. 845 </P> 846 <P> 847 The \A, \Z, and \z assertions differ from the traditional circumflex and 848 dollar (described in the next section) in that they only ever match at the very 849 start and end of the subject string, whatever options are set. Thus, they are 850 independent of multiline mode. These three assertions are not affected by the 851 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the 852 circumflex and dollar metacharacters. However, if the <i>startoffset</i> 853 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start 854 at a point other than the beginning of the subject, \A can never match. The 855 difference between \Z and \z is that \Z matches before a newline at the end 856 of the string as well as at the very end, whereas \z matches only at the end. 857 </P> 858 <P> 859 The \G assertion is true only when the current matching position is at the 860 start point of the match, as specified by the <i>startoffset</i> argument of 861 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is 862 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate 863 arguments, you can mimic Perl's /g option, and it is in this kind of 864 implementation where \G can be useful. 865 </P> 866 <P> 867 Note, however, that PCRE's interpretation of \G, as the start of the current 868 match, is subtly different from Perl's, which defines it as the end of the 869 previous match. In Perl, these can be different when the previously matched 870 string was empty. Because PCRE does just one match at a time, it cannot 871 reproduce this behaviour. 872 </P> 873 <P> 874 If all the alternatives of a pattern begin with \G, the expression is anchored 875 to the starting match position, and the "anchored" flag is set in the compiled 876 regular expression. 877 </P> 878 <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br> 879 <P> 880 Outside a character class, in the default matching mode, the circumflex 881 character is an assertion that is true only if the current matching point is 882 at the start of the subject string. If the <i>startoffset</i> argument of 883 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE 884 option is unset. Inside a character class, circumflex has an entirely different 885 meaning 886 <a href="#characterclass">(see below).</a> 887 </P> 888 <P> 889 Circumflex need not be the first character of the pattern if a number of 890 alternatives are involved, but it should be the first thing in each alternative 891 in which it appears if the pattern is ever to match that branch. If all 892 possible alternatives start with a circumflex, that is, if the pattern is 893 constrained to match only at the start of the subject, it is said to be an 894 "anchored" pattern. (There are also other constructs that can cause a pattern 895 to be anchored.) 896 </P> 897 <P> 898 A dollar character is an assertion that is true only if the current matching 899 point is at the end of the subject string, or immediately before a newline 900 at the end of the string (by default). Dollar need not be the last character of 901 the pattern if a number of alternatives are involved, but it should be the last 902 item in any branch in which it appears. Dollar has no special meaning in a 903 character class. 904 </P> 905 <P> 906 The meaning of dollar can be changed so that it matches only at the very end of 907 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This 908 does not affect the \Z assertion. 909 </P> 910 <P> 911 The meanings of the circumflex and dollar characters are changed if the 912 PCRE_MULTILINE option is set. When this is the case, a circumflex matches 913 immediately after internal newlines as well as at the start of the subject 914 string. It does not match after a newline that ends the string. A dollar 915 matches before any newlines in the string, as well as at the very end, when 916 PCRE_MULTILINE is set. When newline is specified as the two-character 917 sequence CRLF, isolated CR and LF characters do not indicate newlines. 918 </P> 919 <P> 920 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where 921 \n represents a newline) in multiline mode, but not otherwise. Consequently, 922 patterns that are anchored in single line mode because all branches start with 923 ^ are not anchored in multiline mode, and a match for circumflex is possible 924 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The 925 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. 926 </P> 927 <P> 928 Note that the sequences \A, \Z, and \z can be used to match the start and 929 end of the subject in both modes, and if all branches of a pattern start with 930 \A it is always anchored, whether or not PCRE_MULTILINE is set. 931 <a name="fullstopdot"></a></P> 932 <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br> 933 <P> 934 Outside a character class, a dot in the pattern matches any one character in 935 the subject string except (by default) a character that signifies the end of a 936 line. In UTF-8 mode, the matched character may be more than one byte long. 937 </P> 938 <P> 939 When a line ending is defined as a single character, dot never matches that 940 character; when the two-character sequence CRLF is used, dot does not match CR 941 if it is immediately followed by LF, but otherwise it matches all characters 942 (including isolated CRs and LFs). When any Unicode line endings are being 943 recognized, dot does not match CR or LF or any of the other line ending 944 characters. 945 </P> 946 <P> 947 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL 948 option is set, a dot matches any one character, without exception. If the 949 two-character sequence CRLF is present in the subject string, it takes two dots 950 to match it. 951 </P> 952 <P> 953 The handling of dot is entirely independent of the handling of circumflex and 954 dollar, the only relationship being that they both involve newlines. Dot has no 955 special meaning in a character class. 956 </P> 957 <P> 958 The escape sequence \N behaves like a dot, except that it is not affected by 959 the PCRE_DOTALL option. In other words, it matches any character except one 960 that signifies the end of a line. 961 </P> 962 <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br> 963 <P> 964 Outside a character class, the escape sequence \C matches any one byte, both 965 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending 966 characters. The feature is provided in Perl in order to match individual bytes 967 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the 968 rest of the string may start with a malformed UTF-8 character. For this reason, 969 the \C escape sequence is best avoided. 970 </P> 971 <P> 972 PCRE does not allow \C to appear in lookbehind assertions 973 <a href="#lookbehind">(described below),</a> 974 because in UTF-8 mode this would make it impossible to calculate the length of 975 the lookbehind. 976 <a name="characterclass"></a></P> 977 <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br> 978 <P> 979 An opening square bracket introduces a character class, terminated by a closing 980 square bracket. A closing square bracket on its own is not special by default. 981 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square 982 bracket causes a compile-time error. If a closing square bracket is required as 983 a member of the class, it should be the first data character in the class 984 (after an initial circumflex, if present) or escaped with a backslash. 985 </P> 986 <P> 987 A character class matches a single character in the subject. In UTF-8 mode, the 988 character may be more than one byte long. A matched character must be in the 989 set of characters defined by the class, unless the first character in the class 990 definition is a circumflex, in which case the subject character must not be in 991 the set defined by the class. If a circumflex is actually required as a member 992 of the class, ensure it is not the first character, or escape it with a 993 backslash. 994 </P> 995 <P> 996 For example, the character class [aeiou] matches any lower case vowel, while 997 [^aeiou] matches any character that is not a lower case vowel. Note that a 998 circumflex is just a convenient notation for specifying the characters that 999 are in the class by enumerating those that are not. A class that starts with a 1000 circumflex is not an assertion; it still consumes a character from the subject 1001 string, and therefore it fails if the current pointer is at the end of the 1002 string. 1003 </P> 1004 <P> 1005 In UTF-8 mode, characters with values greater than 255 can be included in a 1006 class as a literal string of bytes, or by using the \x{ escaping mechanism. 1007 </P> 1008 <P> 1009 When caseless matching is set, any letters in a class represent both their 1010 upper case and lower case versions, so for example, a caseless [aeiou] matches 1011 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a 1012 caseful version would. In UTF-8 mode, PCRE always understands the concept of 1013 case for characters whose values are less than 128, so caseless matching is 1014 always possible. For characters with higher values, the concept of case is 1015 supported if PCRE is compiled with Unicode property support, but not otherwise. 1016 If you want to use caseless matching in UTF8-mode for characters 128 and above, 1017 you must ensure that PCRE is compiled with Unicode property support as well as 1018 with UTF-8 support. 1019 </P> 1020 <P> 1021 Characters that might indicate line breaks are never treated in any special way 1022 when matching character classes, whatever line-ending sequence is in use, and 1023 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class 1024 such as [^a] always matches one of these characters. 1025 </P> 1026 <P> 1027 The minus (hyphen) character can be used to specify a range of characters in a 1028 character class. For example, [d-m] matches any letter between d and m, 1029 inclusive. If a minus character is required in a class, it must be escaped with 1030 a backslash or appear in a position where it cannot be interpreted as 1031 indicating a range, typically as the first or last character in the class. 1032 </P> 1033 <P> 1034 It is not possible to have the literal character "]" as the end character of a 1035 range. A pattern such as [W-]46] is interpreted as a class of two characters 1036 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or 1037 "-46]". However, if the "]" is escaped with a backslash it is interpreted as 1038 the end of range, so [W-\]46] is interpreted as a class containing a range 1039 followed by two other characters. The octal or hexadecimal representation of 1040 "]" can also be used to end a range. 1041 </P> 1042 <P> 1043 Ranges operate in the collating sequence of character values. They can also be 1044 used for characters specified numerically, for example [\000-\037]. In UTF-8 1045 mode, ranges can include characters whose values are greater than 255, for 1046 example [\x{100}-\x{2ff}]. 1047 </P> 1048 <P> 1049 If a range that includes letters is used when caseless matching is set, it 1050 matches the letters in either case. For example, [W-c] is equivalent to 1051 [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character 1052 tables for a French locale are in use, [\xc8-\xcb] matches accented E 1053 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for 1054 characters with values greater than 128 only when it is compiled with Unicode 1055 property support. 