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

Update to Unicode 7.0.0 release

 1 ph10 1436 .TH PCREPATTERN 3 "08 January 2014" "PCRE 8.35" 2 nigel 63 .SH NAME 3 PCRE - Perl-compatible regular expressions 4 nigel 75 .SH "PCRE REGULAR EXPRESSION DETAILS" 5 nigel 63 .rs 6 .sp 7 ph10 208 The syntax and semantics of the regular expressions that are supported by PCRE 8 are described in detail below. There is a quick-reference syntax summary in the 9 .\" HREF 10 \fBpcresyntax\fP 11 .\" 12 ph10 333 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE 13 also supports some alternative regular expression syntax (which does not 14 conflict with the Perl syntax) in order to provide some compatibility with 15 regular expressions in Python, .NET, and Oniguruma. 16 .P 17 Perl's regular expressions are described in its own documentation, and 18 ph10 208 regular expressions in general are covered in a number of books, some of which 19 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", 20 published by O'Reilly, covers regular expressions in great detail. This 21 description of PCRE's regular expressions is intended as reference material. 22 nigel 75 .P 23 ph10 1314 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 ph10 1335 A number of options that can be passed to \fBpcre_compile()\fP can also be set 41 ph10 1314 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 nigel 75 The original operation of PCRE was on strings of one-byte characters. However, 52 chpe 1055 there is now also support for UTF-8 strings in the original library, an 53 ph10 1219 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 ph10 859 features, PCRE must be built to include appropriate support. When using UTF 56 chpe 1055 strings you must either call the compiling function with the PCRE_UTF8, 57 ph10 1219 PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of 58 chpe 1055 these special sequences: 59 ph10 412 .sp 60 (*UTF8) 61 ph10 903 (*UTF16) 62 chpe 1055 (*UTF32) 63 ph10 1221 (*UTF) 64 ph10 416 .sp 65 ph10 1219 (*UTF) is a generic sequence that can be used with any of the libraries. 66 ph10 859 Starting a pattern with such a sequence is equivalent to setting the relevant 67 ph10 1314 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 nigel 63 .\" HREF 70 ph10 678 \fBpcreunicode\fP 71 nigel 63 .\" 72 page. 73 nigel 75 .P 74 ph10 1335 Some applications that allow their users to supply patterns may wish to 75 ph10 1314 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 ph10 518 .sp 83 ph10 1395 Another special sequence that may appear at the start of a pattern is (*UCP). 84 ph10 535 This has the same effect as setting the PCRE_UCP option: it causes sequences 85 such as \ed and \ew to use Unicode properties to determine character types, 86 instead of recognizing only characters with codes less than 128 via a lookup 87 ph10 518 table. 88 nigel 93 . 89 . 90 ph10 1395 .SS "Disabling auto-possessification" 91 .rs 92 .sp 93 ph10 1404 If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting 94 ph10 1398 the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making 95 ph10 1395 quantifiers possessive when what follows cannot match the repeated item. For 96 example, by default a+b is treated as a++b. For more details, see the 97 .\" HREF 98 \fBpcreapi\fP 99 .\" 100 documentation. 101 . 102 . 103 ph10 1314 .SS "Disabling start-up optimizations" 104 ph10 1033 .rs 105 .sp 106 ph10 1314 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the 107 ph10 1395 PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables 108 several optimizations for quickly reaching "no match" results. For more 109 details, see the 110 .\" HREF 111 \fBpcreapi\fP 112 .\" 113 documentation. 114 ph10 1033 . 115 . 116 ph10 556 .\" HTML 117 ph10 1314 .SS "Newline conventions" 118 ph10 227 .rs 119 .sp 120 PCRE supports five different conventions for indicating line breaks in 121 strings: a single CR (carriage return) character, a single LF (linefeed) 122 character, the two-character sequence CRLF, any of the three preceding, or any 123 Unicode newline sequence. The 124 .\" HREF 125 \fBpcreapi\fP 126 .\" 127 page has 128 .\" HTML 129 .\" 130 further discussion 131 .\" 132 about newlines, and shows how to set the newline convention in the 133 \fIoptions\fP arguments for the compiling and matching functions. 134 .P 135 It is also possible to specify a newline convention by starting a pattern 136 string with one of the following five sequences: 137 .sp 138 (*CR) carriage return 139 (*LF) linefeed 140 (*CRLF) carriage return, followed by linefeed 141 (*ANYCRLF) any of the three above 142 (*ANY) all Unicode newline sequences 143 .sp 144 ph10 859 These override the default and the options given to the compiling function. For 145 example, on a Unix system where LF is the default newline sequence, the pattern 146 ph10 227 .sp 147 (*CR)a.b 148 .sp 149 changes the convention to CR. That pattern matches "a\enb" because LF is no 150 ph10 1314 longer a newline. If more than one of these settings is present, the last one 151 ph10 231 is used. 152 .P 153 ph10 1213 The newline convention affects where the circumflex and dollar assertions are 154 true. It also affects the interpretation of the dot metacharacter when 155 PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect 156 what the \eR escape sequence matches. By default, this is any Unicode newline 157 sequence, for Perl compatibility. However, this can be changed; see the 158 ph10 514 description of \eR in the section entitled 159 ph10 231 .\" HTML 160 .\" 161 "Newline sequences" 162 .\" 163 ph10 247 below. A change of \eR setting can be combined with a change of newline 164 ph10 246 convention. 165 ph10 227 . 166 . 167 ph10 1314 .SS "Setting match and recursion limits" 168 .rs 169 .sp 170 ph10 1335 The caller of \fBpcre_exec()\fP can set a limit on the number of times the 171 internal \fBmatch()\fP function is called and on the maximum depth of 172 ph10 1314 recursive calls. These facilities are provided to catch runaway matches that 173 are provoked by patterns with huge matching trees (a typical example is a 174 pattern with nested unlimited repeats) and to avoid running out of system stack 175 by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP 176 ph10 1335 gives an error return. The limits can also be set by items at the start of the 177 ph10 1314 pattern of the form 178 .sp 179 (*LIMIT_MATCH=d) 180 (*LIMIT_RECURSION=d) 181 .sp 182 ph10 1335 where d is any number of decimal digits. However, the value of the setting must 183 ph10 1391 be less than the value set (or defaulted) by the caller of \fBpcre_exec()\fP 184 for it to have any effect. In other words, the pattern writer can lower the 185 limits set by the programmer, but not raise them. If there is more than one 186 setting of one of these limits, the lower value is used. 187 ph10 1314 . 188 . 189 .SH "EBCDIC CHARACTER CODES" 190 .rs 191 .sp 192 PCRE can be compiled to run in an environment that uses EBCDIC as its character 193 code rather than ASCII or Unicode (typically a mainframe system). In the 194 sections below, character code values are ASCII or Unicode; in an EBCDIC 195 environment these characters may have different code values, and there are no 196 code points greater than 255. 197 . 198 . 199 nigel 93 .SH "CHARACTERS AND METACHARACTERS" 200 .rs 201 .sp 202 nigel 63 A regular expression is a pattern that is matched against a subject string from 203 left to right. Most characters stand for themselves in a pattern, and match the 204 corresponding characters in the subject. As a trivial example, the pattern 205 nigel 75 .sp 206 nigel 63 The quick brown fox 207 nigel 75 .sp 208 nigel 77 matches a portion of a subject string that is identical to itself. When 209 caseless matching is specified (the PCRE_CASELESS option), letters are matched 210 ph10 859 independently of case. In a UTF mode, PCRE always understands the concept of 211 nigel 77 case for characters whose values are less than 128, so caseless matching is 212 always possible. For characters with higher values, the concept of case is 213 supported if PCRE is compiled with Unicode property support, but not otherwise. 214 If you want to use caseless matching for characters 128 and above, you must 215 ensure that PCRE is compiled with Unicode property support as well as with 216 ph10 859 UTF support. 217 nigel 77 .P 218 The power of regular expressions comes from the ability to include alternatives 219 and repetitions in the pattern. These are encoded in the pattern by the use of 220 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are 221 nigel 63 interpreted in some special way. 222 nigel 75 .P 223 There are two different sets of metacharacters: those that are recognized 224 nigel 63 anywhere in the pattern except within square brackets, and those that are 225 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters 226 are as follows: 227 nigel 75 .sp 228 \e general escape character with several uses 229 nigel 63 ^ assert start of string (or line, in multiline mode) 230 $assert end of string (or line, in multiline mode) 231 . match any character except newline (by default) 232 [ start character class definition 233 | start of alternative branch 234 ( start subpattern 235 ) end subpattern 236 ? extends the meaning of ( 237 also 0 or 1 quantifier 238 also quantifier minimizer 239 * 0 or more quantifier 240 + 1 or more quantifier 241 also "possessive quantifier" 242 { start min/max quantifier 243 nigel 75 .sp 244 nigel 63 Part of a pattern that is in square brackets is called a "character class". In 245 nigel 75 a character class the only metacharacters are: 246 .sp 247 \e general escape character 248 nigel 63 ^ negate the class, but only if the first character 249 - indicates character range 250 nigel 75 .\" JOIN 251 nigel 63 [ POSIX character class (only if followed by POSIX 252 syntax) 253 ] terminates the character class 254 nigel 75 .sp 255 The following sections describe the use of each of the metacharacters. 256 . 257 nigel 93 . 258 nigel 63 .SH BACKSLASH 259 .rs 260 .sp 261 The backslash character has several uses. Firstly, if it is followed by a 262 ph10 574 character that is not a number or a letter, it takes away any special meaning 263 that character may have. This use of backslash as an escape character applies 264 ph10 579 both inside and outside character classes. 265 nigel 75 .P 266 For example, if you want to match a * character, you write \e* in the pattern. 267 nigel 63 This escaping action applies whether or not the following character would 268 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a 269 non-alphanumeric with backslash to specify that it stands for itself. In 270 particular, if you want to match a backslash, you write \e\e. 271 .P 272 ph10 859 In a UTF mode, only ASCII numbers and letters have any special meaning after a 273 ph10 579 backslash. All other characters (in particular, those whose codepoints are 274 ph10 574 greater than 127) are treated as literals. 275 .P 276 ph10 1396 If a pattern is compiled with the PCRE_EXTENDED option, most white space in the 277 pattern (other than in a character class), and characters between a # outside a 278 character class and the next newline, inclusive, are ignored. An escaping 279 backslash can be used to include a white space or # character as part of the 280 pattern. 281 nigel 75 .P 282 nigel 63 If you want to remove the special meaning from a sequence of characters, you 283 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in 284 that$ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in 285 nigel 63 Perl, $and @ cause variable interpolation. Note the following examples: 286 nigel 75 .sp 287 nigel 63 Pattern PCRE matches Perl matches 288 nigel 75 .sp 289 .\" JOIN 290 \eQabc$xyz\eE abc$xyz abc followed by the 291 nigel 63 contents of$xyz 292 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz 293 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz 294 .sp 295 The \eQ...\eE sequence is recognized both inside and outside character classes. 296 ph10 654 An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed 297 by \eE later in the pattern, the literal interpretation continues to the end of 298 ph10 607 the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside 299 a character class, this causes an error, because the character class is not 300 terminated. 301 nigel 75 . 302 . 303 .\" HTML 304 .SS "Non-printing characters" 305 .rs 306 .sp 307 nigel 63 A second use of backslash provides a way of encoding non-printing characters 308 in patterns in a visible manner. There is no restriction on the appearance of 309 non-printing characters, apart from the binary zero that terminates a pattern, 310 ph10 456 but when a pattern is being prepared by text editing, it is often easier to use 311 one of the following escape sequences than the binary character it represents: 312 nigel 75 .sp 313 \ea alarm, that is, the BEL character (hex 07) 314 ph10 574 \ecx "control-x", where x is any ASCII character 315 nigel 75 \ee escape (hex 1B) 316 ph10 968 \ef form feed (hex 0C) 317 ph10 227 \en linefeed (hex 0A) 318 nigel 75 \er carriage return (hex 0D) 319 \et tab (hex 09) 320 ph10 1404 \e0dd character with octal code 0dd 321 ph10 488 \eddd character with octal code ddd, or back reference 322 ph10 1404 \eo{ddd..} character with octal code ddd.. 323 nigel 75 \exhh character with hex code hh 324 ph10 745 \ex{hhh..} character with hex code hhh.. (non-JavaScript mode) 325 ph10 836 \euhhhh character with hex code hhhh (JavaScript mode only) 326 nigel 75 .sp 327 ph10 1001 The precise effect of \ecx on ASCII characters is as follows: if x is a lower 328 case letter, it is converted to upper case. Then bit 6 of the character (hex 329 40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A), 330 but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the 331 data item (byte or 16-bit value) following \ec has a value greater than 127, a 332 compile-time error occurs. This locks out non-ASCII characters in all modes. 333 nigel 75 .P 334 ph10 1001 The \ec facility was designed for use with ASCII characters, but with the 335 extension to Unicode it is even less useful than it once was. It is, however, 336 recognized when PCRE is compiled in EBCDIC mode, where data items are always 337 bytes. In this mode, all values are valid after \ec. If the next character is a 338 lower case letter, it is converted to upper case. Then the 0xc0 bits of the 339 byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because 340 ph10 1221 the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other 341 ph10 1001 characters also generate different values. 342 .P 343 nigel 91 After \e0 up to two further octal digits are read. If there are fewer than two 344 digits, just those that are present are used. Thus the sequence \e0\ex\e07 345 specifies two binary zeros followed by a BEL character (code value 7). Make 346 sure you supply two digits after the initial zero if the pattern character that 347 follows is itself an octal digit. 348 nigel 75 .P 349 ph10 1404 The escape \eo must be followed by a sequence of octal digits, enclosed in 350 ph10 1370 braces. An error occurs if this is not the case. This escape is a recent 351 addition to Perl; it provides way of specifying character code points as octal 352 numbers greater than 0777, and it also allows octal numbers and back references 353 to be unambiguously specified. 354 .P 355 For greater clarity and unambiguity, it is best to avoid following \e by a 356 digit greater than zero. Instead, use \eo{} or \ex{} to specify character 357 numbers, and \eg{} to specify back references. The following paragraphs 358 describe the old, ambiguous syntax. 359 .P 360 ph10 1369 The handling of a backslash followed by a digit other than 0 is complicated, 361 and Perl has changed in recent releases, causing PCRE also to change. Outside a 362 character class, PCRE reads the digit and any following digits as a decimal 363 number. If the number is less than 8, or if there have been at least that many 364 nigel 63 previous capturing left parentheses in the expression, the entire sequence is 365 nigel 75 taken as a \fIback reference\fP. A description of how this works is given 366 .\" HTML 367 .\" 368 later, 369 .\" 370 following the discussion of 371 .\" HTML 372 .\" 373 parenthesized subpatterns. 374 .\" 375 .P 376 ph10 1369 Inside a character class, or if the decimal number following \e is greater than 377 7 and there have not been that many capturing subpatterns, PCRE handles \e8 and 378 \e9 as the literal characters "8" and "9", and otherwise re-reads up to three 379 octal digits following the backslash, using them to generate a data character. 380 ph10 1370 Any subsequent digits stand for themselves. For example: 381 nigel 75 .sp 382 ph10 1033 \e040 is another way of writing an ASCII space 383 nigel 75 .\" JOIN 384 \e40 is the same, provided there are fewer than 40 385 nigel 63 previous capturing subpatterns 386 nigel 75 \e7 is always a back reference 387 .\" JOIN 388 \e11 might be a back reference, or another way of 389 nigel 63 writing a tab 390 nigel 75 \e011 is always a tab 391 \e0113 is a tab followed by the character "3" 392 .\" JOIN 393 \e113 might be a back reference, otherwise the 394 nigel 63 character with octal code 113 395 nigel 75 .\" JOIN 396 \e377 might be a back reference, otherwise 397 ph10 859 the value 255 (decimal) 398 nigel 75 .\" JOIN 399 ph10 1369 \e81 is either a back reference, or the two 400 characters "8" and "1" 401 nigel 75 .sp 402 ph10 1370 Note that octal values of 100 or greater that are specified using this syntax 403 must not be introduced by a leading zero, because no more than three octal 404 digits are ever read. 405 nigel 75 .P 406 ph10 1370 By default, after \ex that is not followed by {, from zero to two hexadecimal 407 digits are read (letters can be in upper or lower case). Any number of 408 hexadecimal digits may appear between \ex{ and }. If a character other than 409 a hexadecimal digit appears between \ex{ and }, or if there is no terminating 410 }, an error occurs. 411 .P 412 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is 413 as just described only when it is followed by two hexadecimal digits. 414 Otherwise, it matches a literal "x" character. In JavaScript mode, support for 415 code points greater than 256 is provided by \eu, which must be followed by 416 four hexadecimal digits; otherwise it matches a literal "u" character. 417 .P 418 Characters whose value is less than 256 can be defined by either of the two 419 syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the 420 way they are handled. For example, \exdc is exactly the same as \ex{dc} (or 421 \eu00dc in JavaScript mode). 422 . 423 . 