# Contents of /code/trunk/doc/pcrepattern.3

Thu Apr 10 19:55:57 2008 UTC (5 years, 1 month ago) by ph10
File size: 91051 byte(s)
Add Oniguruma syntax \g<...> and \g'...' for subroutine calls.


 1 nigel 79 .TH PCREPATTERN 3 2 nigel 63 .SH NAME 3 PCRE - Perl-compatible regular expressions 4 nigel 75 .SH "PCRE REGULAR EXPRESSION DETAILS" 5 nigel 63 .rs 6 .sp 7 ph10 208 The syntax and semantics of the regular expressions that are supported by PCRE 8 are described in detail below. There is a quick-reference syntax summary in the 9 .\" HREF 10 \fBpcresyntax\fP 11 .\" 12 ph10 333 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE 13 also supports some alternative regular expression syntax (which does not 14 conflict with the Perl syntax) in order to provide some compatibility with 15 regular expressions in Python, .NET, and Oniguruma. 16 .P 17 Perl's regular expressions are described in its own documentation, and 18 ph10 208 regular expressions in general are covered in a number of books, some of which 19 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", 20 published by O'Reilly, covers regular expressions in great detail. This 21 description of PCRE's regular expressions is intended as reference material. 22 nigel 75 .P 23 The original operation of PCRE was on strings of one-byte characters. However, 24 there is now also support for UTF-8 character strings. To use this, you must 25 build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with 26 the PCRE_UTF8 option. How this affects pattern matching is mentioned in several 27 places below. There is also a summary of UTF-8 features in the 28 nigel 63 .\" HTML 29 .\" 30 section on UTF-8 support 31 .\" 32 in the main 33 .\" HREF 34 nigel 75 \fBpcre\fP 35 nigel 63 .\" 36 page. 37 nigel 75 .P 38 nigel 77 The remainder of this document discusses the patterns that are supported by 39 PCRE when its main matching function, \fBpcre_exec()\fP, is used. 40 From release 6.0, PCRE offers a second matching function, 41 \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not 42 ph10 172 Perl-compatible. Some of the features discussed below are not available when 43 ph10 168 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the 44 alternative function, and how it differs from the normal function, are 45 discussed in the 46 nigel 77 .\" HREF 47 \fBpcrematching\fP 48 .\" 49 page. 50 nigel 93 . 51 . 52 ph10 227 .SH "NEWLINE CONVENTIONS" 53 .rs 54 .sp 55 PCRE supports five different conventions for indicating line breaks in 56 strings: a single CR (carriage return) character, a single LF (linefeed) 57 character, the two-character sequence CRLF, any of the three preceding, or any 58 Unicode newline sequence. The 59 .\" HREF 60 \fBpcreapi\fP 61 .\" 62 page has 63 .\" HTML 64 .\" 65 further discussion 66 .\" 67 about newlines, and shows how to set the newline convention in the 68 \fIoptions\fP arguments for the compiling and matching functions. 69 .P 70 It is also possible to specify a newline convention by starting a pattern 71 string with one of the following five sequences: 72 .sp 73 (*CR) carriage return 74 (*LF) linefeed 75 (*CRLF) carriage return, followed by linefeed 76 (*ANYCRLF) any of the three above 77 (*ANY) all Unicode newline sequences 78 .sp 79 These override the default and the options given to \fBpcre_compile()\fP. For 80 example, on a Unix system where LF is the default newline sequence, the pattern 81 .sp 82 (*CR)a.b 83 .sp 84 changes the convention to CR. That pattern matches "a\enb" because LF is no 85 longer a newline. Note that these special settings, which are not 86 Perl-compatible, are recognized only at the very start of a pattern, and that 87 ph10 231 they must be in upper case. If more than one of them is present, the last one 88 is used. 89 .P 90 The newline convention does not affect what the \eR escape sequence matches. By 91 default, this is any Unicode newline sequence, for Perl compatibility. However, 92 this can be changed; see the description of \eR in the section entitled 93 .\" HTML 94 .\" 95 "Newline sequences" 96 .\" 97 ph10 247 below. A change of \eR setting can be combined with a change of newline 98 ph10 246 convention. 99 ph10 227 . 100 . 101 nigel 93 .SH "CHARACTERS AND METACHARACTERS" 102 .rs 103 .sp 104 nigel 63 A regular expression is a pattern that is matched against a subject string from 105 left to right. Most characters stand for themselves in a pattern, and match the 106 corresponding characters in the subject. As a trivial example, the pattern 107 nigel 75 .sp 108 nigel 63 The quick brown fox 109 nigel 75 .sp 110 nigel 77 matches a portion of a subject string that is identical to itself. When 111 caseless matching is specified (the PCRE_CASELESS option), letters are matched 112 independently of case. In UTF-8 mode, PCRE always understands the concept of 113 case for characters whose values are less than 128, so caseless matching is 114 always possible. For characters with higher values, the concept of case is 115 supported if PCRE is compiled with Unicode property support, but not otherwise. 116 If you want to use caseless matching for characters 128 and above, you must 117 ensure that PCRE is compiled with Unicode property support as well as with 118 UTF-8 support. 119 .P 120 The power of regular expressions comes from the ability to include alternatives 121 and repetitions in the pattern. These are encoded in the pattern by the use of 122 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are 123 nigel 63 interpreted in some special way. 124 nigel 75 .P 125 There are two different sets of metacharacters: those that are recognized 126 nigel 63 anywhere in the pattern except within square brackets, and those that are 127 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters 128 are as follows: 129 nigel 75 .sp 130 \e general escape character with several uses 131 nigel 63 ^ assert start of string (or line, in multiline mode) 132 $assert end of string (or line, in multiline mode) 133 . match any character except newline (by default) 134 [ start character class definition 135 | start of alternative branch 136 ( start subpattern 137 ) end subpattern 138 ? extends the meaning of ( 139 also 0 or 1 quantifier 140 also quantifier minimizer 141 * 0 or more quantifier 142 + 1 or more quantifier 143 also "possessive quantifier" 144 { start min/max quantifier 145 nigel 75 .sp 146 nigel 63 Part of a pattern that is in square brackets is called a "character class". In 147 nigel 75 a character class the only metacharacters are: 148 .sp 149 \e general escape character 150 nigel 63 ^ negate the class, but only if the first character 151 - indicates character range 152 nigel 75 .\" JOIN 153 nigel 63 [ POSIX character class (only if followed by POSIX 154 syntax) 155 ] terminates the character class 156 nigel 75 .sp 157 The following sections describe the use of each of the metacharacters. 158 . 159 nigel 93 . 160 nigel 63 .SH BACKSLASH 161 .rs 162 .sp 163 The backslash character has several uses. Firstly, if it is followed by a 164 nigel 91 non-alphanumeric character, it takes away any special meaning that character 165 may have. This use of backslash as an escape character applies both inside and 166 nigel 63 outside character classes. 167 nigel 75 .P 168 For example, if you want to match a * character, you write \e* in the pattern. 169 nigel 63 This escaping action applies whether or not the following character would 170 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a 171 non-alphanumeric with backslash to specify that it stands for itself. In 172 particular, if you want to match a backslash, you write \e\e. 173 .P 174 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the 175 pattern (other than in a character class) and characters between a # outside 176 nigel 91 a character class and the next newline are ignored. An escaping backslash can 177 be used to include a whitespace or # character as part of the pattern. 178 nigel 75 .P 179 nigel 63 If you want to remove the special meaning from a sequence of characters, you 180 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in 181 that$ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in 182 nigel 63 Perl, $and @ cause variable interpolation. Note the following examples: 183 nigel 75 .sp 184 nigel 63 Pattern PCRE matches Perl matches 185 nigel 75 .sp 186 .\" JOIN 187 \eQabc$xyz\eE abc$xyz abc followed by the 188 nigel 63 contents of$xyz 189 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz 190 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz 191 .sp 192 The \eQ...\eE sequence is recognized both inside and outside character classes. 193 . 194 . 195 .\" HTML 196 .SS "Non-printing characters" 197 .rs 198 .sp 199 nigel 63 A second use of backslash provides a way of encoding non-printing characters 200 in patterns in a visible manner. There is no restriction on the appearance of 201 non-printing characters, apart from the binary zero that terminates a pattern, 202 but when a pattern is being prepared by text editing, it is usually easier to 203 use one of the following escape sequences than the binary character it 204 represents: 205 nigel 75 .sp 206 \ea alarm, that is, the BEL character (hex 07) 207 \ecx "control-x", where x is any character 208 \ee escape (hex 1B) 209 \ef formfeed (hex 0C) 210 ph10 227 \en linefeed (hex 0A) 211 nigel 75 \er carriage return (hex 0D) 212 \et tab (hex 09) 213 \eddd character with octal code ddd, or backreference 214 \exhh character with hex code hh 215 nigel 87 \ex{hhh..} character with hex code hhh.. 216 nigel 75 .sp 217 The precise effect of \ecx is as follows: if x is a lower case letter, it 218 nigel 63 is converted to upper case. Then bit 6 of the character (hex 40) is inverted. 219 nigel 75 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex 220 nigel 63 7B. 221 nigel 75 .P 222 After \ex, from zero to two hexadecimal digits are read (letters can be in 223 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \ex{ 224 and }, but the value of the character code must be less than 256 in non-UTF-8 225 ph10 211 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in 226 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code 227 point, which is 10FFFF. 228 nigel 75 .P 229 ph10 211 If characters other than hexadecimal digits appear between \ex{ and }, or if 230 there is no terminating }, this form of escape is not recognized. Instead, the 231 initial \ex will be interpreted as a basic hexadecimal escape, with no 232 following digits, giving a character whose value is zero. 233 .P 234 nigel 63 Characters whose value is less than 256 can be defined by either of the two 235 nigel 87 syntaxes for \ex. There is no difference in the way they are handled. For 236 example, \exdc is exactly the same as \ex{dc}. 237 nigel 75 .P 238 nigel 91 After \e0 up to two further octal digits are read. If there are fewer than two 239 digits, just those that are present are used. Thus the sequence \e0\ex\e07 240 specifies two binary zeros followed by a BEL character (code value 7). Make 241 sure you supply two digits after the initial zero if the pattern character that 242 follows is itself an octal digit. 243 nigel 75 .P 244 nigel 63 The handling of a backslash followed by a digit other than 0 is complicated. 245 Outside a character class, PCRE reads it and any following digits as a decimal 246 number. If the number is less than 10, or if there have been at least that many 247 previous capturing left parentheses in the expression, the entire sequence is 248 nigel 75 taken as a \fIback reference\fP. A description of how this works is given 249 .\" HTML 250 .\" 251 later, 252 .\" 253 following the discussion of 254 .\" HTML 255 .\" 256 parenthesized subpatterns. 257 .\" 258 .P 259 nigel 63 Inside a character class, or if the decimal number is greater than 9 and there 260 have not been that many capturing subpatterns, PCRE re-reads up to three octal 261 nigel 93 digits following the backslash, and uses them to generate a data character. Any 262 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a 263 character specified in octal must be less than \e400. In UTF-8 mode, values up 264 to \e777 are permitted. For example: 265 nigel 75 .sp 266 \e040 is another way of writing a space 267 .