1056 </P> 1057 <P> 1058 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, 1059 \V, \w, and \W may appear in a character class, and add the characters that 1060 they match to the class. For example, [\dABCDEF] matches any hexadecimal 1061 digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \d, \s, \w 1062 and their upper case partners, just as it does when they appear outside a 1063 character class, as described in the section entitled 1064 <a href="#genericchartypes">"Generic character types"</a> 1065 above. The escape sequence \b has a different meaning inside a character 1066 class; it matches the backspace character. The sequences \B, \N, \R, and \X 1067 are not special inside a character class. Like any other unrecognized escape 1068 sequences, they are treated as the literal characters "B", "N", "R", and "X" by 1069 default, but cause an error if the PCRE_EXTRA option is set. 1070 </P> 1071 <P> 1072 A circumflex can conveniently be used with the upper case character types to 1073 specify a more restricted set of characters than the matching lower case type. 1074 For example, the class [^\W_] matches any letter or digit, but not underscore, 1075 whereas [\w] includes underscore. A positive character class should be read as 1076 "something OR something OR ..." and a negative class as "NOT something AND NOT 1077 something AND NOT ...". 1078 </P> 1079 <P> 1080 The only metacharacters that are recognized in character classes are backslash, 1081 hyphen (only where it can be interpreted as specifying a range), circumflex 1082 (only at the start), opening square bracket (only when it can be interpreted as 1083 introducing a POSIX class name - see the next section), and the terminating 1084 closing square bracket. However, escaping other non-alphanumeric characters 1085 does no harm. 1086 </P> 1087 <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br> 1088 <P> 1089 Perl supports the POSIX notation for character classes. This uses names 1090 enclosed by [: and :] within the enclosing square brackets. PCRE also supports 1091 this notation. For example, 1092 <pre> 1093 [01[:alpha:]%] 1094 </pre> 1095 matches "0", "1", any alphabetic character, or "%". The supported class names 1096 are: 1097 <pre> 1098 alnum letters and digits 1099 alpha letters 1100 ascii character codes 0 - 127 1101 blank space or tab only 1102 cntrl control characters 1103 digit decimal digits (same as \d) 1104 graph printing characters, excluding space 1105 lower lower case letters 1106 print printing characters, including space 1107 punct printing characters, excluding letters and digits and space 1108 space white space (not quite the same as \s) 1109 upper upper case letters 1110 word "word" characters (same as \w) 1111 xdigit hexadecimal digits 1112 </pre> 1113 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and 1114 space (32). Notice that this list includes the VT character (code 11). This 1115 makes "space" different to \s, which does not include VT (for Perl 1116 compatibility). 1117 </P> 1118 <P> 1119 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 1120 5.8. Another Perl extension is negation, which is indicated by a ^ character 1121 after the colon. For example, 1122 <pre> 1123 [12[:^digit:]] 1124 </pre> 1125 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX 1126 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not 1127 supported, and an error is given if they are encountered. 1128 </P> 1129 <P> 1130 By default, in UTF-8 mode, characters with values greater than 128 do not match 1131 any of the POSIX character classes. However, if the PCRE_UCP option is passed 1132 to <b>pcre_compile()</b>, some of the classes are changed so that Unicode 1133 character properties are used. This is achieved by replacing the POSIX classes 1134 by other sequences, as follows: 1135 <pre> 1136 [:alnum:] becomes \p{Xan} 1137 [:alpha:] becomes \p{L} 1138 [:blank:] becomes \h 1139 [:digit:] becomes \p{Nd} 1140 [:lower:] becomes \p{Ll} 1141 [:space:] becomes \p{Xps} 1142 [:upper:] becomes \p{Lu} 1143 [:word:] becomes \p{Xwd} 1144 </pre> 1145 Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX 1146 classes are unchanged, and match only characters with code points less than 1147 128. 1148 </P> 1149 <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br> 1150 <P> 1151 Vertical bar characters are used to separate alternative patterns. For example, 1152 the pattern 1153 <pre> 1154 gilbert|sullivan 1155 </pre> 1156 matches either "gilbert" or "sullivan". Any number of alternatives may appear, 1157 and an empty alternative is permitted (matching the empty string). The matching 1158 process tries each alternative in turn, from left to right, and the first one 1159 that succeeds is used. If the alternatives are within a subpattern 1160 <a href="#subpattern">(defined below),</a> 1161 "succeeds" means matching the rest of the main pattern as well as the 1162 alternative in the subpattern. 1163 </P> 1164 <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br> 1165 <P> 1166 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and 1167 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within 1168 the pattern by a sequence of Perl option letters enclosed between "(?" and ")". 1169 The option letters are 1170 <pre> 1171 i for PCRE_CASELESS 1172 m for PCRE_MULTILINE 1173 s for PCRE_DOTALL 1174 x for PCRE_EXTENDED 1175 </pre> 1176 For example, (?im) sets caseless, multiline matching. It is also possible to 1177 unset these options by preceding the letter with a hyphen, and a combined 1178 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and 1179 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also 1180 permitted. If a letter appears both before and after the hyphen, the option is 1181 unset. 1182 </P> 1183 <P> 1184 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be 1185 changed in the same way as the Perl-compatible options by using the characters 1186 J, U and X respectively. 1187 </P> 1188 <P> 1189 When one of these option changes occurs at top level (that is, not inside 1190 subpattern parentheses), the change applies to the remainder of the pattern 1191 that follows. If the change is placed right at the start of a pattern, PCRE 1192 extracts it into the global options (and it will therefore show up in data 1193 extracted by the <b>pcre_fullinfo()</b> function). 1194 </P> 1195 <P> 1196 An option change within a subpattern (see below for a description of 1197 subpatterns) affects only that part of the subpattern that follows it, so 1198 <pre> 1199 (a(?i)b)c 1200 </pre> 1201 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). 1202 By this means, options can be made to have different settings in different 1203 parts of the pattern. Any changes made in one alternative do carry on 1204 into subsequent branches within the same subpattern. For example, 1205 <pre> 1206 (a(?i)b|c) 1207 </pre> 1208 matches "ab", "aB", "c", and "C", even though when matching "C" the first 1209 branch is abandoned before the option setting. This is because the effects of 1210 option settings happen at compile time. There would be some very weird 1211 behaviour otherwise. 1212 </P> 1213 <P> 1214 <b>Note:</b> There are other PCRE-specific options that can be set by the 1215 application when the compile or match functions are called. In some cases the 1216 pattern can contain special leading sequences such as (*CRLF) to override what 1217 the application has set or what has been defaulted. Details are given in the 1218 section entitled 1219 <a href="#newlineseq">"Newline sequences"</a> 1220 above. There are also the (*UTF8) and (*UCP) leading sequences that can be used 1221 to set UTF-8 and Unicode property modes; they are equivalent to setting the 1222 PCRE_UTF8 and the PCRE_UCP options, respectively. 1223 <a name="subpattern"></a></P> 1224 <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br> 1225 <P> 1226 Subpatterns are delimited by parentheses (round brackets), which can be nested. 1227 Turning part of a pattern into a subpattern does two things: 1228 <br> 1229 <br> 1230 1. It localizes a set of alternatives. For example, the pattern 1231 <pre> 1232 cat(aract|erpillar|) 1233 </pre> 1234 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would 1235 match "cataract", "erpillar" or an empty string. 1236 <br> 1237 <br> 1238 2. It sets up the subpattern as a capturing subpattern. This means that, when 1239 the whole pattern matches, that portion of the subject string that matched the 1240 subpattern is passed back to the caller via the <i>ovector</i> argument of 1241 <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting 1242 from 1) to obtain numbers for the capturing subpatterns. For example, if the 1243 string "the red king" is matched against the pattern 1244 <pre> 1245 the ((red|white) (king|queen)) 1246 </pre> 1247 the captured substrings are "red king", "red", and "king", and are numbered 1, 1248 2, and 3, respectively. 1249 </P> 1250 <P> 1251 The fact that plain parentheses fulfil two functions is not always helpful. 1252 There are often times when a grouping subpattern is required without a 1253 capturing requirement. If an opening parenthesis is followed by a question mark 1254 and a colon, the subpattern does not do any capturing, and is not counted when 1255 computing the number of any subsequent capturing subpatterns. For example, if 1256 the string "the white queen" is matched against the pattern 1257 <pre> 1258 the ((?:red|white) (king|queen)) 1259 </pre> 1260 the captured substrings are "white queen" and "queen", and are numbered 1 and 1261 2. The maximum number of capturing subpatterns is 65535. 1262 </P> 1263 <P> 1264 As a convenient shorthand, if any option settings are required at the start of 1265 a non-capturing subpattern, the option letters may appear between the "?" and 1266 the ":". Thus the two patterns 1267 <pre> 1268 (?i:saturday|sunday) 1269 (?:(?i)saturday|sunday) 1270 </pre> 1271 match exactly the same set of strings. Because alternative branches are tried 1272 from left to right, and options are not reset until the end of the subpattern 1273 is reached, an option setting in one branch does affect subsequent branches, so 1274 the above patterns match "SUNDAY" as well as "Saturday". 1275 <a name="dupsubpatternnumber"></a></P> 1276 <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br> 1277 <P> 1278 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses 1279 the same numbers for its capturing parentheses. Such a subpattern starts with 1280 (?| and is itself a non-capturing subpattern. For example, consider this 1281 pattern: 1282 <pre> 1283 (?|(Sat)ur|(Sun))day 1284 </pre> 1285 Because the two alternatives are inside a (?| group, both sets of capturing 1286 parentheses are numbered one. Thus, when the pattern matches, you can look 1287 at captured substring number one, whichever alternative matched. This construct 1288 is useful when you want to capture part, but not all, of one of a number of 1289 alternatives. Inside a (?| group, parentheses are numbered as usual, but the 1290 number is reset at the start of each branch. The numbers of any capturing 1291 parentheses that follow the subpattern start after the highest number used in 1292 any branch. The following example is taken from the Perl documentation. The 1293 numbers underneath show in which buffer the captured content will be stored. 