424 .SS "Constraints on character values" 425 .rs 426 .sp 427 Characters that are specified using octal or hexadecimal numbers are 428 limited to certain values, as follows: 429 .sp 430 8-bit non-UTF mode less than 0x100 431 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint 432 16-bit non-UTF mode less than 0x10000 433 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint 434 chpe 1373 32-bit non-UTF mode less than 0x100000000 435 ph10 1370 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint 436 .sp 437 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called 438 ph10 1404 "surrogate" codepoints), and 0xffef. 439 ph10 1370 . 440 . 441 .SS "Escape sequences in character classes" 442 .rs 443 .sp 444 nigel 91 All the sequences that define a single character value can be used both inside 445 ph10 836 and outside character classes. In addition, inside a character class, \eb is 446 interpreted as the backspace character (hex 08). 447 .P 448 \eN is not allowed in a character class. \eB, \eR, and \eX are not special 449 inside a character class. Like other unrecognized escape sequences, they are 450 treated as the literal characters "B", "R", and "X" by default, but cause an 451 error if the PCRE_EXTRA option is set. Outside a character class, these 452 sequences have different meanings. 453 nigel 75 . 454 . 455 ph10 745 .SS "Unsupported escape sequences" 456 .rs 457 .sp 458 In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string 459 handler and used to modify the case of following characters. By default, PCRE 460 does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT 461 option is set, \eU matches a "U" character, and \eu can be used to define a 462 character by code point, as described in the previous section. 463 . 464 . 465 nigel 93 .SS "Absolute and relative back references" 466 .rs 467 .sp 468 ph10 208 The sequence \eg followed by an unsigned or a negative number, optionally 469 enclosed in braces, is an absolute or relative back reference. A named back 470 reference can be coded as \eg{name}. Back references are discussed 471 nigel 93 .\" HTML 472 .\" 473 later, 474 .\" 475 following the discussion of 476 .\" HTML 477 .\" 478 parenthesized subpatterns. 479 .\" 480 . 481 . 482 ph10 333 .SS "Absolute and relative subroutine calls" 483 .rs 484 .sp 485 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or 486 a number enclosed either in angle brackets or single quotes, is an alternative 487 syntax for referencing a subpattern as a "subroutine". Details are discussed 488 ph10 333 .\" HTML 489 .\" 490 later. 491 .\" 492 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP 493 ph10 461 synonymous. The former is a back reference; the latter is a 494 ph10 454 .\" HTML 495 .\" 496 subroutine 497 .\" 498 call. 499 ph10 333 . 500 . 501 ph10 518 .\" HTML 502 nigel 75 .SS "Generic character types" 503 .rs 504 .sp 505 ph10 514 Another use of backslash is for specifying generic character types: 506 nigel 75 .sp 507 ph10 182 \ed any decimal digit 508 nigel 75 \eD any character that is not a decimal digit 509 ph10 968 \eh any horizontal white space character 510 \eH any character that is not a horizontal white space character 511 \es any white space character 512 \eS any character that is not a white space character 513 \ev any vertical white space character 514 \eV any character that is not a vertical white space character 515 nigel 75 \ew any "word" character 516 \eW any "non-word" character 517 .sp 518 ph10 535 There is also the single sequence \eN, which matches a non-newline character. 519 This is the same as 520 ph10 514 .\" HTML 521 .\" 522 ph10 535 the "." metacharacter 523 ph10 514 .\" 524 ph10 836 when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name; 525 ph10 745 PCRE does not support this. 526 nigel 75 .P 527 ph10 514 Each pair of lower and upper case escape sequences partitions the complete set 528 of characters into two disjoint sets. Any given character matches one, and only 529 ph10 518 one, of each pair. The sequences can appear both inside and outside character 530 nigel 75 classes. They each match one character of the appropriate type. If the current 531 ph10 518 matching point is at the end of the subject string, all of them fail, because 532 nigel 75 there is no character to match. 533 .P 534 ph10 1364 For compatibility with Perl, \es did not used to match the VT character (code 535 11), which made it different from the the POSIX "space" class. However, Perl 536 ph10 1401 added VT at release 5.18, and PCRE followed suit at release 8.34. The default 537 \es characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space 538 ph10 1404 (32), which are defined as white space in the "C" locale. This list may vary if 539 ph10 1405 locale-specific matching is taking place. For example, in some locales the 540 ph10 1412 "non-breaking space" character (\exA0) is recognized as white space, and in 541 ph10 1405 others the VT character is not. 542 nigel 75 .P 543 ph10 518 A "word" character is an underscore or any character that is a letter or digit. 544 By default, the definition of letters and digits is controlled by PCRE's 545 low-valued character tables, and may vary if locale-specific matching is taking 546 place (see 547 .\" HTML 548 .\" 549 "Locale support" 550 .\" 551 in the 552 .\" HREF 553 \fBpcreapi\fP 554 .\" 555 page). For example, in a French locale such as "fr_FR" in Unix-like systems, 556 ph10 1401 or "french" in Windows, some character codes greater than 127 are used for 557 ph10 518 accented letters, and these are then matched by \ew. The use of locales with 558 Unicode is discouraged. 559 ph10 178 .P 560 ph10 1401 By default, characters whose code points are greater than 127 never match \ed, 561 \es, or \ew, and always match \eD, \eS, and \eW, although this may vary for 562 characters in the range 128-255 when locale-specific matching is happening. 563 These escape sequences retain their original meanings from before Unicode 564 support was available, mainly for efficiency reasons. If PCRE is compiled with 565 Unicode property support, and the PCRE_UCP option is set, the behaviour is 566 changed so that Unicode properties are used to determine character types, as 567 follows: 568 ph10 518 .sp 569 ph10 1376 \ed any character that matches \ep{Nd} (decimal digit) 570 \es any character that matches \ep{Z} or \eh or \ev 571 \ew any character that matches \ep{L} or \ep{N}, plus underscore 572 ph10 518 .sp 573 The upper case escapes match the inverse sets of characters. Note that \ed 574 ph10 535 matches only decimal digits, whereas \ew matches any Unicode digit, as well as 575 ph10 518 any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and 576 \eB because they are defined in terms of \ew and \eW. Matching these sequences 577 is noticeably slower when PCRE_UCP is set. 578 .P 579 ph10 579 The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at 580 ph10 572 release 5.10. In contrast to the other sequences, which match only ASCII 581 ph10 1401 characters by default, these always match certain high-valued code points, 582 ph10 859 whether or not PCRE_UCP is set. The horizontal space characters are: 583 ph10 178 .sp 584 ph10 1033 U+0009 Horizontal tab (HT) 585 ph10 178 U+0020 Space 586 U+00A0 Non-break space 587 U+1680 Ogham space mark 588 U+180E Mongolian vowel separator 589 U+2000 En quad 590 U+2001 Em quad 591 U+2002 En space 592 U+2003 Em space 593 U+2004 Three-per-em space 594 U+2005 Four-per-em space 595 U+2006 Six-per-em space 596 U+2007 Figure space 597 U+2008 Punctuation space 598 U+2009 Thin space 599 U+200A Hair space 600 U+202F Narrow no-break space 601 U+205F Medium mathematical space 602 U+3000 Ideographic space 603 .sp 604 The vertical space characters are: 605 .sp 606 ph10 1033 U+000A Linefeed (LF) 607 U+000B Vertical tab (VT) 608 U+000C Form feed (FF) 609 U+000D Carriage return (CR) 610 U+0085 Next line (NEL) 611 ph10 178 U+2028 Line separator 612 U+2029 Paragraph separator 613 ph10 859 .sp 614 ph10 903 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are 615 relevant. 616 nigel 75 . 617 . 618 ph10 231 .\" HTML 619 nigel 93 .SS "Newline sequences" 620 .rs 621 .sp 622 ph10 231 Outside a character class, by default, the escape sequence \eR matches any 623 ph10 859 Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the 624 following: 625 nigel 93 .sp 626 (?>\er\en|\en|\ex0b|\ef|\er|\ex85) 627 .sp 628 This is an example of an "atomic group", details of which are given 629 .\" HTML 630 .\" 631 below. 632 .\" 633 This particular group matches either the two-character sequence CR followed by 634 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, 635 ph10 968 U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next 636 nigel 93 line, U+0085). The two-character sequence is treated as a single unit that 637 cannot be split. 638 .P 639 ph10 859 In other modes, two additional characters whose codepoints are greater than 255 640 nigel 93 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). 641 Unicode character property support is not needed for these characters to be 642 recognized. 643 .P 644 ph10 231 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the 645 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF 646 ph10 247 either at compile time or when the pattern is matched. (BSR is an abbrevation 647 ph10 246 for "backslash R".) This can be made the default when PCRE is built; if this is 648 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option. 649 It is also possible to specify these settings by starting a pattern string with 650 one of the following sequences: 651 ph10 231 .sp 652 (*BSR_ANYCRLF) CR, LF, or CRLF only 653 (*BSR_UNICODE) any Unicode newline sequence 654 .sp 655 ph10 859 These override the default and the options given to the compiling function, but 656 they can themselves be overridden by options given to a matching function. Note 657 that these special settings, which are not Perl-compatible, are recognized only 658 at the very start of a pattern, and that they must be in upper case. If more 659 than one of them is present, the last one is used. They can be combined with a 660 change of newline convention; for example, a pattern can start with: 661 ph10 246 .sp 662 (*ANY)(*BSR_ANYCRLF) 663 .sp 664 ph10 1219 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or 665 (*UCP) special sequences. Inside a character class, \eR is treated as an 666 unrecognized escape sequence, and so matches the letter "R" by default, but 667 causes an error if PCRE_EXTRA is set. 668 nigel 93 . 669 . 670 nigel 75 .\" HTML 671 .SS Unicode character properties 672 .rs 673 .sp 674 When PCRE is built with Unicode character property support, three additional 675 ph10 184 escape sequences that match characters with specific properties are available. 676 ph10 859 When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing 677 ph10 184 characters whose codepoints are less than 256, but they do work in this mode. 678 The extra escape sequences are: 679 nigel 75 .sp 680 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property 681 \eP{\fIxx\fP} a character without the \fIxx\fP property 682 ph10 1011 \eX a Unicode extended grapheme cluster 683 nigel 75 .sp 684 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode 685 ph10 517 script names, the general category properties, "Any", which matches any 686 character (including newline), and some special PCRE properties (described 687 ph10 535 in the 688 ph10 517 .\" HTML 689 .\" 690 ph10 535 next section). 691 ph10 517 .\" 692 Other Perl properties such as "InMusicalSymbols" are not currently supported by 693 PCRE. Note that \eP{Any} does not match any characters, so always causes a 694 match failure. 695 nigel 75 .P 696 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A 697 character from one of these sets can be matched using a script name. For 698 example: 699 nigel 75 .sp 700 nigel 87 \ep{Greek} 701 \eP{Han} 702 .sp 703 Those that are not part of an identified script are lumped together as 704 "Common". The current list of scripts is: 705 .P 706 Arabic, 707 Armenian, 708 ph10 491 Avestan, 709 nigel 93 Balinese, 710 ph10 491 Bamum, 711 chpe 1490 Bassa_Vah, 712 ph10 942 Batak, 713 nigel 87 Bengali, 714 Bopomofo, 715 ph10 942 Brahmi, 716 nigel 87 Braille, 717 Buginese, 718 Buhid, 719 Canadian_Aboriginal, 720 ph10 491 Carian, 721 chpe 1490 Caucasian_Albanian, 722 ph10 942 Chakma, 723 ph10 491 Cham, 724 nigel 87 Cherokee, 725 Common, 726 Coptic, 727 nigel 93 Cuneiform, 728 nigel 87 Cypriot, 729 Cyrillic, 730 Deseret, 731 Devanagari, 732 chpe 1490 Duployan, 733 ph10 491 Egyptian_Hieroglyphs, 734 chpe 1490 Elbasan, 735 nigel 87 Ethiopic, 736 Georgian, 737 Glagolitic, 738 Gothic, 739 chpe 1490 Grantha, 740 nigel 87 Greek, 741 Gujarati, 742 Gurmukhi, 743 Han, 744 Hangul, 745 Hanunoo, 746 Hebrew, 747 Hiragana, 748 ph10 491 Imperial_Aramaic, 749 nigel 87 Inherited, 750 ph10 491 Inscriptional_Pahlavi, 751 Inscriptional_Parthian, 752 Javanese, 753 Kaithi, 754 nigel 87 Kannada, 755 Katakana, 756 ph10 491 Kayah_Li, 757 nigel 87 Kharoshthi, 758 Khmer, 759 chpe 1490 Khojki, 760 Khudawadi, 761 nigel 87 Lao, 762 Latin, 763 ph10 491 Lepcha, 764 nigel 87 Limbu, 765 chpe 1490 Linear_A, 766 nigel 87 Linear_B, 767 ph10 491 Lisu, 768 Lycian, 769 Lydian, 770 chpe 1490 Mahajani, 771 nigel 87 Malayalam, 772 ph10 942 Mandaic, 773 chpe 1490 Manichaean, 774 ph10 491 Meetei_Mayek, 775 chpe 1490 Mende_Kikakui, 776 ph10 942 Meroitic_Cursive, 777 Meroitic_Hieroglyphs, 778 Miao, 779 chpe 1490 Modi, 780 nigel 87 Mongolian, 781 chpe 1490 Mro, 782 nigel 87 Myanmar, 783 chpe 1490 Nabataean, 784 nigel 87 New_Tai_Lue, 785 nigel 93 Nko, 786 nigel 87 Ogham, 787 chpe 1490 Ol_Chiki, 788 nigel 87 Old_Italic, 789 chpe 1490 Old_North_Arabian, 790 Old_Permic, 791 nigel 87 Old_Persian, 792 ph10 491 Old_South_Arabian, 793 Old_Turkic, 794 nigel 87 Oriya, 795 Osmanya, 796 chpe 1490 Pahawh_Hmong, 797 Palmyrene, 798 Pau_Cin_Hau, 799 nigel 93 Phags_Pa, 800 Phoenician, 801 chpe 1490 Psalter_Pahlavi, 802 ph10 491 Rejang, 803 nigel 87 Runic, 804 ph10 491 Samaritan, 805 Saurashtra, 806 ph10 942 Sharada, 807 nigel 87 Shavian, 808 chpe 1490 Siddham, 809 nigel 87 Sinhala, 810 ph10 942 Sora_Sompeng, 811 ph10 491 Sundanese, 812 nigel 87 Syloti_Nagri, 813 Syriac, 814 Tagalog, 815 Tagbanwa, 816 Tai_Le, 817 ph10 491 Tai_Tham, 818 Tai_Viet, 819 ph10 942 Takri, 820 nigel 87 Tamil, 821 Telugu, 822 Thaana, 823 Thai, 824 Tibetan, 825 Tifinagh, 826 chpe 1490 Tirhuta, 827 nigel 87 Ugaritic, 828 ph10 491 Vai, 829 chpe 1490 Warang_Citi, 830 nigel 87 Yi. 831 .P 832 ph10 517 Each character has exactly one Unicode general category property, specified by 833 a two-letter abbreviation. For compatibility with Perl, negation can be 834 specified by including a circumflex between the opening brace and the property 835 name. For example, \ep{^Lu} is the same as \eP{Lu}. 836 nigel 87 .P 837 If only one letter is specified with \ep or \eP, it includes all the general 838 category properties that start with that letter. In this case, in the absence 839 of negation, the curly brackets in the escape sequence are optional; these two 840 examples have the same effect: 841 .sp 842 nigel 75 \ep{L} 843 \epL 844 .sp 845 nigel 87 The following general category property codes are supported: 846 nigel 75 .sp 847 C Other 848 Cc Control 849 Cf Format 850 Cn Unassigned 851 Co Private use 852 Cs Surrogate 853 .sp 854 L Letter 855 Ll Lower case letter 856 Lm Modifier letter 857 Lo Other letter 858 Lt Title case letter 859 Lu Upper case letter 860 .sp 861 M Mark 862 Mc Spacing mark 863 Me Enclosing mark 864 Mn Non-spacing mark 865 .sp 866 N Number 867 Nd Decimal number 868 Nl Letter number 869 No Other number 870 .sp 871 P Punctuation 872 Pc Connector punctuation 873 Pd Dash punctuation 874 Pe Close punctuation 875 Pf Final punctuation 876 Pi Initial punctuation 877 Po Other punctuation 878 Ps Open punctuation 879 .sp 880 S Symbol 881 Sc Currency symbol 882 Sk Modifier symbol 883 Sm Mathematical symbol 884 So Other symbol 885 .sp 886 Z Separator 887 Zl Line separator 888 Zp Paragraph separator 889 Zs Space separator 890 .sp 891 nigel 87 The special property L& is also supported: it matches a character that has 892 the Lu, Ll, or Lt property, in other words, a letter that is not classified as 893 a modifier or "other". 894 nigel 75 .P 895 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to 896 ph10 859 U+DFFF. Such characters are not valid in Unicode strings and so 897 cannot be tested by PCRE, unless UTF validity checking has been turned off 898 chpe 1055 (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and 899 PCRE_NO_UTF32_CHECK in the 900 ph10 211 .\" HREF 901 \fBpcreapi\fP 902 .\" 903 ph10 451 page). Perl does not support the Cs property. 904 ph10 211 .P 905 ph10 451 The long synonyms for property names that Perl supports (such as \ep{Letter}) 906 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these 907 nigel 87 properties with "Is". 908 .P 909 No character that is in the Unicode table has the Cn (unassigned) property. 910 Instead, this property is assumed for any code point that is not in the 911 Unicode table. 912 .P 913 nigel 75 Specifying caseless matching does not affect these escape sequences. For 914 ph10 1335 example, \ep{Lu} always matches only upper case letters. This is different from 915 ph10 1258 the behaviour of current versions of Perl. 916 nigel 75 .P 917 ph10 1011 Matching characters by Unicode property is not fast, because PCRE has to do a 918 multistage table lookup in order to find a character's property. That is why 919 the traditional escape sequences such as \ed and \ew do not use Unicode 920 properties in PCRE by default, though you can make them do so by setting the 921 PCRE_UCP option or by starting the pattern with (*UCP). 922 . 923 . 924 .SS Extended grapheme clusters 925 .rs 926 nigel 75 .sp 927 ph10 1011 The \eX escape matches any number of Unicode characters that form an "extended 928 grapheme cluster", and treats the sequence as an atomic group 929 nigel 75 .\" HTML 930 .\" 931 (see below). 932 .\" 933 ph10 1011 Up to and including release 8.31, PCRE matched an earlier, simpler definition 934 that was equivalent to 935 .sp 936 (?>\ePM\epM*) 937 .sp 938 That is, it matched a character without the "mark" property, followed by zero 939 or more characters with the "mark" property. Characters with the "mark" 940 property are typically non-spacing accents that affect the preceding character. 941 nigel 75 .P 942 ph10 1011 This simple definition was extended in Unicode to include more complicated 943 kinds of composite character by giving each character a grapheme breaking 944 property, and creating rules that use these properties to define the boundaries 945 of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches 946 one of these clusters. 