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

Load pcre-6.0 into code/trunk.

 1 nigel 63 .TH PCRE 3 2 .SH NAME 3 PCRE - Perl-compatible regular expressions 4 nigel 75 .SH "PCRE REGULAR EXPRESSION DETAILS" 5 nigel 63 .rs 6 .sp 7 The syntax and semantics of the regular expressions supported by PCRE are 8 described below. Regular expressions are also described in the Perl 9 nigel 75 documentation and in a number of books, some of which have copious examples. 10 Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers 11 regular expressions in great detail. This description of PCRE's regular 12 expressions is intended as reference material. 13 .P 14 The original operation of PCRE was on strings of one-byte characters. However, 15 there is now also support for UTF-8 character strings. To use this, you must 16 build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with 17 the PCRE_UTF8 option. How this affects pattern matching is mentioned in several 18 places below. There is also a summary of UTF-8 features in the 19 nigel 63 .\" HTML 20 .\" 21 section on UTF-8 support 22 .\" 23 in the main 24 .\" HREF 25 nigel 75 \fBpcre\fP 26 nigel 63 .\" 27 page. 28 nigel 75 .P 29 nigel 77 The remainder of this document discusses the patterns that are supported by 30 PCRE when its main matching function, \fBpcre_exec()\fP, is used. 31 From release 6.0, PCRE offers a second matching function, 32 \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not 33 Perl-compatible. The advantages and disadvantages of the alternative function, 34 and how it differs from the normal function, are discussed in the 35 .\" HREF 36 \fBpcrematching\fP 37 .\" 38 page. 39 .P 40 nigel 63 A regular expression is a pattern that is matched against a subject string from 41 left to right. Most characters stand for themselves in a pattern, and match the 42 corresponding characters in the subject. As a trivial example, the pattern 43 nigel 75 .sp 44 nigel 63 The quick brown fox 45 nigel 75 .sp 46 nigel 77 matches a portion of a subject string that is identical to itself. When 47 caseless matching is specified (the PCRE_CASELESS option), letters are matched 48 independently of case. In UTF-8 mode, PCRE always understands the concept of 49 case for characters whose values are less than 128, so caseless matching is 50 always possible. For characters with higher values, the concept of case is 51 supported if PCRE is compiled with Unicode property support, but not otherwise. 52 If you want to use caseless matching for characters 128 and above, you must 53 ensure that PCRE is compiled with Unicode property support as well as with 54 UTF-8 support. 55 .P 56 The power of regular expressions comes from the ability to include alternatives 57 and repetitions in the pattern. These are encoded in the pattern by the use of 58 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are 59 nigel 63 interpreted in some special way. 60 nigel 75 .P 61 There are two different sets of metacharacters: those that are recognized 62 nigel 63 anywhere in the pattern except within square brackets, and those that are 63 nigel 75 recognized in square brackets. Outside square brackets, the metacharacters are 64 nigel 63 as follows: 65 nigel 75 .sp 66 \e general escape character with several uses 67 nigel 63 ^ assert start of string (or line, in multiline mode) 68 $assert end of string (or line, in multiline mode) 69 . match any character except newline (by default) 70 [ start character class definition 71 | start of alternative branch 72 ( start subpattern 73 ) end subpattern 74 ? extends the meaning of ( 75 also 0 or 1 quantifier 76 also quantifier minimizer 77 * 0 or more quantifier 78 + 1 or more quantifier 79 also "possessive quantifier" 80 { start min/max quantifier 81 nigel 75 .sp 82 nigel 63 Part of a pattern that is in square brackets is called a "character class". In 83 nigel 75 a character class the only metacharacters are: 84 .sp 85 \e general escape character 86 nigel 63 ^ negate the class, but only if the first character 87 - indicates character range 88 nigel 75 .\" JOIN 89 nigel 63 [ POSIX character class (only if followed by POSIX 90 syntax) 91 ] terminates the character class 92 nigel 75 .sp 93 The following sections describe the use of each of the metacharacters. 94 . 95 nigel 63 .SH BACKSLASH 96 .rs 97 .sp 98 The backslash character has several uses. Firstly, if it is followed by a 99 nigel 75 non-alphanumeric character, it takes away any special meaning that character may 100 nigel 63 have. This use of backslash as an escape character applies both inside and 101 outside character classes. 102 nigel 75 .P 103 For example, if you want to match a * character, you write \e* in the pattern. 104 nigel 63 This escaping action applies whether or not the following character would 105 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a 106 non-alphanumeric with backslash to specify that it stands for itself. In 107 particular, if you want to match a backslash, you write \e\e. 108 .P 109 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the 110 pattern (other than in a character class) and characters between a # outside 111 a character class and the next newline character are ignored. An escaping 112 backslash can be used to include a whitespace or # character as part of the 113 pattern. 114 nigel 75 .P 115 nigel 63 If you want to remove the special meaning from a sequence of characters, you 116 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in 117 that$ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in 118 nigel 63 Perl, $and @ cause variable interpolation. Note the following examples: 119 nigel 75 .sp 120 nigel 63 Pattern PCRE matches Perl matches 121 nigel 75 .sp 122 .\" JOIN 123 \eQabc$xyz\eE abc$xyz abc followed by the 124 nigel 63 contents of$xyz 125 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz 126 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz 127 .sp 128 The \eQ...\eE sequence is recognized both inside and outside character classes. 129 . 130 . 131 .\" HTML 132 .SS "Non-printing characters" 133 .rs 134 .sp 135 nigel 63 A second use of backslash provides a way of encoding non-printing characters 136 in patterns in a visible manner. There is no restriction on the appearance of 137 non-printing characters, apart from the binary zero that terminates a pattern, 138 but when a pattern is being prepared by text editing, it is usually easier to 139 use one of the following escape sequences than the binary character it 140 represents: 141 nigel 75 .sp 142 \ea alarm, that is, the BEL character (hex 07) 143 \ecx "control-x", where x is any character 144 \ee escape (hex 1B) 145 \ef formfeed (hex 0C) 146 \en newline (hex 0A) 147 \er carriage return (hex 0D) 148 \et tab (hex 09) 149 \eddd character with octal code ddd, or backreference 150 \exhh character with hex code hh 151 \ex{hhh..} character with hex code hhh... (UTF-8 mode only) 152 .sp 153 The precise effect of \ecx is as follows: if x is a lower case letter, it 154 nigel 63 is converted to upper case. Then bit 6 of the character (hex 40) is inverted. 155 nigel 75 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex 156 nigel 63 7B. 157 nigel 75 .P 158 After \ex, from zero to two hexadecimal digits are read (letters can be in 159 nigel 63 upper or lower case). In UTF-8 mode, any number of hexadecimal digits may 160 nigel 75 appear between \ex{ and }, but the value of the character code must be less 161 nigel 63 than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters 162 nigel 75 other than hexadecimal digits appear between \ex{ and }, or if there is no 163 nigel 63 terminating }, this form of escape is not recognized. Instead, the initial 164 nigel 75 \ex will be interpreted as a basic hexadecimal escape, with no following 165 digits, giving a character whose value is zero. 166 .P 167 nigel 63 Characters whose value is less than 256 can be defined by either of the two 168 nigel 75 syntaxes for \ex when PCRE is in UTF-8 mode. There is no difference in the 169 way they are handled. For example, \exdc is exactly the same as \ex{dc}. 