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

Tue May 28 09:13:59 2013 UTC (3 weeks, 1 day ago) by ph10
File size: 130605 byte(s)
Final source file tidies for 8.33 release.


 1 .TH PCREPATTERN 3 "26 April 2013" "PCRE 8.33" 2 .SH NAME 3 PCRE - Perl-compatible regular expressions 4 .SH "PCRE REGULAR EXPRESSION DETAILS" 5 .rs 6 .sp 7 The syntax and semantics of the regular expressions that are supported by PCRE 8 are described in detail below. There is a quick-reference syntax summary in the 9 .\" HREF 10 \fBpcresyntax\fP 11 .\" 12 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE 13 also supports some alternative regular expression syntax (which does not 14 conflict with the Perl syntax) in order to provide some compatibility with 15 regular expressions in Python, .NET, and Oniguruma. 16 .P 17 Perl's regular expressions are described in its own documentation, and 18 regular expressions in general are covered in a number of books, some of which 19 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", 20 published by O'Reilly, covers regular expressions in great detail. This 21 description of PCRE's regular expressions is intended as reference material. 22 .P 23 This document discusses the patterns that are supported by PCRE when one its 24 main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16|32]_exec()\fP 25 (16- or 32-bit), is used. PCRE also has alternative matching functions, 26 \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP, which match using a 27 different algorithm that is not Perl-compatible. Some of the features discussed 28 below are not available when DFA matching is used. The advantages and 29 disadvantages of the alternative functions, and how they differ from the normal 30 functions, are discussed in the 31 .\" HREF 32 \fBpcrematching\fP 33 .\" 34 page. 35 . 36 . 37 .SH "SPECIAL START-OF-PATTERN ITEMS" 38 .rs 39 .sp 40 A number of options that can be passed to \fBpcre_compile()\fP can also be set 41 by special items at the start of a pattern. These are not Perl-compatible, but 42 are provided to make these options accessible to pattern writers who are not 43 able to change the program that processes the pattern. Any number of these 44 items may appear, but they must all be together right at the start of the 45 pattern string, and the letters must be in upper case. 46 . 47 . 48 .SS "UTF support" 49 .rs 50 .sp 51 The original operation of PCRE was on strings of one-byte characters. However, 52 there is now also support for UTF-8 strings in the original library, an 53 extra library that supports 16-bit and UTF-16 character strings, and a 54 third library that supports 32-bit and UTF-32 character strings. To use these 55 features, PCRE must be built to include appropriate support. When using UTF 56 strings you must either call the compiling function with the PCRE_UTF8, 57 PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of 58 these special sequences: 59 .sp 60 (*UTF8) 61 (*UTF16) 62 (*UTF32) 63 (*UTF) 64 .sp 65 (*UTF) is a generic sequence that can be used with any of the libraries. 66 Starting a pattern with such a sequence is equivalent to setting the relevant 67 option. How setting a UTF mode affects pattern matching is mentioned in several 68 places below. There is also a summary of features in the 69 .\" HREF 70 \fBpcreunicode\fP 71 .\" 72 page. 73 .P 74 Some applications that allow their users to supply patterns may wish to 75 restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF 76 option is set at compile time, (*UTF) etc. are not allowed, and their 77 appearance causes an error. 78 . 79 . 80 .SS "Unicode property support" 81 .rs 82 .sp 83 Another special sequence that may appear at the start of a pattern is 84 .sp 85 (*UCP) 86 .sp 87 This has the same effect as setting the PCRE_UCP option: it causes sequences 88 such as \ed and \ew to use Unicode properties to determine character types, 89 instead of recognizing only characters with codes less than 128 via a lookup 90 table. 91 . 92 . 93 .SS "Disabling start-up optimizations" 94 .rs 95 .sp 96 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the 97 PCRE_NO_START_OPTIMIZE option either at compile or matching time. 98 . 99 . 100 .\" HTML 101 .SS "Newline conventions" 102 .rs 103 .sp 104 PCRE supports five different conventions for indicating line breaks in 105 strings: a single CR (carriage return) character, a single LF (linefeed) 106 character, the two-character sequence CRLF, any of the three preceding, or any 107 Unicode newline sequence. The 108 .\" HREF 109 \fBpcreapi\fP 110 .\" 111 page has 112 .\" HTML 113 .\" 114 further discussion 115 .\" 116 about newlines, and shows how to set the newline convention in the 117 \fIoptions\fP arguments for the compiling and matching functions. 118 .P 119 It is also possible to specify a newline convention by starting a pattern 120 string with one of the following five sequences: 121 .sp 122 (*CR) carriage return 123 (*LF) linefeed 124 (*CRLF) carriage return, followed by linefeed 125 (*ANYCRLF) any of the three above 126 (*ANY) all Unicode newline sequences 127 .sp 128 These override the default and the options given to the compiling function. For 129 example, on a Unix system where LF is the default newline sequence, the pattern 130 .sp 131 (*CR)a.b 132 .sp 133 changes the convention to CR. That pattern matches "a\enb" because LF is no 134 longer a newline. If more than one of these settings is present, the last one 135 is used. 136 .P 137 The newline convention affects where the circumflex and dollar assertions are 138 true. It also affects the interpretation of the dot metacharacter when 139 PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect 140 what the \eR escape sequence matches. By default, this is any Unicode newline 141 sequence, for Perl compatibility. However, this can be changed; see the 142 description of \eR in the section entitled 143 .\" HTML 144 .\" 145 "Newline sequences" 146 .\" 147 below. A change of \eR setting can be combined with a change of newline 148 convention. 149 . 150 . 151 .SS "Setting match and recursion limits" 152 .rs 153 .sp 154 The caller of \fBpcre_exec()\fP can set a limit on the number of times the 155 internal \fBmatch()\fP function is called and on the maximum depth of 156 recursive calls. These facilities are provided to catch runaway matches that 157 are provoked by patterns with huge matching trees (a typical example is a 158 pattern with nested unlimited repeats) and to avoid running out of system stack 159 by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP 160 gives an error return. The limits can also be set by items at the start of the 161 pattern of the form 162 .sp 163 (*LIMIT_MATCH=d) 164 (*LIMIT_RECURSION=d) 165 .sp 166 where d is any number of decimal digits. However, the value of the setting must 167 be less than the value set by the caller of \fBpcre_exec()\fP for it to have 168 any effect. In other words, the pattern writer can lower the limit set by the 169 programmer, but not raise it. If there is more than one setting of one of these 170 limits, the lower value is used. 171 . 172 . 173 .SH "EBCDIC CHARACTER CODES" 174 .rs 175 .sp 176 PCRE can be compiled to run in an environment that uses EBCDIC as its character 177 code rather than ASCII or Unicode (typically a mainframe system). In the 178 sections below, character code values are ASCII or Unicode; in an EBCDIC 179 environment these characters may have different code values, and there are no 180 code points greater than 255. 181 . 182 . 183 .SH "CHARACTERS AND METACHARACTERS" 184 .rs 185 .sp 186 A regular expression is a pattern that is matched against a subject string from 187 left to right. Most characters stand for themselves in a pattern, and match the 188 corresponding characters in the subject. As a trivial example, the pattern 189 .sp 190 The quick brown fox 191 .sp 192 matches a portion of a subject string that is identical to itself. When 193 caseless matching is specified (the PCRE_CASELESS option), letters are matched 194 independently of case. In a UTF mode, PCRE always understands the concept of 195 case for characters whose values are less than 128, so caseless matching is 196 always possible. For characters with higher values, the concept of case is 197 supported if PCRE is compiled with Unicode property support, but not otherwise. 198 If you want to use caseless matching for characters 128 and above, you must 199 ensure that PCRE is compiled with Unicode property support as well as with 200 UTF support. 201 .P 202 The power of regular expressions comes from the ability to include alternatives 203 and repetitions in the pattern. These are encoded in the pattern by the use of 204 \fImetacharacters\fP, which do not stand for themselves but instead are 205 interpreted in some special way. 206 .P 207 There are two different sets of metacharacters: those that are recognized 208 anywhere in the pattern except within square brackets, and those that are 209 recognized within square brackets. Outside square brackets, the metacharacters 210 are as follows: 211 .sp 212 \e general escape character with several uses 213 ^ assert start of string (or line, in multiline mode) 214 $assert end of string (or line, in multiline mode) 215 . match any character except newline (by default) 216 [ start character class definition 217 | start of alternative branch 218 ( start subpattern 219 ) end subpattern 220 ? extends the meaning of ( 221 also 0 or 1 quantifier 222 also quantifier minimizer 223 * 0 or more quantifier 224 + 1 or more quantifier 225 also "possessive quantifier" 226 { start min/max quantifier 227 .sp 228 Part of a pattern that is in square brackets is called a "character class". In 229 a character class the only metacharacters are: 230 .sp 231 \e general escape character 232 ^ negate the class, but only if the first character 233 - indicates character range 234 .\" JOIN 235 [ POSIX character class (only if followed by POSIX 236 syntax) 237 ] terminates the character class 238 .sp 239 The following sections describe the use of each of the metacharacters. 240 . 241 . 242 .SH BACKSLASH 243 .rs 244 .sp 245 The backslash character has several uses. Firstly, if it is followed by a 246 character that is not a number or a letter, it takes away any special meaning 247 that character may have. This use of backslash as an escape character applies 248 both inside and outside character classes. 249 .P 250 For example, if you want to match a * character, you write \e* in the pattern. 251 This escaping action applies whether or not the following character would 252 otherwise be interpreted as a metacharacter, so it is always safe to precede a 253 non-alphanumeric with backslash to specify that it stands for itself. In 254 particular, if you want to match a backslash, you write \e\e. 255 .P 256 In a UTF mode, only ASCII numbers and letters have any special meaning after a 257 backslash. All other characters (in particular, those whose codepoints are 258 greater than 127) are treated as literals. 259 .P 260 If a pattern is compiled with the PCRE_EXTENDED option, white space in the 261 pattern (other than in a character class) and characters between a # outside 262 a character class and the next newline are ignored. An escaping backslash can 263 be used to include a white space or # character as part of the pattern. 264 .P 265 If you want to remove the special meaning from a sequence of characters, you 266 can do so by putting them between \eQ and \eE. This is different from Perl in 267 that$ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in 268 Perl, $and @ cause variable interpolation. Note the following examples: 269 .sp 270 Pattern PCRE matches Perl matches 271 .sp 272 .\" JOIN 273 \eQabc$xyz\eE abc$xyz abc followed by the 274 contents of$xyz 275 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz 276 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz 277 .sp 278 The \eQ...\eE sequence is recognized both inside and outside character classes. 279 An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed 280 by \eE later in the pattern, the literal interpretation continues to the end of 281 the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside 282 a character class, this causes an error, because the character class is not 283 terminated. 284 . 285 . 286 .\" HTML 287 .SS "Non-printing characters" 288 .rs 289 .sp 290 A second use of backslash provides a way of encoding non-printing characters 291 in patterns in a visible manner. There is no restriction on the appearance of 292 non-printing characters, apart from the binary zero that terminates a pattern, 293 but when a pattern is being prepared by text editing, it is often easier to use 294 one of the following escape sequences than the binary character it represents: 295 .sp 296 \ea alarm, that is, the BEL character (hex 07) 297 \ecx "control-x", where x is any ASCII character 298 \ee escape (hex 1B) 299 \ef form feed (hex 0C) 300 \en linefeed (hex 0A) 301 \er carriage return (hex 0D) 302 \et tab (hex 09) 303 \eddd character with octal code ddd, or back reference 304 \exhh character with hex code hh 305 \ex{hhh..} character with hex code hhh.. (non-JavaScript mode) 306 \euhhhh character with hex code hhhh (JavaScript mode only) 307 .sp 308 The precise effect of \ecx on ASCII characters is as follows: if x is a lower 309 case letter, it is converted to upper case. Then bit 6 of the character (hex 310 40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A), 311 but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the 312 data item (byte or 16-bit value) following \ec has a value greater than 127, a 313 compile-time error occurs. This locks out non-ASCII characters in all modes. 314 .P 315 The \ec facility was designed for use with ASCII characters, but with the 316 extension to Unicode it is even less useful than it once was. It is, however, 317 recognized when PCRE is compiled in EBCDIC mode, where data items are always 318 bytes. In this mode, all values are valid after \ec. If the next character is a 319 lower case letter, it is converted to upper case. Then the 0xc0 bits of the 320 byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because 321 the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other 322 characters also generate different values. 323 .P 324 By default, after \ex, from zero to two hexadecimal digits are read (letters 325 can be in upper or lower case). Any number of hexadecimal digits may appear 326 between \ex{ and }, but the character code is constrained as follows: 327 .sp 328 8-bit non-UTF mode less than 0x100 329 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint 330 16-bit non-UTF mode less than 0x10000 331 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint 332 32-bit non-UTF mode less than 0x80000000 333 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint 334 .sp 335 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called 336 "surrogate" codepoints), and 0xffef. 337 .P 338 If characters other than hexadecimal digits appear between \ex{ and }, or if 339 there is no terminating }, this form of escape is not recognized. Instead, the 340 initial \ex will be interpreted as a basic hexadecimal escape, with no 341 following digits, giving a character whose value is zero. 342 .P 343 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is 344 as just described only when it is followed by two hexadecimal digits. 345 Otherwise, it matches a literal "x" character. In JavaScript mode, support for 346 code points greater than 256 is provided by \eu, which must be followed by 347 four hexadecimal digits; otherwise it matches a literal "u" character. 348 Character codes specified by \eu in JavaScript mode are constrained in the same 349 was as those specified by \ex in non-JavaScript mode. 350 .P 351 Characters whose value is less than 256 can be defined by either of the two 352 syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the 353 way they are handled. For example, \exdc is exactly the same as \ex{dc} (or 354 \eu00dc in JavaScript mode). 355 .P 356 After \e0 up to two further octal digits are read. If there are fewer than two 357 digits, just those that are present are used. Thus the sequence \e0\ex\e07 358 specifies two binary zeros followed by a BEL character (code value 7). Make 359 sure you supply two digits after the initial zero if the pattern character that 360 follows is itself an octal digit. 361 .P 362 The handling of a backslash followed by a digit other than 0 is complicated. 363 Outside a character class, PCRE reads it and any following digits as a decimal 364 number. If the number is less than 10, or if there have been at least that many 365 previous capturing left parentheses in the expression, the entire sequence is 366 taken as a \fIback reference\fP. A description of how this works is given 367 .\" HTML 368 .\" 369 later, 370 .\" 371 following the discussion of 372 .\" HTML 373 .\" 374 parenthesized subpatterns. 375 .