947 ph10 628 .P 948 ph10 1221 \eX always matches at least one character. Then it decides whether to add 949 ph10 1011 additional characters according to the following rules for ending a cluster: 950 .P 951 1. End at the end of the subject string. 952 .P 953 2. Do not end between CR and LF; otherwise end after any control character. 954 .P 955 ph10 1221 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters 956 ph10 1011 are of five types: L, V, T, LV, and LVT. An L character may be followed by an 957 ph10 1221 L, V, LV, or LVT character; an LV or V character may be followed by a V or T 958 ph10 1011 character; an LVT or T character may be follwed only by a T character. 959 .P 960 4. Do not end before extending characters or spacing marks. Characters with 961 the "mark" property always have the "extend" grapheme breaking property. 962 .P 963 5. Do not end after prepend characters. 964 .P 965 6. Otherwise, end the cluster. 966 nigel 75 . 967 . 968 ph10 517 .\" HTML 969 .SS PCRE's additional properties 970 .rs 971 .sp 972 ph10 1011 As well as the standard Unicode properties described above, PCRE supports four 973 more that make it possible to convert traditional escape sequences such as \ew 974 ph10 1387 and \es to use Unicode properties. PCRE uses these non-standard, non-Perl 975 properties internally when PCRE_UCP is set. However, they may also be used 976 explicitly. These properties are: 977 ph10 517 .sp 978 Xan Any alphanumeric character 979 Xps Any POSIX space character 980 Xsp Any Perl space character 981 Xwd Any Perl "word" character 982 .sp 983 ph10 535 Xan matches characters that have either the L (letter) or the N (number) 984 ph10 968 property. Xps matches the characters tab, linefeed, vertical tab, form feed, or 985 ph10 517 carriage return, and any other character that has the Z (separator) property. 986 ph10 1387 Xsp is the same as Xps; it used to exclude vertical tab, for Perl 987 compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd 988 matches the same characters as Xan, plus underscore. 989 ph10 1260 .P 990 ph10 1335 There is another non-standard property, Xuc, which matches any character that 991 can be represented by a Universal Character Name in C++ and other programming 992 languages. These are the characters $, @,  (grave accent), and all characters 993 with Unicode code points greater than or equal to U+00A0, except for the 994 surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are 995 ph10 1260 excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH 996 ph10 1335 where H is a hexadecimal digit. Note that the Xuc property does not match these 997 ph10 1260 sequences but the characters that they represent.) 998 ph10 517 . 999 . 1000 ph10 168 .\" HTML 1001 .SS "Resetting the match start" 1002 .rs 1003 .sp 1004 ph10 572 The escape sequence \eK causes any previously matched characters not to be 1005 included in the final matched sequence. For example, the pattern: 1006 ph10 168 .sp 1007 foo\eKbar 1008 .sp 1009 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is 1010 ph10 168 similar to a lookbehind assertion 1011 .\" HTML 1012 .\" 1013 (described below). 1014 .\" 1015 ph10 172 However, in this case, the part of the subject before the real match does not 1016 have to be of fixed length, as lookbehind assertions do. The use of \eK does 1017 ph10 168 not interfere with the setting of 1018 .\" HTML 1019 .\" 1020 captured substrings. 1021 ph10 172 .\" 1022 ph10 168 For example, when the pattern 1023 .sp 1024 (foo)\eKbar 1025 .sp 1026 ph10 172 matches "foobar", the first substring is still set to "foo". 1027 ph10 500 .P 1028 ph10 507 Perl documents that the use of \eK within assertions is "not well defined". In 1029 PCRE, \eK is acted upon when it occurs inside positive assertions, but is 1030 ph10 1418 ignored in negative assertions. Note that when a pattern such as (?=ab\eK) 1031 ph10 1459 matches, the reported start of the match can be greater than the end of the 1032 ph10 1418 match. 1033 ph10 168 . 1034 . 1035 nigel 75 .\" HTML 1036 .SS "Simple assertions" 1037 .rs 1038 .sp 1039 nigel 93 The final use of backslash is for certain simple assertions. An assertion 1040 nigel 63 specifies a condition that has to be met at a particular point in a match, 1041 without consuming any characters from the subject string. The use of 1042 nigel 75 subpatterns for more complicated assertions is described 1043 .\" HTML 1044 .\" 1045 below. 1046 .\" 1047 nigel 91 The backslashed assertions are: 1048 nigel 75 .sp 1049 \eb matches at a word boundary 1050 \eB matches when not at a word boundary 1051 nigel 93 \eA matches at the start of the subject 1052 \eZ matches at the end of the subject 1053 also matches before a newline at the end of the subject 1054 \ez matches only at the end of the subject 1055 \eG matches at the first matching position in the subject 1056 nigel 75 .sp 1057 ph10 513 Inside a character class, \eb has a different meaning; it matches the backspace 1058 ph10 535 character. If any other of these assertions appears in a character class, by 1059 ph10 513 default it matches the corresponding literal character (for example, \eB 1060 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid 1061 escape sequence" error is generated instead. 1062 nigel 75 .P 1063 nigel 63 A word boundary is a position in the subject string where the current character 1064 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches 1065 \ew and the other matches \eW), or the start or end of the string if the 1066 ph10 859 first or last character matches \ew, respectively. In a UTF mode, the meanings 1067 ph10 518 of \ew and \eW can be changed by setting the PCRE_UCP option. When this is 1068 done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start 1069 of word" or "end of word" metasequence. However, whatever follows \eb normally 1070 determines which it is. For example, the fragment \eba matches "a" at the start 1071 of a word. 1072 nigel 75 .P 1073 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and 1074 dollar (described in the next section) in that they only ever match at the very 1075 start and end of the subject string, whatever options are set. Thus, they are 1076 independent of multiline mode. These three assertions are not affected by the 1077 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the 1078 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP 1079 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start 1080 at a point other than the beginning of the subject, \eA can never match. The 1081 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end 1082 of the string as well as at the very end, whereas \ez matches only at the end. 1083 nigel 75 .P 1084 The \eG assertion is true only when the current matching position is at the 1085 start point of the match, as specified by the \fIstartoffset\fP argument of 1086 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is 1087 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate 1088 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of 1089 nigel 75 implementation where \eG can be useful. 1090 .P 1091 Note, however, that PCRE's interpretation of \eG, as the start of the current 1092 nigel 63 match, is subtly different from Perl's, which defines it as the end of the 1093 previous match. In Perl, these can be different when the previously matched 1094 string was empty. Because PCRE does just one match at a time, it cannot 1095 reproduce this behaviour. 1096 nigel 75 .P 1097 If all the alternatives of a pattern begin with \eG, the expression is anchored 1098 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled 1099 regular expression. 1100 nigel 75 . 1101 . 1102 .SH "CIRCUMFLEX AND DOLLAR" 1103 nigel 63 .rs 1104 .sp 1105 ph10 1221 The circumflex and dollar metacharacters are zero-width assertions. That is, 1106 they test for a particular condition being true without consuming any 1107 ph10 1213 characters from the subject string. 1108 .P 1109 nigel 63 Outside a character class, in the default matching mode, the circumflex 1110 ph10 1213 character is an assertion that is true only if the current matching point is at 1111 the start of the subject string. If the \fIstartoffset\fP argument of 1112 nigel 75 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE 1113 nigel 63 option is unset. Inside a character class, circumflex has an entirely different 1114 nigel 75 meaning 1115 .\" HTML 1116 .\" 1117 (see below). 1118 .\" 1119 .P 1120 nigel 63 Circumflex need not be the first character of the pattern if a number of 1121 alternatives are involved, but it should be the first thing in each alternative 1122 in which it appears if the pattern is ever to match that branch. If all 1123 possible alternatives start with a circumflex, that is, if the pattern is 1124 constrained to match only at the start of the subject, it is said to be an 1125 "anchored" pattern. (There are also other constructs that can cause a pattern 1126 to be anchored.) 1127 nigel 75 .P 1128 ph10 1213 The dollar character is an assertion that is true only if the current matching 1129 point is at the end of the subject string, or immediately before a newline at 1130 the end of the string (by default). Note, however, that it does not actually 1131 match the newline. Dollar need not be the last character of the pattern if a 1132 number of alternatives are involved, but it should be the last item in any 1133 branch in which it appears. Dollar has no special meaning in a character class. 1134 nigel 75 .P 1135 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of 1136 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This 1137 nigel 75 does not affect the \eZ assertion. 1138 .P 1139 nigel 63 The meanings of the circumflex and dollar characters are changed if the 1140 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches 1141 immediately after internal newlines as well as at the start of the subject 1142 string. It does not match after a newline that ends the string. A dollar 1143 matches before any newlines in the string, as well as at the very end, when 1144 PCRE_MULTILINE is set. When newline is specified as the two-character 1145 sequence CRLF, isolated CR and LF characters do not indicate newlines. 1146 nigel 75 .P 1147 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where 1148 \en represents a newline) in multiline mode, but not otherwise. Consequently, 1149 patterns that are anchored in single line mode because all branches start with 1150 ^ are not anchored in multiline mode, and a match for circumflex is possible 1151 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The 1152 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. 1153 .P 1154 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and 1155 nigel 63 end of the subject in both modes, and if all branches of a pattern start with 1156 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set. 1157 nigel 75 . 1158 . 1159 ph10 514 .\" HTML 1160 .SH "FULL STOP (PERIOD, DOT) AND \eN" 1161 nigel 63 .rs 1162 .sp 1163 Outside a character class, a dot in the pattern matches any one character in 1164 nigel 91 the subject string except (by default) a character that signifies the end of a 1165 ph10 903 line. 1166 nigel 91 .P 1167 nigel 93 When a line ending is defined as a single character, dot never matches that 1168 character; when the two-character sequence CRLF is used, dot does not match CR 1169 if it is immediately followed by LF, but otherwise it matches all characters 1170 (including isolated CRs and LFs). When any Unicode line endings are being 1171 recognized, dot does not match CR or LF or any of the other line ending 1172 characters. 1173 .P 1174 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL 1175 nigel 93 option is set, a dot matches any one character, without exception. If the 1176 two-character sequence CRLF is present in the subject string, it takes two dots 1177 to match it. 1178 nigel 91 .P 1179 The handling of dot is entirely independent of the handling of circumflex and 1180 dollar, the only relationship being that they both involve newlines. Dot has no 1181 special meaning in a character class. 1182 ph10 514 .P 1183 ph10 579 The escape sequence \eN behaves like a dot, except that it is not affected by 1184 ph10 572 the PCRE_DOTALL option. In other words, it matches any character except one 1185 ph10 745 that signifies the end of a line. Perl also uses \eN to match characters by 1186 name; PCRE does not support this. 1187 nigel 75 . 1188 . 1189 ph10 859 .SH "MATCHING A SINGLE DATA UNIT" 1190 nigel 63 .rs 1191 .sp 1192 ph10 859 Outside a character class, the escape sequence \eC matches any one data unit, 1193 whether or not a UTF mode is set. In the 8-bit library, one data unit is one 1194 chpe 1055 byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is 1195 a 32-bit unit. Unlike a dot, \eC always 1196 ph10 859 matches line-ending characters. The feature is provided in Perl in order to 1197 match individual bytes in UTF-8 mode, but it is unclear how it can usefully be 1198 used. Because \eC breaks up characters into individual data units, matching one 1199 unit with \eC in a UTF mode means that the rest of the string may start with a 1200 malformed UTF character. This has undefined results, because PCRE assumes that 1201 it is dealing with valid UTF strings (and by default it checks this at the 1202 chpe 1055 start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or 1203 PCRE_NO_UTF32_CHECK option is used). 1204 nigel 75 .P 1205 PCRE does not allow \eC to appear in lookbehind assertions 1206 .\" HTML 1207 .\" 1208 ph10 754 (described below) 1209 nigel 75 .\" 1210 ph10 859 in a UTF mode, because this would make it impossible to calculate the length of 1211 nigel 75 the lookbehind. 1212 ph10 737 .P 1213 ph10 859 In general, the \eC escape sequence is best avoided. However, one 1214 way of using it that avoids the problem of malformed UTF characters is to use a 1215 lookahead to check the length of the next character, as in this pattern, which 1216 could be used with a UTF-8 string (ignore white space and line breaks): 1217 ph10 737 .sp 1218 (?| (?=[\ex00-\ex7f])(\eC) | 1219 ph10 738 (?=[\ex80-\ex{7ff}])(\eC)(\eC) | 1220 (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) | 1221 ph10 737 (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC)) 1222 .sp 1223 ph10 738 A group that starts with (?| resets the capturing parentheses numbers in each 1224 alternative (see 1225 ph10 737 .\" HTML 1226 .\" 1227 "Duplicate Subpattern Numbers" 1228 .\" 1229 ph10 738 below). The assertions at the start of each branch check the next UTF-8 1230 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The 1231 ph10 737 character's individual bytes are then captured by the appropriate number of 1232 groups. 1233 nigel 75 . 1234 . 1235 .\" HTML 1236 .SH "SQUARE BRACKETS AND CHARACTER CLASSES" 1237 nigel 63 .rs 1238 .sp 1239 An opening square bracket introduces a character class, terminated by a closing 1240 ph10 461 square bracket. A closing square bracket on its own is not special by default. 1241 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square 1242 ph10 456 bracket causes a compile-time error. If a closing square bracket is required as 1243 a member of the class, it should be the first data character in the class 1244 (after an initial circumflex, if present) or escaped with a backslash. 1245 nigel 75 .P 1246 ph10 859 A character class matches a single character in the subject. In a UTF mode, the 1247 character may be more than one data unit long. A matched character must be in 1248 the set of characters defined by the class, unless the first character in the 1249 class definition is a circumflex, in which case the subject character must not 1250 be in the set defined by the class. If a circumflex is actually required as a 1251 member of the class, ensure it is not the first character, or escape it with a 1252 nigel 63 backslash. 1253 nigel 75 .P 1254 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while 1255 [^aeiou] matches any character that is not a lower case vowel. Note that a 1256 nigel 75 circumflex is just a convenient notation for specifying the characters that 1257 are in the class by enumerating those that are not. A class that starts with a 1258 ph10 456 circumflex is not an assertion; it still consumes a character from the subject 1259 nigel 75 string, and therefore it fails if the current pointer is at the end of the 1260 string. 1261 .P 1262 chpe 1055 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff) 1263 can be included in a class as a literal string of data units, or by using the 1264 \ex{ escaping mechanism. 1265 nigel 75 .P 1266 nigel 63 When caseless matching is set, any letters in a class represent both their 1267 upper case and lower case versions, so for example, a caseless [aeiou] matches 1268 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a 1269 ph10 859 caseful version would. In a UTF mode, PCRE always understands the concept of 1270 nigel 77 case for characters whose values are less than 128, so caseless matching is 1271 always possible. For characters with higher values, the concept of case is 1272 supported if PCRE is compiled with Unicode property support, but not otherwise. 1273 ph10 859 If you want to use caseless matching in a UTF mode for characters 128 and 1274 above, you must ensure that PCRE is compiled with Unicode property support as 1275 well as with UTF support. 1276 nigel 75 .P 1277 nigel 93 Characters that might indicate line breaks are never treated in any special way 1278 when matching character classes, whatever line-ending sequence is in use, and 1279 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class 1280 such as [^a] always matches one of these characters. 1281 nigel 75 .P 1282 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a 1283 character class. For example, [d-m] matches any letter between d and m, 1284 inclusive. If a minus character is required in a class, it must be escaped with 1285 a backslash or appear in a position where it cannot be interpreted as 1286 ph10 1404 indicating a range, typically as the first or last character in the class, or 1287 ph10 1392 immediately after a range. For example, [b-d-z] matches letters in the range b 1288 to d, a hyphen character, or z. 1289 nigel 75 .P 1290 nigel 63 It is not possible to have the literal character "]" as the end character of a 1291 range. A pattern such as [W-]46] is interpreted as a class of two characters 1292 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or 1293 "-46]". However, if the "]" is escaped with a backslash it is interpreted as 1294 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range 1295 followed by two other characters. The octal or hexadecimal representation of 1296 "]" can also be used to end a range. 