1 <html> 2 <head> 3 <title>pcrepattern specification</title> 4 </head> 5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"> 6 <h1>pcrepattern man page</h1> 7 <p> 8 Return to the <a href="index.html">PCRE index page</a>. 9 </p> 10 <p> 11 This page is part of the PCRE HTML documentation. It was generated automatically 12 from the original man page. If there is any nonsense in it, please consult the 13 man page, in case the conversion went wrong. 14 <br> 15 <ul> 16 <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a> 17 <li><a name="TOC2" href="#SEC2">SPECIAL START-OF-PATTERN ITEMS</a> 18 <li><a name="TOC3" href="#SEC3">EBCDIC CHARACTER CODES</a> 19 <li><a name="TOC4" href="#SEC4">CHARACTERS AND METACHARACTERS</a> 20 <li><a name="TOC5" href="#SEC5">BACKSLASH</a> 21 <li><a name="TOC6" href="#SEC6">CIRCUMFLEX AND DOLLAR</a> 22 <li><a name="TOC7" href="#SEC7">FULL STOP (PERIOD, DOT) AND \N</a> 23 <li><a name="TOC8" href="#SEC8">MATCHING A SINGLE DATA UNIT</a> 24 <li><a name="TOC9" href="#SEC9">SQUARE BRACKETS AND CHARACTER CLASSES</a> 25 <li><a name="TOC10" href="#SEC10">POSIX CHARACTER CLASSES</a> 26 <li><a name="TOC11" href="#SEC11">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a> 27 <li><a name="TOC12" href="#SEC12">VERTICAL BAR</a> 28 <li><a name="TOC13" href="#SEC13">INTERNAL OPTION SETTING</a> 29 <li><a name="TOC14" href="#SEC14">SUBPATTERNS</a> 30 <li><a name="TOC15" href="#SEC15">DUPLICATE SUBPATTERN NUMBERS</a> 31 <li><a name="TOC16" href="#SEC16">NAMED SUBPATTERNS</a> 32 <li><a name="TOC17" href="#SEC17">REPETITION</a> 33 <li><a name="TOC18" href="#SEC18">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a> 34 <li><a name="TOC19" href="#SEC19">BACK REFERENCES</a> 35 <li><a name="TOC20" href="#SEC20">ASSERTIONS</a> 36 <li><a name="TOC21" href="#SEC21">CONDITIONAL SUBPATTERNS</a> 37 <li><a name="TOC22" href="#SEC22">COMMENTS</a> 38 <li><a name="TOC23" href="#SEC23">RECURSIVE PATTERNS</a> 39 <li><a name="TOC24" href="#SEC24">SUBPATTERNS AS SUBROUTINES</a> 40 <li><a name="TOC25" href="#SEC25">ONIGURUMA SUBROUTINE SYNTAX</a> 41 <li><a name="TOC26" href="#SEC26">CALLOUTS</a> 42 <li><a name="TOC27" href="#SEC27">BACKTRACKING CONTROL</a> 43 <li><a name="TOC28" href="#SEC28">SEE ALSO</a> 44 <li><a name="TOC29" href="#SEC29">AUTHOR</a> 45 <li><a name="TOC30" href="#SEC30">REVISION</a> 46 </ul> 47 <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br> 48 <P> 49 The syntax and semantics of the regular expressions that are supported by PCRE 50 are described in detail below. There is a quick-reference syntax summary in the 51 <a href="pcresyntax.html"><b>pcresyntax</b></a> 52 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE 53 also supports some alternative regular expression syntax (which does not 54 conflict with the Perl syntax) in order to provide some compatibility with 55 regular expressions in Python, .NET, and Oniguruma. 56 </P> 57 <P> 58 Perl's regular expressions are described in its own documentation, and 59 regular expressions in general are covered in a number of books, some of which 60 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", 61 published by O'Reilly, covers regular expressions in great detail. This 62 description of PCRE's regular expressions is intended as reference material. 63 </P> 64 <P> 65 This document discusses the patterns that are supported by PCRE when one its 66 main matching functions, <b>pcre_exec()</b> (8-bit) or <b>pcre[16|32]_exec()</b> 67 (16- or 32-bit), is used. PCRE also has alternative matching functions, 68 <b>pcre_dfa_exec()</b> and <b>pcre[16|32_dfa_exec()</b>, which match using a 69 different algorithm that is not Perl-compatible. Some of the features discussed 70 below are not available when DFA matching is used. The advantages and 71 disadvantages of the alternative functions, and how they differ from the normal 72 functions, are discussed in the 73 <a href="pcrematching.html"><b>pcrematching</b></a> 74 page. 75 </P> 76 <br><a name="SEC2" href="#TOC1">SPECIAL START-OF-PATTERN ITEMS</a><br> 77 <P> 78 A number of options that can be passed to <b>pcre_compile()</b> can also be set 79 by special items at the start of a pattern. These are not Perl-compatible, but 80 are provided to make these options accessible to pattern writers who are not 81 able to change the program that processes the pattern. Any number of these 82 items may appear, but they must all be together right at the start of the 83 pattern string, and the letters must be in upper case. 84 </P> 85 <br><b> 86 UTF support 87 </b><br> 88 <P> 89 The original operation of PCRE was on strings of one-byte characters. However, 90 there is now also support for UTF-8 strings in the original library, an 91 extra library that supports 16-bit and UTF-16 character strings, and a 92 third library that supports 32-bit and UTF-32 character strings. To use these 93 features, PCRE must be built to include appropriate support. When using UTF 94 strings you must either call the compiling function with the PCRE_UTF8, 95 PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of 96 these special sequences: 97 <pre> 98 (*UTF8) 99 (*UTF16) 100 (*UTF32) 101 (*UTF) 102 </pre> 103 (*UTF) is a generic sequence that can be used with any of the libraries. 104 Starting a pattern with such a sequence is equivalent to setting the relevant 105 option. How setting a UTF mode affects pattern matching is mentioned in several 106 places below. There is also a summary of features in the 107 <a href="pcreunicode.html"><b>pcreunicode</b></a> 108 page. 109 </P> 110 <P> 111 Some applications that allow their users to supply patterns may wish to 112 restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF 113 option is set at compile time, (*UTF) etc. are not allowed, and their 114 appearance causes an error. 115 </P> 116 <br><b> 117 Unicode property support 118 </b><br> 119 <P> 120 Another special sequence that may appear at the start of a pattern is (*UCP). 121 This has the same effect as setting the PCRE_UCP option: it causes sequences 122 such as \d and \w to use Unicode properties to determine character types, 123 instead of recognizing only characters with codes less than 128 via a lookup 124 table. 125 </P> 126 <br><b> 127 Disabling auto-possessification 128 </b><br> 129 <P> 130 If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting 131 the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making 132 quantifiers possessive when what follows cannot match the repeated item. For 133 example, by default a+b is treated as a++b. For more details, see the 134 <a href="pcreapi.html"><b>pcreapi</b></a> 135 documentation. 136 </P> 137 <br><b> 138 Disabling start-up optimizations 139 </b><br> 140 <P> 141 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the 142 PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables 143 several optimizations for quickly reaching "no match" results. For more 144 details, see the 145 <a href="pcreapi.html"><b>pcreapi</b></a> 146 documentation. 147 <a name="newlines"></a></P> 148 <br><b> 149 Newline conventions 150 </b><br> 151 <P> 152 PCRE supports five different conventions for indicating line breaks in 153 strings: a single CR (carriage return) character, a single LF (linefeed) 154 character, the two-character sequence CRLF, any of the three preceding, or any 155 Unicode newline sequence. The 156 <a href="pcreapi.html"><b>pcreapi</b></a> 157 page has 158 <a href="pcreapi.html#newlines">further discussion</a> 159 about newlines, and shows how to set the newline convention in the 160 <i>options</i> arguments for the compiling and matching functions. 161 </P> 162 <P> 163 It is also possible to specify a newline convention by starting a pattern 164 string with one of the following five sequences: 165 <pre> 166 (*CR) carriage return 167 (*LF) linefeed 168 (*CRLF) carriage return, followed by linefeed 169 (*ANYCRLF) any of the three above 170 (*ANY) all Unicode newline sequences 171 </pre> 172 These override the default and the options given to the compiling function. For 173 example, on a Unix system where LF is the default newline sequence, the pattern 174 <pre> 175 (*CR)a.b 176 </pre> 177 changes the convention to CR. That pattern matches "a\nb" because LF is no 178 longer a newline. If more than one of these settings is present, the last one 179 is used. 180 </P> 181 <P> 182 The newline convention affects where the circumflex and dollar assertions are 183 true. It also affects the interpretation of the dot metacharacter when 184 PCRE_DOTALL is not set, and the behaviour of \N. However, it does not affect 185 what the \R escape sequence matches. By default, this is any Unicode newline 186 sequence, for Perl compatibility. However, this can be changed; see the 187 description of \R in the section entitled 188 <a href="#newlineseq">"Newline sequences"</a> 189 below. A change of \R setting can be combined with a change of newline 190 convention. 191 </P> 192 <br><b> 193 Setting match and recursion limits 194 </b><br> 195 <P> 196 The caller of <b>pcre_exec()</b> can set a limit on the number of times the 197 internal <b>match()</b> function is called and on the maximum depth of 198 recursive calls. These facilities are provided to catch runaway matches that 199 are provoked by patterns with huge matching trees (a typical example is a 200 pattern with nested unlimited repeats) and to avoid running out of system stack 201 by too much recursion. When one of these limits is reached, <b>pcre_exec()</b> 202 gives an error return. The limits can also be set by items at the start of the 203 pattern of the form 204 <pre> 205 (*LIMIT_MATCH=d) 206 (*LIMIT_RECURSION=d) 207 </pre> 208 where d is any number of decimal digits. However, the value of the setting must 209 be less than the value set (or defaulted) by the caller of <b>pcre_exec()</b> 210 for it to have any effect. In other words, the pattern writer can lower the 211 limits set by the programmer, but not raise them. If there is more than one 212 setting of one of these limits, the lower value is used. 213 </P> 214 <br><a name="SEC3" href="#TOC1">EBCDIC CHARACTER CODES</a><br> 215 <P> 216 PCRE can be compiled to run in an environment that uses EBCDIC as its character 217 code rather than ASCII or Unicode (typically a mainframe system). In the 218 sections below, character code values are ASCII or Unicode; in an EBCDIC 219 environment these characters may have different code values, and there are no 220 code points greater than 255. 221 </P> 222 <br><a name="SEC4" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br> 223 <P> 224 A regular expression is a pattern that is matched against a subject string from 225 left to right. Most characters stand for themselves in a pattern, and match the 226 corresponding characters in the subject. As a trivial example, the pattern 227 <pre> 228 The quick brown fox 229 </pre> 230 matches a portion of a subject string that is identical to itself. When 231 caseless matching is specified (the PCRE_CASELESS option), letters are matched 232 independently of case. In a UTF mode, PCRE always understands the concept of 233 case for characters whose values are less than 128, so caseless matching is 234 always possible. For characters with higher values, the concept of case is 235 supported if PCRE is compiled with Unicode property support, but not otherwise. 236 If you want to use caseless matching for characters 128 and above, you must 237 ensure that PCRE is compiled with Unicode property support as well as with 238 UTF support. 239 </P> 240 <P> 241 The power of regular expressions comes from the ability to include alternatives 242 and repetitions in the pattern. These are encoded in the pattern by the use of 243 <i>metacharacters</i>, which do not stand for themselves but instead are 244 interpreted in some special way. 245 </P> 246 <P> 247 There are two different sets of metacharacters: those that are recognized 248 anywhere in the pattern except within square brackets, and those that are 249 recognized within square brackets. Outside square brackets, the metacharacters 250 are as follows: 251 <pre> 252 \ general escape character with several uses 253 ^ assert start of string (or line, in multiline mode) 254 $ assert end of string (or line, in multiline mode) 255 . match any character except newline (by default) 256 [ start character class definition 257 | start of alternative branch 258 ( start subpattern 259 ) end subpattern 260 ? extends the meaning of ( 261 also 0 or 1 quantifier 262 also quantifier minimizer 263 * 0 or more quantifier 264 + 1 or more quantifier 265 also "possessive quantifier" 266 { start min/max quantifier 267 </pre> 268 Part of a pattern that is in square brackets is called a "character class". In 269 a character class the only metacharacters are: 270 <pre> 271 \ general escape character 272 ^ negate the class, but only if the first character 273 - indicates character range 274 [ POSIX character class (only if followed by POSIX syntax) 275 ] terminates the character class 276 </pre> 277 The following sections describe the use of each of the metacharacters. 278 </P> 279 <br><a name="SEC5" href="#TOC1">BACKSLASH</a><br> 280 <P> 281 The backslash character has several uses. Firstly, if it is followed by a 282 character that is not a number or a letter, it takes away any special meaning 283 that character may have. This use of backslash as an escape character applies 284 both inside and outside character classes. 285 </P> 286 <P> 287 For example, if you want to match a * character, you write \* in the pattern. 288 This escaping action applies whether or not the following character would 289 otherwise be interpreted as a metacharacter, so it is always safe to precede a 290 non-alphanumeric with backslash to specify that it stands for itself. In 291 particular, if you want to match a backslash, you write \\. 292 </P> 293 <P> 294 In a UTF mode, only ASCII numbers and letters have any special meaning after a 295 backslash. All other characters (in particular, those whose codepoints are 296 greater than 127) are treated as literals. 297 </P> 298 <P> 299 If a pattern is compiled with the PCRE_EXTENDED option, most white space in the 300 pattern (other than in a character class), and characters between a # outside a 301 character class and the next newline, inclusive, are ignored. An escaping 302 backslash can be used to include a white space or # character as part of the 303 pattern. 304 </P> 305 <P> 306 If you want to remove the special meaning from a sequence of characters, you 307 can do so by putting them between \Q and \E. This is different from Perl in 308 that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in 309 Perl, $ and @ cause variable interpolation. Note the following examples: 310 <pre> 311 Pattern PCRE matches Perl matches 312 313 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz 314 \Qabc\$xyz\E abc\$xyz abc\$xyz 315 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 316 </pre> 317 The \Q...\E sequence is recognized both inside and outside character classes. 318 An isolated \E that is not preceded by \Q is ignored. If \Q is not followed 319 by \E later in the pattern, the literal interpretation continues to the end of 320 the pattern (that is, \E is assumed at the end). If the isolated \Q is inside 321 a character class, this causes an error, because the character class is not 322 terminated. 323 <a name="digitsafterbackslash"></a></P> 324 <br><b> 325 Non-printing characters 326 </b><br> 327 <P> 328 A second use of backslash provides a way of encoding non-printing characters 329 in patterns in a visible manner. There is no restriction on the appearance of 330 non-printing characters, apart from the binary zero that terminates a pattern, 331 but when a pattern is being prepared by text editing, it is often easier to use 332 one of the following escape sequences than the binary character it represents. 333 In an ASCII or Unicode environment, these escapes are as follows: 334 <pre> 335 \a alarm, that is, the BEL character (hex 07) 336 \cx "control-x", where x is any ASCII character 337 \e escape (hex 1B) 338 \f form feed (hex 0C) 339 \n linefeed (hex 0A) 340 \r carriage return (hex 0D) 341 \t tab (hex 09) 342 \0dd character with octal code 0dd 343 \ddd character with octal code ddd, or back reference 344 \o{ddd..} character with octal code ddd.. 345 \xhh character with hex code hh 346 \x{hhh..} character with hex code hhh.. (non-JavaScript mode) 347 \uhhhh character with hex code hhhh (JavaScript mode only) 348 </pre> 349 The precise effect of \cx on ASCII characters is as follows: if x is a lower 350 case letter, it is converted to upper case. Then bit 6 of the character (hex 351 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A), 352 but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the 353 data item (byte or 16-bit value) following \c has a value greater than 127, a 354 compile-time error occurs. This locks out non-ASCII characters in all modes. 355 </P> 356 <P> 357 When PCRE is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t 358 generate the appropriate EBCDIC code values. The \c escape is processed 359 as specified for Perl in the <b>perlebcdic</b> document. The only characters 360 that are allowed after \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?. Any 361 other character provokes a compile-time error. The sequence \@ encodes 362 character code 0; the letters (in either case) encode characters 1-26 (hex 01 363 to hex 1A); [, \, ], ^, and _ encode characters 27-31 (hex 1B to hex 1F), and 364 \? becomes either 255 (hex FF) or 95 (hex 5F). 365 </P> 366 <P> 367 Thus, apart from \?, these escapes generate the same character code values as 368 they do in an ASCII environment, though the meanings of the values mostly 369 differ. For example, \G always generates code value 7, which is BEL in ASCII 370 but DEL in EBCDIC. 371 </P> 372 <P> 373 The sequence \? generates DEL (127, hex 7F) in an ASCII environment, but 374 because 127 is not a control character in EBCDIC, Perl makes it generate the 375 APC character. Unfortunately, there are several variants of EBCDIC. In most of 376 them the APC character has the value 255 (hex FF), but in the one Perl calls 377 POSIX-BC its value is 95 (hex 5F). If certain other characters have POSIX-BC 378 values, PCRE makes \? generate 95; otherwise it generates 255. 379 </P> 380 <P> 381 After \0 up to two further octal digits are read. If there are fewer than two 382 digits, just those that are present are used. Thus the sequence \0\x\015 383 specifies two binary zeros followed by a CR character (code value 13). Make 384 sure you supply two digits after the initial zero if the pattern character that 385 follows is itself an octal digit. 