\" 376 .P 377 Inside a character class, or if the decimal number is greater than 9 and there 378 have not been that many capturing subpatterns, PCRE re-reads up to three octal 379 digits following the backslash, and uses them to generate a data character. Any 380 subsequent digits stand for themselves. The value of the character is 381 constrained in the same way as characters specified in hexadecimal. 382 For example: 383 .sp 384 \e040 is another way of writing an ASCII space 385 .\" JOIN 386 \e40 is the same, provided there are fewer than 40 387 previous capturing subpatterns 388 \e7 is always a back reference 389 .\" JOIN 390 \e11 might be a back reference, or another way of 391 writing a tab 392 \e011 is always a tab 393 \e0113 is a tab followed by the character "3" 394 .\" JOIN 395 \e113 might be a back reference, otherwise the 396 character with octal code 113 397 .\" JOIN 398 \e377 might be a back reference, otherwise 399 the value 255 (decimal) 400 .\" JOIN 401 \e81 is either a back reference, or a binary zero 402 followed by the two characters "8" and "1" 403 .sp 404 Note that octal values of 100 or greater must not be introduced by a leading 405 zero, because no more than three octal digits are ever read. 406 .P 407 All the sequences that define a single character value can be used both inside 408 and outside character classes. In addition, inside a character class, \eb is 409 interpreted as the backspace character (hex 08). 410 .P 411 \eN is not allowed in a character class. \eB, \eR, and \eX are not special 412 inside a character class. Like other unrecognized escape sequences, they are 413 treated as the literal characters "B", "R", and "X" by default, but cause an 414 error if the PCRE_EXTRA option is set. Outside a character class, these 415 sequences have different meanings. 416 . 417 . 418 .SS "Unsupported escape sequences" 419 .rs 420 .sp 421 In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string 422 handler and used to modify the case of following characters. By default, PCRE 423 does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT 424 option is set, \eU matches a "U" character, and \eu can be used to define a 425 character by code point, as described in the previous section. 426 . 427 . 428 .SS "Absolute and relative back references" 429 .rs 430 .sp 431 The sequence \eg followed by an unsigned or a negative number, optionally 432 enclosed in braces, is an absolute or relative back reference. A named back 433 reference can be coded as \eg{name}. Back references are discussed 434 .\" HTML 435 .\" 436 later, 437 .\" 438 following the discussion of 439 .\" HTML 440 .\" 441 parenthesized subpatterns. 442 .\" 443 . 444 . 445 .SS "Absolute and relative subroutine calls" 446 .rs 447 .sp 448 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or 449 a number enclosed either in angle brackets or single quotes, is an alternative 450 syntax for referencing a subpattern as a "subroutine". Details are discussed 451 .\" HTML 452 .\" 453 later. 454 .\" 455 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP 456 synonymous. The former is a back reference; the latter is a 457 .\" HTML 458 .\" 459 subroutine 460 .\" 461 call. 462 . 463 . 464 .\" HTML 465 .SS "Generic character types" 466 .rs 467 .sp 468 Another use of backslash is for specifying generic character types: 469 .sp 470 \ed any decimal digit 471 \eD any character that is not a decimal digit 472 \eh any horizontal white space character 473 \eH any character that is not a horizontal white space character 474 \es any white space character 475 \eS any character that is not a white space character 476 \ev any vertical white space character 477 \eV any character that is not a vertical white space character 478 \ew any "word" character 479 \eW any "non-word" character 480 .sp 481 There is also the single sequence \eN, which matches a non-newline character. 482 This is the same as 483 .\" HTML 484 .\" 485 the "." metacharacter 486 .\" 487 when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name; 488 PCRE does not support this. 489 .P 490 Each pair of lower and upper case escape sequences partitions the complete set 491 of characters into two disjoint sets. Any given character matches one, and only 492 one, of each pair. The sequences can appear both inside and outside character 493 classes. They each match one character of the appropriate type. If the current 494 matching point is at the end of the subject string, all of them fail, because 495 there is no character to match. 496 .P 497 For compatibility with Perl, \es does not match the VT character (code 11). 498 This makes it different from the the POSIX "space" class. The \es characters 499 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is 500 included in a Perl script, \es may match the VT character. In PCRE, it never 501 does. 502 .P 503 A "word" character is an underscore or any character that is a letter or digit. 504 By default, the definition of letters and digits is controlled by PCRE's 505 low-valued character tables, and may vary if locale-specific matching is taking 506 place (see 507 .\" HTML 508 .\" 509 "Locale support" 510 .\" 511 in the 512 .\" HREF 513 \fBpcreapi\fP 514 .\" 515 page). For example, in a French locale such as "fr_FR" in Unix-like systems, 516 or "french" in Windows, some character codes greater than 128 are used for 517 accented letters, and these are then matched by \ew. The use of locales with 518 Unicode is discouraged. 519 .P 520 By default, in a UTF mode, characters with values greater than 128 never match 521 \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain 522 their original meanings from before UTF support was available, mainly for 523 efficiency reasons. However, if PCRE is compiled with Unicode property support, 524 and the PCRE_UCP option is set, the behaviour is changed so that Unicode 525 properties are used to determine character types, as follows: 526 .sp 527 \ed any character that \ep{Nd} matches (decimal digit) 528 \es any character that \ep{Z} matches, plus HT, LF, FF, CR 529 \ew any character that \ep{L} or \ep{N} matches, plus underscore 530 .sp 531 The upper case escapes match the inverse sets of characters. Note that \ed 532 matches only decimal digits, whereas \ew matches any Unicode digit, as well as 533 any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and 534 \eB because they are defined in terms of \ew and \eW. Matching these sequences 535 is noticeably slower when PCRE_UCP is set. 536 .P 537 The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at 538 release 5.10. In contrast to the other sequences, which match only ASCII 539 characters by default, these always match certain high-valued codepoints, 540 whether or not PCRE_UCP is set. The horizontal space characters are: 541 .sp 542 U+0009 Horizontal tab (HT) 543 U+0020 Space 544 U+00A0 Non-break space 545 U+1680 Ogham space mark 546 U+180E Mongolian vowel separator 547 U+2000 En quad 548 U+2001 Em quad 549 U+2002 En space 550 U+2003 Em space 551 U+2004 Three-per-em space 552 U+2005 Four-per-em space 553 U+2006 Six-per-em space 554 U+2007 Figure space 555 U+2008 Punctuation space 556 U+2009 Thin space 557 U+200A Hair space 558 U+202F Narrow no-break space 559 U+205F Medium mathematical space 560 U+3000 Ideographic space 561 .sp 562 The vertical space characters are: 563 .sp 564 U+000A Linefeed (LF) 565 U+000B Vertical tab (VT) 566 U+000C Form feed (FF) 567 U+000D Carriage return (CR) 568 U+0085 Next line (NEL) 569 U+2028 Line separator 570 U+2029 Paragraph separator 571 .sp 572 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are 573 relevant. 574 . 575 . 576 .\" HTML 577 .SS "Newline sequences" 578 .rs 579 .sp 580 Outside a character class, by default, the escape sequence \eR matches any 581 Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the 582 following: 583 .sp 584 (?>\er\en|\en|\ex0b|\ef|\er|\ex85) 585 .sp 586 This is an example of an "atomic group", details of which are given 587 .\" HTML 588 .\" 589 below. 590 .\" 591 This particular group matches either the two-character sequence CR followed by 592 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, 593 U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next 594 line, U+0085). The two-character sequence is treated as a single unit that 595 cannot be split. 596 .P 597 In other modes, two additional characters whose codepoints are greater than 255 598 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). 599 Unicode character property support is not needed for these characters to be 600 recognized. 601 .P 602 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the 603 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF 604 either at compile time or when the pattern is matched. (BSR is an abbrevation 605 for "backslash R".) This can be made the default when PCRE is built; if this is 606 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option. 607 It is also possible to specify these settings by starting a pattern string with 608 one of the following sequences: 609 .sp 610 (*BSR_ANYCRLF) CR, LF, or CRLF only 611 (*BSR_UNICODE) any Unicode newline sequence 612 .sp 613 These override the default and the options given to the compiling function, but 614 they can themselves be overridden by options given to a matching function. Note 615 that these special settings, which are not Perl-compatible, are recognized only 616 at the very start of a pattern, and that they must be in upper case. If more 617 than one of them is present, the last one is used. They can be combined with a 618 change of newline convention; for example, a pattern can start with: 619 .sp 620 (*ANY)(*BSR_ANYCRLF) 621 .sp 622 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or 623 (*UCP) special sequences. Inside a character class, \eR is treated as an 624 unrecognized escape sequence, and so matches the letter "R" by default, but 625 causes an error if PCRE_EXTRA is set. 626 . 627 . 628 .\" HTML 629 .SS Unicode character properties 630 .rs 631 .sp 632 When PCRE is built with Unicode character property support, three additional 633 escape sequences that match characters with specific properties are available. 634 When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing 635 characters whose codepoints are less than 256, but they do work in this mode. 636 The extra escape sequences are: 637 .sp 638 \ep{\fIxx\fP} a character with the \fIxx\fP property 639 \eP{\fIxx\fP} a character without the \fIxx\fP property 640 \eX a Unicode extended grapheme cluster 641 .sp 642 The property names represented by \fIxx\fP above are limited to the Unicode 643 script names, the general category properties, "Any", which matches any 644 character (including newline), and some special PCRE properties (described 645 in the 646 .\" HTML 647 .\" 648 next section). 649 .\" 650 Other Perl properties such as "InMusicalSymbols" are not currently supported by 651 PCRE. Note that \eP{Any} does not match any characters, so always causes a 652 match failure. 653 .P 654 Sets of Unicode characters are defined as belonging to certain scripts. A 655 character from one of these sets can be matched using a script name. For 656 example: 657 .sp 658 \ep{Greek} 659 \eP{Han} 660 .sp 661 Those that are not part of an identified script are lumped together as 662 "Common". The current list of scripts is: 663 .P 664 Arabic, 665 Armenian, 666 Avestan, 667 Balinese, 668 Bamum, 669 Batak, 670 Bengali, 671 Bopomofo, 672 Brahmi, 673 Braille, 674 Buginese, 675 Buhid, 676 Canadian_Aboriginal, 677 Carian, 678 Chakma, 679 Cham, 680 Cherokee, 681 Common, 682 Coptic, 683 Cuneiform, 684 Cypriot, 685 Cyrillic, 686 Deseret, 687 Devanagari, 688 Egyptian_Hieroglyphs, 689 Ethiopic, 690 Georgian, 691 Glagolitic, 692 Gothic, 693 Greek, 694 Gujarati, 695 Gurmukhi, 696 Han, 697 Hangul, 698 Hanunoo, 699 Hebrew, 700 Hiragana, 701 Imperial_Aramaic, 702 Inherited, 703 Inscriptional_Pahlavi, 704 Inscriptional_Parthian, 705 Javanese, 706 Kaithi, 707 Kannada, 708 Katakana, 709 Kayah_Li, 710 Kharoshthi, 711 Khmer, 712 Lao, 713 Latin, 714 Lepcha, 715 Limbu, 716 Linear_B, 717 Lisu, 718 Lycian, 719 Lydian, 720 Malayalam, 721 Mandaic, 722 Meetei_Mayek, 723 Meroitic_Cursive, 724 Meroitic_Hieroglyphs, 725 Miao, 726 Mongolian, 727 Myanmar, 728 New_Tai_Lue, 729 Nko, 730 Ogham, 731 Old_Italic, 732 Old_Persian, 733 Old_South_Arabian, 734 Old_Turkic, 735 Ol_Chiki, 736 Oriya, 737 Osmanya, 738 Phags_Pa, 739 Phoenician, 740 Rejang, 741 Runic, 742 Samaritan, 743 Saurashtra, 744 Sharada, 745 Shavian, 746 Sinhala, 747 Sora_Sompeng, 748 Sundanese, 749 Syloti_Nagri, 750 Syriac, 751 Tagalog, 752 Tagbanwa, 753 Tai_Le, 754 Tai_Tham, 755 Tai_Viet, 756 Takri, 757 Tamil, 758 Telugu, 759 Thaana, 760 Thai, 761 Tibetan, 762 Tifinagh, 763 Ugaritic, 764 Vai, 765 Yi. 766 .P 767 Each character has exactly one Unicode general category property, specified by 768 a two-letter abbreviation. For compatibility with Perl, negation can be 769 specified by including a circumflex between the opening brace and the property 770 name. For example, \ep{^Lu} is the same as \eP{Lu}. 771 .P 772 If only one letter is specified with \ep or \eP, it includes all the general 773 category properties that start with that letter. In this case, in the absence 774 of negation, the curly brackets in the escape sequence are optional; these two 775 examples have the same effect: 776 .sp 777 \ep{L} 778 \epL 779 .sp 780 The following general category property codes are supported: 781 .sp 782 C Other 783 Cc Control 784 Cf Format 785 Cn Unassigned 786 Co Private use 787 Cs Surrogate 788 .sp 789 L Letter 790 Ll Lower case letter 791 Lm Modifier letter 792 Lo Other letter 793 Lt Title case letter 794 Lu Upper case letter 795 .sp 796 M Mark 797 Mc Spacing mark 798 Me Enclosing mark 799 Mn Non-spacing mark 800 .sp 801 N Number 802 Nd Decimal number 803 Nl Letter number 804 No Other number 805 .sp 806 P Punctuation 807 Pc Connector punctuation 808 Pd Dash punctuation 809 Pe Close punctuation 810 Pf Final punctuation 811 Pi Initial punctuation 812 Po Other punctuation 813 Ps Open punctuation 814 .sp 815 S Symbol 816 Sc Currency symbol 817 Sk Modifier symbol 818 Sm Mathematical symbol 819 So Other symbol 820 .sp 821 Z Separator 822 Zl Line separator 823 Zp Paragraph separator 824 Zs Space separator 825 .sp 826 The special property L& is also supported: it matches a character that has 827 the Lu, Ll, or Lt property, in other words, a letter that is not classified as 828 a modifier or "other". 829 .P 830 The Cs (Surrogate) property applies only to characters in the range U+D800 to 831 U+DFFF. Such characters are not valid in Unicode strings and so 832 cannot be tested by PCRE, unless UTF validity checking has been turned off 833 (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and 834 PCRE_NO_UTF32_CHECK in the 835 .\" HREF 836 \fBpcreapi\fP 837 .\" 838 page). Perl does not support the Cs property. 839 .P 840 The long synonyms for property names that Perl supports (such as \ep{Letter}) 841 are not supported by PCRE, nor is it permitted to prefix any of these 842 properties with "Is". 843 .P 844 No character that is in the Unicode table has the Cn (unassigned) property. 845 Instead, this property is assumed for any code point that is not in the 846 Unicode table. 847 .P 848 Specifying caseless matching does not affect these escape sequences. For 849 example, \ep{Lu} always matches only upper case letters. This is different from 850 the behaviour of current versions of Perl. 851 .P 852 Matching characters by Unicode property is not fast, because PCRE has to do a 853 multistage table lookup in order to find a character's property. That is why 854 the traditional escape sequences such as \ed and \ew do not use Unicode 855 properties in PCRE by default, though you can make them do so by setting the 856 PCRE_UCP option or by starting the pattern with (*UCP). 857 . 858 . 859 .SS Extended grapheme clusters 860 .rs 861 .sp 862 The \eX escape matches any number of Unicode characters that form an "extended 863 grapheme cluster", and treats the sequence as an atomic group 864 .\" HTML 865 .\" 866 (see below). 867 .\" 868 Up to and including release 8.31, PCRE matched an earlier, simpler definition 869 that was equivalent to 870 .sp 871 (?>\ePM\epM*) 872 .sp 873 That is, it matched a character without the "mark" property, followed by zero 874 or more characters with the "mark" property. Characters with the "mark" 875 property are typically non-spacing accents that affect the preceding character. 876 .P 877 This simple definition was extended in Unicode to include more complicated 878 kinds of composite character by giving each character a grapheme breaking 879 property, and creating rules that use these properties to define the boundaries 880 of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches 881 one of these clusters. 882 .P 883 \eX always matches at least one character. Then it decides whether to add 884 additional characters according to the following rules for ending a cluster: 885 .P 886 1. End at the end of the subject string. 