170 .P 171 After \e0 up to two further octal digits are read. In both cases, if there 172 nigel 63 are fewer than two digits, just those that are present are used. Thus the 173 nigel 75 sequence \e0\ex\e07 specifies two binary zeros followed by a BEL character 174 nigel 63 (code value 7). Make sure you supply two digits after the initial zero if the 175 nigel 75 pattern character that follows is itself an octal digit. 176 .P 177 nigel 63 The handling of a backslash followed by a digit other than 0 is complicated. 178 Outside a character class, PCRE reads it and any following digits as a decimal 179 number. If the number is less than 10, or if there have been at least that many 180 previous capturing left parentheses in the expression, the entire sequence is 181 nigel 75 taken as a \fIback reference\fP. A description of how this works is given 182 .\" HTML 183 .\" 184 later, 185 .\" 186 following the discussion of 187 .\" HTML 188 .\" 189 parenthesized subpatterns. 190 .\" 191 .P 192 nigel 63 Inside a character class, or if the decimal number is greater than 9 and there 193 have not been that many capturing subpatterns, PCRE re-reads up to three octal 194 digits following the backslash, and generates a single byte from the least 195 significant 8 bits of the value. Any subsequent digits stand for themselves. 196 For example: 197 nigel 75 .sp 198 \e040 is another way of writing a space 199 .\" JOIN 200 \e40 is the same, provided there are fewer than 40 201 nigel 63 previous capturing subpatterns 202 nigel 75 \e7 is always a back reference 203 .\" JOIN 204 \e11 might be a back reference, or another way of 205 nigel 63 writing a tab 206 nigel 75 \e011 is always a tab 207 \e0113 is a tab followed by the character "3" 208 .\" JOIN 209 \e113 might be a back reference, otherwise the 210 nigel 63 character with octal code 113 211 nigel 75 .\" JOIN 212 \e377 might be a back reference, otherwise 213 nigel 63 the byte consisting entirely of 1 bits 214 nigel 75 .\" JOIN 215 \e81 is either a back reference, or a binary zero 216 nigel 63 followed by the two characters "8" and "1" 217 nigel 75 .sp 218 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading 219 zero, because no more than three octal digits are ever read. 220 nigel 75 .P 221 nigel 63 All the sequences that define a single byte value or a single UTF-8 character 222 (in UTF-8 mode) can be used both inside and outside character classes. In 223 nigel 75 addition, inside a character class, the sequence \eb is interpreted as the 224 backspace character (hex 08), and the sequence \eX is interpreted as the 225 character "X". Outside a character class, these sequences have different 226 meanings 227 .\" HTML 228 .\" 229 (see below). 230 .\" 231 . 232 . 233 .SS "Generic character types" 234 .rs 235 .sp 236 The third use of backslash is for specifying generic character types. The 237 following are always recognized: 238 .sp 239 \ed any decimal digit 240 \eD any character that is not a decimal digit 241 \es any whitespace character 242 \eS any character that is not a whitespace character 243 \ew any "word" character 244 \eW any "non-word" character 245 .sp 246 nigel 63 Each pair of escape sequences partitions the complete set of characters into 247 two disjoint sets. Any given character matches one, and only one, of each pair. 248 nigel 75 .P 249 These character type sequences can appear both inside and outside character 250 classes. They each match one character of the appropriate type. If the current 251 matching point is at the end of the subject string, all of them fail, since 252 there is no character to match. 253 .P 254 For compatibility with Perl, \es does not match the VT character (code 11). 255 This makes it different from the the POSIX "space" class. The \es characters 256 nigel 63 are HT (9), LF (10), FF (12), CR (13), and space (32). 257 nigel 75 .P 258 A "word" character is an underscore or any character less than 256 that is a 259 letter or digit. The definition of letters and digits is controlled by PCRE's 260 low-valued character tables, and may vary if locale-specific matching is taking 261 place (see 262 nigel 63 .\" HTML 263 .\" 264 "Locale support" 265 .\" 266 in the 267 .\" HREF 268 nigel 75 \fBpcreapi\fP 269 nigel 63 .\" 270 nigel 75 page). For example, in the "fr_FR" (French) locale, some character codes 271 greater than 128 are used for accented letters, and these are matched by \ew. 272 .P 273 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or 274 \ew, and always match \eD, \eS, and \eW. This is true even when Unicode 275 character property support is available. 276 . 277 . 278 .\" HTML 279 .SS Unicode character properties 280 .rs 281 .sp 282 When PCRE is built with Unicode character property support, three additional 283 escape sequences to match generic character types are available when UTF-8 mode 284 is selected. They are: 285 .sp 286 \ep{\fIxx\fP} a character with the \fIxx\fP property 287 \eP{\fIxx\fP} a character without the \fIxx\fP property 288 \eX an extended Unicode sequence 289 .sp 290 The property names represented by \fIxx\fP above are limited to the 291 Unicode general category properties. Each character has exactly one such 292 property, specified by a two-letter abbreviation. For compatibility with Perl, 293 negation can be specified by including a circumflex between the opening brace 294 and the property name. For example, \ep{^Lu} is the same as \eP{Lu}. 295 .P 296 If only one letter is specified with \ep or \eP, it includes all the properties 297 that start with that letter. In this case, in the absence of negation, the 298 curly brackets in the escape sequence are optional; these two examples have 299 the same effect: 300 .sp 301 \ep{L} 302 \epL 303 .sp 304 The following property codes are supported: 305 .sp 306 C Other 307 Cc Control 308 Cf Format 309 Cn Unassigned 310 Co Private use 311 Cs Surrogate 312 .sp 313 L Letter 314 Ll Lower case letter 315 Lm Modifier letter 316 Lo Other letter 317 Lt Title case letter 318 Lu Upper case letter 319 .sp 320 M Mark 321 Mc Spacing mark 322 Me Enclosing mark 323 Mn Non-spacing mark 324 .sp 325 N Number 326 Nd Decimal number 327 Nl Letter number 328 No Other number 329 .sp 330 P Punctuation 331 Pc Connector punctuation 332 Pd Dash punctuation 333 Pe Close punctuation 334 Pf Final punctuation 335 Pi Initial punctuation 336 Po Other punctuation 337 Ps Open punctuation 338 .sp 339 S Symbol 340 Sc Currency symbol 341 Sk Modifier symbol 342 Sm Mathematical symbol 343 So Other symbol 344 .sp 345 Z Separator 346 Zl Line separator 347 Zp Paragraph separator 348 Zs Space separator 349 .sp 350 Extended properties such as "Greek" or "InMusicalSymbols" are not supported by 351 PCRE. 352 .P 353 Specifying caseless matching does not affect these escape sequences. For 354 example, \ep{Lu} always matches only upper case letters. 355 .P 356 The \eX escape matches any number of Unicode characters that form an extended 357 Unicode sequence. \eX is equivalent to 358 .sp 359 (?>\ePM\epM*) 360 .sp 361 That is, it matches a character without the "mark" property, followed by zero 362 or more characters with the "mark" property, and treats the sequence as an 363 atomic group 364 .\" HTML 365 .\" 366 (see below). 367 .\" 368 Characters with the "mark" property are typically accents that affect the 369 preceding character. 