1 <html> 2 <head> 3 <title>pcre2pattern specification</title> 4 </head> 5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"> 6 <h1>pcre2pattern man page</h1> 7 <p> 8 Return to the <a href="index.html">PCRE2 index page</a>. 9 </p> 10 <p> 11 This page is part of the PCRE2 HTML documentation. It was generated 12 automatically from the original man page. If there is any nonsense in it, 13 please consult the man page, in case the conversion went wrong. 14 <br> 15 <ul> 16 <li><a name="TOC1" href="#SEC1">PCRE2 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 CODE 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">BACKREFERENCES</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">PCRE2 REGULAR EXPRESSION DETAILS</a><br> 48 <P> 49 The syntax and semantics of the regular expressions that are supported by PCRE2 50 are described in detail below. There is a quick-reference syntax summary in the 51 <a href="pcre2syntax.html"><b>pcre2syntax</b></a> 52 page. PCRE2 tries to match Perl syntax and semantics as closely as it can. 53 PCRE2 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 regular 59 expressions in general are covered in a number of books, some of which have 60 copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published 61 by O'Reilly, covers regular expressions in great detail. This description of 62 PCRE2's regular expressions is intended as reference material. 63 </P> 64 <P> 65 This document discusses the patterns that are supported by PCRE2 when its main 66 matching function, <b>pcre2_match()</b>, is used. PCRE2 also has an alternative 67 matching function, <b>pcre2_dfa_match()</b>, which matches using a different 68 algorithm that is not Perl-compatible. Some of the features discussed below are 69 not available when DFA matching is used. The advantages and disadvantages of 70 the alternative function, and how it differs from the normal function, are 71 discussed in the 72 <a href="pcre2matching.html"><b>pcre2matching</b></a> 73 page. 74 </P> 75 <br><a name="SEC2" href="#TOC1">SPECIAL START-OF-PATTERN ITEMS</a><br> 76 <P> 77 A number of options that can be passed to <b>pcre2_compile()</b> can also be set 78 by special items at the start of a pattern. These are not Perl-compatible, but 79 are provided to make these options accessible to pattern writers who are not 80 able to change the program that processes the pattern. Any number of these 81 items may appear, but they must all be together right at the start of the 82 pattern string, and the letters must be in upper case. 83 </P> 84 <br><b> 85 UTF support 86 </b><br> 87 <P> 88 In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either as 89 single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32 can be 90 specified for the 32-bit library, in which case it constrains the character 91 values to valid Unicode code points. To process UTF strings, PCRE2 must be 92 built to include Unicode support (which is the default). When using UTF strings 93 you must either call the compiling function with the PCRE2_UTF option, or the 94 pattern must start with the special sequence (*UTF), which is equivalent to 95 setting the relevant option. How setting a UTF mode affects pattern matching is 96 mentioned in several places below. There is also a summary of features in the 97 <a href="pcre2unicode.html"><b>pcre2unicode</b></a> 98 page. 99 </P> 100 <P> 101 Some applications that allow their users to supply patterns may wish to 102 restrict them to non-UTF data for security reasons. If the PCRE2_NEVER_UTF 103 option is passed to <b>pcre2_compile()</b>, (*UTF) is not allowed, and its 104 appearance in a pattern causes an error. 105 </P> 106 <br><b> 107 Unicode property support 108 </b><br> 109 <P> 110 Another special sequence that may appear at the start of a pattern is (*UCP). 111 This has the same effect as setting the PCRE2_UCP option: it causes sequences 112 such as \d and \w to use Unicode properties to determine character types, 113 instead of recognizing only characters with codes less than 256 via a lookup 114 table. 115 </P> 116 <P> 117 Some applications that allow their users to supply patterns may wish to 118 restrict them for security reasons. If the PCRE2_NEVER_UCP option is passed to 119 <b>pcre2_compile()</b>, (*UCP) is not allowed, and its appearance in a pattern 120 causes an error. 121 </P> 122 <br><b> 123 Locking out empty string matching 124 </b><br> 125 <P> 126 Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same effect 127 as passing the PCRE2_NOTEMPTY or PCRE2_NOTEMPTY_ATSTART option to whichever 128 matching function is subsequently called to match the pattern. These options 129 lock out the matching of empty strings, either entirely, or only at the start 130 of the subject. 131 </P> 132 <br><b> 133 Disabling auto-possessification 134 </b><br> 135 <P> 136 If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting 137 the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from making quantifiers 138 possessive when what follows cannot match the repeated item. For example, by 139 default a+b is treated as a++b. For more details, see the 140 <a href="pcre2api.html"><b>pcre2api</b></a> 141 documentation. 142 </P> 143 <br><b> 144 Disabling start-up optimizations 145 </b><br> 146 <P> 147 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the 148 PCRE2_NO_START_OPTIMIZE option. This disables several optimizations for quickly 149 reaching "no match" results. For more details, see the 150 <a href="pcre2api.html"><b>pcre2api</b></a> 151 documentation. 152 </P> 153 <br><b> 154 Disabling automatic anchoring 155 </b><br> 156 <P> 157 If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the same effect as 158 setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables optimizations that 159 apply to patterns whose top-level branches all start with .* (match any number 160 of arbitrary characters). For more details, see the 161 <a href="pcre2api.html"><b>pcre2api</b></a> 162 documentation. 163 </P> 164 <br><b> 165 Disabling JIT compilation 166 </b><br> 167 <P> 168 If a pattern that starts with (*NO_JIT) is successfully compiled, an attempt by 169 the application to apply the JIT optimization by calling 170 <b>pcre2_jit_compile()</b> is ignored. 171 </P> 172 <br><b> 173 Setting match resource limits 174 </b><br> 175 <P> 176 The <b>pcre2_match()</b> function contains a counter that is incremented every 177 time it goes round its main loop. The caller of <b>pcre2_match()</b> can set a 178 limit on this counter, which therefore limits the amount of computing resource 179 used for a match. The maximum depth of nested backtracking can also be limited; 180 this indirectly restricts the amount of heap memory that is used, but there is 181 also an explicit memory limit that can be set. 182 </P> 183 <P> 184 These facilities are provided to catch runaway matches that are provoked by 185 patterns with huge matching trees (a typical example is a pattern with nested 186 unlimited repeats applied to a long string that does not match). When one of 187 these limits is reached, <b>pcre2_match()</b> gives an error return. The limits 188 can also be set by items at the start of the pattern of the form 189 <pre> 190 (*LIMIT_HEAP=d) 191 (*LIMIT_MATCH=d) 192 (*LIMIT_DEPTH=d) 193 </pre> 194 where d is any number of decimal digits. However, the value of the setting must 195 be less than the value set (or defaulted) by the caller of <b>pcre2_match()</b> 196 for it to have any effect. In other words, the pattern writer can lower the 197 limits set by the programmer, but not raise them. If there is more than one 198 setting of one of these limits, the lower value is used. The heap limit is 199 specified in kibibytes (units of 1024 bytes). 200 </P> 201 <P> 202 Prior to release 10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This name is 203 still recognized for backwards compatibility. 204 </P> 205 <P> 206 The heap limit applies only when the <b>pcre2_match()</b> or 207 <b>pcre2_dfa_match()</b> interpreters are used for matching. It does not apply 208 to JIT. The match limit is used (but in a different way) when JIT is being 209 used, or when <b>pcre2_dfa_match()</b> is called, to limit computing resource 210 usage by those matching functions. The depth limit is ignored by JIT but is 211 relevant for DFA matching, which uses function recursion for recursions within 212 the pattern and for lookaround assertions and atomic groups. In this case, the 213 depth limit controls the depth of such recursion. 214 <a name="newlines"></a></P> 215 <br><b> 216 Newline conventions 217 </b><br> 218 <P> 219 PCRE2 supports six different conventions for indicating line breaks in 220 strings: a single CR (carriage return) character, a single LF (linefeed) 221 character, the two-character sequence CRLF, any of the three preceding, any 222 Unicode newline sequence, or the NUL character (binary zero). The 223 <a href="pcre2api.html"><b>pcre2api</b></a> 224 page has 225 <a href="pcre2api.html#newlines">further discussion</a> 226 about newlines, and shows how to set the newline convention when calling 227 <b>pcre2_compile()</b>. 228 </P> 229 <P> 230 It is also possible to specify a newline convention by starting a pattern 231 string with one of the following sequences: 232 <pre> 233 (*CR) carriage return 234 (*LF) linefeed 235 (*CRLF) carriage return, followed by linefeed 236 (*ANYCRLF) any of the three above 237 (*ANY) all Unicode newline sequences 238 (*NUL) the NUL character (binary zero) 239 </pre> 240 These override the default and the options given to the compiling function. For 241 example, on a Unix system where LF is the default newline sequence, the pattern 242 <pre> 243 (*CR)a.b 244 </pre> 245 changes the convention to CR. That pattern matches "a\nb" because LF is no 246 longer a newline. If more than one of these settings is present, the last one 247 is used. 248 </P> 249 <P> 250 The newline convention affects where the circumflex and dollar assertions are 251 true. It also affects the interpretation of the dot metacharacter when 252 PCRE2_DOTALL is not set, and the behaviour of \N when not followed by an 253 opening brace. However, it does not affect what the \R escape sequence 254 matches. By default, this is any Unicode newline sequence, for Perl 255 compatibility. However, this can be changed; see the next section and the 256 description of \R in the section entitled 257 <a href="#newlineseq">"Newline sequences"</a> 258 below. A change of \R setting can be combined with a change of newline 259 convention. 260 </P> 261 <br><b> 262 Specifying what \R matches 263 </b><br> 264 <P> 265 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the 266 complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF 267 at compile time. This effect can also be achieved by starting a pattern with 268 (*BSR_ANYCRLF). For completeness, (*BSR_UNICODE) is also recognized, 269 corresponding to PCRE2_BSR_UNICODE. 270 </P> 271 <br><a name="SEC3" href="#TOC1">EBCDIC CHARACTER CODES</a><br> 272 <P> 273 PCRE2 can be compiled to run in an environment that uses EBCDIC as its 274 character code instead of ASCII or Unicode (typically a mainframe system). In 275 the sections below, character code values are ASCII or Unicode; in an EBCDIC 276 environment these characters may have different code values, and there are no 277 code points greater than 255. 278 </P> 279 <br><a name="SEC4" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br> 280 <P> 281 A regular expression is a pattern that is matched against a subject string from 282 left to right. Most characters stand for themselves in a pattern, and match the 283 corresponding characters in the subject. As a trivial example, the pattern 284 <pre> 285 The quick brown fox 286 </pre> 287 matches a portion of a subject string that is identical to itself. When 288 caseless matching is specified (the PCRE2_CASELESS option), letters are matched 289 independently of case. 290 </P> 291 <P> 292 The power of regular expressions comes from the ability to include alternatives 293 and repetitions in the pattern. These are encoded in the pattern by the use of 294 <i>metacharacters</i>, which do not stand for themselves but instead are 295 interpreted in some special way. 296 </P> 297 <P> 298 There are two different sets of metacharacters: those that are recognized 299 anywhere in the pattern except within square brackets, and those that are 300 recognized within square brackets. Outside square brackets, the metacharacters 301 are as follows: 302 <pre> 303 \ general escape character with several uses 304 ^ assert start of string (or line, in multiline mode) 305 $ assert end of string (or line, in multiline mode) 306 . match any character except newline (by default) 307 [ start character class definition 308 | start of alternative branch 309 ( start subpattern 310 ) end subpattern 311 ? extends the meaning of ( 312 also 0 or 1 quantifier 313 also quantifier minimizer 314 * 0 or more quantifier 315 + 1 or more quantifier 316 also "possessive quantifier" 317 { start min/max quantifier 318 </pre> 319 Part of a pattern that is in square brackets is called a "character class". In 320 a character class the only metacharacters are: 321 <pre> 322 \ general escape character 323 ^ negate the class, but only if the first character 324 - indicates character range 325 [ POSIX character class (only if followed by POSIX syntax) 326 ] terminates the character class 327 </pre> 328 The following sections describe the use of each of the metacharacters. 329 </P> 330 <br><a name="SEC5" href="#TOC1">BACKSLASH</a><br> 331 <P> 332 The backslash character has several uses. Firstly, if it is followed by a 333 character that is not a number or a letter, it takes away any special meaning 334 that character may have. This use of backslash as an escape character applies 335 both inside and outside character classes. 336 </P> 337 <P> 338 For example, if you want to match a * character, you must write \* in the 339 pattern. This escaping action applies whether or not the following character 340 would otherwise be interpreted as a metacharacter, so it is always safe to 341 precede a non-alphanumeric with backslash to specify that it stands for itself. 342 In particular, if you want to match a backslash, you write \\. 343 </P> 344 <P> 345 In a UTF mode, only ASCII numbers and letters have any special meaning after a 346 backslash. All other characters (in particular, those whose code points are 347 greater than 127) are treated as literals. 348 </P> 349 <P> 350 If a pattern is compiled with the PCRE2_EXTENDED option, most white space in 351 the pattern (other than in a character class), and characters between a # 352 outside a character class and the next newline, inclusive, are ignored. An 353 escaping backslash can be used to include a white space or # character as part 354 of the pattern. 355 </P> 356 <P> 357 If you want to remove the special meaning from a sequence of characters, you 358 can do so by putting them between \Q and \E. This is different from Perl in 359 that $ and @ are handled as literals in \Q...\E sequences in PCRE2, whereas 360 in Perl, $ and @ cause variable interpolation. Also, Perl does "double-quotish 361 backslash interpolation" on any backslashes between \Q and \E which, its 362 documentation says, "may lead to confusing results". PCRE2 treats a backslash 363 between \Q and \E just like any other character. Note the following examples: 364 <pre> 365 Pattern PCRE2 matches Perl matches 366 367 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz 368 \Qabc\$xyz\E abc\$xyz abc\$xyz 369 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 370 \QA\B\E A\B A\B 371 \Q\\E \ \\E 372 </pre> 373 The \Q...\E sequence is recognized both inside and outside character classes. 374 An isolated \E that is not preceded by \Q is ignored. If \Q is not followed 375 by \E later in the pattern, the literal interpretation continues to the end of 376 the pattern (that is, \E is assumed at the end). If the isolated \Q is inside 377 a character class, this causes an error, because the character class is not 378 terminated by a closing square bracket. 379 <a name="digitsafterbackslash"></a></P> 380 <br><b> 381 Non-printing characters 382 </b><br> 383 <P> 384 A second use of backslash provides a way of encoding non-printing characters 385 in patterns in a visible manner. There is no restriction on the appearance of 386 non-printing characters in a pattern, but when a pattern is being prepared by 387 text editing, it is often easier to use one of the following escape sequences 388 than the binary character it represents. In an ASCII or Unicode environment, 389 these escapes are as follows: 390 <pre> 391 \a alarm, that is, the BEL character (hex 07) 392 \cx "control-x", where x is any printable ASCII character 393 \e escape (hex 1B) 394 \f form feed (hex 0C) 395 \n linefeed (hex 0A) 396 \r carriage return (hex 0D) 397 \t tab (hex 09) 398 \0dd character with octal code 0dd 399 \ddd character with octal code ddd, or backreference 400 \o{ddd..} character with octal code ddd.. 401 \xhh character with hex code hh 402 \x{hhh..} character with hex code hhh.. 403 \N{U+hhh..} character with Unicode hex code point hhh.. 404 \uhhhh character with hex code hhhh (when PCRE2_ALT_BSUX is set) 405 </pre> 406 The \N{U+hhh..} escape sequence is recognized only when the PCRE2_UTF option 407 is set, that is, when PCRE2 is operating in a Unicode mode. Perl also uses 408 \N{name} to specify characters by Unicode name; PCRE2 does not support this. 409 Note that when \N is not followed by an opening brace (curly bracket) it has 410 an entirely different meaning, matching any character that is not a newline. 411 </P> 412 <P> 413 The precise effect of \cx on ASCII characters is as follows: if x is a lower 414 case letter, it is converted to upper case. Then bit 6 of the character (hex 415 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A), 416 but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the 417 code unit following \c has a value less than 32 or greater than 126, a 418 compile-time error occurs. 419 </P> 420 <P> 421 When PCRE2 is compiled in EBCDIC mode, \N{U+hhh..} is not supported. \a, \e, 422 \f, \n, \r, and \t generate the appropriate EBCDIC code values. The \c 423 escape is processed as specified for Perl in the <b>perlebcdic</b> document. The 424 only characters that are allowed after \c are A-Z, a-z, or one of @, [, \, ], 425 ^, _, or ?. Any other character provokes a compile-time error. The sequence 426 \c@ encodes character code 0; after \c the letters (in either case) encode 427 characters 1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31 428 (hex 1B to hex 1F), and \c? becomes either 255 (hex FF) or 95 (hex 5F). 429 </P> 430 <P> 431 Thus, apart from \c?, these escapes generate the same character code values as 432 they do in an ASCII environment, though the meanings of the values mostly 433 differ. For example, \cG always generates code value 7, which is BEL in ASCII 434 but DEL in EBCDIC. 435 </P> 436 <P> 437 The sequence \c? generates DEL (127, hex 7F) in an ASCII environment, but 438 because 127 is not a control character in EBCDIC, Perl makes it generate the 439 APC character. Unfortunately, there are several variants of EBCDIC. In most of 440 them the APC character has the value 255 (hex FF), but in the one Perl calls 441 POSIX-BC its value is 95 (hex 5F). If certain other characters have POSIX-BC 442 values, PCRE2 makes \c? generate 95; otherwise it generates 255. 443 </P> 444 <P> 445 After \0 up to two further octal digits are read. If there are fewer than two 446 digits, just those that are present are used. Thus the sequence \0\x\015 447 specifies two binary zeros followed by a CR character (code value 13). Make 448 sure you supply two digits after the initial zero if the pattern character that 449 follows is itself an octal digit. 450 </P> 451 <P> 452 The escape \o must be followed by a sequence of octal digits, enclosed in 453 braces. An error occurs if this is not the case. This escape is a recent 454 addition to Perl; it provides way of specifying character code points as octal 455 numbers greater than 0777, and it also allows octal numbers and backreferences 456 to be unambiguously specified. 457 </P> 458 <P> 459 For greater clarity and unambiguity, it is best to avoid following \ by a 460 digit greater than zero. Instead, use \o{} or \x{} to specify numerical 461 character code points, and \g{} to specify backreferences. The following 462 paragraphs describe the old, ambiguous syntax. 463 </P> 464 <P> 465 The handling of a backslash followed by a digit other than 0 is complicated, 466 and Perl has changed over time, causing PCRE2 also to change. 