887 .P 888 2. Do not end between CR and LF; otherwise end after any control character. 889 .P 890 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters 891 are of five types: L, V, T, LV, and LVT. An L character may be followed by an 892 L, V, LV, or LVT character; an LV or V character may be followed by a V or T 893 character; an LVT or T character may be follwed only by a T character. 894 .P 895 4. Do not end before extending characters or spacing marks. Characters with 896 the "mark" property always have the "extend" grapheme breaking property. 897 .P 898 5. Do not end after prepend characters. 899 .P 900 6. Otherwise, end the cluster. 901 . 902 . 903 .\" HTML 904 .SS PCRE's additional properties 905 .rs 906 .sp 907 As well as the standard Unicode properties described above, PCRE supports four 908 more that make it possible to convert traditional escape sequences such as \ew 909 and \es and POSIX character classes to use Unicode properties. PCRE uses these 910 non-standard, non-Perl properties internally when PCRE_UCP is set. However, 911 they may also be used explicitly. These properties are: 912 .sp 913 Xan Any alphanumeric character 914 Xps Any POSIX space character 915 Xsp Any Perl space character 916 Xwd Any Perl "word" character 917 .sp 918 Xan matches characters that have either the L (letter) or the N (number) 919 property. Xps matches the characters tab, linefeed, vertical tab, form feed, or 920 carriage return, and any other character that has the Z (separator) property. 921 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the 922 same characters as Xan, plus underscore. 923 .P 924 There is another non-standard property, Xuc, which matches any character that 925 can be represented by a Universal Character Name in C++ and other programming 926 languages. These are the characters $, @,  (grave accent), and all characters 927 with Unicode code points greater than or equal to U+00A0, except for the 928 surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are 929 excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH 930 where H is a hexadecimal digit. Note that the Xuc property does not match these 931 sequences but the characters that they represent.) 932 . 933 . 934 .\" HTML 935 .SS "Resetting the match start" 936 .rs 937 .sp 938 The escape sequence \eK causes any previously matched characters not to be 939 included in the final matched sequence. For example, the pattern: 940 .sp 941 foo\eKbar 942 .sp 943 matches "foobar", but reports that it has matched "bar". This feature is 944 similar to a lookbehind assertion 945 .\" HTML 946 .\" 947 (described below). 948 .\" 949 However, in this case, the part of the subject before the real match does not 950 have to be of fixed length, as lookbehind assertions do. The use of \eK does 951 not interfere with the setting of 952 .\" HTML 953 .\" 954 captured substrings. 955 .\" 956 For example, when the pattern 957 .sp 958 (foo)\eKbar 959 .sp 960 matches "foobar", the first substring is still set to "foo". 961 .P 962 Perl documents that the use of \eK within assertions is "not well defined". In 963 PCRE, \eK is acted upon when it occurs inside positive assertions, but is 964 ignored in negative assertions. 965 . 966 . 967 .\" HTML 968 .SS "Simple assertions" 969 .rs 970 .sp 971 The final use of backslash is for certain simple assertions. An assertion 972 specifies a condition that has to be met at a particular point in a match, 973 without consuming any characters from the subject string. The use of 974 subpatterns for more complicated assertions is described 975 .\" HTML 976 .\" 977 below. 978 .\" 979 The backslashed assertions are: 980 .sp 981 \eb matches at a word boundary 982 \eB matches when not at a word boundary 983 \eA matches at the start of the subject 984 \eZ matches at the end of the subject 985 also matches before a newline at the end of the subject 986 \ez matches only at the end of the subject 987 \eG matches at the first matching position in the subject 988 .sp 989 Inside a character class, \eb has a different meaning; it matches the backspace 990 character. If any other of these assertions appears in a character class, by 991 default it matches the corresponding literal character (for example, \eB 992 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid 993 escape sequence" error is generated instead. 994 .P 995 A word boundary is a position in the subject string where the current character 996 and the previous character do not both match \ew or \eW (i.e. one matches 997 \ew and the other matches \eW), or the start or end of the string if the 998 first or last character matches \ew, respectively. In a UTF mode, the meanings 999 of \ew and \eW can be changed by setting the PCRE_UCP option. When this is 1000 done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start 1001 of word" or "end of word" metasequence. However, whatever follows \eb normally 1002 determines which it is. For example, the fragment \eba matches "a" at the start 1003 of a word. 1004 .P 1005 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and 1006 dollar (described in the next section) in that they only ever match at the very 1007 start and end of the subject string, whatever options are set. Thus, they are 1008 independent of multiline mode. These three assertions are not affected by the 1009 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the 1010 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP 1011 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start 1012 at a point other than the beginning of the subject, \eA can never match. The 1013 difference between \eZ and \ez is that \eZ matches before a newline at the end 1014 of the string as well as at the very end, whereas \ez matches only at the end. 1015 .P 1016 The \eG assertion is true only when the current matching position is at the 1017 start point of the match, as specified by the \fIstartoffset\fP argument of 1018 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is 1019 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate 1020 arguments, you can mimic Perl's /g option, and it is in this kind of 1021 implementation where \eG can be useful. 1022 .P 1023 Note, however, that PCRE's interpretation of \eG, as the start of the current 1024 match, is subtly different from Perl's, which defines it as the end of the 1025 previous match. In Perl, these can be different when the previously matched 1026 string was empty. Because PCRE does just one match at a time, it cannot 1027 reproduce this behaviour. 1028 .P 1029 If all the alternatives of a pattern begin with \eG, the expression is anchored 1030 to the starting match position, and the "anchored" flag is set in the compiled 1031 regular expression. 1032 . 1033 . 1034 .SH "CIRCUMFLEX AND DOLLAR" 1035 .rs 1036 .sp 1037 The circumflex and dollar metacharacters are zero-width assertions. That is, 1038 they test for a particular condition being true without consuming any 1039 characters from the subject string. 1040 .P 1041 Outside a character class, in the default matching mode, the circumflex 1042 character is an assertion that is true only if the current matching point is at 1043 the start of the subject string. If the \fIstartoffset\fP argument of 1044 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE 1045 option is unset. Inside a character class, circumflex has an entirely different 1046 meaning 1047 .\" HTML 1048 .\" 1049 (see below). 1050 .\" 1051 .P 1052 Circumflex need not be the first character of the pattern if a number of 1053 alternatives are involved, but it should be the first thing in each alternative 1054 in which it appears if the pattern is ever to match that branch. If all 1055 possible alternatives start with a circumflex, that is, if the pattern is 1056 constrained to match only at the start of the subject, it is said to be an 1057 "anchored" pattern. (There are also other constructs that can cause a pattern 1058 to be anchored.) 1059 .P 1060 The dollar character is an assertion that is true only if the current matching 1061 point is at the end of the subject string, or immediately before a newline at 1062 the end of the string (by default). Note, however, that it does not actually 1063 match the newline. Dollar need not be the last character of the pattern if a 1064 number of alternatives are involved, but it should be the last item in any 1065 branch in which it appears. Dollar has no special meaning in a character class. 1066 .P 1067 The meaning of dollar can be changed so that it matches only at the very end of 1068 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This 1069 does not affect the \eZ assertion. 1070 .P 1071 The meanings of the circumflex and dollar characters are changed if the 1072 PCRE_MULTILINE option is set. When this is the case, a circumflex matches 1073 immediately after internal newlines as well as at the start of the subject 1074 string. It does not match after a newline that ends the string. A dollar 1075 matches before any newlines in the string, as well as at the very end, when 1076 PCRE_MULTILINE is set. When newline is specified as the two-character 1077 sequence CRLF, isolated CR and LF characters do not indicate newlines. 1078 .P 1079 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where 1080 \en represents a newline) in multiline mode, but not otherwise. Consequently, 1081 patterns that are anchored in single line mode because all branches start with 1082 ^ are not anchored in multiline mode, and a match for circumflex is possible 1083 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The 1084 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. 1085 .P 1086 Note that the sequences \eA, \eZ, and \ez can be used to match the start and 1087 end of the subject in both modes, and if all branches of a pattern start with 1088 \eA it is always anchored, whether or not PCRE_MULTILINE is set. 1089 . 1090 . 1091 .\" HTML 1092 .SH "FULL STOP (PERIOD, DOT) AND \eN" 1093 .rs 1094 .sp 1095 Outside a character class, a dot in the pattern matches any one character in 1096 the subject string except (by default) a character that signifies the end of a 1097 line. 1098 .P 1099 When a line ending is defined as a single character, dot never matches that 1100 character; when the two-character sequence CRLF is used, dot does not match CR 1101 if it is immediately followed by LF, but otherwise it matches all characters 1102 (including isolated CRs and LFs). When any Unicode line endings are being 1103 recognized, dot does not match CR or LF or any of the other line ending 1104 characters. 1105 .P 1106 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL 1107 option is set, a dot matches any one character, without exception. If the 1108 two-character sequence CRLF is present in the subject string, it takes two dots 1109 to match it. 1110 .P 1111 The handling of dot is entirely independent of the handling of circumflex and 1112 dollar, the only relationship being that they both involve newlines. Dot has no 1113 special meaning in a character class. 1114 .P 1115 The escape sequence \eN behaves like a dot, except that it is not affected by 1116 the PCRE_DOTALL option. In other words, it matches any character except one 1117 that signifies the end of a line. Perl also uses \eN to match characters by 1118 name; PCRE does not support this. 1119 . 1120 . 1121 .SH "MATCHING A SINGLE DATA UNIT" 1122 .rs 1123 .sp 1124 Outside a character class, the escape sequence \eC matches any one data unit, 1125 whether or not a UTF mode is set. In the 8-bit library, one data unit is one 1126 byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is 1127 a 32-bit unit. Unlike a dot, \eC always 1128 matches line-ending characters. The feature is provided in Perl in order to 1129 match individual bytes in UTF-8 mode, but it is unclear how it can usefully be 1130 used. Because \eC breaks up characters into individual data units, matching one 1131 unit with \eC in a UTF mode means that the rest of the string may start with a 1132 malformed UTF character. This has undefined results, because PCRE assumes that 1133 it is dealing with valid UTF strings (and by default it checks this at the 1134 start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or 1135 PCRE_NO_UTF32_CHECK option is used). 1136 .P 1137 PCRE does not allow \eC to appear in lookbehind assertions 1138 .\" HTML 1139 .\" 1140 (described below) 1141 .\" 1142 in a UTF mode, because this would make it impossible to calculate the length of 1143 the lookbehind. 1144 .P 1145 In general, the \eC escape sequence is best avoided. However, one 1146 way of using it that avoids the problem of malformed UTF characters is to use a 1147 lookahead to check the length of the next character, as in this pattern, which 1148 could be used with a UTF-8 string (ignore white space and line breaks): 1149 .sp 1150 (?| (?=[\ex00-\ex7f])(\eC) | 1151 (?=[\ex80-\ex{7ff}])(\eC)(\eC) | 1152 (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) | 1153 (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC)) 1154 .sp 1155 A group that starts with (?| resets the capturing parentheses numbers in each 1156 alternative (see 1157 .\" HTML 1158 .\" 1159 "Duplicate Subpattern Numbers" 1160 .\" 1161 below). The assertions at the start of each branch check the next UTF-8 1162 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The 1163 character's individual bytes are then captured by the appropriate number of 1164 groups. 1165 . 1166 . 1167 .\" HTML 1168 .SH "SQUARE BRACKETS AND CHARACTER CLASSES" 1169 .rs 1170 .sp 1171 An opening square bracket introduces a character class, terminated by a closing 1172 square bracket. A closing square bracket on its own is not special by default. 1173 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square 1174 bracket causes a compile-time error. If a closing square bracket is required as 1175 a member of the class, it should be the first data character in the class 1176 (after an initial circumflex, if present) or escaped with a backslash. 1177 .P 1178 A character class matches a single character in the subject. In a UTF mode, the 1179 character may be more than one data unit long. A matched character must be in 1180 the set of characters defined by the class, unless the first character in the 1181 class definition is a circumflex, in which case the subject character must not 1182 be in the set defined by the class. If a circumflex is actually required as a 1183 member of the class, ensure it is not the first character, or escape it with a 1184 backslash. 1185 .P 1186 For example, the character class [aeiou] matches any lower case vowel, while 1187 [^aeiou] matches any character that is not a lower case vowel. Note that a 1188 circumflex is just a convenient notation for specifying the characters that 1189 are in the class by enumerating those that are not. A class that starts with a 1190 circumflex is not an assertion; it still consumes a character from the subject 1191 string, and therefore it fails if the current pointer is at the end of the 1192 string. 1193 .P 1194 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff) 1195 can be included in a class as a literal string of data units, or by using the 1196 \ex{ escaping mechanism. 1197 .P 1198 When caseless matching is set, any letters in a class represent both their 1199 upper case and lower case versions, so for example, a caseless [aeiou] matches 1200 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a 1201 caseful version would. In a UTF mode, PCRE always understands the concept of 1202 case for characters whose values are less than 128, so caseless matching is 1203 always possible. For characters with higher values, the concept of case is 1204 supported if PCRE is compiled with Unicode property support, but not otherwise. 1205 If you want to use caseless matching in a UTF mode for characters 128 and 1206 above, you must ensure that PCRE is compiled with Unicode property support as 1207 well as with UTF support. 1208 .P 1209 Characters that might indicate line breaks are never treated in any special way 1210 when matching character classes, whatever line-ending sequence is in use, and 1211 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class 1212 such as [^a] always matches one of these characters. 1213 .P 1214 The minus (hyphen) character can be used to specify a range of characters in a 1215 character class. For example, [d-m] matches any letter between d and m, 1216 inclusive. If a minus character is required in a class, it must be escaped with 1217 a backslash or appear in a position where it cannot be interpreted as 1218 indicating a range, typically as the first or last character in the class. 1219 .P 1220 It is not possible to have the literal character "]" as the end character of a 1221 range. A pattern such as [W-]46] is interpreted as a class of two characters 1222 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or 1223 "-46]". However, if the "]" is escaped with a backslash it is interpreted as 1224 the end of range, so [W-\e]46] is interpreted as a class containing a range 1225 followed by two other characters. The octal or hexadecimal representation of 1226 "]" can also be used to end a range. 