370 .P 371 Matching characters by Unicode property is not fast, because PCRE has to search 372 a structure that contains data for over fifteen thousand characters. That is 373 why the traditional escape sequences such as \ed and \ew do not use Unicode 374 properties in PCRE. 375 . 376 . 377 .\" HTML 378 .SS "Simple assertions" 379 .rs 380 .sp 381 nigel 63 The fourth use of backslash is for certain simple assertions. An assertion 382 specifies a condition that has to be met at a particular point in a match, 383 without consuming any characters from the subject string. The use of 384 nigel 75 subpatterns for more complicated assertions is described 385 .\" HTML 386 .\" 387 below. 388 .\" 389 The backslashed 390 assertions are: 391 .sp 392 \eb matches at a word boundary 393 \eB matches when not at a word boundary 394 \eA matches at start of subject 395 \eZ matches at end of subject or before newline at end 396 \ez matches at end of subject 397 \eG matches at first matching position in subject 398 .sp 399 These assertions may not appear in character classes (but note that \eb has a 400 nigel 63 different meaning, namely the backspace character, inside a character class). 401 nigel 75 .P 402 nigel 63 A word boundary is a position in the subject string where the current character 403 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches 404 \ew and the other matches \eW), or the start or end of the string if the 405 first or last character matches \ew, respectively. 406 .P 407 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and 408 dollar (described in the next section) in that they only ever match at the very 409 start and end of the subject string, whatever options are set. Thus, they are 410 independent of multiline mode. These three assertions are not affected by the 411 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the 412 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP 413 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start 414 at a point other than the beginning of the subject, \eA can never match. The 415 difference between \eZ and \ez is that \eZ matches before a newline that is the 416 last character of the string as well as at the end of the string, whereas \ez 417 matches only at the end. 418 .P 419 The \eG assertion is true only when the current matching position is at the 420 start point of the match, as specified by the \fIstartoffset\fP argument of 421 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is 422 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate 423 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of 424 nigel 75 implementation where \eG can be useful. 425 .P 426 Note, however, that PCRE's interpretation of \eG, as the start of the current 427 nigel 63 match, is subtly different from Perl's, which defines it as the end of the 428 previous match. In Perl, these can be different when the previously matched 429 string was empty. Because PCRE does just one match at a time, it cannot 430 reproduce this behaviour. 431 nigel 75 .P 432 If all the alternatives of a pattern begin with \eG, the expression is anchored 433 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled 434 regular expression. 435 nigel 75 . 436 . 437 .SH "CIRCUMFLEX AND DOLLAR" 438 nigel 63 .rs 439 .sp 440 Outside a character class, in the default matching mode, the circumflex 441 nigel 75 character is an assertion that is true only if the current matching point is 442 at the start of the subject string. If the \fIstartoffset\fP argument of 443 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE 444 nigel 63 option is unset. Inside a character class, circumflex has an entirely different 445 nigel 75 meaning 446 .\" HTML 447 .\" 448 (see below). 449 .\" 450 .P 451 nigel 63 Circumflex need not be the first character of the pattern if a number of 452 alternatives are involved, but it should be the first thing in each alternative 453 in which it appears if the pattern is ever to match that branch. If all 454 possible alternatives start with a circumflex, that is, if the pattern is 455 constrained to match only at the start of the subject, it is said to be an 456 "anchored" pattern. (There are also other constructs that can cause a pattern 457 to be anchored.) 458 nigel 75 .P 459 A dollar character is an assertion that is true only if the current matching 460 nigel 63 point is at the end of the subject string, or immediately before a newline 461 character that is the last character in the string (by default). Dollar need 462 not be the last character of the pattern if a number of alternatives are 463 involved, but it should be the last item in any branch in which it appears. 464 Dollar has no special meaning in a character class. 465 nigel 75 .P 466 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of 467 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This 468 nigel 75 does not affect the \eZ assertion. 469 .P 470 nigel 63 The meanings of the circumflex and dollar characters are changed if the 471 PCRE_MULTILINE option is set. When this is the case, they match immediately 472 after and immediately before an internal newline character, respectively, in 473 addition to matching at the start and end of the subject string. For example, 474 nigel 75 the pattern /^abc$/ matches the subject string "def\enabc" (where \en 475 represents a newline character) in multiline mode, but not otherwise. 476 Consequently, patterns that are anchored in single line mode because all 477 branches start with ^ are not anchored in multiline mode, and a match for 478 circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP 479 is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is 480 set. 481 .P 482 Note that the sequences \eA, \eZ, and \ez can be used to match the start and 483 nigel 63 end of the subject in both modes, and if all branches of a pattern start with 484 nigel 75 \eA it is always anchored, whether PCRE_MULTILINE is set or not. 485 . 486 . 487 .SH "FULL STOP (PERIOD, DOT)" 488 nigel 63 .rs 489 .sp 490 Outside a character class, a dot in the pattern matches any one character in 491 the subject, including a non-printing character, but not (by default) newline. 492 In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one 493 nigel 75 byte long, except (by default) newline. If the PCRE_DOTALL option is set, 494 nigel 63 dots match newlines as well. The handling of dot is entirely independent of the 495 handling of circumflex and dollar, the only relationship being that they both 496 involve newline characters. Dot has no special meaning in a character class. 497 nigel 75 . 498 . 499 .SH "MATCHING A SINGLE BYTE" 500 nigel 63 .rs 501 .sp 502 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both 503 in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is 504 provided in Perl in order to match individual bytes in UTF-8 mode. Because it 505 breaks up UTF-8 characters into individual bytes, what remains in the string 506 may be a malformed UTF-8 string. For this reason, the \eC escape sequence is 507 best avoided. 508 .P 509 PCRE does not allow \eC to appear in lookbehind assertions 510 .\" HTML 511 .\" 512 (described below), 513 .\" 514 because in UTF-8 mode this would make it impossible to calculate the length of 515 the lookbehind. 516 . 517 . 518 .\" HTML 519 .SH "SQUARE BRACKETS AND CHARACTER CLASSES" 520 nigel 63 .rs 521 .sp 522 An opening square bracket introduces a character class, terminated by a closing 523 square bracket. A closing square bracket on its own is not special. If a 524 closing square bracket is required as a member of the class, it should be the 525 first data character in the class (after an initial circumflex, if present) or 526 escaped with a backslash. 