386 </P> 387 <P> 388 The escape \o must be followed by a sequence of octal digits, enclosed in 389 braces. An error occurs if this is not the case. This escape is a recent 390 addition to Perl; it provides way of specifying character code points as octal 391 numbers greater than 0777, and it also allows octal numbers and back references 392 to be unambiguously specified. 393 </P> 394 <P> 395 For greater clarity and unambiguity, it is best to avoid following \ by a 396 digit greater than zero. Instead, use \o{} or \x{} to specify character 397 numbers, and \g{} to specify back references. The following paragraphs 398 describe the old, ambiguous syntax. 399 </P> 400 <P> 401 The handling of a backslash followed by a digit other than 0 is complicated, 402 and Perl has changed in recent releases, causing PCRE also to change. Outside a 403 character class, PCRE reads the digit and any following digits as a decimal 404 number. If the number is less than 8, or if there have been at least that many 405 previous capturing left parentheses in the expression, the entire sequence is 406 taken as a <i>back reference</i>. A description of how this works is given 407 <a href="#backreferences">later,</a> 408 following the discussion of 409 <a href="#subpattern">parenthesized subpatterns.</a> 410 </P> 411 <P> 412 Inside a character class, or if the decimal number following \ is greater than 413 7 and there have not been that many capturing subpatterns, PCRE handles \8 and 414 \9 as the literal characters "8" and "9", and otherwise re-reads up to three 415 octal digits following the backslash, using them to generate a data character. 416 Any subsequent digits stand for themselves. For example: 417 <pre> 418 \040 is another way of writing an ASCII space 419 \40 is the same, provided there are fewer than 40 previous capturing subpatterns 420 \7 is always a back reference 421 \11 might be a back reference, or another way of writing a tab 422 \011 is always a tab 423 \0113 is a tab followed by the character "3" 424 \113 might be a back reference, otherwise the character with octal code 113 425 \377 might be a back reference, otherwise the value 255 (decimal) 426 \81 is either a back reference, or the two characters "8" and "1" 427 </pre> 428 Note that octal values of 100 or greater that are specified using this syntax 429 must not be introduced by a leading zero, because no more than three octal 430 digits are ever read. 431 </P> 432 <P> 433 By default, after \x that is not followed by {, from zero to two hexadecimal 434 digits are read (letters can be in upper or lower case). Any number of 435 hexadecimal digits may appear between \x{ and }. If a character other than 436 a hexadecimal digit appears between \x{ and }, or if there is no terminating 437 }, an error occurs. 438 </P> 439 <P> 440 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is 441 as just described only when it is followed by two hexadecimal digits. 442 Otherwise, it matches a literal "x" character. In JavaScript mode, support for 443 code points greater than 256 is provided by \u, which must be followed by 444 four hexadecimal digits; otherwise it matches a literal "u" character. 445 </P> 446 <P> 447 Characters whose value is less than 256 can be defined by either of the two 448 syntaxes for \x (or by \u in JavaScript mode). There is no difference in the 449 way they are handled. For example, \xdc is exactly the same as \x{dc} (or 450 \u00dc in JavaScript mode). 451 </P> 452 <br><b> 453 Constraints on character values 454 </b><br> 455 <P> 456 Characters that are specified using octal or hexadecimal numbers are 457 limited to certain values, as follows: 458 <pre> 459 8-bit non-UTF mode less than 0x100 460 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint 461 16-bit non-UTF mode less than 0x10000 462 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint 463 32-bit non-UTF mode less than 0x100000000 464 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint 465 </pre> 466 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called 467 "surrogate" codepoints), and 0xffef. 468 </P> 469 <br><b> 470 Escape sequences in character classes 471 </b><br> 472 <P> 473 All the sequences that define a single character value can be used both inside 474 and outside character classes. In addition, inside a character class, \b is 475 interpreted as the backspace character (hex 08). 476 </P> 477 <P> 478 \N is not allowed in a character class. \B, \R, and \X are not special 479 inside a character class. Like other unrecognized escape sequences, they are 480 treated as the literal characters "B", "R", and "X" by default, but cause an 481 error if the PCRE_EXTRA option is set. Outside a character class, these 482 sequences have different meanings. 483 </P> 484 <br><b> 485 Unsupported escape sequences 486 </b><br> 487 <P> 488 In Perl, the sequences \l, \L, \u, and \U are recognized by its string 489 handler and used to modify the case of following characters. By default, PCRE 490 does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT 491 option is set, \U matches a "U" character, and \u can be used to define a 492 character by code point, as described in the previous section. 493 </P> 494 <br><b> 495 Absolute and relative back references 496 </b><br> 497 <P> 498 The sequence \g followed by an unsigned or a negative number, optionally 499 enclosed in braces, is an absolute or relative back reference. A named back 500 reference can be coded as \g{name}. Back references are discussed 501 <a href="#backreferences">later,</a> 502 following the discussion of 503 <a href="#subpattern">parenthesized subpatterns.</a> 504 </P> 505 <br><b> 506 Absolute and relative subroutine calls 507 </b><br> 508 <P> 509 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or 510 a number enclosed either in angle brackets or single quotes, is an alternative 511 syntax for referencing a subpattern as a "subroutine". Details are discussed 512 <a href="#onigurumasubroutines">later.</a> 513 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i> 514 synonymous. The former is a back reference; the latter is a 515 <a href="#subpatternsassubroutines">subroutine</a> 516 call. 517 <a name="genericchartypes"></a></P> 518 <br><b> 519 Generic character types 520 </b><br> 521 <P> 522 Another use of backslash is for specifying generic character types: 523 <pre> 524 \d any decimal digit 525 \D any character that is not a decimal digit 526 \h any horizontal white space character 527 \H any character that is not a horizontal white space character 528 \s any white space character 529 \S any character that is not a white space character 530 \v any vertical white space character 531 \V any character that is not a vertical white space character 532 \w any "word" character 533 \W any "non-word" character 534 </pre> 535 There is also the single sequence \N, which matches a non-newline character. 536 This is the same as 537 <a href="#fullstopdot">the "." metacharacter</a> 538 when PCRE_DOTALL is not set. Perl also uses \N to match characters by name; 539 PCRE does not support this. 540 </P> 541 <P> 542 Each pair of lower and upper case escape sequences partitions the complete set 543 of characters into two disjoint sets. Any given character matches one, and only 544 one, of each pair. The sequences can appear both inside and outside character 545 classes. They each match one character of the appropriate type. If the current 546 matching point is at the end of the subject string, all of them fail, because 547 there is no character to match. 548 </P> 549 <P> 550 For compatibility with Perl, \s did not used to match the VT character (code 551 11), which made it different from the the POSIX "space" class. However, Perl 552 added VT at release 5.18, and PCRE followed suit at release 8.34. The default 553 \s characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space 554 (32), which are defined as white space in the "C" locale. This list may vary if 555 locale-specific matching is taking place. For example, in some locales the 556 "non-breaking space" character (\xA0) is recognized as white space, and in 557 others the VT character is not. 558 </P> 559 <P> 560 A "word" character is an underscore or any character that is a letter or digit. 561 By default, the definition of letters and digits is controlled by PCRE's 562 low-valued character tables, and may vary if locale-specific matching is taking 563 place (see 564 <a href="pcreapi.html#localesupport">"Locale support"</a> 565 in the 566 <a href="pcreapi.html"><b>pcreapi</b></a> 567 page). For example, in a French locale such as "fr_FR" in Unix-like systems, 568 or "french" in Windows, some character codes greater than 127 are used for 569 accented letters, and these are then matched by \w. The use of locales with 570 Unicode is discouraged. 571 </P> 572 <P> 573 By default, characters whose code points are greater than 127 never match \d, 574 \s, or \w, and always match \D, \S, and \W, although this may vary for 575 characters in the range 128-255 when locale-specific matching is happening. 576 These escape sequences retain their original meanings from before Unicode 577 support was available, mainly for efficiency reasons. If PCRE is compiled with 578 Unicode property support, and the PCRE_UCP option is set, the behaviour is 579 changed so that Unicode properties are used to determine character types, as 580 follows: 581 <pre> 582 \d any character that matches \p{Nd} (decimal digit) 583 \s any character that matches \p{Z} or \h or \v 584 \w any character that matches \p{L} or \p{N}, plus underscore 585 </pre> 586 The upper case escapes match the inverse sets of characters. Note that \d 587 matches only decimal digits, whereas \w matches any Unicode digit, as well as 588 any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and 589 \B because they are defined in terms of \w and \W. Matching these sequences 590 is noticeably slower when PCRE_UCP is set. 591 </P> 592 <P> 593 The sequences \h, \H, \v, and \V are features that were added to Perl at 594 release 5.10. In contrast to the other sequences, which match only ASCII 595 characters by default, these always match certain high-valued code points, 596 whether or not PCRE_UCP is set. The horizontal space characters are: 597 <pre> 598 U+0009 Horizontal tab (HT) 599 U+0020 Space 600 U+00A0 Non-break space 601 U+1680 Ogham space mark 602 U+180E Mongolian vowel separator 603 U+2000 En quad 604 U+2001 Em quad 605 U+2002 En space 606 U+2003 Em space 607 U+2004 Three-per-em space 608 U+2005 Four-per-em space 609 U+2006 Six-per-em space 610 U+2007 Figure space 611 U+2008 Punctuation space 612 U+2009 Thin space 613 U+200A Hair space 614 U+202F Narrow no-break space 615 U+205F Medium mathematical space 616 U+3000 Ideographic space 617 </pre> 618 The vertical space characters are: 619 <pre> 620 U+000A Linefeed (LF) 621 U+000B Vertical tab (VT) 622 U+000C Form feed (FF) 623 U+000D Carriage return (CR) 624 U+0085 Next line (NEL) 625 U+2028 Line separator 626 U+2029 Paragraph separator 627 </pre> 628 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are 629 relevant. 630 <a name="newlineseq"></a></P> 631 <br><b> 632 Newline sequences 633 </b><br> 634 <P> 635 Outside a character class, by default, the escape sequence \R matches any 636 Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the 637 following: 638 <pre> 639 (?>\r\n|\n|\x0b|\f|\r|\x85) 640 </pre> 641 This is an example of an "atomic group", details of which are given 642 <a href="#atomicgroup">below.</a> 643 This particular group matches either the two-character sequence CR followed by 644 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, 645 U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next 646 line, U+0085). The two-character sequence is treated as a single unit that 647 cannot be split. 648 </P> 649 <P> 650 In other modes, two additional characters whose codepoints are greater than 255 651 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). 652 Unicode character property support is not needed for these characters to be 653 recognized. 654 </P> 655 <P> 656 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the 657 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF 658 either at compile time or when the pattern is matched. (BSR is an abbrevation 659 for "backslash R".) This can be made the default when PCRE is built; if this is 660 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option. 661 It is also possible to specify these settings by starting a pattern string with 662 one of the following sequences: 663 <pre> 664 (*BSR_ANYCRLF) CR, LF, or CRLF only 665 (*BSR_UNICODE) any Unicode newline sequence 666 </pre> 667 These override the default and the options given to the compiling function, but 668 they can themselves be overridden by options given to a matching function. Note 669 that these special settings, which are not Perl-compatible, are recognized only 670 at the very start of a pattern, and that they must be in upper case. If more 671 than one of them is present, the last one is used. They can be combined with a 672 change of newline convention; for example, a pattern can start with: 673 <pre> 674 (*ANY)(*BSR_ANYCRLF) 675 </pre> 676 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or 677 (*UCP) special sequences. Inside a character class, \R is treated as an 678 unrecognized escape sequence, and so matches the letter "R" by default, but 679 causes an error if PCRE_EXTRA is set. 680 <a name="uniextseq"></a></P> 681 <br><b> 682 Unicode character properties 683 </b><br> 684 <P> 685 When PCRE is built with Unicode character property support, three additional 686 escape sequences that match characters with specific properties are available. 687 When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing 688 characters whose codepoints are less than 256, but they do work in this mode. 689 The extra escape sequences are: 690 <pre> 691 \p{<i>xx</i>} a character with the <i>xx</i> property 692 \P{<i>xx</i>} a character without the <i>xx</i> property 693 \X a Unicode extended grapheme cluster 694 </pre> 695 The property names represented by <i>xx</i> above are limited to the Unicode 696 script names, the general category properties, "Any", which matches any 697 character (including newline), and some special PCRE properties (described 698 in the 699 <a href="#extraprops">next section).</a> 700 Other Perl properties such as "InMusicalSymbols" are not currently supported by 701 PCRE. Note that \P{Any} does not match any characters, so always causes a 702 match failure. 703 </P> 704 <P> 705 Sets of Unicode characters are defined as belonging to certain scripts. A 706 character from one of these sets can be matched using a script name. For 707 example: 708 <pre> 709 \p{Greek} 710 \P{Han} 711 </pre> 712 Those that are not part of an identified script are lumped together as 713 "Common". The current list of scripts is: 714 </P> 715 <P> 716 Arabic, 717 Armenian, 718 Avestan, 719 Balinese, 720 Bamum, 721 Bassa_Vah, 722 Batak, 723 Bengali, 724 Bopomofo, 725 Brahmi, 726 Braille, 727 Buginese, 728 Buhid, 729 Canadian_Aboriginal, 730 Carian, 731 Caucasian_Albanian, 732 Chakma, 733 Cham, 734 Cherokee, 735 Common, 736 Coptic, 737 Cuneiform, 738 Cypriot, 739 Cyrillic, 740 Deseret, 741 Devanagari, 742 Duployan, 743 Egyptian_Hieroglyphs, 744 Elbasan, 745 Ethiopic, 746 Georgian, 747 Glagolitic, 748 Gothic, 749 Grantha, 750 Greek, 751 Gujarati, 752 Gurmukhi, 753 Han, 754 Hangul, 755 Hanunoo, 756 Hebrew, 757 Hiragana, 758 Imperial_Aramaic, 759 Inherited, 760 Inscriptional_Pahlavi, 761 Inscriptional_Parthian, 762 Javanese, 763 Kaithi, 764 Kannada, 765 Katakana, 766 Kayah_Li, 767 Kharoshthi, 768 Khmer, 769 Khojki, 770 Khudawadi, 771 Lao, 772 Latin, 773 Lepcha, 774 Limbu, 775 Linear_A, 776 Linear_B, 777 Lisu, 778 Lycian, 779 Lydian, 780 Mahajani, 781 Malayalam, 782 Mandaic, 783 Manichaean, 784 Meetei_Mayek, 785 Mende_Kikakui, 786 Meroitic_Cursive, 787 Meroitic_Hieroglyphs, 788 Miao, 789 Modi, 790 Mongolian, 791 Mro, 792 Myanmar, 793 Nabataean, 794 New_Tai_Lue, 795 Nko, 796 Ogham, 797 Ol_Chiki, 798 Old_Italic, 799 Old_North_Arabian, 800 Old_Permic, 801 Old_Persian, 802 Old_South_Arabian, 803 Old_Turkic, 804 Oriya, 805 Osmanya, 806 Pahawh_Hmong, 807 Palmyrene, 808 Pau_Cin_Hau, 809 Phags_Pa, 810 Phoenician, 811 Psalter_Pahlavi, 812 Rejang, 813 Runic, 814 Samaritan, 815 Saurashtra, 816 Sharada, 817 Shavian, 818 Siddham, 819 Sinhala, 820 Sora_Sompeng, 821 Sundanese, 822 Syloti_Nagri, 823 Syriac, 824 Tagalog, 825 Tagbanwa, 826 Tai_Le, 827 Tai_Tham, 828 Tai_Viet, 829 Takri, 830 Tamil, 831 Telugu, 832 Thaana, 833 Thai, 834 Tibetan, 835 Tifinagh, 836 Tirhuta, 837 Ugaritic, 838 Vai, 839 Warang_Citi, 840 Yi. 841 </P> 842 <P> 843 Each character has exactly one Unicode general category property, specified by 844 a two-letter abbreviation. For compatibility with Perl, negation can be 845 specified by including a circumflex between the opening brace and the property 846 name. For example, \p{^Lu} is the same as \P{Lu}. 847 </P> 848 <P> 849 If only one letter is specified with \p or \P, it includes all the general 850 category properties that start with that letter. In this case, in the absence 851 of negation, the curly brackets in the escape sequence are optional; these two 852 examples have the same effect: 853 <pre> 854 \p{L} 855 \pL 856 </pre> 857 The following general category property codes are supported: 858 <pre> 859 C Other 860 Cc Control 861 Cf Format 862 Cn Unassigned 863 Co Private use 864 Cs Surrogate 865 866 L Letter 867 Ll Lower case letter 868 Lm Modifier letter 869 Lo Other letter 870 Lt Title case letter 871 Lu Upper case letter 872 873 M Mark 874 Mc Spacing mark 875 Me Enclosing mark 876 Mn Non-spacing mark 877 878 N Number 879 Nd Decimal number 880 Nl Letter number 881 No Other number 882 883 P Punctuation 884 Pc Connector punctuation 885 Pd Dash punctuation 886 Pe Close punctuation 887 Pf Final punctuation 888 Pi Initial punctuation 889 Po Other punctuation 890 Ps Open punctuation 891 892 S Symbol 893 Sc Currency symbol 894 Sk Modifier symbol 895 Sm Mathematical symbol 896 So Other symbol 897 898 Z Separator 899 Zl Line separator 900 Zp Paragraph separator 901 Zs Space separator 902 </pre> 903 The special property L& is also supported: it matches a character that has 904 the Lu, Ll, or Lt property, in other words, a letter that is not classified as 905 a modifier or "other". 906 </P> 907 <P> 908 The Cs (Surrogate) property applies only to characters in the range U+D800 to 909 U+DFFF. Such characters are not valid in Unicode strings and so 910 cannot be tested by PCRE, unless UTF validity checking has been turned off 911 (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and 912 PCRE_NO_UTF32_CHECK in the 913 <a href="pcreapi.html"><b>pcreapi</b></a> 914 page). Perl does not support the Cs property. 