\" JOIN 268 \e40 is the same, provided there are fewer than 40 269 nigel 63 previous capturing subpatterns 270 nigel 75 \e7 is always a back reference 271 .\" JOIN 272 \e11 might be a back reference, or another way of 273 nigel 63 writing a tab 274 nigel 75 \e011 is always a tab 275 \e0113 is a tab followed by the character "3" 276 .\" JOIN 277 \e113 might be a back reference, otherwise the 278 nigel 63 character with octal code 113 279 nigel 75 .\" JOIN 280 \e377 might be a back reference, otherwise 281 nigel 63 the byte consisting entirely of 1 bits 282 nigel 75 .\" JOIN 283 \e81 is either a back reference, or a binary zero 284 nigel 63 followed by the two characters "8" and "1" 285 nigel 75 .sp 286 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading 287 zero, because no more than three octal digits are ever read. 288 nigel 75 .P 289 nigel 91 All the sequences that define a single character value can be used both inside 290 and outside character classes. In addition, inside a character class, the 291 sequence \eb is interpreted as the backspace character (hex 08), and the 292 nigel 93 sequences \eR and \eX are interpreted as the characters "R" and "X", 293 respectively. Outside a character class, these sequences have different 294 meanings 295 nigel 75 .\" HTML 296 .\" 297 (see below). 298 .\" 299 . 300 . 301 nigel 93 .SS "Absolute and relative back references" 302 .rs 303 .sp 304 ph10 208 The sequence \eg followed by an unsigned or a negative number, optionally 305 enclosed in braces, is an absolute or relative back reference. A named back 306 reference can be coded as \eg{name}. Back references are discussed 307 nigel 93 .\" HTML 308 .\" 309 later, 310 .\" 311 following the discussion of 312 .\" HTML 313 .\" 314 parenthesized subpatterns. 315 .\" 316 . 317 . 318 ph10 333 .SS "Absolute and relative subroutine calls" 319 .rs 320 .sp 321 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or 322 a number enclosed either in angle brackets or single quotes, is an alternative 323 syntax for referencing a subpattern as a "subroutine". Details are discussed 324 .\" HTML 325 .\" 326 later. 327 .\" 328 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP 329 synonymous. The former is a back reference; the latter is a subroutine call. 330 . 331 . 332 nigel 75 .SS "Generic character types" 333 .rs 334 .sp 335 nigel 93 Another use of backslash is for specifying generic character types. The 336 nigel 75 following are always recognized: 337 .sp 338 ph10 182 \ed any decimal digit 339 nigel 75 \eD any character that is not a decimal digit 340 ph10 178 \eh any horizontal whitespace character 341 ph10 182 \eH any character that is not a horizontal whitespace character 342 nigel 75 \es any whitespace character 343 \eS any character that is not a whitespace character 344 ph10 178 \ev any vertical whitespace character 345 ph10 182 \eV any character that is not a vertical whitespace character 346 nigel 75 \ew any "word" character 347 \eW any "non-word" character 348 .sp 349 nigel 63 Each pair of escape sequences partitions the complete set of characters into 350 two disjoint sets. Any given character matches one, and only one, of each pair. 351 nigel 75 .P 352 These character type sequences can appear both inside and outside character 353 classes. They each match one character of the appropriate type. If the current 354 matching point is at the end of the subject string, all of them fail, since 355 there is no character to match. 356 .P 357 For compatibility with Perl, \es does not match the VT character (code 11). 358 This makes it different from the the POSIX "space" class. The \es characters 359 ph10 178 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is 360 nigel 91 included in a Perl script, \es may match the VT character. In PCRE, it never 361 ph10 178 does. 362 nigel 75 .P 363 ph10 178 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or 364 \ew, and always match \eD, \eS, and \eW. This is true even when Unicode 365 ph10 182 character property support is available. These sequences retain their original 366 meanings from before UTF-8 support was available, mainly for efficiency 367 ph10 178 reasons. 368 .P 369 ph10 182 The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the 370 ph10 178 other sequences, these do match certain high-valued codepoints in UTF-8 mode. 371 The horizontal space characters are: 372 .sp 373 U+0009 Horizontal tab 374 U+0020 Space 375 U+00A0 Non-break space 376 U+1680 Ogham space mark 377 U+180E Mongolian vowel separator 378 U+2000 En quad 379 U+2001 Em quad 380 U+2002 En space 381 U+2003 Em space 382 U+2004 Three-per-em space 383 U+2005 Four-per-em space 384 U+2006 Six-per-em space 385 U+2007 Figure space 386 U+2008 Punctuation space 387 U+2009 Thin space 388 U+200A Hair space 389 U+202F Narrow no-break space 390 U+205F Medium mathematical space 391 U+3000 Ideographic space 392 .sp 393 The vertical space characters are: 394 .sp 395 U+000A Linefeed 396 U+000B Vertical tab 397 U+000C Formfeed 398 U+000D Carriage return 399 U+0085 Next line 400 U+2028 Line separator 401 U+2029 Paragraph separator 402 .P 403 nigel 75 A "word" character is an underscore or any character less than 256 that is a 404 letter or digit. The definition of letters and digits is controlled by PCRE's 405 low-valued character tables, and may vary if locale-specific matching is taking 406 place (see 407 nigel 63 .\" HTML 408 .\" 409 "Locale support" 410 .\" 411 in the 412 .\" HREF 413 nigel 75 \fBpcreapi\fP 414 nigel 63 .\" 415 ph10 139 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 ph10 178 accented letters, and these are matched by \ew. The use of locales with Unicode 418 is discouraged. 419 nigel 75 . 420 . 421 ph10 231 .\" HTML 422 nigel 93 .SS "Newline sequences" 423 .rs 424 .sp 425 ph10 231 Outside a character class, by default, the escape sequence \eR matches any 426 Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is 427 equivalent to the following: 428 nigel 93 .sp 429 (?>\er\en|\en|\ex0b|\ef|\er|\ex85) 430 .sp 431 This is an example of an "atomic group", details of which are given 432 .\" HTML 433 .\" 434 below. 435 .\" 436 This particular group matches either the two-character sequence CR followed by 437 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, 438 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next 439 line, U+0085). The two-character sequence is treated as a single unit that 440 cannot be split. 441 .P 442 In UTF-8 mode, two additional characters whose codepoints are greater than 255 443 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). 444 Unicode character property support is not needed for these characters to be 445 recognized. 446 .P 447 ph10 231 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the 448 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF 449 ph10 247 either at compile time or when the pattern is matched. (BSR is an abbrevation 450 ph10 246 for "backslash R".) This can be made the default when PCRE is built; if this is 451 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option. 452 It is also possible to specify these settings by starting a pattern string with 453 one of the following sequences: 454 ph10 231 .sp 455 (*BSR_ANYCRLF) CR, LF, or CRLF only 456 (*BSR_UNICODE) any Unicode newline sequence 457 .sp 458 These override the default and the options given to \fBpcre_compile()\fP, but 459 they can be overridden by options given to \fBpcre_exec()\fP. Note that these 460 special settings, which are not Perl-compatible, are recognized only at the 461 very start of a pattern, and that they must be in upper case. If more than one 462 ph10 247 of them is present, the last one is used. They can be combined with a change of 463 ph10 246 newline convention, for example, a pattern can start with: 464 .sp 465 (*ANY)(*BSR_ANYCRLF) 466 .sp 467 nigel 93 Inside a character class, \eR matches the letter "R". 468 . 469 . 470 nigel 75 .\" HTML 471 .SS Unicode character properties 472 .rs 473 .sp 474 When PCRE is built with Unicode character property support, three additional 475 ph10 184 escape sequences that match characters with specific properties are available. 476 When not in UTF-8 mode, these sequences are of course limited to testing 477 characters whose codepoints are less than 256, but they do work in this mode. 478 The extra escape sequences are: 479 nigel 75 .sp 480 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property 481 \eP{\fIxx\fP} a character without the \fIxx\fP property 482 \eX an extended Unicode sequence 483 nigel 75 .sp 484 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode 485 script names, the general category properties, and "Any", which matches any 486 character (including newline). Other properties such as "InMusicalSymbols" are 487 not currently supported by PCRE. Note that \eP{Any} does not match any 488 characters, so always causes a match failure. 489 nigel 75 .P 490 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A 491 character from one of these sets can be matched using a script name. For 492 example: 493 nigel 75 .sp 494 nigel 87 \ep{Greek} 495 \eP{Han} 496 .sp 497 Those that are not part of an identified script are lumped together as 498 "Common". The current list of scripts is: 499 .P 500 Arabic, 501 Armenian, 502 nigel 93 Balinese, 503 nigel 87 Bengali, 504 Bopomofo, 505 Braille, 506 Buginese, 507 Buhid, 508 Canadian_Aboriginal, 509 Cherokee, 510 Common, 511 Coptic, 512 nigel 93 Cuneiform, 513 nigel 87 Cypriot, 514 Cyrillic, 515 Deseret, 516 Devanagari, 517 Ethiopic, 518 Georgian, 519 Glagolitic, 520 Gothic, 521 Greek, 522 Gujarati, 523 Gurmukhi, 524 Han, 525 Hangul, 526 Hanunoo, 527 Hebrew, 528 Hiragana, 529 Inherited, 530 Kannada, 531 Katakana, 532 Kharoshthi, 533 Khmer, 534 Lao, 535 Latin, 536 Limbu, 537 Linear_B, 538 Malayalam, 539 Mongolian, 540 Myanmar, 541 New_Tai_Lue, 542 nigel 93 Nko, 543 nigel 87 Ogham, 544 Old_Italic, 545 Old_Persian, 546 Oriya, 547 Osmanya, 548 nigel 93 Phags_Pa, 549 Phoenician, 550 nigel 87 Runic, 551 Shavian, 552 Sinhala, 553 Syloti_Nagri, 554 Syriac, 555 Tagalog, 556 Tagbanwa, 557 Tai_Le, 558 Tamil, 559 Telugu, 560 Thaana, 561 Thai, 562 Tibetan, 563 Tifinagh, 564 Ugaritic, 565 Yi. 566 .P 567 Each character has exactly one general category property, specified by a 568 two-letter abbreviation. For compatibility with Perl, negation can be specified 569 by including a circumflex between the opening brace and the property name. For 570 example, \ep{^Lu} is the same as \eP{Lu}. 571 .P 572 If only one letter is specified with \ep or \eP, it includes all the general 573 category properties that start with that letter. In this case, in the absence 574 of negation, the curly brackets in the escape sequence are optional; these two 575 examples have the same effect: 576 .sp 577 nigel 75 \ep{L} 578 \epL 579 .sp 580 nigel 87 The following general category property codes are supported: 581 nigel 75 .sp 582 C Other 583 Cc Control 584 Cf Format 585 Cn Unassigned 586 Co Private use 587 Cs Surrogate 588 .sp 589 L Letter 590 Ll Lower case letter 591 Lm Modifier letter 592 Lo Other letter 593 Lt Title case letter 594 Lu Upper case letter 595 .sp 596 M Mark 597 Mc Spacing mark 598 Me Enclosing mark 599 Mn Non-spacing mark 600 .sp 601 N Number 602 Nd Decimal number 603 Nl Letter number 604 No Other number 605 .sp 606 P Punctuation 607 Pc Connector punctuation 608 Pd Dash punctuation 609 Pe Close punctuation 610 Pf Final punctuation 611 Pi Initial punctuation 612 Po Other punctuation 613 Ps Open punctuation 614 .sp 615 S Symbol 616 Sc Currency symbol 617 Sk Modifier symbol 618 Sm Mathematical symbol 619 So Other symbol 620 .sp 621 Z Separator 622 Zl Line separator 623 Zp Paragraph separator 624 Zs Space separator 625 .sp 626 nigel 87 The special property L& is also supported: it matches a character that has 627 the Lu, Ll, or Lt property, in other words, a letter that is not classified as 628 a modifier or "other". 629 nigel 75 .P 630 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to 631 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so 632 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off 633 (see the discussion of PCRE_NO_UTF8_CHECK in the 634 .\" HREF 635 \fBpcreapi\fP 636 .\" 637 page). 