467 </P> 468 <P> 469 Outside a character class, PCRE2 reads the digit and any following digits as a 470 decimal number. If the number is less than 10, begins with the digit 8 or 9, or 471 if there are at least that many previous capturing left parentheses in the 472 expression, the entire sequence is taken as a <i>backreference</i>. A 473 description of how this works is given 474 <a href="#backreferences">later,</a> 475 following the discussion of 476 <a href="#subpattern">parenthesized subpatterns.</a> 477 Otherwise, up to three octal digits are read to form a character code. 478 </P> 479 <P> 480 Inside a character class, PCRE2 handles \8 and \9 as the literal characters 481 "8" and "9", and otherwise reads up to three octal digits following the 482 backslash, using them to generate a data character. Any subsequent digits stand 483 for themselves. For example, outside a character class: 484 <pre> 485 \040 is another way of writing an ASCII space 486 \40 is the same, provided there are fewer than 40 previous capturing subpatterns 487 \7 is always a backreference 488 \11 might be a backreference, or another way of writing a tab 489 \011 is always a tab 490 \0113 is a tab followed by the character "3" 491 \113 might be a backreference, otherwise the character with octal code 113 492 \377 might be a backreference, otherwise the value 255 (decimal) 493 \81 is always a backreference .sp 494 </pre> 495 Note that octal values of 100 or greater that are specified using this syntax 496 must not be introduced by a leading zero, because no more than three octal 497 digits are ever read. 498 </P> 499 <P> 500 By default, after \x that is not followed by {, from zero to two hexadecimal 501 digits are read (letters can be in upper or lower case). Any number of 502 hexadecimal digits may appear between \x{ and }. If a character other than 503 a hexadecimal digit appears between \x{ and }, or if there is no terminating 504 }, an error occurs. 505 </P> 506 <P> 507 If the PCRE2_ALT_BSUX option is set, the interpretation of \x is as just 508 described only when it is followed by two hexadecimal digits. Otherwise, it 509 matches a literal "x" character. In this mode, support for code points greater 510 than 256 is provided by \u, which must be followed by four hexadecimal digits; 511 otherwise it matches a literal "u" character. 512 </P> 513 <P> 514 Characters whose value is less than 256 can be defined by either of the two 515 syntaxes for \x (or by \u in PCRE2_ALT_BSUX mode). There is no difference in 516 the way they are handled. For example, \xdc is exactly the same as \x{dc} (or 517 \u00dc in PCRE2_ALT_BSUX mode). 518 </P> 519 <br><b> 520 Constraints on character values 521 </b><br> 522 <P> 523 Characters that are specified using octal or hexadecimal numbers are 524 limited to certain values, as follows: 525 <pre> 526 8-bit non-UTF mode no greater than 0xff 527 16-bit non-UTF mode no greater than 0xffff 528 32-bit non-UTF mode no greater than 0xffffffff 529 All UTF modes no greater than 0x10ffff and a valid code point 530 </pre> 531 Invalid Unicode code points are all those in the range 0xd800 to 0xdfff (the 532 so-called "surrogate" code points). The check for these can be disabled by the 533 caller of <b>pcre2_compile()</b> by setting the option 534 PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES. However, this is possible only in UTF-8 535 and UTF-32 modes, because these values are not representable in UTF-16. 536 </P> 537 <br><b> 538 Escape sequences in character classes 539 </b><br> 540 <P> 541 All the sequences that define a single character value can be used both inside 542 and outside character classes. In addition, inside a character class, \b is 543 interpreted as the backspace character (hex 08). 544 </P> 545 <P> 546 When not followed by an opening brace, \N is not allowed in a character class. 547 \B, \R, and \X are not special inside a character class. Like other 548 unrecognized alphabetic escape sequences, they cause an error. Outside a 549 character class, these sequences have different meanings. 550 </P> 551 <br><b> 552 Unsupported escape sequences 553 </b><br> 554 <P> 555 In Perl, the sequences \F, \l, \L, \u, and \U are recognized by its string 556 handler and used to modify the case of following characters. By default, PCRE2 557 does not support these escape sequences. However, if the PCRE2_ALT_BSUX option 558 is set, \U matches a "U" character, and \u can be used to define a character 559 by code point, as described above. 560 </P> 561 <br><b> 562 Absolute and relative backreferences 563 </b><br> 564 <P> 565 The sequence \g followed by a signed or unsigned number, optionally enclosed 566 in braces, is an absolute or relative backreference. A named backreference 567 can be coded as \g{name}. Backreferences are discussed 568 <a href="#backreferences">later,</a> 569 following the discussion of 570 <a href="#subpattern">parenthesized subpatterns.</a> 571 </P> 572 <br><b> 573 Absolute and relative subroutine calls 574 </b><br> 575 <P> 576 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or 577 a number enclosed either in angle brackets or single quotes, is an alternative 578 syntax for referencing a subpattern as a "subroutine". Details are discussed 579 <a href="#onigurumasubroutines">later.</a> 580 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i> 581 synonymous. The former is a backreference; the latter is a 582 <a href="#subpatternsassubroutines">subroutine</a> 583 call. 584 <a name="genericchartypes"></a></P> 585 <br><b> 586 Generic character types 587 </b><br> 588 <P> 589 Another use of backslash is for specifying generic character types: 590 <pre> 591 \d any decimal digit 592 \D any character that is not a decimal digit 593 \h any horizontal white space character 594 \H any character that is not a horizontal white space character 595 \N any character that is not a newline 596 \s any white space character 597 \S any character that is not a white space character 598 \v any vertical white space character 599 \V any character that is not a vertical white space character 600 \w any "word" character 601 \W any "non-word" character 602 </pre> 603 The \N escape sequence has the same meaning as 604 <a href="#fullstopdot">the "." metacharacter</a> 605 when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not change the 606 meaning of \N. Note that when \N is followed by an opening brace it has a 607 different meaning. See the section entitled 608 <a href="#digitsafterbackslash">"Non-printing characters"</a> 609 above for details. Perl also uses \N{name} to specify characters by Unicode 610 name; PCRE2 does not support this. 611 </P> 612 <P> 613 Each pair of lower and upper case escape sequences partitions the complete set 614 of characters into two disjoint sets. Any given character matches one, and only 615 one, of each pair. The sequences can appear both inside and outside character 616 classes. They each match one character of the appropriate type. If the current 617 matching point is at the end of the subject string, all of them fail, because 618 there is no character to match. 619 </P> 620 <P> 621 The default \s characters are HT (9), LF (10), VT (11), FF (12), CR (13), and 622 space (32), which are defined as white space in the "C" locale. This list may 623 vary if locale-specific matching is taking place. For example, in some locales 624 the "non-breaking space" character (\xA0) is recognized as white space, and in 625 others the VT character is not. 626 </P> 627 <P> 628 A "word" character is an underscore or any character that is a letter or digit. 629 By default, the definition of letters and digits is controlled by PCRE2's 630 low-valued character tables, and may vary if locale-specific matching is taking 631 place (see 632 <a href="pcre2api.html#localesupport">"Locale support"</a> 633 in the 634 <a href="pcre2api.html"><b>pcre2api</b></a> 635 page). For example, in a French locale such as "fr_FR" in Unix-like systems, 636 or "french" in Windows, some character codes greater than 127 are used for 637 accented letters, and these are then matched by \w. The use of locales with 638 Unicode is discouraged. 639 </P> 640 <P> 641 By default, characters whose code points are greater than 127 never match \d, 642 \s, or \w, and always match \D, \S, and \W, although this may be different 643 for characters in the range 128-255 when locale-specific matching is happening. 644 These escape sequences retain their original meanings from before Unicode 645 support was available, mainly for efficiency reasons. If the PCRE2_UCP option 646 is set, the behaviour is changed so that Unicode properties are used to 647 determine character types, as follows: 648 <pre> 649 \d any character that matches \p{Nd} (decimal digit) 650 \s any character that matches \p{Z} or \h or \v 651 \w any character that matches \p{L} or \p{N}, plus underscore 652 </pre> 653 The upper case escapes match the inverse sets of characters. Note that \d 654 matches only decimal digits, whereas \w matches any Unicode digit, as well as 655 any Unicode letter, and underscore. Note also that PCRE2_UCP affects \b, and 656 \B because they are defined in terms of \w and \W. Matching these sequences 657 is noticeably slower when PCRE2_UCP is set. 658 </P> 659 <P> 660 The sequences \h, \H, \v, and \V, in contrast to the other sequences, which 661 match only ASCII characters by default, always match a specific list of code 662 points, whether or not PCRE2_UCP is set. The horizontal space characters are: 663 <pre> 664 U+0009 Horizontal tab (HT) 665 U+0020 Space 666 U+00A0 Non-break space 667 U+1680 Ogham space mark 668 U+180E Mongolian vowel separator 669 U+2000 En quad 670 U+2001 Em quad 671 U+2002 En space 672 U+2003 Em space 673 U+2004 Three-per-em space 674 U+2005 Four-per-em space 675 U+2006 Six-per-em space 676 U+2007 Figure space 677 U+2008 Punctuation space 678 U+2009 Thin space 679 U+200A Hair space 680 U+202F Narrow no-break space 681 U+205F Medium mathematical space 682 U+3000 Ideographic space 683 </pre> 684 The vertical space characters are: 685 <pre> 686 U+000A Linefeed (LF) 687 U+000B Vertical tab (VT) 688 U+000C Form feed (FF) 689 U+000D Carriage return (CR) 690 U+0085 Next line (NEL) 691 U+2028 Line separator 692 U+2029 Paragraph separator 693 </pre> 694 In 8-bit, non-UTF-8 mode, only the characters with code points less than 256 695 are relevant. 696 <a name="newlineseq"></a></P> 697 <br><b> 698 Newline sequences 699 </b><br> 700 <P> 701 Outside a character class, by default, the escape sequence \R matches any 702 Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the 703 following: 704 <pre> 705 (?>\r\n|\n|\x0b|\f|\r|\x85) 706 </pre> 707 This is an example of an "atomic group", details of which are given 708 <a href="#atomicgroup">below.</a> 709 This particular group matches either the two-character sequence CR followed by 710 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, 711 U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next 712 line, U+0085). Because this is an atomic group, the two-character sequence is 713 treated as a single unit that cannot be split. 714 </P> 715 <P> 716 In other modes, two additional characters whose code points are greater than 255 717 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). 718 Unicode support is not needed for these characters to be recognized. 719 </P> 720 <P> 721 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the 722 complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF 723 at compile time. (BSR is an abbrevation for "backslash R".) This can be made 724 the default when PCRE2 is built; if this is the case, the other behaviour can 725 be requested via the PCRE2_BSR_UNICODE option. It is also possible to specify 726 these settings by starting a pattern string with one of the following 727 sequences: 728 <pre> 729 (*BSR_ANYCRLF) CR, LF, or CRLF only 730 (*BSR_UNICODE) any Unicode newline sequence 731 </pre> 732 These override the default and the options given to the compiling function. 733 Note that these special settings, which are not Perl-compatible, are recognized 734 only at the very start of a pattern, and that they must be in upper case. If 735 more than one of them is present, the last one is used. They can be combined 736 with a change of newline convention; for example, a pattern can start with: 737 <pre> 738 (*ANY)(*BSR_ANYCRLF) 739 </pre> 740 They can also be combined with the (*UTF) or (*UCP) special sequences. Inside a 741 character class, \R is treated as an unrecognized escape sequence, and causes 742 an error. 743 <a name="uniextseq"></a></P> 744 <br><b> 745 Unicode character properties 746 </b><br> 747 <P> 748 When PCRE2 is built with Unicode support (the default), three additional escape 749 sequences that match characters with specific properties are available. In 750 8-bit non-UTF-8 mode, these sequences are of course limited to testing 751 characters whose code points are less than 256, but they do work in this mode. 752 In 32-bit non-UTF mode, code points greater than 0x10ffff (the Unicode limit) 753 may be encountered. These are all treated as being in the Common script and 754 with an unassigned type. The extra escape sequences are: 755 <pre> 756 \p{<i>xx</i>} a character with the <i>xx</i> property 757 \P{<i>xx</i>} a character without the <i>xx</i> property 758 \X a Unicode extended grapheme cluster 759 </pre> 760 The property names represented by <i>xx</i> above are limited to the Unicode 761 script names, the general category properties, "Any", which matches any 762 character (including newline), and some special PCRE2 properties (described 763 in the 764 <a href="#extraprops">next section).</a> 765 Other Perl properties such as "InMusicalSymbols" are not supported by PCRE2. 766 Note that \P{Any} does not match any characters, so always causes a match 767 failure. 768 </P> 769 <P> 770 Sets of Unicode characters are defined as belonging to certain scripts. A 771 character from one of these sets can be matched using a script name. For 772 example: 773 <pre> 774 \p{Greek} 775 \P{Han} 776 </pre> 777 Those that are not part of an identified script are lumped together as 778 "Common". The current list of scripts is: 779 </P> 780 <P> 781 Adlam, 782 Ahom, 783 Anatolian_Hieroglyphs, 784 Arabic, 785 Armenian, 786 Avestan, 787 Balinese, 788 Bamum, 789 Bassa_Vah, 790 Batak, 791 Bengali, 792 Bhaiksuki, 793 Bopomofo, 794 Brahmi, 795 Braille, 796 Buginese, 797 Buhid, 798 Canadian_Aboriginal, 799 Carian, 800 Caucasian_Albanian, 801 Chakma, 802 Cham, 803 Cherokee, 804 Common, 805 Coptic, 806 Cuneiform, 807 Cypriot, 808 Cyrillic, 809 Deseret, 810 Devanagari, 811 Dogra, 812 Duployan, 813 Egyptian_Hieroglyphs, 814 Elbasan, 815 Ethiopic, 816 Georgian, 817 Glagolitic, 818 Gothic, 819 Grantha, 820 Greek, 821 Gujarati, 822 Gunjala_Gondi, 823 Gurmukhi, 824 Han, 825 Hangul, 826 Hanifi_Rohingya, 827 Hanunoo, 828 Hatran, 829 Hebrew, 830 Hiragana, 831 Imperial_Aramaic, 832 Inherited, 833 Inscriptional_Pahlavi, 834 Inscriptional_Parthian, 835 Javanese, 836 Kaithi, 837 Kannada, 838 Katakana, 839 Kayah_Li, 840 Kharoshthi, 841 Khmer, 842 Khojki, 843 Khudawadi, 844 Lao, 845 Latin, 846 Lepcha, 847 Limbu, 848 Linear_A, 849 Linear_B, 850 Lisu, 851 Lycian, 852 Lydian, 853 Mahajani, 854 Makasar, 855 Malayalam, 856 Mandaic, 857 Manichaean, 858 Marchen, 859 Masaram_Gondi, 860 Medefaidrin, 861 Meetei_Mayek, 862 Mende_Kikakui, 863 Meroitic_Cursive, 864 Meroitic_Hieroglyphs, 865 Miao, 866 Modi, 867 Mongolian, 868 Mro, 869 Multani, 870 Myanmar, 871 Nabataean, 872 New_Tai_Lue, 873 Newa, 874 Nko, 875 Nushu, 876 Ogham, 877 Ol_Chiki, 878 Old_Hungarian, 879 Old_Italic, 880 Old_North_Arabian, 881 Old_Permic, 882 Old_Persian, 883 Old_Sogdian, 884 Old_South_Arabian, 885 Old_Turkic, 886 Oriya, 887 Osage, 888 Osmanya, 889 Pahawh_Hmong, 890 Palmyrene, 891 Pau_Cin_Hau, 892 Phags_Pa, 893 Phoenician, 894 Psalter_Pahlavi, 895 Rejang, 896 Runic, 897 Samaritan, 898 Saurashtra, 899 Sharada, 900 Shavian, 901 Siddham, 902 SignWriting, 903 Sinhala, 904 Sogdian, 905 Sora_Sompeng, 906 Soyombo, 907 Sundanese, 908 Syloti_Nagri, 909 Syriac, 910 Tagalog, 911 Tagbanwa, 912 Tai_Le, 913 Tai_Tham, 914 Tai_Viet, 915 Takri, 916 Tamil, 917 Tangut, 918 Telugu, 919 Thaana, 920 Thai, 921 Tibetan, 922 Tifinagh, 923 Tirhuta, 924 Ugaritic, 925 Vai, 926 Warang_Citi, 927 Yi, 928 Zanabazar_Square. 929 </P> 930 <P> 931 Each character has exactly one Unicode general category property, specified by 932 a two-letter abbreviation. For compatibility with Perl, negation can be 933 specified by including a circumflex between the opening brace and the property 934 name. For example, \p{^Lu} is the same as \P{Lu}. 935 </P> 936 <P> 937 If only one letter is specified with \p or \P, it includes all the general 938 category properties that start with that letter. In this case, in the absence 939 of negation, the curly brackets in the escape sequence are optional; these two 940 examples have the same effect: 941 <pre> 942 \p{L} 943 \pL 944 </pre> 945 The following general category property codes are supported: 946 <pre> 947 C Other 948 Cc Control 949 Cf Format 950 Cn Unassigned 951 Co Private use 952 Cs Surrogate 953 954 L Letter 955 Ll Lower case letter 956 Lm Modifier letter 957 Lo Other letter 958 Lt Title case letter 959 Lu Upper case letter 960 961 M Mark 962 Mc Spacing mark 963 Me Enclosing mark 964 Mn Non-spacing mark 965 966 N Number 967 Nd Decimal number 968 Nl Letter number 969 No Other number 970 971 P Punctuation 972 Pc Connector punctuation 973 Pd Dash punctuation 974 Pe Close punctuation 975 Pf Final punctuation 976 Pi Initial punctuation 977 Po Other punctuation 978 Ps Open punctuation 979 980 S Symbol 981 Sc Currency symbol 982 Sk Modifier symbol 983 Sm Mathematical symbol 984 So Other symbol 985 986 Z Separator 987 Zl Line separator 988 Zp Paragraph separator 989 Zs Space separator 990 </pre> 991 The special property L& is also supported: it matches a character that has 992 the Lu, Ll, or Lt property, in other words, a letter that is not classified as 993 a modifier or "other". 994 </P> 995 <P> 996 The Cs (Surrogate) property applies only to characters in the range U+D800 to 997 U+DFFF. Such characters are not valid in Unicode strings and so 998 cannot be tested by PCRE2, unless UTF validity checking has been turned off 999 (see the discussion of PCRE2_NO_UTF_CHECK in the 1000 <a href="pcre2api.html"><b>pcre2api</b></a> 1001 page). Perl does not support the Cs property. 1002 </P> 1003 <P> 1004 The long synonyms for property names that Perl supports (such as \p{Letter}) 1005 are not supported by PCRE2, nor is it permitted to prefix any of these 1006 properties with "Is". 1007 </P> 1008 <P> 1009 No character that is in the Unicode table has the Cn (unassigned) property. 1010 Instead, this property is assumed for any code point that is not in the 1011 Unicode table. 1012 </P> 1013 <P> 1014 Specifying caseless matching does not affect these escape sequences. For 1015 example, \p{Lu} always matches only upper case letters. This is different from 1016 the behaviour of current versions of Perl. 1017 </P> 1018 <P> 1019 Matching characters by Unicode property is not fast, because PCRE2 has to do a 1020 multistage table lookup in order to find a character's property. That is why 1021 the traditional escape sequences such as \d and \w do not use Unicode 1022 properties in PCRE2 by default, though you can make them do so by setting the 1023 PCRE2_UCP option or by starting the pattern with (*UCP). 1024 </P> 1025 <br><b> 1026 Extended grapheme clusters 1027 </b><br> 1028 <P> 1029 The \X escape matches any number of Unicode characters that form an "extended 1030 grapheme cluster", and treats the sequence as an atomic group 1031 <a href="#atomicgroup">(see below).</a> 1032 Unicode supports various kinds of composite character by giving each character 1033 a grapheme breaking property, and having rules that use these properties to 1034 define the boundaries of extended grapheme clusters. The rules are defined in 1035 Unicode Standard Annex 29, "Unicode Text Segmentation". Unicode 11.0.0 1036 abandoned the use of some previous properties that had been used for emojis. 1037 Instead it introduced various emoji-specific properties. PCRE2 uses only the 1038 Extended Pictographic property. 1039 </P> 1040 <P> 1041 \X always matches at least one character. Then it decides whether to add 1042 additional characters according to the following rules for ending a cluster: 1043 </P> 1044 <P> 1045 1. End at the end of the subject string. 1046 </P> 1047 <P> 1048 2. Do not end between CR and LF; otherwise end after any control character. 1049 </P> 1050 <P> 1051 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters 1052 are of five types: L, V, T, LV, and LVT. An L character may be followed by an 1053 L, V, LV, or LVT character; an LV or V character may be followed by a V or T 1054 character; an LVT or T character may be follwed only by a T character. 1055 </P> 1056 <P> 1057 4. Do not end before extending characters or spacing marks or the "zero-width 1058 joiner" character. Characters with the "mark" property always have the 1059 "extend" grapheme breaking property. 