915 </P> 916 <P> 917 The long synonyms for property names that Perl supports (such as \p{Letter}) 918 are not supported by PCRE, nor is it permitted to prefix any of these 919 properties with "Is". 920 </P> 921 <P> 922 No character that is in the Unicode table has the Cn (unassigned) property. 923 Instead, this property is assumed for any code point that is not in the 924 Unicode table. 925 </P> 926 <P> 927 Specifying caseless matching does not affect these escape sequences. For 928 example, \p{Lu} always matches only upper case letters. This is different from 929 the behaviour of current versions of Perl. 930 </P> 931 <P> 932 Matching characters by Unicode property is not fast, because PCRE has to do a 933 multistage table lookup in order to find a character's property. That is why 934 the traditional escape sequences such as \d and \w do not use Unicode 935 properties in PCRE by default, though you can make them do so by setting the 936 PCRE_UCP option or by starting the pattern with (*UCP). 937 </P> 938 <br><b> 939 Extended grapheme clusters 940 </b><br> 941 <P> 942 The \X escape matches any number of Unicode characters that form an "extended 943 grapheme cluster", and treats the sequence as an atomic group 944 <a href="#atomicgroup">(see below).</a> 945 Up to and including release 8.31, PCRE matched an earlier, simpler definition 946 that was equivalent to 947 <pre> 948 (?>\PM\pM*) 949 </pre> 950 That is, it matched a character without the "mark" property, followed by zero 951 or more characters with the "mark" property. Characters with the "mark" 952 property are typically non-spacing accents that affect the preceding character. 953 </P> 954 <P> 955 This simple definition was extended in Unicode to include more complicated 956 kinds of composite character by giving each character a grapheme breaking 957 property, and creating rules that use these properties to define the boundaries 958 of extended grapheme clusters. In releases of PCRE later than 8.31, \X matches 959 one of these clusters. 960 </P> 961 <P> 962 \X always matches at least one character. Then it decides whether to add 963 additional characters according to the following rules for ending a cluster: 964 </P> 965 <P> 966 1. End at the end of the subject string. 967 </P> 968 <P> 969 2. Do not end between CR and LF; otherwise end after any control character. 970 </P> 971 <P> 972 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters 973 are of five types: L, V, T, LV, and LVT. An L character may be followed by an 974 L, V, LV, or LVT character; an LV or V character may be followed by a V or T 975 character; an LVT or T character may be follwed only by a T character. 976 </P> 977 <P> 978 4. Do not end before extending characters or spacing marks. Characters with 979 the "mark" property always have the "extend" grapheme breaking property. 980 </P> 981 <P> 982 5. Do not end after prepend characters. 983 </P> 984 <P> 985 6. Otherwise, end the cluster. 986 <a name="extraprops"></a></P> 987 <br><b> 988 PCRE's additional properties 989 </b><br> 990 <P> 991 As well as the standard Unicode properties described above, PCRE supports four 992 more that make it possible to convert traditional escape sequences such as \w 993 and \s to use Unicode properties. PCRE uses these non-standard, non-Perl 994 properties internally when PCRE_UCP is set. However, they may also be used 995 explicitly. These properties are: 996 <pre> 997 Xan Any alphanumeric character 998 Xps Any POSIX space character 999 Xsp Any Perl space character 1000 Xwd Any Perl "word" character 1001 </pre> 1002 Xan matches characters that have either the L (letter) or the N (number) 1003 property. Xps matches the characters tab, linefeed, vertical tab, form feed, or 1004 carriage return, and any other character that has the Z (separator) property. 1005 Xsp is the same as Xps; it used to exclude vertical tab, for Perl 1006 compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd 1007 matches the same characters as Xan, plus underscore. 1008 </P> 1009 <P> 1010 There is another non-standard property, Xuc, which matches any character that 1011 can be represented by a Universal Character Name in C++ and other programming 1012 languages. These are the characters $, @, ` (grave accent), and all characters 1013 with Unicode code points greater than or equal to U+00A0, except for the 1014 surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are 1015 excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH 1016 where H is a hexadecimal digit. Note that the Xuc property does not match these 1017 sequences but the characters that they represent.) 1018 <a name="resetmatchstart"></a></P> 1019 <br><b> 1020 Resetting the match start 1021 </b><br> 1022 <P> 1023 The escape sequence \K causes any previously matched characters not to be 1024 included in the final matched sequence. For example, the pattern: 1025 <pre> 1026 foo\Kbar 1027 </pre> 1028 matches "foobar", but reports that it has matched "bar". This feature is 1029 similar to a lookbehind assertion 1030 <a href="#lookbehind">(described below).</a> 1031 However, in this case, the part of the subject before the real match does not 1032 have to be of fixed length, as lookbehind assertions do. The use of \K does 1033 not interfere with the setting of 1034 <a href="#subpattern">captured substrings.</a> 1035 For example, when the pattern 1036 <pre> 1037 (foo)\Kbar 1038 </pre> 1039 matches "foobar", the first substring is still set to "foo". 1040 </P> 1041 <P> 1042 Perl documents that the use of \K within assertions is "not well defined". In 1043 PCRE, \K is acted upon when it occurs inside positive assertions, but is 1044 ignored in negative assertions. Note that when a pattern such as (?=ab\K) 1045 matches, the reported start of the match can be greater than the end of the 1046 match. 1047 <a name="smallassertions"></a></P> 1048 <br><b> 1049 Simple assertions 1050 </b><br> 1051 <P> 1052 The final use of backslash is for certain simple assertions. An assertion 1053 specifies a condition that has to be met at a particular point in a match, 1054 without consuming any characters from the subject string. The use of 1055 subpatterns for more complicated assertions is described 1056 <a href="#bigassertions">below.</a> 1057 The backslashed assertions are: 1058 <pre> 1059 \b matches at a word boundary 1060 \B matches when not at a word boundary 1061 \A matches at the start of the subject 1062 \Z matches at the end of the subject 1063 also matches before a newline at the end of the subject 1064 \z matches only at the end of the subject 1065 \G matches at the first matching position in the subject 1066 </pre> 1067 Inside a character class, \b has a different meaning; it matches the backspace 1068 character. If any other of these assertions appears in a character class, by 1069 default it matches the corresponding literal character (for example, \B 1070 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid 1071 escape sequence" error is generated instead. 1072 </P> 1073 <P> 1074 A word boundary is a position in the subject string where the current character 1075 and the previous character do not both match \w or \W (i.e. one matches 1076 \w and the other matches \W), or the start or end of the string if the 1077 first or last character matches \w, respectively. In a UTF mode, the meanings 1078 of \w and \W can be changed by setting the PCRE_UCP option. When this is 1079 done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start 1080 of word" or "end of word" metasequence. However, whatever follows \b normally 1081 determines which it is. For example, the fragment \ba matches "a" at the start 1082 of a word. 1083 </P> 1084 <P> 1085 The \A, \Z, and \z assertions differ from the traditional circumflex and 1086 dollar (described in the next section) in that they only ever match at the very 1087 start and end of the subject string, whatever options are set. Thus, they are 1088 independent of multiline mode. These three assertions are not affected by the 1089 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the 1090 circumflex and dollar metacharacters. However, if the <i>startoffset</i> 1091 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start 1092 at a point other than the beginning of the subject, \A can never match. The 1093 difference between \Z and \z is that \Z matches before a newline at the end 1094 of the string as well as at the very end, whereas \z matches only at the end. 1095 </P> 1096 <P> 1097 The \G assertion is true only when the current matching position is at the 1098 start point of the match, as specified by the <i>startoffset</i> argument of 1099 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is 1100 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate 1101 arguments, you can mimic Perl's /g option, and it is in this kind of 1102 implementation where \G can be useful. 1103 </P> 1104 <P> 1105 Note, however, that PCRE's interpretation of \G, as the start of the current 1106 match, is subtly different from Perl's, which defines it as the end of the 1107 previous match. In Perl, these can be different when the previously matched 1108 string was empty. Because PCRE does just one match at a time, it cannot 1109 reproduce this behaviour. 1110 </P> 1111 <P> 1112 If all the alternatives of a pattern begin with \G, the expression is anchored 1113 to the starting match position, and the "anchored" flag is set in the compiled 1114 regular expression. 1115 </P> 1116 <br><a name="SEC6" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br> 1117 <P> 1118 The circumflex and dollar metacharacters are zero-width assertions. That is, 1119 they test for a particular condition being true without consuming any 1120 characters from the subject string. 1121 </P> 1122 <P> 1123 Outside a character class, in the default matching mode, the circumflex 1124 character is an assertion that is true only if the current matching point is at 1125 the start of the subject string. If the <i>startoffset</i> argument of 1126 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE 1127 option is unset. Inside a character class, circumflex has an entirely different 1128 meaning 1129 <a href="#characterclass">(see below).</a> 1130 </P> 1131 <P> 1132 Circumflex need not be the first character of the pattern if a number of 1133 alternatives are involved, but it should be the first thing in each alternative 1134 in which it appears if the pattern is ever to match that branch. If all 1135 possible alternatives start with a circumflex, that is, if the pattern is 1136 constrained to match only at the start of the subject, it is said to be an 1137 "anchored" pattern. (There are also other constructs that can cause a pattern 1138 to be anchored.) 1139 </P> 1140 <P> 1141 The dollar character is an assertion that is true only if the current matching 1142 point is at the end of the subject string, or immediately before a newline at 1143 the end of the string (by default). Note, however, that it does not actually 1144 match the newline. Dollar need not be the last character of the pattern if a 1145 number of alternatives are involved, but it should be the last item in any 1146 branch in which it appears. Dollar has no special meaning in a character class. 1147 </P> 1148 <P> 1149 The meaning of dollar can be changed so that it matches only at the very end of 1150 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This 1151 does not affect the \Z assertion. 1152 </P> 1153 <P> 1154 The meanings of the circumflex and dollar characters are changed if the 1155 PCRE_MULTILINE option is set. When this is the case, a circumflex matches 1156 immediately after internal newlines as well as at the start of the subject 1157 string. It does not match after a newline that ends the string. A dollar 1158 matches before any newlines in the string, as well as at the very end, when 1159 PCRE_MULTILINE is set. When newline is specified as the two-character 1160 sequence CRLF, isolated CR and LF characters do not indicate newlines. 1161 </P> 1162 <P> 1163 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where 1164 \n represents a newline) in multiline mode, but not otherwise. Consequently, 1165 patterns that are anchored in single line mode because all branches start with 1166 ^ are not anchored in multiline mode, and a match for circumflex is possible 1167 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The 1168 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. 1169 </P> 1170 <P> 1171 Note that the sequences \A, \Z, and \z can be used to match the start and 1172 end of the subject in both modes, and if all branches of a pattern start with 1173 \A it is always anchored, whether or not PCRE_MULTILINE is set. 1174 <a name="fullstopdot"></a></P> 1175 <br><a name="SEC7" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br> 1176 <P> 1177 Outside a character class, a dot in the pattern matches any one character in 1178 the subject string except (by default) a character that signifies the end of a 1179 line. 1180 </P> 1181 <P> 1182 When a line ending is defined as a single character, dot never matches that 1183 character; when the two-character sequence CRLF is used, dot does not match CR 1184 if it is immediately followed by LF, but otherwise it matches all characters 1185 (including isolated CRs and LFs). When any Unicode line endings are being 1186 recognized, dot does not match CR or LF or any of the other line ending 1187 characters. 1188 </P> 1189 <P> 1190 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL 1191 option is set, a dot matches any one character, without exception. If the 1192 two-character sequence CRLF is present in the subject string, it takes two dots 1193 to match it. 1194 </P> 1195 <P> 1196 The handling of dot is entirely independent of the handling of circumflex and 1197 dollar, the only relationship being that they both involve newlines. Dot has no 1198 special meaning in a character class. 1199 </P> 1200 <P> 1201 The escape sequence \N behaves like a dot, except that it is not affected by 1202 the PCRE_DOTALL option. In other words, it matches any character except one 1203 that signifies the end of a line. Perl also uses \N to match characters by 1204 name; PCRE does not support this. 1205 </P> 1206 <br><a name="SEC8" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br> 1207 <P> 1208 Outside a character class, the escape sequence \C matches any one data unit, 1209 whether or not a UTF mode is set. In the 8-bit library, one data unit is one 1210 byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is 1211 a 32-bit unit. Unlike a dot, \C always 1212 matches line-ending characters. The feature is provided in Perl in order to 1213 match individual bytes in UTF-8 mode, but it is unclear how it can usefully be 1214 used. Because \C breaks up characters into individual data units, matching one 1215 unit with \C in a UTF mode means that the rest of the string may start with a 1216 malformed UTF character. This has undefined results, because PCRE assumes that 1217 it is dealing with valid UTF strings (and by default it checks this at the 1218 start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or 1219 PCRE_NO_UTF32_CHECK option is used). 1220 </P> 1221 <P> 1222 PCRE does not allow \C to appear in lookbehind assertions 1223 <a href="#lookbehind">(described below)</a> 1224 in a UTF mode, because this would make it impossible to calculate the length of 1225 the lookbehind. 1226 </P> 1227 <P> 1228 In general, the \C escape sequence is best avoided. However, one 1229 way of using it that avoids the problem of malformed UTF characters is to use a 1230 lookahead to check the length of the next character, as in this pattern, which 1231 could be used with a UTF-8 string (ignore white space and line breaks): 1232 <pre> 1233 (?| (?=[\x00-\x7f])(\C) | 1234 (?=[\x80-\x{7ff}])(\C)(\C) | 1235 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | 1236 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) 1237 </pre> 1238 A group that starts with (?| resets the capturing parentheses numbers in each 1239 alternative (see 1240 <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a> 1241 below). The assertions at the start of each branch check the next UTF-8 1242 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The 1243 character's individual bytes are then captured by the appropriate number of 1244 groups. 1245 <a name="characterclass"></a></P> 1246 <br><a name="SEC9" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br> 1247 <P> 1248 An opening square bracket introduces a character class, terminated by a closing 1249 square bracket. A closing square bracket on its own is not special by default. 1250 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square 1251 bracket causes a compile-time error. If a closing square bracket is required as 1252 a member of the class, it should be the first data character in the class 1253 (after an initial circumflex, if present) or escaped with a backslash. 1254 </P> 1255 <P> 1256 A character class matches a single character in the subject. In a UTF mode, the 1257 character may be more than one data unit long. A matched character must be in 1258 the set of characters defined by the class, unless the first character in the 1259 class definition is a circumflex, in which case the subject character must not 1260 be in the set defined by the class. If a circumflex is actually required as a 1261 member of the class, ensure it is not the first character, or escape it with a 1262 backslash. 1263 </P> 1264 <P> 1265 For example, the character class [aeiou] matches any lower case vowel, while 1266 [^aeiou] matches any character that is not a lower case vowel. Note that a 1267 circumflex is just a convenient notation for specifying the characters that 1268 are in the class by enumerating those that are not. A class that starts with a 1269 circumflex is not an assertion; it still consumes a character from the subject 1270 string, and therefore it fails if the current pointer is at the end of the 1271 string. 1272 </P> 1273 <P> 1274 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff) 1275 can be included in a class as a literal string of data units, or by using the 1276 \x{ escaping mechanism. 1277 </P> 1278 <P> 1279 When caseless matching is set, any letters in a class represent both their 1280 upper case and lower case versions, so for example, a caseless [aeiou] matches 1281 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a 1282 caseful version would. In a UTF mode, PCRE always understands the concept of 1283 case for characters whose values are less than 128, so caseless matching is 1284 always possible. For characters with higher values, the concept of case is 1285 supported if PCRE is compiled with Unicode property support, but not otherwise. 