1297 .P 1298 ph10 1392 An error is generated if a POSIX character class (see below) or an escape 1299 sequence other than one that defines a single character appears at a point 1300 where a range ending character is expected. For example, [z-\exff] is valid, 1301 but [A-\ed] and [A-[:digit:]] are not. 1302 .P 1303 nigel 63 Ranges operate in the collating sequence of character values. They can also be 1304 ph10 903 used for characters specified numerically, for example [\e000-\e037]. Ranges 1305 ph10 859 can include any characters that are valid for the current mode. 1306 nigel 75 .P 1307 nigel 63 If a range that includes letters is used when caseless matching is set, it 1308 matches the letters in either case. For example, [W-c] is equivalent to 1309 ph10 859 [][\e\e^_wxyzabc], matched caselessly, and in a non-UTF mode, if character 1310 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E 1311 ph10 859 characters in both cases. In UTF modes, PCRE supports the concept of case for 1312 nigel 75 characters with values greater than 128 only when it is compiled with Unicode 1313 property support. 1314 .P 1315 ph10 575 The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, 1316 \eV, \ew, and \eW may appear in a character class, and add the characters that 1317 they match to the class. For example, [\edABCDEF] matches any hexadecimal 1318 ph10 859 digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew 1319 ph10 575 and their upper case partners, just as it does when they appear outside a 1320 character class, as described in the section entitled 1321 .\" HTML 1322 .\" 1323 "Generic character types" 1324 .\" 1325 above. The escape sequence \eb has a different meaning inside a character 1326 class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX 1327 are not special inside a character class. Like any other unrecognized escape 1328 sequences, they are treated as the literal characters "B", "N", "R", and "X" by 1329 default, but cause an error if the PCRE_EXTRA option is set. 1330 .P 1331 A circumflex can conveniently be used with the upper case character types to 1332 ph10 518 specify a more restricted set of characters than the matching lower case type. 1333 ph10 575 For example, the class [^\eW_] matches any letter or digit, but not underscore, 1334 ph10 579 whereas [\ew] includes underscore. A positive character class should be read as 1335 "something OR something OR ..." and a negative class as "NOT something AND NOT 1336 ph10 575 something AND NOT ...". 1337 nigel 75 .P 1338 The only metacharacters that are recognized in character classes are backslash, 1339 hyphen (only where it can be interpreted as specifying a range), circumflex 1340 (only at the start), opening square bracket (only when it can be interpreted as 1341 ph10 1408 introducing a POSIX class name, or for a special compatibility feature - see 1342 the next two sections), and the terminating closing square bracket. However, 1343 escaping other non-alphanumeric characters does no harm. 1344 nigel 75 . 1345 . 1346 .SH "POSIX CHARACTER CLASSES" 1347 nigel 63 .rs 1348 .sp 1349 nigel 75 Perl supports the POSIX notation for character classes. This uses names 1350 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports 1351 this notation. For example, 1352 nigel 75 .sp 1353 nigel 63 [01[:alpha:]%] 1354 nigel 75 .sp 1355 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names 1356 ph10 518 are: 1357 nigel 75 .sp 1358 nigel 63 alnum letters and digits 1359 alpha letters 1360 ascii character codes 0 - 127 1361 blank space or tab only 1362 cntrl control characters 1363 nigel 75 digit decimal digits (same as \ed) 1364 nigel 63 graph printing characters, excluding space 1365 lower lower case letters 1366 print printing characters, including space 1367 ph10 518 punct printing characters, excluding letters and digits and space 1368 ph10 1376 space white space (the same as \es from PCRE 8.34) 1369 nigel 63 upper upper case letters 1370 nigel 75 word "word" characters (same as \ew) 1371 nigel 63 xdigit hexadecimal digits 1372 nigel 75 .sp 1373 ph10 1401 The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), 1374 ph10 1405 and space (32). If locale-specific matching is taking place, the list of space 1375 characters may be different; there may be fewer or more of them. "Space" used 1376 to be different to \es, which did not include VT, for Perl compatibility. 1377 However, Perl changed at release 5.18, and PCRE followed at release 8.34. 1378 "Space" and \es now match the same set of characters. 1379 nigel 75 .P 1380 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 1381 5.8. Another Perl extension is negation, which is indicated by a ^ character 1382 after the colon. For example, 1383 nigel 75 .sp 1384 nigel 63 [12[:^digit:]] 1385 nigel 75 .sp 1386 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX 1387 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not 1388 supported, and an error is given if they are encountered. 1389 nigel 75 .P 1390 ph10 1401 By default, characters with values greater than 128 do not match any of the 1391 POSIX character classes. However, if the PCRE_UCP option is passed to 1392 \fBpcre_compile()\fP, some of the classes are changed so that Unicode character 1393 properties are used. This is achieved by replacing certain POSIX classes by 1394 other sequences, as follows: 1395 ph10 518 .sp 1396 [:alnum:] becomes \ep{Xan} 1397 [:alpha:] becomes \ep{L} 1398 ph10 535 [:blank:] becomes \eh 1399 ph10 518 [:digit:] becomes \ep{Nd} 1400 [:lower:] becomes \ep{Ll} 1401 ph10 535 [:space:] becomes \ep{Xps} 1402 ph10 518 [:upper:] becomes \ep{Lu} 1403 [:word:] becomes \ep{Xwd} 1404 .sp 1405 ph10 1404 Negated versions, such as [:^alpha:] use \eP instead of \ep. Three other POSIX 1406 ph10 1387 classes are handled specially in UCP mode: 1407 .TP 10 1408 [:graph:] 1409 ph10 1404 This matches characters that have glyphs that mark the page when printed. In 1410 Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf 1411 ph10 1387 properties, except for: 1412 .sp 1413 U+061C Arabic Letter Mark 1414 ph10 1404 U+180E Mongolian Vowel Separator 1415 ph10 1387 U+2066 - U+2069 Various "isolate"s 1416 .sp 1417 .TP 10 1418 [:print:] 1419 ph10 1404 This matches the same characters as [:graph:] plus space characters that are 1420 ph10 1387 not controls, that is, characters with the Zs property. 1421 .TP 10 1422 [:punct:] 1423 This matches all characters that have the Unicode P (punctuation) property, 1424 plus those characters whose code points are less than 128 that have the S 1425 (Symbol) property. 1426 .P 1427 The other POSIX classes are unchanged, and match only characters with code 1428 points less than 128. 1429 nigel 75 . 1430 . 1431 ph10 1408 .SH "COMPATIBILITY FEATURE FOR WORD BOUNDARIES" 1432 .rs 1433 .sp 1434 ph10 1412 In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly 1435 syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of 1436 ph10 1408 word". PCRE treats these items as follows: 1437 .sp 1438 [[:<:]] is converted to \eb(?=\ew) 1439 [[:>:]] is converted to \eb(?<=\ew) 1440 .sp 1441 Only these exact character sequences are recognized. A sequence such as 1442 ph10 1412 [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is 1443 not compatible with Perl. It is provided to help migrations from other 1444 environments, and is best not used in any new patterns. Note that \eb matches 1445 at the start and the end of a word (see 1446 ph10 1408 .\" HTML 1447 .\" 1448 "Simple assertions" 1449 .\" 1450 ph10 1412 above), and in a Perl-style pattern the preceding or following character 1451 normally shows which is wanted, without the need for the assertions that are 1452 ph10 1408 used above in order to give exactly the POSIX behaviour. 1453 . 1454 . 1455 nigel 75 .SH "VERTICAL BAR" 1456 nigel 63 .rs 1457 .sp 1458 Vertical bar characters are used to separate alternative patterns. For example, 1459 the pattern 1460 nigel 75 .sp 1461 nigel 63 gilbert|sullivan 1462 nigel 75 .sp 1463 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear, 1464 nigel 91 and an empty alternative is permitted (matching the empty string). The matching 1465 process tries each alternative in turn, from left to right, and the first one 1466 that succeeds is used. If the alternatives are within a subpattern 1467 nigel 75 .\" HTML 1468 .\" 1469 (defined below), 1470 .\" 1471 "succeeds" means matching the rest of the main pattern as well as the 1472 alternative in the subpattern. 1473 . 1474 . 1475 .SH "INTERNAL OPTION SETTING" 1476 nigel 63 .rs 1477 .sp 1478 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and 1479 ph10 231 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within 1480 the pattern by a sequence of Perl option letters enclosed between "(?" and ")". 1481 The option letters are 1482 nigel 75 .sp 1483 nigel 63 i for PCRE_CASELESS 1484 m for PCRE_MULTILINE 1485 s for PCRE_DOTALL 1486 x for PCRE_EXTENDED 1487 nigel 75 .sp 1488 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to 1489 unset these options by preceding the letter with a hyphen, and a combined 1490 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and 1491 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also 1492 permitted. If a letter appears both before and after the hyphen, the option is 1493 unset. 1494 nigel 75 .P 1495 ph10 231 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be 1496 changed in the same way as the Perl-compatible options by using the characters 1497 J, U and X respectively. 1498 .P 1499 ph10 412 When one of these option changes occurs at top level (that is, not inside 1500 subpattern parentheses), the change applies to the remainder of the pattern 1501 that follows. If the change is placed right at the start of a pattern, PCRE 1502 extracts it into the global options (and it will therefore show up in data 1503 extracted by the \fBpcre_fullinfo()\fP function). 1504 nigel 75 .P 1505 nigel 93 An option change within a subpattern (see below for a description of 1506 ph10 572 subpatterns) affects only that part of the subpattern that follows it, so 1507 nigel 75 .sp 1508 nigel 63 (a(?i)b)c 1509 nigel 75 .sp 1510 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). 1511 By this means, options can be made to have different settings in different 1512 parts of the pattern. Any changes made in one alternative do carry on 1513 into subsequent branches within the same subpattern. For example, 1514 nigel 75 .sp 1515 nigel 63 (a(?i)b|c) 1516 nigel 75 .sp 1517 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first 1518 branch is abandoned before the option setting. This is because the effects of 1519 option settings happen at compile time. There would be some very weird 1520 behaviour otherwise. 1521 ph10 251 .P 1522 \fBNote:\fP There are other PCRE-specific options that can be set by the 1523 ph10 859 application when the compiling or matching functions are called. In some cases 1524 the pattern can contain special leading sequences such as (*CRLF) to override 1525 what the application has set or what has been defaulted. Details are given in 1526 the section entitled 1527 ph10 251 .\" HTML 1528 .\" 1529 "Newline sequences" 1530 .\" 1531 ph10 1219 above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading 1532 chpe 1055 sequences that can be used to set UTF and Unicode property modes; they are 1533 equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP 1534 ph10 1219 options, respectively. The (*UTF) sequence is a generic version that can be 1535 ph10 1335 used with any of the libraries. However, the application can set the 1536 ph10 1309 PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences. 1537 nigel 75 . 1538 . 1539 .\" HTML 1540 nigel 63 .SH SUBPATTERNS 1541 .rs 1542 .sp 1543 Subpatterns are delimited by parentheses (round brackets), which can be nested. 1544 nigel 75 Turning part of a pattern into a subpattern does two things: 1545 .sp 1546 nigel 63 1. It localizes a set of alternatives. For example, the pattern 1547 nigel 75 .sp 1548 nigel 63 cat(aract|erpillar|) 1549 nigel 75 .sp 1550 ph10 572 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would 1551 match "cataract", "erpillar" or an empty string. 1552 nigel 75 .sp 1553 2. It sets up the subpattern as a capturing subpattern. This means that, when 1554 the whole pattern matches, that portion of the subject string that matched the 1555 ph10 903 subpattern is passed back to the caller via the \fIovector\fP argument of the 1556 matching function. (This applies only to the traditional matching functions; 1557 ph10 859 the DFA matching functions do not support capturing.) 1558 .P 1559 Opening parentheses are counted from left to right (starting from 1) to obtain 1560 numbers for the capturing subpatterns. For example, if the string "the red 1561 king" is matched against the pattern 1562 nigel 75 .sp 1563 nigel 63 the ((red|white) (king|queen)) 1564 nigel 75 .sp 1565 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1, 1566 2, and 3, respectively. 1567 nigel 75 .P 1568 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful. 1569 There are often times when a grouping subpattern is required without a 1570 capturing requirement. If an opening parenthesis is followed by a question mark 1571 and a colon, the subpattern does not do any capturing, and is not counted when 1572 computing the number of any subsequent capturing subpatterns. For example, if 1573 the string "the white queen" is matched against the pattern 1574 nigel 75 .sp 1575 nigel 63 the ((?:red|white) (king|queen)) 1576 nigel 75 .sp 1577 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and 1578 nigel 93 2. The maximum number of capturing subpatterns is 65535. 1579 nigel 75 .P 1580 nigel 63 As a convenient shorthand, if any option settings are required at the start of 1581 a non-capturing subpattern, the option letters may appear between the "?" and 1582 the ":". Thus the two patterns 1583 nigel 75 .sp 1584 nigel 63 (?i:saturday|sunday) 1585 (?:(?i)saturday|sunday) 1586 nigel 75 .sp 1587 nigel 63 match exactly the same set of strings. Because alternative branches are tried 1588 from left to right, and options are not reset until the end of the subpattern 1589 is reached, an option setting in one branch does affect subsequent branches, so 1590 the above patterns match "SUNDAY" as well as "Saturday". 1591 nigel 75 . 1592 . 1593 ph10 456 .\" HTML 1594 ph10 175 .SH "DUPLICATE SUBPATTERN NUMBERS" 1595 .rs 1596 .sp 1597 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses 1598 the same numbers for its capturing parentheses. Such a subpattern starts with 1599 (?| and is itself a non-capturing subpattern. For example, consider this 1600 ph10 175 pattern: 1601 .sp 1602 (?|(Sat)ur|(Sun))day 1603 ph10 182 .sp 1604 Because the two alternatives are inside a (?| group, both sets of capturing 1605 parentheses are numbered one. Thus, when the pattern matches, you can look 1606 at captured substring number one, whichever alternative matched. This construct 1607 is useful when you want to capture part, but not all, of one of a number of 1608 alternatives. Inside a (?| group, parentheses are numbered as usual, but the 1609 ph10 175 number is reset at the start of each branch. The numbers of any capturing 1610 ph10 572 parentheses that follow the subpattern start after the highest number used in 1611 any branch. The following example is taken from the Perl documentation. The 1612 numbers underneath show in which buffer the captured content will be stored. 1613 ph10 175 .sp 1614 # before ---------------branch-reset----------- after 1615 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x 1616 # 1 2 2 3 2 3 4 1617 ph10 182 .sp 1618 ph10 488 A back reference to a numbered subpattern uses the most recent value that is 1619 set for that number by any subpattern. The following pattern matches "abcabc" 1620 or "defdef": 1621 ph10 456 .sp 1622 ph10 461 /(?|(abc)|(def))\e1/ 1623 ph10 456 .sp 1624 ph10 716 In contrast, a subroutine call to a numbered subpattern always refers to the 1625 first one in the pattern with the given number. The following pattern matches 1626 "abcabc" or "defabc": 1627 ph10 456 .sp 1628 /(?|(abc)|(def))(?1)/ 1629 .sp 1630 ph10 459 If a 1631 .\" HTML 1632 .\" 1633 condition test 1634 .\" 1635 for a subpattern's having matched refers to a non-unique number, the test is 1636 true if any of the subpatterns of that number have matched. 1637 .P 1638 An alternative approach to using this "branch reset" feature is to use 1639 ph10 175 duplicate named subpatterns, as described in the next section. 1640 . 1641 . 1642 nigel 75 .SH "NAMED SUBPATTERNS" 1643 nigel 63 .rs 1644 .sp 1645 Identifying capturing parentheses by number is simple, but it can be very hard 1646 to keep track of the numbers in complicated regular expressions. Furthermore, 1647 nigel 75 if an expression is modified, the numbers may change. To help with this 1648 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not 1649 added to Perl until release 5.10. Python had the feature earlier, and PCRE 1650 introduced it at release 4.0, using the Python syntax. PCRE now supports both 1651 ph10 459 the Perl and the Python syntax. Perl allows identically numbered subpatterns to 1652 have different names, but PCRE does not. 1653 nigel 93 .P 1654 In PCRE, a subpattern can be named in one of three ways: (?...) or 1655 (?'name'...) as in Perl, or (?P...) as in Python. References to capturing 1656 nigel 91 parentheses from other parts of the pattern, such as 1657 .\" HTML 1658 .\" 1659 ph10 488 back references, 1660 nigel 91 .\" 1661 .\" HTML 1662 .\" 1663 recursion, 1664 .\" 1665 and 1666 .\" HTML 1667 .\" 1668 conditions, 1669 .\" 1670 can be made by name as well as by number. 1671 nigel 75 .P 1672 ph10 1404 Names consist of up to 32 alphanumeric characters and underscores, but must 1673 ph10 1394 start with a non-digit. Named capturing parentheses are still allocated numbers 1674 as well as names, exactly as if the names were not present. The PCRE API 1675 provides function calls for extracting the name-to-number translation table 1676 from a compiled pattern. There is also a convenience function for extracting a 1677 captured substring by name. 1678 nigel 91 .P 1679 By default, a name must be unique within a pattern, but it is possible to relax 1680 ph10 457 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate 1681 ph10 461 names are also always permitted for subpatterns with the same number, set up as 1682 ph10 457 described in the previous section.) Duplicate names can be useful for patterns 1683 where only one instance of the named parentheses can match. Suppose you want to 1684 match the name of a weekday, either as a 3-letter abbreviation or as the full 1685 name, and in both cases you want to extract the abbreviation. This pattern 1686 (ignoring the line breaks) does the job: 1687 nigel 91 .sp 1688 nigel 93 (?Mon|Fri|Sun)(?:day)?