1294 <pre> 1295 # before ---------------branch-reset----------- after 1296 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x 1297 # 1 2 2 3 2 3 4 1298 </pre> 1299 A back reference to a numbered subpattern uses the most recent value that is 1300 set for that number by any subpattern. The following pattern matches "abcabc" 1301 or "defdef": 1302 <pre> 1303 /(?|(abc)|(def))\1/ 1304 </pre> 1305 In contrast, a recursive or "subroutine" call to a numbered subpattern always 1306 refers to the first one in the pattern with the given number. The following 1307 pattern matches "abcabc" or "defabc": 1308 <pre> 1309 /(?|(abc)|(def))(?1)/ 1310 </pre> 1311 If a 1312 <a href="#conditions">condition test</a> 1313 for a subpattern's having matched refers to a non-unique number, the test is 1314 true if any of the subpatterns of that number have matched. 1315 </P> 1316 <P> 1317 An alternative approach to using this "branch reset" feature is to use 1318 duplicate named subpatterns, as described in the next section. 1319 </P> 1320 <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br> 1321 <P> 1322 Identifying capturing parentheses by number is simple, but it can be very hard 1323 to keep track of the numbers in complicated regular expressions. Furthermore, 1324 if an expression is modified, the numbers may change. To help with this 1325 difficulty, PCRE supports the naming of subpatterns. This feature was not 1326 added to Perl until release 5.10. Python had the feature earlier, and PCRE 1327 introduced it at release 4.0, using the Python syntax. PCRE now supports both 1328 the Perl and the Python syntax. Perl allows identically numbered subpatterns to 1329 have different names, but PCRE does not. 1330 </P> 1331 <P> 1332 In PCRE, a subpattern can be named in one of three ways: (?<name>...) or 1333 (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing 1334 parentheses from other parts of the pattern, such as 1335 <a href="#backreferences">back references,</a> 1336 <a href="#recursion">recursion,</a> 1337 and 1338 <a href="#conditions">conditions,</a> 1339 can be made by name as well as by number. 1340 </P> 1341 <P> 1342 Names consist of up to 32 alphanumeric characters and underscores. Named 1343 capturing parentheses are still allocated numbers as well as names, exactly as 1344 if the names were not present. The PCRE API provides function calls for 1345 extracting the name-to-number translation table from a compiled pattern. There 1346 is also a convenience function for extracting a captured substring by name. 1347 </P> 1348 <P> 1349 By default, a name must be unique within a pattern, but it is possible to relax 1350 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate 1351 names are also always permitted for subpatterns with the same number, set up as 1352 described in the previous section.) Duplicate names can be useful for patterns 1353 where only one instance of the named parentheses can match. Suppose you want to 1354 match the name of a weekday, either as a 3-letter abbreviation or as the full 1355 name, and in both cases you want to extract the abbreviation. This pattern 1356 (ignoring the line breaks) does the job: 1357 <pre> 1358 (?<DN>Mon|Fri|Sun)(?:day)?| 1359 (?<DN>Tue)(?:sday)?| 1360 (?<DN>Wed)(?:nesday)?| 1361 (?<DN>Thu)(?:rsday)?| 1362 (?<DN>Sat)(?:urday)? 1363 </pre> 1364 There are five capturing substrings, but only one is ever set after a match. 1365 (An alternative way of solving this problem is to use a "branch reset" 1366 subpattern, as described in the previous section.) 1367 </P> 1368 <P> 1369 The convenience function for extracting the data by name returns the substring 1370 for the first (and in this example, the only) subpattern of that name that 1371 matched. This saves searching to find which numbered subpattern it was. 1372 </P> 1373 <P> 1374 If you make a back reference to a non-unique named subpattern from elsewhere in 1375 the pattern, the one that corresponds to the first occurrence of the name is 1376 used. In the absence of duplicate numbers (see the previous section) this is 1377 the one with the lowest number. If you use a named reference in a condition 1378 test (see the 1379 <a href="#conditions">section about conditions</a> 1380 below), either to check whether a subpattern has matched, or to check for 1381 recursion, all subpatterns with the same name are tested. If the condition is 1382 true for any one of them, the overall condition is true. This is the same 1383 behaviour as testing by number. For further details of the interfaces for 1384 handling named subpatterns, see the 1385 <a href="pcreapi.html"><b>pcreapi</b></a> 1386 documentation. 1387 </P> 1388 <P> 1389 <b>Warning:</b> You cannot use different names to distinguish between two 1390 subpatterns with the same number because PCRE uses only the numbers when 1391 matching. For this reason, an error is given at compile time if different names 1392 are given to subpatterns with the same number. However, you can give the same 1393 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set. 1394 </P> 1395 <br><a name="SEC15" href="#TOC1">REPETITION</a><br> 1396 <P> 1397 Repetition is specified by quantifiers, which can follow any of the following 1398 items: 1399 <pre> 1400 a literal data character 1401 the dot metacharacter 1402 the \C escape sequence 1403 the \X escape sequence (in UTF-8 mode with Unicode properties) 1404 the \R escape sequence 1405 an escape such as \d or \pL that matches a single character 1406 a character class 1407 a back reference (see next section) 1408 a parenthesized subpattern (unless it is an assertion) 1409 a recursive or "subroutine" call to a subpattern 1410 </pre> 1411 The general repetition quantifier specifies a minimum and maximum number of 1412 permitted matches, by giving the two numbers in curly brackets (braces), 1413 separated by a comma. The numbers must be less than 65536, and the first must 1414 be less than or equal to the second. For example: 1415 <pre> 1416 z{2,4} 1417 </pre> 1418 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special 1419 character. If the second number is omitted, but the comma is present, there is 1420 no upper limit; if the second number and the comma are both omitted, the 1421 quantifier specifies an exact number of required matches. Thus 1422 <pre> 1423 [aeiou]{3,} 1424 </pre> 1425 matches at least 3 successive vowels, but may match many more, while 1426 <pre> 1427 \d{8} 1428 </pre> 1429 matches exactly 8 digits. An opening curly bracket that appears in a position 1430 where a quantifier is not allowed, or one that does not match the syntax of a 1431 quantifier, is taken as a literal character. For example, {,6} is not a 1432 quantifier, but a literal string of four characters. 1433 </P> 1434 <P> 1435 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual 1436 bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of 1437 which is represented by a two-byte sequence. Similarly, when Unicode property 1438 support is available, \X{3} matches three Unicode extended sequences, each of 1439 which may be several bytes long (and they may be of different lengths). 1440 </P> 1441 <P> 1442 The quantifier {0} is permitted, causing the expression to behave as if the 1443 previous item and the quantifier were not present. This may be useful for 1444 subpatterns that are referenced as 1445 <a href="#subpatternsassubroutines">subroutines</a> 1446 from elsewhere in the pattern (but see also the section entitled 1447 <a href="#subdefine">"Defining subpatterns for use by reference only"</a> 1448 below). Items other than subpatterns that have a {0} quantifier are omitted 1449 from the compiled pattern. 1450 </P> 1451 <P> 1452 For convenience, the three most common quantifiers have single-character 1453 abbreviations: 1454 <pre> 1455 * is equivalent to {0,} 1456 + is equivalent to {1,} 1457 ? is equivalent to {0,1} 1458 </pre> 1459 It is possible to construct infinite loops by following a subpattern that can 1460 match no characters with a quantifier that has no upper limit, for example: 1461 <pre> 1462 (a?)* 1463 </pre> 1464 Earlier versions of Perl and PCRE used to give an error at compile time for 1465 such patterns. However, because there are cases where this can be useful, such 1466 patterns are now accepted, but if any repetition of the subpattern does in fact 1467 match no characters, the loop is forcibly broken. 1468 </P> 1469 <P> 1470 By default, the quantifiers are "greedy", that is, they match as much as 1471 possible (up to the maximum number of permitted times), without causing the 1472 rest of the pattern to fail. The classic example of where this gives problems 1473 is in trying to match comments in C programs. These appear between /* and */ 1474 and within the comment, individual * and / characters may appear. An attempt to 1475 match C comments by applying the pattern 1476 <pre> 1477 /\*.*\*/ 1478 </pre> 1479 to the string 1480 <pre> 1481 /* first comment */ not comment /* second comment */ 1482 </pre> 1483 fails, because it matches the entire string owing to the greediness of the .* 1484 item. 1485 </P> 1486 <P> 1487 However, if a quantifier is followed by a question mark, it ceases to be 1488 greedy, and instead matches the minimum number of times possible, so the 1489 pattern 1490 <pre> 1491 /\*.*?\*/ 1492 </pre> 1493 does the right thing with the C comments. The meaning of the various 1494 quantifiers is not otherwise changed, just the preferred number of matches. 1495 Do not confuse this use of question mark with its use as a quantifier in its 1496 own right. Because it has two uses, it can sometimes appear doubled, as in 1497 <pre> 1498 \d??\d 1499 </pre> 1500 which matches one digit by preference, but can match two if that is the only 1501 way the rest of the pattern matches. 1502 </P> 1503 <P> 1504 If the PCRE_UNGREEDY option is set (an option that is not available in Perl), 1505 the quantifiers are not greedy by default, but individual ones can be made 1506 greedy by following them with a question mark. In other words, it inverts the 1507 default behaviour. 1508 </P> 1509 <P> 1510 When a parenthesized subpattern is quantified with a minimum repeat count that 1511 is greater than 1 or with a limited maximum, more memory is required for the 1512 compiled pattern, in proportion to the size of the minimum or maximum. 1513 </P> 1514 <P> 1515 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent 1516 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is 1517 implicitly anchored, because whatever follows will be tried against every 1518 character position in the subject string, so there is no point in retrying the 1519 overall match at any position after the first. PCRE normally treats such a 1520 pattern as though it were preceded by \A. 1521 </P> 1522 <P> 1523 In cases where it is known that the subject string contains no newlines, it is 1524 worth setting PCRE_DOTALL in order to obtain this optimization, or 1525 alternatively using ^ to indicate anchoring explicitly. 1526 </P> 1527 <P> 1528 However, there is one situation where the optimization cannot be used. When .