1286 If you want to use caseless matching in a UTF mode for characters 128 and 1287 above, you must ensure that PCRE is compiled with Unicode property support as 1288 well as with UTF support. 1289 </P> 1290 <P> 1291 Characters that might indicate line breaks are never treated in any special way 1292 when matching character classes, whatever line-ending sequence is in use, and 1293 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class 1294 such as [^a] always matches one of these characters. 1295 </P> 1296 <P> 1297 The minus (hyphen) character can be used to specify a range of characters in a 1298 character class. For example, [d-m] matches any letter between d and m, 1299 inclusive. If a minus character is required in a class, it must be escaped with 1300 a backslash or appear in a position where it cannot be interpreted as 1301 indicating a range, typically as the first or last character in the class, or 1302 immediately after a range. For example, [b-d-z] matches letters in the range b 1303 to d, a hyphen character, or z. 1304 </P> 1305 <P> 1306 It is not possible to have the literal character "]" as the end character of a 1307 range. A pattern such as [W-]46] is interpreted as a class of two characters 1308 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or 1309 "-46]". However, if the "]" is escaped with a backslash it is interpreted as 1310 the end of range, so [W-\]46] is interpreted as a class containing a range 1311 followed by two other characters. The octal or hexadecimal representation of 1312 "]" can also be used to end a range. 1313 </P> 1314 <P> 1315 An error is generated if a POSIX character class (see below) or an escape 1316 sequence other than one that defines a single character appears at a point 1317 where a range ending character is expected. For example, [z-\xff] is valid, 1318 but [A-\d] and [A-[:digit:]] are not. 1319 </P> 1320 <P> 1321 Ranges operate in the collating sequence of character values. They can also be 1322 used for characters specified numerically, for example [\000-\037]. Ranges 1323 can include any characters that are valid for the current mode. 1324 </P> 1325 <P> 1326 If a range that includes letters is used when caseless matching is set, it 1327 matches the letters in either case. For example, [W-c] is equivalent to 1328 [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character 1329 tables for a French locale are in use, [\xc8-\xcb] matches accented E 1330 characters in both cases. In UTF modes, PCRE supports the concept of case for 1331 characters with values greater than 128 only when it is compiled with Unicode 1332 property support. 1333 </P> 1334 <P> 1335 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, 1336 \V, \w, and \W may appear in a character class, and add the characters that 1337 they match to the class. For example, [\dABCDEF] matches any hexadecimal 1338 digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w 1339 and their upper case partners, just as it does when they appear outside a 1340 character class, as described in the section entitled 1341 <a href="#genericchartypes">"Generic character types"</a> 1342 above. The escape sequence \b has a different meaning inside a character 1343 class; it matches the backspace character. The sequences \B, \N, \R, and \X 1344 are not special inside a character class. Like any other unrecognized escape 1345 sequences, they are treated as the literal characters "B", "N", "R", and "X" by 1346 default, but cause an error if the PCRE_EXTRA option is set. 1347 </P> 1348 <P> 1349 A circumflex can conveniently be used with the upper case character types to 1350 specify a more restricted set of characters than the matching lower case type. 1351 For example, the class [^\W_] matches any letter or digit, but not underscore, 1352 whereas [\w] includes underscore. A positive character class should be read as 1353 "something OR something OR ..." and a negative class as "NOT something AND NOT 1354 something AND NOT ...". 1355 </P> 1356 <P> 1357 The only metacharacters that are recognized in character classes are backslash, 1358 hyphen (only where it can be interpreted as specifying a range), circumflex 1359 (only at the start), opening square bracket (only when it can be interpreted as 1360 introducing a POSIX class name, or for a special compatibility feature - see 1361 the next two sections), and the terminating closing square bracket. However, 1362 escaping other non-alphanumeric characters does no harm. 1363 </P> 1364 <br><a name="SEC10" href="#TOC1">POSIX CHARACTER CLASSES</a><br> 1365 <P> 1366 Perl supports the POSIX notation for character classes. This uses names 1367 enclosed by [: and :] within the enclosing square brackets. PCRE also supports 1368 this notation. For example, 1369 <pre> 1370 [01[:alpha:]%] 1371 </pre> 1372 matches "0", "1", any alphabetic character, or "%". The supported class names 1373 are: 1374 <pre> 1375 alnum letters and digits 1376 alpha letters 1377 ascii character codes 0 - 127 1378 blank space or tab only 1379 cntrl control characters 1380 digit decimal digits (same as \d) 1381 graph printing characters, excluding space 1382 lower lower case letters 1383 print printing characters, including space 1384 punct printing characters, excluding letters and digits and space 1385 space white space (the same as \s from PCRE 8.34) 1386 upper upper case letters 1387 word "word" characters (same as \w) 1388 xdigit hexadecimal digits 1389 </pre> 1390 The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), 1391 and space (32). If locale-specific matching is taking place, the list of space 1392 characters may be different; there may be fewer or more of them. "Space" used 1393 to be different to \s, which did not include VT, for Perl compatibility. 1394 However, Perl changed at release 5.18, and PCRE followed at release 8.34. 1395 "Space" and \s now match the same set of characters. 1396 </P> 1397 <P> 1398 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 1399 5.8. Another Perl extension is negation, which is indicated by a ^ character 1400 after the colon. For example, 1401 <pre> 1402 [12[:^digit:]] 1403 </pre> 1404 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX 1405 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not 1406 supported, and an error is given if they are encountered. 1407 </P> 1408 <P> 1409 By default, characters with values greater than 128 do not match any of the 1410 POSIX character classes. However, if the PCRE_UCP option is passed to 1411 <b>pcre_compile()</b>, some of the classes are changed so that Unicode character 1412 properties are used. This is achieved by replacing certain POSIX classes by 1413 other sequences, as follows: 1414 <pre> 1415 [:alnum:] becomes \p{Xan} 1416 [:alpha:] becomes \p{L} 1417 [:blank:] becomes \h 1418 [:digit:] becomes \p{Nd} 1419 [:lower:] becomes \p{Ll} 1420 [:space:] becomes \p{Xps} 1421 [:upper:] becomes \p{Lu} 1422 [:word:] becomes \p{Xwd} 1423 </pre> 1424 Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX 1425 classes are handled specially in UCP mode: 1426 </P> 1427 <P> 1428 [:graph:] 1429 This matches characters that have glyphs that mark the page when printed. In 1430 Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf 1431 properties, except for: 1432 <pre> 1433 U+061C Arabic Letter Mark 1434 U+180E Mongolian Vowel Separator 1435 U+2066 - U+2069 Various "isolate"s 1436 1437 </PRE> 1438 </P> 1439 <P> 1440 [:print:] 1441 This matches the same characters as [:graph:] plus space characters that are 1442 not controls, that is, characters with the Zs property. 1443 </P> 1444 <P> 1445 [:punct:] 1446 This matches all characters that have the Unicode P (punctuation) property, 1447 plus those characters whose code points are less than 128 that have the S 1448 (Symbol) property. 1449 </P> 1450 <P> 1451 The other POSIX classes are unchanged, and match only characters with code 1452 points less than 128. 1453 </P> 1454 <br><a name="SEC11" href="#TOC1">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a><br> 1455 <P> 1456 In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly 1457 syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of 1458 word". PCRE treats these items as follows: 1459 <pre> 1460 [[:<:]] is converted to \b(?=\w) 1461 [[:>:]] is converted to \b(?<=\w) 1462 </pre> 1463 Only these exact character sequences are recognized. A sequence such as 1464 [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is 1465 not compatible with Perl. It is provided to help migrations from other 1466 environments, and is best not used in any new patterns. Note that \b matches 1467 at the start and the end of a word (see 1468 <a href="#smallassertions">"Simple assertions"</a> 1469 above), and in a Perl-style pattern the preceding or following character 1470 normally shows which is wanted, without the need for the assertions that are 1471 used above in order to give exactly the POSIX behaviour. 1472 </P> 1473 <br><a name="SEC12" href="#TOC1">VERTICAL BAR</a><br> 1474 <P> 1475 Vertical bar characters are used to separate alternative patterns. For example, 1476 the pattern 1477 <pre> 1478 gilbert|sullivan 1479 </pre> 1480 matches either "gilbert" or "sullivan". Any number of alternatives may appear, 1481 and an empty alternative is permitted (matching the empty string). The matching 1482 process tries each alternative in turn, from left to right, and the first one 1483 that succeeds is used. If the alternatives are within a subpattern 1484 <a href="#subpattern">(defined below),</a> 1485 "succeeds" means matching the rest of the main pattern as well as the 1486 alternative in the subpattern. 1487 </P> 1488 <br><a name="SEC13" href="#TOC1">INTERNAL OPTION SETTING</a><br> 1489 <P> 1490 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and 1491 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within 1492 the pattern by a sequence of Perl option letters enclosed between "(?" and ")". 1493 The option letters are 1494 <pre> 1495 i for PCRE_CASELESS 1496 m for PCRE_MULTILINE 1497 s for PCRE_DOTALL 1498 x for PCRE_EXTENDED 1499 </pre> 1500 For example, (?im) sets caseless, multiline matching. It is also possible to 1501 unset these options by preceding the letter with a hyphen, and a combined 1502 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and 1503 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also 1504 permitted. If a letter appears both before and after the hyphen, the option is 1505 unset. 1506 </P> 1507 <P> 1508 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be 1509 changed in the same way as the Perl-compatible options by using the characters 1510 J, U and X respectively. 1511 </P> 1512 <P> 1513 When one of these option changes occurs at top level (that is, not inside 1514 subpattern parentheses), the change applies to the remainder of the pattern 1515 that follows. If the change is placed right at the start of a pattern, PCRE 1516 extracts it into the global options (and it will therefore show up in data 1517 extracted by the <b>pcre_fullinfo()</b> function). 1518 </P> 1519 <P> 1520 An option change within a subpattern (see below for a description of 1521 subpatterns) affects only that part of the subpattern that follows it, so 1522 <pre> 1523 (a(?i)b)c 1524 </pre> 1525 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). 1526 By this means, options can be made to have different settings in different 1527 parts of the pattern. Any changes made in one alternative do carry on 1528 into subsequent branches within the same subpattern. For example, 1529 <pre> 1530 (a(?i)b|c) 1531 </pre> 1532 matches "ab", "aB", "c", and "C", even though when matching "C" the first 1533 branch is abandoned before the option setting. This is because the effects of 1534 option settings happen at compile time. There would be some very weird 1535 behaviour otherwise. 1536 </P> 1537 <P> 1538 <b>Note:</b> There are other PCRE-specific options that can be set by the 1539 application when the compiling or matching functions are called. In some cases 1540 the pattern can contain special leading sequences such as (*CRLF) to override 1541 what the application has set or what has been defaulted. Details are given in 1542 the section entitled 1543 <a href="#newlineseq">"Newline sequences"</a> 1544 above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading 1545 sequences that can be used to set UTF and Unicode property modes; they are 1546 equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP 1547 options, respectively. The (*UTF) sequence is a generic version that can be 1548 used with any of the libraries. However, the application can set the 1549 PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences. 1550 <a name="subpattern"></a></P> 1551 <br><a name="SEC14" href="#TOC1">SUBPATTERNS</a><br> 1552 <P> 1553 Subpatterns are delimited by parentheses (round brackets), which can be nested. 1554 Turning part of a pattern into a subpattern does two things: 1555 <br> 1556 <br> 1557 1. It localizes a set of alternatives. For example, the pattern 1558 <pre> 1559 cat(aract|erpillar|) 1560 </pre> 1561 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would 1562 match "cataract", "erpillar" or an empty string. 1563 <br> 1564 <br> 1565 2. It sets up the subpattern as a capturing subpattern. This means that, when 1566 the whole pattern matches, that portion of the subject string that matched the 1567 subpattern is passed back to the caller via the <i>ovector</i> argument of the 1568 matching function. (This applies only to the traditional matching functions; 1569 the DFA matching functions do not support capturing.) 1570 </P> 1571 <P> 1572 Opening parentheses are counted from left to right (starting from 1) to obtain 1573 numbers for the capturing subpatterns. For example, if the string "the red 1574 king" is matched against the pattern 1575 <pre> 1576 the ((red|white) (king|queen)) 1577 </pre> 1578 the captured substrings are "red king", "red", and "king", and are numbered 1, 1579 2, and 3, respectively. 1580 </P> 1581 <P> 1582 The fact that plain parentheses fulfil two functions is not always helpful. 1583 There are often times when a grouping subpattern is required without a 1584 capturing requirement. If an opening parenthesis is followed by a question mark 1585 and a colon, the subpattern does not do any capturing, and is not counted when 1586 computing the number of any subsequent capturing subpatterns. For example, if 1587 the string "the white queen" is matched against the pattern 1588 <pre> 1589 the ((?:red|white) (king|queen)) 1590 </pre> 1591 the captured substrings are "white queen" and "queen", and are numbered 1 and 1592 2. The maximum number of capturing subpatterns is 65535. 1593 </P> 1594 <P> 1595 As a convenient shorthand, if any option settings are required at the start of 1596 a non-capturing subpattern, the option letters may appear between the "?" and 1597 the ":". Thus the two patterns 1598 <pre> 1599 (?i:saturday|sunday) 1600 (?:(?i)saturday|sunday) 1601 </pre> 1602 match exactly the same set of strings. Because alternative branches are tried 1603 from left to right, and options are not reset until the end of the subpattern 1604 is reached, an option setting in one branch does affect subsequent branches, so 1605 the above patterns match "SUNDAY" as well as "Saturday". 1606 <a name="dupsubpatternnumber"></a></P> 1607 <br><a name="SEC15" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br> 1608 <P> 1609 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses 1610 the same numbers for its capturing parentheses. Such a subpattern starts with 1611 (?| and is itself a non-capturing subpattern. For example, consider this 1612 pattern: 1613 <pre> 1614 (?|(Sat)ur|(Sun))day 1615 </pre> 1616 Because the two alternatives are inside a (?| group, both sets of capturing 1617 parentheses are numbered one. Thus, when the pattern matches, you can look 1618 at captured substring number one, whichever alternative matched. This construct 1619 is useful when you want to capture part, but not all, of one of a number of 1620 alternatives. Inside a (?| group, parentheses are numbered as usual, but the 1621 number is reset at the start of each branch. The numbers of any capturing 1622 parentheses that follow the subpattern start after the highest number used in 1623 any branch. The following example is taken from the Perl documentation. The 1624 numbers underneath show in which buffer the captured content will be stored. 1625 <pre> 1626 # before ---------------branch-reset----------- after 1627 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x 1628 # 1 2 2 3 2 3 4 1629 </pre> 1630 A back reference to a numbered subpattern uses the most recent value that is 1631 set for that number by any subpattern. The following pattern matches "abcabc" 1632 or "defdef": 1633 <pre> 1634 /(?|(abc)|(def))\1/ 1635 </pre> 1636 In contrast, a subroutine call to a numbered subpattern always refers to the 1637 first one in the pattern with the given number. The following pattern matches 1638 "abcabc" or "defabc": 1639 <pre> 1640 /(?|(abc)|(def))(?1)/ 1641 </pre> 1642 If a 1643 <a href="#conditions">condition test</a> 1644 for a subpattern's having matched refers to a non-unique number, the test is 1645 true if any of the subpatterns of that number have matched. 1646 </P> 1647 <P> 1648 An alternative approach to using this "branch reset" feature is to use 1649 duplicate named subpatterns, as described in the next section. 1650 </P> 1651 <br><a name="SEC16" href="#TOC1">NAMED SUBPATTERNS</a><br> 1652 <P> 1653 Identifying capturing parentheses by number is simple, but it can be very hard 1654 to keep track of the numbers in complicated regular expressions. Furthermore, 1655 if an expression is modified, the numbers may change. To help with this 1656 difficulty, PCRE supports the naming of subpatterns. This feature was not 1657 added to Perl until release 5.10. Python had the feature earlier, and PCRE 1658 introduced it at release 4.0, using the Python syntax. PCRE now supports both 1659 the Perl and the Python syntax. Perl allows identically numbered subpatterns to 1660 have different names, but PCRE does not. 1661 </P> 1662 <P> 1663 In PCRE, a subpattern can be named in one of three ways: (?<name>...) or 1664 (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing 1665 parentheses from other parts of the pattern, such as 1666 <a href="#backreferences">back references,</a> 1667 <a href="#recursion">recursion,</a> 1668 and 1669 <a href="#conditions">conditions,</a> 1670 can be made by name as well as by number. 