1227 .P 1228 Ranges operate in the collating sequence of character values. They can also be 1229 used for characters specified numerically, for example [\e000-\e037]. Ranges 1230 can include any characters that are valid for the current mode. 1231 .P 1232 If a range that includes letters is used when caseless matching is set, it 1233 matches the letters in either case. For example, [W-c] is equivalent to 1234 [][\e\e^_wxyzabc], matched caselessly, and in a non-UTF mode, if character 1235 tables for a French locale are in use, [\exc8-\excb] matches accented E 1236 characters in both cases. In UTF modes, PCRE supports the concept of case for 1237 characters with values greater than 128 only when it is compiled with Unicode 1238 property support. 1239 .P 1240 The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, 1241 \eV, \ew, and \eW may appear in a character class, and add the characters that 1242 they match to the class. For example, [\edABCDEF] matches any hexadecimal 1243 digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew 1244 and their upper case partners, just as it does when they appear outside a 1245 character class, as described in the section entitled 1246 .\" HTML 1247 .\" 1248 "Generic character types" 1249 .\" 1250 above. The escape sequence \eb has a different meaning inside a character 1251 class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX 1252 are not special inside a character class. Like any other unrecognized escape 1253 sequences, they are treated as the literal characters "B", "N", "R", and "X" by 1254 default, but cause an error if the PCRE_EXTRA option is set. 1255 .P 1256 A circumflex can conveniently be used with the upper case character types to 1257 specify a more restricted set of characters than the matching lower case type. 1258 For example, the class [^\eW_] matches any letter or digit, but not underscore, 1259 whereas [\ew] includes underscore. A positive character class should be read as 1260 "something OR something OR ..." and a negative class as "NOT something AND NOT 1261 something AND NOT ...". 1262 .P 1263 The only metacharacters that are recognized in character classes are backslash, 1264 hyphen (only where it can be interpreted as specifying a range), circumflex 1265 (only at the start), opening square bracket (only when it can be interpreted as 1266 introducing a POSIX class name - see the next section), and the terminating 1267 closing square bracket. However, escaping other non-alphanumeric characters 1268 does no harm. 1269 . 1270 . 1271 .SH "POSIX CHARACTER CLASSES" 1272 .rs 1273 .sp 1274 Perl supports the POSIX notation for character classes. This uses names 1275 enclosed by [: and :] within the enclosing square brackets. PCRE also supports 1276 this notation. For example, 1277 .sp 1278 [01[:alpha:]%] 1279 .sp 1280 matches "0", "1", any alphabetic character, or "%". The supported class names 1281 are: 1282 .sp 1283 alnum letters and digits 1284 alpha letters 1285 ascii character codes 0 - 127 1286 blank space or tab only 1287 cntrl control characters 1288 digit decimal digits (same as \ed) 1289 graph printing characters, excluding space 1290 lower lower case letters 1291 print printing characters, including space 1292 punct printing characters, excluding letters and digits and space 1293 space white space (not quite the same as \es) 1294 upper upper case letters 1295 word "word" characters (same as \ew) 1296 xdigit hexadecimal digits 1297 .sp 1298 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and 1299 space (32). Notice that this list includes the VT character (code 11). This 1300 makes "space" different to \es, which does not include VT (for Perl 1301 compatibility). 1302 .P 1303 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 1304 5.8. Another Perl extension is negation, which is indicated by a ^ character 1305 after the colon. For example, 1306 .sp 1307 [12[:^digit:]] 1308 .sp 1309 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX 1310 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not 1311 supported, and an error is given if they are encountered. 1312 .P 1313 By default, in UTF modes, characters with values greater than 128 do not match 1314 any of the POSIX character classes. However, if the PCRE_UCP option is passed 1315 to \fBpcre_compile()\fP, some of the classes are changed so that Unicode 1316 character properties are used. This is achieved by replacing the POSIX classes 1317 by other sequences, as follows: 1318 .sp 1319 [:alnum:] becomes \ep{Xan} 1320 [:alpha:] becomes \ep{L} 1321 [:blank:] becomes \eh 1322 [:digit:] becomes \ep{Nd} 1323 [:lower:] becomes \ep{Ll} 1324 [:space:] becomes \ep{Xps} 1325 [:upper:] becomes \ep{Lu} 1326 [:word:] becomes \ep{Xwd} 1327 .sp 1328 Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX 1329 classes are unchanged, and match only characters with code points less than 1330 128. 1331 . 1332 . 1333 .SH "VERTICAL BAR" 1334 .rs 1335 .sp 1336 Vertical bar characters are used to separate alternative patterns. For example, 1337 the pattern 1338 .sp 1339 gilbert|sullivan 1340 .sp 1341 matches either "gilbert" or "sullivan". Any number of alternatives may appear, 1342 and an empty alternative is permitted (matching the empty string). The matching 1343 process tries each alternative in turn, from left to right, and the first one 1344 that succeeds is used. If the alternatives are within a subpattern 1345 .\" HTML 1346 .\" 1347 (defined below), 1348 .\" 1349 "succeeds" means matching the rest of the main pattern as well as the 1350 alternative in the subpattern. 1351 . 1352 . 1353 .SH "INTERNAL OPTION SETTING" 1354 .rs 1355 .sp 1356 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and 1357 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within 1358 the pattern by a sequence of Perl option letters enclosed between "(?" and ")". 1359 The option letters are 1360 .sp 1361 i for PCRE_CASELESS 1362 m for PCRE_MULTILINE 1363 s for PCRE_DOTALL 1364 x for PCRE_EXTENDED 1365 .sp 1366 For example, (?im) sets caseless, multiline matching. It is also possible to 1367 unset these options by preceding the letter with a hyphen, and a combined 1368 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and 1369 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also 1370 permitted. If a letter appears both before and after the hyphen, the option is 1371 unset. 1372 .P 1373 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be 1374 changed in the same way as the Perl-compatible options by using the characters 1375 J, U and X respectively. 1376 .P 1377 When one of these option changes occurs at top level (that is, not inside 1378 subpattern parentheses), the change applies to the remainder of the pattern 1379 that follows. If the change is placed right at the start of a pattern, PCRE 1380 extracts it into the global options (and it will therefore show up in data 1381 extracted by the \fBpcre_fullinfo()\fP function). 1382 .P 1383 An option change within a subpattern (see below for a description of 1384 subpatterns) affects only that part of the subpattern that follows it, so 1385 .sp 1386 (a(?i)b)c 1387 .sp 1388 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). 1389 By this means, options can be made to have different settings in different 1390 parts of the pattern. Any changes made in one alternative do carry on 1391 into subsequent branches within the same subpattern. For example, 1392 .sp 1393 (a(?i)b|c) 1394 .sp 1395 matches "ab", "aB", "c", and "C", even though when matching "C" the first 1396 branch is abandoned before the option setting. This is because the effects of 1397 option settings happen at compile time. There would be some very weird 1398 behaviour otherwise. 1399 .P 1400 \fBNote:\fP There are other PCRE-specific options that can be set by the 1401 application when the compiling or matching functions are called. In some cases 1402 the pattern can contain special leading sequences such as (*CRLF) to override 1403 what the application has set or what has been defaulted. Details are given in 1404 the section entitled 1405 .\" HTML 1406 .\" 1407 "Newline sequences" 1408 .\" 1409 above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading 1410 sequences that can be used to set UTF and Unicode property modes; they are 1411 equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP 1412 options, respectively. The (*UTF) sequence is a generic version that can be 1413 used with any of the libraries. However, the application can set the 1414 PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences. 1415 . 1416 . 1417 .\" HTML 1418 .SH SUBPATTERNS 1419 .rs 1420 .sp 1421 Subpatterns are delimited by parentheses (round brackets), which can be nested. 1422 Turning part of a pattern into a subpattern does two things: 1423 .sp 1424 1. It localizes a set of alternatives. For example, the pattern 1425 .sp 1426 cat(aract|erpillar|) 1427 .sp 1428 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would 1429 match "cataract", "erpillar" or an empty string. 1430 .sp 1431 2. It sets up the subpattern as a capturing subpattern. This means that, when 1432 the whole pattern matches, that portion of the subject string that matched the 1433 subpattern is passed back to the caller via the \fIovector\fP argument of the 1434 matching function. (This applies only to the traditional matching functions; 1435 the DFA matching functions do not support capturing.) 1436 .P 1437 Opening parentheses are counted from left to right (starting from 1) to obtain 1438 numbers for the capturing subpatterns. For example, if the string "the red 1439 king" is matched against the pattern 1440 .sp 1441 the ((red|white) (king|queen)) 1442 .sp 1443 the captured substrings are "red king", "red", and "king", and are numbered 1, 1444 2, and 3, respectively. 1445 .P 1446 The fact that plain parentheses fulfil two functions is not always helpful. 1447 There are often times when a grouping subpattern is required without a 1448 capturing requirement. If an opening parenthesis is followed by a question mark 1449 and a colon, the subpattern does not do any capturing, and is not counted when 1450 computing the number of any subsequent capturing subpatterns. For example, if 1451 the string "the white queen" is matched against the pattern 1452 .sp 1453 the ((?:red|white) (king|queen)) 1454 .sp 1455 the captured substrings are "white queen" and "queen", and are numbered 1 and 1456 2. The maximum number of capturing subpatterns is 65535. 1457 .P 1458 As a convenient shorthand, if any option settings are required at the start of 1459 a non-capturing subpattern, the option letters may appear between the "?" and 1460 the ":". Thus the two patterns 1461 .sp 1462 (?i:saturday|sunday) 1463 (?:(?i)saturday|sunday) 1464 .sp 1465 match exactly the same set of strings. Because alternative branches are tried 1466 from left to right, and options are not reset until the end of the subpattern 1467 is reached, an option setting in one branch does affect subsequent branches, so 1468 the above patterns match "SUNDAY" as well as "Saturday". 1469 . 1470 . 1471 .\" HTML 1472 .SH "DUPLICATE SUBPATTERN NUMBERS" 1473 .rs 1474 .sp 1475 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses 1476 the same numbers for its capturing parentheses. Such a subpattern starts with 1477 (?| and is itself a non-capturing subpattern. For example, consider this 1478 pattern: 1479 .sp 1480 (?|(Sat)ur|(Sun))day 1481 .sp 1482 Because the two alternatives are inside a (?| group, both sets of capturing 1483 parentheses are numbered one. Thus, when the pattern matches, you can look 1484 at captured substring number one, whichever alternative matched. This construct 1485 is useful when you want to capture part, but not all, of one of a number of 1486 alternatives. Inside a (?| group, parentheses are numbered as usual, but the 1487 number is reset at the start of each branch. The numbers of any capturing 1488 parentheses that follow the subpattern start after the highest number used in 1489 any branch. The following example is taken from the Perl documentation. The 1490 numbers underneath show in which buffer the captured content will be stored. 1491 .sp 1492 # before ---------------branch-reset----------- after 1493 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x 1494 # 1 2 2 3 2 3 4 1495 .sp 1496 A back reference to a numbered subpattern uses the most recent value that is 1497 set for that number by any subpattern. The following pattern matches "abcabc" 1498 or "defdef": 1499 .sp 1500 /(?|(abc)|(def))\e1/ 1501 .sp 1502 In contrast, a subroutine call to a numbered subpattern always refers to the 1503 first one in the pattern with the given number. The following pattern matches 1504 "abcabc" or "defabc": 1505 .sp 1506 /(?|(abc)|(def))(?1)/ 1507 .sp 1508 If a 1509 .\" HTML 1510 .\" 1511 condition test 1512 .\" 1513 for a subpattern's having matched refers to a non-unique number, the test is 1514 true if any of the subpatterns of that number have matched. 1515 .P 1516 An alternative approach to using this "branch reset" feature is to use 1517 duplicate named subpatterns, as described in the next section. 1518 . 1519 . 1520 .SH "NAMED SUBPATTERNS" 1521 .rs 1522 .sp 1523 Identifying capturing parentheses by number is simple, but it can be very hard 1524 to keep track of the numbers in complicated regular expressions. Furthermore, 1525 if an expression is modified, the numbers may change. To help with this 1526 difficulty, PCRE supports the naming of subpatterns. This feature was not 1527 added to Perl until release 5.10. Python had the feature earlier, and PCRE 1528 introduced it at release 4.0, using the Python syntax. PCRE now supports both 1529 the Perl and the Python syntax. Perl allows identically numbered subpatterns to 1530 have different names, but PCRE does not. 1531 .P 1532 In PCRE, a subpattern can be named in one of three ways: (?...) or 1533 (?'name'...) as in Perl, or (?P...) as in Python. References to capturing 1534 parentheses from other parts of the pattern, such as 1535 .\" HTML 1536 .\" 1537 back references, 1538 .\" 1539 .\" HTML 1540 .\" 1541 recursion, 1542 .\" 1543 and 1544 .\" HTML 1545 .\" 1546 conditions, 1547 .\" 1548 can be made by name as well as by number. 1549 .P 1550 Names consist of up to 32 alphanumeric characters and underscores. Named 1551 capturing parentheses are still allocated numbers as well as names, exactly as 1552 if the names were not present. The PCRE API provides function calls for 1553 extracting the name-to-number translation table from a compiled pattern. There 1554 is also a convenience function for extracting a captured substring by name. 1555 .P 1556 By default, a name must be unique within a pattern, but it is possible to relax 1557 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate 1558 names are also always permitted for subpatterns with the same number, set up as 1559 described in the previous section.) Duplicate names can be useful for patterns 1560 where only one instance of the named parentheses can match. Suppose you want to 1561 match the name of a weekday, either as a 3-letter abbreviation or as the full 1562 name, and in both cases you want to extract the abbreviation. This pattern 1563 (ignoring the line breaks) does the job: 1564 .sp 1565 (?Mon|Fri|Sun)(?:day)?| 1566 (?Tue)(?:sday)?| 1567 (?Wed)(?:nesday)?| 1568 (?Thu)(?:rsday)?| 1569 (?Sat)(?:urday)? 1570 .sp 1571 There are five capturing substrings, but only one is ever set after a match. 1572 (An alternative way of solving this problem is to use a "branch reset" 1573 subpattern, as described in the previous section.) 1574 .P 1575 The convenience function for extracting the data by name returns the substring 1576 for the first (and in this example, the only) subpattern of that name that 1577 matched. This saves searching to find which numbered subpattern it was. 1578 .P 1579 If you make a back reference to a non-unique named subpattern from elsewhere in 1580 the pattern, the one that corresponds to the first occurrence of the name is 1581 used. In the absence of duplicate numbers (see the previous section) this is 1582 the one with the lowest number. If you use a named reference in a condition 1583 test (see the 1584 .\" 1585 .\" HTML 1586 .\" 1587 section about conditions 1588 .\" 1589 below), either to check whether a subpattern has matched, or to check for 1590 recursion, all subpatterns with the same name are tested. If the condition is 1591 true for any one of them, the overall condition is true. This is the same 1592 behaviour as testing by number. For further details of the interfaces for 1593 handling named subpatterns, see the 1594 .\" HREF 1595 \fBpcreapi\fP 1596 .\" 1597 documentation. 