1060 </P> 1061 <P> 1062 5. Do not end after prepend characters. 1063 </P> 1064 <P> 1065 6. Do not break within emoji modifier sequences or emoji zwj sequences. That 1066 is, do not break between characters with the Extended_Pictographic property. 1067 Extend and ZWJ characters are allowed between the characters. 1068 </P> 1069 <P> 1070 7. Do not break within emoji flag sequences. That is, do not break between 1071 regional indicator (RI) characters if there are an odd number of RI characters 1072 before the break point. 1073 </P> 1074 <P> 1075 8. Otherwise, end the cluster. 1076 <a name="extraprops"></a></P> 1077 <br><b> 1078 PCRE2's additional properties 1079 </b><br> 1080 <P> 1081 As well as the standard Unicode properties described above, PCRE2 supports four 1082 more that make it possible to convert traditional escape sequences such as \w 1083 and \s to use Unicode properties. PCRE2 uses these non-standard, non-Perl 1084 properties internally when PCRE2_UCP is set. However, they may also be used 1085 explicitly. These properties are: 1086 <pre> 1087 Xan Any alphanumeric character 1088 Xps Any POSIX space character 1089 Xsp Any Perl space character 1090 Xwd Any Perl "word" character 1091 </pre> 1092 Xan matches characters that have either the L (letter) or the N (number) 1093 property. Xps matches the characters tab, linefeed, vertical tab, form feed, or 1094 carriage return, and any other character that has the Z (separator) property. 1095 Xsp is the same as Xps; in PCRE1 it used to exclude vertical tab, for Perl 1096 compatibility, but Perl changed. Xwd matches the same characters as Xan, plus 1097 underscore. 1098 </P> 1099 <P> 1100 There is another non-standard property, Xuc, which matches any character that 1101 can be represented by a Universal Character Name in C++ and other programming 1102 languages. These are the characters $, @, ` (grave accent), and all characters 1103 with Unicode code points greater than or equal to U+00A0, except for the 1104 surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are 1105 excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH 1106 where H is a hexadecimal digit. Note that the Xuc property does not match these 1107 sequences but the characters that they represent.) 1108 <a name="resetmatchstart"></a></P> 1109 <br><b> 1110 Resetting the match start 1111 </b><br> 1112 <P> 1113 In normal use, the escape sequence \K causes any previously matched characters 1114 not to be included in the final matched sequence that is returned. For example, 1115 the pattern: 1116 <pre> 1117 foo\Kbar 1118 </pre> 1119 matches "foobar", but reports that it has matched "bar". \K does not interact 1120 with anchoring in any way. The pattern: 1121 <pre> 1122 ^foo\Kbar 1123 </pre> 1124 matches only when the subject begins with "foobar" (in single line mode), 1125 though it again reports the matched string as "bar". This feature is similar to 1126 a lookbehind assertion 1127 <a href="#lookbehind">(described below).</a> 1128 However, in this case, the part of the subject before the real match does not 1129 have to be of fixed length, as lookbehind assertions do. The use of \K does 1130 not interfere with the setting of 1131 <a href="#subpattern">captured substrings.</a> 1132 For example, when the pattern 1133 <pre> 1134 (foo)\Kbar 1135 </pre> 1136 matches "foobar", the first substring is still set to "foo". 1137 </P> 1138 <P> 1139 Perl documents that the use of \K within assertions is "not well defined". In 1140 PCRE2, \K is acted upon when it occurs inside positive assertions, but is 1141 ignored in negative assertions. Note that when a pattern such as (?=ab\K) 1142 matches, the reported start of the match can be greater than the end of the 1143 match. Using \K in a lookbehind assertion at the start of a pattern can also 1144 lead to odd effects. For example, consider this pattern: 1145 <pre> 1146 (?<=\Kfoo)bar 1147 </pre> 1148 If the subject is "foobar", a call to <b>pcre2_match()</b> with a starting 1149 offset of 3 succeeds and reports the matching string as "foobar", that is, the 1150 start of the reported match is earlier than where the match started. 1151 <a name="smallassertions"></a></P> 1152 <br><b> 1153 Simple assertions 1154 </b><br> 1155 <P> 1156 The final use of backslash is for certain simple assertions. An assertion 1157 specifies a condition that has to be met at a particular point in a match, 1158 without consuming any characters from the subject string. The use of 1159 subpatterns for more complicated assertions is described 1160 <a href="#bigassertions">below.</a> 1161 The backslashed assertions are: 1162 <pre> 1163 \b matches at a word boundary 1164 \B matches when not at a word boundary 1165 \A matches at the start of the subject 1166 \Z matches at the end of the subject 1167 also matches before a newline at the end of the subject 1168 \z matches only at the end of the subject 1169 \G matches at the first matching position in the subject 1170 </pre> 1171 Inside a character class, \b has a different meaning; it matches the backspace 1172 character. If any other of these assertions appears in a character class, an 1173 "invalid escape sequence" error is generated. 1174 </P> 1175 <P> 1176 A word boundary is a position in the subject string where the current character 1177 and the previous character do not both match \w or \W (i.e. one matches 1178 \w and the other matches \W), or the start or end of the string if the 1179 first or last character matches \w, respectively. In a UTF mode, the meanings 1180 of \w and \W can be changed by setting the PCRE2_UCP option. When this is 1181 done, it also affects \b and \B. Neither PCRE2 nor Perl has a separate "start 1182 of word" or "end of word" metasequence. However, whatever follows \b normally 1183 determines which it is. For example, the fragment \ba matches "a" at the start 1184 of a word. 1185 </P> 1186 <P> 1187 The \A, \Z, and \z assertions differ from the traditional circumflex and 1188 dollar (described in the next section) in that they only ever match at the very 1189 start and end of the subject string, whatever options are set. Thus, they are 1190 independent of multiline mode. These three assertions are not affected by the 1191 PCRE2_NOTBOL or PCRE2_NOTEOL options, which affect only the behaviour of the 1192 circumflex and dollar metacharacters. However, if the <i>startoffset</i> 1193 argument of <b>pcre2_match()</b> is non-zero, indicating that matching is to 1194 start at a point other than the beginning of the subject, \A can never match. 1195 The difference between \Z and \z is that \Z matches before a newline at the 1196 end of the string as well as at the very end, whereas \z matches only at the 1197 end. 1198 </P> 1199 <P> 1200 The \G assertion is true only when the current matching position is at the 1201 start point of the matching process, as specified by the <i>startoffset</i> 1202 argument of <b>pcre2_match()</b>. It differs from \A when the value of 1203 <i>startoffset</i> is non-zero. By calling <b>pcre2_match()</b> multiple times 1204 with appropriate arguments, you can mimic Perl's /g option, and it is in this 1205 kind of implementation where \G can be useful. 1206 </P> 1207 <P> 1208 Note, however, that PCRE2's implementation of \G, being true at the starting 1209 character of the matching process, is subtly different from Perl's, which 1210 defines it as true at the end of the previous match. In Perl, these can be 1211 different when the previously matched string was empty. Because PCRE2 does just 1212 one match at a time, it cannot reproduce this behaviour. 1213 </P> 1214 <P> 1215 If all the alternatives of a pattern begin with \G, the expression is anchored 1216 to the starting match position, and the "anchored" flag is set in the compiled 1217 regular expression. 1218 </P> 1219 <br><a name="SEC6" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br> 1220 <P> 1221 The circumflex and dollar metacharacters are zero-width assertions. That is, 1222 they test for a particular condition being true without consuming any 1223 characters from the subject string. These two metacharacters are concerned with 1224 matching the starts and ends of lines. If the newline convention is set so that 1225 only the two-character sequence CRLF is recognized as a newline, isolated CR 1226 and LF characters are treated as ordinary data characters, and are not 1227 recognized as newlines. 1228 </P> 1229 <P> 1230 Outside a character class, in the default matching mode, the circumflex 1231 character is an assertion that is true only if the current matching point is at 1232 the start of the subject string. If the <i>startoffset</i> argument of 1233 <b>pcre2_match()</b> is non-zero, or if PCRE2_NOTBOL is set, circumflex can 1234 never match if the PCRE2_MULTILINE option is unset. Inside a character class, 1235 circumflex has an entirely different meaning 1236 <a href="#characterclass">(see below).</a> 1237 </P> 1238 <P> 1239 Circumflex need not be the first character of the pattern if a number of 1240 alternatives are involved, but it should be the first thing in each alternative 1241 in which it appears if the pattern is ever to match that branch. If all 1242 possible alternatives start with a circumflex, that is, if the pattern is 1243 constrained to match only at the start of the subject, it is said to be an 1244 "anchored" pattern. (There are also other constructs that can cause a pattern 1245 to be anchored.) 1246 </P> 1247 <P> 1248 The dollar character is an assertion that is true only if the current matching 1249 point is at the end of the subject string, or immediately before a newline at 1250 the end of the string (by default), unless PCRE2_NOTEOL is set. Note, however, 1251 that it does not actually match the newline. Dollar need not be the last 1252 character of the pattern if a number of alternatives are involved, but it 1253 should be the last item in any branch in which it appears. Dollar has no 1254 special meaning in a character class. 1255 </P> 1256 <P> 1257 The meaning of dollar can be changed so that it matches only at the very end of 1258 the string, by setting the PCRE2_DOLLAR_ENDONLY option at compile time. This 1259 does not affect the \Z assertion. 1260 </P> 1261 <P> 1262 The meanings of the circumflex and dollar metacharacters are changed if the 1263 PCRE2_MULTILINE option is set. When this is the case, a dollar character 1264 matches before any newlines in the string, as well as at the very end, and a 1265 circumflex matches immediately after internal newlines as well as at the start 1266 of the subject string. It does not match after a newline that ends the string, 1267 for compatibility with Perl. However, this can be changed by setting the 1268 PCRE2_ALT_CIRCUMFLEX option. 1269 </P> 1270 <P> 1271 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where 1272 \n represents a newline) in multiline mode, but not otherwise. Consequently, 1273 patterns that are anchored in single line mode because all branches start with 1274 ^ are not anchored in multiline mode, and a match for circumflex is possible 1275 when the <i>startoffset</i> argument of <b>pcre2_match()</b> is non-zero. The 1276 PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set. 1277 </P> 1278 <P> 1279 When the newline convention (see 1280 <a href="#newlines">"Newline conventions"</a> 1281 below) recognizes the two-character sequence CRLF as a newline, this is 1282 preferred, even if the single characters CR and LF are also recognized as 1283 newlines. For example, if the newline convention is "any", a multiline mode 1284 circumflex matches before "xyz" in the string "abc\r\nxyz" rather than after 1285 CR, even though CR on its own is a valid newline. (It also matches at the very 1286 start of the string, of course.) 1287 </P> 1288 <P> 1289 Note that the sequences \A, \Z, and \z can be used to match the start and 1290 end of the subject in both modes, and if all branches of a pattern start with 1291 \A it is always anchored, whether or not PCRE2_MULTILINE is set. 1292 <a name="fullstopdot"></a></P> 1293 <br><a name="SEC7" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br> 1294 <P> 1295 Outside a character class, a dot in the pattern matches any one character in 1296 the subject string except (by default) a character that signifies the end of a 1297 line. 1298 </P> 1299 <P> 1300 When a line ending is defined as a single character, dot never matches that 1301 character; when the two-character sequence CRLF is used, dot does not match CR 1302 if it is immediately followed by LF, but otherwise it matches all characters 1303 (including isolated CRs and LFs). When any Unicode line endings are being 1304 recognized, dot does not match CR or LF or any of the other line ending 1305 characters. 1306 </P> 1307 <P> 1308 The behaviour of dot with regard to newlines can be changed. If the 1309 PCRE2_DOTALL option is set, a dot matches any one character, without exception. 1310 If the two-character sequence CRLF is present in the subject string, it takes 1311 two dots to match it. 1312 </P> 1313 <P> 1314 The handling of dot is entirely independent of the handling of circumflex and 1315 dollar, the only relationship being that they both involve newlines. Dot has no 1316 special meaning in a character class. 1317 </P> 1318 <P> 1319 The escape sequence \N when not followed by an opening brace behaves like a 1320 dot, except that it is not affected by the PCRE2_DOTALL option. In other words, 1321 it matches any character except one that signifies the end of a line. 1322 </P> 1323 <P> 1324 When \N is followed by an opening brace it has a different meaning. See the 1325 section entitled 1326 <a href="digitsafterbackslash">"Non-printing characters"</a> 1327 above for details. Perl also uses \N{name} to specify characters by Unicode 1328 name; PCRE2 does not support this. 1329 </P> 1330 <br><a name="SEC8" href="#TOC1">MATCHING A SINGLE CODE UNIT</a><br> 1331 <P> 1332 Outside a character class, the escape sequence \C matches any one code unit, 1333 whether or not a UTF mode is set. In the 8-bit library, one code unit is one 1334 byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is a 1335 32-bit unit. Unlike a dot, \C always matches line-ending characters. The 1336 feature is provided in Perl in order to match individual bytes in UTF-8 mode, 1337 but it is unclear how it can usefully be used. 1338 </P> 1339 <P> 1340 Because \C breaks up characters into individual code units, matching one unit 1341 with \C in UTF-8 or UTF-16 mode means that the rest of the string may start 1342 with a malformed UTF character. This has undefined results, because PCRE2 1343 assumes that it is matching character by character in a valid UTF string (by 1344 default it checks the subject string's validity at the start of processing 1345 unless the PCRE2_NO_UTF_CHECK option is used). 1346 </P> 1347 <P> 1348 An application can lock out the use of \C by setting the 1349 PCRE2_NEVER_BACKSLASH_C option when compiling a pattern. It is also possible to 1350 build PCRE2 with the use of \C permanently disabled. 1351 </P> 1352 <P> 1353 PCRE2 does not allow \C to appear in lookbehind assertions 1354 <a href="#lookbehind">(described below)</a> 1355 in UTF-8 or UTF-16 modes, because this would make it impossible to calculate 1356 the length of the lookbehind. Neither the alternative matching function 1357 <b>pcre2_dfa_match()</b> nor the JIT optimizer support \C in these UTF modes. 1358 The former gives a match-time error; the latter fails to optimize and so the 1359 match is always run using the interpreter. 1360 </P> 1361 <P> 1362 In the 32-bit library, however, \C is always supported (when not explicitly 1363 locked out) because it always matches a single code unit, whether or not UTF-32 1364 is specified. 1365 </P> 1366 <P> 1367 In general, the \C escape sequence is best avoided. However, one way of using 1368 it that avoids the problem of malformed UTF-8 or UTF-16 characters is to use a 1369 lookahead to check the length of the next character, as in this pattern, which 1370 could be used with a UTF-8 string (ignore white space and line breaks): 1371 <pre> 1372 (?| (?=[\x00-\x7f])(\C) | 1373 (?=[\x80-\x{7ff}])(\C)(\C) | 1374 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | 1375 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) 1376 </pre> 1377 In this example, a group that starts with (?| resets the capturing parentheses 1378 numbers in each alternative (see 1379 <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a> 1380 below). The assertions at the start of each branch check the next UTF-8 1381 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The 1382 character's individual bytes are then captured by the appropriate number of 1383 \C groups. 1384 <a name="characterclass"></a></P> 1385 <br><a name="SEC9" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br> 1386 <P> 1387 An opening square bracket introduces a character class, terminated by a closing 1388 square bracket. A closing square bracket on its own is not special by default. 1389 If a closing square bracket is required as a member of the class, it should be 1390 the first data character in the class (after an initial circumflex, if present) 1391 or escaped with a backslash. This means that, by default, an empty class cannot 1392 be defined. However, if the PCRE2_ALLOW_EMPTY_CLASS option is set, a closing 1393 square bracket at the start does end the (empty) class. 1394 </P> 1395 <P> 1396 A character class matches a single character in the subject. A matched 1397 character must be in the set of characters defined by the class, unless the 1398 first character in the class definition is a circumflex, in which case the 1399 subject character must not be in the set defined by the class. If a circumflex 1400 is actually required as a member of the class, ensure it is not the first 1401 character, or escape it with a backslash. 1402 </P> 1403 <P> 1404 For example, the character class [aeiou] matches any lower case vowel, while 1405 [^aeiou] matches any character that is not a lower case vowel. Note that a 1406 circumflex is just a convenient notation for specifying the characters that 1407 are in the class by enumerating those that are not. A class that starts with a 1408 circumflex is not an assertion; it still consumes a character from the subject 1409 string, and therefore it fails if the current pointer is at the end of the 1410 string. 1411 </P> 1412 <P> 1413 Characters in a class may be specified by their code points using \o, \x, or 1414 \N{U+hh..} in the usual way. When caseless matching is set, any letters in a 1415 class represent both their upper case and lower case versions, so for example, 1416 a caseless [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not 1417 match "A", whereas a caseful version would. 1418 </P> 1419 <P> 1420 Characters that might indicate line breaks are never treated in any special way 1421 when matching character classes, whatever line-ending sequence is in use, and 1422 whatever setting of the PCRE2_DOTALL and PCRE2_MULTILINE options is used. A 1423 class such as [^a] always matches one of these characters. 1424 </P> 1425 <P> 1426 The generic character type escape sequences \d, \D, \h, \H, \p, \P, \s, 1427 \S, \v, \V, \w, and \W may appear in a character class, and add the 1428 characters that they match to the class. For example, [\dABCDEF] matches any 1429 hexadecimal digit. In UTF modes, the PCRE2_UCP option affects the meanings of 1430 \d, \s, \w and their upper case partners, just as it does when they appear 1431 outside a character class, as described in the section entitled 1432 <a href="#genericchartypes">"Generic character types"</a> 1433 above. The escape sequence \b has a different meaning inside a character 1434 class; it matches the backspace character. The sequences \B, \R, and \X are 1435 not special inside a character class. Like any other unrecognized escape 1436 sequences, they cause an error. The same is true for \N when not followed by 1437 an opening brace. 1438 </P> 1439 <P> 1440 The minus (hyphen) character can be used to specify a range of characters in a 1441 character class. For example, [d-m] matches any letter between d and m, 1442 inclusive. If a minus character is required in a class, it must be escaped with 1443 a backslash or appear in a position where it cannot be interpreted as 1444 indicating a range, typically as the first or last character in the class, 1445 or immediately after a range. For example, [b-d-z] matches letters in the range 1446 b to d, a hyphen character, or z. 1447 </P> 1448 <P> 1449 Perl treats a hyphen as a literal if it appears before or after a POSIX class 1450 (see below) or before or after a character type escape such as as \d or \H. 1451 However, unless the hyphen is the last character in the class, Perl outputs a 1452 warning in its warning mode, as this is most likely a user error. As PCRE2 has 1453 no facility for warning, an error is given in these cases. 