638 .P 639 nigel 87 The long synonyms for these properties that Perl supports (such as \ep{Letter}) 640 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these 641 nigel 87 properties with "Is". 642 .P 643 No character that is in the Unicode table has the Cn (unassigned) property. 644 Instead, this property is assumed for any code point that is not in the 645 Unicode table. 646 .P 647 nigel 75 Specifying caseless matching does not affect these escape sequences. For 648 example, \ep{Lu} always matches only upper case letters. 649 .P 650 The \eX escape matches any number of Unicode characters that form an extended 651 Unicode sequence. \eX is equivalent to 652 .sp 653 (?>\ePM\epM*) 654 .sp 655 That is, it matches a character without the "mark" property, followed by zero 656 or more characters with the "mark" property, and treats the sequence as an 657 atomic group 658 .\" HTML 659 .\" 660 (see below). 661 .\" 662 Characters with the "mark" property are typically accents that affect the 663 ph10 185 preceding character. None of them have codepoints less than 256, so in 664 ph10 184 non-UTF-8 mode \eX matches any one character. 665 nigel 75 .P 666 Matching characters by Unicode property is not fast, because PCRE has to search 667 a structure that contains data for over fifteen thousand characters. That is 668 why the traditional escape sequences such as \ed and \ew do not use Unicode 669 properties in PCRE. 670 . 671 . 672 ph10 168 .\" HTML 673 .SS "Resetting the match start" 674 .rs 675 .sp 676 ph10 172 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously 677 matched characters not to be included in the final matched sequence. For 678 ph10 168 example, the pattern: 679 .sp 680 foo\eKbar 681 .sp 682 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is 683 ph10 168 similar to a lookbehind assertion 684 .\" HTML 685 .\" 686 (described below). 687 .\" 688 ph10 172 However, in this case, the part of the subject before the real match does not 689 have to be of fixed length, as lookbehind assertions do. The use of \eK does 690 ph10 168 not interfere with the setting of 691 .\" HTML 692 .\" 693 captured substrings. 694 ph10 172 .\" 695 ph10 168 For example, when the pattern 696 .sp 697 (foo)\eKbar 698 .sp 699 ph10 172 matches "foobar", the first substring is still set to "foo". 700 ph10 168 . 701 . 702 nigel 75 .\" HTML 703 .SS "Simple assertions" 704 .rs 705 .sp 706 nigel 93 The final use of backslash is for certain simple assertions. An assertion 707 nigel 63 specifies a condition that has to be met at a particular point in a match, 708 without consuming any characters from the subject string. The use of 709 nigel 75 subpatterns for more complicated assertions is described 710 .\" HTML 711 .\" 712 below. 713 .\" 714 nigel 91 The backslashed assertions are: 715 nigel 75 .sp 716 \eb matches at a word boundary 717 \eB matches when not at a word boundary 718 nigel 93 \eA matches at the start of the subject 719 \eZ matches at the end of the subject 720 also matches before a newline at the end of the subject 721 \ez matches only at the end of the subject 722 \eG matches at the first matching position in the subject 723 nigel 75 .sp 724 These assertions may not appear in character classes (but note that \eb has a 725 nigel 63 different meaning, namely the backspace character, inside a character class). 726 nigel 75 .P 727 nigel 63 A word boundary is a position in the subject string where the current character 728 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches 729 \ew and the other matches \eW), or the start or end of the string if the 730 first or last character matches \ew, respectively. 731 .P 732 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and 733 dollar (described in the next section) in that they only ever match at the very 734 start and end of the subject string, whatever options are set. Thus, they are 735 independent of multiline mode. These three assertions are not affected by the 736 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the 737 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP 738 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start 739 at a point other than the beginning of the subject, \eA can never match. The 740 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end 741 of the string as well as at the very end, whereas \ez matches only at the end. 742 nigel 75 .P 743 The \eG assertion is true only when the current matching position is at the 744 start point of the match, as specified by the \fIstartoffset\fP argument of 745 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is 746 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate 747 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of 748 nigel 75 implementation where \eG can be useful. 749 .P 750 Note, however, that PCRE's interpretation of \eG, as the start of the current 751 nigel 63 match, is subtly different from Perl's, which defines it as the end of the 752 previous match. In Perl, these can be different when the previously matched 753 string was empty. Because PCRE does just one match at a time, it cannot 754 reproduce this behaviour. 755 nigel 75 .P 756 If all the alternatives of a pattern begin with \eG, the expression is anchored 757 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled 758 regular expression. 759 nigel 75 . 760 . 761 .SH "CIRCUMFLEX AND DOLLAR" 762 nigel 63 .rs 763 .sp 764 Outside a character class, in the default matching mode, the circumflex 765 nigel 75 character is an assertion that is true only if the current matching point is 766 at the start of the subject string. If the \fIstartoffset\fP argument of 767 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE 768 nigel 63 option is unset. Inside a character class, circumflex has an entirely different 769 nigel 75 meaning 770 .\" HTML 771 .\" 772 (see below). 773 .\" 774 .P 775 nigel 63 Circumflex need not be the first character of the pattern if a number of 776 alternatives are involved, but it should be the first thing in each alternative 777 in which it appears if the pattern is ever to match that branch. If all 778 possible alternatives start with a circumflex, that is, if the pattern is 779 constrained to match only at the start of the subject, it is said to be an 780 "anchored" pattern. (There are also other constructs that can cause a pattern 781 to be anchored.) 782 nigel 75 .P 783 A dollar character is an assertion that is true only if the current matching 784 nigel 63 point is at the end of the subject string, or immediately before a newline 785 nigel 91 at the end of the string (by default). Dollar need not be the last character of 786 the pattern if a number of alternatives are involved, but it should be the last 787 item in any branch in which it appears. Dollar has no special meaning in a 788 character class. 789 nigel 75 .P 790 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of 791 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This 792 nigel 75 does not affect the \eZ assertion. 793 .P 794 nigel 63 The meanings of the circumflex and dollar characters are changed if the 795 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches 796 immediately after internal newlines as well as at the start of the subject 797 string. It does not match after a newline that ends the string. A dollar 798 matches before any newlines in the string, as well as at the very end, when 799 PCRE_MULTILINE is set. When newline is specified as the two-character 800 sequence CRLF, isolated CR and LF characters do not indicate newlines. 801 nigel 75 .P 802 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where 803 \en represents a newline) in multiline mode, but not otherwise. Consequently, 804 patterns that are anchored in single line mode because all branches start with 805 ^ are not anchored in multiline mode, and a match for circumflex is possible 806 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The 807 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. 808 .P 809 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and 810 nigel 63 end of the subject in both modes, and if all branches of a pattern start with 811 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set. 812 nigel 75 . 813 . 814 .SH "FULL STOP (PERIOD, DOT)" 815 nigel 63 .rs 816 .sp 817 Outside a character class, a dot in the pattern matches any one character in 818 nigel 91 the subject string except (by default) a character that signifies the end of a 819 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long. 820 nigel 91 .P 821 nigel 93 When a line ending is defined as a single character, dot never matches that 822 character; when the two-character sequence CRLF is used, dot does not match CR 823 if it is immediately followed by LF, but otherwise it matches all characters 824 (including isolated CRs and LFs). When any Unicode line endings are being 825 recognized, dot does not match CR or LF or any of the other line ending 826 characters. 827 .P 828 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL 829 nigel 93 option is set, a dot matches any one character, without exception. If the 830 two-character sequence CRLF is present in the subject string, it takes two dots 831 to match it. 832 nigel 91 .P 833 The handling of dot is entirely independent of the handling of circumflex and 834 dollar, the only relationship being that they both involve newlines. Dot has no 835 special meaning in a character class. 836 nigel 75 . 837 . 838 .SH "MATCHING A SINGLE BYTE" 839 nigel 63 .rs 840 .sp 841 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both 842 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending 843 characters. The feature is provided in Perl in order to match individual bytes 844 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, 845 what remains in the string may be a malformed UTF-8 string. For this reason, 846 the \eC escape sequence is best avoided. 847 nigel 75 .P 848 PCRE does not allow \eC to appear in lookbehind assertions 849 .\" HTML 850 .\" 851 (described below), 852 .\" 853 because in UTF-8 mode this would make it impossible to calculate the length of 854 the lookbehind. 855 . 856 . 857 .\" HTML 858 .SH "SQUARE BRACKETS AND CHARACTER CLASSES" 859 nigel 63 .rs 860 .sp 861 An opening square bracket introduces a character class, terminated by a closing 862 square bracket. A closing square bracket on its own is not special. If a 863 closing square bracket is required as a member of the class, it should be the 864 first data character in the class (after an initial circumflex, if present) or 865 escaped with a backslash. 866 nigel 75 .P 867 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the 868 character may occupy more than one byte. A matched character must be in the set 869 of characters defined by the class, unless the first character in the class 870 definition is a circumflex, in which case the subject character must not be in 871 the set defined by the class. If a circumflex is actually required as a member 872 of the class, ensure it is not the first character, or escape it with a 873 backslash. 874 nigel 75 .P 875 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while 876 [^aeiou] matches any character that is not a lower case vowel. Note that a 877 nigel 75 circumflex is just a convenient notation for specifying the characters that 878 are in the class by enumerating those that are not. A class that starts with a 879 circumflex is not an assertion: it still consumes a character from the subject 880 string, and therefore it fails if the current pointer is at the end of the 881 string. 882 .P 883 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a 884 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism. 885 .P 886 nigel 63 When caseless matching is set, any letters in a class represent both their 887 upper case and lower case versions, so for example, a caseless [aeiou] matches 888 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a 889 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of 890 case for characters whose values are less than 128, so caseless matching is 891 always possible. For characters with higher values, the concept of case is 892 supported if PCRE is compiled with Unicode property support, but not otherwise. 