1598 .P 1599 \fBWarning:\fP You cannot use different names to distinguish between two 1600 subpatterns with the same number because PCRE uses only the numbers when 1601 matching. For this reason, an error is given at compile time if different names 1602 are given to subpatterns with the same number. However, you can give the same 1603 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set. 1604 . 1605 . 1606 .SH REPETITION 1607 .rs 1608 .sp 1609 Repetition is specified by quantifiers, which can follow any of the following 1610 items: 1611 .sp 1612 a literal data character 1613 the dot metacharacter 1614 the \eC escape sequence 1615 the \eX escape sequence 1616 the \eR escape sequence 1617 an escape such as \ed or \epL that matches a single character 1618 a character class 1619 a back reference (see next section) 1620 a parenthesized subpattern (including assertions) 1621 a subroutine call to a subpattern (recursive or otherwise) 1622 .sp 1623 The general repetition quantifier specifies a minimum and maximum number of 1624 permitted matches, by giving the two numbers in curly brackets (braces), 1625 separated by a comma. The numbers must be less than 65536, and the first must 1626 be less than or equal to the second. For example: 1627 .sp 1628 z{2,4} 1629 .sp 1630 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special 1631 character. If the second number is omitted, but the comma is present, there is 1632 no upper limit; if the second number and the comma are both omitted, the 1633 quantifier specifies an exact number of required matches. Thus 1634 .sp 1635 [aeiou]{3,} 1636 .sp 1637 matches at least 3 successive vowels, but may match many more, while 1638 .sp 1639 \ed{8} 1640 .sp 1641 matches exactly 8 digits. An opening curly bracket that appears in a position 1642 where a quantifier is not allowed, or one that does not match the syntax of a 1643 quantifier, is taken as a literal character. For example, {,6} is not a 1644 quantifier, but a literal string of four characters. 1645 .P 1646 In UTF modes, quantifiers apply to characters rather than to individual data 1647 units. Thus, for example, \ex{100}{2} matches two characters, each of 1648 which is represented by a two-byte sequence in a UTF-8 string. Similarly, 1649 \eX{3} matches three Unicode extended grapheme clusters, each of which may be 1650 several data units long (and they may be of different lengths). 1651 .P 1652 The quantifier {0} is permitted, causing the expression to behave as if the 1653 previous item and the quantifier were not present. This may be useful for 1654 subpatterns that are referenced as 1655 .\" HTML 1656 .\" 1657 subroutines 1658 .\" 1659 from elsewhere in the pattern (but see also the section entitled 1660 .\" HTML 1661 .\" 1662 "Defining subpatterns for use by reference only" 1663 .\" 1664 below). Items other than subpatterns that have a {0} quantifier are omitted 1665 from the compiled pattern. 1666 .P 1667 For convenience, the three most common quantifiers have single-character 1668 abbreviations: 1669 .sp 1670 * is equivalent to {0,} 1671 + is equivalent to {1,} 1672 ? is equivalent to {0,1} 1673 .sp 1674 It is possible to construct infinite loops by following a subpattern that can 1675 match no characters with a quantifier that has no upper limit, for example: 1676 .sp 1677 (a?)* 1678 .sp 1679 Earlier versions of Perl and PCRE used to give an error at compile time for 1680 such patterns. However, because there are cases where this can be useful, such 1681 patterns are now accepted, but if any repetition of the subpattern does in fact 1682 match no characters, the loop is forcibly broken. 1683 .P 1684 By default, the quantifiers are "greedy", that is, they match as much as 1685 possible (up to the maximum number of permitted times), without causing the 1686 rest of the pattern to fail. The classic example of where this gives problems 1687 is in trying to match comments in C programs. These appear between /* and */ 1688 and within the comment, individual * and / characters may appear. An attempt to 1689 match C comments by applying the pattern 1690 .sp 1691 /\e*.*\e*/ 1692 .sp 1693 to the string 1694 .sp 1695 /* first comment */ not comment /* second comment */ 1696 .sp 1697 fails, because it matches the entire string owing to the greediness of the .* 1698 item. 1699 .P 1700 However, if a quantifier is followed by a question mark, it ceases to be 1701 greedy, and instead matches the minimum number of times possible, so the 1702 pattern 1703 .sp 1704 /\e*.*?\e*/ 1705 .sp 1706 does the right thing with the C comments. The meaning of the various 1707 quantifiers is not otherwise changed, just the preferred number of matches. 1708 Do not confuse this use of question mark with its use as a quantifier in its 1709 own right. Because it has two uses, it can sometimes appear doubled, as in 1710 .sp 1711 \ed??\ed 1712 .sp 1713 which matches one digit by preference, but can match two if that is the only 1714 way the rest of the pattern matches. 1715 .P 1716 If the PCRE_UNGREEDY option is set (an option that is not available in Perl), 1717 the quantifiers are not greedy by default, but individual ones can be made 1718 greedy by following them with a question mark. In other words, it inverts the 1719 default behaviour. 1720 .P 1721 When a parenthesized subpattern is quantified with a minimum repeat count that 1722 is greater than 1 or with a limited maximum, more memory is required for the 1723 compiled pattern, in proportion to the size of the minimum or maximum. 1724 .P 1725 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent 1726 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is 1727 implicitly anchored, because whatever follows will be tried against every 1728 character position in the subject string, so there is no point in retrying the 1729 overall match at any position after the first. PCRE normally treats such a 1730 pattern as though it were preceded by \eA. 1731 .P 1732 In cases where it is known that the subject string contains no newlines, it is 1733 worth setting PCRE_DOTALL in order to obtain this optimization, or 1734 alternatively using ^ to indicate anchoring explicitly. 1735 .P 1736 However, there are some cases where the optimization cannot be used. When .* 1737 is inside capturing parentheses that are the subject of a back reference 1738 elsewhere in the pattern, a match at the start may fail where a later one 1739 succeeds. Consider, for example: 1740 .sp 1741 (.*)abc\e1 1742 .sp 1743 If the subject is "xyz123abc123" the match point is the fourth character. For 1744 this reason, such a pattern is not implicitly anchored. 1745 .P 1746 Another case where implicit anchoring is not applied is when the leading .* is 1747 inside an atomic group. Once again, a match at the start may fail where a later 1748 one succeeds. Consider this pattern: 1749 .sp 1750 (?>.*?a)b 1751 .sp 1752 It matches "ab" in the subject "aab". The use of the backtracking control verbs 1753 (*PRUNE) and (*SKIP) also disable this optimization. 1754 .P 1755 When a capturing subpattern is repeated, the value captured is the substring 1756 that matched the final iteration. For example, after 1757 .sp 1758 (tweedle[dume]{3}\es*)+ 1759 .sp 1760 has matched "tweedledum tweedledee" the value of the captured substring is 1761 "tweedledee". However, if there are nested capturing subpatterns, the 1762 corresponding captured values may have been set in previous iterations. For 1763 example, after 1764 .sp 1765 /(a|(b))+/ 1766 .sp 1767 matches "aba" the value of the second captured substring is "b". 1768 . 1769 . 1770 .\" HTML 1771 .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS" 1772 .rs 1773 .sp 1774 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 1775 repetition, failure of what follows normally causes the repeated item to be 1776 re-evaluated to see if a different number of repeats allows the rest of the 1777 pattern to match. Sometimes it is useful to prevent this, either to change the 1778 nature of the match, or to cause it fail earlier than it otherwise might, when 1779 the author of the pattern knows there is no point in carrying on. 1780 .P 1781 Consider, for example, the pattern \ed+foo when applied to the subject line 1782 .sp 1783 123456bar 1784 .sp 1785 After matching all 6 digits and then failing to match "foo", the normal 1786 action of the matcher is to try again with only 5 digits matching the \ed+ 1787 item, and then with 4, and so on, before ultimately failing. "Atomic grouping" 1788 (a term taken from Jeffrey Friedl's book) provides the means for specifying 1789 that once a subpattern has matched, it is not to be re-evaluated in this way. 1790 .P 1791 If we use atomic grouping for the previous example, the matcher gives up 1792 immediately on failing to match "foo" the first time. The notation is a kind of 1793 special parenthesis, starting with (?> as in this example: 1794 .sp 1795 (?>\ed+)foo 1796 .sp 1797 This kind of parenthesis "locks up" the part of the pattern it contains once 1798 it has matched, and a failure further into the pattern is prevented from 1799 backtracking into it. Backtracking past it to previous items, however, works as 1800 normal. 1801 .P 1802 An alternative description is that a subpattern of this type matches the string 1803 of characters that an identical standalone pattern would match, if anchored at 1804 the current point in the subject string. 1805 .P 1806 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as 1807 the above example can be thought of as a maximizing repeat that must swallow 1808 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the 1809 number of digits they match in order to make the rest of the pattern match, 1810 (?>\ed+) can only match an entire sequence of digits. 1811 .P 1812 Atomic groups in general can of course contain arbitrarily complicated 1813 subpatterns, and can be nested. However, when the subpattern for an atomic 1814 group is just a single repeated item, as in the example above, a simpler 1815 notation, called a "possessive quantifier" can be used. This consists of an 1816 additional + character following a quantifier. Using this notation, the 1817 previous example can be rewritten as 1818 .sp 1819 \ed++foo 1820 .sp 1821 Note that a possessive quantifier can be used with an entire group, for 1822 example: 1823 .sp 1824 (abc|xyz){2,3}+ 1825 .sp 1826 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY 1827 option is ignored. They are a convenient notation for the simpler forms of 1828 atomic group. However, there is no difference in the meaning of a possessive 1829 quantifier and the equivalent atomic group, though there may be a performance 1830 difference; possessive quantifiers should be slightly faster. 1831 .P 1832 The possessive quantifier syntax is an extension to the Perl 5.8 syntax. 1833 Jeffrey Friedl originated the idea (and the name) in the first edition of his 1834 book. Mike McCloskey liked it, so implemented it when he built Sun's Java 1835 package, and PCRE copied it from there. It ultimately found its way into Perl 1836 at release 5.10. 1837 .P 1838 PCRE has an optimization that automatically "possessifies" certain simple 1839 pattern constructs. For example, the sequence A+B is treated as A++B because 1840 there is no point in backtracking into a sequence of A's when B must follow. 1841 .P 1842 When a pattern contains an unlimited repeat inside a subpattern that can itself 1843 be repeated an unlimited number of times, the use of an atomic group is the 1844 only way to avoid some failing matches taking a very long time indeed. The 1845 pattern 1846 .sp 1847 (\eD+|<\ed+>)*[!?] 1848 .sp 1849 matches an unlimited number of substrings that either consist of non-digits, or 1850 digits enclosed in <>, followed by either ! or ?. When it matches, it runs 1851 quickly. However, if it is applied to 1852 .sp 1853 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 1854 .sp 1855 it takes a long time before reporting failure. This is because the string can 1856 be divided between the internal \eD+ repeat and the external * repeat in a 1857 large number of ways, and all have to be tried. (The example uses [!?] rather 1858 than a single character at the end, because both PCRE and Perl have an 1859 optimization that allows for fast failure when a single character is used. They 1860 remember the last single character that is required for a match, and fail early 1861 if it is not present in the string.) If the pattern is changed so that it uses 1862 an atomic group, like this: 1863 .sp 1864 ((?>\eD+)|<\ed+>)*[!?] 1865 .sp 1866 sequences of non-digits cannot be broken, and failure happens quickly. 1867 . 1868 . 1869 .\" HTML 1870 .SH "BACK REFERENCES" 1871 .rs 1872 .sp 1873 Outside a character class, a backslash followed by a digit greater than 0 (and 1874 possibly further digits) is a back reference to a capturing subpattern earlier 1875 (that is, to its left) in the pattern, provided there have been that many 1876 previous capturing left parentheses. 1877 .P 1878 However, if the decimal number following the backslash is less than 10, it is 1879 always taken as a back reference, and causes an error only if there are not 1880 that many capturing left parentheses in the entire pattern. In other words, the 1881 parentheses that are referenced need not be to the left of the reference for 1882 numbers less than 10. A "forward back reference" of this type can make sense 1883 when a repetition is involved and the subpattern to the right has participated 1884 in an earlier iteration. 1885 .P 1886 It is not possible to have a numerical "forward back reference" to a subpattern 1887 whose number is 10 or more using this syntax because a sequence such as \e50 is 1888 interpreted as a character defined in octal. See the subsection entitled 1889 "Non-printing characters" 1890 .\" HTML 1891 .\" 1892 above 1893 .\" 1894 for further details of the handling of digits following a backslash. There is 1895 no such problem when named parentheses are used. A back reference to any 1896 subpattern is possible using named parentheses (see below). 1897 .P 1898 Another way of avoiding the ambiguity inherent in the use of digits following a 1899 backslash is to use the \eg escape sequence. This escape must be followed by an 1900 unsigned number or a negative number, optionally enclosed in braces. These 1901 examples are all identical: 1902 .sp 1903 (ring), \e1 1904 (ring), \eg1 1905 (ring), \eg{1} 1906 .sp 1907 An unsigned number specifies an absolute reference without the ambiguity that 1908 is present in the older syntax. It is also useful when literal digits follow 1909 the reference. A negative number is a relative reference. Consider this 1910 example: 1911 .sp 1912 (abc(def)ghi)\eg{-1} 1913 .sp 1914 The sequence \eg{-1} is a reference to the most recently started capturing 1915 subpattern before \eg, that is, is it equivalent to \e2 in this example. 1916 Similarly, \eg{-2} would be equivalent to \e1. The use of relative references 1917 can be helpful in long patterns, and also in patterns that are created by 1918 joining together fragments that contain references within themselves. 1919 .P 1920 A back reference matches whatever actually matched the capturing subpattern in 1921 the current subject string, rather than anything matching the subpattern 1922 itself (see 1923 .\" HTML 1924 .\" 1925 "Subpatterns as subroutines" 1926 .\" 1927 below for a way of doing that). So the pattern 1928 .sp 1929 (sens|respons)e and \e1ibility 1930 .sp 1931 matches "sense and sensibility" and "response and responsibility", but not 1932 "sense and responsibility". If caseful matching is in force at the time of the 1933 back reference, the case of letters is relevant. For example, 1934 .sp 1935 ((?i)rah)\es+\e1 1936 .sp 1937 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original 1938 capturing subpattern is matched caselessly. 1939 .P 1940 There are several different ways of writing back references to named 1941 subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek or 1942 \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified 1943 back reference syntax, in which \eg can be used for both numeric and named 1944 references, is also supported. We could rewrite the above example in any of 1945 the following ways: 1946 .sp 1947 (?(?i)rah)\es+\ek 1948 (?'p1'(?i)rah)\es+\ek{p1} 1949 (?P(?i)rah)\es+(?P=p1) 1950 (?(?i)rah)\es+\eg{p1} 1951 .sp 1952 A subpattern that is referenced by name may appear in the pattern before or 1953 after the reference. 1954 .P 1955 There may be more than one back reference to the same subpattern. If a 1956 subpattern has not actually been used in a particular match, any back 1957 references to it always fail by default. For example, the pattern 1958 .sp 1959 (a|(bc))\e2 1960 .sp 1961 always fails if it starts to match "a" rather than "bc". However, if the 1962 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an 1963 unset value matches an empty string. 