527 nigel 75 .P 528 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the 529 character may occupy more than one byte. A matched character must be in the set 530 of characters defined by the class, unless the first character in the class 531 definition is a circumflex, in which case the subject character must not be in 532 the set defined by the class. If a circumflex is actually required as a member 533 of the class, ensure it is not the first character, or escape it with a 534 backslash. 535 nigel 75 .P 536 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while 537 [^aeiou] matches any character that is not a lower case vowel. Note that a 538 nigel 75 circumflex is just a convenient notation for specifying the characters that 539 are in the class by enumerating those that are not. A class that starts with a 540 circumflex is not an assertion: it still consumes a character from the subject 541 string, and therefore it fails if the current pointer is at the end of the 542 string. 543 .P 544 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a 545 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism. 546 .P 547 nigel 63 When caseless matching is set, any letters in a class represent both their 548 upper case and lower case versions, so for example, a caseless [aeiou] matches 549 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a 550 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of 551 case for characters whose values are less than 128, so caseless matching is 552 always possible. For characters with higher values, the concept of case is 553 supported if PCRE is compiled with Unicode property support, but not otherwise. 554 If you want to use caseless matching for characters 128 and above, you must 555 ensure that PCRE is compiled with Unicode property support as well as with 556 UTF-8 support. 557 nigel 75 .P 558 nigel 63 The newline character is never treated in any special way in character classes, 559 whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class 560 such as [^a] will always match a newline. 561 nigel 75 .P 562 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a 563 character class. For example, [d-m] matches any letter between d and m, 564 inclusive. If a minus character is required in a class, it must be escaped with 565 a backslash or appear in a position where it cannot be interpreted as 566 indicating a range, typically as the first or last character in the class. 567 nigel 75 .P 568 nigel 63 It is not possible to have the literal character "]" as the end character of a 569 range. A pattern such as [W-]46] is interpreted as a class of two characters 570 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or 571 "-46]". However, if the "]" is escaped with a backslash it is interpreted as 572 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range 573 followed by two other characters. The octal or hexadecimal representation of 574 "]" can also be used to end a range. 575 .P 576 nigel 63 Ranges operate in the collating sequence of character values. They can also be 577 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8 578 nigel 63 mode, ranges can include characters whose values are greater than 255, for 579 nigel 75 example [\ex{100}-\ex{2ff}]. 580 .P 581 nigel 63 If a range that includes letters is used when caseless matching is set, it 582 matches the letters in either case. For example, [W-c] is equivalent to 583 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character 584 tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E 585 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for 586 characters with values greater than 128 only when it is compiled with Unicode 587 property support. 588 .P 589 The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear 590 in a character class, and add the characters that they match to the class. For 591 example, [\edABCDEF] matches any hexadecimal digit. A circumflex can 592 nigel 63 conveniently be used with the upper case character types to specify a more 593 restricted set of characters than the matching lower case type. For example, 594 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore. 595 .P 596 The only metacharacters that are recognized in character classes are backslash, 597 hyphen (only where it can be interpreted as specifying a range), circumflex 598 (only at the start), opening square bracket (only when it can be interpreted as 599 introducing a POSIX class name - see the next section), and the terminating 600 closing square bracket. However, escaping other non-alphanumeric characters 601 does no harm. 602 . 603 . 604 .SH "POSIX CHARACTER CLASSES" 605 nigel 63 .rs 606 .sp 607 nigel 75 Perl supports the POSIX notation for character classes. This uses names 608 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports 609 this notation. For example, 610 nigel 75 .sp 611 nigel 63 [01[:alpha:]%] 612 nigel 75 .sp 613 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names 614 are 615 nigel 75 .sp 616 nigel 63 alnum letters and digits 617 alpha letters 618 ascii character codes 0 - 127 619 blank space or tab only 620 cntrl control characters 621 nigel 75 digit decimal digits (same as \ed) 622 nigel 63 graph printing characters, excluding space 623 lower lower case letters 624 print printing characters, including space 625 punct printing characters, excluding letters and digits 626 nigel 75 space white space (not quite the same as \es) 627 nigel 63 upper upper case letters 628 nigel 75 word "word" characters (same as \ew) 629 nigel 63 xdigit hexadecimal digits 630 nigel 75 .sp 631 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and 632 space (32). Notice that this list includes the VT character (code 11). This 633 nigel 75 makes "space" different to \es, which does not include VT (for Perl 634 nigel 63 compatibility). 635 nigel 75 .P 636 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 637 5.8. Another Perl extension is negation, which is indicated by a ^ character 638 after the colon. For example, 639 nigel 75 .sp 640 nigel 63 [12[:^digit:]] 641 nigel 75 .sp 642 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX 643 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not 644 supported, and an error is given if they are encountered. 645 nigel 75 .P 646 In UTF-8 mode, characters with values greater than 128 do not match any of 647 nigel 63 the POSIX character classes. 648 nigel 75 . 649 . 650 .SH "VERTICAL BAR" 651 nigel 63 .rs 652 .sp 653 Vertical bar characters are used to separate alternative patterns. For example, 654 the pattern 655 nigel 75 .sp 656 nigel 63 gilbert|sullivan 657 nigel 75 .sp 658 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear, 659 and an empty alternative is permitted (matching the empty string). 660 The matching process tries each alternative in turn, from left to right, 661 and the first one that succeeds is used. If the alternatives are within a 662 nigel 75 subpattern 663 .\" HTML 664 .\" 665 (defined below), 666 .\" 667 "succeeds" means matching the rest of the main pattern as well as the 668 alternative in the subpattern. 669 . 670 . 671 .SH "INTERNAL OPTION SETTING" 672 nigel 63 .rs 673 .sp 674 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and 675 PCRE_EXTENDED options can be changed from within the pattern by a sequence of 676 Perl option letters enclosed between "(?" and ")". The option letters are 677 nigel 75 .sp 678 nigel 63 i for PCRE_CASELESS 679 m for PCRE_MULTILINE 680 s for PCRE_DOTALL 681 x for PCRE_EXTENDED 682 nigel 75 .sp 683 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to 684 unset these options by preceding the letter with a hyphen, and a combined 685 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and 686 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also 687 permitted. If a letter appears both before and after the hyphen, the option is 688 unset. 