1671 </P> 1672 <P> 1673 Names consist of up to 32 alphanumeric characters and underscores, but must 1674 start with a non-digit. Named capturing parentheses are still allocated numbers 1675 as well as names, exactly as if the names were not present. The PCRE API 1676 provides function calls for extracting the name-to-number translation table 1677 from a compiled pattern. There is also a convenience function for extracting a 1678 captured substring by name. 1679 </P> 1680 <P> 1681 By default, a name must be unique within a pattern, but it is possible to relax 1682 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate 1683 names are also always permitted for subpatterns with the same number, set up as 1684 described in the previous section.) Duplicate names can be useful for patterns 1685 where only one instance of the named parentheses can match. Suppose you want to 1686 match the name of a weekday, either as a 3-letter abbreviation or as the full 1687 name, and in both cases you want to extract the abbreviation. This pattern 1688 (ignoring the line breaks) does the job: 1689 <pre> 1690 (?<DN>Mon|Fri|Sun)(?:day)?| 1691 (?<DN>Tue)(?:sday)?| 1692 (?<DN>Wed)(?:nesday)?| 1693 (?<DN>Thu)(?:rsday)?| 1694 (?<DN>Sat)(?:urday)? 1695 </pre> 1696 There are five capturing substrings, but only one is ever set after a match. 1697 (An alternative way of solving this problem is to use a "branch reset" 1698 subpattern, as described in the previous section.) 1699 </P> 1700 <P> 1701 The convenience function for extracting the data by name returns the substring 1702 for the first (and in this example, the only) subpattern of that name that 1703 matched. This saves searching to find which numbered subpattern it was. 1704 </P> 1705 <P> 1706 If you make a back reference to a non-unique named subpattern from elsewhere in 1707 the pattern, the subpatterns to which the name refers are checked in the order 1708 in which they appear in the overall pattern. The first one that is set is used 1709 for the reference. For example, this pattern matches both "foofoo" and 1710 "barbar" but not "foobar" or "barfoo": 1711 <pre> 1712 (?:(?<n>foo)|(?<n>bar))\k<n> 1713 1714 </PRE> 1715 </P> 1716 <P> 1717 If you make a subroutine call to a non-unique named subpattern, the one that 1718 corresponds to the first occurrence of the name is used. In the absence of 1719 duplicate numbers (see the previous section) this is the one with the lowest 1720 number. 1721 </P> 1722 <P> 1723 If you use a named reference in a condition 1724 test (see the 1725 <a href="#conditions">section about conditions</a> 1726 below), either to check whether a subpattern has matched, or to check for 1727 recursion, all subpatterns with the same name are tested. If the condition is 1728 true for any one of them, the overall condition is true. This is the same 1729 behaviour as testing by number. For further details of the interfaces for 1730 handling named subpatterns, see the 1731 <a href="pcreapi.html"><b>pcreapi</b></a> 1732 documentation. 1733 </P> 1734 <P> 1735 <b>Warning:</b> You cannot use different names to distinguish between two 1736 subpatterns with the same number because PCRE uses only the numbers when 1737 matching. For this reason, an error is given at compile time if different names 1738 are given to subpatterns with the same number. However, you can always give the 1739 same name to subpatterns with the same number, even when PCRE_DUPNAMES is not 1740 set. 1741 </P> 1742 <br><a name="SEC17" href="#TOC1">REPETITION</a><br> 1743 <P> 1744 Repetition is specified by quantifiers, which can follow any of the following 1745 items: 1746 <pre> 1747 a literal data character 1748 the dot metacharacter 1749 the \C escape sequence 1750 the \X escape sequence 1751 the \R escape sequence 1752 an escape such as \d or \pL that matches a single character 1753 a character class 1754 a back reference (see next section) 1755 a parenthesized subpattern (including assertions) 1756 a subroutine call to a subpattern (recursive or otherwise) 1757 </pre> 1758 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 <pre> 1763 z{2,4} 1764 </pre> 1765 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 <pre> 1770 [aeiou]{3,} 1771 </pre> 1772 matches at least 3 successive vowels, but may match many more, while 1773 <pre> 1774 \d{8} 1775 </pre> 1776 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 </P> 1781 <P> 1782 In UTF modes, quantifiers apply to characters rather than to individual data 1783 units. Thus, for example, \x{100}{2} matches two characters, each of 1784 which is represented by a two-byte sequence in a UTF-8 string. Similarly, 1785 \X{3} matches three Unicode extended grapheme clusters, each of which may be 1786 several data units long (and they may be of different lengths). 1787 </P> 1788 <P> 1789 The quantifier {0} is permitted, causing the expression to behave as if the 1790 previous item and the quantifier were not present. This may be useful for 1791 subpatterns that are referenced as 1792 <a href="#subpatternsassubroutines">subroutines</a> 1793 from elsewhere in the pattern (but see also the section entitled 1794 <a href="#subdefine">"Defining subpatterns for use by reference only"</a> 1795 below). Items other than subpatterns that have a {0} quantifier are omitted 1796 from the compiled pattern. 1797 </P> 1798 <P> 1799 For convenience, the three most common quantifiers have single-character 1800 abbreviations: 1801 <pre> 1802 * is equivalent to {0,} 1803 + is equivalent to {1,} 1804 ? is equivalent to {0,1} 1805 </pre> 1806 It is possible to construct infinite loops by following a subpattern that can 1807 match no characters with a quantifier that has no upper limit, for example: 1808 <pre> 1809 (a?)* 1810 </pre> 1811 Earlier versions of Perl and PCRE used to give an error at compile time for 1812 such patterns. However, because there are cases where this can be useful, such 1813 patterns are now accepted, but if any repetition of the subpattern does in fact 1814 match no characters, the loop is forcibly broken. 1815 </P> 1816 <P> 1817 By default, the quantifiers are "greedy", that is, they match as much as 1818 possible (up to the maximum number of permitted times), without causing the 1819 rest of the pattern to fail. The classic example of where this gives problems 1820 is in trying to match comments in C programs. These appear between /* and */ 1821 and within the comment, individual * and / characters may appear. An attempt to 1822 match C comments by applying the pattern 1823 <pre> 1824 /\*.*\*/ 1825 </pre> 1826 to the string 1827 <pre> 1828 /* first comment */ not comment /* second comment */ 1829 </pre> 1830 fails, because it matches the entire string owing to the greediness of the .* 1831 item. 1832 </P> 1833 <P> 1834 However, if a quantifier is followed by a question mark, it ceases to be 1835 greedy, and instead matches the minimum number of times possible, so the 1836 pattern 1837 <pre> 1838 /\*.*?\*/ 1839 </pre> 1840 does the right thing with the C comments. The meaning of the various 1841 quantifiers is not otherwise changed, just the preferred number of matches. 1842 Do not confuse this use of question mark with its use as a quantifier in its 1843 own right. Because it has two uses, it can sometimes appear doubled, as in 1844 <pre> 1845 \d??\d 1846 </pre> 1847 which matches one digit by preference, but can match two if that is the only 1848 way the rest of the pattern matches. 1849 </P> 1850 <P> 1851 If the PCRE_UNGREEDY option is set (an option that is not available in Perl), 1852 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 </P> 1856 <P> 1857 When a parenthesized subpattern is quantified with a minimum repeat count that 1858 is greater than 1 or with a limited maximum, more memory is required for the 1859 compiled pattern, in proportion to the size of the minimum or maximum. 1860 </P> 1861 <P> 1862 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent 1863 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is 1864 implicitly anchored, because whatever follows will be tried against every 1865 character position in the subject string, so there is no point in retrying the 1866 overall match at any position after the first. PCRE normally treats such a 1867 pattern as though it were preceded by \A. 1868 </P> 1869 <P> 1870 In cases where it is known that the subject string contains no newlines, it is 1871 worth setting PCRE_DOTALL in order to obtain this optimization, or 1872 alternatively using ^ to indicate anchoring explicitly. 1873 </P> 1874 <P> 1875 However, there are some cases where the optimization cannot be used. When .* 1876 is inside capturing parentheses that are the subject of a back reference 1877 elsewhere in the pattern, a match at the start may fail where a later one 1878 succeeds. Consider, for example: 1879 <pre> 1880 (.*)abc\1 1881 </pre> 1882 If the subject is "xyz123abc123" the match point is the fourth character. For 1883 this reason, such a pattern is not implicitly anchored. 1884 </P> 1885 <P> 1886 Another case where implicit anchoring is not applied is when the leading .* is 1887 inside an atomic group. Once again, a match at the start may fail where a later 1888 one succeeds. Consider this pattern: 1889 <pre> 1890 (?>.*?a)b 1891 </pre> 1892 It matches "ab" in the subject "aab". The use of the backtracking control verbs 1893 (*PRUNE) and (*SKIP) also disable this optimization. 1894 </P> 1895 <P> 1896 When a capturing subpattern is repeated, the value captured is the substring 1897 that matched the final iteration. For example, after 1898 <pre> 1899 (tweedle[dume]{3}\s*)+ 1900 </pre> 1901 has matched "tweedledum tweedledee" the value of the captured substring is 1902 "tweedledee". However, if there are nested capturing subpatterns, the 1903 corresponding captured values may have been set in previous iterations. For 1904 example, after 1905 <pre> 1906 /(a|(b))+/ 1907 </pre> 1908 matches "aba" the value of the second captured substring is "b". 1909 <a name="atomicgroup"></a></P> 1910 <br><a name="SEC18" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br> 1911 <P> 1912 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 1913 repetition, failure of what follows normally causes the repeated item to be 1914 re-evaluated to see if a different number of repeats allows the rest of the 1915 pattern to match. Sometimes it is useful to prevent this, either to change the 1916 nature of the match, or to cause it fail earlier than it otherwise might, when 1917 the author of the pattern knows there is no point in carrying on. 1918 </P> 1919 <P> 1920 Consider, for example, the pattern \d+foo when applied to the subject line 1921 <pre> 1922 123456bar 1923 </pre> 1924 After matching all 6 digits and then failing to match "foo", the normal 1925 action of the matcher is to try again with only 5 digits matching the \d+ 1926 item, and then with 4, and so on, before ultimately failing. "Atomic grouping" 1927 (a term taken from Jeffrey Friedl's book) provides the means for specifying 1928 that once a subpattern has matched, it is not to be re-evaluated in this way. 1929 </P> 1930 <P> 1931 If we use atomic grouping for the previous example, the matcher gives up 1932 immediately on failing to match "foo" the first time. The notation is a kind of 1933 special parenthesis, starting with (?> as in this example: 1934 <pre> 1935 (?>\d+)foo 1936 </pre> 1937 This kind of parenthesis "locks up" the part of the pattern it contains once 1938 it has matched, and a failure further into the pattern is prevented from 1939 backtracking into it. Backtracking past it to previous items, however, works as 1940 normal. 1941 </P> 1942 <P> 1943 An alternative description is that a subpattern of this type matches the string 1944 of characters that an identical standalone pattern would match, if anchored at 1945 the current point in the subject string. 1946 </P> 1947 <P> 1948 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as 1949 the above example can be thought of as a maximizing repeat that must swallow 1950 everything it can. So, while both \d+ and \d+? are prepared to adjust the 1951 number of digits they match in order to make the rest of the pattern match, 1952 (?>\d+) can only match an entire sequence of digits. 1953 </P> 1954 <P> 1955 Atomic groups in general can of course contain arbitrarily complicated 1956 subpatterns, and can be nested. However, when the subpattern for an atomic 1957 group is just a single repeated item, as in the example above, a simpler 1958 notation, called a "possessive quantifier" can be used. This consists of an 1959 additional + character following a quantifier. Using this notation, the 1960 previous example can be rewritten as 1961 <pre> 1962 \d++foo 1963 </pre> 1964 Note that a possessive quantifier can be used with an entire group, for 1965 example: 1966 <pre> 1967 (abc|xyz){2,3}+ 1968 </pre> 1969 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY 1970 option is ignored. They are a convenient notation for the simpler forms of 1971 atomic group. However, there is no difference in the meaning of a possessive 1972 quantifier and the equivalent atomic group, though there may be a performance 1973 difference; possessive quantifiers should be slightly faster. 1974 </P> 1975 <P> 1976 The possessive quantifier syntax is an extension to the Perl 5.8 syntax. 1977 Jeffrey Friedl originated the idea (and the name) in the first edition of his 1978 book. Mike McCloskey liked it, so implemented it when he built Sun's Java 1979 package, and PCRE copied it from there. It ultimately found its way into Perl 1980 at release 5.10. 1981 </P> 1982 <P> 1983 PCRE has an optimization that automatically "possessifies" certain simple 1984 pattern constructs. For example, the sequence A+B is treated as A++B because 1985 there is no point in backtracking into a sequence of A's when B must follow. 1986 </P> 1987 <P> 1988 When a pattern contains an unlimited repeat inside a subpattern that can itself 1989 be repeated an unlimited number of times, the use of an atomic group is the 1990 only way to avoid some failing matches taking a very long time indeed. The 1991 pattern 1992 <pre> 1993 (\D+|<\d+>)*[!?] 1994 </pre> 1995 matches an unlimited number of substrings that either consist of non-digits, or 1996 digits enclosed in <>, followed by either ! or ?. When it matches, it runs 1997 quickly. However, if it is applied to 1998 <pre> 1999 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 2000 </pre> 2001 it takes a long time before reporting failure. This is because the string can 2002 be divided between the internal \D+ repeat and the external * repeat in a 2003 large number of ways, and all have to be tried. (The example uses [!?] rather 2004 than a single character at the end, because both PCRE and Perl have an 2005 optimization that allows for fast failure when a single character is used. They 2006 remember the last single character that is required for a match, and fail early 2007 if it is not present in the string.) If the pattern is changed so that it uses 2008 an atomic group, like this: 2009 <pre> 2010 ((?>\D+)|<\d+>)*[!?] 2011 </pre> 2012 sequences of non-digits cannot be broken, and failure happens quickly. 2013 <a name="backreferences"></a></P> 2014 <br><a name="SEC19" href="#TOC1">BACK REFERENCES</a><br> 2015 <P> 2016 Outside a character class, a backslash followed by a digit greater than 0 (and 2017 possibly further digits) is a back reference to a capturing subpattern earlier 2018 (that is, to its left) in the pattern, provided there have been that many 2019 previous capturing left parentheses. 2020 </P> 2021 <P> 2022 However, if the decimal number following the backslash is less than 10, it is 2023 always taken as a back reference, and causes an error only if there are not 2024 that many capturing left parentheses in the entire pattern. In other words, the 2025 parentheses that are referenced need not be to the left of the reference for 2026 numbers less than 10. A "forward back reference" of this type can make sense 2027 when a repetition is involved and the subpattern to the right has participated 2028 in an earlier iteration. 2029 </P> 2030 <P> 2031 It is not possible to have a numerical "forward back reference" to a subpattern 2032 whose number is 10 or more using this syntax because a sequence such as \50 is 2033 interpreted as a character defined in octal. See the subsection entitled 2034 "Non-printing characters" 2035 <a href="#digitsafterbackslash">above</a> 2036 for further details of the handling of digits following a backslash. There is 2037 no such problem when named parentheses are used. A back reference to any 2038 subpattern is possible using named parentheses (see below). 2039 </P> 2040 <P> 2041 Another way of avoiding the ambiguity inherent in the use of digits following a 2042 backslash is to use the \g escape sequence. This escape must be followed by an 2043 unsigned number or a negative number, optionally enclosed in braces. These 2044 examples are all identical: 2045 <pre> 2046 (ring), \1 2047 (ring), \g1 2048 (ring), \g{1} 2049 </pre> 2050 An unsigned number specifies an absolute reference without the ambiguity that 2051 is present in the older syntax. It is also useful when literal digits follow 2052 the reference. A negative number is a relative reference. Consider this 2053 example: 2054 <pre> 2055 (abc(def)ghi)\g{-1} 2056 </pre> 2057 The sequence \g{-1} is a reference to the most recently started capturing 2058 subpattern before \g, that is, is it equivalent to \2 in this example. 2059 Similarly, \g{-2} would be equivalent to \1. The use of relative references 2060 can be helpful in long patterns, and also in patterns that are created by 2061 joining together fragments that contain references within themselves. 2062 </P> 2063 <P> 2064 A back reference matches whatever actually matched the capturing subpattern in 2065 the current subject string, rather than anything matching the subpattern 2066 itself (see 2067 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a> 2068 below for a way of doing that). So the pattern 2069 <pre> 2070 (sens|respons)e and \1ibility 2071 </pre> 2072 matches "sense and sensibility" and "response and responsibility", but not 2073 "sense and responsibility". If caseful matching is in force at the time of the 2074 back reference, the case of letters is relevant. For example, 2075 <pre> 2076 ((?i)rah)\s+\1 2077 </pre> 2078 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original 2079 capturing subpattern is matched caselessly. 