Tristan Matthews | 0461646 | 2013-11-14 16:09:34 -0500 | [diff] [blame] | 1 | <html> |
| 2 | <head> |
| 3 | <title>pcrepattern specification</title> |
| 4 | </head> |
| 5 | <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"> |
| 6 | <h1>pcrepattern man page</h1> |
| 7 | <p> |
| 8 | Return to the <a href="index.html">PCRE index page</a>. |
| 9 | </p> |
| 10 | <p> |
| 11 | This page is part of the PCRE HTML documentation. It was generated automatically |
| 12 | from the original man page. If there is any nonsense in it, please consult the |
| 13 | man page, in case the conversion went wrong. |
| 14 | <br> |
| 15 | <ul> |
| 16 | <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a> |
| 17 | <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a> |
| 18 | <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a> |
| 19 | <li><a name="TOC4" href="#SEC4">BACKSLASH</a> |
| 20 | <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a> |
| 21 | <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT) AND \N</a> |
| 22 | <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a> |
| 23 | <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a> |
| 24 | <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a> |
| 25 | <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a> |
| 26 | <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a> |
| 27 | <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a> |
| 28 | <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a> |
| 29 | <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a> |
| 30 | <li><a name="TOC15" href="#SEC15">REPETITION</a> |
| 31 | <li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a> |
| 32 | <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a> |
| 33 | <li><a name="TOC18" href="#SEC18">ASSERTIONS</a> |
| 34 | <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a> |
| 35 | <li><a name="TOC20" href="#SEC20">COMMENTS</a> |
| 36 | <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a> |
| 37 | <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a> |
| 38 | <li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a> |
| 39 | <li><a name="TOC24" href="#SEC24">CALLOUTS</a> |
| 40 | <li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a> |
| 41 | <li><a name="TOC26" href="#SEC26">SEE ALSO</a> |
| 42 | <li><a name="TOC27" href="#SEC27">AUTHOR</a> |
| 43 | <li><a name="TOC28" href="#SEC28">REVISION</a> |
| 44 | </ul> |
| 45 | <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br> |
| 46 | <P> |
| 47 | The syntax and semantics of the regular expressions that are supported by PCRE |
| 48 | are described in detail below. There is a quick-reference syntax summary in the |
| 49 | <a href="pcresyntax.html"><b>pcresyntax</b></a> |
| 50 | page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE |
| 51 | also supports some alternative regular expression syntax (which does not |
| 52 | conflict with the Perl syntax) in order to provide some compatibility with |
| 53 | regular expressions in Python, .NET, and Oniguruma. |
| 54 | </P> |
| 55 | <P> |
| 56 | Perl's regular expressions are described in its own documentation, and |
| 57 | regular expressions in general are covered in a number of books, some of which |
| 58 | have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", |
| 59 | published by O'Reilly, covers regular expressions in great detail. This |
| 60 | description of PCRE's regular expressions is intended as reference material. |
| 61 | </P> |
| 62 | <P> |
| 63 | The original operation of PCRE was on strings of one-byte characters. However, |
| 64 | there is now also support for UTF-8 character strings. To use this, |
| 65 | PCRE must be built to include UTF-8 support, and you must call |
| 66 | <b>pcre_compile()</b> or <b>pcre_compile2()</b> with the PCRE_UTF8 option. There |
| 67 | is also a special sequence that can be given at the start of a pattern: |
| 68 | <pre> |
| 69 | (*UTF8) |
| 70 | </pre> |
| 71 | Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8 |
| 72 | option. This feature is not Perl-compatible. How setting UTF-8 mode affects |
| 73 | pattern matching is mentioned in several places below. There is also a summary |
| 74 | of UTF-8 features in the |
| 75 | <a href="pcreunicode.html"><b>pcreunicode</b></a> |
| 76 | page. |
| 77 | </P> |
| 78 | <P> |
| 79 | Another special sequence that may appear at the start of a pattern or in |
| 80 | combination with (*UTF8) is: |
| 81 | <pre> |
| 82 | (*UCP) |
| 83 | </pre> |
| 84 | This has the same effect as setting the PCRE_UCP option: it causes sequences |
| 85 | such as \d and \w to use Unicode properties to determine character types, |