893 If you want to use caseless matching for characters 128 and above, you must 894 ensure that PCRE is compiled with Unicode property support as well as with 895 UTF-8 support. 896 nigel 75 .P 897 nigel 93 Characters that might indicate line breaks are never treated in any special way 898 when matching character classes, whatever line-ending sequence is in use, and 899 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class 900 such as [^a] always matches one of these characters. 901 nigel 75 .P 902 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a 903 character class. For example, [d-m] matches any letter between d and m, 904 inclusive. If a minus character is required in a class, it must be escaped with 905 a backslash or appear in a position where it cannot be interpreted as 906 indicating a range, typically as the first or last character in the class. 907 nigel 75 .P 908 nigel 63 It is not possible to have the literal character "]" as the end character of a 909 range. A pattern such as [W-]46] is interpreted as a class of two characters 910 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or 911 "-46]". However, if the "]" is escaped with a backslash it is interpreted as 912 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range 913 followed by two other characters. The octal or hexadecimal representation of 914 "]" can also be used to end a range. 915 .P 916 nigel 63 Ranges operate in the collating sequence of character values. They can also be 917 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8 918 nigel 63 mode, ranges can include characters whose values are greater than 255, for 919 nigel 75 example [\ex{100}-\ex{2ff}]. 920 .P 921 nigel 63 If a range that includes letters is used when caseless matching is set, it 922 matches the letters in either case. For example, [W-c] is equivalent to 923 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character 924 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E 925 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for 926 characters with values greater than 128 only when it is compiled with Unicode 927 property support. 928 .P 929 The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear 930 in a character class, and add the characters that they match to the class. For 931 example, [\edABCDEF] matches any hexadecimal digit. A circumflex can 932 nigel 63 conveniently be used with the upper case character types to specify a more 933 restricted set of characters than the matching lower case type. For example, 934 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore. 935 .P 936 The only metacharacters that are recognized in character classes are backslash, 937 hyphen (only where it can be interpreted as specifying a range), circumflex 938 (only at the start), opening square bracket (only when it can be interpreted as 939 introducing a POSIX class name - see the next section), and the terminating 940 closing square bracket. However, escaping other non-alphanumeric characters 941 does no harm. 942 . 943 . 944 .SH "POSIX CHARACTER CLASSES" 945 nigel 63 .rs 946 .sp 947 nigel 75 Perl supports the POSIX notation for character classes. This uses names 948 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports 949 this notation. For example, 950 nigel 75 .sp 951 nigel 63 [01[:alpha:]%] 952 nigel 75 .sp 953 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names 954 are 955 nigel 75 .sp 956 nigel 63 alnum letters and digits 957 alpha letters 958 ascii character codes 0 - 127 959 blank space or tab only 960 cntrl control characters 961 nigel 75 digit decimal digits (same as \ed) 962 nigel 63 graph printing characters, excluding space 963 lower lower case letters 964 print printing characters, including space 965 punct printing characters, excluding letters and digits 966 nigel 75 space white space (not quite the same as \es) 967 nigel 63 upper upper case letters 968 nigel 75 word "word" characters (same as \ew) 969 nigel 63 xdigit hexadecimal digits 970 nigel 75 .sp 971 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and 972 space (32). Notice that this list includes the VT character (code 11). This 973 nigel 75 makes "space" different to \es, which does not include VT (for Perl 974 nigel 63 compatibility). 975 nigel 75 .P 976 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 977 5.8. Another Perl extension is negation, which is indicated by a ^ character 978 after the colon. For example, 979 nigel 75 .sp 980 nigel 63 [12[:^digit:]] 981 nigel 75 .sp 982 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX 983 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not 984 supported, and an error is given if they are encountered. 985 nigel 75 .P 986 In UTF-8 mode, characters with values greater than 128 do not match any of 987 nigel 63 the POSIX character classes. 988 nigel 75 . 989 . 990 .SH "VERTICAL BAR" 991 nigel 63 .rs 992 .sp 993 Vertical bar characters are used to separate alternative patterns. For example, 994 the pattern 995 nigel 75 .sp 996 nigel 63 gilbert|sullivan 997 nigel 75 .sp 998 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear, 999 nigel 91 and an empty alternative is permitted (matching the empty string). The matching 1000 process tries each alternative in turn, from left to right, and the first one 1001 that succeeds is used. If the alternatives are within a subpattern 1002 nigel 75 .\" HTML 1003 .\" 1004 (defined below), 1005 .\" 1006 "succeeds" means matching the rest of the main pattern as well as the 1007 alternative in the subpattern. 1008 . 1009 . 1010 .SH "INTERNAL OPTION SETTING" 1011 nigel 63 .rs 1012 .sp 1013 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and 1014 ph10 231 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within 1015 the pattern by a sequence of Perl option letters enclosed between "(?" and ")". 1016 The option letters are 1017 nigel 75 .sp 1018 nigel 63 i for PCRE_CASELESS 1019 m for PCRE_MULTILINE 1020 s for PCRE_DOTALL 1021 x for PCRE_EXTENDED 1022 nigel 75 .sp 1023 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to 1024 unset these options by preceding the letter with a hyphen, and a combined 1025 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and 1026 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also 1027 permitted. If a letter appears both before and after the hyphen, the option is 1028 unset. 1029 nigel 75 .P 1030 ph10 231 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be 1031 changed in the same way as the Perl-compatible options by using the characters 1032 J, U and X respectively. 1033 .P 1034 nigel 63 When an option change occurs at top level (that is, not inside subpattern 1035 parentheses), the change applies to the remainder of the pattern that follows. 1036 If the change is placed right at the start of a pattern, PCRE extracts it into 1037 the global options (and it will therefore show up in data extracted by the 1038 nigel 75 \fBpcre_fullinfo()\fP function). 1039 .P 1040 nigel 93 An option change within a subpattern (see below for a description of 1041 subpatterns) affects only that part of the current pattern that follows it, so 1042 nigel 75 .sp 1043 nigel 63 (a(?i)b)c 1044 nigel 75 .sp 1045 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). 1046 By this means, options can be made to have different settings in different 1047 parts of the pattern. Any changes made in one alternative do carry on 1048 into subsequent branches within the same subpattern. For example, 1049 nigel 75 .sp 1050 nigel 63 (a(?i)b|c) 1051 nigel 75 .sp 1052 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first 1053 branch is abandoned before the option setting. This is because the effects of 1054 option settings happen at compile time. There would be some very weird 1055 behaviour otherwise. 1056 ph10 251 .P 1057 \fBNote:\fP There are other PCRE-specific options that can be set by the 1058 application when the compile or match functions are called. In some cases the 1059 pattern can contain special leading sequences to override what the application 1060 has set or what has been defaulted. Details are given in the section entitled 1061 .\" HTML 1062 .\" 1063 "Newline sequences" 1064 .\" 1065 ph10 259 above. 1066 nigel 75 . 1067 . 1068 .\" HTML 1069 nigel 63 .SH SUBPATTERNS 1070 .rs 1071 .sp 1072 Subpatterns are delimited by parentheses (round brackets), which can be nested. 1073 nigel 75 Turning part of a pattern into a subpattern does two things: 1074 .sp 1075 nigel 63 1. It localizes a set of alternatives. For example, the pattern 1076 nigel 75 .sp 1077 nigel 63 cat(aract|erpillar|) 1078 nigel 75 .sp 1079 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the 1080 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string. 1081 nigel 75 .sp 1082 2. It sets up the subpattern as a capturing subpattern. This means that, when 1083 the whole pattern matches, that portion of the subject string that matched the 1084 subpattern is passed back to the caller via the \fIovector\fP argument of 1085 \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting 1086 from 1) to obtain numbers for the capturing subpatterns. 1087 .P 1088 nigel 63 For example, if the string "the red king" is matched against the pattern 1089 nigel 75 .sp 1090 nigel 63 the ((red|white) (king|queen)) 1091 nigel 75 .sp 1092 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1, 1093 2, and 3, respectively. 1094 nigel 75 .P 1095 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful. 1096 There are often times when a grouping subpattern is required without a 1097 capturing requirement. If an opening parenthesis is followed by a question mark 1098 and a colon, the subpattern does not do any capturing, and is not counted when 1099 computing the number of any subsequent capturing subpatterns. For example, if 1100 the string "the white queen" is matched against the pattern 1101 nigel 75 .sp 1102 nigel 63 the ((?:red|white) (king|queen)) 1103 nigel 75 .sp 1104 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and 1105 nigel 93 2. The maximum number of capturing subpatterns is 65535. 1106 nigel 75 .P 1107 nigel 63 As a convenient shorthand, if any option settings are required at the start of 1108 a non-capturing subpattern, the option letters may appear between the "?" and 1109 the ":". Thus the two patterns 1110 nigel 75 .sp 1111 nigel 63 (?i:saturday|sunday) 1112 (?:(?i)saturday|sunday) 1113 nigel 75 .sp 1114 nigel 63 match exactly the same set of strings. Because alternative branches are tried 1115 from left to right, and options are not reset until the end of the subpattern 1116 is reached, an option setting in one branch does affect subsequent branches, so 1117 the above patterns match "SUNDAY" as well as "Saturday". 1118 nigel 75 . 1119 . 1120 ph10 175 .SH "DUPLICATE SUBPATTERN NUMBERS" 1121 .rs 1122 .sp 1123 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses 1124 the same numbers for its capturing parentheses. Such a subpattern starts with 1125 (?| and is itself a non-capturing subpattern. For example, consider this 1126 ph10 175 pattern: 1127 .sp 1128 (?|(Sat)ur|(Sun))day 1129 ph10 182 .sp 1130 Because the two alternatives are inside a (?| group, both sets of capturing 1131 parentheses are numbered one. Thus, when the pattern matches, you can look 1132 at captured substring number one, whichever alternative matched. This construct 1133 is useful when you want to capture part, but not all, of one of a number of 1134 alternatives. Inside a (?| group, parentheses are numbered as usual, but the 1135 ph10 175 number is reset at the start of each branch. The numbers of any capturing 1136 ph10 182 buffers that follow the subpattern start after the highest number used in any 1137 branch. The following example is taken from the Perl documentation. 1138 ph10 175 The numbers underneath show in which buffer the captured content will be 1139 stored. 1140 .sp 1141 # before ---------------branch-reset----------- after 1142 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x 1143 # 1 2 2 3 2 3 4 1144 ph10 182 .sp 1145 ph10 175 A backreference or a recursive call to a numbered subpattern always refers to 1146 the first one in the pattern with the given number. 