* 1529 is inside capturing parentheses that are the subject of a back reference 1530 elsewhere in the pattern, a match at the start may fail where a later one 1531 succeeds. Consider, for example: 1532 <pre> 1533 (.*)abc\1 1534 </pre> 1535 If the subject is "xyz123abc123" the match point is the fourth character. For 1536 this reason, such a pattern is not implicitly anchored. 1537 </P> 1538 <P> 1539 When a capturing subpattern is repeated, the value captured is the substring 1540 that matched the final iteration. For example, after 1541 <pre> 1542 (tweedle[dume]{3}\s*)+ 1543 </pre> 1544 has matched "tweedledum tweedledee" the value of the captured substring is 1545 "tweedledee". However, if there are nested capturing subpatterns, the 1546 corresponding captured values may have been set in previous iterations. For 1547 example, after 1548 <pre> 1549 /(a|(b))+/ 1550 </pre> 1551 matches "aba" the value of the second captured substring is "b". 1552 <a name="atomicgroup"></a></P> 1553 <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br> 1554 <P> 1555 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 1556 repetition, failure of what follows normally causes the repeated item to be 1557 re-evaluated to see if a different number of repeats allows the rest of the 1558 pattern to match. Sometimes it is useful to prevent this, either to change the 1559 nature of the match, or to cause it fail earlier than it otherwise might, when 1560 the author of the pattern knows there is no point in carrying on. 1561 </P> 1562 <P> 1563 Consider, for example, the pattern \d+foo when applied to the subject line 1564 <pre> 1565 123456bar 1566 </pre> 1567 After matching all 6 digits and then failing to match "foo", the normal 1568 action of the matcher is to try again with only 5 digits matching the \d+ 1569 item, and then with 4, and so on, before ultimately failing. "Atomic grouping" 1570 (a term taken from Jeffrey Friedl's book) provides the means for specifying 1571 that once a subpattern has matched, it is not to be re-evaluated in this way. 1572 </P> 1573 <P> 1574 If we use atomic grouping for the previous example, the matcher gives up 1575 immediately on failing to match "foo" the first time. The notation is a kind of 1576 special parenthesis, starting with (?> as in this example: 1577 <pre> 1578 (?>\d+)foo 1579 </pre> 1580 This kind of parenthesis "locks up" the part of the pattern it contains once 1581 it has matched, and a failure further into the pattern is prevented from 1582 backtracking into it. Backtracking past it to previous items, however, works as 1583 normal. 1584 </P> 1585 <P> 1586 An alternative description is that a subpattern of this type matches the string 1587 of characters that an identical standalone pattern would match, if anchored at 1588 the current point in the subject string. 1589 </P> 1590 <P> 1591 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as 1592 the above example can be thought of as a maximizing repeat that must swallow 1593 everything it can. So, while both \d+ and \d+? are prepared to adjust the 1594 number of digits they match in order to make the rest of the pattern match, 1595 (?>\d+) can only match an entire sequence of digits. 1596 </P> 1597 <P> 1598 Atomic groups in general can of course contain arbitrarily complicated 1599 subpatterns, and can be nested. However, when the subpattern for an atomic 1600 group is just a single repeated item, as in the example above, a simpler 1601 notation, called a "possessive quantifier" can be used. This consists of an 1602 additional + character following a quantifier. Using this notation, the 1603 previous example can be rewritten as 1604 <pre> 1605 \d++foo 1606 </pre> 1607 Note that a possessive quantifier can be used with an entire group, for 1608 example: 1609 <pre> 1610 (abc|xyz){2,3}+ 1611 </pre> 1612 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY 1613 option is ignored. They are a convenient notation for the simpler forms of 1614 atomic group. However, there is no difference in the meaning of a possessive 1615 quantifier and the equivalent atomic group, though there may be a performance 1616 difference; possessive quantifiers should be slightly faster. 1617 </P> 1618 <P> 1619 The possessive quantifier syntax is an extension to the Perl 5.8 syntax. 1620 Jeffrey Friedl originated the idea (and the name) in the first edition of his 1621 book. Mike McCloskey liked it, so implemented it when he built Sun's Java 1622 package, and PCRE copied it from there. It ultimately found its way into Perl 1623 at release 5.10. 1624 </P> 1625 <P> 1626 PCRE has an optimization that automatically "possessifies" certain simple 1627 pattern constructs. For example, the sequence A+B is treated as A++B because 1628 there is no point in backtracking into a sequence of A's when B must follow. 1629 </P> 1630 <P> 1631 When a pattern contains an unlimited repeat inside a subpattern that can itself 1632 be repeated an unlimited number of times, the use of an atomic group is the 1633 only way to avoid some failing matches taking a very long time indeed. The 1634 pattern 1635 <pre> 1636 (\D+|<\d+>)*[!?] 1637 </pre> 1638 matches an unlimited number of substrings that either consist of non-digits, or 1639 digits enclosed in <>, followed by either ! or ?. When it matches, it runs 1640 quickly. However, if it is applied to 1641 <pre> 1642 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 1643 </pre> 1644 it takes a long time before reporting failure. This is because the string can 1645 be divided between the internal \D+ repeat and the external * repeat in a 1646 large number of ways, and all have to be tried. (The example uses [!?] rather 1647 than a single character at the end, because both PCRE and Perl have an 1648 optimization that allows for fast failure when a single character is used. They 1649 remember the last single character that is required for a match, and fail early 1650 if it is not present in the string.) If the pattern is changed so that it uses 1651 an atomic group, like this: 1652 <pre> 1653 ((?>\D+)|<\d+>)*[!?] 1654 </pre> 1655 sequences of non-digits cannot be broken, and failure happens quickly. 1656 <a name="backreferences"></a></P> 1657 <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br> 1658 <P> 1659 Outside a character class, a backslash followed by a digit greater than 0 (and 1660 possibly further digits) is a back reference to a capturing subpattern earlier 1661 (that is, to its left) in the pattern, provided there have been that many 1662 previous capturing left parentheses. 1663 </P> 1664 <P> 1665 However, if the decimal number following the backslash is less than 10, it is 1666 always taken as a back reference, and causes an error only if there are not 1667 that many capturing left parentheses in the entire pattern. In other words, the 1668 parentheses that are referenced need not be to the left of the reference for 1669 numbers less than 10. A "forward back reference" of this type can make sense 1670 when a repetition is involved and the subpattern to the right has participated 1671 in an earlier iteration. 1672 </P> 1673 <P> 1674 It is not possible to have a numerical "forward back reference" to a subpattern 1675 whose number is 10 or more using this syntax because a sequence such as \50 is 1676 interpreted as a character defined in octal. See the subsection entitled 1677 "Non-printing characters" 1678 <a href="#digitsafterbackslash">above</a> 1679 for further details of the handling of digits following a backslash. There is 1680 no such problem when named parentheses are used. A back reference to any 1681 subpattern is possible using named parentheses (see below). 1682 </P> 1683 <P> 1684 Another way of avoiding the ambiguity inherent in the use of digits following a 1685 backslash is to use the \g escape sequence. This escape must be followed by an 1686 unsigned number or a negative number, optionally enclosed in braces. These 1687 examples are all identical: 1688 <pre> 1689 (ring), \1 1690 (ring), \g1 1691 (ring), \g{1} 1692 </pre> 1693 An unsigned number specifies an absolute reference without the ambiguity that 1694 is present in the older syntax. It is also useful when literal digits follow 1695 the reference. A negative number is a relative reference. Consider this 1696 example: 1697 <pre> 1698 (abc(def)ghi)\g{-1} 1699 </pre> 1700 The sequence \g{-1} is a reference to the most recently started capturing 1701 subpattern before \g, that is, is it equivalent to \2 in this example. 1702 Similarly, \g{-2} would be equivalent to \1. The use of relative references 1703 can be helpful in long patterns, and also in patterns that are created by 1704 joining together fragments that contain references within themselves. 1705 </P> 1706 <P> 1707 A back reference matches whatever actually matched the capturing subpattern in 1708 the current subject string, rather than anything matching the subpattern 1709 itself (see 1710 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a> 1711 below for a way of doing that). So the pattern 1712 <pre> 1713 (sens|respons)e and \1ibility 1714 </pre> 1715 matches "sense and sensibility" and "response and responsibility", but not 1716 "sense and responsibility". If caseful matching is in force at the time of the 1717 back reference, the case of letters is relevant. For example, 1718 <pre> 1719 ((?i)rah)\s+\1 1720 </pre> 1721 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original 1722 capturing subpattern is matched caselessly. 1723 </P> 1724 <P> 1725 There are several different ways of writing back references to named 1726 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or 1727 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified 1728 back reference syntax, in which \g can be used for both numeric and named 1729 references, is also supported. We could rewrite the above example in any of 1730 the following ways: 1731 <pre> 1732 (?<p1>(?i)rah)\s+\k<p1> 1733 (?'p1'(?i)rah)\s+\k{p1} 1734 (?P<p1>(?i)rah)\s+(?P=p1) 1735 (?<p1>(?i)rah)\s+\g{p1} 1736 </pre> 1737 A subpattern that is referenced by name may appear in the pattern before or 1738 after the reference. 1739 </P> 1740 <P> 1741 There may be more than one back reference to the same subpattern. If a 1742 subpattern has not actually been used in a particular match, any back 1743 references to it always fail by default. For example, the pattern 1744 <pre> 1745 (a|(bc))\2 1746 </pre> 1747 always fails if it starts to match "a" rather than "bc". However, if the 1748 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an 1749 unset value matches an empty string. 1750 </P> 1751 <P> 1752 Because there may be many capturing parentheses in a pattern, all digits 1753 following a backslash are taken as part of a potential back reference number. 1754 If the pattern continues with a digit character, some delimiter must be used to 1755 terminate the back reference. If the PCRE_EXTENDED option is set, this can be 1756 whitespace. Otherwise, the \g{ syntax or an empty comment (see 1757 <a href="#comments">"Comments"</a> 1758 below) can be used. 1759 </P> 1760 <br><b> 1761 Recursive back references 1762 </b><br> 1763 <P> 1764 A back reference that occurs inside the parentheses to which it refers fails 1765 when the subpattern is first used, so, for example, (a\1) never matches. 