689 nigel 75 .P 690 nigel 63 When an option change occurs at top level (that is, not inside subpattern 691 parentheses), the change applies to the remainder of the pattern that follows. 692 If the change is placed right at the start of a pattern, PCRE extracts it into 693 the global options (and it will therefore show up in data extracted by the 694 nigel 75 \fBpcre_fullinfo()\fP function). 695 .P 696 nigel 63 An option change within a subpattern affects only that part of the current 697 pattern that follows it, so 698 nigel 75 .sp 699 nigel 63 (a(?i)b)c 700 nigel 75 .sp 701 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). 702 By this means, options can be made to have different settings in different 703 parts of the pattern. Any changes made in one alternative do carry on 704 into subsequent branches within the same subpattern. For example, 705 nigel 75 .sp 706 nigel 63 (a(?i)b|c) 707 nigel 75 .sp 708 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first 709 branch is abandoned before the option setting. This is because the effects of 710 option settings happen at compile time. There would be some very weird 711 behaviour otherwise. 712 nigel 75 .P 713 nigel 63 The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the 714 same way as the Perl-compatible options by using the characters U and X 715 respectively. The (?X) flag setting is special in that it must always occur 716 earlier in the pattern than any of the additional features it turns on, even 717 nigel 75 when it is at top level. It is best to put it at the start. 718 . 719 . 720 .\" HTML 721 nigel 63 .SH SUBPATTERNS 722 .rs 723 .sp 724 Subpatterns are delimited by parentheses (round brackets), which can be nested. 725 nigel 75 Turning part of a pattern into a subpattern does two things: 726 .sp 727 nigel 63 1. It localizes a set of alternatives. For example, the pattern 728 nigel 75 .sp 729 nigel 63 cat(aract|erpillar|) 730 nigel 75 .sp 731 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the 732 parentheses, it would match "cataract", "erpillar" or the empty string. 733 nigel 75 .sp 734 2. It sets up the subpattern as a capturing subpattern. This means that, when 735 the whole pattern matches, that portion of the subject string that matched the 736 subpattern is passed back to the caller via the \fIovector\fP argument of 737 \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting 738 from 1) to obtain numbers for the capturing subpatterns. 739 .P 740 nigel 63 For example, if the string "the red king" is matched against the pattern 741 nigel 75 .sp 742 nigel 63 the ((red|white) (king|queen)) 743 nigel 75 .sp 744 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1, 745 2, and 3, respectively. 746 nigel 75 .P 747 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful. 748 There are often times when a grouping subpattern is required without a 749 capturing requirement. If an opening parenthesis is followed by a question mark 750 and a colon, the subpattern does not do any capturing, and is not counted when 751 computing the number of any subsequent capturing subpatterns. For example, if 752 the string "the white queen" is matched against the pattern 753 nigel 75 .sp 754 nigel 63 the ((?:red|white) (king|queen)) 755 nigel 75 .sp 756 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and 757 2. The maximum number of capturing subpatterns is 65535, and the maximum depth 758 of nesting of all subpatterns, both capturing and non-capturing, is 200. 759 nigel 75 .P 760 nigel 63 As a convenient shorthand, if any option settings are required at the start of 761 a non-capturing subpattern, the option letters may appear between the "?" and 762 the ":". Thus the two patterns 763 nigel 75 .sp 764 nigel 63 (?i:saturday|sunday) 765 (?:(?i)saturday|sunday) 766 nigel 75 .sp 767 nigel 63 match exactly the same set of strings. Because alternative branches are tried 768 from left to right, and options are not reset until the end of the subpattern 769 is reached, an option setting in one branch does affect subsequent branches, so 770 the above patterns match "SUNDAY" as well as "Saturday". 771 nigel 75 . 772 . 773 .SH "NAMED SUBPATTERNS" 774 nigel 63 .rs 775 .sp 776 Identifying capturing parentheses by number is simple, but it can be very hard 777 to keep track of the numbers in complicated regular expressions. Furthermore, 778 nigel 75 if an expression is modified, the numbers may change. To help with this 779 nigel 63 difficulty, PCRE supports the naming of subpatterns, something that Perl does 780 not provide. The Python syntax (?P...) is used. Names consist of 781 alphanumeric characters and underscores, and must be unique within a pattern. 782 nigel 75 .P 783 nigel 63 Named capturing parentheses are still allocated numbers as well as names. The 784 PCRE API provides function calls for extracting the name-to-number translation 785 nigel 75 table from a compiled pattern. There is also a convenience function for 786 extracting a captured substring by name. For further details see the 787 nigel 63 .\" HREF 788 nigel 75 \fBpcreapi\fP 789 nigel 63 .\" 790 documentation. 791 nigel 75 . 792 . 793 nigel 63 .SH REPETITION 794 .rs 795 .sp 796 Repetition is specified by quantifiers, which can follow any of the following 797 items: 798 nigel 75 .sp 799 nigel 63 a literal data character 800 the . metacharacter 801 nigel 75 the \eC escape sequence 802 the \eX escape sequence (in UTF-8 mode with Unicode properties) 803 an escape such as \ed that matches a single character 804 nigel 63 a character class 805 a back reference (see next section) 806 a parenthesized subpattern (unless it is an assertion) 807 nigel 75 .sp 808 nigel 63 The general repetition quantifier specifies a minimum and maximum number of 809 permitted matches, by giving the two numbers in curly brackets (braces), 810 separated by a comma. The numbers must be less than 65536, and the first must 811 be less than or equal to the second. For example: 812 nigel 75 .sp 813 nigel 63 z{2,4} 814 nigel 75 .sp 815 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special 816 character. If the second number is omitted, but the comma is present, there is 817 no upper limit; if the second number and the comma are both omitted, the 818 quantifier specifies an exact number of required matches. Thus 819 nigel 75 .sp 820 nigel 63 [aeiou]{3,} 821 nigel 75 .sp 822 nigel 63 matches at least 3 successive vowels, but may match many more, while 823 nigel 75 .sp 824 \ed{8} 825 .sp 826 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position 827 where a quantifier is not allowed, or one that does not match the syntax of a 828 quantifier, is taken as a literal character. For example, {,6} is not a 829 quantifier, but a literal string of four characters. 830 nigel 75 .P 831 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual 832 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of 833 which is represented by a two-byte sequence. Similarly, when Unicode property 834 support is available, \eX{3} matches three Unicode extended sequences, each of 835 which may be several bytes long (and they may be of different lengths). 836 .P 837 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the 838 previous item and the quantifier were not present. 839 nigel 75 .P 840 nigel 63 For convenience (and historical compatibility) the three most common 841 quantifiers have single-character abbreviations: 842 nigel 75 .sp 843 nigel 63 * is equivalent to {0,} 844 + is equivalent to {1,} 845 ? is equivalent to {0,1} 846 nigel 75 .sp 847 nigel 63 It is possible to construct infinite loops by following a subpattern that can 848 match no characters with a quantifier that has no upper limit, for example: 849 nigel 75 .sp 850 nigel 63 (a?)