1454 </P> 1455 <P> 1456 It is not possible to have the literal character "]" as the end character of a 1457 range. A pattern such as [W-]46] is interpreted as a class of two characters 1458 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or 1459 "-46]". However, if the "]" is escaped with a backslash it is interpreted as 1460 the end of range, so [W-\]46] is interpreted as a class containing a range 1461 followed by two other characters. The octal or hexadecimal representation of 1462 "]" can also be used to end a range. 1463 </P> 1464 <P> 1465 Ranges normally include all code points between the start and end characters, 1466 inclusive. They can also be used for code points specified numerically, for 1467 example [\000-\037]. Ranges can include any characters that are valid for the 1468 current mode. In any UTF mode, the so-called "surrogate" characters (those 1469 whose code points lie between 0xd800 and 0xdfff inclusive) may not be specified 1470 explicitly by default (the PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES option disables 1471 this check). However, ranges such as [\x{d7ff}-\x{e000}], which include the 1472 surrogates, are always permitted. 1473 </P> 1474 <P> 1475 There is a special case in EBCDIC environments for ranges whose end points are 1476 both specified as literal letters in the same case. For compatibility with 1477 Perl, EBCDIC code points within the range that are not letters are omitted. For 1478 example, [h-k] matches only four characters, even though the codes for h and k 1479 are 0x88 and 0x92, a range of 11 code points. However, if the range is 1480 specified numerically, for example, [\x88-\x92] or [h-\x92], all code points 1481 are included. 1482 </P> 1483 <P> 1484 If a range that includes letters is used when caseless matching is set, it 1485 matches the letters in either case. For example, [W-c] is equivalent to 1486 [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character 1487 tables for a French locale are in use, [\xc8-\xcb] matches accented E 1488 characters in both cases. 1489 </P> 1490 <P> 1491 A circumflex can conveniently be used with the upper case character types to 1492 specify a more restricted set of characters than the matching lower case type. 1493 For example, the class [^\W_] matches any letter or digit, but not underscore, 1494 whereas [\w] includes underscore. A positive character class should be read as 1495 "something OR something OR ..." and a negative class as "NOT something AND NOT 1496 something AND NOT ...". 1497 </P> 1498 <P> 1499 The only metacharacters that are recognized in character classes are backslash, 1500 hyphen (only where it can be interpreted as specifying a range), circumflex 1501 (only at the start), opening square bracket (only when it can be interpreted as 1502 introducing a POSIX class name, or for a special compatibility feature - see 1503 the next two sections), and the terminating closing square bracket. However, 1504 escaping other non-alphanumeric characters does no harm. 1505 </P> 1506 <br><a name="SEC10" href="#TOC1">POSIX CHARACTER CLASSES</a><br> 1507 <P> 1508 Perl supports the POSIX notation for character classes. This uses names 1509 enclosed by [: and :] within the enclosing square brackets. PCRE2 also supports 1510 this notation. For example, 1511 <pre> 1512 [01[:alpha:]%] 1513 </pre> 1514 matches "0", "1", any alphabetic character, or "%". The supported class names 1515 are: 1516 <pre> 1517 alnum letters and digits 1518 alpha letters 1519 ascii character codes 0 - 127 1520 blank space or tab only 1521 cntrl control characters 1522 digit decimal digits (same as \d) 1523 graph printing characters, excluding space 1524 lower lower case letters 1525 print printing characters, including space 1526 punct printing characters, excluding letters and digits and space 1527 space white space (the same as \s from PCRE2 8.34) 1528 upper upper case letters 1529 word "word" characters (same as \w) 1530 xdigit hexadecimal digits 1531 </pre> 1532 The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), 1533 and space (32). If locale-specific matching is taking place, the list of space 1534 characters may be different; there may be fewer or more of them. "Space" and 1535 \s match the same set of characters. 1536 </P> 1537 <P> 1538 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 1539 5.8. Another Perl extension is negation, which is indicated by a ^ character 1540 after the colon. For example, 1541 <pre> 1542 [12[:^digit:]] 1543 </pre> 1544 matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the POSIX 1545 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not 1546 supported, and an error is given if they are encountered. 1547 </P> 1548 <P> 1549 By default, characters with values greater than 127 do not match any of the 1550 POSIX character classes, although this may be different for characters in the 1551 range 128-255 when locale-specific matching is happening. However, if the 1552 PCRE2_UCP option is passed to <b>pcre2_compile()</b>, some of the classes are 1553 changed so that Unicode character properties are used. This is achieved by 1554 replacing certain POSIX classes with other sequences, as follows: 1555 <pre> 1556 [:alnum:] becomes \p{Xan} 1557 [:alpha:] becomes \p{L} 1558 [:blank:] becomes \h 1559 [:cntrl:] becomes \p{Cc} 1560 [:digit:] becomes \p{Nd} 1561 [:lower:] becomes \p{Ll} 1562 [:space:] becomes \p{Xps} 1563 [:upper:] becomes \p{Lu} 1564 [:word:] becomes \p{Xwd} 1565 </pre> 1566 Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX 1567 classes are handled specially in UCP mode: 1568 </P> 1569 <P> 1570 [:graph:] 1571 This matches characters that have glyphs that mark the page when printed. In 1572 Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf 1573 properties, except for: 1574 <pre> 1575 U+061C Arabic Letter Mark 1576 U+180E Mongolian Vowel Separator 1577 U+2066 - U+2069 Various "isolate"s 1578 1579 </PRE> 1580 </P> 1581 <P> 1582 [:print:] 1583 This matches the same characters as [:graph:] plus space characters that are 1584 not controls, that is, characters with the Zs property. 1585 </P> 1586 <P> 1587 [:punct:] 1588 This matches all characters that have the Unicode P (punctuation) property, 1589 plus those characters with code points less than 256 that have the S (Symbol) 1590 property. 1591 </P> 1592 <P> 1593 The other POSIX classes are unchanged, and match only characters with code 1594 points less than 256. 1595 </P> 1596 <br><a name="SEC11" href="#TOC1">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a><br> 1597 <P> 1598 In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly 1599 syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of 1600 word". PCRE2 treats these items as follows: 1601 <pre> 1602 [[:<:]] is converted to \b(?=\w) 1603 [[:>:]] is converted to \b(?<=\w) 1604 </pre> 1605 Only these exact character sequences are recognized. A sequence such as 1606 [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is 1607 not compatible with Perl. It is provided to help migrations from other 1608 environments, and is best not used in any new patterns. Note that \b matches 1609 at the start and the end of a word (see 1610 <a href="#smallassertions">"Simple assertions"</a> 1611 above), and in a Perl-style pattern the preceding or following character 1612 normally shows which is wanted, without the need for the assertions that are 1613 used above in order to give exactly the POSIX behaviour. 1614 </P> 1615 <br><a name="SEC12" href="#TOC1">VERTICAL BAR</a><br> 1616 <P> 1617 Vertical bar characters are used to separate alternative patterns. For example, 1618 the pattern 1619 <pre> 1620 gilbert|sullivan 1621 </pre> 1622 matches either "gilbert" or "sullivan". Any number of alternatives may appear, 1623 and an empty alternative is permitted (matching the empty string). The matching 1624 process tries each alternative in turn, from left to right, and the first one 1625 that succeeds is used. If the alternatives are within a subpattern 1626 <a href="#subpattern">(defined below),</a> 1627 "succeeds" means matching the rest of the main pattern as well as the 1628 alternative in the subpattern. 1629 </P> 1630 <br><a name="SEC13" href="#TOC1">INTERNAL OPTION SETTING</a><br> 1631 <P> 1632 The settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL, 1633 PCRE2_EXTENDED, PCRE2_EXTENDED_MORE, and PCRE2_NO_AUTO_CAPTURE options can be 1634 changed from within the pattern by a sequence of letters enclosed between "(?" 1635 and ")". These options are Perl-compatible, and are described in detail in the 1636 <a href="pcre2api.html"><b>pcre2api</b></a> 1637 documentation. The option letters are: 1638 <pre> 1639 i for PCRE2_CASELESS 1640 m for PCRE2_MULTILINE 1641 n for PCRE2_NO_AUTO_CAPTURE 1642 s for PCRE2_DOTALL 1643 x for PCRE2_EXTENDED 1644 xx for PCRE2_EXTENDED_MORE 1645 </pre> 1646 For example, (?im) sets caseless, multiline matching. It is also possible to 1647 unset these options by preceding the relevant letters with a hyphen, for 1648 example (?-im). The two "extended" options are not independent; unsetting either 1649 one cancels the effects of both of them. 1650 </P> 1651 <P> 1652 A combined setting and unsetting such as (?im-sx), which sets PCRE2_CASELESS 1653 and PCRE2_MULTILINE while unsetting PCRE2_DOTALL and PCRE2_EXTENDED, is also 1654 permitted. Only one hyphen may appear in the options string. If a letter 1655 appears both before and after the hyphen, the option is unset. An empty options 1656 setting "(?)" is allowed. Needless to say, it has no effect. 1657 </P> 1658 <P> 1659 If the first character following (? is a circumflex, it causes all of the above 1660 options to be unset. Thus, (?^) is equivalent to (?-imnsx). Letters may follow 1661 the circumflex to cause some options to be re-instated, but a hyphen may not 1662 appear. 1663 </P> 1664 <P> 1665 The PCRE2-specific options PCRE2_DUPNAMES and PCRE2_UNGREEDY can be changed in 1666 the same way as the Perl-compatible options by using the characters J and U 1667 respectively. However, these are not unset by (?^). 1668 </P> 1669 <P> 1670 When one of these option changes occurs at top level (that is, not inside 1671 subpattern parentheses), the change applies to the remainder of the pattern 1672 that follows. An option change within a subpattern (see below for a description 1673 of subpatterns) affects only that part of the subpattern that follows it, so 1674 <pre> 1675 (a(?i)b)c 1676 </pre> 1677 matches abc and aBc and no other strings (assuming PCRE2_CASELESS is not used). 1678 By this means, options can be made to have different settings in different 1679 parts of the pattern. Any changes made in one alternative do carry on 1680 into subsequent branches within the same subpattern. For example, 1681 <pre> 1682 (a(?i)b|c) 1683 </pre> 1684 matches "ab", "aB", "c", and "C", even though when matching "C" the first 1685 branch is abandoned before the option setting. This is because the effects of 1686 option settings happen at compile time. There would be some very weird 1687 behaviour otherwise. 1688 </P> 1689 <P> 1690 As a convenient shorthand, if any option settings are required at the start of 1691 a non-capturing subpattern (see the next section), the option letters may 1692 appear between the "?" and the ":". Thus the two patterns 1693 <pre> 1694 (?i:saturday|sunday) 1695 (?:(?i)saturday|sunday) 1696 </pre> 1697 match exactly the same set of strings. 1698 </P> 1699 <P> 1700 <b>Note:</b> There are other PCRE2-specific options that can be set by the 1701 application when the compiling function is called. The pattern can contain 1702 special leading sequences such as (*CRLF) to override what the application has 1703 set or what has been defaulted. Details are given in the section entitled 1704 <a href="#newlineseq">"Newline sequences"</a> 1705 above. There are also the (*UTF) and (*UCP) leading sequences that can be used 1706 to set UTF and Unicode property modes; they are equivalent to setting the 1707 PCRE2_UTF and PCRE2_UCP options, respectively. However, the application can set 1708 the PCRE2_NEVER_UTF and PCRE2_NEVER_UCP options, which lock out the use of the 1709 (*UTF) and (*UCP) sequences. 1710 <a name="subpattern"></a></P> 1711 <br><a name="SEC14" href="#TOC1">SUBPATTERNS</a><br> 1712 <P> 1713 Subpatterns are delimited by parentheses (round brackets), which can be nested. 1714 Turning part of a pattern into a subpattern does two things: 1715 <br> 1716 <br> 1717 1. It localizes a set of alternatives. For example, the pattern 1718 <pre> 1719 cat(aract|erpillar|) 1720 </pre> 1721 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would 1722 match "cataract", "erpillar" or an empty string. 1723 <br> 1724 <br> 1725 2. It sets up the subpattern as a capturing subpattern. This means that, when 1726 the whole pattern matches, the portion of the subject string that matched the 1727 subpattern is passed back to the caller, separately from the portion that 1728 matched the whole pattern. (This applies only to the traditional matching 1729 function; the DFA matching function does not support capturing.) 1730 </P> 1731 <P> 1732 Opening parentheses are counted from left to right (starting from 1) to obtain 1733 numbers for the capturing subpatterns. For example, if the string "the red 1734 king" is matched against the pattern 1735 <pre> 1736 the ((red|white) (king|queen)) 1737 </pre> 1738 the captured substrings are "red king", "red", and "king", and are numbered 1, 1739 2, and 3, respectively. 1740 </P> 1741 <P> 1742 The fact that plain parentheses fulfil two functions is not always helpful. 1743 There are often times when a grouping subpattern is required without a 1744 capturing requirement. If an opening parenthesis is followed by a question mark 1745 and a colon, the subpattern does not do any capturing, and is not counted when 1746 computing the number of any subsequent capturing subpatterns. For example, if 1747 the string "the white queen" is matched against the pattern 1748 <pre> 1749 the ((?:red|white) (king|queen)) 1750 </pre> 1751 the captured substrings are "white queen" and "queen", and are numbered 1 and 1752 2. The maximum number of capturing subpatterns is 65535. 1753 </P> 1754 <P> 1755 As a convenient shorthand, if any option settings are required at the start of 1756 a non-capturing subpattern, the option letters may appear between the "?" and 1757 the ":". Thus the two patterns 1758 <pre> 1759 (?i:saturday|sunday) 1760 (?:(?i)saturday|sunday) 1761 </pre> 1762 match exactly the same set of strings. Because alternative branches are tried 1763 from left to right, and options are not reset until the end of the subpattern 1764 is reached, an option setting in one branch does affect subsequent branches, so 1765 the above patterns match "SUNDAY" as well as "Saturday". 1766 <a name="dupsubpatternnumber"></a></P> 1767 <br><a name="SEC15" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br> 1768 <P> 1769 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses 1770 the same numbers for its capturing parentheses. Such a subpattern starts with 1771 (?| and is itself a non-capturing subpattern. For example, consider this 1772 pattern: 1773 <pre> 1774 (?|(Sat)ur|(Sun))day 1775 </pre> 1776 Because the two alternatives are inside a (?| group, both sets of capturing 1777 parentheses are numbered one. Thus, when the pattern matches, you can look 1778 at captured substring number one, whichever alternative matched. This construct 1779 is useful when you want to capture part, but not all, of one of a number of 1780 alternatives. Inside a (?| group, parentheses are numbered as usual, but the 1781 number is reset at the start of each branch. The numbers of any capturing 1782 parentheses that follow the subpattern start after the highest number used in 1783 any branch. The following example is taken from the Perl documentation. The 1784 numbers underneath show in which buffer the captured content will be stored. 1785 <pre> 1786 # before ---------------branch-reset----------- after 1787 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x 1788 # 1 2 2 3 2 3 4 1789 </pre> 1790 A backreference to a numbered subpattern uses the most recent value that is 1791 set for that number by any subpattern. The following pattern matches "abcabc" 1792 or "defdef": 1793 <pre> 1794 /(?|(abc)|(def))\1/ 1795 </pre> 1796 In contrast, a subroutine call to a numbered subpattern always refers to the 1797 first one in the pattern with the given number. The following pattern matches 1798 "abcabc" or "defabc": 1799 <pre> 1800 /(?|(abc)|(def))(?1)/ 1801 </pre> 1802 A relative reference such as (?-1) is no different: it is just a convenient way 1803 of computing an absolute group number. 1804 </P> 1805 <P> 1806 If a 1807 <a href="#conditions">condition test</a> 1808 for a subpattern's having matched refers to a non-unique number, the test is 1809 true if any of the subpatterns of that number have matched. 1810 </P> 1811 <P> 1812 An alternative approach to using this "branch reset" feature is to use 1813 duplicate named subpatterns, as described in the next section. 1814 </P> 1815 <br><a name="SEC16" href="#TOC1">NAMED SUBPATTERNS</a><br> 1816 <P> 1817 Identifying capturing parentheses by number is simple, but it can be very hard 1818 to keep track of the numbers in complicated patterns. Furthermore, if an 1819 expression is modified, the numbers may change. To help with this difficulty, 1820 PCRE2 supports the naming of capturing subpatterns. This feature was not added 1821 to Perl until release 5.10. Python had the feature earlier, and PCRE1 1822 introduced it at release 4.0, using the Python syntax. PCRE2 supports both the 1823 Perl and the Python syntax. 1824 </P> 1825 <P> 1826 In PCRE2, a capturing subpattern can be named in one of three ways: 1827 (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python. Names 1828 consist of up to 32 alphanumeric characters and underscores, but must start 1829 with a non-digit. References to capturing parentheses from other parts of the 1830 pattern, such as 1831 <a href="#backreferences">backreferences,</a> 1832 <a href="#recursion">recursion,</a> 1833 and 1834 <a href="#conditions">conditions,</a> 1835 can all be made by name as well as by number. 1836 </P> 1837 <P> 1838 Named capturing parentheses are allocated numbers as well as names, exactly as 1839 if the names were not present. In both PCRE2 and Perl, capturing subpatterns 1840 are primarily identified by numbers; any names are just aliases for these 1841 numbers. The PCRE2 API provides function calls for extracting the complete 1842 name-to-number translation table from a compiled pattern, as well as 1843 convenience functions for extracting captured substrings by name. 1844 </P> 1845 <P> 1846 <b>Warning:</b> When more than one subpattern has the same number, as described 1847 in the previous section, a name given to one of them applies to all of them. 1848 Perl allows identically numbered subpatterns to have different names. Consider 1849 this pattern, where there are two capturing subpatterns, both numbered 1: 1850 <pre> 1851 (?|(?<AA>aa)|(?<BB>bb)) 1852 </pre> 1853 Perl allows this, with both names AA and BB as aliases of group 1. Thus, after 1854 a successful match, both names yield the same value (either "aa" or "bb"). 1855 </P> 1856 <P> 1857 In an attempt to reduce confusion, PCRE2 does not allow the same group number 1858 to be associated with more than one name. The example above provokes a 1859 compile-time error. However, there is still scope for confusion. Consider this 1860 pattern: 1861 <pre> 1862 (?|(?<AA>aa)|(bb)) 1863 </pre> 1864 Although the second subpattern number 1 is not explicitly named, the name AA is 1865 still an alias for subpattern 1. Whether the pattern matches "aa" or "bb", a 1866 reference by name to group AA yields the matched string. 1867 </P> 1868 <P> 1869 By default, a name must be unique within a pattern, except that duplicate names 1870 are permitted for subpatterns with the same number, for example: 1871 <pre> 1872 (?|(?<AA>aa)|(?<AA>bb)) 1873 </pre> 1874 The duplicate name constraint can be disabled by setting the PCRE2_DUPNAMES 1875 option at compile time, or by the use of (?J) within the pattern. Duplicate 1876 names can be useful for patterns where only one instance of the named 1877 parentheses can match. Suppose you want to match the name of a weekday, either 1878 as a 3-letter abbreviation or as the full name, and in both cases you want to 1879 extract the abbreviation. This pattern (ignoring the line breaks) does the job: 1880 <pre> 1881 (?<DN>Mon|Fri|Sun)(?:day)?| 1882 (?<DN>Tue)(?:sday)?| 1883 (?<DN>Wed)(?:nesday)?| 1884 (?<DN>Thu)(?:rsday)?| 1885 (?<DN>Sat)(?:urday)? 