| 86 | instead of recognizing only characters with codes less than 128 via a lookup |
| 87 | table. |
| 88 | </P> |
| 89 | <P> |
| 90 | If a pattern starts with (*NO_START_OPT), it has the same effect as setting the |
| 91 | PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are |
| 92 | also some more of these special sequences that are concerned with the handling |
| 93 | of newlines; they are described below. |
| 94 | </P> |
| 95 | <P> |
| 96 | The remainder of this document discusses the patterns that are supported by |
| 97 | PCRE when its main matching function, <b>pcre_exec()</b>, is used. |
| 98 | From release 6.0, PCRE offers a second matching function, |
| 99 | <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not |
| 100 | Perl-compatible. Some of the features discussed below are not available when |
| 101 | <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the |
| 102 | alternative function, and how it differs from the normal function, are |
| 103 | discussed in the |
| 104 | <a href="pcrematching.html"><b>pcrematching</b></a> |
| 105 | page. |
| 106 | <a name="newlines"></a></P> |
| 107 | <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br> |
| 108 | <P> |
| 109 | PCRE supports five different conventions for indicating line breaks in |
| 110 | strings: a single CR (carriage return) character, a single LF (linefeed) |
| 111 | character, the two-character sequence CRLF, any of the three preceding, or any |
| 112 | Unicode newline sequence. The |
| 113 | <a href="pcreapi.html"><b>pcreapi</b></a> |
| 114 | page has |
| 115 | <a href="pcreapi.html#newlines">further discussion</a> |
| 116 | about newlines, and shows how to set the newline convention in the |
| 117 | <i>options</i> arguments for the compiling and matching functions. |
| 118 | </P> |
| 119 | <P> |
| 120 | It is also possible to specify a newline convention by starting a pattern |
| 121 | string with one of the following five sequences: |
| 122 | <pre> |
| 123 | (*CR) carriage return |
| 124 | (*LF) linefeed |
| 125 | (*CRLF) carriage return, followed by linefeed |
| 126 | (*ANYCRLF) any of the three above |
| 127 | (*ANY) all Unicode newline sequences |
| 128 | </pre> |
| 129 | These override the default and the options given to <b>pcre_compile()</b> or |
| 130 | <b>pcre_compile2()</b>. For example, on a Unix system where LF is the default |
| 131 | newline sequence, the pattern |
| 132 | <pre> |
| 133 | (*CR)a.b |
| 134 | </pre> |
| 135 | changes the convention to CR. That pattern matches "a\nb" because LF is no |
| 136 | longer a newline. Note that these special settings, which are not |
| 137 | Perl-compatible, are recognized only at the very start of a pattern, and that |
| 138 | they must be in upper case. If more than one of them is present, the last one |
| 139 | is used. |
| 140 | </P> |
| 141 | <P> |
| 142 | The newline convention affects the interpretation of the dot metacharacter when |
| 143 | PCRE_DOTALL is not set, and also the behaviour of \N. However, it does not |
| 144 | affect what the \R escape sequence matches. By default, this is any Unicode |
| 145 | newline sequence, for Perl compatibility. However, this can be changed; see the |
| 146 | description of \R in the section entitled |
| 147 | <a href="#newlineseq">"Newline sequences"</a> |
| 148 | below. A change of \R setting can be combined with a change of newline |
| 149 | convention. |
| 150 | </P> |
| 151 | <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br> |
| 152 | <P> |
| 153 | A regular expression is a pattern that is matched against a subject string from |
| 154 | left to right. Most characters stand for themselves in a pattern, and match the |
| 155 | corresponding characters in the subject. As a trivial example, the pattern |
| 156 | <pre> |
| 157 | The quick brown fox |
| 158 | </pre> |
| 159 | matches a portion of a subject string that is identical to itself. When |
| 160 | caseless matching is specified (the PCRE_CASELESS option), letters are matched |
| 161 | independently of case. In UTF-8 mode, PCRE always understands the concept of |
| 162 | case for characters whose values are less than 128, so caseless matching is |
| 163 | always possible. For characters with higher values, the concept of case is |
| 164 | supported if PCRE is compiled with Unicode property support, but not otherwise. |
| 165 | If you want to use caseless matching for characters 128 and above, you must |
| 166 | ensure that PCRE is compiled with Unicode property support as well as with |
| 167 | UTF-8 support. |
| 168 | </P> |
| 169 | <P> |