1766 However, such references can be useful inside repeated subpatterns. For 1767 example, the pattern 1768 <pre> 1769 (a|b\1)+ 1770 </pre> 1771 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of 1772 the subpattern, the back reference matches the character string corresponding 1773 to the previous iteration. In order for this to work, the pattern must be such 1774 that the first iteration does not need to match the back reference. This can be 1775 done using alternation, as in the example above, or by a quantifier with a 1776 minimum of zero. 1777 </P> 1778 <P> 1779 Back references of this type cause the group that they reference to be treated 1780 as an 1781 <a href="#atomicgroup">atomic group.</a> 1782 Once the whole group has been matched, a subsequent matching failure cannot 1783 cause backtracking into the middle of the group. 1784 <a name="bigassertions"></a></P> 1785 <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br> 1786 <P> 1787 An assertion is a test on the characters following or preceding the current 1788 matching point that does not actually consume any characters. The simple 1789 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described 1790 <a href="#smallassertions">above.</a> 1791 </P> 1792 <P> 1793 More complicated assertions are coded as subpatterns. There are two kinds: 1794 those that look ahead of the current position in the subject string, and those 1795 that look behind it. An assertion subpattern is matched in the normal way, 1796 except that it does not cause the current matching position to be changed. 1797 </P> 1798 <P> 1799 Assertion subpatterns are not capturing subpatterns, and may not be repeated, 1800 because it makes no sense to assert the same thing several times. If any kind 1801 of assertion contains capturing subpatterns within it, these are counted for 1802 the purposes of numbering the capturing subpatterns in the whole pattern. 1803 However, substring capturing is carried out only for positive assertions, 1804 because it does not make sense for negative assertions. 1805 </P> 1806 <br><b> 1807 Lookahead assertions 1808 </b><br> 1809 <P> 1810 Lookahead assertions start with (?= for positive assertions and (?! for 1811 negative assertions. For example, 1812 <pre> 1813 \w+(?=;) 1814 </pre> 1815 matches a word followed by a semicolon, but does not include the semicolon in 1816 the match, and 1817 <pre> 1818 foo(?!bar) 1819 </pre> 1820 matches any occurrence of "foo" that is not followed by "bar". Note that the 1821 apparently similar pattern 1822 <pre> 1823 (?!foo)bar 1824 </pre> 1825 does not find an occurrence of "bar" that is preceded by something other than 1826 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion 1827 (?!foo) is always true when the next three characters are "bar". A 1828 lookbehind assertion is needed to achieve the other effect. 1829 </P> 1830 <P> 1831 If you want to force a matching failure at some point in a pattern, the most 1832 convenient way to do it is with (?!) because an empty string always matches, so 1833 an assertion that requires there not to be an empty string must always fail. 1834 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!). 1835 <a name="lookbehind"></a></P> 1836 <br><b> 1837 Lookbehind assertions 1838 </b><br> 1839 <P> 1840 Lookbehind assertions start with (?<= for positive assertions and (?<! for 1841 negative assertions. For example, 1842 <pre> 1843 (?<!foo)bar 1844 </pre> 1845 does find an occurrence of "bar" that is not preceded by "foo". The contents of 1846 a lookbehind assertion are restricted such that all the strings it matches must 1847 have a fixed length. However, if there are several top-level alternatives, they 1848 do not all have to have the same fixed length. Thus 1849 <pre> 1850 (?<=bullock|donkey) 1851 </pre> 1852 is permitted, but 1853 <pre> 1854 (?<!dogs?|cats?) 1855 </pre> 1856 causes an error at compile time. Branches that match different length strings 1857 are permitted only at the top level of a lookbehind assertion. This is an 1858 extension compared with Perl, which requires all branches to match the same 1859 length of string. An assertion such as 1860 <pre> 1861 (?<=ab(c|de)) 1862 </pre> 1863 is not permitted, because its single top-level branch can match two different 1864 lengths, but it is acceptable to PCRE if rewritten to use two top-level 1865 branches: 1866 <pre> 1867 (?<=abc|abde) 1868 </pre> 1869 In some cases, the escape sequence \K 1870 <a href="#resetmatchstart">(see above)</a> 1871 can be used instead of a lookbehind assertion to get round the fixed-length 1872 restriction. 1873 </P> 1874 <P> 1875 The implementation of lookbehind assertions is, for each alternative, to 1876 temporarily move the current position back by the fixed length and then try to 1877 match. If there are insufficient characters before the current position, the 1878 assertion fails. 1879 </P> 1880 <P> 1881 PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode) 1882 to appear in lookbehind assertions, because it makes it impossible to calculate 1883 the length of the lookbehind. The \X and \R escapes, which can match 1884 different numbers of bytes, are also not permitted. 1885 </P> 1886 <P> 1887 <a href="#subpatternsassubroutines">"Subroutine"</a> 1888 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long 1889 as the subpattern matches a fixed-length string. 1890 <a href="#recursion">Recursion,</a> 1891 however, is not supported. 1892 </P> 1893 <P> 1894 Possessive quantifiers can be used in conjunction with lookbehind assertions to 1895 specify efficient matching of fixed-length strings at the end of subject 1896 strings. Consider a simple pattern such as 1897 <pre> 1898 abcd$ 1899 </pre> 1900 when applied to a long string that does not match. Because matching proceeds 1901 from left to right, PCRE will look for each "a" in the subject and then see if 1902 what follows matches the rest of the pattern. If the pattern is specified as 1903 <pre> 1904 ^.*abcd$ 1905 </pre> 1906 the initial .* matches the entire string at first, but when this fails (because 1907 there is no following "a"), it backtracks to match all but the last character, 1908 then all but the last two characters, and so on. Once again the search for "a" 1909 covers the entire string, from right to left, so we are no better off. However, 1910 if the pattern is written as 1911 <pre> 1912 ^.*+(?<=abcd) 1913 </pre> 1914 there can be no backtracking for the .*+ item; it can match only the entire 1915 string. The subsequent lookbehind assertion does a single test on the last four 1916 characters. If it fails, the match fails immediately. For long strings, this 1917 approach makes a significant difference to the processing time. 1918 </P> 1919 <br><b> 1920 Using multiple assertions 1921 </b><br> 1922 <P> 1923 Several assertions (of any sort) may occur in succession. For example, 1924 <pre> 1925 (?<=\d{3})(?<!999)foo 1926 </pre> 1927 matches "foo" preceded by three digits that are not "999". Notice that each of 1928 the assertions is applied independently at the same point in the subject 1929 string. First there is a check that the previous three characters are all 1930 digits, and then there is a check that the same three characters are not "999". 1931 This pattern does <i>not</i> match "foo" preceded by six characters, the first 1932 of which are digits and the last three of which are not "999". For example, it 1933 doesn't match "123abcfoo". A pattern to do that is 1934 <pre> 1935 (?<=\d{3}...)(?<!999)foo 1936 </pre> 1937 This time the first assertion looks at the preceding six characters, checking 1938 that the first three are digits, and then the second assertion checks that the 1939 preceding three characters are not "999". 1940 </P> 1941 <P> 1942 Assertions can be nested in any combination. For example, 1943 <pre> 1944 (?<=(?<!foo)bar)baz 1945 </pre> 1946 matches an occurrence of "baz" that is preceded by "bar" which in turn is not 1947 preceded by "foo", while 1948 <pre> 1949 (?<=\d{3}(?!999)...)foo 1950 </pre> 1951 is another pattern that matches "foo" preceded by three digits and any three 1952 characters that are not "999". 1953 <a name="conditions"></a></P> 1954 <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br> 1955 <P> 1956 It is possible to cause the matching process to obey a subpattern 1957 conditionally or to choose between two alternative subpatterns, depending on 1958 the result of an assertion, or whether a specific capturing subpattern has 1959 already been matched. The two possible forms of conditional subpattern are: 1960 <pre> 1961 (?(condition)yes-pattern) 1962 (?(condition)yes-pattern|no-pattern) 1963 </pre> 1964 If the condition is satisfied, the yes-pattern is used; otherwise the 1965 no-pattern (if present) is used. If there are more than two alternatives in the 1966 subpattern, a compile-time error occurs. Each of the two alternatives may 1967 itself contain nested subpatterns of any form, including conditional 1968 subpatterns; the restriction to two alternatives applies only at the level of 1969 the condition. This pattern fragment is an example where the alternatives are 1970 complex: 1971 <pre> 1972 (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) 1973 1974 </PRE> 1975 </P> 1976 <P> 1977 There are four kinds of condition: references to subpatterns, references to 1978 recursion, a pseudo-condition called DEFINE, and assertions. 1979 </P> 1980 <br><b> 1981 Checking for a used subpattern by number 1982 </b><br> 1983 <P> 1984 If the text between the parentheses consists of a sequence of digits, the 1985 condition is true if a capturing subpattern of that number has previously 1986 matched. If there is more than one capturing subpattern with the same number 1987 (see the earlier 1988 <a href="#recursion">section about duplicate subpattern numbers),</a> 1989 the condition is true if any of them have matched. An alternative notation is 1990 to precede the digits with a plus or minus sign. In this case, the subpattern 1991 number is relative rather than absolute. The most recently opened parentheses 1992 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside 1993 loops it can also make sense to refer to subsequent groups. The next 1994 parentheses to be opened can be referenced as (?(+1), and so on. (The value 1995 zero in any of these forms is not used; it provokes a compile-time error.) 1996 </P> 1997 <P> 1998 Consider the following pattern, which contains non-significant white space to 1999 make it more readable (assume the PCRE_EXTENDED option) and to divide it into 2000 three parts for ease of discussion: 2001 <pre> 2002 ( \( )? [^()]+ (?(1) \) ) 2003 </pre> 2004 The first part matches an optional opening parenthesis, and if that 2005 character is present, sets it as the first captured substring. The second part 2006 matches one or more characters that are not parentheses. The third part is a 2007 conditional subpattern that tests whether or not the first set of parentheses 2008 matched. If they did, that is, if subject started with an opening parenthesis, 2009 the condition is true, and so the yes-pattern is executed and a closing 2010 parenthesis is required. Otherwise, since no-pattern is not present, the 2011 subpattern matches nothing. In other words, this pattern matches a sequence of 2012 non-parentheses, optionally enclosed in parentheses. 2013 </P> 2014 <P> 2015 If you were embedding this pattern in a larger one, you could use a relative 2016 reference: 2017 <pre> 2018 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... 