| 1689 (?Tue)(?:sday)?| 1690 (?Wed)(?:nesday)?| 1691 (?Thu)(?:rsday)?| 1692 (?Sat)(?:urday)? 1693 nigel 91 .sp 1694 There are five capturing substrings, but only one is ever set after a match. 1695 ph10 182 (An alternative way of solving this problem is to use a "branch reset" 1696 ph10 175 subpattern, as described in the previous section.) 1697 .P 1698 nigel 91 The convenience function for extracting the data by name returns the substring 1699 nigel 93 for the first (and in this example, the only) subpattern of that name that 1700 ph10 461 matched. This saves searching to find which numbered subpattern it was. 1701 ph10 459 .P 1702 ph10 488 If you make a back reference to a non-unique named subpattern from elsewhere in 1703 ph10 1404 the pattern, the subpatterns to which the name refers are checked in the order 1704 in which they appear in the overall pattern. The first one that is set is used 1705 for the reference. For example, this pattern matches both "foofoo" and 1706 ph10 1361 "barbar" but not "foobar" or "barfoo": 1707 .sp 1708 ph10 1404 (?:(?foo)|(?bar))\ek 1709 ph10 1361 .sp 1710 .P 1711 If you make a subroutine call to a non-unique named subpattern, the one that 1712 corresponds to the first occurrence of the name is used. In the absence of 1713 duplicate numbers (see the previous section) this is the one with the lowest 1714 number. 1715 .P 1716 If you use a named reference in a condition 1717 ph10 459 test (see the 1718 .\" 1719 .\" HTML 1720 .\" 1721 section about conditions 1722 .\" 1723 ph10 461 below), either to check whether a subpattern has matched, or to check for 1724 ph10 459 recursion, all subpatterns with the same name are tested. If the condition is 1725 true for any one of them, the overall condition is true. This is the same 1726 behaviour as testing by number. For further details of the interfaces for 1727 handling named subpatterns, see the 1728 nigel 63 .\" HREF 1729 nigel 75 \fBpcreapi\fP 1730 nigel 63 .\" 1731 documentation. 1732 ph10 385 .P 1733 \fBWarning:\fP You cannot use different names to distinguish between two 1734 ph10 457 subpatterns with the same number because PCRE uses only the numbers when 1735 matching. For this reason, an error is given at compile time if different names 1736 ph10 1361 are given to subpatterns with the same number. However, you can always give the 1737 same name to subpatterns with the same number, even when PCRE_DUPNAMES is not 1738 set. 1739 nigel 75 . 1740 . 1741 nigel 63 .SH REPETITION 1742 .rs 1743 .sp 1744 Repetition is specified by quantifiers, which can follow any of the following 1745 items: 1746 nigel 75 .sp 1747 nigel 63 a literal data character 1748 nigel 93 the dot metacharacter 1749 nigel 75 the \eC escape sequence 1750 ph10 859 the \eX escape sequence 1751 nigel 93 the \eR escape sequence 1752 ph10 572 an escape such as \ed or \epL that matches a single character 1753 nigel 63 a character class 1754 a back reference (see next section) 1755 ph10 637 a parenthesized subpattern (including assertions) 1756 ph10 716 a subroutine call to a subpattern (recursive or otherwise) 1757 nigel 75 .sp 1758 nigel 63 The general repetition quantifier specifies a minimum and maximum number of 1759 permitted matches, by giving the two numbers in curly brackets (braces), 1760 separated by a comma. The numbers must be less than 65536, and the first must 1761 be less than or equal to the second. For example: 1762 nigel 75 .sp 1763 nigel 63 z{2,4} 1764 nigel 75 .sp 1765 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special 1766 character. If the second number is omitted, but the comma is present, there is 1767 no upper limit; if the second number and the comma are both omitted, the 1768 quantifier specifies an exact number of required matches. Thus 1769 nigel 75 .sp 1770 nigel 63 [aeiou]{3,} 1771 nigel 75 .sp 1772 nigel 63 matches at least 3 successive vowels, but may match many more, while 1773 nigel 75 .sp 1774 \ed{8} 1775 .sp 1776 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position 1777 where a quantifier is not allowed, or one that does not match the syntax of a 1778 quantifier, is taken as a literal character. For example, {,6} is not a 1779 quantifier, but a literal string of four characters. 1780 nigel 75 .P 1781 ph10 859 In UTF modes, quantifiers apply to characters rather than to individual data 1782 units. Thus, for example, \ex{100}{2} matches two characters, each of 1783 which is represented by a two-byte sequence in a UTF-8 string. Similarly, 1784 ph10 1011 \eX{3} matches three Unicode extended grapheme clusters, each of which may be 1785 several data units long (and they may be of different lengths). 1786 nigel 75 .P 1787 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the 1788 ph10 345 previous item and the quantifier were not present. This may be useful for 1789 subpatterns that are referenced as 1790 ph10 335 .\" HTML 1791 .\" 1792 subroutines 1793 .\" 1794 ph10 572 from elsewhere in the pattern (but see also the section entitled 1795 .\" HTML 1796 .\" 1797 "Defining subpatterns for use by reference only" 1798 .\" 1799 below). Items other than subpatterns that have a {0} quantifier are omitted 1800 from the compiled pattern. 1801 nigel 75 .P 1802 nigel 93 For convenience, the three most common quantifiers have single-character 1803 abbreviations: 1804 nigel 75 .sp 1805 nigel 63 * is equivalent to {0,} 1806 + is equivalent to {1,} 1807 ? is equivalent to {0,1} 1808 nigel 75 .sp 1809 nigel 63 It is possible to construct infinite loops by following a subpattern that can 1810 match no characters with a quantifier that has no upper limit, for example: 1811 nigel 75 .sp 1812 nigel 63 (a?)* 1813 nigel 75 .sp 1814 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for 1815 such patterns. However, because there are cases where this can be useful, such 1816 patterns are now accepted, but if any repetition of the subpattern does in fact 1817 match no characters, the loop is forcibly broken. 1818 nigel 75 .P 1819 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as 1820 possible (up to the maximum number of permitted times), without causing the 1821 rest of the pattern to fail. The classic example of where this gives problems 1822 nigel 75 is in trying to match comments in C programs. These appear between /* and */ 1823 and within the comment, individual * and / characters may appear. An attempt to 1824 match C comments by applying the pattern 1825 .sp 1826 /\e*.*\e*/ 1827 .sp 1828 nigel 63 to the string 1829 nigel 75 .sp 1830 /* first comment */ not comment /* second comment */ 1831 .sp 1832 nigel 63 fails, because it matches the entire string owing to the greediness of the .* 1833 item. 1834 nigel 75 .P 1835 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be 1836 greedy, and instead matches the minimum number of times possible, so the 1837 pattern 1838 nigel 75 .sp 1839 /\e*.*?\e*/ 1840 .sp 1841 nigel 63 does the right thing with the C comments. The meaning of the various 1842 quantifiers is not otherwise changed, just the preferred number of matches. 1843 Do not confuse this use of question mark with its use as a quantifier in its 1844 own right. Because it has two uses, it can sometimes appear doubled, as in 1845 nigel 75 .sp 1846 \ed??\ed 1847 .sp 1848 nigel 63 which matches one digit by preference, but can match two if that is the only 1849 way the rest of the pattern matches. 1850 nigel 75 .P 1851 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl), 1852 nigel 63 the quantifiers are not greedy by default, but individual ones can be made 1853 greedy by following them with a question mark. In other words, it inverts the 1854 default behaviour. 1855 nigel 75 .P 1856 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that 1857 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the 1858 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum. 1859 nigel 75 .P 1860 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent 1861 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is 1862 nigel 63 implicitly anchored, because whatever follows will be tried against every 1863 character position in the subject string, so there is no point in retrying the 1864 overall match at any position after the first. PCRE normally treats such a 1865 nigel 75 pattern as though it were preceded by \eA. 1866 .P 1867 nigel 63 In cases where it is known that the subject string contains no newlines, it is 1868 worth setting PCRE_DOTALL in order to obtain this optimization, or 1869 alternatively using ^ to indicate anchoring explicitly. 1870 nigel 75 .P 1871 ph10 994 However, there are some cases where the optimization cannot be used. When .* 1872 ph10 488 is inside capturing parentheses that are the subject of a back reference 1873 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one 1874 succeeds. Consider, for example: 1875 nigel 75 .sp 1876 (.*)abc\e1 1877 .sp 1878 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For 1879 this reason, such a pattern is not implicitly anchored. 1880 nigel 75 .P 1881 ph10 994 Another case where implicit anchoring is not applied is when the leading .* is 1882 inside an atomic group. Once again, a match at the start may fail where a later 1883 one succeeds. Consider this pattern: 1884 .sp 1885 (?>.*?a)b 1886 .sp 1887 ph10 1221 It matches "ab" in the subject "aab". The use of the backtracking control verbs 1888 ph10 994 (*PRUNE) and (*SKIP) also disable this optimization. 1889 .P 1890 nigel 63 When a capturing subpattern is repeated, the value captured is the substring 1891 that matched the final iteration. For example, after 1892 nigel 75 .sp 1893 (tweedle[dume]{3}\es*)+ 1894 .sp 1895 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is 1896 "tweedledee". However, if there are nested capturing subpatterns, the 1897 corresponding captured values may have been set in previous iterations. For 1898 example, after 1899 nigel 75 .sp 1900 nigel 63 /(a|(b))+/ 1901 nigel 75 .sp 1902 nigel 63 matches "aba" the value of the second captured substring is "b". 1903 nigel 75 . 1904 . 1905 .\" HTML 1906 .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS" 1907 nigel 63 .rs 1908 .sp 1909 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 1910 repetition, failure of what follows normally causes the repeated item to be 1911 re-evaluated to see if a different number of repeats allows the rest of the 1912 pattern to match. Sometimes it is useful to prevent this, either to change the 1913 nature of the match, or to cause it fail earlier than it otherwise might, when 1914 the author of the pattern knows there is no point in carrying on. 1915 nigel 75 .P 1916 Consider, for example, the pattern \ed+foo when applied to the subject line 1917 .sp 1918 nigel 63 123456bar 1919 nigel 75 .sp 1920 nigel 63 After matching all 6 digits and then failing to match "foo", the normal 1921 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+ 1922 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping" 1923 (a term taken from Jeffrey Friedl's book) provides the means for specifying 1924 that once a subpattern has matched, it is not to be re-evaluated in this way. 1925 nigel 75 .P 1926 nigel 93 If we use atomic grouping for the previous example, the matcher gives up 1927 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of 1928 special parenthesis, starting with (?> as in this example: 1929 nigel 75 .sp 1930 (?>\ed+)foo 1931 .sp 1932 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once 1933 it has matched, and a failure further into the pattern is prevented from 1934 backtracking into it. Backtracking past it to previous items, however, works as 1935 normal. 1936 nigel 75 .P 1937 nigel 63 An alternative description is that a subpattern of this type matches the string 1938 of characters that an identical standalone pattern would match, if anchored at 1939 the current point in the subject string. 1940 nigel 75 .P 1941 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as 1942 the above example can be thought of as a maximizing repeat that must swallow 1943 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the 1944 nigel 63 number of digits they match in order to make the rest of the pattern match, 1945 nigel 75 (?>\ed+) can only match an entire sequence of digits. 1946 .P 1947 nigel 63 Atomic groups in general can of course contain arbitrarily complicated 1948 subpatterns, and can be nested. However, when the subpattern for an atomic 1949 group is just a single repeated item, as in the example above, a simpler 1950 notation, called a "possessive quantifier" can be used. This consists of an 1951 additional + character following a quantifier. Using this notation, the 1952 previous example can be rewritten as 1953 nigel 75 .sp 1954 \ed++foo 1955 .sp 1956 ph10 208 Note that a possessive quantifier can be used with an entire group, for 1957 example: 1958 .sp 1959 (abc|xyz){2,3}+ 1960 .sp 1961 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY 1962 option is ignored. They are a convenient notation for the simpler forms of 1963 nigel 93 atomic group. However, there is no difference in the meaning of a possessive 1964 quantifier and the equivalent atomic group, though there may be a performance 1965 difference; possessive quantifiers should be slightly faster. 1966 nigel 75 .P 1967 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax. 1968 Jeffrey Friedl originated the idea (and the name) in the first edition of his 1969 book. Mike McCloskey liked it, so implemented it when he built Sun's Java 1970 package, and PCRE copied it from there. It ultimately found its way into Perl 1971 at release 5.10. 1972 nigel 75 .P 1973 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple 1974 pattern constructs. For example, the sequence A+B is treated as A++B because 1975 there is no point in backtracking into a sequence of A's when B must follow. 1976 .P 1977 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself 1978 be repeated an unlimited number of times, the use of an atomic group is the 1979 only way to avoid some failing matches taking a very long time indeed. The 1980 pattern 1981 nigel 75 .sp 1982 (\eD+|<\ed+>)*[!?] 1983 .sp 1984 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or 1985 digits enclosed in <>, followed by either ! or ?. When it matches, it runs 1986 quickly. However, if it is applied to 1987 nigel 75 .sp 1988 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 1989 nigel 75 .sp 1990 nigel 63 it takes a long time before reporting failure. This is because the string can 1991 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a 1992 large number of ways, and all have to be tried. (The example uses [!?] rather 1993 than a single character at the end, because both PCRE and Perl have an 1994 optimization that allows for fast failure when a single character is used. They 1995 remember the last single character that is required for a match, and fail early 1996 if it is not present in the string.) If the pattern is changed so that it uses 1997 an atomic group, like this: 1998 .sp 1999 ((?>\eD+)|<\ed+>)*[!?] 2000 .sp 2001 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly. 2002 nigel 75 . 2003 . 2004 .\" HTML 2005 .SH "BACK REFERENCES" 2006 nigel 63 .rs 2007 .sp 2008 Outside a character class, a backslash followed by a digit greater than 0 (and 2009 possibly further digits) is a back reference to a capturing subpattern earlier 2010 (that is, to its left) in the pattern, provided there have been that many 2011 previous capturing left parentheses. 2012 nigel 75 .P 2013 nigel 63 However, if the decimal number following the backslash is less than 10, it is 2014 always taken as a back reference, and causes an error only if there are not 2015 that many capturing left parentheses in the entire pattern. In other words, the 2016 parentheses that are referenced need not be to the left of the reference for 2017 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense 2018 when a repetition is involved and the subpattern to the right has participated 2019 in an earlier iteration. 2020 .P 2021 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern 2022 whose number is 10 or more using this syntax because a sequence such as \e50 is 2023 interpreted as a character defined in octal. See the subsection entitled 2024 nigel 91 "Non-printing characters" 2025 nigel 75 .\" HTML 2026 .\" 2027 above 2028 .\" 2029 nigel 93 for further details of the handling of digits following a backslash. There is 2030 no such problem when named parentheses are used. A back reference to any 2031 subpattern is possible using named parentheses (see below). 2032 nigel 75 .P 2033 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a 2034 ph10 572 backslash is to use the \eg escape sequence. This escape must be followed by an 2035 unsigned number or a negative number, optionally enclosed in braces. These 2036 examples are all identical: 2037 nigel 93 .sp 2038 (ring), \e1 2039 (ring), \eg1 2040 (ring), \eg{1} 2041 .sp 2042 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that 2043 is present in the older syntax. It is also useful when literal digits follow 2044 the reference. A negative number is a relative reference. Consider this 2045 example: 2046 nigel 93 .sp 2047 (abc(def)ghi)\eg{-1} 2048 .sp 2049 The sequence \eg{-1} is a reference to the most recently started capturing 2050 ph10 572 subpattern before \eg, that is, is it equivalent to \e2 in this example. 2051 Similarly, \eg{-2} would be equivalent to \e1. The use of relative references 2052 can be helpful in long patterns, and also in patterns that are created by 2053 joining together fragments that contain references within themselves. 2054 nigel 93 .P 2055 nigel 63 A back reference matches whatever actually matched the capturing subpattern in 2056 the current subject string, rather than anything matching the subpattern 2057 itself (see 2058 .\" HTML 2059 .\" 2060 "Subpatterns as subroutines" 2061 .\" 2062 below for a way of doing that). So the pattern 2063 nigel 75 .sp 2064 (sens|respons)e and \e1ibility 2065 .sp 2066 nigel 63 matches "sense and sensibility" and "response and responsibility", but not 2067 "sense and responsibility". If caseful matching is in force at the time of the 2068 back reference, the case of letters is relevant. For example, 2069 nigel 75 .sp 2070 ((?i)rah)\es+\e1 2071 .sp 2072 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original 2073 capturing subpattern is matched caselessly. 