1886 </pre> 1887 There are five capturing substrings, but only one is ever set after a match. 1888 The convenience functions for extracting the data by name returns the substring 1889 for the first (and in this example, the only) subpattern of that name that 1890 matched. This saves searching to find which numbered subpattern it was. (An 1891 alternative way of solving this problem is to use a "branch reset" subpattern, 1892 as described in the previous section.) 1893 </P> 1894 <P> 1895 If you make a backreference to a non-unique named subpattern from elsewhere in 1896 the pattern, the subpatterns to which the name refers are checked in the order 1897 in which they appear in the overall pattern. The first one that is set is used 1898 for the reference. For example, this pattern matches both "foofoo" and 1899 "barbar" but not "foobar" or "barfoo": 1900 <pre> 1901 (?:(?<n>foo)|(?<n>bar))\k<n> 1902 1903 </PRE> 1904 </P> 1905 <P> 1906 If you make a subroutine call to a non-unique named subpattern, the one that 1907 corresponds to the first occurrence of the name is used. In the absence of 1908 duplicate numbers this is the one with the lowest number. 1909 </P> 1910 <P> 1911 If you use a named reference in a condition 1912 test (see the 1913 <a href="#conditions">section about conditions</a> 1914 below), either to check whether a subpattern has matched, or to check for 1915 recursion, all subpatterns with the same name are tested. If the condition is 1916 true for any one of them, the overall condition is true. This is the same 1917 behaviour as testing by number. For further details of the interfaces for 1918 handling named subpatterns, see the 1919 <a href="pcre2api.html"><b>pcre2api</b></a> 1920 documentation. 1921 </P> 1922 <br><a name="SEC17" href="#TOC1">REPETITION</a><br> 1923 <P> 1924 Repetition is specified by quantifiers, which can follow any of the following 1925 items: 1926 <pre> 1927 a literal data character 1928 the dot metacharacter 1929 the \C escape sequence 1930 the \X escape sequence 1931 the \R escape sequence 1932 an escape such as \d or \pL that matches a single character 1933 a character class 1934 a backreference 1935 a parenthesized subpattern (including most assertions) 1936 a subroutine call to a subpattern (recursive or otherwise) 1937 </pre> 1938 The general repetition quantifier specifies a minimum and maximum number of 1939 permitted matches, by giving the two numbers in curly brackets (braces), 1940 separated by a comma. The numbers must be less than 65536, and the first must 1941 be less than or equal to the second. For example: 1942 <pre> 1943 z{2,4} 1944 </pre> 1945 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special 1946 character. If the second number is omitted, but the comma is present, there is 1947 no upper limit; if the second number and the comma are both omitted, the 1948 quantifier specifies an exact number of required matches. Thus 1949 <pre> 1950 [aeiou]{3,} 1951 </pre> 1952 matches at least 3 successive vowels, but may match many more, whereas 1953 <pre> 1954 \d{8} 1955 </pre> 1956 matches exactly 8 digits. An opening curly bracket that appears in a position 1957 where a quantifier is not allowed, or one that does not match the syntax of a 1958 quantifier, is taken as a literal character. For example, {,6} is not a 1959 quantifier, but a literal string of four characters. 1960 </P> 1961 <P> 1962 In UTF modes, quantifiers apply to characters rather than to individual code 1963 units. Thus, for example, \x{100}{2} matches two characters, each of 1964 which is represented by a two-byte sequence in a UTF-8 string. Similarly, 1965 \X{3} matches three Unicode extended grapheme clusters, each of which may be 1966 several code units long (and they may be of different lengths). 1967 </P> 1968 <P> 1969 The quantifier {0} is permitted, causing the expression to behave as if the 1970 previous item and the quantifier were not present. This may be useful for 1971 subpatterns that are referenced as 1972 <a href="#subpatternsassubroutines">subroutines</a> 1973 from elsewhere in the pattern (but see also the section entitled 1974 <a href="#subdefine">"Defining subpatterns for use by reference only"</a> 1975 below). Items other than subpatterns that have a {0} quantifier are omitted 1976 from the compiled pattern. 1977 </P> 1978 <P> 1979 For convenience, the three most common quantifiers have single-character 1980 abbreviations: 1981 <pre> 1982 * is equivalent to {0,} 1983 + is equivalent to {1,} 1984 ? is equivalent to {0,1} 1985 </pre> 1986 It is possible to construct infinite loops by following a subpattern that can 1987 match no characters with a quantifier that has no upper limit, for example: 1988 <pre> 1989 (a?)* 1990 </pre> 1991 Earlier versions of Perl and PCRE1 used to give an error at compile time for 1992 such patterns. However, because there are cases where this can be useful, such 1993 patterns are now accepted, but if any repetition of the subpattern does in fact 1994 match no characters, the loop is forcibly broken. 1995 </P> 1996 <P> 1997 By default, the quantifiers are "greedy", that is, they match as much as 1998 possible (up to the maximum number of permitted times), without causing the 1999 rest of the pattern to fail. The classic example of where this gives problems 2000 is in trying to match comments in C programs. These appear between /* and */ 2001 and within the comment, individual * and / characters may appear. An attempt to 2002 match C comments by applying the pattern 2003 <pre> 2004 /\*.*\*/ 2005 </pre> 2006 to the string 2007 <pre> 2008 /* first comment */ not comment /* second comment */ 2009 </pre> 2010 fails, because it matches the entire string owing to the greediness of the .* 2011 item. 2012 </P> 2013 <P> 2014 If a quantifier is followed by a question mark, it ceases to be greedy, and 2015 instead matches the minimum number of times possible, so the pattern 2016 <pre> 2017 /\*.*?\*/ 2018 </pre> 2019 does the right thing with the C comments. The meaning of the various 2020 quantifiers is not otherwise changed, just the preferred number of matches. 2021 Do not confuse this use of question mark with its use as a quantifier in its 2022 own right. Because it has two uses, it can sometimes appear doubled, as in 2023 <pre> 2024 \d??\d 2025 </pre> 2026 which matches one digit by preference, but can match two if that is the only 2027 way the rest of the pattern matches. 2028 </P> 2029 <P> 2030 If the PCRE2_UNGREEDY option is set (an option that is not available in Perl), 2031 the quantifiers are not greedy by default, but individual ones can be made 2032 greedy by following them with a question mark. In other words, it inverts the 2033 default behaviour. 2034 </P> 2035 <P> 2036 When a parenthesized subpattern is quantified with a minimum repeat count that 2037 is greater than 1 or with a limited maximum, more memory is required for the 2038 compiled pattern, in proportion to the size of the minimum or maximum. 2039 </P> 2040 <P> 2041 If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option (equivalent 2042 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is 2043 implicitly anchored, because whatever follows will be tried against every 2044 character position in the subject string, so there is no point in retrying the 2045 overall match at any position after the first. PCRE2 normally treats such a 2046 pattern as though it were preceded by \A. 2047 </P> 2048 <P> 2049 In cases where it is known that the subject string contains no newlines, it is 2050 worth setting PCRE2_DOTALL in order to obtain this optimization, or 2051 alternatively, using ^ to indicate anchoring explicitly. 2052 </P> 2053 <P> 2054 However, there are some cases where the optimization cannot be used. When .* 2055 is inside capturing parentheses that are the subject of a backreference 2056 elsewhere in the pattern, a match at the start may fail where a later one 2057 succeeds. Consider, for example: 2058 <pre> 2059 (.*)abc\1 2060 </pre> 2061 If the subject is "xyz123abc123" the match point is the fourth character. For 2062 this reason, such a pattern is not implicitly anchored. 2063 </P> 2064 <P> 2065 Another case where implicit anchoring is not applied is when the leading .* is 2066 inside an atomic group. Once again, a match at the start may fail where a later 2067 one succeeds. Consider this pattern: 2068 <pre> 2069 (?>.*?a)b 2070 </pre> 2071 It matches "ab" in the subject "aab". The use of the backtracking control verbs 2072 (*PRUNE) and (*SKIP) also disable this optimization, and there is an option, 2073 PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly. 2074 </P> 2075 <P> 2076 When a capturing subpattern is repeated, the value captured is the substring 2077 that matched the final iteration. For example, after 2078 <pre> 2079 (tweedle[dume]{3}\s*)+ 2080 </pre> 2081 has matched "tweedledum tweedledee" the value of the captured substring is 2082 "tweedledee". However, if there are nested capturing subpatterns, the 2083 corresponding captured values may have been set in previous iterations. For 2084 example, after 2085 <pre> 2086 (a|(b))+ 2087 </pre> 2088 matches "aba" the value of the second captured substring is "b". 2089 <a name="atomicgroup"></a></P> 2090 <br><a name="SEC18" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br> 2091 <P> 2092 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 2093 repetition, failure of what follows normally causes the repeated item to be 2094 re-evaluated to see if a different number of repeats allows the rest of the 2095 pattern to match. Sometimes it is useful to prevent this, either to change the 2096 nature of the match, or to cause it fail earlier than it otherwise might, when 2097 the author of the pattern knows there is no point in carrying on. 2098 </P> 2099 <P> 2100 Consider, for example, the pattern \d+foo when applied to the subject line 2101 <pre> 2102 123456bar 2103 </pre> 2104 After matching all 6 digits and then failing to match "foo", the normal 2105 action of the matcher is to try again with only 5 digits matching the \d+ 2106 item, and then with 4, and so on, before ultimately failing. "Atomic grouping" 2107 (a term taken from Jeffrey Friedl's book) provides the means for specifying 2108 that once a subpattern has matched, it is not to be re-evaluated in this way. 2109 </P> 2110 <P> 2111 If we use atomic grouping for the previous example, the matcher gives up 2112 immediately on failing to match "foo" the first time. The notation is a kind of 2113 special parenthesis, starting with (?> as in this example: 2114 <pre> 2115 (?>\d+)foo 2116 </pre> 2117 This kind of parenthesis "locks up" the part of the pattern it contains once 2118 it has matched, and a failure further into the pattern is prevented from 2119 backtracking into it. Backtracking past it to previous items, however, works as 2120 normal. 2121 </P> 2122 <P> 2123 An alternative description is that a subpattern of this type matches exactly 2124 the string of characters that an identical standalone pattern would match, if 2125 anchored at the current point in the subject string. 2126 </P> 2127 <P> 2128 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as 2129 the above example can be thought of as a maximizing repeat that must swallow 2130 everything it can. So, while both \d+ and \d+? are prepared to adjust the 2131 number of digits they match in order to make the rest of the pattern match, 2132 (?>\d+) can only match an entire sequence of digits. 2133 </P> 2134 <P> 2135 Atomic groups in general can of course contain arbitrarily complicated 2136 subpatterns, and can be nested. However, when the subpattern for an atomic 2137 group is just a single repeated item, as in the example above, a simpler 2138 notation, called a "possessive quantifier" can be used. This consists of an 2139 additional + character following a quantifier. Using this notation, the 2140 previous example can be rewritten as 2141 <pre> 2142 \d++foo 2143 </pre> 2144 Note that a possessive quantifier can be used with an entire group, for 2145 example: 2146 <pre> 2147 (abc|xyz){2,3}+ 2148 </pre> 2149 Possessive quantifiers are always greedy; the setting of the PCRE2_UNGREEDY 2150 option is ignored. They are a convenient notation for the simpler forms of 2151 atomic group. However, there is no difference in the meaning of a possessive 2152 quantifier and the equivalent atomic group, though there may be a performance 2153 difference; possessive quantifiers should be slightly faster. 2154 </P> 2155 <P> 2156 The possessive quantifier syntax is an extension to the Perl 5.8 syntax. 2157 Jeffrey Friedl originated the idea (and the name) in the first edition of his 2158 book. Mike McCloskey liked it, so implemented it when he built Sun's Java 2159 package, and PCRE1 copied it from there. It ultimately found its way into Perl 2160 at release 5.10. 2161 </P> 2162 <P> 2163 PCRE2 has an optimization that automatically "possessifies" certain simple 2164 pattern constructs. For example, the sequence A+B is treated as A++B because 2165 there is no point in backtracking into a sequence of A's when B must follow. 2166 This feature can be disabled by the PCRE2_NO_AUTOPOSSESS option, or starting 2167 the pattern with (*NO_AUTO_POSSESS). 2168 </P> 2169 <P> 2170 When a pattern contains an unlimited repeat inside a subpattern that can itself 2171 be repeated an unlimited number of times, the use of an atomic group is the 2172 only way to avoid some failing matches taking a very long time indeed. The 2173 pattern 2174 <pre> 2175 (\D+|<\d+>)*[!?] 2176 </pre> 2177 matches an unlimited number of substrings that either consist of non-digits, or 2178 digits enclosed in <>, followed by either ! or ?. When it matches, it runs 2179 quickly. However, if it is applied to 2180 <pre> 2181 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 2182 </pre> 2183 it takes a long time before reporting failure. This is because the string can 2184 be divided between the internal \D+ repeat and the external * repeat in a 2185 large number of ways, and all have to be tried. (The example uses [!?] rather 2186 than a single character at the end, because both PCRE2 and Perl have an 2187 optimization that allows for fast failure when a single character is used. They 2188 remember the last single character that is required for a match, and fail early 2189 if it is not present in the string.) If the pattern is changed so that it uses 2190 an atomic group, like this: 2191 <pre> 2192 ((?>\D+)|<\d+>)*[!?] 2193 </pre> 2194 sequences of non-digits cannot be broken, and failure happens quickly. 2195 <a name="backreferences"></a></P> 2196 <br><a name="SEC19" href="#TOC1">BACKREFERENCES</a><br> 2197 <P> 2198 Outside a character class, a backslash followed by a digit greater than 0 (and 2199 possibly further digits) is a backreference to a capturing subpattern earlier 2200 (that is, to its left) in the pattern, provided there have been that many 2201 previous capturing left parentheses. 2202 </P> 2203 <P> 2204 However, if the decimal number following the backslash is less than 8, it is 2205 always taken as a backreference, and causes an error only if there are not 2206 that many capturing left parentheses in the entire pattern. In other words, the 2207 parentheses that are referenced need not be to the left of the reference for 2208 numbers less than 8. A "forward backreference" of this type can make sense 2209 when a repetition is involved and the subpattern to the right has participated 2210 in an earlier iteration. 2211 </P> 2212 <P> 2213 It is not possible to have a numerical "forward backreference" to a subpattern 2214 whose number is 8 or more using this syntax because a sequence such as \50 is 2215 interpreted as a character defined in octal. See the subsection entitled 2216 "Non-printing characters" 2217 <a href="#digitsafterbackslash">above</a> 2218 for further details of the handling of digits following a backslash. There is 2219 no such problem when named parentheses are used. A backreference to any 2220 subpattern is possible using named parentheses (see below). 2221 </P> 2222 <P> 2223 Another way of avoiding the ambiguity inherent in the use of digits following a 2224 backslash is to use the \g escape sequence. This escape must be followed by a 2225 signed or unsigned number, optionally enclosed in braces. These examples are 2226 all identical: 2227 <pre> 2228 (ring), \1 2229 (ring), \g1 2230 (ring), \g{1} 2231 </pre> 2232 An unsigned number specifies an absolute reference without the ambiguity that 2233 is present in the older syntax. It is also useful when literal digits follow 2234 the reference. A signed number is a relative reference. Consider this example: 2235 <pre> 2236 (abc(def)ghi)\g{-1} 2237 </pre> 2238 The sequence \g{-1} is a reference to the most recently started capturing 2239 subpattern before \g, that is, is it equivalent to \2 in this example. 2240 Similarly, \g{-2} would be equivalent to \1. The use of relative references 2241 can be helpful in long patterns, and also in patterns that are created by 2242 joining together fragments that contain references within themselves. 2243 </P> 2244 <P> 2245 The sequence \g{+1} is a reference to the next capturing subpattern. This kind 2246 of forward reference can be useful it patterns that repeat. Perl does not 2247 support the use of + in this way. 2248 </P> 2249 <P> 2250 A backreference matches whatever actually matched the capturing subpattern in 2251 the current subject string, rather than anything matching the subpattern 2252 itself (see 2253 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a> 2254 below for a way of doing that). So the pattern 2255 <pre> 2256 (sens|respons)e and \1ibility 2257 </pre> 2258 matches "sense and sensibility" and "response and responsibility", but not 2259 "sense and responsibility". If caseful matching is in force at the time of the 2260 backreference, the case of letters is relevant. For example, 2261 <pre> 2262 ((?i)rah)\s+\1 2263 </pre> 2264 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original 2265 capturing subpattern is matched caselessly. 2266 </P> 2267 <P> 2268 There are several different ways of writing backreferences to named 2269 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or 2270 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified 2271 backreference syntax, in which \g can be used for both numeric and named 2272 references, is also supported. We could rewrite the above example in any of 2273 the following ways: 2274 <pre> 2275 (?<p1>(?i)rah)\s+\k<p1> 2276 (?'p1'(?i)rah)\s+\k{p1} 2277 (?P<p1>(?i)rah)\s+(?P=p1) 2278 (?<p1>(?i)rah)\s+\g{p1} 2279 </pre> 2280 A subpattern that is referenced by name may appear in the pattern before or 2281 after the reference. 2282 </P> 2283 <P> 2284 There may be more than one backreference to the same subpattern. If a 2285 subpattern has not actually been used in a particular match, any backreferences 2286 to it always fail by default. For example, the pattern 2287 <pre> 2288 (a|(bc))\2 2289 </pre> 2290 always fails if it starts to match "a" rather than "bc". However, if the 2291 PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backreference to an 2292 unset value matches an empty string. 2293 </P> 2294 <P> 2295 Because there may be many capturing parentheses in a pattern, all digits 2296 following a backslash are taken as part of a potential backreference number. 2297 If the pattern continues with a digit character, some delimiter must be used to 2298 terminate the backreference. If the PCRE2_EXTENDED or PCRE2_EXTENDED_MORE 2299 option is set, this can be white space. Otherwise, the \g{ syntax or an empty 2300 comment (see 2301 <a href="#comments">"Comments"</a> 2302 below) can be used. 2303 </P> 2304 <br><b> 2305 Recursive backreferences 2306 </b><br> 2307 <P> 2308 A backreference that occurs inside the parentheses to which it refers fails 2309 when the subpattern is first used, so, for example, (a\1) never matches. 2310 However, such references can be useful inside repeated subpatterns. For 2311 example, the pattern 2312 <pre> 2313 (a|b\1)+ 2314 </pre> 2315 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of 2316 the subpattern, the backreference matches the character string corresponding 2317 to the previous iteration. In order for this to work, the pattern must be such 2318 that the first iteration does not need to match the backreference. This can be 2319 done using alternation, as in the example above, or by a quantifier with a 2320 minimum of zero. 2321 </P> 2322 <P> 2323 Backreferences of this type cause the group that they reference to be treated 2324 as an 2325 <a href="#atomicgroup">atomic group.</a> 2326 Once the whole group has been matched, a subsequent matching failure cannot 2327 cause backtracking into the middle of the group. 