* 851 nigel 75 .sp 852 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for 853 such patterns. However, because there are cases where this can be useful, such 854 patterns are now accepted, but if any repetition of the subpattern does in fact 855 match no characters, the loop is forcibly broken. 856 nigel 75 .P 857 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as 858 possible (up to the maximum number of permitted times), without causing the 859 rest of the pattern to fail. The classic example of where this gives problems 860 nigel 75 is in trying to match comments in C programs. These appear between /* and */ 861 and within the comment, individual * and / characters may appear. An attempt to 862 match C comments by applying the pattern 863 .sp 864 /\e*.*\e*/ 865 .sp 866 nigel 63 to the string 867 nigel 75 .sp 868 /* first comment */ not comment /* second comment */ 869 .sp 870 nigel 63 fails, because it matches the entire string owing to the greediness of the .* 871 item. 872 nigel 75 .P 873 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be 874 greedy, and instead matches the minimum number of times possible, so the 875 pattern 876 nigel 75 .sp 877 /\e*.*?\e*/ 878 .sp 879 nigel 63 does the right thing with the C comments. The meaning of the various 880 quantifiers is not otherwise changed, just the preferred number of matches. 881 Do not confuse this use of question mark with its use as a quantifier in its 882 own right. Because it has two uses, it can sometimes appear doubled, as in 883 nigel 75 .sp 884 \ed??\ed 885 .sp 886 nigel 63 which matches one digit by preference, but can match two if that is the only 887 way the rest of the pattern matches. 888 nigel 75 .P 889 nigel 63 If the PCRE_UNGREEDY option is set (an option which is not available in Perl), 890 the quantifiers are not greedy by default, but individual ones can be made 891 greedy by following them with a question mark. In other words, it inverts the 892 default behaviour. 893 nigel 75 .P 894 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that 895 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the 896 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum. 897 nigel 75 .P 898 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent 899 to Perl's /s) is set, thus allowing the . to match newlines, the pattern is 900 implicitly anchored, because whatever follows will be tried against every 901 character position in the subject string, so there is no point in retrying the 902 overall match at any position after the first. PCRE normally treats such a 903 nigel 75 pattern as though it were preceded by \eA. 904 .P 905 nigel 63 In cases where it is known that the subject string contains no newlines, it is 906 worth setting PCRE_DOTALL in order to obtain this optimization, or 907 alternatively using ^ to indicate anchoring explicitly. 908 nigel 75 .P 909 nigel 63 However, there is one situation where the optimization cannot be used. When .* 910 is inside capturing parentheses that are the subject of a backreference 911 elsewhere in the pattern, a match at the start may fail, and a later one 912 succeed. Consider, for example: 913 nigel 75 .sp 914 (.*)abc\e1 915 .sp 916 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For 917 this reason, such a pattern is not implicitly anchored. 918 nigel 75 .P 919 nigel 63 When a capturing subpattern is repeated, the value captured is the substring 920 that matched the final iteration. For example, after 921 nigel 75 .sp 922 (tweedle[dume]{3}\es*)+ 923 .sp 924 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is 925 "tweedledee". However, if there are nested capturing subpatterns, the 926 corresponding captured values may have been set in previous iterations. For 927 example, after 928 nigel 75 .sp 929 nigel 63 /(a|(b))+/ 930 nigel 75 .sp 931 nigel 63 matches "aba" the value of the second captured substring is "b". 932 nigel 75 . 933 . 934 .\" HTML 935 .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS" 936 nigel 63 .rs 937 .sp 938 With both maximizing and minimizing repetition, failure of what follows 939 normally causes the repeated item to be re-evaluated to see if a different 940 number of repeats allows the rest of the pattern to match. Sometimes it is 941 useful to prevent this, either to change the nature of the match, or to cause 942 it fail earlier than it otherwise might, when the author of the pattern knows 943 there is no point in carrying on. 944 nigel 75 .P 945 Consider, for example, the pattern \ed+foo when applied to the subject line 946 .sp 947 nigel 63 123456bar 948 nigel 75 .sp 949 nigel 63 After matching all 6 digits and then failing to match "foo", the normal 950 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+ 951 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping" 952 (a term taken from Jeffrey Friedl's book) provides the means for specifying 953 that once a subpattern has matched, it is not to be re-evaluated in this way. 954 nigel 75 .P 955 nigel 63 If we use atomic grouping for the previous example, the matcher would give up 956 immediately on failing to match "foo" the first time. The notation is a kind of 957 special parenthesis, starting with (?> as in this example: 958 nigel 75 .sp 959 (?>\ed+)foo 960 .sp 961 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once 962 it has matched, and a failure further into the pattern is prevented from 963 backtracking into it. Backtracking past it to previous items, however, works as 964 normal. 965 nigel 75 .P 966 nigel 63 An alternative description is that a subpattern of this type matches the string 967 of characters that an identical standalone pattern would match, if anchored at 968 the current point in the subject string. 969 nigel 75 .P 970 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as 971 the above example can be thought of as a maximizing repeat that must swallow 972 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the 973 nigel 63 number of digits they match in order to make the rest of the pattern match, 974 nigel 75 (?>\ed+) can only match an entire sequence of digits. 975 .P 976 nigel 63 Atomic groups in general can of course contain arbitrarily complicated 977 subpatterns, and can be nested. However, when the subpattern for an atomic 978 group is just a single repeated item, as in the example above, a simpler 979 notation, called a "possessive quantifier" can be used. This consists of an 980 additional + character following a quantifier. Using this notation, the 981 previous example can be rewritten as 982 nigel 75 .sp 983 \ed++foo 984 .sp 985 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY 986 option is ignored. They are a convenient notation for the simpler forms of 987 atomic group. However, there is no difference in the meaning or processing of a 988 possessive quantifier and the equivalent atomic group. 989 nigel 75 .P 990 nigel 63 The possessive quantifier syntax is an extension to the Perl syntax. It 991 originates in Sun's Java package. 992 nigel 75 .P 993 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself 994 be repeated an unlimited number of times, the use of an atomic group is the 995 only way to avoid some failing matches taking a very long time indeed. The 996 pattern 997 nigel 75 .sp 998 (\eD+|<\ed+>)*[!?] 