2074 nigel 75 .P 2075 ph10 171 There are several different ways of writing back references to named 2076 subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek or 2077 \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified 2078 back reference syntax, in which \eg can be used for both numeric and named 2079 references, is also supported. We could rewrite the above example in any of 2080 nigel 93 the following ways: 2081 nigel 75 .sp 2082 nigel 93 (?(?i)rah)\es+\ek 2083 ph10 171 (?'p1'(?i)rah)\es+\ek{p1} 2084 nigel 91 (?P(?i)rah)\es+(?P=p1) 2085 ph10 171 (?(?i)rah)\es+\eg{p1} 2086 nigel 75 .sp 2087 nigel 91 A subpattern that is referenced by name may appear in the pattern before or 2088 after the reference. 2089 .P 2090 nigel 63 There may be more than one back reference to the same subpattern. If a 2091 subpattern has not actually been used in a particular match, any back 2092 ph10 456 references to it always fail by default. For example, the pattern 2093 nigel 75 .sp 2094 (a|(bc))\e2 2095 .sp 2096 ph10 461 always fails if it starts to match "a" rather than "bc". However, if the 2097 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an 2098 ph10 456 unset value matches an empty string. 2099 .P 2100 Because there may be many capturing parentheses in a pattern, all digits 2101 following a backslash are taken as part of a potential back reference number. 2102 If the pattern continues with a digit character, some delimiter must be used to 2103 terminate the back reference. If the PCRE_EXTENDED option is set, this can be 2104 ph10 968 white space. Otherwise, the \eg{ syntax or an empty comment (see 2105 nigel 75 .\" HTML 2106 .\" 2107 "Comments" 2108 .\" 2109 below) can be used. 2110 ph10 488 . 2111 .SS "Recursive back references" 2112 .rs 2113 .sp 2114 nigel 63 A back reference that occurs inside the parentheses to which it refers fails 2115 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches. 2116 nigel 63 However, such references can be useful inside repeated subpatterns. For 2117 example, the pattern 2118 nigel 75 .sp 2119 (a|b\e1)+ 2120 .sp 2121 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of 2122 the subpattern, the back reference matches the character string corresponding 2123 to the previous iteration. In order for this to work, the pattern must be such 2124 that the first iteration does not need to match the back reference. This can be 2125 done using alternation, as in the example above, or by a quantifier with a 2126 minimum of zero. 2127 ph10 488 .P 2128 Back references of this type cause the group that they reference to be treated 2129 as an 2130 .\" HTML 2131 .\" 2132 atomic group. 2133 .\" 2134 Once the whole group has been matched, a subsequent matching failure cannot 2135 cause backtracking into the middle of the group. 2136 nigel 75 . 2137 . 2138 .\" HTML 2139 nigel 63 .SH ASSERTIONS 2140 .rs 2141 .sp 2142 An assertion is a test on the characters following or preceding the current 2143 matching point that does not actually consume any characters. The simple 2144 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $are described 2145 .\" HTML 2146 .\" 2147 above. 2148 .\" 2149 .P 2150 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds: 2151 those that look ahead of the current position in the subject string, and those 2152 nigel 75 that look behind it. An assertion subpattern is matched in the normal way, 2153 except that it does not cause the current matching position to be changed. 2154 .P 2155 ph10 637 Assertion subpatterns are not capturing subpatterns. If such an assertion 2156 contains capturing subpatterns within it, these are counted for the purposes of 2157 numbering the capturing subpatterns in the whole pattern. However, substring 2158 ph10 1335 capturing is carried out only for positive assertions. (Perl sometimes, but not 2159 ph10 1292 always, does do capturing in negative assertions.) 2160 ph10 637 .P 2161 ph10 643 For compatibility with Perl, assertion subpatterns may be repeated; though 2162 ph10 654 it makes no sense to assert the same thing several times, the side effect of 2163 ph10 643 capturing parentheses may occasionally be useful. In practice, there only three 2164 cases: 2165 ph10 637 .sp 2166 ph10 654 (1) If the quantifier is {0}, the assertion is never obeyed during matching. 2167 However, it may contain internal capturing parenthesized groups that are called 2168 ph10 637 from elsewhere via the 2169 .\" HTML 2170 .\" 2171 subroutine mechanism. 2172 .\" 2173 .sp 2174 ph10 654 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it 2175 were {0,1}. At run time, the rest of the pattern match is tried with and 2176 ph10 637 without the assertion, the order depending on the greediness of the quantifier. 2177 .sp 2178 ph10 654 (3) If the minimum repetition is greater than zero, the quantifier is ignored. 2179 ph10 637 The assertion is obeyed just once when encountered during matching. 2180 nigel 75 . 2181 . 2182 .SS "Lookahead assertions" 2183 .rs 2184 .sp 2185 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for 2186 negative assertions. For example, 2187 nigel 75 .sp 2188 \ew+(?=;) 2189 .sp 2190 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in 2191 the match, and 2192 nigel 75 .sp 2193 nigel 63 foo(?!bar) 2194 nigel 75 .sp 2195 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the 2196 apparently similar pattern 2197 nigel 75 .sp 2198 nigel 63 (?!foo)bar 2199 nigel 75 .sp 2200 nigel 63 does not find an occurrence of "bar" that is preceded by something other than 2201 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion 2202 (?!foo) is always true when the next three characters are "bar". A 2203 nigel 75 lookbehind assertion is needed to achieve the other effect. 2204 .P 2205 nigel 63 If you want to force a matching failure at some point in a pattern, the most 2206 convenient way to do it is with (?!) because an empty string always matches, so 2207 an assertion that requires there not to be an empty string must always fail. 2208 ph10 572 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!). 2209 nigel 75 . 2210 . 2211 .\" HTML 2212 .SS "Lookbehind assertions" 2213 .rs 2214 .sp 2215 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (? 2246 .\" 2247 (see above) 2248 .\" 2249 ph10 461 can be used instead of a lookbehind assertion to get round the fixed-length 2250 ph10 454 restriction. 2251 ph10 168 .P 2252 nigel 63 The implementation of lookbehind assertions is, for each alternative, to 2253 nigel 93 temporarily move the current position back by the fixed length and then try to 2254 nigel 63 match. If there are insufficient characters before the current position, the 2255 nigel 93 assertion fails. 2256 nigel 75 .P 2257 ph10 859 In a UTF mode, PCRE does not allow the \eC escape (which matches a single data 2258 unit even in a UTF mode) to appear in lookbehind assertions, because it makes 2259 it impossible to calculate the length of the lookbehind. The \eX and \eR 2260 escapes, which can match different numbers of data units, are also not 2261 permitted. 2262 nigel 75 .P 2263 ph10 454 .\" HTML 2264 .\" 2265 "Subroutine" 2266 .\" 2267 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long 2268 ph10 461 as the subpattern matches a fixed-length string. 2269 ph10 454 .\" HTML 2270 .\" 2271 Recursion, 2272 .\" 2273 however, is not supported. 2274 .P 2275 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to 2276 ph10 456 specify efficient matching of fixed-length strings at the end of subject 2277 strings. Consider a simple pattern such as 2278 nigel 75 .sp 2279 nigel 63 abcd$ 2280 nigel 75 .sp 2281 nigel 63 when applied to a long string that does not match. Because matching proceeds 2282 from left to right, PCRE will look for each "a" in the subject and then see if 2283 what follows matches the rest of the pattern. If the pattern is specified as 2284 nigel 75 .sp 2285 nigel 63 ^.*abcd$2286 nigel 75 .sp 2287 nigel 63 the initial .* matches the entire string at first, but when this fails (because 2288 there is no following "a"), it backtracks to match all but the last character, 2289 then all but the last two characters, and so on. Once again the search for "a" 2290 covers the entire string, from right to left, so we are no better off. However, 2291 if the pattern is written as 2292 nigel 75 .sp 2293 nigel 63 ^.*+(?<=abcd) 2294 nigel 75 .sp 2295 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire 2296 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four 2297 characters. If it fails, the match fails immediately. For long strings, this 2298 approach makes a significant difference to the processing time. 2299 nigel 75 . 2300 . 2301 .SS "Using multiple assertions" 2302 .rs 2303 .sp 2304 nigel 63 Several assertions (of any sort) may occur in succession. For example, 2305 nigel 75 .sp 2306 (?<=\ed{3})(? 2336 nigel 75 .SH "CONDITIONAL SUBPATTERNS" 2337 nigel 63 .rs 2338 .sp 2339 It is possible to cause the matching process to obey a subpattern 2340 conditionally or to choose between two alternative subpatterns, depending on 2341 ph10 461 the result of an assertion, or whether a specific capturing subpattern has 2342 ph10 456 already been matched. The two possible forms of conditional subpattern are: 2343 nigel 75 .sp 2344 nigel 63 (?(condition)yes-pattern) 2345 (?(condition)yes-pattern|no-pattern) 2346 nigel 75 .sp 2347 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the 2348 no-pattern (if present) is used. If there are more than two alternatives in the 2349 ph10 557 subpattern, a compile-time error occurs. Each of the two alternatives may 2350 ph10 579 itself contain nested subpatterns of any form, including conditional 2351 ph10 557 subpatterns; the restriction to two alternatives applies only at the level of 2352 ph10 579 the condition. This pattern fragment is an example where the alternatives are 2353 ph10 557 complex: 2354 .sp 2355 (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) 2356 .sp 2357 nigel 75 .P 2358 nigel 93 There are four kinds of condition: references to subpatterns, references to 2359 recursion, a pseudo-condition called DEFINE, and assertions. 2360 . 2361 .SS "Checking for a used subpattern by number" 2362 .rs 2363 .sp 2364 If the text between the parentheses consists of a sequence of digits, the 2365 ph10 456 condition is true if a capturing subpattern of that number has previously 2366 ph10 461 matched. If there is more than one capturing subpattern with the same number 2367 (see the earlier 2368 ph10 456 .\" 2369 .\" HTML 2370 .\" 2371 section about duplicate subpattern numbers), 2372 .\" 2373 ph10 572 the condition is true if any of them have matched. An alternative notation is 2374 ph10 456 to precede the digits with a plus or minus sign. In this case, the subpattern 2375 number is relative rather than absolute. The most recently opened parentheses 2376 ph10 572 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside 2377 ph10 579 loops it can also make sense to refer to subsequent groups. The next 2378 ph10 572 parentheses to be opened can be referenced as (?(+1), and so on. (The value 2379 zero in any of these forms is not used; it provokes a compile-time error.) 2380 nigel 91 .P 2381 Consider the following pattern, which contains non-significant white space to 2382 make it more readable (assume the PCRE_EXTENDED option) and to divide it into 2383 three parts for ease of discussion: 2384 nigel 75 .sp 2385 ( \e( )? [^()]+ (?(1) \e) ) 2386 .sp 2387 nigel 63 The first part matches an optional opening parenthesis, and if that 2388 character is present, sets it as the first captured substring. The second part 2389 matches one or more characters that are not parentheses. The third part is a 2390 ph10 572 conditional subpattern that tests whether or not the first set of parentheses 2391 matched. If they did, that is, if subject started with an opening parenthesis, 2392 nigel 63 the condition is true, and so the yes-pattern is executed and a closing 2393 parenthesis is required. Otherwise, since no-pattern is not present, the 2394 subpattern matches nothing. In other words, this pattern matches a sequence of 2395 nigel 93 non-parentheses, optionally enclosed in parentheses. 2396 ph10 167 .P 2397 ph10 172 If you were embedding this pattern in a larger one, you could use a relative 2398 ph10 167 reference: 2399 .sp 2400 ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ... 2401 .sp 2402 This makes the fragment independent of the parentheses in the larger pattern. 2403 nigel 93 . 2404 .SS "Checking for a used subpattern by name" 2405 .rs 2406 nigel 91 .sp 2407 nigel 93 Perl uses the syntax (?()...) or (?('name')...) to test for a used 2408 subpattern by name. For compatibility with earlier versions of PCRE, which had 2409 ph10 1404 this facility before Perl, the syntax (?(name)...) is also recognized. 2410 nigel 93 .P 2411 Rewriting the above example to use a named subpattern gives this: 2412 nigel 91 .sp 2413 nigel 93 (? \e( )? [^()]+ (?() \e) ) 2414 .sp 2415 ph10 461 If the name used in a condition of this kind is a duplicate, the test is 2416 applied to all subpatterns of the same name, and is true if any one of them has 2417 ph10 459 matched. 2418 nigel 93 . 2419 .SS "Checking for pattern recursion" 2420 .rs 2421 .sp 2422 nigel 91 If the condition is the string (R), and there is no subpattern with the name R, 2423 nigel 93 the condition is true if a recursive call to the whole pattern or any 2424 subpattern has been made. If digits or a name preceded by ampersand follow the 2425 letter R, for example: 2426 .sp 2427 (?(R3)...) or (?(R&name)...) 2428 .sp 2429 ph10 456 the condition is true if the most recent recursion is into a subpattern whose 2430 nigel 93 number or name is given. This condition does not check the entire recursion 2431 ph10 461 stack. If the name used in a condition of this kind is a duplicate, the test is 2432 applied to all subpatterns of the same name, and is true if any one of them is 2433 the most recent recursion. 2434 nigel 75 .P 2435 ph10 461 At "top level", all these recursion test conditions are false. 2436 ph10 454 .\" HTML 2437 .\" 2438 ph10 459 The syntax for recursive patterns 2439 ph10 454 .\" 2440 ph10 459 is described below. 2441 nigel 93 . 2442 ph10 572 .\" HTML 2443 nigel 93 .SS "Defining subpatterns for use by reference only" 2444 .rs 2445 .sp 2446 If the condition is the string (DEFINE), and there is no subpattern with the 2447 name DEFINE, the condition is always false. In this case, there may be only one 2448 alternative in the subpattern. It is always skipped if control reaches this 2449 point in the pattern; the idea of DEFINE is that it can be used to define 2450 ph10 716 subroutines that can be referenced from elsewhere. (The use of 2451 ph10 454 .\" HTML 2452 .\" 2453 ph10 716 subroutines 2454 ph10 454 .\" 2455 ph10 572 is described below.) For example, a pattern to match an IPv4 address such as 2456 ph10 968 "192.168.23.245" could be written like this (ignore white space and line 2457 ph10 572 breaks): 2458 nigel 93 .sp 2459 (?(DEFINE) (? 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) ) 2460 \eb (?&byte) (\e.(?&byte)){3} \eb 2461 .sp 2462 The first part of the pattern is a DEFINE group inside which a another group 2463 named "byte" is defined. This matches an individual component of an IPv4 2464 address (a number less than 256). When matching takes place, this part of the 2465 ph10 456 pattern is skipped because DEFINE acts like a false condition. The rest of the 2466 pattern uses references to the named group to match the four dot-separated 2467 components of an IPv4 address, insisting on a word boundary at each end. 2468 nigel 93 . 2469 .SS "Assertion conditions" 2470 .rs 2471 .sp 2472 If the condition is not in any of the above formats, it must be an assertion. 2473 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider 2474 this pattern, again containing non-significant white space, and with the two 2475 alternatives on the second line: 2476 nigel 75 .sp 2477 nigel 63 (?(?=[^a-z]*[a-z]) 2478 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} ) 2479 .sp 2480 nigel 63 The condition is a positive lookahead assertion that matches an optional 2481 sequence of non-letters followed by a letter. In other words, it tests for the 2482 presence of at least one letter in the subject. If a letter is found, the 2483 subject is matched against the first alternative; otherwise it is matched 2484 against the second. This pattern matches strings in one of the two forms 2485 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. 2486 nigel 75 . 2487 . 2488 .\" HTML 2489 nigel 63 .SH COMMENTS 2490 .rs 2491 .sp 2492 ph10 579 There are two ways of including comments in patterns that are processed by 2493 ph10 562 PCRE. In both cases, the start of the comment must not be in a character class, 2494 nor in the middle of any other sequence of related characters such as (?: or a 2495 subpattern name or number. The characters that make up a comment play no part 2496 in the pattern matching. 2497 .P 2498 nigel 75 The sequence (?# marks the start of a comment that continues up to the next 2499 ph10 562 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED 2500 option is set, an unescaped # character also introduces a comment, which in 2501 this case continues to immediately after the next newline character or 2502 character sequence in the pattern. Which characters are interpreted as newlines 2503 ph10 859 is controlled by the options passed to a compiling function or by a special 2504 ph10 562 sequence at the start of the pattern, as described in the section entitled 2505 ph10 572 .\" HTML 2506 ph10 556 .\" 2507 "Newline conventions" 2508 .\" 2509 ph10 572 above. Note that the end of this type of comment is a literal newline sequence 2510 in the pattern; escape sequences that happen to represent a newline do not 2511 count. For example, consider this pattern when PCRE_EXTENDED is set, and the 2512 default newline convention is in force: 2513 ph10 556 .sp 2514 abc #comment \en still comment 2515 .sp 2516 ph10 579 On encountering the # character, \fBpcre_compile()\fP skips along, looking for 2517 ph10 556 a newline in the pattern. The sequence \en is still literal at this stage, so 2518 it does not terminate the comment. Only an actual character with the code value 2519 ph10 562 0x0a (the default newline) does so. 2520 nigel 75 . 2521 . 2522 nigel 91 .\" HTML 2523 nigel 75 .SH "RECURSIVE PATTERNS" 2524 nigel 63 .rs 2525 .sp 2526 Consider the problem of matching a string in parentheses, allowing for 2527 unlimited nested parentheses. Without the use of recursion, the best that can 2528 be done is to use a pattern that matches up to some fixed depth of nesting. It 2529 nigel 93 is not possible to handle an arbitrary nesting depth. 