2328 <a name="bigassertions"></a></P> 2329 <br><a name="SEC20" href="#TOC1">ASSERTIONS</a><br> 2330 <P> 2331 An assertion is a test on the characters following or preceding the current 2332 matching point that does not consume any characters. The simple assertions 2333 coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described 2334 <a href="#smallassertions">above.</a> 2335 </P> 2336 <P> 2337 More complicated assertions are coded as subpatterns. There are two kinds: 2338 those that look ahead of the current position in the subject string, and those 2339 that look behind it, and in each case an assertion may be positive (must 2340 succeed for matching to continue) or negative (must not succeed for matching to 2341 continue). An assertion subpattern is matched in the normal way, except that, 2342 when matching continues after a successful assertion, the matching position in 2343 the subject string is as it was before the assertion was processed. 2344 </P> 2345 <P> 2346 Assertion subpatterns are not capturing subpatterns. If an assertion contains 2347 capturing subpatterns within it, these are counted for the purposes of 2348 numbering the capturing subpatterns in the whole pattern. Within each branch of 2349 an assertion, locally captured substrings may be referenced in the usual way. 2350 For example, a sequence such as (.)\g{-1} can be used to check that two 2351 adjacent characters are the same. 2352 </P> 2353 <P> 2354 When a branch within an assertion fails to match, any substrings that were 2355 captured are discarded (as happens with any pattern branch that fails to 2356 match). A negative assertion succeeds only when all its branches fail to match; 2357 this means that no captured substrings are ever retained after a successful 2358 negative assertion. When an assertion contains a matching branch, what happens 2359 depends on the type of assertion. 2360 </P> 2361 <P> 2362 For a positive assertion, internally captured substrings in the successful 2363 branch are retained, and matching continues with the next pattern item after 2364 the assertion. For a negative assertion, a matching branch means that the 2365 assertion has failed. If the assertion is being used as a condition in a 2366 <a href="#conditions">conditional subpattern</a> 2367 (see below), captured substrings are retained, because matching continues with 2368 the "no" branch of the condition. For other failing negative assertions, 2369 control passes to the previous backtracking point, thus discarding any captured 2370 strings within the assertion. 2371 </P> 2372 <P> 2373 For compatibility with Perl, most assertion subpatterns may be repeated; though 2374 it makes no sense to assert the same thing several times, the side effect of 2375 capturing parentheses may occasionally be useful. However, an assertion that 2376 forms the condition for a conditional subpattern may not be quantified. In 2377 practice, for other assertions, there only three cases: 2378 <br> 2379 <br> 2380 (1) If the quantifier is {0}, the assertion is never obeyed during matching. 2381 However, it may contain internal capturing parenthesized groups that are called 2382 from elsewhere via the 2383 <a href="#subpatternsassubroutines">subroutine mechanism.</a> 2384 <br> 2385 <br> 2386 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it 2387 were {0,1}. At run time, the rest of the pattern match is tried with and 2388 without the assertion, the order depending on the greediness of the quantifier. 2389 <br> 2390 <br> 2391 (3) If the minimum repetition is greater than zero, the quantifier is ignored. 2392 The assertion is obeyed just once when encountered during matching. 2393 </P> 2394 <br><b> 2395 Lookahead assertions 2396 </b><br> 2397 <P> 2398 Lookahead assertions start with (?= for positive assertions and (?! for 2399 negative assertions. For example, 2400 <pre> 2401 \w+(?=;) 2402 </pre> 2403 matches a word followed by a semicolon, but does not include the semicolon in 2404 the match, and 2405 <pre> 2406 foo(?!bar) 2407 </pre> 2408 matches any occurrence of "foo" that is not followed by "bar". Note that the 2409 apparently similar pattern 2410 <pre> 2411 (?!foo)bar 2412 </pre> 2413 does not find an occurrence of "bar" that is preceded by something other than 2414 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion 2415 (?!foo) is always true when the next three characters are "bar". A 2416 lookbehind assertion is needed to achieve the other effect. 2417 </P> 2418 <P> 2419 If you want to force a matching failure at some point in a pattern, the most 2420 convenient way to do it is with (?!) because an empty string always matches, so 2421 an assertion that requires there not to be an empty string must always fail. 2422 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!). 2423 <a name="lookbehind"></a></P> 2424 <br><b> 2425 Lookbehind assertions 2426 </b><br> 2427 <P> 2428 Lookbehind assertions start with (?<= for positive assertions and (?<! for 2429 negative assertions. For example, 2430 <pre> 2431 (?<!foo)bar 2432 </pre> 2433 does find an occurrence of "bar" that is not preceded by "foo". The contents of 2434 a lookbehind assertion are restricted such that all the strings it matches must 2435 have a fixed length. However, if there are several top-level alternatives, they 2436 do not all have to have the same fixed length. Thus 2437 <pre> 2438 (?<=bullock|donkey) 2439 </pre> 2440 is permitted, but 2441 <pre> 2442 (?<!dogs?|cats?) 2443 </pre> 2444 causes an error at compile time. Branches that match different length strings 2445 are permitted only at the top level of a lookbehind assertion. This is an 2446 extension compared with Perl, which requires all branches to match the same 2447 length of string. An assertion such as 2448 <pre> 2449 (?<=ab(c|de)) 2450 </pre> 2451 is not permitted, because its single top-level branch can match two different 2452 lengths, but it is acceptable to PCRE2 if rewritten to use two top-level 2453 branches: 2454 <pre> 2455 (?<=abc|abde) 2456 </pre> 2457 In some cases, the escape sequence \K 2458 <a href="#resetmatchstart">(see above)</a> 2459 can be used instead of a lookbehind assertion to get round the fixed-length 2460 restriction. 2461 </P> 2462 <P> 2463 The implementation of lookbehind assertions is, for each alternative, to 2464 temporarily move the current position back by the fixed length and then try to 2465 match. If there are insufficient characters before the current position, the 2466 assertion fails. 2467 </P> 2468 <P> 2469 In UTF-8 and UTF-16 modes, PCRE2 does not allow the \C escape (which matches a 2470 single code unit even in a UTF mode) to appear in lookbehind assertions, 2471 because it makes it impossible to calculate the length of the lookbehind. The 2472 \X and \R escapes, which can match different numbers of code units, are never 2473 permitted in lookbehinds. 2474 </P> 2475 <P> 2476 <a href="#subpatternsassubroutines">"Subroutine"</a> 2477 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long 2478 as the subpattern matches a fixed-length string. However, 2479 <a href="#recursion">recursion,</a> 2480 that is, a "subroutine" call into a group that is already active, 2481 is not supported. 2482 </P> 2483 <P> 2484 Perl does not support backreferences in lookbehinds. PCRE2 does support them, 2485 but only if certain conditions are met. The PCRE2_MATCH_UNSET_BACKREF option 2486 must not be set, there must be no use of (?| in the pattern (it creates 2487 duplicate subpattern numbers), and if the backreference is by name, the name 2488 must be unique. Of course, the referenced subpattern must itself be of fixed 2489 length. The following pattern matches words containing at least two characters 2490 that begin and end with the same character: 2491 <pre> 2492 \b(\w)\w++(?<=\1) 2493 </PRE> 2494 </P> 2495 <P> 2496 Possessive quantifiers can be used in conjunction with lookbehind assertions to 2497 specify efficient matching of fixed-length strings at the end of subject 2498 strings. Consider a simple pattern such as 2499 <pre> 2500 abcd$ 2501 </pre> 2502 when applied to a long string that does not match. Because matching proceeds 2503 from left to right, PCRE2 will look for each "a" in the subject and then see if 2504 what follows matches the rest of the pattern. If the pattern is specified as 2505 <pre> 2506 ^.*abcd$ 2507 </pre> 2508 the initial .* matches the entire string at first, but when this fails (because 2509 there is no following "a"), it backtracks to match all but the last character, 2510 then all but the last two characters, and so on. Once again the search for "a" 2511 covers the entire string, from right to left, so we are no better off. However, 2512 if the pattern is written as 2513 <pre> 2514 ^.*+(?<=abcd) 2515 </pre> 2516 there can be no backtracking for the .*+ item because of the possessive 2517 quantifier; it can match only the entire string. The subsequent lookbehind 2518 assertion does a single test on the last four characters. If it fails, the 2519 match fails immediately. For long strings, this approach makes a significant 2520 difference to the processing time. 2521 </P> 2522 <br><b> 2523 Using multiple assertions 2524 </b><br> 2525 <P> 2526 Several assertions (of any sort) may occur in succession. For example, 2527 <pre> 2528 (?<=\d{3})(?<!999)foo 2529 </pre> 2530 matches "foo" preceded by three digits that are not "999". Notice that each of 2531 the assertions is applied independently at the same point in the subject 2532 string. First there is a check that the previous three characters are all 2533 digits, and then there is a check that the same three characters are not "999". 2534 This pattern does <i>not</i> match "foo" preceded by six characters, the first 2535 of which are digits and the last three of which are not "999". For example, it 2536 doesn't match "123abcfoo". A pattern to do that is 2537 <pre> 2538 (?<=\d{3}...)(?<!999)foo 2539 </pre> 2540 This time the first assertion looks at the preceding six characters, checking 2541 that the first three are digits, and then the second assertion checks that the 2542 preceding three characters are not "999". 2543 </P> 2544 <P> 2545 Assertions can be nested in any combination. For example, 2546 <pre> 2547 (?<=(?<!foo)bar)baz 2548 </pre> 2549 matches an occurrence of "baz" that is preceded by "bar" which in turn is not 2550 preceded by "foo", while 2551 <pre> 2552 (?<=\d{3}(?!999)...)foo 2553 </pre> 2554 is another pattern that matches "foo" preceded by three digits and any three 2555 characters that are not "999". 2556 <a name="conditions"></a></P> 2557 <br><a name="SEC21" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br> 2558 <P> 2559 It is possible to cause the matching process to obey a subpattern 2560 conditionally or to choose between two alternative subpatterns, depending on 2561 the result of an assertion, or whether a specific capturing subpattern has 2562 already been matched. The two possible forms of conditional subpattern are: 2563 <pre> 2564 (?(condition)yes-pattern) 2565 (?(condition)yes-pattern|no-pattern) 2566 </pre> 2567 If the condition is satisfied, the yes-pattern is used; otherwise the 2568 no-pattern (if present) is used. An absent no-pattern is equivalent to an empty 2569 string (it always matches). If there are more than two alternatives in the 2570 subpattern, a compile-time error occurs. Each of the two alternatives may 2571 itself contain nested subpatterns of any form, including conditional 2572 subpatterns; the restriction to two alternatives applies only at the level of 2573 the condition. This pattern fragment is an example where the alternatives are 2574 complex: 2575 <pre> 2576 (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) 2577 2578 </PRE> 2579 </P> 2580 <P> 2581 There are five kinds of condition: references to subpatterns, references to 2582 recursion, two pseudo-conditions called DEFINE and VERSION, and assertions. 2583 </P> 2584 <br><b> 2585 Checking for a used subpattern by number 2586 </b><br> 2587 <P> 2588 If the text between the parentheses consists of a sequence of digits, the 2589 condition is true if a capturing subpattern of that number has previously 2590 matched. If there is more than one capturing subpattern with the same number 2591 (see the earlier 2592 <a href="#recursion">section about duplicate subpattern numbers),</a> 2593 the condition is true if any of them have matched. An alternative notation is 2594 to precede the digits with a plus or minus sign. In this case, the subpattern 2595 number is relative rather than absolute. The most recently opened parentheses 2596 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside 2597 loops it can also make sense to refer to subsequent groups. The next 2598 parentheses to be opened can be referenced as (?(+1), and so on. (The value 2599 zero in any of these forms is not used; it provokes a compile-time error.) 2600 </P> 2601 <P> 2602 Consider the following pattern, which contains non-significant white space to 2603 make it more readable (assume the PCRE2_EXTENDED option) and to divide it into 2604 three parts for ease of discussion: 2605 <pre> 2606 ( \( )? [^()]+ (?(1) \) ) 2607 </pre> 2608 The first part matches an optional opening parenthesis, and if that 2609 character is present, sets it as the first captured substring. The second part 2610 matches one or more characters that are not parentheses. The third part is a 2611 conditional subpattern that tests whether or not the first set of parentheses 2612 matched. If they did, that is, if subject started with an opening parenthesis, 2613 the condition is true, and so the yes-pattern is executed and a closing 2614 parenthesis is required. Otherwise, since no-pattern is not present, the 2615 subpattern matches nothing. In other words, this pattern matches a sequence of 2616 non-parentheses, optionally enclosed in parentheses. 2617 </P> 2618 <P> 2619 If you were embedding this pattern in a larger one, you could use a relative 2620 reference: 2621 <pre> 2622 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... 2623 </pre> 2624 This makes the fragment independent of the parentheses in the larger pattern. 2625 </P> 2626 <br><b> 2627 Checking for a used subpattern by name 2628 </b><br> 2629 <P> 2630 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used 2631 subpattern by name. For compatibility with earlier versions of PCRE1, which had 2632 this facility before Perl, the syntax (?(name)...) is also recognized. Note, 2633 however, that undelimited names consisting of the letter R followed by digits 2634 are ambiguous (see the following section). 2635 </P> 2636 <P> 2637 Rewriting the above example to use a named subpattern gives this: 2638 <pre> 2639 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) 2640 </pre> 2641 If the name used in a condition of this kind is a duplicate, the test is 2642 applied to all subpatterns of the same name, and is true if any one of them has 2643 matched. 2644 </P> 2645 <br><b> 2646 Checking for pattern recursion 2647 </b><br> 2648 <P> 2649 "Recursion" in this sense refers to any subroutine-like call from one part of 2650 the pattern to another, whether or not it is actually recursive. See the 2651 sections entitled 2652 <a href="#recursion">"Recursive patterns"</a> 2653 and 2654 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a> 2655 below for details of recursion and subpattern calls. 2656 </P> 2657 <P> 2658 If a condition is the string (R), and there is no subpattern with the name R, 2659 the condition is true if matching is currently in a recursion or subroutine 2660 call to the whole pattern or any subpattern. If digits follow the letter R, and 2661 there is no subpattern with that name, the condition is true if the most recent 2662 call is into a subpattern with the given number, which must exist somewhere in 2663 the overall pattern. This is a contrived example that is equivalent to a+b: 2664 <pre> 2665 ((?(R1)a+|(?1)b)) 2666 </pre> 2667 However, in both cases, if there is a subpattern with a matching name, the 2668 condition tests for its being set, as described in the section above, instead 2669 of testing for recursion. For example, creating a group with the name R1 by 2670 adding (?<R1>) to the above pattern completely changes its meaning. 2671 </P> 2672 <P> 2673 If a name preceded by ampersand follows the letter R, for example: 2674 <pre> 2675 (?(R&name)...) 2676 </pre> 2677 the condition is true if the most recent recursion is into a subpattern of that 2678 name (which must exist within the pattern). 2679 </P> 2680 <P> 2681 This condition does not check the entire recursion stack. It tests only the 2682 current level. If the name used in a condition of this kind is a duplicate, the 2683 test is applied to all subpatterns of the same name, and is true if any one of 2684 them is the most recent recursion. 2685 </P> 2686 <P> 2687 At "top level", all these recursion test conditions are false. 2688 <a name="subdefine"></a></P> 2689 <br><b> 2690 Defining subpatterns for use by reference only 2691 </b><br> 2692 <P> 2693 If the condition is the string (DEFINE), the condition is always false, even if 2694 there is a group with the name DEFINE. In this case, there may be only one 2695 alternative in the subpattern. It is always skipped if control reaches this 2696 point in the pattern; the idea of DEFINE is that it can be used to define 2697 subroutines that can be referenced from elsewhere. (The use of 2698 <a href="#subpatternsassubroutines">subroutines</a> 2699 is described below.) For example, a pattern to match an IPv4 address such as 2700 "192.168.23.245" could be written like this (ignore white space and line 2701 breaks): 2702 <pre> 2703 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) 2704 \b (?&byte) (\.(?&byte)){3} \b 2705 </pre> 2706 The first part of the pattern is a DEFINE group inside which a another group 2707 named "byte" is defined. This matches an individual component of an IPv4 2708 address (a number less than 256). When matching takes place, this part of the 2709 pattern is skipped because DEFINE acts like a false condition. The rest of the 2710 pattern uses references to the named group to match the four dot-separated 2711 components of an IPv4 address, insisting on a word boundary at each end. 2712 </P> 2713 <br><b> 2714 Checking the PCRE2 version 2715 </b><br> 2716 <P> 2717 Programs that link with a PCRE2 library can check the version by calling 2718 <b>pcre2_config()</b> with appropriate arguments. Users of applications that do 2719 not have access to the underlying code cannot do this. A special "condition" 2720 called VERSION exists to allow such users to discover which version of PCRE2 2721 they are dealing with by using this condition to match a string such as 2722 "yesno". VERSION must be followed either by "=" or ">=" and a version number. 2723 For example: 2724 <pre> 2725 (?(VERSION>=10.4)yes|no) 2726 </pre> 2727 This pattern matches "yes" if the PCRE2 version is greater or equal to 10.4, or 2728 "no" otherwise. The fractional part of the version number may not contain more 2729 than two digits. 2730 </P> 2731 <br><b> 2732 Assertion conditions 2733 </b><br> 2734 <P> 2735 If the condition is not in any of the above formats, it must be an assertion. 2736 This may be a positive or negative lookahead or lookbehind assertion. Consider 2737 this pattern, again containing non-significant white space, and with the two 2738 alternatives on the second line: 2739 <pre> 2740 (?(?=[^a-z]*[a-z]) 2741 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) 2742 </pre> 2743 The condition is a positive lookahead assertion that matches an optional 2744 sequence of non-letters followed by a letter. In other words, it tests for the 2745 presence of at least one letter in the subject. If a letter is found, the 2746 subject is matched against the first alternative; otherwise it is matched 2747 against the second. This pattern matches strings in one of the two forms 2748 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. 2749 </P> 2750 <P> 2751 When an assertion that is a condition contains capturing subpatterns, any 2752 capturing that occurs in a matching branch is retained afterwards, for both 2753 positive and negative assertions, because matching always continues after the 2754 assertion, whether it succeeds or fails. (Compare non-conditional assertions, 2755 when captures are retained only for positive assertions that succeed.) 2756 <a name="comments"></a></P> 2757 <br><a name="SEC22" href="#TOC1">COMMENTS</a><br> 2758 <P> 2759 There are two ways of including comments in patterns that are processed by 2760 PCRE2. In both cases, the start of the comment must not be in a character 2761 class, nor in the middle of any other sequence of related characters such as 2762 (?: or a subpattern name or number. The characters that make up a comment play 2763 no part in the pattern matching. 2764 </P> 2765 <P> 2766 The sequence (?