2080 </P> 2081 <P> 2082 There are several different ways of writing back references to named 2083 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or 2084 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified 2085 back reference syntax, in which \g can be used for both numeric and named 2086 references, is also supported. We could rewrite the above example in any of 2087 the following ways: 2088 <pre> 2089 (?<p1>(?i)rah)\s+\k<p1> 2090 (?'p1'(?i)rah)\s+\k{p1} 2091 (?P<p1>(?i)rah)\s+(?P=p1) 2092 (?<p1>(?i)rah)\s+\g{p1} 2093 </pre> 2094 A subpattern that is referenced by name may appear in the pattern before or 2095 after the reference. 2096 </P> 2097 <P> 2098 There may be more than one back reference to the same subpattern. If a 2099 subpattern has not actually been used in a particular match, any back 2100 references to it always fail by default. For example, the pattern 2101 <pre> 2102 (a|(bc))\2 2103 </pre> 2104 always fails if it starts to match "a" rather than "bc". However, if the 2105 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an 2106 unset value matches an empty string. 2107 </P> 2108 <P> 2109 Because there may be many capturing parentheses in a pattern, all digits 2110 following a backslash are taken as part of a potential back reference number. 2111 If the pattern continues with a digit character, some delimiter must be used to 2112 terminate the back reference. If the PCRE_EXTENDED option is set, this can be 2113 white space. Otherwise, the \g{ syntax or an empty comment (see 2114 <a href="#comments">"Comments"</a> 2115 below) can be used. 2116 </P> 2117 <br><b> 2118 Recursive back references 2119 </b><br> 2120 <P> 2121 A back reference that occurs inside the parentheses to which it refers fails 2122 when the subpattern is first used, so, for example, (a\1) never matches. 2123 However, such references can be useful inside repeated subpatterns. For 2124 example, the pattern 2125 <pre> 2126 (a|b\1)+ 2127 </pre> 2128 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of 2129 the subpattern, the back reference matches the character string corresponding 2130 to the previous iteration. In order for this to work, the pattern must be such 2131 that the first iteration does not need to match the back reference. This can be 2132 done using alternation, as in the example above, or by a quantifier with a 2133 minimum of zero. 2134 </P> 2135 <P> 2136 Back references of this type cause the group that they reference to be treated 2137 as an 2138 <a href="#atomicgroup">atomic group.</a> 2139 Once the whole group has been matched, a subsequent matching failure cannot 2140 cause backtracking into the middle of the group. 2141 <a name="bigassertions"></a></P> 2142 <br><a name="SEC20" href="#TOC1">ASSERTIONS</a><br> 2143 <P> 2144 An assertion is a test on the characters following or preceding the current 2145 matching point that does not actually consume any characters. The simple 2146 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described 2147 <a href="#smallassertions">above.</a> 2148 </P> 2149 <P> 2150 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 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 <P> 2156 Assertion subpatterns are not capturing subpatterns. If such an assertion 2157 contains capturing subpatterns within it, these are counted for the purposes of 2158 numbering the capturing subpatterns in the whole pattern. However, substring 2159 capturing is carried out only for positive assertions. (Perl sometimes, but not 2160 always, does do capturing in negative assertions.) 2161 </P> 2162 <P> 2163 For compatibility with Perl, assertion subpatterns may be repeated; though 2164 it makes no sense to assert the same thing several times, the side effect of 2165 capturing parentheses may occasionally be useful. In practice, there only three 2166 cases: 2167 <br> 2168 <br> 2169 (1) If the quantifier is {0}, the assertion is never obeyed during matching. 2170 However, it may contain internal capturing parenthesized groups that are called 2171 from elsewhere via the 2172 <a href="#subpatternsassubroutines">subroutine mechanism.</a> 2173 <br> 2174 <br> 2175 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it 2176 were {0,1}. At run time, the rest of the pattern match is tried with and 2177 without the assertion, the order depending on the greediness of the quantifier. 2178 <br> 2179 <br> 2180 (3) If the minimum repetition is greater than zero, the quantifier is ignored. 2181 The assertion is obeyed just once when encountered during matching. 2182 </P> 2183 <br><b> 2184 Lookahead assertions 2185 </b><br> 2186 <P> 2187 Lookahead assertions start with (?= for positive assertions and (?! for 2188 negative assertions. For example, 2189 <pre> 2190 \w+(?=;) 2191 </pre> 2192 matches a word followed by a semicolon, but does not include the semicolon in 2193 the match, and 2194 <pre> 2195 foo(?!bar) 2196 </pre> 2197 matches any occurrence of "foo" that is not followed by "bar". Note that the 2198 apparently similar pattern 2199 <pre> 2200 (?!foo)bar 2201 </pre> 2202 does not find an occurrence of "bar" that is preceded by something other than 2203 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion 2204 (?!foo) is always true when the next three characters are "bar". A 2205 lookbehind assertion is needed to achieve the other effect. 2206 </P> 2207 <P> 2208 If you want to force a matching failure at some point in a pattern, the most 2209 convenient way to do it is with (?!) because an empty string always matches, so 2210 an assertion that requires there not to be an empty string must always fail. 2211 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!). 2212 <a name="lookbehind"></a></P> 2213 <br><b> 2214 Lookbehind assertions 2215 </b><br> 2216 <P> 2217 Lookbehind assertions start with (?<= for positive assertions and (?<! for 2218 negative assertions. For example, 2219 <pre> 2220 (?<!foo)bar 2221 </pre> 2222 does find an occurrence of "bar" that is not preceded by "foo". The contents of 2223 a lookbehind assertion are restricted such that all the strings it matches must 2224 have a fixed length. However, if there are several top-level alternatives, they 2225 do not all have to have the same fixed length. Thus 2226 <pre> 2227 (?<=bullock|donkey) 2228 </pre> 2229 is permitted, but 2230 <pre> 2231 (?<!dogs?|cats?) 2232 </pre> 2233 causes an error at compile time. Branches that match different length strings 2234 are permitted only at the top level of a lookbehind assertion. This is an 2235 extension compared with Perl, which requires all branches to match the same 2236 length of string. An assertion such as 2237 <pre> 2238 (?<=ab(c|de)) 2239 </pre> 2240 is not permitted, because its single top-level branch can match two different 2241 lengths, but it is acceptable to PCRE if rewritten to use two top-level 2242 branches: 2243 <pre> 2244 (?<=abc|abde) 2245 </pre> 2246 In some cases, the escape sequence \K 2247 <a href="#resetmatchstart">(see above)</a> 2248 can be used instead of a lookbehind assertion to get round the fixed-length 2249 restriction. 2250 </P> 2251 <P> 2252 The implementation of lookbehind assertions is, for each alternative, to 2253 temporarily move the current position back by the fixed length and then try to 2254 match. If there are insufficient characters before the current position, the 2255 assertion fails. 2256 </P> 2257 <P> 2258 In a UTF mode, PCRE does not allow the \C escape (which matches a single data 2259 unit even in a UTF mode) to appear in lookbehind assertions, because it makes 2260 it impossible to calculate the length of the lookbehind. The \X and \R 2261 escapes, which can match different numbers of data units, are also not 2262 permitted. 2263 </P> 2264 <P> 2265 <a href="#subpatternsassubroutines">"Subroutine"</a> 2266 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long 2267 as the subpattern matches a fixed-length string. 2268 <a href="#recursion">Recursion,</a> 2269 however, is not supported. 2270 </P> 2271 <P> 2272 Possessive quantifiers can be used in conjunction with lookbehind assertions to 2273 specify efficient matching of fixed-length strings at the end of subject 2274 strings. Consider a simple pattern such as 2275 <pre> 2276 abcd$ 2277 </pre> 2278 when applied to a long string that does not match. Because matching proceeds 2279 from left to right, PCRE will look for each "a" in the subject and then see if 2280 what follows matches the rest of the pattern. If the pattern is specified as 2281 <pre> 2282 ^.*abcd$ 2283 </pre> 2284 the initial .* matches the entire string at first, but when this fails (because 2285 there is no following "a"), it backtracks to match all but the last character, 2286 then all but the last two characters, and so on. Once again the search for "a" 2287 covers the entire string, from right to left, so we are no better off. However, 2288 if the pattern is written as 2289 <pre> 2290 ^.*+(?<=abcd) 2291 </pre> 2292 there can be no backtracking for the .*+ item; it can match only the entire 2293 string. The subsequent lookbehind assertion does a single test on the last four 2294 characters. If it fails, the match fails immediately. For long strings, this 2295 approach makes a significant difference to the processing time. 2296 </P> 2297 <br><b> 2298 Using multiple assertions 2299 </b><br> 2300 <P> 2301 Several assertions (of any sort) may occur in succession. For example, 2302 <pre> 2303 (?<=\d{3})(?<!999)foo 2304 </pre> 2305 matches "foo" preceded by three digits that are not "999". Notice that each of 2306 the assertions is applied independently at the same point in the subject 2307 string. First there is a check that the previous three characters are all 2308 digits, and then there is a check that the same three characters are not "999". 2309 This pattern does <i>not</i> match "foo" preceded by six characters, the first 2310 of which are digits and the last three of which are not "999". For example, it 2311 doesn't match "123abcfoo". A pattern to do that is 2312 <pre> 2313 (?<=\d{3}...)(?<!999)foo 2314 </pre> 2315 This time the first assertion looks at the preceding six characters, checking 2316 that the first three are digits, and then the second assertion checks that the 2317 preceding three characters are not "999". 2318 </P> 2319 <P> 2320 Assertions can be nested in any combination. For example, 2321 <pre> 2322 (?<=(?<!foo)bar)baz 2323 </pre> 2324 matches an occurrence of "baz" that is preceded by "bar" which in turn is not 2325 preceded by "foo", while 2326 <pre> 2327 (?<=\d{3}(?!999)...)foo 2328 </pre> 2329 is another pattern that matches "foo" preceded by three digits and any three 2330 characters that are not "999". 2331 <a name="conditions"></a></P> 2332 <br><a name="SEC21" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br> 2333 <P> 2334 It is possible to cause the matching process to obey a subpattern 2335 conditionally or to choose between two alternative subpatterns, depending on 2336 the result of an assertion, or whether a specific capturing subpattern has 2337 already been matched. The two possible forms of conditional subpattern are: 2338 <pre> 2339 (?(condition)yes-pattern) 2340 (?(condition)yes-pattern|no-pattern) 2341 </pre> 2342 If the condition is satisfied, the yes-pattern is used; otherwise the 2343 no-pattern (if present) is used. If there are more than two alternatives in the 2344 subpattern, a compile-time error occurs. Each of the two alternatives may 2345 itself contain nested subpatterns of any form, including conditional 2346 subpatterns; the restriction to two alternatives applies only at the level of 2347 the condition. This pattern fragment is an example where the alternatives are 2348 complex: 2349 <pre> 2350 (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) 2351 2352 </PRE> 2353 </P> 2354 <P> 2355 There are four kinds of condition: references to subpatterns, references to 2356 recursion, a pseudo-condition called DEFINE, and assertions. 2357 </P> 2358 <br><b> 2359 Checking for a used subpattern by number 2360 </b><br> 2361 <P> 2362 If the text between the parentheses consists of a sequence of digits, the 2363 condition is true if a capturing subpattern of that number has previously 2364 matched. If there is more than one capturing subpattern with the same number 2365 (see the earlier 2366 <a href="#recursion">section about duplicate subpattern numbers),</a> 2367 the condition is true if any of them have matched. An alternative notation is 2368 to precede the digits with a plus or minus sign. In this case, the subpattern 2369 number is relative rather than absolute. The most recently opened parentheses 2370 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside 2371 loops it can also make sense to refer to subsequent groups. The next 2372 parentheses to be opened can be referenced as (?(+1), and so on. (The value 2373 zero in any of these forms is not used; it provokes a compile-time error.) 2374 </P> 2375 <P> 2376 Consider the following pattern, which contains non-significant white space to 2377 make it more readable (assume the PCRE_EXTENDED option) and to divide it into 2378 three parts for ease of discussion: 2379 <pre> 2380 ( \( )? [^()]+ (?(1) \) ) 2381 </pre> 2382 The first part matches an optional opening parenthesis, and if that 2383 character is present, sets it as the first captured substring. The second part 2384 matches one or more characters that are not parentheses. The third part is a 2385 conditional subpattern that tests whether or not the first set of parentheses 2386 matched. If they did, that is, if subject started with an opening parenthesis, 2387 the condition is true, and so the yes-pattern is executed and a closing 2388 parenthesis is required. Otherwise, since no-pattern is not present, the 2389 subpattern matches nothing. In other words, this pattern matches a sequence of 2390 non-parentheses, optionally enclosed in parentheses. 2391 </P> 2392 <P> 2393 If you were embedding this pattern in a larger one, you could use a relative 2394 reference: 2395 <pre> 2396 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... 2397 </pre> 2398 This makes the fragment independent of the parentheses in the larger pattern. 2399 </P> 2400 <br><b> 2401 Checking for a used subpattern by name 2402 </b><br> 2403 <P> 2404 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used 2405 subpattern by name. For compatibility with earlier versions of PCRE, which had 2406 this facility before Perl, the syntax (?(name)...) is also recognized. 2407 </P> 2408 <P> 2409 Rewriting the above example to use a named subpattern gives this: 2410 <pre> 2411 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) 2412 </pre> 2413 If the name used in a condition of this kind is a duplicate, the test is 2414 applied to all subpatterns of the same name, and is true if any one of them has 2415 matched. 2416 </P> 2417 <br><b> 2418 Checking for pattern recursion 2419 </b><br> 2420 <P> 2421 If the condition is the string (R), and there is no subpattern with the name R, 2422 the condition is true if a recursive call to the whole pattern or any 2423 subpattern has been made. If digits or a name preceded by ampersand follow the 2424 letter R, for example: 2425 <pre> 2426 (?(R3)...) or (?(R&name)...) 2427 </pre> 2428 the condition is true if the most recent recursion is into a subpattern whose 2429 number or name is given. This condition does not check the entire recursion 2430 stack. If the name used in a condition of this kind is a duplicate, the test is 2431 applied to all subpatterns of the same name, and is true if any one of them is 2432 the most recent recursion. 2433 </P> 2434 <P> 2435 At "top level", all these recursion test conditions are false. 2436 <a href="#recursion">The syntax for recursive patterns</a> 2437 is described below. 2438 <a name="subdefine"></a></P> 2439 <br><b> 2440 Defining subpatterns for use by reference only 2441 </b><br> 2442 <P> 2443 If the condition is the string (DEFINE), and there is no subpattern with the 2444 name DEFINE, the condition is always false. In this case, there may be only one 2445 alternative in the subpattern. It is always skipped if control reaches this 2446 point in the pattern; the idea of DEFINE is that it can be used to define 2447 subroutines that can be referenced from elsewhere. (The use of 2448 <a href="#subpatternsassubroutines">subroutines</a> 2449 is described below.) For example, a pattern to match an IPv4 address such as 2450 "192.168.23.245" could be written like this (ignore white space and line 2451 breaks): 2452 <pre> 2453 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) 2454 \b (?&byte) (\.(?&byte)){3} \b 2455 </pre> 2456 The first part of the pattern is a DEFINE group inside which a another group 2457 named "byte" is defined. This matches an individual component of an IPv4 2458 address (a number less than 256). When matching takes place, this part of the 2459 pattern is skipped because DEFINE acts like a false condition. The rest of the 2460 pattern uses references to the named group to match the four dot-separated 2461 components of an IPv4 address, insisting on a word boundary at each end. 2462 </P> 2463 <br><b> 2464 Assertion conditions 2465 </b><br> 2466 <P> 2467 If the condition is not in any of the above formats, it must be an assertion. 2468 This may be a positive or negative lookahead or lookbehind assertion. Consider 2469 this pattern, again containing non-significant white space, and with the two 2470 alternatives on the second line: 2471 <pre> 2472 (?(?=[^a-z]*[a-z]) 2473 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) 2474 </pre> 2475 The condition is a positive lookahead assertion that matches an optional 2476 sequence of non-letters followed by a letter. In other words, it tests for the 2477 presence of at least one letter in the subject. If a letter is found, the 2478 subject is matched against the first alternative; otherwise it is matched 2479 against the second. This pattern matches strings in one of the two forms 2480 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. 2481 <a name="comments"></a></P> 2482 <br><a name="SEC22" href="#TOC1">COMMENTS</a><br> 2483 <P> 2484 There are two ways of including comments in patterns that are processed by 2485 PCRE. In both cases, the start of the comment must not be in a character class, 2486 nor in the middle of any other sequence of related characters such as (?: or a 2487 subpattern name or number. The characters that make up a comment play no part 2488 in the pattern matching. 2489 </P> 2490 <P> 2491 The sequence (?