2530 .P 2531 For some time, Perl has provided a facility that allows regular expressions to 2532 recurse (amongst other things). It does this by interpolating Perl code in the 2533 expression at run time, and the code can refer to the expression itself. A Perl 2534 pattern using code interpolation to solve the parentheses problem can be 2535 created like this: 2536 nigel 75 .sp 2537$re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x; 2538 .sp 2539 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers 2540 nigel 93 recursively to the pattern in which it appears. 2541 nigel 75 .P 2542 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it 2543 supports special syntax for recursion of the entire pattern, and also for 2544 individual subpattern recursion. After its introduction in PCRE and Python, 2545 ph10 453 this kind of recursion was subsequently introduced into Perl at release 5.10. 2546 nigel 75 .P 2547 nigel 93 A special item that consists of (? followed by a number greater than zero and a 2548 ph10 716 closing parenthesis is a recursive subroutine call of the subpattern of the 2549 given number, provided that it occurs inside that subpattern. (If not, it is a 2550 ph10 454 .\" HTML 2551 .\" 2552 ph10 716 non-recursive subroutine 2553 ph10 454 .\" 2554 nigel 93 call, which is described in the next section.) The special item (?R) or (?0) is 2555 a recursive call of the entire regular expression. 2556 nigel 87 .P 2557 This PCRE pattern solves the nested parentheses problem (assume the 2558 PCRE_EXTENDED option is set so that white space is ignored): 2559 nigel 75 .sp 2560 ph10 456 \e( ( [^()]++ | (?R) )* \e) 2561 nigel 75 .sp 2562 nigel 63 First it matches an opening parenthesis. Then it matches any number of 2563 substrings which can either be a sequence of non-parentheses, or a recursive 2564 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring). 2565 ph10 461 Finally there is a closing parenthesis. Note the use of a possessive quantifier 2566 ph10 456 to avoid backtracking into sequences of non-parentheses. 2567 nigel 75 .P 2568 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire 2569 pattern, so instead you could use this: 2570 nigel 75 .sp 2571 ph10 456 ( \e( ( [^()]++ | (?1) )* \e) ) 2572 nigel 75 .sp 2573 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to 2574 ph10 172 them instead of the whole pattern. 2575 ph10 166 .P 2576 In a larger pattern, keeping track of parenthesis numbers can be tricky. This 2577 ph10 572 is made easier by the use of relative references. Instead of (?1) in the 2578 pattern above you can write (?-2) to refer to the second most recently opened 2579 parentheses preceding the recursion. In other words, a negative number counts 2580 capturing parentheses leftwards from the point at which it is encountered. 2581 ph10 166 .P 2582 It is also possible to refer to subsequently opened parentheses, by writing 2583 references such as (?+2). However, these cannot be recursive because the 2584 reference is not inside the parentheses that are referenced. They are always 2585 ph10 454 .\" HTML 2586 .\" 2587 ph10 716 non-recursive subroutine 2588 ph10 454 .\" 2589 calls, as described in the next section. 2590 ph10 166 .P 2591 An alternative approach is to use named parentheses instead. The Perl syntax 2592 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We 2593 could rewrite the above example as follows: 2594 nigel 75 .sp 2595 ph10 456 (? \e( ( [^()]++ | (?&pn) )* \e) ) 2596 nigel 75 .sp 2597 nigel 93 If there is more than one subpattern with the same name, the earliest one is 2598 ph10 172 used. 2599 ph10 166 .P 2600 This particular example pattern that we have been looking at contains nested 2601 ph10 456 unlimited repeats, and so the use of a possessive quantifier for matching 2602 strings of non-parentheses is important when applying the pattern to strings 2603 that do not match. For example, when this pattern is applied to 2604 nigel 75 .sp 2605 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 2606 nigel 75 .sp 2607 ph10 456 it yields "no match" quickly. However, if a possessive quantifier is not used, 2608 nigel 63 the match runs for a very long time indeed because there are so many different 2609 ways the + and * repeats can carve up the subject, and all have to be tested 2610 before failure can be reported. 2611 nigel 75 .P 2612 ph10 464 At the end of a match, the values of capturing parentheses are those from 2613 the outermost level. If you want to obtain intermediate values, a callout 2614 function can be used (see below and the 2615 nigel 63 .\" HREF 2616 nigel 75 \fBpcrecallout\fP 2617 nigel 63 .\" 2618 documentation). If the pattern above is matched against 2619 nigel 75 .sp 2620 nigel 63 (ab(cd)ef) 2621 nigel 75 .sp 2622 ph10 464 the value for the inner capturing parentheses (numbered 2) is "ef", which is 2623 the last value taken on at the top level. If a capturing subpattern is not 2624 ph10 724 matched at the top level, its final captured value is unset, even if it was 2625 (temporarily) set at a deeper level during the matching process. 2626 nigel 75 .P 2627 ph10 464 If there are more than 15 capturing parentheses in a pattern, PCRE has to 2628 obtain extra memory to store data during a recursion, which it does by using 2629 \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can 2630 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. 2631 .P 2632 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion. 2633 Consider this pattern, which matches text in angle brackets, allowing for 2634 arbitrary nesting. Only digits are allowed in nested brackets (that is, when 2635 recursing), whereas any characters are permitted at the outer level. 2636 nigel 75 .sp 2637 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * > 2638 .sp 2639 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two 2640 different alternatives for the recursive and non-recursive cases. The (?R) item 2641 is the actual recursive call. 2642 nigel 75 . 2643 . 2644 ph10 453 .\" HTML 2645 ph10 724 .SS "Differences in recursion processing between PCRE and Perl" 2646 ph10 453 .rs 2647 .sp 2648 ph10 724 Recursion processing in PCRE differs from Perl in two important ways. In PCRE 2649 (like Python, but unlike Perl), a recursive subpattern call is always treated 2650 as an atomic group. That is, once it has matched some of the subject string, it 2651 is never re-entered, even if it contains untried alternatives and there is a 2652 subsequent matching failure. This can be illustrated by the following pattern, 2653 which purports to match a palindromic string that contains an odd number of 2654 characters (for example, "a", "aba", "abcba", "abcdcba"): 2655 ph10 453 .sp 2656 ^(.|(.)(?1)\e2)$ 2657 .sp 2658 ph10 461 The idea is that it either matches a single character, or two identical 2659 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE 2660 ph10 453 it does not if the pattern is longer than three characters. Consider the 2661 subject string "abcba": 2662 .P 2663 ph10 461 At the top level, the first character is matched, but as it is not at the end 2664 ph10 453 of the string, the first alternative fails; the second alternative is taken 2665 and the recursion kicks in. The recursive call to subpattern 1 successfully 2666 matches the next character ("b"). (Note that the beginning and end of line 2667 tests are not part of the recursion). 2668 .P 2669 Back at the top level, the next character ("c") is compared with what 2670 ph10 461 subpattern 2 matched, which was "a". This fails. Because the recursion is 2671 ph10 453 treated as an atomic group, there are now no backtracking points, and so the 2672 entire match fails. (Perl is able, at this point, to re-enter the recursion and 2673 try the second alternative.) However, if the pattern is written with the 2674 alternatives in the other order, things are different: 2675 .sp 2676 ^((.)(?1)\e2|.)$2677 .sp 2678 ph10 461 This time, the recursing alternative is tried first, and continues to recurse 2679 until it runs out of characters, at which point the recursion fails. But this 2680 time we do have another alternative to try at the higher level. That is the big 2681 ph10 453 difference: in the previous case the remaining alternative is at a deeper 2682 recursion level, which PCRE cannot use. 2683 .P 2684 ph10 572 To change the pattern so that it matches all palindromic strings, not just 2685 those with an odd number of characters, it is tempting to change the pattern to 2686 this: 2687 ph10 453 .sp 2688 ^((.)(?1)\e2|.?)$ 2689 .sp 2690 ph10 461 Again, this works in Perl, but not in PCRE, and for the same reason. When a 2691 deeper recursion has matched a single character, it cannot be entered again in 2692 order to match an empty string. The solution is to separate the two cases, and 2693 ph10 453 write out the odd and even cases as alternatives at the higher level: 2694 .sp 2695 ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.)) 2696 ph10 461 .sp 2697 If you want to match typical palindromic phrases, the pattern has to ignore all 2698 ph10 453 non-word characters, which can be done like this: 2699 .sp 2700 ph10 461 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+\$ 2701 ph10 453 .sp 2702 ph10 461 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A 2703 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note 2704 the use of the possessive quantifier *+ to avoid backtracking into sequences of 2705 ph10 453 non-word characters. Without this, PCRE takes a great deal longer (ten times or 2706 more) to match typical phrases, and Perl takes so long that you think it has 2707 gone into a loop. 2708 ph10 456 .P 2709 \fBWARNING\fP: The palindrome-matching patterns above work only if the subject 2710 string does not start with a palindrome that is shorter than the entire string. 2711 For example, although "abcba" is correctly matched, if the subject is "ababa", 2712 PCRE finds the palindrome "aba" at the start, then fails at top level because 2713 the end of the string does not follow. Once again, it cannot jump back into the 2714 recursion to try other alternatives, so the entire match fails. 2715 ph10 724 .P 2716 ph10 733 The second way in which PCRE and Perl differ in their recursion processing is 2717 in the handling of captured values. In Perl, when a subpattern is called 2718 recursively or as a subpattern (see the next section), it has no access to any 2719 values that were captured outside the recursion, whereas in PCRE these values 2720 ph10 724 can be referenced. Consider this pattern: 2721 .sp 2722 ^(.)(\e1|a(?2)) 2723 .sp 2724 ph10 733 In PCRE, this pattern matches "bab". The first capturing parentheses match "b", 2725 ph10 724 then in the second group, when the back reference \e1 fails to match "b", the 2726 second alternative matches "a" and then recurses. In the recursion, \e1 does 2727 now match "b" and so the whole match succeeds. In Perl, the pattern fails to 2728 match because inside the recursive call \e1 cannot access the externally set 2729 value. 2730 ph10 453 . 2731 . 2732 nigel 63 .\" HTML 2733 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES" 2734 nigel 63 .rs 2735 .sp 2736 ph10 716 If the syntax for a recursive subpattern call (either by number or by 2737 nigel 63 name) is used outside the parentheses to which it refers, it operates like a 2738 ph10 716 subroutine in a programming language. The called subpattern may be defined 2739 ph10 166 before or after the reference. A numbered reference can be absolute or 2740 relative, as in these examples: 2741 nigel 75 .sp 2742 ph10 166 (...(absolute)...)...(?2)... 2743 (...(relative)...)...(?-1)... 2744 ph10 172 (...(?+1)...(relative)... 2745 ph10 166 .sp 2746 An earlier example pointed out that the pattern 2747 .sp 2748 nigel 75 (sens|respons)e and \e1ibility 2749 .sp 2750 nigel 63 matches "sense and sensibility" and "response and responsibility", but not 2751 "sense and responsibility". If instead the pattern 2752 nigel 75 .sp 2753 nigel 63 (sens|respons)e and (?1)ibility 2754 nigel 75 .sp 2755 nigel 63 is used, it does match "sense and responsibility" as well as the other two 2756 nigel 93 strings. Another example is given in the discussion of DEFINE above. 2757 nigel 87 .P 2758 ph10 716 All subroutine calls, whether recursive or not, are always treated as atomic 2759 groups. That is, once a subroutine has matched some of the subject string, it 2760 is never re-entered, even if it contains untried alternatives and there is a 2761 subsequent matching failure. Any capturing parentheses that are set during the 2762 subroutine call revert to their previous values afterwards. 2763 nigel 93 .P 2764 ph10 716 Processing options such as case-independence are fixed when a subpattern is 2765 defined, so if it is used as a subroutine, such options cannot be changed for 2766 different calls. For example, consider this pattern: 2767 nigel 93 .sp 2768 ph10 166 (abc)(?i:(?-1)) 2769 nigel 93 .sp 2770 It matches "abcabc". It does not match "abcABC" because the change of 2771 processing option does not affect the called subpattern. 2772 nigel 75 . 2773 . 2774 ph10 333 .\" HTML 2775 .SH "ONIGURUMA SUBROUTINE SYNTAX" 2776 .rs 2777 .sp 2778 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or 2779 a number enclosed either in angle brackets or single quotes, is an alternative 2780 syntax for referencing a subpattern as a subroutine, possibly recursively. Here 2781 ph10 333 are two of the examples used above, rewritten using this syntax: 2782 .sp 2783 (? \e( ( (?>[^()]+) | \eg )* \e) ) 2784 (sens|respons)e and \eg'1'ibility 2785 .sp 2786 ph10 345 PCRE supports an extension to Oniguruma: if a number is preceded by a 2787 ph10 333 plus or a minus sign it is taken as a relative reference. For example: 2788 .sp 2789 (abc)(?i:\eg<-1>) 2790 .sp 2791 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP 2792 ph10 333 synonymous. The former is a back reference; the latter is a subroutine call. 2793 . 2794 . 2795 nigel 63 .SH CALLOUTS 2796 .rs 2797 .sp 2798 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl 2799 code to be obeyed in the middle of matching a regular expression. This makes it 2800 possible, amongst other things, to extract different substrings that match the 2801 same pair of parentheses when there is a repetition. 2802 nigel 75 .P 2803 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl 2804 code. The feature is called "callout". The caller of PCRE provides an external 2805 ph10 903 function by putting its entry point in the global variable \fIpcre_callout\fP 2806 chpe 1055 (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library). 2807 By default, this variable contains NULL, which disables all calling out. 2808 nigel 75 .P 2809 nigel 63 Within a regular expression, (?C) indicates the points at which the external 2810 function is to be called. If you want to identify different callout points, you 2811 can put a number less than 256 after the letter C. The default value is zero. 2812 For example, this pattern has two callout points: 2813 nigel 75 .sp 2814 ph10 155 (?C1)abc(?C2)def 2815 nigel 75 .sp 2816 ph10 859 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are 2817 nigel 75 automatically installed before each item in the pattern. They are all numbered 2818 ph10 1266 255. If there is a conditional group in the pattern whose condition is an 2819 assertion, an additional callout is inserted just before the condition. An 2820 explicit callout may also be set at this position, as in this example: 2821 .sp 2822 (?(?C9)(?=a)abc|def) 2823 .sp 2824 ph10 1335 Note that this applies only to assertion conditions, not to other types of 2825 condition. 2826 nigel 75 .P 2827 ph10 859 During matching, when PCRE reaches a callout point, the external function is 2828 called. It is provided with the number of the callout, the position in the 2829 pattern, and, optionally, one item of data originally supplied by the caller of 2830 the matching function. The callout function may cause matching to proceed, to 2831 ph10 1395 backtrack, or to fail altogether. 2832 .P 2833 By default, PCRE implements a number of optimizations at compile time and 2834 matching time, and one side-effect is that sometimes callouts are skipped. If 2835 you need all possible callouts to happen, you need to set options that disable 2836 the relevant optimizations. More details, and a complete description of the 2837 interface to the callout function, are given in the 2838 nigel 63 .\" HREF 2839 nigel 75 \fBpcrecallout\fP 2840 nigel 63 .\" 2841 documentation. 2842 nigel 93 . 2843 . 2844 ph10 510 .\" HTML 2845 ph10 235 .SH "BACKTRACKING CONTROL" 2846 ph10 210 .rs 2847 .sp 2848 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which 2849 ph10 1297 are still described in the Perl documentation as "experimental and subject to 2850 change or removal in a future version of Perl". It goes on to say: "Their usage 2851 in production code should be noted to avoid problems during upgrades." The same 2852 ph10 210 remarks apply to the PCRE features described in this section. 2853 .P 2854 ph10 211 The new verbs make use of what was previously invalid syntax: an opening 2855 ph10 510 parenthesis followed by an asterisk. They are generally of the form 2856 ph10 1335 (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving 2857 ph10 1302 differently depending on whether or not a name is present. A name is any 2858 sequence of characters that does not include a closing parenthesis. The maximum 2859 length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit 2860 libraries. If the name is empty, that is, if the closing parenthesis 2861 immediately follows the colon, the effect is as if the colon were not there. 2862 Any number of these verbs may occur in a pattern. 2863 ph10 1297 .P 2864 Since these verbs are specifically related to backtracking, most of them can be 2865 used only when the pattern is to be matched using one of the traditional 2866 matching functions, because these use a backtracking algorithm. With the 2867 ph10 1335 exception of (*FAIL), which behaves like a failing negative assertion, the 2868 ph10 1297 backtracking control verbs cause an error if encountered by a DFA matching 2869 function. 2870 .P 2871 ph10 1335 The behaviour of these verbs in 2872 ph10 1298 .\" HTML 2873 .\" 2874 ph10 1335 repeated groups, 2875 ph10 1298 .\" 2876 .\" HTML 2877 .\" 2878 ph10 1335 assertions, 2879 ph10 1298 .\" 2880 ph10 1335 and in 2881 ph10 1298 .\" HTML 2882 .\" 2883 subpatterns called as subroutines 2884 .\" 2885 (whether or not recursively) is documented below. 2886 ph10 930 . 2887 . 2888 .