# marks the start of a comment that continues up to the next 2767 closing parenthesis. Nested parentheses are not permitted. If the 2768 PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, an unescaped # character 2769 also introduces a comment, which in this case continues to immediately after 2770 the next newline character or character sequence in the pattern. Which 2771 characters are interpreted as newlines is controlled by an option passed to the 2772 compiling function or by a special sequence at the start of the pattern, as 2773 described in the section entitled 2774 <a href="#newlines">"Newline conventions"</a> 2775 above. Note that the end of this type of comment is a literal newline sequence 2776 in the pattern; escape sequences that happen to represent a newline do not 2777 count. For example, consider this pattern when PCRE2_EXTENDED is set, and the 2778 default newline convention (a single linefeed character) is in force: 2779 <pre> 2780 abc #comment \n still comment 2781 </pre> 2782 On encountering the # character, <b>pcre2_compile()</b> skips along, looking for 2783 a newline in the pattern. The sequence \n is still literal at this stage, so 2784 it does not terminate the comment. Only an actual character with the code value 2785 0x0a (the default newline) does so. 2786 <a name="recursion"></a></P> 2787 <br><a name="SEC23" href="#TOC1">RECURSIVE PATTERNS</a><br> 2788 <P> 2789 Consider the problem of matching a string in parentheses, allowing for 2790 unlimited nested parentheses. Without the use of recursion, the best that can 2791 be done is to use a pattern that matches up to some fixed depth of nesting. It 2792 is not possible to handle an arbitrary nesting depth. 2793 </P> 2794 <P> 2795 For some time, Perl has provided a facility that allows regular expressions to 2796 recurse (amongst other things). It does this by interpolating Perl code in the 2797 expression at run time, and the code can refer to the expression itself. A Perl 2798 pattern using code interpolation to solve the parentheses problem can be 2799 created like this: 2800 <pre> 2801 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; 2802 </pre> 2803 The (?p{...}) item interpolates Perl code at run time, and in this case refers 2804 recursively to the pattern in which it appears. 2805 </P> 2806 <P> 2807 Obviously, PCRE2 cannot support the interpolation of Perl code. Instead, it 2808 supports special syntax for recursion of the entire pattern, and also for 2809 individual subpattern recursion. After its introduction in PCRE1 and Python, 2810 this kind of recursion was subsequently introduced into Perl at release 5.10. 2811 </P> 2812 <P> 2813 A special item that consists of (? followed by a number greater than zero and a 2814 closing parenthesis is a recursive subroutine call of the subpattern of the 2815 given number, provided that it occurs inside that subpattern. (If not, it is a 2816 <a href="#subpatternsassubroutines">non-recursive subroutine</a> 2817 call, which is described in the next section.) The special item (?R) or (?0) is 2818 a recursive call of the entire regular expression. 2819 </P> 2820 <P> 2821 This PCRE2 pattern solves the nested parentheses problem (assume the 2822 PCRE2_EXTENDED option is set so that white space is ignored): 2823 <pre> 2824 \( ( [^()]++ | (?R) )* \) 2825 </pre> 2826 First it matches an opening parenthesis. Then it matches any number of 2827 substrings which can either be a sequence of non-parentheses, or a recursive 2828 match of the pattern itself (that is, a correctly parenthesized substring). 2829 Finally there is a closing parenthesis. Note the use of a possessive quantifier 2830 to avoid backtracking into sequences of non-parentheses. 2831 </P> 2832 <P> 2833 If this were part of a larger pattern, you would not want to recurse the entire 2834 pattern, so instead you could use this: 2835 <pre> 2836 ( \( ( [^()]++ | (?1) )* \) ) 2837 </pre> 2838 We have put the pattern into parentheses, and caused the recursion to refer to 2839 them instead of the whole pattern. 2840 </P> 2841 <P> 2842 In a larger pattern, keeping track of parenthesis numbers can be tricky. This 2843 is made easier by the use of relative references. Instead of (?1) in the 2844 pattern above you can write (?-2) to refer to the second most recently opened 2845 parentheses preceding the recursion. In other words, a negative number counts 2846 capturing parentheses leftwards from the point at which it is encountered. 2847 </P> 2848 <P> 2849 Be aware however, that if 2850 <a href="#dupsubpatternnumber">duplicate subpattern numbers</a> 2851 are in use, relative references refer to the earliest subpattern with the 2852 appropriate number. Consider, for example: 2853 <pre> 2854 (?|(a)|(b)) (c) (?-2) 2855 </pre> 2856 The first two capturing groups (a) and (b) are both numbered 1, and group (c) 2857 is number 2. When the reference (?-2) is encountered, the second most recently 2858 opened parentheses has the number 1, but it is the first such group (the (a) 2859 group) to which the recursion refers. This would be the same if an absolute 2860 reference (?1) was used. In other words, relative references are just a 2861 shorthand for computing a group number. 2862 </P> 2863 <P> 2864 It is also possible to refer to subsequently opened parentheses, by writing 2865 references such as (?+2). However, these cannot be recursive because the 2866 reference is not inside the parentheses that are referenced. They are always 2867 <a href="#subpatternsassubroutines">non-recursive subroutine</a> 2868 calls, as described in the next section. 2869 </P> 2870 <P> 2871 An alternative approach is to use named parentheses. The Perl syntax for this 2872 is (?&name); PCRE1's earlier syntax (?P>name) is also supported. We could 2873 rewrite the above example as follows: 2874 <pre> 2875 (?<pn> \( ( [^()]++ | (?&pn) )* \) ) 2876 </pre> 2877 If there is more than one subpattern with the same name, the earliest one is 2878 used. 2879 </P> 2880 <P> 2881 The example pattern that we have been looking at contains nested unlimited 2882 repeats, and so the use of a possessive quantifier for matching strings of 2883 non-parentheses is important when applying the pattern to strings that do not 2884 match. For example, when this pattern is applied to 2885 <pre> 2886 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 2887 </pre> 2888 it yields "no match" quickly. However, if a possessive quantifier is not used, 2889 the match runs for a very long time indeed because there are so many different 2890 ways the + and * repeats can carve up the subject, and all have to be tested 2891 before failure can be reported. 2892 </P> 2893 <P> 2894 At the end of a match, the values of capturing parentheses are those from 2895 the outermost level. If you want to obtain intermediate values, a callout 2896 function can be used (see below and the 2897 <a href="pcre2callout.html"><b>pcre2callout</b></a> 2898 documentation). If the pattern above is matched against 2899 <pre> 2900 (ab(cd)ef) 2901 </pre> 2902 the value for the inner capturing parentheses (numbered 2) is "ef", which is 2903 the last value taken on at the top level. If a capturing subpattern is not 2904 matched at the top level, its final captured value is unset, even if it was 2905 (temporarily) set at a deeper level during the matching process. 2906 </P> 2907 <P> 2908 Do not confuse the (?R) item with the condition (R), which tests for recursion. 2909 Consider this pattern, which matches text in angle brackets, allowing for 2910 arbitrary nesting. Only digits are allowed in nested brackets (that is, when 2911 recursing), whereas any characters are permitted at the outer level. 2912 <pre> 2913 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > 2914 </pre> 2915 In this pattern, (?(R) is the start of a conditional subpattern, with two 2916 different alternatives for the recursive and non-recursive cases. The (?R) item 2917 is the actual recursive call. 2918 <a name="recursiondifference"></a></P> 2919 <br><b> 2920 Differences in recursion processing between PCRE2 and Perl 2921 </b><br> 2922 <P> 2923 Some former differences between PCRE2 and Perl no longer exist. 2924 </P> 2925 <P> 2926 Before release 10.30, recursion processing in PCRE2 differed from Perl in that 2927 a recursive subpattern call was always treated as an atomic group. That is, 2928 once it had matched some of the subject string, it was never re-entered, even 2929 if it contained untried alternatives and there was a subsequent matching 2930 failure. (Historical note: PCRE implemented recursion before Perl did.) 2931 </P> 2932 <P> 2933 Starting with release 10.30, recursive subroutine calls are no longer treated 2934 as atomic. That is, they can be re-entered to try unused alternatives if there 2935 is a matching failure later in the pattern. This is now compatible with the way 2936 Perl works. If you want a subroutine call to be atomic, you must explicitly 2937 enclose it in an atomic group. 2938 </P> 2939 <P> 2940 Supporting backtracking into recursions simplifies certain types of recursive 2941 pattern. For example, this pattern matches palindromic strings: 2942 <pre> 2943 ^((.)(?1)\2|.?)$ 2944 </pre> 2945 The second branch in the group matches a single central character in the 2946 palindrome when there are an odd number of characters, or nothing when there 2947 are an even number of characters, but in order to work it has to be able to try 2948 the second case when the rest of the pattern match fails. If you want to match 2949 typical palindromic phrases, the pattern has to ignore all non-word characters, 2950 which can be done like this: 2951 <pre> 2952 ^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$ 2953 </pre> 2954 If run with the PCRE2_CASELESS option, this pattern matches phrases such as "A 2955 man, a plan, a canal: Panama!". Note the use of the possessive quantifier *+ to 2956 avoid backtracking into sequences of non-word characters. Without this, PCRE2 2957 takes a great deal longer (ten times or more) to match typical phrases, and 2958 Perl takes so long that you think it has gone into a loop. 2959 </P> 2960 <P> 2961 Another way in which PCRE2 and Perl used to differ in their recursion 2962 processing is in the handling of captured values. Formerly in Perl, when a 2963 subpattern was called recursively or as a subpattern (see the next section), it 2964 had no access to any values that were captured outside the recursion, whereas 2965 in PCRE2 these values can be referenced. Consider this pattern: 2966 <pre> 2967 ^(.)(\1|a(?2)) 2968 </pre> 2969 This pattern matches "bab". The first capturing parentheses match "b", then in 2970 the second group, when the backreference \1 fails to match "b", the second 2971 alternative matches "a" and then recurses. In the recursion, \1 does now match 2972 "b" and so the whole match succeeds. This match used to fail in Perl, but in 2973 later versions (I tried 5.024) it now works. 2974 <a name="subpatternsassubroutines"></a></P> 2975 <br><a name="SEC24" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br> 2976 <P> 2977 If the syntax for a recursive subpattern call (either by number or by 2978 name) is used outside the parentheses to which it refers, it operates a bit 2979 like a subroutine in a programming language. More accurately, PCRE2 treats the 2980 referenced subpattern as an independent subpattern which it tries to match at 2981 the current matching position. The called subpattern may be defined before or 2982 after the reference. A numbered reference can be absolute or relative, as in 2983 these examples: 2984 <pre> 2985 (...(absolute)...)...(?2)... 2986 (...(relative)...)...(?-1)... 2987 (...(?+1)...(relative)... 2988 </pre> 2989 An earlier example pointed out that the pattern 2990 <pre> 2991 (sens|respons)e and \1ibility 2992 </pre> 2993 matches "sense and sensibility" and "response and responsibility", but not 2994 "sense and responsibility". If instead the pattern 2995 <pre> 2996 (sens|respons)e and (?1)ibility 2997 </pre> 2998 is used, it does match "sense and responsibility" as well as the other two 2999 strings. Another example is given in the discussion of DEFINE above. 3000 </P> 3001 <P> 3002 Like recursions, subroutine calls used to be treated as atomic, but this 3003 changed at PCRE2 release 10.30, so backtracking into subroutine calls can now 3004 occur. However, any capturing parentheses that are set during the subroutine 3005 call revert to their previous values afterwards. 3006 </P> 3007 <P> 3008 Processing options such as case-independence are fixed when a subpattern is 3009 defined, so if it is used as a subroutine, such options cannot be changed for 3010 different calls. For example, consider this pattern: 3011 <pre> 3012 (abc)(?i:(?-1)) 3013 </pre> 3014 It matches "abcabc". It does not match "abcABC" because the change of 3015 processing option does not affect the called subpattern. 3016 </P> 3017 <P> 3018 The behaviour of 3019 <a href="#backtrackcontrol">backtracking control verbs</a> 3020 in subpatterns when called as subroutines is described in the section entitled 3021 <a href="#btsub">"Backtracking verbs in subroutines"</a> 3022 below. 3023 <a name="onigurumasubroutines"></a></P> 3024 <br><a name="SEC25" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br> 3025 <P> 3026 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or 3027 a number enclosed either in angle brackets or single quotes, is an alternative 3028 syntax for referencing a subpattern as a subroutine, possibly recursively. Here 3029 are two of the examples used above, rewritten using this syntax: 3030 <pre> 3031 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) 3032 (sens|respons)e and \g'1'ibility 3033 </pre> 3034 PCRE2 supports an extension to Oniguruma: if a number is preceded by a 3035 plus or a minus sign it is taken as a relative reference. For example: 3036 <pre> 3037 (abc)(?i:\g<-1>) 3038 </pre> 3039 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i> 3040 synonymous. The former is a backreference; the latter is a subroutine call. 3041 </P> 3042 <br><a name="SEC26" href="#TOC1">CALLOUTS</a><br> 3043 <P> 3044 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl 3045 code to be obeyed in the middle of matching a regular expression. This makes it 3046 possible, amongst other things, to extract different substrings that match the 3047 same pair of parentheses when there is a repetition. 3048 </P> 3049 <P> 3050 PCRE2 provides a similar feature, but of course it cannot obey arbitrary Perl 3051 code. The feature is called "callout". The caller of PCRE2 provides an external 3052 function by putting its entry point in a match context using the function 3053 <b>pcre2_set_callout()</b>, and then passing that context to <b>pcre2_match()</b> 3054 or <b>pcre2_dfa_match()</b>. If no match context is passed, or if the callout 3055 entry point is set to NULL, callouts are disabled. 3056 </P> 3057 <P> 3058 Within a regular expression, (?C<arg>) indicates a point at which the external 3059 function is to be called. There are two kinds of callout: those with a 3060 numerical argument and those with a string argument. (?C) on its own with no 3061 argument is treated as (?C0). A numerical argument allows the application to 3062 distinguish between different callouts. String arguments were added for release 3063 10.20 to make it possible for script languages that use PCRE2 to embed short 3064 scripts within patterns in a similar way to Perl. 3065 </P> 3066 <P> 3067 During matching, when PCRE2 reaches a callout point, the external function is 3068 called. It is provided with the number or string argument of the callout, the 3069 position in the pattern, and one item of data that is also set in the match 3070 block. The callout function may cause matching to proceed, to backtrack, or to 3071 fail. 3072 </P> 3073 <P> 3074 By default, PCRE2 implements a number of optimizations at matching time, and 3075 one side-effect is that sometimes callouts are skipped. If you need all 3076 possible callouts to happen, you need to set options that disable the relevant 3077 optimizations. More details, including a complete description of the 3078 programming interface to the callout function, are given in the 3079 <a href="pcre2callout.html"><b>pcre2callout</b></a> 3080 documentation. 3081 </P> 3082 <br><b> 3083 Callouts with numerical arguments 3084 </b><br> 3085 <P> 3086 If you just want to have a means of identifying different callout points, put a 3087 number less than 256 after the letter C. For example, this pattern has two 3088 callout points: 3089 <pre> 3090 (?C1)abc(?C2)def 3091 </pre> 3092 If the PCRE2_AUTO_CALLOUT flag is passed to <b>pcre2_compile()</b>, numerical 3093 callouts are automatically installed before each item in the pattern. They are 3094 all numbered 255. If there is a conditional group in the pattern whose 3095 condition is an assertion, an additional callout is inserted just before the 3096 condition. An explicit callout may also be set at this position, as in this 3097 example: 3098 <pre> 3099 (?(?C9)(?=a)abc|def) 3100 </pre> 3101 Note that this applies only to assertion conditions, not to other types of 3102 condition. 3103 </P> 3104 <br><b> 3105 Callouts with string arguments 3106 </b><br> 3107 <P> 3108 A delimited string may be used instead of a number as a callout argument. The 3109 starting delimiter must be one of ` ' " ^ % # $ { and the ending delimiter is 3110 the same as the start, except for {, where the ending delimiter is }. If the 3111 ending delimiter is needed within the string, it must be doubled. For 3112 example: 3113 <pre> 3114 (?C'ab ''c'' d')xyz(?C{any text})pqr 3115 </pre> 3116 The doubling is removed before the string is passed to the callout function. 3117 <a name="backtrackcontrol"></a></P> 3118 <br><a name="SEC27" href="#TOC1">BACKTRACKING CONTROL</a><br> 3119 <P> 3120 There are a number of special "Backtracking Control Verbs" (to use Perl's 3121 terminology) that modify the behaviour of backtracking during matching. They 3122 are generally of the form (*VERB) or (*VERB:NAME). Some verbs take either form, 3123 possibly behaving differently depending on whether or not a name is present. 3124 </P> 3125 <P> 3126 By default, for compatibility with Perl, a name is any sequence of characters 3127 that does not include a closing parenthesis. The name is not processed in 3128 any way, and it is not possible to include a closing parenthesis in the name. 3129 This can be changed by setting the PCRE2_ALT_VERBNAMES option, but the result 3130 is no longer Perl-compatible. 3131 </P> 3132 <P> 3133 When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to verb names 3134 and only an unescaped closing parenthesis terminates the name. However, the 3135 only backslash items that are permitted are \Q, \E, and sequences such as 3136 \x{100} that define character code points. Character type escapes such as \d 3137 are faulted. 3138 </P> 3139 <P> 3140 A closing parenthesis can be included in a name either as \) or between \Q 3141 and \E. In addition to backslash processing, if the PCRE2_EXTENDED or 3142 PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb names is 3143 skipped, and #-comments are recognized, exactly as in the rest of the pattern. 3144 PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect verb names unless 3145 PCRE2_ALT_VERBNAMES is also set. 3146 </P> 3147 <P> 3148 The maximum length of a name is 255 in the 8-bit library and 65535 in the 3149 16-bit and 32-bit libraries. If the name is empty, that is, if the closing 3150 parenthesis immediately follows the colon, the effect is as if the colon were 3151 not there. Any number of these verbs may occur in a pattern. 3152 </P> 3153 <P> 3154 Since these verbs are specifically related to backtracking, most of them can be 3155 used only when the pattern is to be matched using the traditional matching 3156 function, because that uses a backtracking algorithm. With the exception of 3157 (*FAIL), which behaves like a failing negative assertion, the backtracking 3158 control verbs cause an error if encountered by the DFA matching function. 3159 </P> 3160 <P> 3161 The behaviour of these verbs in 3162 <a href="#btrepeat">repeated groups,</a> 3163 <a href="#btassert">assertions,</a> 3164 and in 3165 <a href="#btsub">subpatterns called as subroutines</a> 3166 (whether or not recursively) is documented below. 3167 <a name="nooptimize"></a></P> 3168 <br><b> 3169 Optimizations that affect backtracking verbs 3170 </b><br> 3171 <P> 3172 PCRE2 contains some optimizations that are used to speed up matching by running 3173 some checks at the start of each match attempt. For example, it may know the 3174 minimum length of matching subject, or that a particular character must be 3175 present. When one of these optimizations bypasses the running of a match, any 3176 included backtracking verbs will not, of course, be processed. You can suppress 3177 the start-of-match optimizations by setting the PCRE2_NO_START_OPTIMIZE option 3178 when calling <b>pcre2_compile()</b>, or by starting the pattern with 3179 (*NO_START_OPT). There is more discussion of this option in the section 3180 entitled 3181 <a href="pcre2api.html#compiling">"Compiling a pattern"</a> 3182 in the 3183 <a href="pcre2api.html"><b>pcre2api</b></a> 3184 documentation. 