1147 .P 1148 An alternative approach to using this "branch reset" feature is to use 1149 duplicate named subpatterns, as described in the next section. 1150 . 1151 . 1152 nigel 75 .SH "NAMED SUBPATTERNS" 1153 nigel 63 .rs 1154 .sp 1155 Identifying capturing parentheses by number is simple, but it can be very hard 1156 to keep track of the numbers in complicated regular expressions. Furthermore, 1157 nigel 75 if an expression is modified, the numbers may change. To help with this 1158 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not 1159 added to Perl until release 5.10. Python had the feature earlier, and PCRE 1160 introduced it at release 4.0, using the Python syntax. PCRE now supports both 1161 the Perl and the Python syntax. 1162 .P 1163 In PCRE, a subpattern can be named in one of three ways: (?...) or 1164 (?'name'...) as in Perl, or (?P...) as in Python. References to capturing 1165 nigel 91 parentheses from other parts of the pattern, such as 1166 .\" HTML 1167 .\" 1168 backreferences, 1169 .\" 1170 .\" HTML 1171 .\" 1172 recursion, 1173 .\" 1174 and 1175 .\" HTML 1176 .\" 1177 conditions, 1178 .\" 1179 can be made by name as well as by number. 1180 nigel 75 .P 1181 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named 1182 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as 1183 if the names were not present. The PCRE API provides function calls for 1184 extracting the name-to-number translation table from a compiled pattern. There 1185 is also a convenience function for extracting a captured substring by name. 1186 nigel 91 .P 1187 By default, a name must be unique within a pattern, but it is possible to relax 1188 this constraint by setting the PCRE_DUPNAMES option at compile time. This can 1189 be useful for patterns where only one instance of the named parentheses can 1190 match. Suppose you want to match the name of a weekday, either as a 3-letter 1191 abbreviation or as the full name, and in both cases you want to extract the 1192 abbreviation. This pattern (ignoring the line breaks) does the job: 1193 .sp 1194 nigel 93 (?Mon|Fri|Sun)(?:day)?| 1195 (?Tue)(?:sday)?| 1196 (?Wed)(?:nesday)?| 1197 (?Thu)(?:rsday)?| 1198 (?Sat)(?:urday)? 1199 nigel 91 .sp 1200 There are five capturing substrings, but only one is ever set after a match. 1201 ph10 182 (An alternative way of solving this problem is to use a "branch reset" 1202 ph10 175 subpattern, as described in the previous section.) 1203 .P 1204 nigel 91 The convenience function for extracting the data by name returns the substring 1205 nigel 93 for the first (and in this example, the only) subpattern of that name that 1206 nigel 91 matched. This saves searching to find which numbered subpattern it was. If you 1207 make a reference to a non-unique named subpattern from elsewhere in the 1208 pattern, the one that corresponds to the lowest number is used. For further 1209 details of the interfaces for handling named subpatterns, see the 1210 nigel 63 .\" HREF 1211 nigel 75 \fBpcreapi\fP 1212 nigel 63 .\" 1213 documentation. 1214 nigel 75 . 1215 . 1216 nigel 63 .SH REPETITION 1217 .rs 1218 .sp 1219 Repetition is specified by quantifiers, which can follow any of the following 1220 items: 1221 nigel 75 .sp 1222 nigel 63 a literal data character 1223 nigel 93 the dot metacharacter 1224 nigel 75 the \eC escape sequence 1225 the \eX escape sequence (in UTF-8 mode with Unicode properties) 1226 nigel 93 the \eR escape sequence 1227 nigel 75 an escape such as \ed that matches a single character 1228 nigel 63 a character class 1229 a back reference (see next section) 1230 a parenthesized subpattern (unless it is an assertion) 1231 nigel 75 .sp 1232 nigel 63 The general repetition quantifier specifies a minimum and maximum number of 1233 permitted matches, by giving the two numbers in curly brackets (braces), 1234 separated by a comma. The numbers must be less than 65536, and the first must 1235 be less than or equal to the second. For example: 1236 nigel 75 .sp 1237 nigel 63 z{2,4} 1238 nigel 75 .sp 1239 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special 1240 character. If the second number is omitted, but the comma is present, there is 1241 no upper limit; if the second number and the comma are both omitted, the 1242 quantifier specifies an exact number of required matches. Thus 1243 nigel 75 .sp 1244 nigel 63 [aeiou]{3,} 1245 nigel 75 .sp 1246 nigel 63 matches at least 3 successive vowels, but may match many more, while 1247 nigel 75 .sp 1248 \ed{8} 1249 .sp 1250 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position 1251 where a quantifier is not allowed, or one that does not match the syntax of a 1252 quantifier, is taken as a literal character. For example, {,6} is not a 1253 quantifier, but a literal string of four characters. 1254 nigel 75 .P 1255 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual 1256 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of 1257 which is represented by a two-byte sequence. Similarly, when Unicode property 1258 support is available, \eX{3} matches three Unicode extended sequences, each of 1259 which may be several bytes long (and they may be of different lengths). 1260 .P 1261 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the 1262 previous item and the quantifier were not present. 1263 nigel 75 .P 1264 nigel 93 For convenience, the three most common quantifiers have single-character 1265 abbreviations: 1266 nigel 75 .sp 1267 nigel 63 * is equivalent to {0,} 1268 + is equivalent to {1,} 1269 ? is equivalent to {0,1} 1270 nigel 75 .sp 1271 nigel 63 It is possible to construct infinite loops by following a subpattern that can 1272 match no characters with a quantifier that has no upper limit, for example: 1273 nigel 75 .sp 1274 nigel 63 (a?)* 1275 nigel 75 .sp 1276 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for 1277 such patterns. However, because there are cases where this can be useful, such 1278 patterns are now accepted, but if any repetition of the subpattern does in fact 1279 match no characters, the loop is forcibly broken. 1280 nigel 75 .P 1281 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as 1282 possible (up to the maximum number of permitted times), without causing the 1283 rest of the pattern to fail. The classic example of where this gives problems 1284 nigel 75 is in trying to match comments in C programs. These appear between /* and */ 1285 and within the comment, individual * and / characters may appear. An attempt to 1286 match C comments by applying the pattern 1287 .sp 1288 /\e*.*\e*/ 1289 .sp 1290 nigel 63 to the string 1291 nigel 75 .sp 1292 /* first comment */ not comment /* second comment */ 1293 .sp 1294 nigel 63 fails, because it matches the entire string owing to the greediness of the .* 1295 item. 1296 nigel 75 .P 1297 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be 1298 greedy, and instead matches the minimum number of times possible, so the 1299 pattern 1300 nigel 75 .sp 1301 /\e*.*?\e*/ 1302 .sp 1303 nigel 63 does the right thing with the C comments. The meaning of the various 1304 quantifiers is not otherwise changed, just the preferred number of matches. 1305 Do not confuse this use of question mark with its use as a quantifier in its 1306 own right. Because it has two uses, it can sometimes appear doubled, as in 1307 nigel 75 .sp 1308 \ed??\ed 1309 .sp 1310 nigel 63 which matches one digit by preference, but can match two if that is the only 1311 way the rest of the pattern matches. 1312 nigel 75 .P 1313 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl), 1314 nigel 63 the quantifiers are not greedy by default, but individual ones can be made 1315 greedy by following them with a question mark. In other words, it inverts the 1316 default behaviour. 1317 nigel 75 .P 1318 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that 1319 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the 1320 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum. 1321 nigel 75 .P 1322 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent 1323 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is 1324 nigel 63 implicitly anchored, because whatever follows will be tried against every 1325 character position in the subject string, so there is no point in retrying the 1326 overall match at any position after the first. PCRE normally treats such a 1327 nigel 75 pattern as though it were preceded by \eA. 1328 .P 1329 nigel 63 In cases where it is known that the subject string contains no newlines, it is 1330 worth setting PCRE_DOTALL in order to obtain this optimization, or 1331 alternatively using ^ to indicate anchoring explicitly. 1332 nigel 75 .P 1333 nigel 63 However, there is one situation where the optimization cannot be used. When .* 1334 is inside capturing parentheses that are the subject of a backreference 1335 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one 1336 succeeds. Consider, for example: 1337 nigel 75 .sp 1338 (.*)abc\e1 1339 .sp 1340 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For 1341 this reason, such a pattern is not implicitly anchored. 1342 nigel 75 .P 1343 nigel 63 When a capturing subpattern is repeated, the value captured is the substring 1344 that matched the final iteration. For example, after 1345 nigel 75 .sp 1346 (tweedle[dume]{3}\es*)+ 1347 .sp 1348 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is 1349 "tweedledee". However, if there are nested capturing subpatterns, the 1350 corresponding captured values may have been set in previous iterations. For 1351 example, after 1352 nigel 75 .sp 1353 nigel 63 /(a|(b))+/ 1354 nigel 75 .sp 1355 nigel 63 matches "aba" the value of the second captured substring is "b". 1356 nigel 75 . 1357 . 1358 .\" HTML 1359 .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS" 1360 nigel 63 .rs 1361 .sp 1362 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 1363 repetition, failure of what follows normally causes the repeated item to be 1364 re-evaluated to see if a different number of repeats allows the rest of the 1365 pattern to match. Sometimes it is useful to prevent this, either to change the 1366 nature of the match, or to cause it fail earlier than it otherwise might, when 1367 the author of the pattern knows there is no point in carrying on. 1368 nigel 75 .P 1369 Consider, for example, the pattern \ed+foo when applied to the subject line 1370 .sp 1371 nigel 63 123456bar 1372 nigel 75 .sp 1373 nigel 63 After matching all 6 digits and then failing to match "foo", the normal 1374 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+ 1375 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping" 1376 (a term taken from Jeffrey Friedl's book) provides the means for specifying 1377 that once a subpattern has matched, it is not to be re-evaluated in this way. 1378 nigel 75 .P 1379 nigel 93 If we use atomic grouping for the previous example, the matcher gives up 1380 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of 1381 special parenthesis, starting with (?> as in this example: 1382 nigel 75 .sp 1383 (?>\ed+)foo 1384 .sp 1385 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once 1386 it has matched, and a failure further into the pattern is prevented from 1387 backtracking into it. Backtracking past it to previous items, however, works as 1388 normal. 1389 nigel 75 .P 1390 nigel 63 An alternative description is that a subpattern of this type matches the string 1391 of characters that an identical standalone pattern would match, if anchored at 1392 the current point in the subject string. 1393 nigel 75 .P 1394 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as 1395 the above example can be thought of as a maximizing repeat that must swallow 1396 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the 1397 nigel 63 number of digits they match in order to make the rest of the pattern match, 1398 nigel 75 (?>\ed+) can only match an entire sequence of digits. 