2019 </pre> 2020 This makes the fragment independent of the parentheses in the larger pattern. 2021 </P> 2022 <br><b> 2023 Checking for a used subpattern by name 2024 </b><br> 2025 <P> 2026 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used 2027 subpattern by name. For compatibility with earlier versions of PCRE, which had 2028 this facility before Perl, the syntax (?(name)...) is also recognized. However, 2029 there is a possible ambiguity with this syntax, because subpattern names may 2030 consist entirely of digits. PCRE looks first for a named subpattern; if it 2031 cannot find one and the name consists entirely of digits, PCRE looks for a 2032 subpattern of that number, which must be greater than zero. Using subpattern 2033 names that consist entirely of digits is not recommended. 2034 </P> 2035 <P> 2036 Rewriting the above example to use a named subpattern gives this: 2037 <pre> 2038 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) 2039 </pre> 2040 If the name used in a condition of this kind is a duplicate, the test is 2041 applied to all subpatterns of the same name, and is true if any one of them has 2042 matched. 2043 </P> 2044 <br><b> 2045 Checking for pattern recursion 2046 </b><br> 2047 <P> 2048 If the condition is the string (R), and there is no subpattern with the name R, 2049 the condition is true if a recursive call to the whole pattern or any 2050 subpattern has been made. If digits or a name preceded by ampersand follow the 2051 letter R, for example: 2052 <pre> 2053 (?(R3)...) or (?(R&name)...) 2054 </pre> 2055 the condition is true if the most recent recursion is into a subpattern whose 2056 number or name is given. This condition does not check the entire recursion 2057 stack. If the name used in a condition of this kind is a duplicate, the test is 2058 applied to all subpatterns of the same name, and is true if any one of them is 2059 the most recent recursion. 2060 </P> 2061 <P> 2062 At "top level", all these recursion test conditions are false. 2063 <a href="#recursion">The syntax for recursive patterns</a> 2064 is described below. 2065 <a name="subdefine"></a></P> 2066 <br><b> 2067 Defining subpatterns for use by reference only 2068 </b><br> 2069 <P> 2070 If the condition is the string (DEFINE), and there is no subpattern with the 2071 name DEFINE, the condition is always false. In this case, there may be only one 2072 alternative in the subpattern. It is always skipped if control reaches this 2073 point in the pattern; the idea of DEFINE is that it can be used to define 2074 "subroutines" that can be referenced from elsewhere. (The use of 2075 <a href="#subpatternsassubroutines">"subroutines"</a> 2076 is described below.) For example, a pattern to match an IPv4 address such as 2077 "192.168.23.245" could be written like this (ignore whitespace and line 2078 breaks): 2079 <pre> 2080 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) 2081 \b (?&byte) (\.(?&byte)){3} \b 2082 </pre> 2083 The first part of the pattern is a DEFINE group inside which a another group 2084 named "byte" is defined. This matches an individual component of an IPv4 2085 address (a number less than 256). When matching takes place, this part of the 2086 pattern is skipped because DEFINE acts like a false condition. The rest of the 2087 pattern uses references to the named group to match the four dot-separated 2088 components of an IPv4 address, insisting on a word boundary at each end. 2089 </P> 2090 <br><b> 2091 Assertion conditions 2092 </b><br> 2093 <P> 2094 If the condition is not in any of the above formats, it must be an assertion. 2095 This may be a positive or negative lookahead or lookbehind assertion. Consider 2096 this pattern, again containing non-significant white space, and with the two 2097 alternatives on the second line: 2098 <pre> 2099 (?(?=[^a-z]*[a-z]) 2100 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) 2101 </pre> 2102 The condition is a positive lookahead assertion that matches an optional 2103 sequence of non-letters followed by a letter. In other words, it tests for the 2104 presence of at least one letter in the subject. If a letter is found, the 2105 subject is matched against the first alternative; otherwise it is matched 2106 against the second. This pattern matches strings in one of the two forms 2107 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. 2108 <a name="comments"></a></P> 2109 <br><a name="SEC20" href="#TOC1">COMMENTS</a><br> 2110 <P> 2111 There are two ways of including comments in patterns that are processed by 2112 PCRE. In both cases, the start of the comment must not be in a character class, 2113 nor in the middle of any other sequence of related characters such as (?: or a 2114 subpattern name or number. The characters that make up a comment play no part 2115 in the pattern matching. 2116 </P> 2117 <P> 2118 The sequence (?# marks the start of a comment that continues up to the next 2119 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED 2120 option is set, an unescaped # character also introduces a comment, which in 2121 this case continues to immediately after the next newline character or 2122 character sequence in the pattern. Which characters are interpreted as newlines 2123 is controlled by the options passed to <b>pcre_compile()</b> or by a special 2124 sequence at the start of the pattern, as described in the section entitled 2125 <a href="#newlines">"Newline conventions"</a> 2126 above. Note that the end of this type of comment is a literal newline sequence 2127 in the pattern; escape sequences that happen to represent a newline do not 2128 count. For example, consider this pattern when PCRE_EXTENDED is set, and the 2129 default newline convention is in force: 2130 <pre> 2131 abc #comment \n still comment 2132 </pre> 2133 On encountering the # character, <b>pcre_compile()</b> skips along, looking for 2134 a newline in the pattern. The sequence \n is still literal at this stage, so 2135 it does not terminate the comment. Only an actual character with the code value 2136 0x0a (the default newline) does so. 2137 <a name="recursion"></a></P> 2138 <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br> 2139 <P> 2140 Consider the problem of matching a string in parentheses, allowing for 2141 unlimited nested parentheses. Without the use of recursion, the best that can 2142 be done is to use a pattern that matches up to some fixed depth of nesting. It 2143 is not possible to handle an arbitrary nesting depth. 2144 </P> 2145 <P> 2146 For some time, Perl has provided a facility that allows regular expressions to 2147 recurse (amongst other things). It does this by interpolating Perl code in the 2148 expression at run time, and the code can refer to the expression itself. A Perl 2149 pattern using code interpolation to solve the parentheses problem can be 2150 created like this: 2151 <pre> 2152 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; 2153 </pre> 2154 The (?p{...}) item interpolates Perl code at run time, and in this case refers 2155 recursively to the pattern in which it appears. 2156 </P> 2157 <P> 2158 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it 2159 supports special syntax for recursion of the entire pattern, and also for 2160 individual subpattern recursion. After its introduction in PCRE and Python, 2161 this kind of recursion was subsequently introduced into Perl at release 5.10. 2162 </P> 2163 <P> 2164 A special item that consists of (? followed by a number greater than zero and a 2165 closing parenthesis is a recursive call of the subpattern of the given number, 2166 provided that it occurs inside that subpattern. (If not, it is a 2167 <a href="#subpatternsassubroutines">"subroutine"</a> 2168 call, which is described in the next section.) The special item (?R) or (?0) is 2169 a recursive call of the entire regular expression. 2170 </P> 2171 <P> 2172 This PCRE pattern solves the nested parentheses problem (assume the 2173 PCRE_EXTENDED option is set so that white space is ignored): 2174 <pre> 2175 \( ( [^()]++ | (?R) )* \) 2176 </pre> 2177 First it matches an opening parenthesis. Then it matches any number of 2178 substrings which can either be a sequence of non-parentheses, or a recursive 2179 match of the pattern itself (that is, a correctly parenthesized substring). 2180 Finally there is a closing parenthesis. Note the use of a possessive quantifier 2181 to avoid backtracking into sequences of non-parentheses. 2182 </P> 2183 <P> 2184 If this were part of a larger pattern, you would not want to recurse the entire 2185 pattern, so instead you could use this: 2186 <pre> 2187 ( \( ( [^()]++ | (?1) )* \) ) 2188 </pre> 2189 We have put the pattern into parentheses, and caused the recursion to refer to 2190 them instead of the whole pattern. 2191 </P> 2192 <P> 2193 In a larger pattern, keeping track of parenthesis numbers can be tricky. This 2194 is made easier by the use of relative references. Instead of (?1) in the 2195 pattern above you can write (?-2) to refer to the second most recently opened 2196 parentheses preceding the recursion. In other words, a negative number counts 2197 capturing parentheses leftwards from the point at which it is encountered. 2198 </P> 2199 <P> 2200 It is also possible to refer to subsequently opened parentheses, by writing 2201 references such as (?+2). However, these cannot be recursive because the 2202 reference is not inside the parentheses that are referenced. They are always 2203 <a href="#subpatternsassubroutines">"subroutine"</a> 2204 calls, as described in the next section. 2205 </P> 2206 <P> 2207 An alternative approach is to use named parentheses instead. The Perl syntax 2208 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We 2209 could rewrite the above example as follows: 2210 <pre> 2211 (?<pn> \( ( [^()]++ | (?&pn) )* \) ) 2212 </pre> 2213 If there is more than one subpattern with the same name, the earliest one is 2214 used. 2215 </P> 2216 <P> 2217 This particular example pattern that we have been looking at contains nested 2218 unlimited repeats, and so the use of a possessive quantifier for matching 2219 strings of non-parentheses is important when applying the pattern to strings 2220 that do not match. For example, when this pattern is applied to 2221 <pre> 2222 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 2223 </pre> 2224 it yields "no match" quickly. However, if a possessive quantifier is not used, 2225 the match runs for a very long time indeed because there are so many different 2226 ways the + and * repeats can carve up the subject, and all have to be tested 2227 before failure can be reported. 2228 </P> 2229 <P> 2230 At the end of a match, the values of capturing parentheses are those from 2231 the outermost level. If you want to obtain intermediate values, a callout 2232 function can be used (see below and the 2233 <a href="pcrecallout.html"><b>pcrecallout</b></a> 2234 documentation). If the pattern above is matched against 2235 <pre> 2236 (ab(cd)ef) 2237 </pre> 2238 the value for the inner capturing parentheses (numbered 2) is "ef", which is 2239 the last value taken on at the top level. If a capturing subpattern is not 2240 matched at the top level, its final value is unset, even if it is (temporarily) 2241 set at a deeper level. 