# marks the start of a comment that continues up to the next 2492 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED 2493 option is set, an unescaped # character also introduces a comment, which in 2494 this case continues to immediately after the next newline character or 2495 character sequence in the pattern. Which characters are interpreted as newlines 2496 is controlled by the options passed to a compiling function or by a special 2497 sequence at the start of the pattern, as described in the section entitled 2498 <a href="#newlines">"Newline conventions"</a> 2499 above. Note that the end of this type of comment is a literal newline sequence 2500 in the pattern; escape sequences that happen to represent a newline do not 2501 count. For example, consider this pattern when PCRE_EXTENDED is set, and the 2502 default newline convention is in force: 2503 <pre> 2504 abc #comment \n still comment 2505 </pre> 2506 On encountering the # character, <b>pcre_compile()</b> skips along, looking for 2507 a newline in the pattern. The sequence \n is still literal at this stage, so 2508 it does not terminate the comment. Only an actual character with the code value 2509 0x0a (the default newline) does so. 2510 <a name="recursion"></a></P> 2511 <br><a name="SEC23" href="#TOC1">RECURSIVE PATTERNS</a><br> 2512 <P> 2513 Consider the problem of matching a string in parentheses, allowing for 2514 unlimited nested parentheses. Without the use of recursion, the best that can 2515 be done is to use a pattern that matches up to some fixed depth of nesting. It 2516 is not possible to handle an arbitrary nesting depth. 2517 </P> 2518 <P> 2519 For some time, Perl has provided a facility that allows regular expressions to 2520 recurse (amongst other things). It does this by interpolating Perl code in the 2521 expression at run time, and the code can refer to the expression itself. A Perl 2522 pattern using code interpolation to solve the parentheses problem can be 2523 created like this: 2524 <pre> 2525 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; 2526 </pre> 2527 The (?p{...}) item interpolates Perl code at run time, and in this case refers 2528 recursively to the pattern in which it appears. 2529 </P> 2530 <P> 2531 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it 2532 supports special syntax for recursion of the entire pattern, and also for 2533 individual subpattern recursion. After its introduction in PCRE and Python, 2534 this kind of recursion was subsequently introduced into Perl at release 5.10. 2535 </P> 2536 <P> 2537 A special item that consists of (? followed by a number greater than zero and a 2538 closing parenthesis is a recursive subroutine call of the subpattern of the 2539 given number, provided that it occurs inside that subpattern. (If not, it is a 2540 <a href="#subpatternsassubroutines">non-recursive subroutine</a> 2541 call, which is described in the next section.) The special item (?R) or (?0) is 2542 a recursive call of the entire regular expression. 2543 </P> 2544 <P> 2545 This PCRE pattern solves the nested parentheses problem (assume the 2546 PCRE_EXTENDED option is set so that white space is ignored): 2547 <pre> 2548 \( ( [^()]++ | (?R) )* \) 2549 </pre> 2550 First it matches an opening parenthesis. Then it matches any number of 2551 substrings which can either be a sequence of non-parentheses, or a recursive 2552 match of the pattern itself (that is, a correctly parenthesized substring). 2553 Finally there is a closing parenthesis. Note the use of a possessive quantifier 2554 to avoid backtracking into sequences of non-parentheses. 2555 </P> 2556 <P> 2557 If this were part of a larger pattern, you would not want to recurse the entire 2558 pattern, so instead you could use this: 2559 <pre> 2560 ( \( ( [^()]++ | (?1) )* \) ) 2561 </pre> 2562 We have put the pattern into parentheses, and caused the recursion to refer to 2563 them instead of the whole pattern. 2564 </P> 2565 <P> 2566 In a larger pattern, keeping track of parenthesis numbers can be tricky. This 2567 is made easier by the use of relative references. Instead of (?1) in the 2568 pattern above you can write (?-2) to refer to the second most recently opened 2569 parentheses preceding the recursion. In other words, a negative number counts 2570 capturing parentheses leftwards from the point at which it is encountered. 2571 </P> 2572 <P> 2573 It is also possible to refer to subsequently opened parentheses, by writing 2574 references such as (?+2). However, these cannot be recursive because the 2575 reference is not inside the parentheses that are referenced. They are always 2576 <a href="#subpatternsassubroutines">non-recursive subroutine</a> 2577 calls, as described in the next section. 2578 </P> 2579 <P> 2580 An alternative approach is to use named parentheses instead. The Perl syntax 2581 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We 2582 could rewrite the above example as follows: 2583 <pre> 2584 (?<pn> \( ( [^()]++ | (?&pn) )* \) ) 2585 </pre> 2586 If there is more than one subpattern with the same name, the earliest one is 2587 used. 2588 </P> 2589 <P> 2590 This particular example pattern that we have been looking at contains nested 2591 unlimited repeats, and so the use of a possessive quantifier for matching 2592 strings of non-parentheses is important when applying the pattern to strings 2593 that do not match. For example, when this pattern is applied to 2594 <pre> 2595 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 2596 </pre> 2597 it yields "no match" quickly. However, if a possessive quantifier is not used, 2598 the match runs for a very long time indeed because there are so many different 2599 ways the + and * repeats can carve up the subject, and all have to be tested 2600 before failure can be reported. 2601 </P> 2602 <P> 2603 At the end of a match, the values of capturing parentheses are those from 2604 the outermost level. If you want to obtain intermediate values, a callout 2605 function can be used (see below and the 2606 <a href="pcrecallout.html"><b>pcrecallout</b></a> 2607 documentation). If the pattern above is matched against 2608 <pre> 2609 (ab(cd)ef) 2610 </pre> 2611 the value for the inner capturing parentheses (numbered 2) is "ef", which is 2612 the last value taken on at the top level. If a capturing subpattern is not 2613 matched at the top level, its final captured value is unset, even if it was 2614 (temporarily) set at a deeper level during the matching process. 2615 </P> 2616 <P> 2617 If there are more than 15 capturing parentheses in a pattern, PCRE has to 2618 obtain extra memory to store data during a recursion, which it does by using 2619 <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can 2620 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. 2621 </P> 2622 <P> 2623 Do not confuse the (?R) item with the condition (R), which tests for recursion. 2624 Consider this pattern, which matches text in angle brackets, allowing for 2625 arbitrary nesting. Only digits are allowed in nested brackets (that is, when 2626 recursing), whereas any characters are permitted at the outer level. 2627 <pre> 2628 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > 2629 </pre> 2630 In this pattern, (?(R) is the start of a conditional subpattern, with two 2631 different alternatives for the recursive and non-recursive cases. The (?R) item 2632 is the actual recursive call. 2633 <a name="recursiondifference"></a></P> 2634 <br><b> 2635 Differences in recursion processing between PCRE and Perl 2636 </b><br> 2637 <P> 2638 Recursion processing in PCRE differs from Perl in two important ways. In PCRE 2639 (like Python, but unlike Perl), a recursive subpattern call is always treated 2640 as an atomic group. That is, once it has matched some of the subject string, it 2641 is never re-entered, even if it contains untried alternatives and there is a 2642 subsequent matching failure. This can be illustrated by the following pattern, 2643 which purports to match a palindromic string that contains an odd number of 2644 characters (for example, "a", "aba", "abcba", "abcdcba"): 2645 <pre> 2646 ^(.|(.)(?1)\2)$ 2647 </pre> 2648 The idea is that it either matches a single character, or two identical 2649 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE 2650 it does not if the pattern is longer than three characters. Consider the 2651 subject string "abcba": 2652 </P> 2653 <P> 2654 At the top level, the first character is matched, but as it is not at the end 2655 of the string, the first alternative fails; the second alternative is taken 2656 and the recursion kicks in. The recursive call to subpattern 1 successfully 2657 matches the next character ("b"). (Note that the beginning and end of line 2658 tests are not part of the recursion). 2659 </P> 2660 <P> 2661 Back at the top level, the next character ("c") is compared with what 2662 subpattern 2 matched, which was "a". This fails. Because the recursion is 2663 treated as an atomic group, there are now no backtracking points, and so the 2664 entire match fails. (Perl is able, at this point, to re-enter the recursion and 2665 try the second alternative.) However, if the pattern is written with the 2666 alternatives in the other order, things are different: 2667 <pre> 2668 ^((.)(?1)\2|.)$ 2669 </pre> 2670 This time, the recursing alternative is tried first, and continues to recurse 2671 until it runs out of characters, at which point the recursion fails. But this 2672 time we do have another alternative to try at the higher level. That is the big 2673 difference: in the previous case the remaining alternative is at a deeper 2674 recursion level, which PCRE cannot use. 2675 </P> 2676 <P> 2677 To change the pattern so that it matches all palindromic strings, not just 2678 those with an odd number of characters, it is tempting to change the pattern to 2679 this: 2680 <pre> 2681 ^((.)(?1)\2|.?)$ 2682 </pre> 2683 Again, this works in Perl, but not in PCRE, and for the same reason. When a 2684 deeper recursion has matched a single character, it cannot be entered again in 2685 order to match an empty string. The solution is to separate the two cases, and 2686 write out the odd and even cases as alternatives at the higher level: 2687 <pre> 2688 ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) 2689 </pre> 2690 If you want to match typical palindromic phrases, the pattern has to ignore all 2691 non-word characters, which can be done like this: 2692 <pre> 2693 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ 2694 </pre> 2695 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A 2696 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note 2697 the use of the possessive quantifier *+ to avoid backtracking into sequences of 2698 non-word characters. Without this, PCRE takes a great deal longer (ten times or 2699 more) to match typical phrases, and Perl takes so long that you think it has 2700 gone into a loop. 2701 </P> 2702 <P> 2703 <b>WARNING</b>: The palindrome-matching patterns above work only if the subject 2704 string does not start with a palindrome that is shorter than the entire string. 2705 For example, although "abcba" is correctly matched, if the subject is "ababa", 2706 PCRE finds the palindrome "aba" at the start, then fails at top level because 2707 the end of the string does not follow. Once again, it cannot jump back into the 2708 recursion to try other alternatives, so the entire match fails. 2709 </P> 2710 <P> 2711 The second way in which PCRE and Perl differ in their recursion processing is 2712 in the handling of captured values. In Perl, when a subpattern is called 2713 recursively or as a subpattern (see the next section), it has no access to any 2714 values that were captured outside the recursion, whereas in PCRE these values 2715 can be referenced. Consider this pattern: 2716 <pre> 2717 ^(.)(\1|a(?2)) 2718 </pre> 2719 In PCRE, this pattern matches "bab". The first capturing parentheses match "b", 2720 then in the second group, when the back reference \1 fails to match "b", the 2721 second alternative matches "a" and then recurses. In the recursion, \1 does 2722 now match "b" and so the whole match succeeds. In Perl, the pattern fails to 2723 match because inside the recursive call \1 cannot access the externally set 2724 value. 2725 <a name="subpatternsassubroutines"></a></P> 2726 <br><a name="SEC24" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br> 2727 <P> 2728 If the syntax for a recursive subpattern call (either by number or by 2729 name) is used outside the parentheses to which it refers, it operates like a 2730 subroutine in a programming language. The called subpattern may be defined 2731 before or after the reference. A numbered reference can be absolute or 2732 relative, as in these examples: 2733 <pre> 2734 (...(absolute)...)...(?2)... 2735 (...(relative)...)...(?-1)... 2736 (...(?+1)...(relative)... 2737 </pre> 2738 An earlier example pointed out that the pattern 2739 <pre> 2740 (sens|respons)e and \1ibility 2741 </pre> 2742 matches "sense and sensibility" and "response and responsibility", but not 2743 "sense and responsibility". If instead the pattern 2744 <pre> 2745 (sens|respons)e and (?1)ibility 2746 </pre> 2747 is used, it does match "sense and responsibility" as well as the other two 2748 strings. Another example is given in the discussion of DEFINE above. 2749 </P> 2750 <P> 2751 All subroutine calls, whether recursive or not, are always treated as atomic 2752 groups. That is, once a subroutine has matched some of the subject string, it 2753 is never re-entered, even if it contains untried alternatives and there is a 2754 subsequent matching failure. Any capturing parentheses that are set during the 2755 subroutine call revert to their previous values afterwards. 2756 </P> 2757 <P> 2758 Processing options such as case-independence are fixed when a subpattern is 2759 defined, so if it is used as a subroutine, such options cannot be changed for 2760 different calls. For example, consider this pattern: 2761 <pre> 2762 (abc)(?i:(?-1)) 2763 </pre> 2764 It matches "abcabc". It does not match "abcABC" because the change of 2765 processing option does not affect the called subpattern. 2766 <a name="onigurumasubroutines"></a></P> 2767 <br><a name="SEC25" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br> 2768 <P> 2769 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or 2770 a number enclosed either in angle brackets or single quotes, is an alternative 2771 syntax for referencing a subpattern as a subroutine, possibly recursively. Here 2772 are two of the examples used above, rewritten using this syntax: 2773 <pre> 2774 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) 2775 (sens|respons)e and \g'1'ibility 2776 </pre> 2777 PCRE supports an extension to Oniguruma: if a number is preceded by a 2778 plus or a minus sign it is taken as a relative reference. For example: 2779 <pre> 2780 (abc)(?i:\g<-1>) 2781 </pre> 2782 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i> 2783 synonymous. The former is a back reference; the latter is a subroutine call. 2784 </P> 2785 <br><a name="SEC26" href="#TOC1">CALLOUTS</a><br> 2786 <P> 2787 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl 2788 code to be obeyed in the middle of matching a regular expression. This makes it 2789 possible, amongst other things, to extract different substrings that match the 2790 same pair of parentheses when there is a repetition. 2791 </P> 2792 <P> 2793 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl 2794 code. The feature is called "callout". The caller of PCRE provides an external 2795 function by putting its entry point in the global variable <i>pcre_callout</i> 2796 (8-bit library) or <i>pcre[16|32]_callout</i> (16-bit or 32-bit library). 2797 By default, this variable contains NULL, which disables all calling out. 2798 </P> 2799 <P> 2800 Within a regular expression, (?C) indicates the points at which the external 2801 function is to be called. If you want to identify different callout points, you 2802 can put a number less than 256 after the letter C. The default value is zero. 2803 For example, this pattern has two callout points: 2804 <pre> 2805 (?C1)abc(?C2)def 2806 </pre> 2807 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are 2808 automatically installed before each item in the pattern. They are all numbered 2809 255. If there is a conditional group in the pattern whose condition is an 2810 assertion, an additional callout is inserted just before the condition. An 2811 explicit callout may also be set at this position, as in this example: 2812 <pre> 2813 (?(?C9)(?=a)abc|def) 2814 </pre> 2815 Note that this applies only to assertion conditions, not to other types of 2816 condition. 2817 </P> 2818 <P> 2819 During matching, when PCRE reaches a callout point, the external function is 2820 called. It is provided with the number of the callout, the position in the 2821 pattern, and, optionally, one item of data originally supplied by the caller of 2822 the matching function. The callout function may cause matching to proceed, to 2823 backtrack, or to fail altogether. 2824 </P> 2825 <P> 2826 By default, PCRE implements a number of optimizations at compile time and 2827 matching time, and one side-effect is that sometimes callouts are skipped. If 2828 you need all possible callouts to happen, you need to set options that disable 2829 the relevant optimizations. More details, and a complete description of the 2830 interface to the callout function, are given in the 2831 <a href="pcrecallout.html"><b>pcrecallout</b></a> 2832 documentation. 2833 <a name="backtrackcontrol"></a></P> 2834 <br><a name="SEC27" href="#TOC1">BACKTRACKING CONTROL</a><br> 2835 <P> 2836 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which 2837 are still described in the Perl documentation as "experimental and subject to 2838 change or removal in a future version of Perl". It goes on to say: "Their usage 2839 in production code should be noted to avoid problems during upgrades." The same 2840 remarks apply to the PCRE features described in this section. 2841 </P> 2842 <P> 2843 The new verbs make use of what was previously invalid syntax: an opening 2844 parenthesis followed by an asterisk. They are generally of the form 2845 (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving 2846 differently depending on whether or not a name is present. A name is any 2847 sequence of characters that does not include a closing parenthesis. The maximum 2848 length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit 2849 libraries. If the name is empty, that is, if the closing parenthesis 2850 immediately follows the colon, the effect is as if the colon were not there. 2851 Any number of these verbs may occur in a pattern. 