1964 .P 1965 Because there may be many capturing parentheses in a pattern, all digits 1966 following a backslash are taken as part of a potential back reference number. 1967 If the pattern continues with a digit character, some delimiter must be used to 1968 terminate the back reference. If the PCRE_EXTENDED option is set, this can be 1969 white space. Otherwise, the \eg{ syntax or an empty comment (see 1970 .\" HTML 1971 .\" 1972 "Comments" 1973 .\" 1974 below) can be used. 1975 . 1976 .SS "Recursive back references" 1977 .rs 1978 .sp 1979 A back reference that occurs inside the parentheses to which it refers fails 1980 when the subpattern is first used, so, for example, (a\e1) never matches. 1981 However, such references can be useful inside repeated subpatterns. For 1982 example, the pattern 1983 .sp 1984 (a|b\e1)+ 1985 .sp 1986 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of 1987 the subpattern, the back reference matches the character string corresponding 1988 to the previous iteration. In order for this to work, the pattern must be such 1989 that the first iteration does not need to match the back reference. This can be 1990 done using alternation, as in the example above, or by a quantifier with a 1991 minimum of zero. 1992 .P 1993 Back references of this type cause the group that they reference to be treated 1994 as an 1995 .\" HTML 1996 .\" 1997 atomic group. 1998 .\" 1999 Once the whole group has been matched, a subsequent matching failure cannot 2000 cause backtracking into the middle of the group. 2001 . 2002 . 2003 .\" HTML 2004 .SH ASSERTIONS 2005 .rs 2006 .sp 2007 An assertion is a test on the characters following or preceding the current 2008 matching point that does not actually consume any characters. The simple 2009 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $are described 2010 .\" HTML 2011 .\" 2012 above. 2013 .\" 2014 .P 2015 More complicated assertions are coded as subpatterns. There are two kinds: 2016 those that look ahead of the current position in the subject string, and those 2017 that look behind it. An assertion subpattern is matched in the normal way, 2018 except that it does not cause the current matching position to be changed. 2019 .P 2020 Assertion subpatterns are not capturing subpatterns. If such an assertion 2021 contains capturing subpatterns within it, these are counted for the purposes of 2022 numbering the capturing subpatterns in the whole pattern. However, substring 2023 capturing is carried out only for positive assertions. (Perl sometimes, but not 2024 always, does do capturing in negative assertions.) 2025 .P 2026 For compatibility with Perl, assertion subpatterns may be repeated; though 2027 it makes no sense to assert the same thing several times, the side effect of 2028 capturing parentheses may occasionally be useful. In practice, there only three 2029 cases: 2030 .sp 2031 (1) If the quantifier is {0}, the assertion is never obeyed during matching. 2032 However, it may contain internal capturing parenthesized groups that are called 2033 from elsewhere via the 2034 .\" HTML 2035 .\" 2036 subroutine mechanism. 2037 .\" 2038 .sp 2039 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it 2040 were {0,1}. At run time, the rest of the pattern match is tried with and 2041 without the assertion, the order depending on the greediness of the quantifier. 2042 .sp 2043 (3) If the minimum repetition is greater than zero, the quantifier is ignored. 2044 The assertion is obeyed just once when encountered during matching. 2045 . 2046 . 2047 .SS "Lookahead assertions" 2048 .rs 2049 .sp 2050 Lookahead assertions start with (?= for positive assertions and (?! for 2051 negative assertions. For example, 2052 .sp 2053 \ew+(?=;) 2054 .sp 2055 matches a word followed by a semicolon, but does not include the semicolon in 2056 the match, and 2057 .sp 2058 foo(?!bar) 2059 .sp 2060 matches any occurrence of "foo" that is not followed by "bar". Note that the 2061 apparently similar pattern 2062 .sp 2063 (?!foo)bar 2064 .sp 2065 does not find an occurrence of "bar" that is preceded by something other than 2066 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion 2067 (?!foo) is always true when the next three characters are "bar". A 2068 lookbehind assertion is needed to achieve the other effect. 2069 .P 2070 If you want to force a matching failure at some point in a pattern, the most 2071 convenient way to do it is with (?!) because an empty string always matches, so 2072 an assertion that requires there not to be an empty string must always fail. 2073 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!). 2074 . 2075 . 2076 .\" HTML 2077 .SS "Lookbehind assertions" 2078 .rs 2079 .sp 2080 Lookbehind assertions start with (?<= for positive assertions and (? 2111 .\" 2112 (see above) 2113 .\" 2114 can be used instead of a lookbehind assertion to get round the fixed-length 2115 restriction. 2116 .P 2117 The implementation of lookbehind assertions is, for each alternative, to 2118 temporarily move the current position back by the fixed length and then try to 2119 match. If there are insufficient characters before the current position, the 2120 assertion fails. 2121 .P 2122 In a UTF mode, PCRE does not allow the \eC escape (which matches a single data 2123 unit even in a UTF mode) to appear in lookbehind assertions, because it makes 2124 it impossible to calculate the length of the lookbehind. The \eX and \eR 2125 escapes, which can match different numbers of data units, are also not 2126 permitted. 2127 .P 2128 .\" HTML 2129 .\" 2130 "Subroutine" 2131 .\" 2132 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long 2133 as the subpattern matches a fixed-length string. 2134 .\" HTML 2135 .\" 2136 Recursion, 2137 .\" 2138 however, is not supported. 2139 .P 2140 Possessive quantifiers can be used in conjunction with lookbehind assertions to 2141 specify efficient matching of fixed-length strings at the end of subject 2142 strings. Consider a simple pattern such as 2143 .sp 2144 abcd$ 2145 .sp 2146 when applied to a long string that does not match. Because matching proceeds 2147 from left to right, PCRE will look for each "a" in the subject and then see if 2148 what follows matches the rest of the pattern. If the pattern is specified as 2149 .sp 2150 ^.*abcd$2151 .sp 2152 the initial .* matches the entire string at first, but when this fails (because 2153 there is no following "a"), it backtracks to match all but the last character, 2154 then all but the last two characters, and so on. Once again the search for "a" 2155 covers the entire string, from right to left, so we are no better off. However, 2156 if the pattern is written as 2157 .sp 2158 ^.*+(?<=abcd) 2159 .sp 2160 there can be no backtracking for the .*+ item; it can match only the entire 2161 string. The subsequent lookbehind assertion does a single test on the last four 2162 characters. If it fails, the match fails immediately. For long strings, this 2163 approach makes a significant difference to the processing time. 2164 . 2165 . 2166 .SS "Using multiple assertions" 2167 .rs 2168 .sp 2169 Several assertions (of any sort) may occur in succession. For example, 2170 .sp 2171 (?<=\ed{3})(? 2201 .SH "CONDITIONAL SUBPATTERNS" 2202 .rs 2203 .sp 2204 It is possible to cause the matching process to obey a subpattern 2205 conditionally or to choose between two alternative subpatterns, depending on 2206 the result of an assertion, or whether a specific capturing subpattern has 2207 already been matched. The two possible forms of conditional subpattern are: 2208 .sp 2209 (?(condition)yes-pattern) 2210 (?(condition)yes-pattern|no-pattern) 2211 .sp 2212 If the condition is satisfied, the yes-pattern is used; otherwise the 2213 no-pattern (if present) is used. If there are more than two alternatives in the 2214 subpattern, a compile-time error occurs. Each of the two alternatives may 2215 itself contain nested subpatterns of any form, including conditional 2216 subpatterns; the restriction to two alternatives applies only at the level of 2217 the condition. This pattern fragment is an example where the alternatives are 2218 complex: 2219 .sp 2220 (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) 2221 .sp 2222 .P 2223 There are four kinds of condition: references to subpatterns, references to 2224 recursion, a pseudo-condition called DEFINE, and assertions. 2225 . 2226 .SS "Checking for a used subpattern by number" 2227 .rs 2228 .sp 2229 If the text between the parentheses consists of a sequence of digits, the 2230 condition is true if a capturing subpattern of that number has previously 2231 matched. If there is more than one capturing subpattern with the same number 2232 (see the earlier 2233 .\" 2234 .\" HTML 2235 .\" 2236 section about duplicate subpattern numbers), 2237 .\" 2238 the condition is true if any of them have matched. An alternative notation is 2239 to precede the digits with a plus or minus sign. In this case, the subpattern 2240 number is relative rather than absolute. The most recently opened parentheses 2241 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside 2242 loops it can also make sense to refer to subsequent groups. The next 2243 parentheses to be opened can be referenced as (?(+1), and so on. (The value 2244 zero in any of these forms is not used; it provokes a compile-time error.) 2245 .P 2246 Consider the following pattern, which contains non-significant white space to 2247 make it more readable (assume the PCRE_EXTENDED option) and to divide it into 2248 three parts for ease of discussion: 2249 .sp 2250 ( \e( )? [^()]+ (?(1) \e) ) 2251 .sp 2252 The first part matches an optional opening parenthesis, and if that 2253 character is present, sets it as the first captured substring. The second part 2254 matches one or more characters that are not parentheses. The third part is a 2255 conditional subpattern that tests whether or not the first set of parentheses 2256 matched. If they did, that is, if subject started with an opening parenthesis, 2257 the condition is true, and so the yes-pattern is executed and a closing 2258 parenthesis is required. Otherwise, since no-pattern is not present, the 2259 subpattern matches nothing. In other words, this pattern matches a sequence of 2260 non-parentheses, optionally enclosed in parentheses. 2261 .P 2262 If you were embedding this pattern in a larger one, you could use a relative 2263 reference: 2264 .sp 2265 ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ... 2266 .sp 2267 This makes the fragment independent of the parentheses in the larger pattern. 2268 . 2269 .SS "Checking for a used subpattern by name" 2270 .rs 2271 .sp 2272 Perl uses the syntax (?()...) or (?('name')...) to test for a used 2273 subpattern by name. For compatibility with earlier versions of PCRE, which had 2274 this facility before Perl, the syntax (?(name)...) is also recognized. However, 2275 there is a possible ambiguity with this syntax, because subpattern names may 2276 consist entirely of digits. PCRE looks first for a named subpattern; if it 2277 cannot find one and the name consists entirely of digits, PCRE looks for a 2278 subpattern of that number, which must be greater than zero. Using subpattern 2279 names that consist entirely of digits is not recommended. 2280 .P 2281 Rewriting the above example to use a named subpattern gives this: 2282 .sp 2283 (? \e( )? [^()]+ (?() \e) ) 2284 .sp 2285 If the name used in a condition of this kind is a duplicate, the test is 2286 applied to all subpatterns of the same name, and is true if any one of them has 2287 matched. 2288 . 2289 .SS "Checking for pattern recursion" 2290 .rs 2291 .sp 2292 If the condition is the string (R), and there is no subpattern with the name R, 2293 the condition is true if a recursive call to the whole pattern or any 2294 subpattern has been made. If digits or a name preceded by ampersand follow the 2295 letter R, for example: 2296 .sp 2297 (?(R3)...) or (?(R&name)...) 2298 .sp 2299 the condition is true if the most recent recursion is into a subpattern whose 2300 number or name is given. This condition does not check the entire recursion 2301 stack. If the name used in a condition of this kind is a duplicate, the test is 2302 applied to all subpatterns of the same name, and is true if any one of them is 2303 the most recent recursion. 2304 .P 2305 At "top level", all these recursion test conditions are false. 2306 .\" HTML 2307 .\" 2308 The syntax for recursive patterns 2309 .\" 2310 is described below. 2311 . 2312 .\" HTML 2313 .SS "Defining subpatterns for use by reference only" 2314 .rs 2315 .sp 2316 If the condition is the string (DEFINE), and there is no subpattern with the 2317 name DEFINE, the condition is always false. In this case, there may be only one 2318 alternative in the subpattern. It is always skipped if control reaches this 2319 point in the pattern; the idea of DEFINE is that it can be used to define 2320 subroutines that can be referenced from elsewhere. (The use of 2321 .\" HTML 2322 .\" 2323 subroutines 2324 .\" 2325 is described below.) For example, a pattern to match an IPv4 address such as 2326 "192.168.23.245" could be written like this (ignore white space and line 2327 breaks): 2328 .sp 2329 (?(DEFINE) (? 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) ) 2330 \eb (?&byte) (\e.(?&byte)){3} \eb 2331 .sp 2332 The first part of the pattern is a DEFINE group inside which a another group 2333 named "byte" is defined. This matches an individual component of an IPv4 2334 address (a number less than 256). When matching takes place, this part of the 2335 pattern is skipped because DEFINE acts like a false condition. The rest of the 2336 pattern uses references to the named group to match the four dot-separated 2337 components of an IPv4 address, insisting on a word boundary at each end. 2338 . 2339 .SS "Assertion conditions" 2340 .rs 2341 .sp 2342 If the condition is not in any of the above formats, it must be an assertion. 2343 This may be a positive or negative lookahead or lookbehind assertion. Consider 2344 this pattern, again containing non-significant white space, and with the two 2345 alternatives on the second line: 2346 .sp 2347 (?(?=[^a-z]*[a-z]) 2348 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} ) 2349 .sp 2350 The condition is a positive lookahead assertion that matches an optional 2351 sequence of non-letters followed by a letter. In other words, it tests for the 2352 presence of at least one letter in the subject. If a letter is found, the 2353 subject is matched against the first alternative; otherwise it is matched 2354 against the second. This pattern matches strings in one of the two forms 2355 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. 2356 . 2357 . 2358 .\" HTML 2359 .SH COMMENTS 2360 .rs 2361 .sp 2362 There are two ways of including comments in patterns that are processed by 2363 PCRE. In both cases, the start of the comment must not be in a character class, 2364 nor in the middle of any other sequence of related characters such as (?: or a 2365 subpattern name or number. The characters that make up a comment play no part 2366 in the pattern matching. 2367 .P 2368 The sequence (?# marks the start of a comment that continues up to the next 2369 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED 2370 option is set, an unescaped # character also introduces a comment, which in 2371 this case continues to immediately after the next newline character or 2372 character sequence in the pattern. Which characters are interpreted as newlines 2373 is controlled by the options passed to a compiling function or by a special 2374 sequence at the start of the pattern, as described in the section entitled 2375 .\" HTML 2376 .\" 2377 "Newline conventions" 2378 .\" 2379 above. Note that the end of this type of comment is a literal newline sequence 2380 in the pattern; escape sequences that happen to represent a newline do not 2381 count. For example, consider this pattern when PCRE_EXTENDED is set, and the 2382 default newline convention is in force: 2383 .sp 2384 abc #comment \en still comment 2385 .sp 2386 On encountering the # character, \fBpcre_compile()\fP skips along, looking for 2387 a newline in the pattern. The sequence \en is still literal at this stage, so 2388 it does not terminate the comment. Only an actual character with the code value 2389 0x0a (the default newline) does so. 2390 . 2391 . 2392 .\" HTML 2393 .SH "RECURSIVE PATTERNS" 2394 .rs 2395 .sp 2396 Consider the problem of matching a string in parentheses, allowing for 2397 unlimited nested parentheses. Without the use of recursion, the best that can 2398 be done is to use a pattern that matches up to some fixed depth of nesting. It 2399 is not possible to handle an arbitrary nesting depth. 2400 .P 2401 For some time, Perl has provided a facility that allows regular expressions to 2402 recurse (amongst other things). It does this by interpolating Perl code in the 2403 expression at run time, and the code can refer to the expression itself. A Perl 2404 pattern using code interpolation to solve the parentheses problem can be 2405 created like this: 2406 .sp 2407$re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x; 2408 .sp 2409 The (?p{...