3185 </P> 3186 <P> 3187 Experiments with Perl suggest that it too has similar optimizations, and like 3188 PCRE2, turning them off can change the result of a match. 3189 </P> 3190 <br><b> 3191 Verbs that act immediately 3192 </b><br> 3193 <P> 3194 The following verbs act as soon as they are encountered. 3195 <pre> 3196 (*ACCEPT) or (*ACCEPT:NAME) 3197 </pre> 3198 This verb causes the match to end successfully, skipping the remainder of the 3199 pattern. However, when it is inside a subpattern that is called as a 3200 subroutine, only that subpattern is ended successfully. Matching then continues 3201 at the outer level. If (*ACCEPT) in triggered in a positive assertion, the 3202 assertion succeeds; in a negative assertion, the assertion fails. 3203 </P> 3204 <P> 3205 If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For 3206 example: 3207 <pre> 3208 A((?:A|B(*ACCEPT)|C)D) 3209 </pre> 3210 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by 3211 the outer parentheses. 3212 <pre> 3213 (*FAIL) or (*FAIL:NAME) 3214 </pre> 3215 This verb causes a matching failure, forcing backtracking to occur. It may be 3216 abbreviated to (*F). It is equivalent to (?!) but easier to read. The Perl 3217 documentation notes that it is probably useful only when combined with (?{}) or 3218 (??{}). Those are, of course, Perl features that are not present in PCRE2. The 3219 nearest equivalent is the callout feature, as for example in this pattern: 3220 <pre> 3221 a+(?C)(*FAIL) 3222 </pre> 3223 A match with the string "aaaa" always fails, but the callout is taken before 3224 each backtrack happens (in this example, 10 times). 3225 </P> 3226 <P> 3227 (*ACCEPT:NAME) and (*FAIL:NAME) behave exactly the same as 3228 (*MARK:NAME)(*ACCEPT) and (*MARK:NAME)(*FAIL), respectively. 3229 </P> 3230 <br><b> 3231 Recording which path was taken 3232 </b><br> 3233 <P> 3234 There is one verb whose main purpose is to track how a match was arrived at, 3235 though it also has a secondary use in conjunction with advancing the match 3236 starting point (see (*SKIP) below). 3237 <pre> 3238 (*MARK:NAME) or (*:NAME) 3239 </pre> 3240 A name is always required with this verb. There may be as many instances of 3241 (*MARK) as you like in a pattern, and their names do not have to be unique. 3242 </P> 3243 <P> 3244 When a match succeeds, the name of the last-encountered (*MARK:NAME) on the 3245 matching path is passed back to the caller as described in the section entitled 3246 <a href="pcre2api.html#matchotherdata">"Other information about the match"</a> 3247 in the 3248 <a href="pcre2api.html"><b>pcre2api</b></a> 3249 documentation. This applies to all instances of (*MARK), including those inside 3250 assertions and atomic groups. (There are differences in those cases when 3251 (*MARK) is used in conjunction with (*SKIP) as described below.) 3252 </P> 3253 <P> 3254 As well as (*MARK), the (*COMMIT), (*PRUNE) and (*THEN) verbs may have 3255 associated NAME arguments. Whichever is last on the matching path is passed 3256 back. See below for more details of these other verbs. 3257 </P> 3258 <P> 3259 Here is an example of <b>pcre2test</b> output, where the "mark" modifier 3260 requests the retrieval and outputting of (*MARK) data: 3261 <pre> 3262 re> /X(*MARK:A)Y|X(*MARK:B)Z/mark 3263 data> XY 3264 0: XY 3265 MK: A 3266 XZ 3267 0: XZ 3268 MK: B 3269 </pre> 3270 The (*MARK) name is tagged with "MK:" in this output, and in this example it 3271 indicates which of the two alternatives matched. This is a more efficient way 3272 of obtaining this information than putting each alternative in its own 3273 capturing parentheses. 3274 </P> 3275 <P> 3276 If a verb with a name is encountered in a positive assertion that is true, the 3277 name is recorded and passed back if it is the last-encountered. This does not 3278 happen for negative assertions or failing positive assertions. 3279 </P> 3280 <P> 3281 After a partial match or a failed match, the last encountered name in the 3282 entire match process is returned. For example: 3283 <pre> 3284 re> /X(*MARK:A)Y|X(*MARK:B)Z/mark 3285 data> XP 3286 No match, mark = B 3287 </pre> 3288 Note that in this unanchored example the mark is retained from the match 3289 attempt that started at the letter "X" in the subject. Subsequent match 3290 attempts starting at "P" and then with an empty string do not get as far as the 3291 (*MARK) item, but nevertheless do not reset it. 3292 </P> 3293 <P> 3294 If you are interested in (*MARK) values after failed matches, you should 3295 probably set the PCRE2_NO_START_OPTIMIZE option 3296 <a href="#nooptimize">(see above)</a> 3297 to ensure that the match is always attempted. 3298 </P> 3299 <br><b> 3300 Verbs that act after backtracking 3301 </b><br> 3302 <P> 3303 The following verbs do nothing when they are encountered. Matching continues 3304 with what follows, but if there is a subsequent match failure, causing a 3305 backtrack to the verb, a failure is forced. That is, backtracking cannot pass 3306 to the left of the verb. However, when one of these verbs appears inside an 3307 atomic group or in a lookaround assertion that is true, its effect is confined 3308 to that group, because once the group has been matched, there is never any 3309 backtracking into it. Backtracking from beyond an assertion or an atomic group 3310 ignores the entire group, and seeks a preceeding backtracking point. 3311 </P> 3312 <P> 3313 These verbs differ in exactly what kind of failure occurs when backtracking 3314 reaches them. The behaviour described below is what happens when the verb is 3315 not in a subroutine or an assertion. Subsequent sections cover these special 3316 cases. 3317 <pre> 3318 (*COMMIT) or (*COMMIT:NAME) 3319 </pre> 3320 This verb causes the whole match to fail outright if there is a later matching 3321 failure that causes backtracking to reach it. Even if the pattern is 3322 unanchored, no further attempts to find a match by advancing the starting point 3323 take place. If (*COMMIT) is the only backtracking verb that is encountered, 3324 once it has been passed <b>pcre2_match()</b> is committed to finding a match at 3325 the current starting point, or not at all. For example: 3326 <pre> 3327 a+(*COMMIT)b 3328 </pre> 3329 This matches "xxaab" but not "aacaab". It can be thought of as a kind of 3330 dynamic anchor, or "I've started, so I must finish." 3331 </P> 3332 <P> 3333 The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COMMIT). It is 3334 like (*MARK:NAME) in that the name is remembered for passing back to the 3335 caller. However, (*SKIP:NAME) searches only for names set with (*MARK), 3336 ignoring those set by (*COMMIT), (*PRUNE) and (*THEN). 3337 </P> 3338 <P> 3339 If there is more than one backtracking verb in a pattern, a different one that 3340 follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a 3341 match does not always guarantee that a match must be at this starting point. 3342 </P> 3343 <P> 3344 Note that (*COMMIT) at the start of a pattern is not the same as an anchor, 3345 unless PCRE2's start-of-match optimizations are turned off, as shown in this 3346 output from <b>pcre2test</b>: 3347 <pre> 3348 re> /(*COMMIT)abc/ 3349 data> xyzabc 3350 0: abc 3351 data> 3352 re> /(*COMMIT)abc/no_start_optimize 3353 data> xyzabc 3354 No match 3355 </pre> 3356 For the first pattern, PCRE2 knows that any match must start with "a", so the 3357 optimization skips along the subject to "a" before applying the pattern to the 3358 first set of data. The match attempt then succeeds. The second pattern disables 3359 the optimization that skips along to the first character. The pattern is now 3360 applied starting at "x", and so the (*COMMIT) causes the match to fail without 3361 trying any other starting points. 3362 <pre> 3363 (*PRUNE) or (*PRUNE:NAME) 3364 </pre> 3365 This verb causes the match to fail at the current starting position in the 3366 subject if there is a later matching failure that causes backtracking to reach 3367 it. If the pattern is unanchored, the normal "bumpalong" advance to the next 3368 starting character then happens. Backtracking can occur as usual to the left of 3369 (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but 3370 if there is no match to the right, backtracking cannot cross (*PRUNE). In 3371 simple cases, the use of (*PRUNE) is just an alternative to an atomic group or 3372 possessive quantifier, but there are some uses of (*PRUNE) that cannot be 3373 expressed in any other way. In an anchored pattern (*PRUNE) has the same effect 3374 as (*COMMIT). 3375 </P> 3376 <P> 3377 The behaviour of (*PRUNE:NAME) is not the same as (*MARK:NAME)(*PRUNE). It is 3378 like (*MARK:NAME) in that the name is remembered for passing back to the 3379 caller. However, (*SKIP:NAME) searches only for names set with (*MARK), 3380 ignoring those set by (*COMMIT), (*PRUNE) or (*THEN). 3381 <pre> 3382 (*SKIP) 3383 </pre> 3384 This verb, when given without a name, is like (*PRUNE), except that if the 3385 pattern is unanchored, the "bumpalong" advance is not to the next character, 3386 but to the position in the subject where (*SKIP) was encountered. (*SKIP) 3387 signifies that whatever text was matched leading up to it cannot be part of a 3388 successful match if there is a later mismatch. Consider: 3389 <pre> 3390 a+(*SKIP)b 3391 </pre> 3392 If the subject is "aaaac...", after the first match attempt fails (starting at 3393 the first character in the string), the starting point skips on to start the 3394 next attempt at "c". Note that a possessive quantifer does not have the same 3395 effect as this example; although it would suppress backtracking during the 3396 first match attempt, the second attempt would start at the second character 3397 instead of skipping on to "c". 3398 <pre> 3399 (*SKIP:NAME) 3400 </pre> 3401 When (*SKIP) has an associated name, its behaviour is modified. When such a 3402 (*SKIP) is triggered, the previous path through the pattern is searched for the 3403 most recent (*MARK) that has the same name. If one is found, the "bumpalong" 3404 advance is to the subject position that corresponds to that (*MARK) instead of 3405 to where (*SKIP) was encountered. If no (*MARK) with a matching name is found, 3406 the (*SKIP) is ignored. 3407 </P> 3408 <P> 3409 The search for a (*MARK) name uses the normal backtracking mechanism, which 3410 means that it does not see (*MARK) settings that are inside atomic groups or 3411 assertions, because they are never re-entered by backtracking. Compare the 3412 following <b>pcre2test</b> examples: 3413 <pre> 3414 re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/ 3415 data: abc 3416 0: a 3417 1: a 3418 data: 3419 re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/ 3420 data: abc 3421 0: b 3422 1: b 3423 </pre> 3424 In the first example, the (*MARK) setting is in an atomic group, so it is not 3425 seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored. This allows 3426 the second branch of the pattern to be tried at the first character position. 3427 In the second example, the (*MARK) setting is not in an atomic group. This 3428 allows (*SKIP:X) to find the (*MARK) when it backtracks, and this causes a new 3429 matching attempt to start at the second character. This time, the (*MARK) is 3430 never seen because "a" does not match "b", so the matcher immediately jumps to 3431 the second branch of the pattern. 3432 </P> 3433 <P> 3434 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores 3435 names that are set by (*COMMIT:NAME), (*PRUNE:NAME) or (*THEN:NAME). 3436 <pre> 3437 (*THEN) or (*THEN:NAME) 3438 </pre> 3439 This verb causes a skip to the next innermost alternative when backtracking 3440 reaches it. That is, it cancels any further backtracking within the current 3441 alternative. Its name comes from the observation that it can be used for a 3442 pattern-based if-then-else block: 3443 <pre> 3444 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... 3445 </pre> 3446 If the COND1 pattern matches, FOO is tried (and possibly further items after 3447 the end of the group if FOO succeeds); on failure, the matcher skips to the 3448 second alternative and tries COND2, without backtracking into COND1. If that 3449 succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no 3450 more alternatives, so there is a backtrack to whatever came before the entire 3451 group. If (*THEN) is not inside an alternation, it acts like (*PRUNE). 3452 </P> 3453 <P> 3454 The behaviour of (*THEN:NAME) is not the same as (*MARK:NAME)(*THEN). It is 3455 like (*MARK:NAME) in that the name is remembered for passing back to the 3456 caller. However, (*SKIP:NAME) searches only for names set with (*MARK), 3457 ignoring those set by (*COMMIT), (*PRUNE) and (*THEN). 3458 </P> 3459 <P> 3460 A subpattern that does not contain a | character is just a part of the 3461 enclosing alternative; it is not a nested alternation with only one 3462 alternative. The effect of (*THEN) extends beyond such a subpattern to the 3463 enclosing alternative. Consider this pattern, where A, B, etc. are complex 3464 pattern fragments that do not contain any | characters at this level: 3465 <pre> 3466 A (B(*THEN)C) | D 3467 </pre> 3468 If A and B are matched, but there is a failure in C, matching does not 3469 backtrack into A; instead it moves to the next alternative, that is, D. 3470 However, if the subpattern containing (*THEN) is given an alternative, it 3471 behaves differently: 3472 <pre> 3473 A (B(*THEN)C | (*FAIL)) | D 3474 </pre> 3475 The effect of (*THEN) is now confined to the inner subpattern. After a failure 3476 in C, matching moves to (*FAIL), which causes the whole subpattern to fail 3477 because there are no more alternatives to try. In this case, matching does now 3478 backtrack into A. 3479 </P> 3480 <P> 3481 Note that a conditional subpattern is not considered as having two 3482 alternatives, because only one is ever used. In other words, the | character in 3483 a conditional subpattern has a different meaning. Ignoring white space, 3484 consider: 3485 <pre> 3486 ^.*? (?(?=a) a | b(*THEN)c ) 3487 </pre> 3488 If the subject is "ba", this pattern does not match. Because .*? is ungreedy, 3489 it initially matches zero characters. The condition (?=a) then fails, the 3490 character "b" is matched, but "c" is not. At this point, matching does not 3491 backtrack to .*? as might perhaps be expected from the presence of the | 3492 character. The conditional subpattern is part of the single alternative that 3493 comprises the whole pattern, and so the match fails. (If there was a backtrack 3494 into .*?, allowing it to match "b", the match would succeed.) 3495 </P> 3496 <P> 3497 The verbs just described provide four different "strengths" of control when 3498 subsequent matching fails. (*THEN) is the weakest, carrying on the match at the 3499 next alternative. (*PRUNE) comes next, failing the match at the current 3500 starting position, but allowing an advance to the next character (for an 3501 unanchored pattern). (*SKIP) is similar, except that the advance may be more 3502 than one character. (*COMMIT) is the strongest, causing the entire match to 3503 fail. 3504 </P> 3505 <br><b> 3506 More than one backtracking verb 3507 </b><br> 3508 <P> 3509 If more than one backtracking verb is present in a pattern, the one that is 3510 backtracked onto first acts. For example, consider this pattern, where A, B, 3511 etc. are complex pattern fragments: 3512 <pre> 3513 (A(*COMMIT)B(*THEN)C|ABD) 3514 </pre> 3515 If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to 3516 fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes 3517 the next alternative (ABD) to be tried. This behaviour is consistent, but is 3518 not always the same as Perl's. It means that if two or more backtracking verbs 3519 appear in succession, all the the last of them has no effect. Consider this 3520 example: 3521 <pre> 3522 ...(*COMMIT)(*PRUNE)... 3523 </pre> 3524 If there is a matching failure to the right, backtracking onto (*PRUNE) causes 3525 it to be triggered, and its action is taken. There can never be a backtrack 3526 onto (*COMMIT). 3527 <a name="btrepeat"></a></P> 3528 <br><b> 3529 Backtracking verbs in repeated groups 3530 </b><br> 3531 <P> 3532 PCRE2 sometimes differs from Perl in its handling of backtracking verbs in 3533 repeated groups. For example, consider: 3534 <pre> 3535 /(a(*COMMIT)b)+ac/ 3536 </pre> 3537 If the subject is "abac", Perl matches unless its optimizations are disabled, 3538 but PCRE2 always fails because the (*COMMIT) in the second repeat of the group 3539 acts. 3540 <a name="btassert"></a></P> 3541 <br><b> 3542 Backtracking verbs in assertions 3543 </b><br> 3544 <P> 3545 (*FAIL) in any assertion has its normal effect: it forces an immediate 3546 backtrack. The behaviour of the other backtracking verbs depends on whether or 3547 not the assertion is standalone or acting as the condition in a conditional 3548 subpattern. 3549 </P> 3550 <P> 3551 (*ACCEPT) in a standalone positive assertion causes the assertion to succeed 3552 without any further processing; captured strings and a (*MARK) name (if set) 3553 are retained. In a standalone negative assertion, (*ACCEPT) causes the 3554 assertion to fail without any further processing; captured substrings and any 3555 (*MARK) name are discarded. 3556 </P> 3557 <P> 3558 If the assertion is a condition, (*ACCEPT) causes the condition to be true for 3559 a positive assertion and false for a negative one; captured substrings are 3560 retained in both cases. 3561 </P> 3562 <P> 3563 The remaining verbs act only when a later failure causes a backtrack to 3564 reach them. This means that their effect is confined to the assertion, 3565 because lookaround assertions are atomic. A backtrack that occurs after an 3566 assertion is complete does not jump back into the assertion. Note in particular 3567 that a (*MARK) name that is set in an assertion is not "seen" by an instance of 3568 (*SKIP:NAME) latter in the pattern. 3569 </P> 3570 <P> 3571 The effect of (*THEN) is not allowed to escape beyond an assertion. If there 3572 are no more branches to try, (*THEN) causes a positive assertion to be false, 3573 and a negative assertion to be true. 3574 </P> 3575 <P> 3576 The other backtracking verbs are not treated specially if they appear in a 3577 standalone positive assertion. In a conditional positive assertion, 3578 backtracking (from within the assertion) into (*COMMIT), (*SKIP), or (*PRUNE) 3579 causes the condition to be false. However, for both standalone and conditional 3580 negative assertions, backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes 3581 the assertion to be true, without considering any further alternative branches. 3582 <a name="btsub"></a></P> 3583 <br><b> 3584 Backtracking verbs in subroutines 3585 </b><br> 3586 <P> 3587 These behaviours occur whether or not the subpattern is called recursively. 3588 </P> 3589 <P> 3590 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to 3591 succeed without any further processing. Matching then continues after the 3592 subroutine call. Perl documents this behaviour. Perl's treatment of the other 3593 verbs in subroutines is different in some cases. 3594 </P> 3595 <P> 3596 (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces 3597 an immediate backtrack. 3598 </P> 3599 <P> 3600 (*COMMIT), (*SKIP), and (*PRUNE) cause the subroutine match to fail when 3601 triggered by being backtracked to in a subpattern called as a subroutine. There 3602 is then a backtrack at the outer level. 3603 </P> 3604 <P> 3605 (*THEN), when triggered, skips to the next alternative in the innermost 3606 enclosing group within the subpattern that has alternatives (its normal 3607 behaviour). However, if there is no such group within the subroutine 3608 subpattern, the subroutine match fails and there is a backtrack at the outer 3609 level. 3610 </P> 3611 <br><a name="SEC28" href="#TOC1">SEE ALSO</a><br> 3612 <P> 3613 <b>pcre2api</b>(3), <b>pcre2callout</b>(3), <b>pcre2matching</b>(3), 3614 <b>pcre2syntax</b>(3), <b>pcre2</b>(3). 3615 </P> 3616 <br><a name="SEC29" href="#TOC1">AUTHOR</a><br> 3617 <P> 3618 Philip Hazel 3619 <br> 3620 University Computing Service 3621 <br> 3622 Cambridge, England. 3623 <br> 3624 </P> 3625 <br><a name="SEC30" href="#TOC1">REVISION</a><br> 3626 <P> 3627 Last updated: 04 September 2018 3628 <br> 3629 Copyright © 1997-2018 University of Cambridge. 3630 <br> 3631 <p> 3632 Return to the <a href="index.html">PCRE2 index page</a>. 3633 </p> 3634