2852 </P> 2853 <P> 2854 Since these verbs are specifically related to backtracking, most of them can be 2855 used only when the pattern is to be matched using one of the traditional 2856 matching functions, because these use a backtracking algorithm. With the 2857 exception of (*FAIL), which behaves like a failing negative assertion, the 2858 backtracking control verbs cause an error if encountered by a DFA matching 2859 function. 2860 </P> 2861 <P> 2862 The behaviour of these verbs in 2863 <a href="#btrepeat">repeated groups,</a> 2864 <a href="#btassert">assertions,</a> 2865 and in 2866 <a href="#btsub">subpatterns called as subroutines</a> 2867 (whether or not recursively) is documented below. 2868 <a name="nooptimize"></a></P> 2869 <br><b> 2870 Optimizations that affect backtracking verbs 2871 </b><br> 2872 <P> 2873 PCRE contains some optimizations that are used to speed up matching by running 2874 some checks at the start of each match attempt. For example, it may know the 2875 minimum length of matching subject, or that a particular character must be 2876 present. When one of these optimizations bypasses the running of a match, any 2877 included backtracking verbs will not, of course, be processed. You can suppress 2878 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option 2879 when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the 2880 pattern with (*NO_START_OPT). There is more discussion of this option in the 2881 section entitled 2882 <a href="pcreapi.html#execoptions">"Option bits for <b>pcre_exec()</b>"</a> 2883 in the 2884 <a href="pcreapi.html"><b>pcreapi</b></a> 2885 documentation. 2886 </P> 2887 <P> 2888 Experiments with Perl suggest that it too has similar optimizations, sometimes 2889 leading to anomalous results. 2890 </P> 2891 <br><b> 2892 Verbs that act immediately 2893 </b><br> 2894 <P> 2895 The following verbs act as soon as they are encountered. They may not be 2896 followed by a name. 2897 <pre> 2898 (*ACCEPT) 2899 </pre> 2900 This verb causes the match to end successfully, skipping the remainder of the 2901 pattern. However, when it is inside a subpattern that is called as a 2902 subroutine, only that subpattern is ended successfully. Matching then continues 2903 at the outer level. If (*ACCEPT) in triggered in a positive assertion, the 2904 assertion succeeds; in a negative assertion, the assertion fails. 2905 </P> 2906 <P> 2907 If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For 2908 example: 2909 <pre> 2910 A((?:A|B(*ACCEPT)|C)D) 2911 </pre> 2912 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by 2913 the outer parentheses. 2914 <pre> 2915 (*FAIL) or (*F) 2916 </pre> 2917 This verb causes a matching failure, forcing backtracking to occur. It is 2918 equivalent to (?!) but easier to read. The Perl documentation notes that it is 2919 probably useful only when combined with (?{}) or (??{}). Those are, of course, 2920 Perl features that are not present in PCRE. The nearest equivalent is the 2921 callout feature, as for example in this pattern: 2922 <pre> 2923 a+(?C)(*FAIL) 2924 </pre> 2925 A match with the string "aaaa" always fails, but the callout is taken before 2926 each backtrack happens (in this example, 10 times). 2927 </P> 2928 <br><b> 2929 Recording which path was taken 2930 </b><br> 2931 <P> 2932 There is one verb whose main purpose is to track how a match was arrived at, 2933 though it also has a secondary use in conjunction with advancing the match 2934 starting point (see (*SKIP) below). 2935 <pre> 2936 (*MARK:NAME) or (*:NAME) 2937 </pre> 2938 A name is always required with this verb. There may be as many instances of 2939 (*MARK) as you like in a pattern, and their names do not have to be unique. 2940 </P> 2941 <P> 2942 When a match succeeds, the name of the last-encountered (*MARK:NAME), 2943 (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the 2944 caller as described in the section entitled 2945 <a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a> 2946 in the 2947 <a href="pcreapi.html"><b>pcreapi</b></a> 2948 documentation. Here is an example of <b>pcretest</b> output, where the /K 2949 modifier requests the retrieval and outputting of (*MARK) data: 2950 <pre> 2951 re> /X(*MARK:A)Y|X(*MARK:B)Z/K 2952 data> XY 2953 0: XY 2954 MK: A 2955 XZ 2956 0: XZ 2957 MK: B 2958 </pre> 2959 The (*MARK) name is tagged with "MK:" in this output, and in this example it 2960 indicates which of the two alternatives matched. This is a more efficient way 2961 of obtaining this information than putting each alternative in its own 2962 capturing parentheses. 2963 </P> 2964 <P> 2965 If a verb with a name is encountered in a positive assertion that is true, the 2966 name is recorded and passed back if it is the last-encountered. This does not 2967 happen for negative assertions or failing positive assertions. 2968 </P> 2969 <P> 2970 After a partial match or a failed match, the last encountered name in the 2971 entire match process is returned. For example: 2972 <pre> 2973 re> /X(*MARK:A)Y|X(*MARK:B)Z/K 2974 data> XP 2975 No match, mark = B 2976 </pre> 2977 Note that in this unanchored example the mark is retained from the match 2978 attempt that started at the letter "X" in the subject. Subsequent match 2979 attempts starting at "P" and then with an empty string do not get as far as the 2980 (*MARK) item, but nevertheless do not reset it. 2981 </P> 2982 <P> 2983 If you are interested in (*MARK) values after failed matches, you should 2984 probably set the PCRE_NO_START_OPTIMIZE option 2985 <a href="#nooptimize">(see above)</a> 2986 to ensure that the match is always attempted. 2987 </P> 2988 <br><b> 2989 Verbs that act after backtracking 2990 </b><br> 2991 <P> 2992 The following verbs do nothing when they are encountered. Matching continues 2993 with what follows, but if there is no subsequent match, causing a backtrack to 2994 the verb, a failure is forced. That is, backtracking cannot pass to the left of 2995 the verb. However, when one of these verbs appears inside an atomic group or an 2996 assertion that is true, its effect is confined to that group, because once the 2997 group has been matched, there is never any backtracking into it. In this 2998 situation, backtracking can "jump back" to the left of the entire atomic group 2999 or assertion. (Remember also, as stated above, that this localization also 3000 applies in subroutine calls.) 3001 </P> 3002 <P> 3003 These verbs differ in exactly what kind of failure occurs when backtracking 3004 reaches them. The behaviour described below is what happens when the verb is 3005 not in a subroutine or an assertion. Subsequent sections cover these special 3006 cases. 3007 <pre> 3008 (*COMMIT) 3009 </pre> 3010 This verb, which may not be followed by a name, causes the whole match to fail 3011 outright if there is a later matching failure that causes backtracking to reach 3012 it. Even if the pattern is unanchored, no further attempts to find a match by 3013 advancing the starting point take place. If (*COMMIT) is the only backtracking 3014 verb that is encountered, once it has been passed <b>pcre_exec()</b> is 3015 committed to finding a match at the current starting point, or not at all. For 3016 example: 3017 <pre> 3018 a+(*COMMIT)b 3019 </pre> 3020 This matches "xxaab" but not "aacaab". It can be thought of as a kind of 3021 dynamic anchor, or "I've started, so I must finish." The name of the most 3022 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a 3023 match failure. 3024 </P> 3025 <P> 3026 If there is more than one backtracking verb in a pattern, a different one that 3027 follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a 3028 match does not always guarantee that a match must be at this starting point. 3029 </P> 3030 <P> 3031 Note that (*COMMIT) at the start of a pattern is not the same as an anchor, 3032 unless PCRE's start-of-match optimizations are turned off, as shown in this 3033 output from <b>pcretest</b>: 3034 <pre> 3035 re> /(*COMMIT)abc/ 3036 data> xyzabc 3037 0: abc 3038 data> xyzabc\Y 3039 No match 3040 </pre> 3041 For this pattern, PCRE knows that any match must start with "a", so the 3042 optimization skips along the subject to "a" before applying the pattern to the 3043 first set of data. The match attempt then succeeds. In the second set of data, 3044 the escape sequence \Y is interpreted by the <b>pcretest</b> program. It causes 3045 the PCRE_NO_START_OPTIMIZE option to be set when <b>pcre_exec()</b> is called. 3046 This disables the optimization that skips along to the first character. The 3047 pattern is now applied starting at "x", and so the (*COMMIT) causes the match 3048 to fail without trying any other starting points. 3049 <pre> 3050 (*PRUNE) or (*PRUNE:NAME) 3051 </pre> 3052 This verb causes the match to fail at the current starting position in the 3053 subject if there is a later matching failure that causes backtracking to reach 3054 it. If the pattern is unanchored, the normal "bumpalong" advance to the next 3055 starting character then happens. Backtracking can occur as usual to the left of 3056 (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but 3057 if there is no match to the right, backtracking cannot cross (*PRUNE). In 3058 simple cases, the use of (*PRUNE) is just an alternative to an atomic group or 3059 possessive quantifier, but there are some uses of (*PRUNE) that cannot be 3060 expressed in any other way. In an anchored pattern (*PRUNE) has the same effect 3061 as (*COMMIT). 3062 </P> 3063 <P> 3064 The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE). 3065 It is like (*MARK:NAME) in that the name is remembered for passing back to the 3066 caller. However, (*SKIP:NAME) searches only for names set with (*MARK). 3067 <pre> 3068 (*SKIP) 3069 </pre> 3070 This verb, when given without a name, is like (*PRUNE), except that if the 3071 pattern is unanchored, the "bumpalong" advance is not to the next character, 3072 but to the position in the subject where (*SKIP) was encountered. (*SKIP) 3073 signifies that whatever text was matched leading up to it cannot be part of a 3074 successful match. Consider: 3075 <pre> 3076 a+(*SKIP)b 3077 </pre> 3078 If the subject is "aaaac...", after the first match attempt fails (starting at 3079 the first character in the string), the starting point skips on to start the 3080 next attempt at "c". Note that a possessive quantifer does not have the same 3081 effect as this example; although it would suppress backtracking during the 3082 first match attempt, the second attempt would start at the second character 3083 instead of skipping on to "c". 3084 <pre> 3085 (*SKIP:NAME) 3086 </pre> 3087 When (*SKIP) has an associated name, its behaviour is modified. When it is 3088 triggered, the previous path through the pattern is searched for the most 3089 recent (*MARK) that has the same name. If one is found, the "bumpalong" advance 3090 is to the subject position that corresponds to that (*MARK) instead of to where 3091 (*SKIP) was encountered. If no (*MARK) with a matching name is found, the 3092 (*SKIP) is ignored. 3093 </P> 3094 <P> 3095 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores 3096 names that are set by (*PRUNE:NAME) or (*THEN:NAME). 3097 <pre> 3098 (*THEN) or (*THEN:NAME) 3099 </pre> 3100 This verb causes a skip to the next innermost alternative when backtracking 3101 reaches it. That is, it cancels any further backtracking within the current 3102 alternative. Its name comes from the observation that it can be used for a 3103 pattern-based if-then-else block: 3104 <pre> 3105 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... 3106 </pre> 3107 If the COND1 pattern matches, FOO is tried (and possibly further items after 3108 the end of the group if FOO succeeds); on failure, the matcher skips to the 3109 second alternative and tries COND2, without backtracking into COND1. If that 3110 succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no 3111 more alternatives, so there is a backtrack to whatever came before the entire 3112 group. If (*THEN) is not inside an alternation, it acts like (*PRUNE). 3113 </P> 3114 <P> 3115 The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN). 3116 It is like (*MARK:NAME) in that the name is remembered for passing back to the 3117 caller. However, (*SKIP:NAME) searches only for names set with (*MARK). 3118 </P> 3119 <P> 3120 A subpattern that does not contain a | character is just a part of the 3121 enclosing alternative; it is not a nested alternation with only one 3122 alternative. The effect of (*THEN) extends beyond such a subpattern to the 3123 enclosing alternative. Consider this pattern, where A, B, etc. are complex 3124 pattern fragments that do not contain any | characters at this level: 3125 <pre> 3126 A (B(*THEN)C) | D 3127 </pre> 3128 If A and B are matched, but there is a failure in C, matching does not 3129 backtrack into A; instead it moves to the next alternative, that is, D. 3130 However, if the subpattern containing (*THEN) is given an alternative, it 3131 behaves differently: 3132 <pre> 3133 A (B(*THEN)C | (*FAIL)) | D 3134 </pre> 3135 The effect of (*THEN) is now confined to the inner subpattern. After a failure 3136 in C, matching moves to (*FAIL), which causes the whole subpattern to fail 3137 because there are no more alternatives to try. In this case, matching does now 3138 backtrack into A. 3139 </P> 3140 <P> 3141 Note that a conditional subpattern is not considered as having two 3142 alternatives, because only one is ever used. In other words, the | character in 3143 a conditional subpattern has a different meaning. Ignoring white space, 3144 consider: 3145 <pre> 3146 ^.*? (?(?=a) a | b(*THEN)c ) 3147 </pre> 3148 If the subject is "ba", this pattern does not match. Because .*? is ungreedy, 3149 it initially matches zero characters. The condition (?=a) then fails, the 3150 character "b" is matched, but "c" is not. At this point, matching does not 3151 backtrack to .*? as might perhaps be expected from the presence of the | 3152 character. The conditional subpattern is part of the single alternative that 3153 comprises the whole pattern, and so the match fails. (If there was a backtrack 3154 into .*?, allowing it to match "b", the match would succeed.) 3155 </P> 3156 <P> 3157 The verbs just described provide four different "strengths" of control when 3158 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the 3159 next alternative. (*PRUNE) comes next, failing the match at the current 3160 starting position, but allowing an advance to the next character (for an 3161 unanchored pattern). (*SKIP) is similar, except that the advance may be more 3162 than one character. (*COMMIT) is the strongest, causing the entire match to 3163 fail. 3164 </P> 3165 <br><b> 3166 More than one backtracking verb 3167 </b><br> 3168 <P> 3169 If more than one backtracking verb is present in a pattern, the one that is 3170 backtracked onto first acts. For example, consider this pattern, where A, B, 3171 etc. are complex pattern fragments: 3172 <pre> 3173 (A(*COMMIT)B(*THEN)C|ABD) 3174 </pre> 3175 If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to 3176 fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes 3177 the next alternative (ABD) to be tried. This behaviour is consistent, but is 3178 not always the same as Perl's. It means that if two or more backtracking verbs 3179 appear in succession, all the the last of them has no effect. Consider this 3180 example: 3181 <pre> 3182 ...(*COMMIT)(*PRUNE)... 3183 </pre> 3184 If there is a matching failure to the right, backtracking onto (*PRUNE) causes 3185 it to be triggered, and its action is taken. There can never be a backtrack 3186 onto (*COMMIT). 3187 <a name="btrepeat"></a></P> 3188 <br><b> 3189 Backtracking verbs in repeated groups 3190 </b><br> 3191 <P> 3192 PCRE differs from Perl in its handling of backtracking verbs in repeated 3193 groups. For example, consider: 3194 <pre> 3195 /(a(*COMMIT)b)+ac/ 3196 </pre> 3197 If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in 3198 the second repeat of the group acts. 3199 <a name="btassert"></a></P> 3200 <br><b> 3201 Backtracking verbs in assertions 3202 </b><br> 3203 <P> 3204 (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack. 3205 </P> 3206 <P> 3207 (*ACCEPT) in a positive assertion causes the assertion to succeed without any 3208 further processing. In a negative assertion, (*ACCEPT) causes the assertion to 3209 fail without any further processing. 3210 </P> 3211 <P> 3212 The other backtracking verbs are not treated specially if they appear in a 3213 positive assertion. In particular, (*THEN) skips to the next alternative in the 3214 innermost enclosing group that has alternations, whether or not this is within 3215 the assertion. 3216 </P> 3217 <P> 3218 Negative assertions are, however, different, in order to ensure that changing a 3219 positive assertion into a negative assertion changes its result. Backtracking 3220 into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true, 3221 without considering any further alternative branches in the assertion. 3222 Backtracking into (*THEN) causes it to skip to the next enclosing alternative 3223 within the assertion (the normal behaviour), but if the assertion does not have 3224 such an alternative, (*THEN) behaves like (*PRUNE). 3225 <a name="btsub"></a></P> 3226 <br><b> 3227 Backtracking verbs in subroutines 3228 </b><br> 3229 <P> 3230 These behaviours occur whether or not the subpattern is called recursively. 3231 Perl's treatment of subroutines is different in some cases. 3232 </P> 3233 <P> 3234 (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces 3235 an immediate backtrack. 3236 </P> 3237 <P> 3238 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to 3239 succeed without any further processing. Matching then continues after the 3240 subroutine call. 3241 </P> 3242 <P> 3243 (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause 3244 the subroutine match to fail. 3245 </P> 3246 <P> 3247 (*THEN) skips to the next alternative in the innermost enclosing group within 3248 the subpattern that has alternatives. If there is no such group within the 3249 subpattern, (*THEN) causes the subroutine match to fail. 3250 </P> 3251 <br><a name="SEC28" href="#TOC1">SEE ALSO</a><br> 3252 <P> 3253 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), 3254 <b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>, <b>pcre32(3)</b>. 3255 </P> 3256 <br><a name="SEC29" href="#TOC1">AUTHOR</a><br> 3257 <P> 3258 Philip Hazel 3259 <br> 3260 University Computing Service 3261 <br> 3262 Cambridge CB2 3QH, England. 3263 <br> 3264 </P> 3265 <br><a name="SEC30" href="#TOC1">REVISION</a><br> 3266 <P> 3267 Last updated: 14 June 2015 3268 <br> 3269 Copyright © 1997-2015 University of Cambridge. 3270 <br> 3271 <p> 3272 Return to the <a href="index.html">PCRE index page</a>. 3273 </p> 3274