}) item interpolates Perl code at run time, and in this case refers 2410 recursively to the pattern in which it appears. 2411 .P 2412 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it 2413 supports special syntax for recursion of the entire pattern, and also for 2414 individual subpattern recursion. After its introduction in PCRE and Python, 2415 this kind of recursion was subsequently introduced into Perl at release 5.10. 2416 .P 2417 A special item that consists of (? followed by a number greater than zero and a 2418 closing parenthesis is a recursive subroutine call of the subpattern of the 2419 given number, provided that it occurs inside that subpattern. (If not, it is a 2420 .\" HTML 2421 .\" 2422 non-recursive subroutine 2423 .\" 2424 call, which is described in the next section.) The special item (?R) or (?0) is 2425 a recursive call of the entire regular expression. 2426 .P 2427 This PCRE pattern solves the nested parentheses problem (assume the 2428 PCRE_EXTENDED option is set so that white space is ignored): 2429 .sp 2430 \e( ( [^()]++ | (?R) )* \e) 2431 .sp 2432 First it matches an opening parenthesis. Then it matches any number of 2433 substrings which can either be a sequence of non-parentheses, or a recursive 2434 match of the pattern itself (that is, a correctly parenthesized substring). 2435 Finally there is a closing parenthesis. Note the use of a possessive quantifier 2436 to avoid backtracking into sequences of non-parentheses. 2437 .P 2438 If this were part of a larger pattern, you would not want to recurse the entire 2439 pattern, so instead you could use this: 2440 .sp 2441 ( \e( ( [^()]++ | (?1) )* \e) ) 2442 .sp 2443 We have put the pattern into parentheses, and caused the recursion to refer to 2444 them instead of the whole pattern. 2445 .P 2446 In a larger pattern, keeping track of parenthesis numbers can be tricky. This 2447 is made easier by the use of relative references. Instead of (?1) in the 2448 pattern above you can write (?-2) to refer to the second most recently opened 2449 parentheses preceding the recursion. In other words, a negative number counts 2450 capturing parentheses leftwards from the point at which it is encountered. 2451 .P 2452 It is also possible to refer to subsequently opened parentheses, by writing 2453 references such as (?+2). However, these cannot be recursive because the 2454 reference is not inside the parentheses that are referenced. They are always 2455 .\" HTML 2456 .\" 2457 non-recursive subroutine 2458 .\" 2459 calls, as described in the next section. 2460 .P 2461 An alternative approach is to use named parentheses instead. The Perl syntax 2462 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We 2463 could rewrite the above example as follows: 2464 .sp 2465 (? \e( ( [^()]++ | (?&pn) )* \e) ) 2466 .sp 2467 If there is more than one subpattern with the same name, the earliest one is 2468 used. 2469 .P 2470 This particular example pattern that we have been looking at contains nested 2471 unlimited repeats, and so the use of a possessive quantifier for matching 2472 strings of non-parentheses is important when applying the pattern to strings 2473 that do not match. For example, when this pattern is applied to 2474 .sp 2475 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 2476 .sp 2477 it yields "no match" quickly. However, if a possessive quantifier is not used, 2478 the match runs for a very long time indeed because there are so many different 2479 ways the + and * repeats can carve up the subject, and all have to be tested 2480 before failure can be reported. 2481 .P 2482 At the end of a match, the values of capturing parentheses are those from 2483 the outermost level. If you want to obtain intermediate values, a callout 2484 function can be used (see below and the 2485 .\" HREF 2486 \fBpcrecallout\fP 2487 .\" 2488 documentation). If the pattern above is matched against 2489 .sp 2490 (ab(cd)ef) 2491 .sp 2492 the value for the inner capturing parentheses (numbered 2) is "ef", which is 2493 the last value taken on at the top level. If a capturing subpattern is not 2494 matched at the top level, its final captured value is unset, even if it was 2495 (temporarily) set at a deeper level during the matching process. 2496 .P 2497 If there are more than 15 capturing parentheses in a pattern, PCRE has to 2498 obtain extra memory to store data during a recursion, which it does by using 2499 \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can 2500 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. 2501 .P 2502 Do not confuse the (?R) item with the condition (R), which tests for recursion. 2503 Consider this pattern, which matches text in angle brackets, allowing for 2504 arbitrary nesting. Only digits are allowed in nested brackets (that is, when 2505 recursing), whereas any characters are permitted at the outer level. 2506 .sp 2507 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * > 2508 .sp 2509 In this pattern, (?(R) is the start of a conditional subpattern, with two 2510 different alternatives for the recursive and non-recursive cases. The (?R) item 2511 is the actual recursive call. 2512 . 2513 . 2514 .\" HTML 2515 .SS "Differences in recursion processing between PCRE and Perl" 2516 .rs 2517 .sp 2518 Recursion processing in PCRE differs from Perl in two important ways. In PCRE 2519 (like Python, but unlike Perl), a recursive subpattern call is always treated 2520 as an atomic group. That is, once it has matched some of the subject string, it 2521 is never re-entered, even if it contains untried alternatives and there is a 2522 subsequent matching failure. This can be illustrated by the following pattern, 2523 which purports to match a palindromic string that contains an odd number of 2524 characters (for example, "a", "aba", "abcba", "abcdcba"): 2525 .sp 2526 ^(.|(.)(?1)\e2)$ 2527 .sp 2528 The idea is that it either matches a single character, or two identical 2529 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE 2530 it does not if the pattern is longer than three characters. Consider the 2531 subject string "abcba": 2532 .P 2533 At the top level, the first character is matched, but as it is not at the end 2534 of the string, the first alternative fails; the second alternative is taken 2535 and the recursion kicks in. The recursive call to subpattern 1 successfully 2536 matches the next character ("b"). (Note that the beginning and end of line 2537 tests are not part of the recursion). 2538 .P 2539 Back at the top level, the next character ("c") is compared with what 2540 subpattern 2 matched, which was "a". This fails. Because the recursion is 2541 treated as an atomic group, there are now no backtracking points, and so the 2542 entire match fails. (Perl is able, at this point, to re-enter the recursion and 2543 try the second alternative.) However, if the pattern is written with the 2544 alternatives in the other order, things are different: 2545 .sp 2546 ^((.)(?1)\e2|.)$2547 .sp 2548 This time, the recursing alternative is tried first, and continues to recurse 2549 until it runs out of characters, at which point the recursion fails. But this 2550 time we do have another alternative to try at the higher level. That is the big 2551 difference: in the previous case the remaining alternative is at a deeper 2552 recursion level, which PCRE cannot use. 2553 .P 2554 To change the pattern so that it matches all palindromic strings, not just 2555 those with an odd number of characters, it is tempting to change the pattern to 2556 this: 2557 .sp 2558 ^((.)(?1)\e2|.?)$ 2559 .sp 2560 Again, this works in Perl, but not in PCRE, and for the same reason. When a 2561 deeper recursion has matched a single character, it cannot be entered again in 2562 order to match an empty string. The solution is to separate the two cases, and 2563 write out the odd and even cases as alternatives at the higher level: 2564 .sp 2565 ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.)) 2566 .sp 2567 If you want to match typical palindromic phrases, the pattern has to ignore all 2568 non-word characters, which can be done like this: 2569 .sp 2570 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+\$ 2571 .sp 2572 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A 2573 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note 2574 the use of the possessive quantifier *+ to avoid backtracking into sequences of 2575 non-word characters. Without this, PCRE takes a great deal longer (ten times or 2576 more) to match typical phrases, and Perl takes so long that you think it has 2577 gone into a loop. 2578 .P 2579 \fBWARNING\fP: The palindrome-matching patterns above work only if the subject 2580 string does not start with a palindrome that is shorter than the entire string. 2581 For example, although "abcba" is correctly matched, if the subject is "ababa", 2582 PCRE finds the palindrome "aba" at the start, then fails at top level because 2583 the end of the string does not follow. Once again, it cannot jump back into the 2584 recursion to try other alternatives, so the entire match fails. 2585 .P 2586 The second way in which PCRE and Perl differ in their recursion processing is 2587 in the handling of captured values. In Perl, when a subpattern is called 2588 recursively or as a subpattern (see the next section), it has no access to any 2589 values that were captured outside the recursion, whereas in PCRE these values 2590 can be referenced. Consider this pattern: 2591 .sp 2592 ^(.)(\e1|a(?2)) 2593 .sp 2594 In PCRE, this pattern matches "bab". The first capturing parentheses match "b", 2595 then in the second group, when the back reference \e1 fails to match "b", the 2596 second alternative matches "a" and then recurses. In the recursion, \e1 does 2597 now match "b" and so the whole match succeeds. In Perl, the pattern fails to 2598 match because inside the recursive call \e1 cannot access the externally set 2599 value. 2600 . 2601 . 2602 .\" HTML 2603 .SH "SUBPATTERNS AS SUBROUTINES" 2604 .rs 2605 .sp 2606 If the syntax for a recursive subpattern call (either by number or by 2607 name) is used outside the parentheses to which it refers, it operates like a 2608 subroutine in a programming language. The called subpattern may be defined 2609 before or after the reference. A numbered reference can be absolute or 2610 relative, as in these examples: 2611 .sp 2612 (...(absolute)...)...(?2)... 2613 (...(relative)...)...(?-1)... 2614 (...(?+1)...(relative)... 2615 .sp 2616 An earlier example pointed out that the pattern 2617 .sp 2618 (sens|respons)e and \e1ibility 2619 .sp 2620 matches "sense and sensibility" and "response and responsibility", but not 2621 "sense and responsibility". If instead the pattern 2622 .sp 2623 (sens|respons)e and (?1)ibility 2624 .sp 2625 is used, it does match "sense and responsibility" as well as the other two 2626 strings. Another example is given in the discussion of DEFINE above. 2627 .P 2628 All subroutine calls, whether recursive or not, are always treated as atomic 2629 groups. That is, once a subroutine has matched some of the subject string, it 2630 is never re-entered, even if it contains untried alternatives and there is a 2631 subsequent matching failure. Any capturing parentheses that are set during the 2632 subroutine call revert to their previous values afterwards. 2633 .P 2634 Processing options such as case-independence are fixed when a subpattern is 2635 defined, so if it is used as a subroutine, such options cannot be changed for 2636 different calls. For example, consider this pattern: 2637 .sp 2638 (abc)(?i:(?-1)) 2639 .sp 2640 It matches "abcabc". It does not match "abcABC" because the change of 2641 processing option does not affect the called subpattern. 2642 . 2643 . 2644 .\" HTML 2645 .SH "ONIGURUMA SUBROUTINE SYNTAX" 2646 .rs 2647 .sp 2648 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or 2649 a number enclosed either in angle brackets or single quotes, is an alternative 2650 syntax for referencing a subpattern as a subroutine, possibly recursively. Here 2651 are two of the examples used above, rewritten using this syntax: 2652 .sp 2653 (? \e( ( (?>[^()]+) | \eg )* \e) ) 2654 (sens|respons)e and \eg'1'ibility 2655 .sp 2656 PCRE supports an extension to Oniguruma: if a number is preceded by a 2657 plus or a minus sign it is taken as a relative reference. For example: 2658 .sp 2659 (abc)(?i:\eg<-1>) 2660 .sp 2661 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP 2662 synonymous. The former is a back reference; the latter is a subroutine call. 2663 . 2664 . 2665 .SH CALLOUTS 2666 .rs 2667 .sp 2668 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl 2669 code to be obeyed in the middle of matching a regular expression. This makes it 2670 possible, amongst other things, to extract different substrings that match the 2671 same pair of parentheses when there is a repetition. 2672 .P 2673 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl 2674 code. The feature is called "callout". The caller of PCRE provides an external 2675 function by putting its entry point in the global variable \fIpcre_callout\fP 2676 (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library). 2677 By default, this variable contains NULL, which disables all calling out. 2678 .P 2679 Within a regular expression, (?C) indicates the points at which the external 2680 function is to be called. If you want to identify different callout points, you 2681 can put a number less than 256 after the letter C. The default value is zero. 2682 For example, this pattern has two callout points: 2683 .sp 2684 (?C1)abc(?C2)def 2685 .sp 2686 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are 2687 automatically installed before each item in the pattern. They are all numbered 2688 255. If there is a conditional group in the pattern whose condition is an 2689 assertion, an additional callout is inserted just before the condition. An 2690 explicit callout may also be set at this position, as in this example: 2691 .sp 2692 (?(?C9)(?=a)abc|def) 2693 .sp 2694 Note that this applies only to assertion conditions, not to other types of 2695 condition. 2696 .P 2697 During matching, when PCRE reaches a callout point, the external function is 2698 called. It is provided with the number of the callout, the position in the 2699 pattern, and, optionally, one item of data originally supplied by the caller of 2700 the matching function. The callout function may cause matching to proceed, to 2701 backtrack, or to fail altogether. A complete description of the interface to 2702 the callout function is given in the 2703 .\" HREF 2704 \fBpcrecallout\fP 2705 .\" 2706 documentation. 2707 . 2708 . 2709 .\" HTML 2710 .SH "BACKTRACKING CONTROL" 2711 .rs 2712 .sp 2713 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which 2714 are still described in the Perl documentation as "experimental and subject to 2715 change or removal in a future version of Perl". It goes on to say: "Their usage 2716 in production code should be noted to avoid problems during upgrades." The same 2717 remarks apply to the PCRE features described in this section. 2718 .P 2719 The new verbs make use of what was previously invalid syntax: an opening 2720 parenthesis followed by an asterisk. They are generally of the form 2721 (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving 2722 differently depending on whether or not a name is present. A name is any 2723 sequence of characters that does not include a closing parenthesis. The maximum 2724 length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit 2725 libraries. If the name is empty, that is, if the closing parenthesis 2726 immediately follows the colon, the effect is as if the colon were not there. 2727 Any number of these verbs may occur in a pattern. 2728 .P 2729 Since these verbs are specifically related to backtracking, most of them can be 2730 used only when the pattern is to be matched using one of the traditional 2731 matching functions, because these use a backtracking algorithm. With the 2732 exception of (*FAIL), which behaves like a failing negative assertion, the 2733 backtracking control verbs cause an error if encountered by a DFA matching 2734 function. 2735 .P 2736 The behaviour of these verbs in 2737 .\" HTML 2738 .\" 2739 repeated groups, 2740 .\" 2741 .\" HTML 2742 .\" 2743 assertions, 2744 .\" 2745 and in 2746 .\" HTML 2747 .\" 2748 subpatterns called as subroutines 2749 .\" 2750 (whether or not recursively) is documented below. 2751 . 2752 . 2753 .