1399 .P 1400 nigel 63 Atomic groups in general can of course contain arbitrarily complicated 1401 subpatterns, and can be nested. However, when the subpattern for an atomic 1402 group is just a single repeated item, as in the example above, a simpler 1403 notation, called a "possessive quantifier" can be used. This consists of an 1404 additional + character following a quantifier. Using this notation, the 1405 previous example can be rewritten as 1406 nigel 75 .sp 1407 \ed++foo 1408 .sp 1409 ph10 208 Note that a possessive quantifier can be used with an entire group, for 1410 example: 1411 .sp 1412 (abc|xyz){2,3}+ 1413 .sp 1414 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY 1415 option is ignored. They are a convenient notation for the simpler forms of 1416 nigel 93 atomic group. However, there is no difference in the meaning of a possessive 1417 quantifier and the equivalent atomic group, though there may be a performance 1418 difference; possessive quantifiers should be slightly faster. 1419 nigel 75 .P 1420 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax. 1421 Jeffrey Friedl originated the idea (and the name) in the first edition of his 1422 book. Mike McCloskey liked it, so implemented it when he built Sun's Java 1423 package, and PCRE copied it from there. It ultimately found its way into Perl 1424 at release 5.10. 1425 nigel 75 .P 1426 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple 1427 pattern constructs. For example, the sequence A+B is treated as A++B because 1428 there is no point in backtracking into a sequence of A's when B must follow. 1429 .P 1430 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself 1431 be repeated an unlimited number of times, the use of an atomic group is the 1432 only way to avoid some failing matches taking a very long time indeed. The 1433 pattern 1434 nigel 75 .sp 1435 (\eD+|<\ed+>)*[!?] 1436 .sp 1437 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or 1438 digits enclosed in <>, followed by either ! or ?. When it matches, it runs 1439 quickly. However, if it is applied to 1440 nigel 75 .sp 1441 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 1442 nigel 75 .sp 1443 nigel 63 it takes a long time before reporting failure. This is because the string can 1444 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a 1445 large number of ways, and all have to be tried. (The example uses [!?] rather 1446 than a single character at the end, because both PCRE and Perl have an 1447 optimization that allows for fast failure when a single character is used. They 1448 remember the last single character that is required for a match, and fail early 1449 if it is not present in the string.) If the pattern is changed so that it uses 1450 an atomic group, like this: 1451 .sp 1452 ((?>\eD+)|<\ed+>)*[!?] 1453 .sp 1454 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly. 1455 nigel 75 . 1456 . 1457 .\" HTML 1458 .SH "BACK REFERENCES" 1459 nigel 63 .rs 1460 .sp 1461 Outside a character class, a backslash followed by a digit greater than 0 (and 1462 possibly further digits) is a back reference to a capturing subpattern earlier 1463 (that is, to its left) in the pattern, provided there have been that many 1464 previous capturing left parentheses. 1465 nigel 75 .P 1466 nigel 63 However, if the decimal number following the backslash is less than 10, it is 1467 always taken as a back reference, and causes an error only if there are not 1468 that many capturing left parentheses in the entire pattern. In other words, the 1469 parentheses that are referenced need not be to the left of the reference for 1470 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense 1471 when a repetition is involved and the subpattern to the right has participated 1472 in an earlier iteration. 1473 .P 1474 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern 1475 whose number is 10 or more using this syntax because a sequence such as \e50 is 1476 interpreted as a character defined in octal. See the subsection entitled 1477 nigel 91 "Non-printing characters" 1478 nigel 75 .\" HTML 1479 .\" 1480 above 1481 .\" 1482 nigel 93 for further details of the handling of digits following a backslash. There is 1483 no such problem when named parentheses are used. A back reference to any 1484 subpattern is possible using named parentheses (see below). 1485 nigel 75 .P 1486 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a 1487 backslash is to use the \eg escape sequence, which is a feature introduced in 1488 ph10 208 Perl 5.10. This escape must be followed by an unsigned number or a negative 1489 number, optionally enclosed in braces. These examples are all identical: 1490 nigel 93 .sp 1491 (ring), \e1 1492 (ring), \eg1 1493 (ring), \eg{1} 1494 .sp 1495 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that 1496 is present in the older syntax. It is also useful when literal digits follow 1497 the reference. A negative number is a relative reference. Consider this 1498 example: 1499 nigel 93 .sp 1500 (abc(def)ghi)\eg{-1} 1501 .sp 1502 The sequence \eg{-1} is a reference to the most recently started capturing 1503 subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2} 1504 would be equivalent to \e1. The use of relative references can be helpful in 1505 long patterns, and also in patterns that are created by joining together 1506 fragments that contain references within themselves. 1507 .P 1508 nigel 63 A back reference matches whatever actually matched the capturing subpattern in 1509 the current subject string, rather than anything matching the subpattern 1510 itself (see 1511 .\" HTML 1512 .\" 1513 "Subpatterns as subroutines" 1514 .\" 1515 below for a way of doing that). So the pattern 1516 nigel 75 .sp 1517 (sens|respons)e and \e1ibility 1518 .sp 1519 nigel 63 matches "sense and sensibility" and "response and responsibility", but not 1520 "sense and responsibility". If caseful matching is in force at the time of the 1521 back reference, the case of letters is relevant. For example, 1522 nigel 75 .sp 1523 ((?i)rah)\es+\e1 1524 .sp 1525 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original 1526 capturing subpattern is matched caselessly. 1527 nigel 75 .P 1528 ph10 171 There are several different ways of writing back references to named 1529 subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek or 1530 \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified 1531 back reference syntax, in which \eg can be used for both numeric and named 1532 references, is also supported. We could rewrite the above example in any of 1533 nigel 93 the following ways: 1534 nigel 75 .sp 1535 nigel 93 (?(?i)rah)\es+\ek 1536 ph10 171 (?'p1'(?i)rah)\es+\ek{p1} 1537 nigel 91 (?P(?i)rah)\es+(?P=p1) 1538 ph10 171 (?(?i)rah)\es+\eg{p1} 1539 nigel 75 .sp 1540 nigel 91 A subpattern that is referenced by name may appear in the pattern before or 1541 after the reference. 1542 .P 1543 nigel 63 There may be more than one back reference to the same subpattern. If a 1544 subpattern has not actually been used in a particular match, any back 1545 references to it always fail. For example, the pattern 1546 nigel 75 .sp 1547 (a|(bc))\e2 1548 .sp 1549 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be 1550 many capturing parentheses in a pattern, all digits following the backslash are 1551 taken as part of a potential back reference number. If the pattern continues 1552 with a digit character, some delimiter must be used to terminate the back 1553 reference. If the PCRE_EXTENDED option is set, this can be whitespace. 1554 nigel 75 Otherwise an empty comment (see 1555 .\" HTML 1556 .\" 1557 "Comments" 1558 .\" 1559 below) can be used. 1560 .P 1561 nigel 63 A back reference that occurs inside the parentheses to which it refers fails 1562 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches. 1563 nigel 63 However, such references can be useful inside repeated subpatterns. For 1564 example, the pattern 1565 nigel 75 .sp 1566 (a|b\e1)+ 1567 .sp 1568 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of 1569 the subpattern, the back reference matches the character string corresponding 1570 to the previous iteration. In order for this to work, the pattern must be such 1571 that the first iteration does not need to match the back reference. This can be 1572 done using alternation, as in the example above, or by a quantifier with a 1573 minimum of zero. 1574 nigel 75 . 1575 . 1576 .\" HTML 1577 nigel 63 .SH ASSERTIONS 1578 .rs 1579 .sp 1580 An assertion is a test on the characters following or preceding the current 1581 matching point that does not actually consume any characters. The simple 1582 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and$ are described 1583 .\" HTML 1584 .\" 1585 above. 1586 .\" 1587 .P 1588 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds: 1589 those that look ahead of the current position in the subject string, and those 1590 nigel 75 that look behind it. An assertion subpattern is matched in the normal way, 1591 except that it does not cause the current matching position to be changed. 1592 .P 1593 Assertion subpatterns are not capturing subpatterns, and may not be repeated, 1594 because it makes no sense to assert the same thing several times. If any kind 1595 of assertion contains capturing subpatterns within it, these are counted for 1596 the purposes of numbering the capturing subpatterns in the whole pattern. 1597 However, substring capturing is carried out only for positive assertions, 1598 because it does not make sense for negative assertions. 1599 . 1600 . 1601 .SS "Lookahead assertions" 1602 .rs 1603 .sp 1604 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for 1605 negative assertions. For example, 1606 nigel 75 .sp 1607 \ew+(?=;) 1608 .sp 1609 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in 1610 the match, and 1611 nigel 75 .sp 1612 nigel 63 foo(?!bar) 1613 nigel 75 .sp 1614 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the 1615 apparently similar pattern 1616 nigel 75 .sp 1617 nigel 63 (?!foo)bar 1618 nigel 75 .sp 1619 nigel 63 does not find an occurrence of "bar" that is preceded by something other than 1620 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion 1621 (?!foo) is always true when the next three characters are "bar". A 1622 nigel 75 lookbehind assertion is needed to achieve the other effect. 1623 .P 1624 nigel 63 If you want to force a matching failure at some point in a pattern, the most 1625 convenient way to do it is with (?!) because an empty string always matches, so 1626 an assertion that requires there not to be an empty string must always fail. 1627 nigel 75 . 1628 . 1629 .\" HTML 1630 .SS "Lookbehind assertions" 1631 .rs 1632 .sp 1633 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (? 1663 .\" 1664 (see above) 1665 .\" 1666 can be used instead of a lookbehind assertion; this is not restricted to a 1667 fixed-length. 1668 .P 1669 nigel 63 The implementation of lookbehind assertions is, for each alternative, to 1670 nigel 93 temporarily move the current position back by the fixed length and then try to 1671 nigel 63 match. If there are insufficient characters before the current position, the 1672 nigel 93 assertion fails. 1673 nigel 75 .P 1674 PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode) 1675 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate 1676 nigel 93 the length of the lookbehind. The \eX and \eR escapes, which can match 1677 different numbers of bytes, are also not permitted. 1678 nigel 75 .P 1679 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to 1680 specify efficient matching at the end of the subject string. Consider a simple 1681 pattern such as 1682 nigel 75 .sp 1683 nigel 63 abcd$1684 nigel 75 .sp 1685 nigel 63 when applied to a long string that does not match. Because matching proceeds 1686 from left to right, PCRE will look for each "a" in the subject and then see if 1687 what follows matches the rest of the pattern. If the pattern is specified as 1688 nigel 75 .sp 1689 nigel 63 ^.*abcd$ 1690 nigel 75 .sp 1691 nigel 63 the initial .* matches the entire string at first, but when this fails (because 1692 there is no following "a"), it backtracks to match all but the last character, 1693 then all but the last two characters, and so on. Once again the search for "a" 1694 covers the entire string, from right to left, so we are no better off. However, 1695 if the pattern is written as 1696 nigel 75 .sp 1697 nigel 63 ^.*+(?