| 170 | The power of regular expressions comes from the ability to include alternatives |
| 171 | and repetitions in the pattern. These are encoded in the pattern by the use of |
| 172 | <i>metacharacters</i>, which do not stand for themselves but instead are |
| 173 | interpreted in some special way. |
| 174 | </P> |
| 175 | <P> |
| 176 | There are two different sets of metacharacters: those that are recognized |
| 177 | anywhere in the pattern except within square brackets, and those that are |
| 178 | recognized within square brackets. Outside square brackets, the metacharacters |
| 179 | are as follows: |
| 180 | <pre> |
| 181 | \ general escape character with several uses |
| 182 | ^ assert start of string (or line, in multiline mode) |
| 183 | $ assert end of string (or line, in multiline mode) |
| 184 | . match any character except newline (by default) |
| 185 | [ start character class definition |
| 186 | | start of alternative branch |
| 187 | ( start subpattern |
| 188 | ) end subpattern |
| 189 | ? extends the meaning of ( |
| 190 | also 0 or 1 quantifier |
| 191 | also quantifier minimizer |
| 192 | * 0 or more quantifier |
| 193 | + 1 or more quantifier |
| 194 | also "possessive quantifier" |
| 195 | { start min/max quantifier |
| 196 | </pre> |
| 197 | Part of a pattern that is in square brackets is called a "character class". In |
| 198 | a character class the only metacharacters are: |
| 199 | <pre> |
| 200 | \ general escape character |
| 201 | ^ negate the class, but only if the first character |
| 202 | - indicates character range |
| 203 | [ POSIX character class (only if followed by POSIX syntax) |
| 204 | ] terminates the character class |
| 205 | </pre> |
| 206 | The following sections describe the use of each of the metacharacters. |
| 207 | </P> |
| 208 | <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br> |
| 209 | <P> |
| 210 | The backslash character has several uses. Firstly, if it is followed by a |
| 211 | character that is not a number or a letter, it takes away any special meaning |
| 212 | that character may have. This use of backslash as an escape character applies |
| 213 | both inside and outside character classes. |
| 214 | </P> |
| 215 | <P> |
| 216 | For example, if you want to match a * character, you write \* in the pattern. |
| 217 | This escaping action applies whether or not the following character would |
| 218 | otherwise be interpreted as a metacharacter, so it is always safe to precede a |
| 219 | non-alphanumeric with backslash to specify that it stands for itself. In |
| 220 | particular, if you want to match a backslash, you write \\. |
| 221 | </P> |
| 222 | <P> |
| 223 | In UTF-8 mode, only ASCII numbers and letters have any special meaning after a |
| 224 | backslash. All other characters (in particular, those whose codepoints are |
| 225 | greater than 127) are treated as literals. |
| 226 | </P> |
| 227 | <P> |
| 228 | If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the |
| 229 | pattern (other than in a character class) and characters between a # outside |
| 230 | a character class and the next newline are ignored. An escaping backslash can |
| 231 | be used to include a whitespace or # character as part of the pattern. |
| 232 | </P> |
| 233 | <P> |
| 234 | If you want to remove the special meaning from a sequence of characters, you |
| 235 | can do so by putting them between \Q and \E. This is different from Perl in |
| 236 | that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in |
| 237 | Perl, $ and @ cause variable interpolation. Note the following examples: |
| 238 | <pre> |
| 239 | Pattern PCRE matches Perl matches |
| 240 | |
| 241 | \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz |
| 242 | \Qabc\$xyz\E abc\$xyz abc\$xyz |
| 243 | \Qabc\E\$\Qxyz\E abc$xyz abc$xyz |
| 244 | </pre> |
| 245 | The \Q...\E sequence is recognized both inside and outside character classes. |
| 246 | An isolated \E that is not preceded by \Q is ignored. If \Q is not followed |
| 247 | by \E later in the pattern, the literal interpretation continues to the end of |
| 248 | the pattern (that is, \E is assumed at the end). If the isolated \Q is inside |
| 249 | a character class, this causes an error, because the character class is not |
| 250 | terminated. |
| 251 | <a name="digitsafterbackslash"></a></P> |
| 252 | <br><b> |
| 253 | Non-printing characters |
| 254 | </b><br> |
| 255 | <P> |
| 256 | A second use of backslash provides a way of encoding non-printing characters |
| 257 | in patterns in a visible manner. There is no restriction on the appearance of |