999 .sp 1000 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or 1001 digits enclosed in <>, followed by either ! or ?. When it matches, it runs 1002 quickly. However, if it is applied to 1003 nigel 75 .sp 1004 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 1005 nigel 75 .sp 1006 nigel 63 it takes a long time before reporting failure. This is because the string can 1007 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a 1008 large number of ways, and all have to be tried. (The example uses [!?] rather 1009 than a single character at the end, because both PCRE and Perl have an 1010 optimization that allows for fast failure when a single character is used. They 1011 remember the last single character that is required for a match, and fail early 1012 if it is not present in the string.) If the pattern is changed so that it uses 1013 an atomic group, like this: 1014 .sp 1015 ((?>\eD+)|<\ed+>)*[!?] 1016 .sp 1017 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly. 1018 nigel 75 . 1019 . 1020 .\" HTML 1021 .SH "BACK REFERENCES" 1022 nigel 63 .rs 1023 .sp 1024 Outside a character class, a backslash followed by a digit greater than 0 (and 1025 possibly further digits) is a back reference to a capturing subpattern earlier 1026 (that is, to its left) in the pattern, provided there have been that many 1027 previous capturing left parentheses. 1028 nigel 75 .P 1029 nigel 63 However, if the decimal number following the backslash is less than 10, it is 1030 always taken as a back reference, and causes an error only if there are not 1031 that many capturing left parentheses in the entire pattern. In other words, the 1032 parentheses that are referenced need not be to the left of the reference for 1033 nigel 75 numbers less than 10. See the subsection entitled "Non-printing characters" 1034 .\" HTML 1035 .\" 1036 above 1037 .\" 1038 for further details of the handling of digits following a backslash. 1039 .P 1040 nigel 63 A back reference matches whatever actually matched the capturing subpattern in 1041 the current subject string, rather than anything matching the subpattern 1042 itself (see 1043 .\" HTML 1044 .\" 1045 "Subpatterns as subroutines" 1046 .\" 1047 below for a way of doing that). So the pattern 1048 nigel 75 .sp 1049 (sens|respons)e and \e1ibility 1050 .sp 1051 nigel 63 matches "sense and sensibility" and "response and responsibility", but not 1052 "sense and responsibility". If caseful matching is in force at the time of the 1053 back reference, the case of letters is relevant. For example, 1054 nigel 75 .sp 1055 ((?i)rah)\es+\e1 1056 .sp 1057 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original 1058 capturing subpattern is matched caselessly. 1059 nigel 75 .P 1060 nigel 63 Back references to named subpatterns use the Python syntax (?P=name). We could 1061 rewrite the above example as follows: 1062 nigel 75 .sp 1063 (?(?i)rah)\es+(?P=p1) 1064 .sp 1065 nigel 63 There may be more than one back reference to the same subpattern. If a 1066 subpattern has not actually been used in a particular match, any back 1067 references to it always fail. For example, the pattern 1068 nigel 75 .sp 1069 (a|(bc))\e2 1070 .sp 1071 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be 1072 many capturing parentheses in a pattern, all digits following the backslash are 1073 taken as part of a potential back reference number. If the pattern continues 1074 with a digit character, some delimiter must be used to terminate the back 1075 reference. If the PCRE_EXTENDED option is set, this can be whitespace. 1076 nigel 75 Otherwise an empty comment (see 1077 .\" HTML 1078 .\" 1079 "Comments" 1080 .\" 1081 below) can be used. 1082 .P 1083 nigel 63 A back reference that occurs inside the parentheses to which it refers fails 1084 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches. 1085 nigel 63 However, such references can be useful inside repeated subpatterns. For 1086 example, the pattern 1087 nigel 75 .sp 1088 (a|b\e1)+ 1089 .sp 1090 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of 1091 the subpattern, the back reference matches the character string corresponding 1092 to the previous iteration. In order for this to work, the pattern must be such 1093 that the first iteration does not need to match the back reference. This can be 1094 done using alternation, as in the example above, or by a quantifier with a 1095 minimum of zero. 1096 nigel 75 . 1097 . 1098 .\" HTML 1099 nigel 63 .SH ASSERTIONS 1100 .rs 1101 .sp 1102 An assertion is a test on the characters following or preceding the current 1103 matching point that does not actually consume any characters. The simple 1104 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and$ are described 1105 .\" HTML 1106 .\" 1107 above. 1108 .\" 1109 .P 1110 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds: 1111 those that look ahead of the current position in the subject string, and those 1112 nigel 75 that look behind it. An assertion subpattern is matched in the normal way, 1113 except that it does not cause the current matching position to be changed. 1114 .P 1115 Assertion subpatterns are not capturing subpatterns, and may not be repeated, 1116 because it makes no sense to assert the same thing several times. If any kind 1117 of assertion contains capturing subpatterns within it, these are counted for 1118 the purposes of numbering the capturing subpatterns in the whole pattern. 1119 However, substring capturing is carried out only for positive assertions, 1120 because it does not make sense for negative assertions. 1121 . 1122 . 1123 .SS "Lookahead assertions" 1124 .rs 1125 .sp 1126 Lookahead assertions start 1127 nigel 63 with (?= for positive assertions and (?! for negative assertions. For example, 1128 nigel 75 .sp 1129 \ew+(?=;) 1130 .sp 1131 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in 1132 the match, and 1133 nigel 75 .sp 1134 nigel 63 foo(?!bar) 1135 nigel 75 .sp 1136 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the 1137 apparently similar pattern 1138 nigel 75 .sp 1139 nigel 63 (?!foo)bar 1140 nigel 75 .sp 1141 nigel 63 does not find an occurrence of "bar" that is preceded by something other than 1142 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion 1143 (?!foo) is always true when the next three characters are "bar". A 1144 nigel 75 lookbehind assertion is needed to achieve the other effect. 1145 .P 1146 nigel 63 If you want to force a matching failure at some point in a pattern, the most 1147 convenient way to do it is with (?!) because an empty string always matches, so 1148 an assertion that requires there not to be an empty string must always fail. 1149 nigel 75 . 1150 . 1151 .\" HTML 1152 .SS "Lookbehind assertions" 1153 .rs 1154 .sp 1155 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?.*)(?<=abcd) 1212 nigel 75 .sp 1213 or, equivalently, using the possessive quantifier syntax, 1214 .sp 1215 nigel 63 ^.*+(?<=abcd) 1216 nigel 75 .sp 1217 nigel 63 there can be no backtracking for the .* item; it can match only the entire 1218 string. The subsequent lookbehind assertion does a single test on the last four 1219 characters. If it fails, the match fails immediately. For long strings, this 1220 approach makes a significant difference to the processing time. 1221 nigel 75 . 1222 . 1223 .SS "Using multiple assertions" 1224 .rs 1225 .sp 1226 nigel 63 Several assertions (of any sort) may occur in succession. For example, 1227 nigel 75 .sp 1228 (?<=\ed{3})(? 1312 nigel 63 .SH COMMENTS 1313 .rs 1314 .sp 1315 nigel 75 The sequence (?# marks the start of a comment that continues up to the next 1316 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters 1317 that make up a comment play no part in the pattern matching at all. 1318 nigel 75 .P 1319 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a 1320 character class introduces a comment that continues up to the next newline 1321 character in the pattern. 1322 nigel 75 . 1323 . 