<=abcd) 1698 nigel 75 .sp 1699 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire 1700 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four 1701 characters. If it fails, the match fails immediately. For long strings, this 1702 approach makes a significant difference to the processing time. 1703 nigel 75 . 1704 . 1705 .SS "Using multiple assertions" 1706 .rs 1707 .sp 1708 nigel 63 Several assertions (of any sort) may occur in succession. For example, 1709 nigel 75 .sp 1710 (?<=\ed{3})(? 1740 nigel 75 .SH "CONDITIONAL SUBPATTERNS" 1741 nigel 63 .rs 1742 .sp 1743 It is possible to cause the matching process to obey a subpattern 1744 conditionally or to choose between two alternative subpatterns, depending on 1745 the result of an assertion, or whether a previous capturing subpattern matched 1746 or not. The two possible forms of conditional subpattern are 1747 nigel 75 .sp 1748 nigel 63 (?(condition)yes-pattern) 1749 (?(condition)yes-pattern|no-pattern) 1750 nigel 75 .sp 1751 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the 1752 no-pattern (if present) is used. If there are more than two alternatives in the 1753 subpattern, a compile-time error occurs. 1754 nigel 75 .P 1755 nigel 93 There are four kinds of condition: references to subpatterns, references to 1756 recursion, a pseudo-condition called DEFINE, and assertions. 1757 . 1758 .SS "Checking for a used subpattern by number" 1759 .rs 1760 .sp 1761 If the text between the parentheses consists of a sequence of digits, the 1762 condition is true if the capturing subpattern of that number has previously 1763 ph10 172 matched. An alternative notation is to precede the digits with a plus or minus 1764 ph10 167 sign. In this case, the subpattern number is relative rather than absolute. 1765 ph10 172 The most recently opened parentheses can be referenced by (?(-1), the next most 1766 ph10 167 recent by (?(-2), and so on. In looping constructs it can also make sense to 1767 refer to subsequent groups with constructs such as (?(+2). 1768 nigel 91 .P 1769 Consider the following pattern, which contains non-significant white space to 1770 make it more readable (assume the PCRE_EXTENDED option) and to divide it into 1771 three parts for ease of discussion: 1772 nigel 75 .sp 1773 ( \e( )? [^()]+ (?(1) \e) ) 1774 .sp 1775 nigel 63 The first part matches an optional opening parenthesis, and if that 1776 character is present, sets it as the first captured substring. The second part 1777 matches one or more characters that are not parentheses. The third part is a 1778 conditional subpattern that tests whether the first set of parentheses matched 1779 or not. If they did, that is, if subject started with an opening parenthesis, 1780 the condition is true, and so the yes-pattern is executed and a closing 1781 parenthesis is required. Otherwise, since no-pattern is not present, the 1782 subpattern matches nothing. In other words, this pattern matches a sequence of 1783 nigel 93 non-parentheses, optionally enclosed in parentheses. 1784 ph10 167 .P 1785 ph10 172 If you were embedding this pattern in a larger one, you could use a relative 1786 ph10 167 reference: 1787 .sp 1788 ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ... 1789 .sp 1790 This makes the fragment independent of the parentheses in the larger pattern. 1791 nigel 93 . 1792 .SS "Checking for a used subpattern by name" 1793 .rs 1794 nigel 91 .sp 1795 nigel 93 Perl uses the syntax (?()...) or (?('name')...) to test for a used 1796 subpattern by name. For compatibility with earlier versions of PCRE, which had 1797 this facility before Perl, the syntax (?(name)...) is also recognized. However, 1798 there is a possible ambiguity with this syntax, because subpattern names may 1799 consist entirely of digits. PCRE looks first for a named subpattern; if it 1800 cannot find one and the name consists entirely of digits, PCRE looks for a 1801 subpattern of that number, which must be greater than zero. Using subpattern 1802 names that consist entirely of digits is not recommended. 1803 .P 1804 Rewriting the above example to use a named subpattern gives this: 1805 nigel 91 .sp 1806 nigel 93 (? \e( )? [^()]+ (?() \e) ) 1807 .sp 1808 . 1809 .SS "Checking for pattern recursion" 1810 .rs 1811 .sp 1812 nigel 91 If the condition is the string (R), and there is no subpattern with the name R, 1813 nigel 93 the condition is true if a recursive call to the whole pattern or any 1814 subpattern has been made. If digits or a name preceded by ampersand follow the 1815 letter R, for example: 1816 .sp 1817 (?(R3)...) or (?(R&name)...) 1818 .sp 1819 the condition is true if the most recent recursion is into the subpattern whose 1820 number or name is given. This condition does not check the entire recursion 1821 stack. 1822 nigel 75 .P 1823 nigel 93 At "top level", all these recursion test conditions are false. Recursive 1824 patterns are described below. 1825 . 1826 .SS "Defining subpatterns for use by reference only" 1827 .rs 1828 .sp 1829 If the condition is the string (DEFINE), and there is no subpattern with the 1830 name DEFINE, the condition is always false. In this case, there may be only one 1831 alternative in the subpattern. It is always skipped if control reaches this 1832 point in the pattern; the idea of DEFINE is that it can be used to define 1833 "subroutines" that can be referenced from elsewhere. (The use of "subroutines" 1834 is described below.) For example, a pattern to match an IPv4 address could be 1835 written like this (ignore whitespace and line breaks): 1836 .sp 1837 (?(DEFINE) (? 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) ) 1838 \eb (?&byte) (\e.(?&byte)){3} \eb 1839 .sp 1840 The first part of the pattern is a DEFINE group inside which a another group 1841 named "byte" is defined. This matches an individual component of an IPv4 1842 address (a number less than 256). When matching takes place, this part of the 1843 pattern is skipped because DEFINE acts like a false condition. 1844 .P 1845 The rest of the pattern uses references to the named group to match the four 1846 dot-separated components of an IPv4 address, insisting on a word boundary at 1847 each end. 1848 . 1849 .SS "Assertion conditions" 1850 .rs 1851 .sp 1852 If the condition is not in any of the above formats, it must be an assertion. 1853 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider 1854 this pattern, again containing non-significant white space, and with the two 1855 alternatives on the second line: 1856 nigel 75 .sp 1857 nigel 63 (?(?=[^a-z]*[a-z]) 1858 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} ) 1859 .sp 1860 nigel 63 The condition is a positive lookahead assertion that matches an optional 1861 sequence of non-letters followed by a letter. In other words, it tests for the 1862 presence of at least one letter in the subject. If a letter is found, the 1863 subject is matched against the first alternative; otherwise it is matched 1864 against the second. This pattern matches strings in one of the two forms 1865 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. 1866 nigel 75 . 1867 . 1868 .\" HTML 1869 nigel 63 .SH COMMENTS 1870 .rs 1871 .sp 1872 nigel 75 The sequence (?# marks the start of a comment that continues up to the next 1873 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters 1874 that make up a comment play no part in the pattern matching at all. 1875 nigel 75 .P 1876 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a 1877 nigel 91 character class introduces a comment that continues to immediately after the 1878 next newline in the pattern. 1879 nigel 75 . 1880 . 1881 nigel 91 .\" HTML 1882 nigel 75 .SH "RECURSIVE PATTERNS" 1883 nigel 63 .rs 1884 .sp 1885 Consider the problem of matching a string in parentheses, allowing for 1886 unlimited nested parentheses. Without the use of recursion, the best that can 1887 be done is to use a pattern that matches up to some fixed depth of nesting. It 1888 nigel 93 is not possible to handle an arbitrary nesting depth. 1889 .P 1890 For some time, Perl has provided a facility that allows regular expressions to 1891 recurse (amongst other things). It does this by interpolating Perl code in the 1892 expression at run time, and the code can refer to the expression itself. A Perl 1893 pattern using code interpolation to solve the parentheses problem can be 1894 created like this: 1895 nigel 75 .sp 1896 $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x; 1897 .sp 1898 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers 1899 nigel 93 recursively to the pattern in which it appears. 1900 nigel 75 .P 1901 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it 1902 supports special syntax for recursion of the entire pattern, and also for 1903 individual subpattern recursion. After its introduction in PCRE and Python, 1904 this kind of recursion was introduced into Perl at release 5.10. 1905 nigel 75 .P 1906 nigel 93 A special item that consists of (? followed by a number greater than zero and a 1907 closing parenthesis is a recursive call of the subpattern of the given number, 1908 provided that it occurs inside that subpattern. (If not, it is a "subroutine" 1909 call, which is described in the next section.) The special item (?R) or (?0) is 1910 a recursive call of the entire regular expression. 1911 nigel 87 .P 1912 nigel 93 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always 1913 treated as an atomic group. That is, once it has matched some of the subject 1914 string, it is never re-entered, even if it contains untried alternatives and 1915 there is a subsequent matching failure. 1916 .P 1917 nigel 87 This PCRE pattern solves the nested parentheses problem (assume the 1918 PCRE_EXTENDED option is set so that white space is ignored): 1919 nigel 75 .sp 1920 \e( ( (?>[^()]+) | (?R) )* \e) 1921 .sp 1922 nigel 63 First it matches an opening parenthesis. Then it matches any number of 1923 substrings which can either be a sequence of non-parentheses, or a recursive 1924 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring). 1925 nigel 63 Finally there is a closing parenthesis. 1926 nigel 75 .P 1927 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire 1928 pattern, so instead you could use this: 1929 nigel 75 .sp 1930 ( \e( ( (?>[^()]+) | (?1) )* \e) ) 1931 .sp 1932 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to 1933 ph10 172 them instead of the whole pattern. 1934 ph10 166 .P 1935 In a larger pattern, keeping track of parenthesis numbers can be tricky. This 1936 is made easier by the use of relative references. (A Perl 5.10 feature.) 1937 Instead of (?1) in the pattern above you can write (?-2) to refer to the second 1938 most recently opened parentheses preceding the recursion. In other words, a 1939 negative number counts capturing parentheses leftwards from the point at which 1940 it is encountered. 1941 .P 1942 It is also possible to refer to subsequently opened parentheses, by writing 1943 references such as (?+2). However, these cannot be recursive because the 1944 reference is not inside the parentheses that are referenced. They are always 1945 "subroutine" calls, as described in the next section. 1946 .P 1947 An alternative approach is to use named parentheses instead. The Perl syntax 1948 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We 1949 could rewrite the above example as follows: 1950 nigel 75 .sp 1951 nigel 93 (? \e( ( (?>[^()]+) | (?&pn) )* \e) ) 1952 nigel 75 .sp 1953 nigel 93 If there is more than one subpattern with the same name, the earliest one is 1954 ph10 172 used. 1955 ph10 166 .P 1956 This particular example pattern that we have been looking at contains nested 1957 unlimited repeats, and so the use of atomic grouping for matching strings of 1958 non-parentheses is important when applying the pattern to strings that do not 1959 match. For example, when this pattern is applied to 1960 nigel 75 .sp 1961 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 1962 nigel 75 .sp 1963 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used, 1964 the match runs for a very long time indeed because there are so many different 1965 ways the + and * repeats can carve up the subject, and all have to be tested 1966 before failure can be reported. 