2242 </P> 2243 <P> 2244 If there are more than 15 capturing parentheses in a pattern, PCRE has to 2245 obtain extra memory to store data during a recursion, which it does by using 2246 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can 2247 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. 2248 </P> 2249 <P> 2250 Do not confuse the (?R) item with the condition (R), which tests for recursion. 2251 Consider this pattern, which matches text in angle brackets, allowing for 2252 arbitrary nesting. Only digits are allowed in nested brackets (that is, when 2253 recursing), whereas any characters are permitted at the outer level. 2254 <pre> 2255 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > 2256 </pre> 2257 In this pattern, (?(R) is the start of a conditional subpattern, with two 2258 different alternatives for the recursive and non-recursive cases. The (?R) item 2259 is the actual recursive call. 2260 <a name="recursiondifference"></a></P> 2261 <br><b> 2262 Recursion difference from Perl 2263 </b><br> 2264 <P> 2265 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always 2266 treated as an atomic group. That is, once it has matched some of the subject 2267 string, it is never re-entered, even if it contains untried alternatives and 2268 there is a subsequent matching failure. This can be illustrated by the 2269 following pattern, which purports to match a palindromic string that contains 2270 an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"): 2271 <pre> 2272 ^(.|(.)(?1)\2)$ 2273 </pre> 2274 The idea is that it either matches a single character, or two identical 2275 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE 2276 it does not if the pattern is longer than three characters. Consider the 2277 subject string "abcba": 2278 </P> 2279 <P> 2280 At the top level, the first character is matched, but as it is not at the end 2281 of the string, the first alternative fails; the second alternative is taken 2282 and the recursion kicks in. The recursive call to subpattern 1 successfully 2283 matches the next character ("b"). (Note that the beginning and end of line 2284 tests are not part of the recursion). 2285 </P> 2286 <P> 2287 Back at the top level, the next character ("c") is compared with what 2288 subpattern 2 matched, which was "a". This fails. Because the recursion is 2289 treated as an atomic group, there are now no backtracking points, and so the 2290 entire match fails. (Perl is able, at this point, to re-enter the recursion and 2291 try the second alternative.) However, if the pattern is written with the 2292 alternatives in the other order, things are different: 2293 <pre> 2294 ^((.)(?1)\2|.)$ 2295 </pre> 2296 This time, the recursing alternative is tried first, and continues to recurse 2297 until it runs out of characters, at which point the recursion fails. But this 2298 time we do have another alternative to try at the higher level. That is the big 2299 difference: in the previous case the remaining alternative is at a deeper 2300 recursion level, which PCRE cannot use. 2301 </P> 2302 <P> 2303 To change the pattern so that it matches all palindromic strings, not just 2304 those with an odd number of characters, it is tempting to change the pattern to 2305 this: 2306 <pre> 2307 ^((.)(?1)\2|.?)$ 2308 </pre> 2309 Again, this works in Perl, but not in PCRE, and for the same reason. When a 2310 deeper recursion has matched a single character, it cannot be entered again in 2311 order to match an empty string. The solution is to separate the two cases, and 2312 write out the odd and even cases as alternatives at the higher level: 2313 <pre> 2314 ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) 2315 </pre> 2316 If you want to match typical palindromic phrases, the pattern has to ignore all 2317 non-word characters, which can be done like this: 2318 <pre> 2319 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ 2320 </pre> 2321 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A 2322 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note 2323 the use of the possessive quantifier *+ to avoid backtracking into sequences of 2324 non-word characters. Without this, PCRE takes a great deal longer (ten times or 2325 more) to match typical phrases, and Perl takes so long that you think it has 2326 gone into a loop. 2327 </P> 2328 <P> 2329 <b>WARNING</b>: The palindrome-matching patterns above work only if the subject 2330 string does not start with a palindrome that is shorter than the entire string. 2331 For example, although "abcba" is correctly matched, if the subject is "ababa", 2332 PCRE finds the palindrome "aba" at the start, then fails at top level because 2333 the end of the string does not follow. Once again, it cannot jump back into the 2334 recursion to try other alternatives, so the entire match fails. 2335 <a name="subpatternsassubroutines"></a></P> 2336 <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br> 2337 <P> 2338 If the syntax for a recursive subpattern reference (either by number or by 2339 name) is used outside the parentheses to which it refers, it operates like a 2340 subroutine in a programming language. The "called" subpattern may be defined 2341 before or after the reference. A numbered reference can be absolute or 2342 relative, as in these examples: 2343 <pre> 2344 (...(absolute)...)...(?2)... 2345 (...(relative)...)...(?-1)... 2346 (...(?+1)...(relative)... 2347 </pre> 2348 An earlier example pointed out that the pattern 2349 <pre> 2350 (sens|respons)e and \1ibility 2351 </pre> 2352 matches "sense and sensibility" and "response and responsibility", but not 2353 "sense and responsibility". If instead the pattern 2354 <pre> 2355 (sens|respons)e and (?1)ibility 2356 </pre> 2357 is used, it does match "sense and responsibility" as well as the other two 2358 strings. Another example is given in the discussion of DEFINE above. 2359 </P> 2360 <P> 2361 Like recursive subpatterns, a subroutine call is always treated as an atomic 2362 group. That is, once it has matched some of the subject string, it is never 2363 re-entered, even if it contains untried alternatives and there is a subsequent 2364 matching failure. Any capturing parentheses that are set during the subroutine 2365 call revert to their previous values afterwards. 2366 </P> 2367 <P> 2368 When a subpattern is used as a subroutine, processing options such as 2369 case-independence are fixed when the subpattern is defined. They cannot be 2370 changed for different calls. For example, consider this pattern: 2371 <pre> 2372 (abc)(?i:(?-1)) 2373 </pre> 2374 It matches "abcabc". It does not match "abcABC" because the change of 2375 processing option does not affect the called subpattern. 2376 <a name="onigurumasubroutines"></a></P> 2377 <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br> 2378 <P> 2379 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or 2380 a number enclosed either in angle brackets or single quotes, is an alternative 2381 syntax for referencing a subpattern as a subroutine, possibly recursively. Here 2382 are two of the examples used above, rewritten using this syntax: 2383 <pre> 2384 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) 2385 (sens|respons)e and \g'1'ibility 2386 </pre> 2387 PCRE supports an extension to Oniguruma: if a number is preceded by a 2388 plus or a minus sign it is taken as a relative reference. For example: 2389 <pre> 2390 (abc)(?i:\g<-1>) 2391 </pre> 2392 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i> 2393 synonymous. The former is a back reference; the latter is a subroutine call. 2394 </P> 2395 <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br> 2396 <P> 2397 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl 2398 code to be obeyed in the middle of matching a regular expression. This makes it 2399 possible, amongst other things, to extract different substrings that match the 2400 same pair of parentheses when there is a repetition. 2401 </P> 2402 <P> 2403 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl 2404 code. The feature is called "callout". The caller of PCRE provides an external 2405 function by putting its entry point in the global variable <i>pcre_callout</i>. 2406 By default, this variable contains NULL, which disables all calling out. 2407 </P> 2408 <P> 2409 Within a regular expression, (?C) indicates the points at which the external 2410 function is to be called. If you want to identify different callout points, you 2411 can put a number less than 256 after the letter C. The default value is zero. 2412 For example, this pattern has two callout points: 2413 <pre> 2414 (?C1)abc(?C2)def 2415 </pre> 2416 If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are 2417 automatically installed before each item in the pattern. They are all numbered 2418 255. 2419 </P> 2420 <P> 2421 During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is 2422 set), the external function is called. It is provided with the number of the 2423 callout, the position in the pattern, and, optionally, one item of data 2424 originally supplied by the caller of <b>pcre_exec()</b>. The callout function 2425 may cause matching to proceed, to backtrack, or to fail altogether. A complete 2426 description of the interface to the callout function is given in the 2427 <a href="pcrecallout.html"><b>pcrecallout</b></a> 2428 documentation. 2429 <a name="backtrackcontrol"></a></P> 2430 <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br> 2431 <P> 2432 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which 2433 are described in the Perl documentation as "experimental and subject to change 2434 or removal in a future version of Perl". It goes on to say: "Their usage in 2435 production code should be noted to avoid problems during upgrades." The same 2436 remarks apply to the PCRE features described in this section. 2437 </P> 2438 <P> 2439 Since these verbs are specifically related to backtracking, most of them can be 2440 used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses 2441 a backtracking algorithm. With the exception of (*FAIL), which behaves like a 2442 failing negative assertion, they cause an error if encountered by 2443 <b>pcre_dfa_exec()</b>. 2444 </P> 2445 <P> 2446 If any of these verbs are used in an assertion or subroutine subpattern 2447 (including recursive subpatterns), their effect is confined to that subpattern; 2448 it does not extend to the surrounding pattern. Note that such subpatterns are 2449 processed as anchored at the point where they are tested. 2450 </P> 2451 <P> 2452 The new verbs make use of what was previously invalid syntax: an opening 2453 parenthesis followed by an asterisk. They are generally of the form 2454 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour, 2455 depending on whether or not an argument is present. An name is a sequence of 2456 letters, digits, and underscores. If the name is empty, that is, if the closing 2457 parenthesis immediately follows the colon, the effect is as if the colon were 2458 not there. Any number of these verbs may occur in a pattern. 