\" HTML 2889 .SS "Optimizations that affect backtracking verbs" 2890 .rs 2891 .sp 2892 ph10 512 PCRE contains some optimizations that are used to speed up matching by running 2893 some checks at the start of each match attempt. For example, it may know the 2894 minimum length of matching subject, or that a particular character must be 2895 ph10 1297 present. When one of these optimizations bypasses the running of a match, any 2896 ph10 512 included backtracking verbs will not, of course, be processed. You can suppress 2897 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option 2898 ph10 577 when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the 2899 ph10 930 pattern with (*NO_START_OPT). There is more discussion of this option in the 2900 section entitled 2901 .\" HTML 2902 .\" 2903 "Option bits for \fBpcre_exec()\fP" 2904 .\" 2905 in the 2906 .\" HREF 2907 \fBpcreapi\fP 2908 .\" 2909 ph10 975 documentation. 2910 ph10 836 .P 2911 Experiments with Perl suggest that it too has similar optimizations, sometimes 2912 leading to anomalous results. 2913 ph10 210 . 2914 ph10 510 . 2915 ph10 210 .SS "Verbs that act immediately" 2916 .rs 2917 .sp 2918 ph10 512 The following verbs act as soon as they are encountered. They may not be 2919 ph10 510 followed by a name. 2920 ph10 210 .sp 2921 (*ACCEPT) 2922 .sp 2923 This verb causes the match to end successfully, skipping the remainder of the 2924 ph10 716 pattern. However, when it is inside a subpattern that is called as a 2925 subroutine, only that subpattern is ended successfully. Matching then continues 2926 ph10 1335 at the outer level. If (*ACCEPT) in triggered in a positive assertion, the 2927 ph10 1297 assertion succeeds; in a negative assertion, the assertion fails. 2928 .P 2929 If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For 2930 example: 2931 ph10 210 .sp 2932 ph10 447 A((?:A|B(*ACCEPT)|C)D) 2933 ph10 210 .sp 2934 ph10 461 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by 2935 ph10 447 the outer parentheses. 2936 ph10 210 .sp 2937 (*FAIL) or (*F) 2938 .sp 2939 ph10 716 This verb causes a matching failure, forcing backtracking to occur. It is 2940 ph10 210 equivalent to (?!) but easier to read. The Perl documentation notes that it is 2941 probably useful only when combined with (?{}) or (??{}). Those are, of course, 2942 Perl features that are not present in PCRE. The nearest equivalent is the 2943 callout feature, as for example in this pattern: 2944 .sp 2945 a+(?C)(*FAIL) 2946 .sp 2947 ph10 211 A match with the string "aaaa" always fails, but the callout is taken before 2948 each backtrack happens (in this example, 10 times). 2949 ph10 210 . 2950 ph10 510 . 2951 .SS "Recording which path was taken" 2952 .rs 2953 .sp 2954 ph10 512 There is one verb whose main purpose is to track how a match was arrived at, 2955 though it also has a secondary use in conjunction with advancing the match 2956 ph10 510 starting point (see (*SKIP) below). 2957 .sp 2958 (*MARK:NAME) or (*:NAME) 2959 .sp 2960 A name is always required with this verb. There may be as many instances of 2961 (*MARK) as you like in a pattern, and their names do not have to be unique. 2962 .P 2963 ph10 1287 When a match succeeds, the name of the last-encountered (*MARK:NAME), 2964 (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the 2965 caller as described in the section entitled 2966 ph10 510 .\" HTML 2967 .\" 2968 ph10 859 "Extra data for \fBpcre_exec()\fP" 2969 ph10 510 .\" 2970 ph10 512 in the 2971 ph10 510 .\" HREF 2972 \fBpcreapi\fP 2973 .\" 2974 ph10 836 documentation. Here is an example of \fBpcretest\fP output, where the /K 2975 modifier requests the retrieval and outputting of (*MARK) data: 2976 ph10 510 .sp 2977 ph10 836 re> /X(*MARK:A)Y|X(*MARK:B)Z/K 2978 data> XY 2979 ph10 510 0: XY 2980 MK: A 2981 XZ 2982 0: XZ 2983 MK: B 2984 .sp 2985 ph10 512 The (*MARK) name is tagged with "MK:" in this output, and in this example it 2986 indicates which of the two alternatives matched. This is a more efficient way 2987 ph10 510 of obtaining this information than putting each alternative in its own 2988 capturing parentheses. 2989 .P 2990 ph10 1302 If a verb with a name is encountered in a positive assertion that is true, the 2991 name is recorded and passed back if it is the last-encountered. This does not 2992 happen for negative assertions or failing positive assertions. 2993 ph10 630 .P 2994 ph10 1287 After a partial match or a failed match, the last encountered name in the 2995 entire match process is returned. For example: 2996 ph10 510 .sp 2997 ph10 836 re> /X(*MARK:A)Y|X(*MARK:B)Z/K 2998 data> XP 2999 ph10 510 No match, mark = B 3000 .sp 3001 ph10 836 Note that in this unanchored example the mark is retained from the match 3002 ph10 930 attempt that started at the letter "X" in the subject. Subsequent match 3003 attempts starting at "P" and then with an empty string do not get as far as the 3004 (*MARK) item, but nevertheless do not reset it. 3005 .P 3006 If you are interested in (*MARK) values after failed matches, you should 3007 ph10 975 probably set the PCRE_NO_START_OPTIMIZE option 3008 ph10 930 .\" HTML 3009 .\" 3010 ph10 975 (see above) 3011 ph10 930 .\" 3012 to ensure that the match is always attempted. 3013 ph10 510 . 3014 . 3015 ph10 210 .SS "Verbs that act after backtracking" 3016 .rs 3017 .sp 3018 ph10 211 The following verbs do nothing when they are encountered. Matching continues 3019 ph10 510 with what follows, but if there is no subsequent match, causing a backtrack to 3020 the verb, a failure is forced. That is, backtracking cannot pass to the left of 3021 ph10 1335 the verb. However, when one of these verbs appears inside an atomic group or an 3022 ph10 1302 assertion that is true, its effect is confined to that group, because once the 3023 group has been matched, there is never any backtracking into it. In this 3024 situation, backtracking can "jump back" to the left of the entire atomic group 3025 or assertion. (Remember also, as stated above, that this localization also 3026 applies in subroutine calls.) 3027 ph10 510 .P 3028 These verbs differ in exactly what kind of failure occurs when backtracking 3029 ph10 1302 reaches them. The behaviour described below is what happens when the verb is 3030 ph10 1335 not in a subroutine or an assertion. Subsequent sections cover these special 3031 ph10 1302 cases. 3032 ph10 210 .sp 3033 (*COMMIT) 3034 .sp 3035 ph10 510 This verb, which may not be followed by a name, causes the whole match to fail 3036 ph10 1335 outright if there is a later matching failure that causes backtracking to reach 3037 ph10 1297 it. Even if the pattern is unanchored, no further attempts to find a match by 3038 advancing the starting point take place. If (*COMMIT) is the only backtracking 3039 verb that is encountered, once it has been passed \fBpcre_exec()\fP is 3040 committed to finding a match at the current starting point, or not at all. For 3041 example: 3042 ph10 210 .sp 3043 a+(*COMMIT)b 3044 .sp 3045 ph10 211 This matches "xxaab" but not "aacaab". It can be thought of as a kind of 3046 ph10 512 dynamic anchor, or "I've started, so I must finish." The name of the most 3047 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a 3048 ph10 510 match failure. 3049 .P 3050 ph10 1335 If there is more than one backtracking verb in a pattern, a different one that 3051 follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a 3052 ph10 1297 match does not always guarantee that a match must be at this starting point. 3053 .P 3054 ph10 512 Note that (*COMMIT) at the start of a pattern is not the same as an anchor, 3055 unless PCRE's start-of-match optimizations are turned off, as shown in this 3056 ph10 1436 output from \fBpcretest\fP: 3057 ph10 210 .sp 3058 ph10 836 re> /(*COMMIT)abc/ 3059 data> xyzabc 3060 ph10 510 0: abc 3061 ph10 1436 data> xyzabc\eY 3062 ph10 510 No match 3063 ph10 210 .sp 3064 ph10 1436 For this pattern, PCRE knows that any match must start with "a", so the 3065 optimization skips along the subject to "a" before applying the pattern to the 3066 first set of data. The match attempt then succeeds. In the second set of data, 3067 the escape sequence \eY is interpreted by the \fBpcretest\fP program. It causes 3068 the PCRE_NO_START_OPTIMIZE option to be set when \fBpcre_exec()\fP is called. 3069 This disables the optimization that skips along to the first character. The 3070 pattern is now applied starting at "x", and so the (*COMMIT) causes the match 3071 to fail without trying any other starting points. 3072 ph10 210 .sp 3073 ph10 510 (*PRUNE) or (*PRUNE:NAME) 3074 .sp 3075 ph10 512 This verb causes the match to fail at the current starting position in the 3076 ph10 1297 subject if there is a later matching failure that causes backtracking to reach 3077 it. If the pattern is unanchored, the normal "bumpalong" advance to the next 3078 starting character then happens. Backtracking can occur as usual to the left of 3079 (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but 3080 if there is no match to the right, backtracking cannot cross (*PRUNE). In 3081 simple cases, the use of (*PRUNE) is just an alternative to an atomic group or 3082 possessive quantifier, but there are some uses of (*PRUNE) that cannot be 3083 expressed in any other way. In an anchored pattern (*PRUNE) has the same effect 3084 as (*COMMIT). 3085 ph10 1287 .P 3086 ph10 1335 The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE). 3087 ph10 1287 It is like (*MARK:NAME) in that the name is remembered for passing back to the 3088 caller. However, (*SKIP:NAME) searches only for names set with (*MARK). 3089 ph10 510 .sp 3090 ph10 210 (*SKIP) 3091 .sp 3092 ph10 510 This verb, when given without a name, is like (*PRUNE), except that if the 3093 pattern is unanchored, the "bumpalong" advance is not to the next character, 3094 but to the position in the subject where (*SKIP) was encountered. (*SKIP) 3095 signifies that whatever text was matched leading up to it cannot be part of a 3096 successful match. Consider: 3097 ph10 210 .sp 3098 a+(*SKIP)b 3099 .sp 3100 ph10 211 If the subject is "aaaac...", after the first match attempt fails (starting at 3101 ph10 210 the first character in the string), the starting point skips on to start the 3102 ph10 211 next attempt at "c". Note that a possessive quantifer does not have the same 3103 ph10 456 effect as this example; although it would suppress backtracking during the 3104 ph10 210 first match attempt, the second attempt would start at the second character 3105 instead of skipping on to "c". 3106 .sp 3107 ph10 510 (*SKIP:NAME) 3108 ph10 211 .sp 3109 ph10 1335 When (*SKIP) has an associated name, its behaviour is modified. When it is 3110 ph10 1297 triggered, the previous path through the pattern is searched for the most 3111 recent (*MARK) that has the same name. If one is found, the "bumpalong" advance 3112 is to the subject position that corresponds to that (*MARK) instead of to where 3113 (*SKIP) was encountered. If no (*MARK) with a matching name is found, the 3114 (*SKIP) is ignored. 3115 ph10 1287 .P 3116 ph10 1335 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores 3117 ph10 1287 names that are set by (*PRUNE:NAME) or (*THEN:NAME). 3118 ph10 510 .sp 3119 (*THEN) or (*THEN:NAME) 3120 .sp 3121 ph10 1335 This verb causes a skip to the next innermost alternative when backtracking 3122 ph10 1297 reaches it. That is, it cancels any further backtracking within the current 3123 alternative. Its name comes from the observation that it can be used for a 3124 pattern-based if-then-else block: 3125 ph10 210 .sp 3126 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... 3127 .sp 3128 ph10 211 If the COND1 pattern matches, FOO is tried (and possibly further items after 3129 ph10 716 the end of the group if FOO succeeds); on failure, the matcher skips to the 3130 ph10 1302 second alternative and tries COND2, without backtracking into COND1. If that 3131 succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no 3132 more alternatives, so there is a backtrack to whatever came before the entire 3133 group. If (*THEN) is not inside an alternation, it acts like (*PRUNE). 3134 ph10 551 .P 3135 ph10 1335 The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN). 3136 ph10 1287 It is like (*MARK:NAME) in that the name is remembered for passing back to the 3137 caller. However, (*SKIP:NAME) searches only for names set with (*MARK). 3138 .P 3139 ph10 1297 A subpattern that does not contain a | character is just a part of the 3140 enclosing alternative; it is not a nested alternation with only one 3141 ph10 716 alternative. The effect of (*THEN) extends beyond such a subpattern to the 3142 enclosing alternative. Consider this pattern, where A, B, etc. are complex 3143 pattern fragments that do not contain any | characters at this level: 3144 .sp 3145 A (B(*THEN)C) | D 3146 .sp 3147 ph10 733 If A and B are matched, but there is a failure in C, matching does not 3148 ph10 716 backtrack into A; instead it moves to the next alternative, that is, D. 3149 However, if the subpattern containing (*THEN) is given an alternative, it 3150 behaves differently: 3151 .sp 3152 A (B(*THEN)C | (*FAIL)) | D 3153 .sp 3154 The effect of (*THEN) is now confined to the inner subpattern. After a failure 3155 ph10 733 in C, matching moves to (*FAIL), which causes the whole subpattern to fail 3156 because there are no more alternatives to try. In this case, matching does now 3157 ph10 716 backtrack into A. 3158 ph10 551 .P 3159 ph10 1297 Note that a conditional subpattern is not considered as having two 3160 ph10 733 alternatives, because only one is ever used. In other words, the | character in 3161 ph10 716 a conditional subpattern has a different meaning. Ignoring white space, 3162 consider: 3163 ph10 551 .sp 3164 ph10 716 ^.*? (?(?=a) a | b(*THEN)c ) 3165 .sp 3166 ph10 733 If the subject is "ba", this pattern does not match. Because .*? is ungreedy, 3167 it initially matches zero characters. The condition (?=a) then fails, the 3168 ph10 716 character "b" is matched, but "c" is not. At this point, matching does not 3169 backtrack to .*? as might perhaps be expected from the presence of the | 3170 character. The conditional subpattern is part of the single alternative that 3171 ph10 733 comprises the whole pattern, and so the match fails. (If there was a backtrack 3172 ph10 716 into .*?, allowing it to match "b", the match would succeed.) 3173 .P 3174 The verbs just described provide four different "strengths" of control when 3175 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the 3176 next alternative. (*PRUNE) comes next, failing the match at the current 3177 starting position, but allowing an advance to the next character (for an 3178 unanchored pattern). (*SKIP) is similar, except that the advance may be more 3179 than one character. (*COMMIT) is the strongest, causing the entire match to 3180 fail. 3181 ph10 1297 . 3182 . 3183 .SS "More than one backtracking verb" 3184 .rs 3185 ph10 716 .sp 3186 ph10 1297 If more than one backtracking verb is present in a pattern, the one that is 3187 backtracked onto first acts. For example, consider this pattern, where A, B, 3188 etc. are complex pattern fragments: 3189 .sp 3190 ph10 1293 (A(*COMMIT)B(*THEN)C|ABD) 3191 ph10 551 .sp 3192 ph10 1335 If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to 3193 fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes 3194 ph10 1293 the next alternative (ABD) to be tried. This behaviour is consistent, but is 3195 ph10 1297 not always the same as Perl's. It means that if two or more backtracking verbs 3196 appear in succession, all the the last of them has no effect. Consider this 3197 example: 3198 .sp 3199 ...(*COMMIT)(*PRUNE)... 3200 .sp 3201 ph10 1341 If there is a matching failure to the right, backtracking onto (*PRUNE) causes 3202 ph10 1335 it to be triggered, and its action is taken. There can never be a backtrack 3203 onto (*COMMIT). 3204 ph10 210 . 3205 ph10 551 . 3206 ph10 1298 .\" HTML 3207 ph10 1297 .SS "Backtracking verbs in repeated groups" 3208 .rs 3209 .sp 3210 ph10 1335 PCRE differs from Perl in its handling of backtracking verbs in repeated 3211 ph10 1297 groups. For example, consider: 3212 .sp 3213 /(a(*COMMIT)b)+ac/ 3214 .sp 3215 ph10 1335 If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in 3216 the second repeat of the group acts. 3217 ph10 1297 . 3218 . 3219 ph10 1298 .\" HTML 3220 .SS "Backtracking verbs in assertions" 3221 .rs 3222 .sp 3223 (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack. 3224 .P 3225 ph10 1335 (*ACCEPT) in a positive assertion causes the assertion to succeed without any 3226 further processing. In a negative assertion, (*ACCEPT) causes the assertion to 3227 ph10 1298 fail without any further processing. 3228 .P 3229 ph10 1302 The other backtracking verbs are not treated specially if they appear in a 3230 positive assertion. In particular, (*THEN) skips to the next alternative in the 3231 ph10 1298 innermost enclosing group that has alternations, whether or not this is within 3232 the assertion. 3233 ph10 1302 .P 3234 Negative assertions are, however, different, in order to ensure that changing a 3235 positive assertion into a negative assertion changes its result. Backtracking 3236 ph10 1335 into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true, 3237 without considering any further alternative branches in the assertion. 3238 ph10 1302 Backtracking into (*THEN) causes it to skip to the next enclosing alternative 3239 ph10 1335 within the assertion (the normal behaviour), but if the assertion does not have 3240 ph10 1302 such an alternative, (*THEN) behaves like (*PRUNE). 3241 ph10 1298 . 3242 . 3243 .\" HTML 3244 .SS "Backtracking verbs in subroutines" 3245 .rs 3246 .sp 3247 ph10 1335 These behaviours occur whether or not the subpattern is called recursively. 3248 ph10 1298 Perl's treatment of subroutines is different in some cases. 3249 .P 3250 (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces 3251 an immediate backtrack. 3252 .P 3253 ph10 1335 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to 3254 succeed without any further processing. Matching then continues after the 3255 ph10 1298 subroutine call. 3256 .P 3257 (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause 3258 the subroutine match to fail. 3259 .P 3260 (*THEN) skips to the next alternative in the innermost enclosing group within 3261 ph10 1335 the subpattern that has alternatives. If there is no such group within the 3262 ph10 1298 subpattern, (*THEN) causes the subroutine match to fail. 3263 . 3264 . 3265 nigel 93 .SH "SEE ALSO" 3266 .rs 3267 .sp 3268 ph10 461 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), 3269 chpe 1055 \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP. 3270 ph10 99 . 3271 . 3272 .SH AUTHOR 3273 .rs 3274 .sp 3275 .nf 3276 Philip Hazel 3277 University Computing Service 3278 Cambridge CB2 3QH, England. 3279 .fi 3280 . 3281 . 3282 .SH REVISION 3283 .rs 3284 .sp 3285 .nf 3286 ph10 1436 Last updated: 08 January 2014 3287 Copyright (c) 1997-2014 University of Cambridge. 3288 ph10 99 .fi