\" HTML 2754 .SS "Optimizations that affect backtracking verbs" 2755 .rs 2756 .sp 2757 PCRE contains some optimizations that are used to speed up matching by running 2758 some checks at the start of each match attempt. For example, it may know the 2759 minimum length of matching subject, or that a particular character must be 2760 present. When one of these optimizations bypasses the running of a match, any 2761 included backtracking verbs will not, of course, be processed. You can suppress 2762 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option 2763 when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the 2764 pattern with (*NO_START_OPT). There is more discussion of this option in the 2765 section entitled 2766 .\" HTML 2767 .\" 2768 "Option bits for \fBpcre_exec()\fP" 2769 .\" 2770 in the 2771 .\" HREF 2772 \fBpcreapi\fP 2773 .\" 2774 documentation. 2775 .P 2776 Experiments with Perl suggest that it too has similar optimizations, sometimes 2777 leading to anomalous results. 2778 . 2779 . 2780 .SS "Verbs that act immediately" 2781 .rs 2782 .sp 2783 The following verbs act as soon as they are encountered. They may not be 2784 followed by a name. 2785 .sp 2786 (*ACCEPT) 2787 .sp 2788 This verb causes the match to end successfully, skipping the remainder of the 2789 pattern. However, when it is inside a subpattern that is called as a 2790 subroutine, only that subpattern is ended successfully. Matching then continues 2791 at the outer level. If (*ACCEPT) in triggered in a positive assertion, the 2792 assertion succeeds; in a negative assertion, the assertion fails. 2793 .P 2794 If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For 2795 example: 2796 .sp 2797 A((?:A|B(*ACCEPT)|C)D) 2798 .sp 2799 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by 2800 the outer parentheses. 2801 .sp 2802 (*FAIL) or (*F) 2803 .sp 2804 This verb causes a matching failure, forcing backtracking to occur. It is 2805 equivalent to (?!) but easier to read. The Perl documentation notes that it is 2806 probably useful only when combined with (?{}) or (??{}). Those are, of course, 2807 Perl features that are not present in PCRE. The nearest equivalent is the 2808 callout feature, as for example in this pattern: 2809 .sp 2810 a+(?C)(*FAIL) 2811 .sp 2812 A match with the string "aaaa" always fails, but the callout is taken before 2813 each backtrack happens (in this example, 10 times). 2814 . 2815 . 2816 .SS "Recording which path was taken" 2817 .rs 2818 .sp 2819 There is one verb whose main purpose is to track how a match was arrived at, 2820 though it also has a secondary use in conjunction with advancing the match 2821 starting point (see (*SKIP) below). 2822 .sp 2823 (*MARK:NAME) or (*:NAME) 2824 .sp 2825 A name is always required with this verb. There may be as many instances of 2826 (*MARK) as you like in a pattern, and their names do not have to be unique. 2827 .P 2828 When a match succeeds, the name of the last-encountered (*MARK:NAME), 2829 (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the 2830 caller as described in the section entitled 2831 .\" HTML 2832 .\" 2833 "Extra data for \fBpcre_exec()\fP" 2834 .\" 2835 in the 2836 .\" HREF 2837 \fBpcreapi\fP 2838 .\" 2839 documentation. Here is an example of \fBpcretest\fP output, where the /K 2840 modifier requests the retrieval and outputting of (*MARK) data: 2841 .sp 2842 re> /X(*MARK:A)Y|X(*MARK:B)Z/K 2843 data> XY 2844 0: XY 2845 MK: A 2846 XZ 2847 0: XZ 2848 MK: B 2849 .sp 2850 The (*MARK) name is tagged with "MK:" in this output, and in this example it 2851 indicates which of the two alternatives matched. This is a more efficient way 2852 of obtaining this information than putting each alternative in its own 2853 capturing parentheses. 2854 .P 2855 If a verb with a name is encountered in a positive assertion that is true, the 2856 name is recorded and passed back if it is the last-encountered. This does not 2857 happen for negative assertions or failing positive assertions. 2858 .P 2859 After a partial match or a failed match, the last encountered name in the 2860 entire match process is returned. For example: 2861 .sp 2862 re> /X(*MARK:A)Y|X(*MARK:B)Z/K 2863 data> XP 2864 No match, mark = B 2865 .sp 2866 Note that in this unanchored example the mark is retained from the match 2867 attempt that started at the letter "X" in the subject. Subsequent match 2868 attempts starting at "P" and then with an empty string do not get as far as the 2869 (*MARK) item, but nevertheless do not reset it. 2870 .P 2871 If you are interested in (*MARK) values after failed matches, you should 2872 probably set the PCRE_NO_START_OPTIMIZE option 2873 .\" HTML 2874 .\" 2875 (see above) 2876 .\" 2877 to ensure that the match is always attempted. 2878 . 2879 . 2880 .SS "Verbs that act after backtracking" 2881 .rs 2882 .sp 2883 The following verbs do nothing when they are encountered. Matching continues 2884 with what follows, but if there is no subsequent match, causing a backtrack to 2885 the verb, a failure is forced. That is, backtracking cannot pass to the left of 2886 the verb. However, when one of these verbs appears inside an atomic group or an 2887 assertion that is true, its effect is confined to that group, because once the 2888 group has been matched, there is never any backtracking into it. In this 2889 situation, backtracking can "jump back" to the left of the entire atomic group 2890 or assertion. (Remember also, as stated above, that this localization also 2891 applies in subroutine calls.) 2892 .P 2893 These verbs differ in exactly what kind of failure occurs when backtracking 2894 reaches them. The behaviour described below is what happens when the verb is 2895 not in a subroutine or an assertion. Subsequent sections cover these special 2896 cases. 2897 .sp 2898 (*COMMIT) 2899 .sp 2900 This verb, which may not be followed by a name, causes the whole match to fail 2901 outright if there is a later matching failure that causes backtracking to reach 2902 it. Even if the pattern is unanchored, no further attempts to find a match by 2903 advancing the starting point take place. If (*COMMIT) is the only backtracking 2904 verb that is encountered, once it has been passed \fBpcre_exec()\fP is 2905 committed to finding a match at the current starting point, or not at all. For 2906 example: 2907 .sp 2908 a+(*COMMIT)b 2909 .sp 2910 This matches "xxaab" but not "aacaab". It can be thought of as a kind of 2911 dynamic anchor, or "I've started, so I must finish." The name of the most 2912 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a 2913 match failure. 2914 .P 2915 If there is more than one backtracking verb in a pattern, a different one that 2916 follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a 2917 match does not always guarantee that a match must be at this starting point. 2918 .P 2919 Note that (*COMMIT) at the start of a pattern is not the same as an anchor, 2920 unless PCRE's start-of-match optimizations are turned off, as shown in this 2921 \fBpcretest\fP example: 2922 .sp 2923 re> /(*COMMIT)abc/ 2924 data> xyzabc 2925 0: abc 2926 xyzabc\eY 2927 No match 2928 .sp 2929 PCRE knows that any match must start with "a", so the optimization skips along 2930 the subject to "a" before running the first match attempt, which succeeds. When 2931 the optimization is disabled by the \eY escape in the second subject, the match 2932 starts at "x" and so the (*COMMIT) causes it to fail without trying any other 2933 starting points. 2934 .sp 2935 (*PRUNE) or (*PRUNE:NAME) 2936 .sp 2937 This verb causes the match to fail at the current starting position in the 2938 subject if there is a later matching failure that causes backtracking to reach 2939 it. If the pattern is unanchored, the normal "bumpalong" advance to the next 2940 starting character then happens. Backtracking can occur as usual to the left of 2941 (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but 2942 if there is no match to the right, backtracking cannot cross (*PRUNE). In 2943 simple cases, the use of (*PRUNE) is just an alternative to an atomic group or 2944 possessive quantifier, but there are some uses of (*PRUNE) that cannot be 2945 expressed in any other way. In an anchored pattern (*PRUNE) has the same effect 2946 as (*COMMIT). 2947 .P 2948 The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE). 2949 It is like (*MARK:NAME) in that the name is remembered for passing back to the 2950 caller. However, (*SKIP:NAME) searches only for names set with (*MARK). 2951 .sp 2952 (*SKIP) 2953 .sp 2954 This verb, when given without a name, is like (*PRUNE), except that if the 2955 pattern is unanchored, the "bumpalong" advance is not to the next character, 2956 but to the position in the subject where (*SKIP) was encountered. (*SKIP) 2957 signifies that whatever text was matched leading up to it cannot be part of a 2958 successful match. Consider: 2959 .sp 2960 a+(*SKIP)b 2961 .sp 2962 If the subject is "aaaac...", after the first match attempt fails (starting at 2963 the first character in the string), the starting point skips on to start the 2964 next attempt at "c". Note that a possessive quantifer does not have the same 2965 effect as this example; although it would suppress backtracking during the 2966 first match attempt, the second attempt would start at the second character 2967 instead of skipping on to "c". 2968 .sp 2969 (*SKIP:NAME) 2970 .sp 2971 When (*SKIP) has an associated name, its behaviour is modified. When it is 2972 triggered, the previous path through the pattern is searched for the most 2973 recent (*MARK) that has the same name. If one is found, the "bumpalong" advance 2974 is to the subject position that corresponds to that (*MARK) instead of to where 2975 (*SKIP) was encountered. If no (*MARK) with a matching name is found, the 2976 (*SKIP) is ignored. 2977 .P 2978 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores 2979 names that are set by (*PRUNE:NAME) or (*THEN:NAME). 2980 .sp 2981 (*THEN) or (*THEN:NAME) 2982 .sp 2983 This verb causes a skip to the next innermost alternative when backtracking 2984 reaches it. That is, it cancels any further backtracking within the current 2985 alternative. Its name comes from the observation that it can be used for a 2986 pattern-based if-then-else block: 2987 .sp 2988 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... 2989 .sp 2990 If the COND1 pattern matches, FOO is tried (and possibly further items after 2991 the end of the group if FOO succeeds); on failure, the matcher skips to the 2992 second alternative and tries COND2, without backtracking into COND1. If that 2993 succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no 2994 more alternatives, so there is a backtrack to whatever came before the entire 2995 group. If (*THEN) is not inside an alternation, it acts like (*PRUNE). 2996 .P 2997 The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN). 2998 It is like (*MARK:NAME) in that the name is remembered for passing back to the 2999 caller. However, (*SKIP:NAME) searches only for names set with (*MARK). 3000 .P 3001 A subpattern that does not contain a | character is just a part of the 3002 enclosing alternative; it is not a nested alternation with only one 3003 alternative. The effect of (*THEN) extends beyond such a subpattern to the 3004 enclosing alternative. Consider this pattern, where A, B, etc. are complex 3005 pattern fragments that do not contain any | characters at this level: 3006 .sp 3007 A (B(*THEN)C) | D 3008 .sp 3009 If A and B are matched, but there is a failure in C, matching does not 3010 backtrack into A; instead it moves to the next alternative, that is, D. 3011 However, if the subpattern containing (*THEN) is given an alternative, it 3012 behaves differently: 3013 .sp 3014 A (B(*THEN)C | (*FAIL)) | D 3015 .sp 3016 The effect of (*THEN) is now confined to the inner subpattern. After a failure 3017 in C, matching moves to (*FAIL), which causes the whole subpattern to fail 3018 because there are no more alternatives to try. In this case, matching does now 3019 backtrack into A. 3020 .P 3021 Note that a conditional subpattern is not considered as having two 3022 alternatives, because only one is ever used. In other words, the | character in 3023 a conditional subpattern has a different meaning. Ignoring white space, 3024 consider: 3025 .sp 3026 ^.*? (?(?=a) a | b(*THEN)c ) 3027 .sp 3028 If the subject is "ba", this pattern does not match. Because .*? is ungreedy, 3029 it initially matches zero characters. The condition (?=a) then fails, the 3030 character "b" is matched, but "c" is not. At this point, matching does not 3031 backtrack to .*? as might perhaps be expected from the presence of the | 3032 character. The conditional subpattern is part of the single alternative that 3033 comprises the whole pattern, and so the match fails. (If there was a backtrack 3034 into .*?, allowing it to match "b", the match would succeed.) 3035 .P 3036 The verbs just described provide four different "strengths" of control when 3037 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the 3038 next alternative. (*PRUNE) comes next, failing the match at the current 3039 starting position, but allowing an advance to the next character (for an 3040 unanchored pattern). (*SKIP) is similar, except that the advance may be more 3041 than one character. (*COMMIT) is the strongest, causing the entire match to 3042 fail. 3043 . 3044 . 3045 .SS "More than one backtracking verb" 3046 .rs 3047 .sp 3048 If more than one backtracking verb is present in a pattern, the one that is 3049 backtracked onto first acts. For example, consider this pattern, where A, B, 3050 etc. are complex pattern fragments: 3051 .sp 3052 (A(*COMMIT)B(*THEN)C|ABD) 3053 .sp 3054 If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to 3055 fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes 3056 the next alternative (ABD) to be tried. This behaviour is consistent, but is 3057 not always the same as Perl's. It means that if two or more backtracking verbs 3058 appear in succession, all the the last of them has no effect. Consider this 3059 example: 3060 .sp 3061 ...(*COMMIT)(*PRUNE)... 3062 .sp 3063 If there is a matching failure to the right, backtracking onto (*PRUNE) cases 3064 it to be triggered, and its action is taken. There can never be a backtrack 3065 onto (*COMMIT). 3066 . 3067 . 3068 .\" HTML 3069 .SS "Backtracking verbs in repeated groups" 3070 .rs 3071 .sp 3072 PCRE differs from Perl in its handling of backtracking verbs in repeated 3073 groups. For example, consider: 3074 .sp 3075 /(a(*COMMIT)b)+ac/ 3076 .sp 3077 If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in 3078 the second repeat of the group acts. 3079 . 3080 . 3081 .\" HTML 3082 .SS "Backtracking verbs in assertions" 3083 .rs 3084 .sp 3085 (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack. 3086 .P 3087 (*ACCEPT) in a positive assertion causes the assertion to succeed without any 3088 further processing. In a negative assertion, (*ACCEPT) causes the assertion to 3089 fail without any further processing. 3090 .P 3091 The other backtracking verbs are not treated specially if they appear in a 3092 positive assertion. In particular, (*THEN) skips to the next alternative in the 3093 innermost enclosing group that has alternations, whether or not this is within 3094 the assertion. 3095 .P 3096 Negative assertions are, however, different, in order to ensure that changing a 3097 positive assertion into a negative assertion changes its result. Backtracking 3098 into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true, 3099 without considering any further alternative branches in the assertion. 3100 Backtracking into (*THEN) causes it to skip to the next enclosing alternative 3101 within the assertion (the normal behaviour), but if the assertion does not have 3102 such an alternative, (*THEN) behaves like (*PRUNE). 3103 . 3104 . 3105 .\" HTML 3106 .SS "Backtracking verbs in subroutines" 3107 .rs 3108 .sp 3109 These behaviours occur whether or not the subpattern is called recursively. 3110 Perl's treatment of subroutines is different in some cases. 3111 .P 3112 (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces 3113 an immediate backtrack. 3114 .P 3115 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to 3116 succeed without any further processing. Matching then continues after the 3117 subroutine call. 3118 .P 3119 (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause 3120 the subroutine match to fail. 3121 .P 3122 (*THEN) skips to the next alternative in the innermost enclosing group within 3123 the subpattern that has alternatives. If there is no such group within the 3124 subpattern, (*THEN) causes the subroutine match to fail. 3125 . 3126 . 3127 .SH "SEE ALSO" 3128 .rs 3129 .sp 3130 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), 3131 \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP. 3132 . 3133 . 3134 .SH AUTHOR 3135 .rs 3136 .sp 3137 .nf 3138 Philip Hazel 3139 University Computing Service 3140 Cambridge CB2 3QH, England. 3141 .fi 3142 . 3143 . 3144 .SH REVISION 3145 .rs 3146 .sp 3147 .nf 3148 Last updated: 26 April 2013 3149 Copyright (c) 1997-2013 University of Cambridge. 3150 .fi

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