2459 </P> 2460 <P> 2461 PCRE contains some optimizations that are used to speed up matching by running 2462 some checks at the start of each match attempt. For example, it may know the 2463 minimum length of matching subject, or that a particular character must be 2464 present. When one of these optimizations suppresses the running of a match, any 2465 included backtracking verbs will not, of course, be processed. You can suppress 2466 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option 2467 when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the 2468 pattern with (*NO_START_OPT). 2469 </P> 2470 <br><b> 2471 Verbs that act immediately 2472 </b><br> 2473 <P> 2474 The following verbs act as soon as they are encountered. They may not be 2475 followed by a name. 2476 <pre> 2477 (*ACCEPT) 2478 </pre> 2479 This verb causes the match to end successfully, skipping the remainder of the 2480 pattern. When inside a recursion, only the innermost pattern is ended 2481 immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is 2482 captured. (This feature was added to PCRE at release 8.00.) For example: 2483 <pre> 2484 A((?:A|B(*ACCEPT)|C)D) 2485 </pre> 2486 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by 2487 the outer parentheses. 2488 <pre> 2489 (*FAIL) or (*F) 2490 </pre> 2491 This verb causes the match to fail, forcing backtracking to occur. It is 2492 equivalent to (?!) but easier to read. The Perl documentation notes that it is 2493 probably useful only when combined with (?{}) or (??{}). Those are, of course, 2494 Perl features that are not present in PCRE. The nearest equivalent is the 2495 callout feature, as for example in this pattern: 2496 <pre> 2497 a+(?C)(*FAIL) 2498 </pre> 2499 A match with the string "aaaa" always fails, but the callout is taken before 2500 each backtrack happens (in this example, 10 times). 2501 </P> 2502 <br><b> 2503 Recording which path was taken 2504 </b><br> 2505 <P> 2506 There is one verb whose main purpose is to track how a match was arrived at, 2507 though it also has a secondary use in conjunction with advancing the match 2508 starting point (see (*SKIP) below). 2509 <pre> 2510 (*MARK:NAME) or (*:NAME) 2511 </pre> 2512 A name is always required with this verb. There may be as many instances of 2513 (*MARK) as you like in a pattern, and their names do not have to be unique. 2514 </P> 2515 <P> 2516 When a match succeeds, the name of the last-encountered (*MARK) is passed back 2517 to the caller via the <i>pcre_extra</i> data structure, as described in the 2518 <a href="pcreapi.html#extradata">section on <i>pcre_extra</i></a> 2519 in the 2520 <a href="pcreapi.html"><b>pcreapi</b></a> 2521 documentation. No data is returned for a partial match. Here is an example of 2522 <b>pcretest</b> output, where the /K modifier requests the retrieval and 2523 outputting of (*MARK) data: 2524 <pre> 2525 /X(*MARK:A)Y|X(*MARK:B)Z/K 2526 XY 2527 0: XY 2528 MK: A 2529 XZ 2530 0: XZ 2531 MK: B 2532 </pre> 2533 The (*MARK) name is tagged with "MK:" in this output, and in this example it 2534 indicates which of the two alternatives matched. This is a more efficient way 2535 of obtaining this information than putting each alternative in its own 2536 capturing parentheses. 2537 </P> 2538 <P> 2539 A name may also be returned after a failed match if the final path through the 2540 pattern involves (*MARK). However, unless (*MARK) used in conjunction with 2541 (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the 2542 starting point for matching is advanced, the final check is often with an empty 2543 string, causing a failure before (*MARK) is reached. For example: 2544 <pre> 2545 /X(*MARK:A)Y|X(*MARK:B)Z/K 2546 XP 2547 No match 2548 </pre> 2549 There are three potential starting points for this match (starting with X, 2550 starting with P, and with an empty string). If the pattern is anchored, the 2551 result is different: 2552 <pre> 2553 /^X(*MARK:A)Y|^X(*MARK:B)Z/K 2554 XP 2555 No match, mark = B 2556 </pre> 2557 PCRE's start-of-match optimizations can also interfere with this. For example, 2558 if, as a result of a call to <b>pcre_study()</b>, it knows the minimum 2559 subject length for a match, a shorter subject will not be scanned at all. 2560 </P> 2561 <P> 2562 Note that similar anomalies (though different in detail) exist in Perl, no 2563 doubt for the same reasons. The use of (*MARK) data after a failed match of an 2564 unanchored pattern is not recommended, unless (*COMMIT) is involved. 2565 </P> 2566 <br><b> 2567 Verbs that act after backtracking 2568 </b><br> 2569 <P> 2570 The following verbs do nothing when they are encountered. Matching continues 2571 with what follows, but if there is no subsequent match, causing a backtrack to 2572 the verb, a failure is forced. That is, backtracking cannot pass to the left of 2573 the verb. However, when one of these verbs appears inside an atomic group, its 2574 effect is confined to that group, because once the group has been matched, 2575 there is never any backtracking into it. In this situation, backtracking can 2576 "jump back" to the left of the entire atomic group. (Remember also, as stated 2577 above, that this localization also applies in subroutine calls and assertions.) 2578 </P> 2579 <P> 2580 These verbs differ in exactly what kind of failure occurs when backtracking 2581 reaches them. 2582 <pre> 2583 (*COMMIT) 2584 </pre> 2585 This verb, which may not be followed by a name, causes the whole match to fail 2586 outright if the rest of the pattern does not match. Even if the pattern is 2587 unanchored, no further attempts to find a match by advancing the starting point 2588 take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to 2589 finding a match at the current starting point, or not at all. For example: 2590 <pre> 2591 a+(*COMMIT)b 2592 </pre> 2593 This matches "xxaab" but not "aacaab". It can be thought of as a kind of 2594 dynamic anchor, or "I've started, so I must finish." The name of the most 2595 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a 2596 match failure. 2597 </P> 2598 <P> 2599 Note that (*COMMIT) at the start of a pattern is not the same as an anchor, 2600 unless PCRE's start-of-match optimizations are turned off, as shown in this 2601 <b>pcretest</b> example: 2602 <pre> 2603 /(*COMMIT)abc/ 2604 xyzabc 2605 0: abc 2606 xyzabc\Y 2607 No match 2608 </pre> 2609 PCRE knows that any match must start with "a", so the optimization skips along 2610 the subject to "a" before running the first match attempt, which succeeds. When 2611 the optimization is disabled by the \Y escape in the second subject, the match 2612 starts at "x" and so the (*COMMIT) causes it to fail without trying any other 2613 starting points. 2614 <pre> 2615 (*PRUNE) or (*PRUNE:NAME) 2616 </pre> 2617 This verb causes the match to fail at the current starting position in the 2618 subject if the rest of the pattern does not match. If the pattern is 2619 unanchored, the normal "bumpalong" advance to the next starting character then 2620 happens. Backtracking can occur as usual to the left of (*PRUNE), before it is 2621 reached, or when matching to the right of (*PRUNE), but if there is no match to 2622 the right, backtracking cannot cross (*PRUNE). In simple cases, the use of 2623 (*PRUNE) is just an alternative to an atomic group or possessive quantifier, 2624 but there are some uses of (*PRUNE) that cannot be expressed in any other way. 2625 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the 2626 match fails completely; the name is passed back if this is the final attempt. 2627 (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored 2628 pattern (*PRUNE) has the same effect as (*COMMIT). 2629 <pre> 2630 (*SKIP) 2631 </pre> 2632 This verb, when given without a name, is like (*PRUNE), except that if the 2633 pattern is unanchored, the "bumpalong" advance is not to the next character, 2634 but to the position in the subject where (*SKIP) was encountered. (*SKIP) 2635 signifies that whatever text was matched leading up to it cannot be part of a 2636 successful match. Consider: 2637 <pre> 2638 a+(*SKIP)b 2639 </pre> 2640 If the subject is "aaaac...", after the first match attempt fails (starting at 2641 the first character in the string), the starting point skips on to start the 2642 next attempt at "c". Note that a possessive quantifer does not have the same 2643 effect as this example; although it would suppress backtracking during the 2644 first match attempt, the second attempt would start at the second character 2645 instead of skipping on to "c". 2646 <pre> 2647 (*SKIP:NAME) 2648 </pre> 2649 When (*SKIP) has an associated name, its behaviour is modified. If the 2650 following pattern fails to match, the previous path through the pattern is 2651 searched for the most recent (*MARK) that has the same name. If one is found, 2652 the "bumpalong" advance is to the subject position that corresponds to that 2653 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a 2654 matching name is found, normal "bumpalong" of one character happens (the 2655 (*SKIP) is ignored). 2656 <pre> 2657 (*THEN) or (*THEN:NAME) 2658 </pre> 2659 This verb causes a skip to the next alternation in the innermost enclosing 2660 group if the rest of the pattern does not match. That is, it cancels pending 2661 backtracking, but only within the current alternation. Its name comes from the 2662 observation that it can be used for a pattern-based if-then-else block: 2663 <pre> 2664 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... 2665 </pre> 2666 If the COND1 pattern matches, FOO is tried (and possibly further items after 2667 the end of the group if FOO succeeds); on failure the matcher skips to the 2668 second alternative and tries COND2, without backtracking into COND1. The 2669 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the 2670 overall match fails. If (*THEN) is not directly inside an alternation, it acts 2671 like (*PRUNE). 2672 </P> 2673 <P> 2674 The above verbs provide four different "strengths" of control when subsequent 2675 matching fails. (*THEN) is the weakest, carrying on the match at the next 2676 alternation. (*PRUNE) comes next, failing the match at the current starting 2677 position, but allowing an advance to the next character (for an unanchored 2678 pattern). (*SKIP) is similar, except that the advance may be more than one 2679 character. (*COMMIT) is the strongest, causing the entire match to fail. 2680 </P> 2681 <P> 2682 If more than one is present in a pattern, the "stongest" one wins. For example, 2683 consider this pattern, where A, B, etc. are complex pattern fragments: 2684 <pre> 2685 (A(*COMMIT)B(*THEN)C|D) 2686 </pre> 2687 Once A has matched, PCRE is committed to this match, at the current starting 2688 position. If subsequently B matches, but C does not, the normal (*THEN) action 2689 of trying the next alternation (that is, D) does not happen because (*COMMIT) 2690 overrides. 2691 </P> 2692 <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br> 2693 <P> 2694 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), 2695 <b>pcresyntax</b>(3), <b>pcre</b>(3). 2696 </P> 2697 <br><a name="SEC27" href="#TOC1">AUTHOR</a><br> 2698 <P> 2699 Philip Hazel 2700 <br> 2701 University Computing Service 2702 <br> 2703 Cambridge CB2 3QH, England. 2704 <br> 2705 </P> 2706 <br><a name="SEC28" href="#TOC1">REVISION</a><br> 2707 <P> 2708 Last updated: 21 November 2010 2709 <br> 2710 Copyright © 1997-2010 University of Cambridge. 2711 <br> 2712 <p> 2713 Return to the <a href="index.html">PCRE index page</a>. 2714 </p> 2715