1967 nigel 75 .P 1968 nigel 63 At the end of a match, the values set for any capturing subpatterns are those 1969 from the outermost level of the recursion at which the subpattern value is set. 1970 If you want to obtain intermediate values, a callout function can be used (see 1971 nigel 93 below and the 1972 nigel 63 .\" HREF 1973 nigel 75 \fBpcrecallout\fP 1974 nigel 63 .\" 1975 documentation). If the pattern above is matched against 1976 nigel 75 .sp 1977 nigel 63 (ab(cd)ef) 1978 nigel 75 .sp 1979 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken 1980 on at the top level. If additional parentheses are added, giving 1981 nigel 75 .sp 1982 \e( ( ( (?>[^()]+) | (?R) )* ) \e) 1983 nigel 63 ^ ^ 1984 ^ ^ 1985 nigel 75 .sp 1986 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level 1987 parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE 1988 has to obtain extra memory to store data during a recursion, which it does by 1989 nigel 75 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no 1990 nigel 63 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. 1991 nigel 75 .P 1992 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion. 1993 Consider this pattern, which matches text in angle brackets, allowing for 1994 arbitrary nesting. Only digits are allowed in nested brackets (that is, when 1995 recursing), whereas any characters are permitted at the outer level. 1996 nigel 75 .sp 1997 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * > 1998 .sp 1999 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two 2000 different alternatives for the recursive and non-recursive cases. The (?R) item 2001 is the actual recursive call. 2002 nigel 75 . 2003 . 2004 nigel 63 .\" HTML 2005 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES" 2006 nigel 63 .rs 2007 .sp 2008 If the syntax for a recursive subpattern reference (either by number or by 2009 name) is used outside the parentheses to which it refers, it operates like a 2010 nigel 93 subroutine in a programming language. The "called" subpattern may be defined 2011 ph10 166 before or after the reference. A numbered reference can be absolute or 2012 relative, as in these examples: 2013 nigel 75 .sp 2014 ph10 166 (...(absolute)...)...(?2)... 2015 (...(relative)...)...(?-1)... 2016 ph10 172 (...(?+1)...(relative)... 2017 ph10 166 .sp 2018 An earlier example pointed out that the pattern 2019 .sp 2020 nigel 75 (sens|respons)e and \e1ibility 2021 .sp 2022 nigel 63 matches "sense and sensibility" and "response and responsibility", but not 2023 "sense and responsibility". If instead the pattern 2024 nigel 75 .sp 2025 nigel 63 (sens|respons)e and (?1)ibility 2026 nigel 75 .sp 2027 nigel 63 is used, it does match "sense and responsibility" as well as the other two 2028 nigel 93 strings. Another example is given in the discussion of DEFINE above. 2029 nigel 87 .P 2030 Like recursive subpatterns, a "subroutine" call is always treated as an atomic 2031 group. That is, once it has matched some of the subject string, it is never 2032 re-entered, even if it contains untried alternatives and there is a subsequent 2033 matching failure. 2034 nigel 93 .P 2035 When a subpattern is used as a subroutine, processing options such as 2036 case-independence are fixed when the subpattern is defined. They cannot be 2037 changed for different calls. For example, consider this pattern: 2038 .sp 2039 ph10 166 (abc)(?i:(?-1)) 2040 nigel 93 .sp 2041 It matches "abcabc". It does not match "abcABC" because the change of 2042 processing option does not affect the called subpattern. 2043 nigel 75 . 2044 . 2045 ph10 333 .\" HTML 2046 .SH "ONIGURUMA SUBROUTINE SYNTAX" 2047 .rs 2048 .sp 2049 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or 2050 a number enclosed either in angle brackets or single quotes, is an alternative 2051 syntax for referencing a subpattern as a subroutine, possibly recursively. Here 2052 are two of the examples used above, rewritten using this syntax: 2053 .sp 2054 (? \e( ( (?>[^()]+) | \eg )* \e) ) 2055 (sens|respons)e and \eg'1'ibility 2056 .sp 2057 PCRE supports an extension to Oniguruma: if a number is preceded by a 2058 plus or a minus sign it is taken as a relative reference. For example: 2059 .sp 2060 (abc)(?i:\eg<-1>) 2061 .sp 2062 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP 2063 synonymous. The former is a back reference; the latter is a subroutine call. 2064 . 2065 . 2066 nigel 63 .SH CALLOUTS 2067 .rs 2068 .sp 2069 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl 2070 code to be obeyed in the middle of matching a regular expression. This makes it 2071 possible, amongst other things, to extract different substrings that match the 2072 same pair of parentheses when there is a repetition. 2073 nigel 75 .P 2074 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl 2075 code. The feature is called "callout". The caller of PCRE provides an external 2076 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP. 2077 nigel 63 By default, this variable contains NULL, which disables all calling out. 2078 nigel 75 .P 2079 nigel 63 Within a regular expression, (?C) indicates the points at which the external 2080 function is to be called. If you want to identify different callout points, you 2081 can put a number less than 256 after the letter C. The default value is zero. 2082 For example, this pattern has two callout points: 2083 nigel 75 .sp 2084 ph10 155 (?C1)abc(?C2)def 2085 nigel 75 .sp 2086 If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are 2087 automatically installed before each item in the pattern. They are all numbered 2088 255. 2089 .P 2090 During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is 2091 nigel 63 set), the external function is called. It is provided with the number of the 2092 nigel 75 callout, the position in the pattern, and, optionally, one item of data 2093 originally supplied by the caller of \fBpcre_exec()\fP. The callout function 2094 may cause matching to proceed, to backtrack, or to fail altogether. A complete 2095 description of the interface to the callout function is given in the 2096 nigel 63 .\" HREF 2097 nigel 75 \fBpcrecallout\fP 2098 nigel 63 .\" 2099 documentation. 2100 nigel 93 . 2101 . 2102 ph10 235 .SH "BACKTRACKING CONTROL" 2103 ph10 210 .rs 2104 .sp 2105 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which 2106 ph10 210 are described in the Perl documentation as "experimental and subject to change 2107 ph10 211 or removal in a future version of Perl". It goes on to say: "Their usage in 2108 production code should be noted to avoid problems during upgrades." The same 2109 ph10 210 remarks apply to the PCRE features described in this section. 2110 .P 2111 ph10 211 Since these verbs are specifically related to backtracking, they can be used 2112 only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a 2113 backtracking algorithm. They cause an error if encountered by 2114 ph10 210 \fBpcre_dfa_exec()\fP. 2115 .P 2116 ph10 211 The new verbs make use of what was previously invalid syntax: an opening 2117 ph10 210 parenthesis followed by an asterisk. In Perl, they are generally of the form 2118 (*VERB:ARG) but PCRE does not support the use of arguments, so its general 2119 form is just (*VERB). Any number of these verbs may occur in a pattern. There 2120 are two kinds: 2121 . 2122 .SS "Verbs that act immediately" 2123 .rs 2124 .sp 2125 The following verbs act as soon as they are encountered: 2126 .sp 2127 (*ACCEPT) 2128 .sp 2129 This verb causes the match to end successfully, skipping the remainder of the 2130 ph10 211 pattern. When inside a recursion, only the innermost pattern is ended 2131 immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside 2132 ph10 210 capturing parentheses. In Perl, the data so far is captured: in PCRE no data is 2133 captured. For example: 2134 .sp 2135 A(A|B(*ACCEPT)|C)D 2136 .sp 2137 ph10 211 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is 2138 ph10 210 captured. 2139 .sp 2140 (*FAIL) or (*F) 2141 .sp 2142 ph10 211 This verb causes the match to fail, forcing backtracking to occur. It is 2143 ph10 210 equivalent to (?!) but easier to read. The Perl documentation notes that it is 2144 probably useful only when combined with (?{}) or (??{}). Those are, of course, 2145 Perl features that are not present in PCRE. The nearest equivalent is the 2146 callout feature, as for example in this pattern: 2147 .sp 2148 a+(?C)(*FAIL) 2149 .sp 2150 ph10 211 A match with the string "aaaa" always fails, but the callout is taken before 2151 each backtrack happens (in this example, 10 times). 2152 ph10 210 . 2153 .SS "Verbs that act after backtracking" 2154 .rs 2155 .sp 2156 ph10 211 The following verbs do nothing when they are encountered. Matching continues 2157 with what follows, but if there is no subsequent match, a failure is forced. 2158 ph10 210 The verbs differ in exactly what kind of failure occurs. 2159 .sp 2160 (*COMMIT) 2161 .sp 2162 ph10 211 This verb causes the whole match to fail outright if the rest of the pattern 2163 ph10 210 does not match. Even if the pattern is unanchored, no further attempts to find 2164 a match by advancing the start point take place. Once (*COMMIT) has been 2165 ph10 211 passed, \fBpcre_exec()\fP is committed to finding a match at the current 2166 ph10 210 starting point, or not at all. For example: 2167 .sp 2168 a+(*COMMIT)b 2169 .sp 2170 ph10 211 This matches "xxaab" but not "aacaab". It can be thought of as a kind of 2171 ph10 210 dynamic anchor, or "I've started, so I must finish." 2172 .sp 2173 (*PRUNE) 2174 .sp 2175 ph10 211 This verb causes the match to fail at the current position if the rest of the 2176 pattern does not match. If the pattern is unanchored, the normal "bumpalong" 2177 ph10 210 advance to the next starting character then happens. Backtracking can occur as 2178 usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but 2179 if there is no match to the right, backtracking cannot cross (*PRUNE). 2180 ph10 211 In simple cases, the use of (*PRUNE) is just an alternative to an atomic 2181 ph10 210 group or possessive quantifier, but there are some uses of (*PRUNE) that cannot 2182 be expressed in any other way. 2183 .sp 2184 (*SKIP) 2185 .sp 2186 ph10 211 This verb is like (*PRUNE), except that if the pattern is unanchored, the 2187 ph10 210 "bumpalong" advance is not to the next character, but to the position in the 2188 subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text 2189 was matched leading up to it cannot be part of a successful match. Consider: 2190 .sp 2191 a+(*SKIP)b 2192 .sp 2193 ph10 211 If the subject is "aaaac...", after the first match attempt fails (starting at 2194 ph10 210 the first character in the string), the starting point skips on to start the 2195 ph10 211 next attempt at "c". Note that a possessive quantifer does not have the same 2196 ph10 210 effect in this example; although it would suppress backtracking during the 2197 first match attempt, the second attempt would start at the second character 2198 instead of skipping on to "c". 2199 .sp 2200 (*THEN) 2201 ph10 211 .sp 2202 ph10 210 This verb causes a skip to the next alternation if the rest of the pattern does 2203 not match. That is, it cancels pending backtracking, but only within the 2204 current alternation. Its name comes from the observation that it can be used 2205 for a pattern-based if-then-else block: 2206 .sp 2207 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... 2208 .sp 2209 ph10 211 If the COND1 pattern matches, FOO is tried (and possibly further items after 2210 ph10 210 the end of the group if FOO succeeds); on failure the matcher skips to the 2211 ph10 211 second alternative and tries COND2, without backtracking into COND1. If (*THEN) 2212 ph10 210 is used outside of any alternation, it acts exactly like (*PRUNE). 2213 . 2214 . 2215 nigel 93 .SH "SEE ALSO" 2216 .rs 2217 .sp 2218 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3). 2219 ph10 99 . 2220 . 2221 .SH AUTHOR 2222 .rs 2223 .sp 2224 .nf 2225 Philip Hazel 2226 University Computing Service 2227 Cambridge CB2 3QH, England. 2228 .fi 2229 . 2230 . 2231 .SH REVISION 2232 .rs 2233 .sp 2234 .nf 2235 ph10 333 Last updated: 10 April 2008 2236 Copyright (c) 1997-2008 University of Cambridge. 2237 ph10 99 .fi

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