1324 .SH "RECURSIVE PATTERNS" 1325 nigel 63 .rs 1326 .sp 1327 Consider the problem of matching a string in parentheses, allowing for 1328 unlimited nested parentheses. Without the use of recursion, the best that can 1329 be done is to use a pattern that matches up to some fixed depth of nesting. It 1330 nigel 75 is not possible to handle an arbitrary nesting depth. Perl provides a facility 1331 that allows regular expressions to recurse (amongst other things). It does this 1332 by interpolating Perl code in the expression at run time, and the code can 1333 refer to the expression itself. A Perl pattern to solve the parentheses problem 1334 can be created like this: 1335 .sp 1336 $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x; 1337 .sp 1338 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers 1339 recursively to the pattern in which it appears. Obviously, PCRE cannot support 1340 the interpolation of Perl code. Instead, it supports some special syntax for 1341 recursion of the entire pattern, and also for individual subpattern recursion. 1342 nigel 75 .P 1343 nigel 63 The special item that consists of (? followed by a number greater than zero and 1344 a closing parenthesis is a recursive call of the subpattern of the given 1345 number, provided that it occurs inside that subpattern. (If not, it is a 1346 "subroutine" call, which is described in the next section.) The special item 1347 (?R) is a recursive call of the entire regular expression. 1348 nigel 75 .P 1349 nigel 63 For example, this PCRE pattern solves the nested parentheses problem (assume 1350 the PCRE_EXTENDED option is set so that white space is ignored): 1351 nigel 75 .sp 1352 \e( ( (?>[^()]+) | (?R) )* \e) 1353 .sp 1354 nigel 63 First it matches an opening parenthesis. Then it matches any number of 1355 substrings which can either be a sequence of non-parentheses, or a recursive 1356 match of the pattern itself (that is a correctly parenthesized substring). 1357 Finally there is a closing parenthesis. 1358 nigel 75 .P 1359 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire 1360 pattern, so instead you could use this: 1361 nigel 75 .sp 1362 ( \e( ( (?>[^()]+) | (?1) )* \e) ) 1363 .sp 1364 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to 1365 them instead of the whole pattern. In a larger pattern, keeping track of 1366 parenthesis numbers can be tricky. It may be more convenient to use named 1367 parentheses instead. For this, PCRE uses (?P>name), which is an extension to 1368 the Python syntax that PCRE uses for named parentheses (Perl does not provide 1369 named parentheses). We could rewrite the above example as follows: 1370 nigel 75 .sp 1371 (?P \e( ( (?>[^()]+) | (?P>pn) )* \e) ) 1372 .sp 1373 nigel 63 This particular example pattern contains nested unlimited repeats, and so the 1374 use of atomic grouping for matching strings of non-parentheses is important 1375 when applying the pattern to strings that do not match. For example, when this 1376 pattern is applied to 1377 nigel 75 .sp 1378 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 1379 nigel 75 .sp 1380 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used, 1381 the match runs for a very long time indeed because there are so many different 1382 ways the + and * repeats can carve up the subject, and all have to be tested 1383 before failure can be reported. 1384 nigel 75 .P 1385 nigel 63 At the end of a match, the values set for any capturing subpatterns are those 1386 from the outermost level of the recursion at which the subpattern value is set. 1387 If you want to obtain intermediate values, a callout function can be used (see 1388 nigel 75 the next section and the 1389 nigel 63 .\" HREF 1390 nigel 75 \fBpcrecallout\fP 1391 nigel 63 .\" 1392 documentation). If the pattern above is matched against 1393 nigel 75 .sp 1394 nigel 63 (ab(cd)ef) 1395 nigel 75 .sp 1396 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken 1397 on at the top level. If additional parentheses are added, giving 1398 nigel 75 .sp 1399 \e( ( ( (?>[^()]+) | (?R) )* ) \e) 1400 nigel 63 ^ ^ 1401 ^ ^ 1402 nigel 75 .sp 1403 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level 1404 parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE 1405 has to obtain extra memory to store data during a recursion, which it does by 1406 nigel 75 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no 1407 nigel 63 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. 1408 nigel 75 .P 1409 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion. 1410 Consider this pattern, which matches text in angle brackets, allowing for 1411 arbitrary nesting. Only digits are allowed in nested brackets (that is, when 1412 recursing), whereas any characters are permitted at the outer level. 1413 nigel 75 .sp 1414 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * > 1415 .sp 1416 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two 1417 different alternatives for the recursive and non-recursive cases. The (?R) item 1418 is the actual recursive call. 1419 nigel 75 . 1420 . 1421 nigel 63 .\" HTML 1422 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES" 1423 nigel 63 .rs 1424 .sp 1425 If the syntax for a recursive subpattern reference (either by number or by 1426 name) is used outside the parentheses to which it refers, it operates like a 1427 subroutine in a programming language. An earlier example pointed out that the 1428 pattern 1429 nigel 75 .sp 1430 (sens|respons)e and \e1ibility 1431 .sp 1432 nigel 63 matches "sense and sensibility" and "response and responsibility", but not 1433 "sense and responsibility". If instead the pattern 1434 nigel 75 .sp 1435 nigel 63 (sens|respons)e and (?1)ibility 1436 nigel 75 .sp 1437 nigel 63 is used, it does match "sense and responsibility" as well as the other two 1438 strings. Such references must, however, follow the subpattern to which they 1439 refer. 1440 nigel 75 . 1441 . 1442 nigel 63 .SH CALLOUTS 1443 .rs 1444 .sp 1445 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl 1446 code to be obeyed in the middle of matching a regular expression. This makes it 1447 possible, amongst other things, to extract different substrings that match the 1448 same pair of parentheses when there is a repetition. 1449 nigel 75 .P 1450 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl 1451 code. The feature is called "callout". The caller of PCRE provides an external 1452 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP. 1453 nigel 63 By default, this variable contains NULL, which disables all calling out. 1454 nigel 75 .P 1455 nigel 63 Within a regular expression, (?C) indicates the points at which the external 1456 function is to be called. If you want to identify different callout points, you 1457 can put a number less than 256 after the letter C. The default value is zero. 1458 For example, this pattern has two callout points: 1459 nigel 75 .sp 1460 nigel 63 (?C1)\dabc(?C2)def 1461 nigel 75 .sp 1462 If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are 1463 automatically installed before each item in the pattern. They are all numbered 1464 255. 1465 .P 1466 During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is 1467 nigel 63 set), the external function is called. It is provided with the number of the 1468 nigel 75 callout, the position in the pattern, and, optionally, one item of data 1469 originally supplied by the caller of \fBpcre_exec()\fP. The callout function 1470 may cause matching to proceed, to backtrack, or to fail altogether. A complete 1471 description of the interface to the callout function is given in the 1472 nigel 63 .\" HREF 1473 nigel 75 \fBpcrecallout\fP 1474 nigel 63 .\" 1475 documentation. 1476 nigel 75 .P 1477 nigel 63 .in 0 1478 nigel 77 Last updated: 28 February 2005 1479 nigel 63 .br 1480 nigel 77 Copyright (c) 1997-2005 University of Cambridge.