| 258 | non-printing characters, apart from the binary zero that terminates a pattern, |
| 259 | but when a pattern is being prepared by text editing, it is often easier to use |
| 260 | one of the following escape sequences than the binary character it represents: |
| 261 | <pre> |
| 262 | \a alarm, that is, the BEL character (hex 07) |
| 263 | \cx "control-x", where x is any ASCII character |
| 264 | \e escape (hex 1B) |
| 265 | \f formfeed (hex 0C) |
| 266 | \n linefeed (hex 0A) |
| 267 | \r carriage return (hex 0D) |
| 268 | \t tab (hex 09) |
| 269 | \ddd character with octal code ddd, or back reference |
| 270 | \xhh character with hex code hh |
| 271 | \x{hhh..} character with hex code hhh.. (non-JavaScript mode) |
| 272 | \uhhhh character with hex code hhhh (JavaScript mode only) |
| 273 | </pre> |
| 274 | The precise effect of \cx is as follows: if x is a lower case letter, it |
| 275 | is converted to upper case. Then bit 6 of the character (hex 40) is inverted. |
| 276 | Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ is 7B), while |
| 277 | \c; becomes hex 7B (; is 3B). If the byte following \c has a value greater |
| 278 | than 127, a compile-time error occurs. This locks out non-ASCII characters in |
| 279 | both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte |
| 280 | values are valid. A lower case letter is converted to upper case, and then the |
| 281 | 0xc0 bits are flipped.) |
| 282 | </P> |
| 283 | <P> |
| 284 | By default, after \x, from zero to two hexadecimal digits are read (letters |
| 285 | can be in upper or lower case). Any number of hexadecimal digits may appear |
| 286 | between \x{ and }, but the value of the character code must be less than 256 |
| 287 | in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum |
| 288 | value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest |
| 289 | Unicode code point, which is 10FFFF. |
| 290 | </P> |
| 291 | <P> |
| 292 | If characters other than hexadecimal digits appear between \x{ and }, or if |
| 293 | there is no terminating }, this form of escape is not recognized. Instead, the |
| 294 | initial \x will be interpreted as a basic hexadecimal escape, with no |
| 295 | following digits, giving a character whose value is zero. |
| 296 | </P> |
| 297 | <P> |
| 298 | If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is |
| 299 | as just described only when it is followed by two hexadecimal digits. |
| 300 | Otherwise, it matches a literal "x" character. In JavaScript mode, support for |
| 301 | code points greater than 256 is provided by \u, which must be followed by |
| 302 | four hexadecimal digits; otherwise it matches a literal "u" character. |
| 303 | </P> |
| 304 | <P> |
| 305 | Characters whose value is less than 256 can be defined by either of the two |
| 306 | syntaxes for \x (or by \u in JavaScript mode). There is no difference in the |
| 307 | way they are handled. For example, \xdc is exactly the same as \x{dc} (or |
| 308 | \u00dc in JavaScript mode). |
| 309 | </P> |
| 310 | <P> |
| 311 | After \0 up to two further octal digits are read. If there are fewer than two |
| 312 | digits, just those that are present are used. Thus the sequence \0\x\07 |
| 313 | specifies two binary zeros followed by a BEL character (code value 7). Make |
| 314 | sure you supply two digits after the initial zero if the pattern character that |
| 315 | follows is itself an octal digit. |
| 316 | </P> |
| 317 | <P> |
| 318 | The handling of a backslash followed by a digit other than 0 is complicated. |
| 319 | Outside a character class, PCRE reads it and any following digits as a decimal |
| 320 | number. If the number is less than 10, or if there have been at least that many |
| 321 | previous capturing left parentheses in the expression, the entire sequence is |
| 322 | taken as a <i>back reference</i>. A description of how this works is given |
| 323 | <a href="#backreferences">later,</a> |
| 324 | following the discussion of |
| 325 | <a href="#subpattern">parenthesized subpatterns.</a> |
| 326 | </P> |
| 327 | <P> |
| 328 | Inside a character class, or if the decimal number is greater than 9 and there |
| 329 | have not been that many capturing subpatterns, PCRE re-reads up to three octal |
| 330 | digits following the backslash, and uses them to generate a data character. Any |
| 331 | subsequent digits stand for themselves. In non-UTF-8 mode, the value of a |
| 332 | character specified in octal must be less than \400. In UTF-8 mode, values up |
| 333 | to \777 are permitted. For example: |
| 334 | <pre> |
| 335 | \040 is another way of writing a space |
| 336 | \40 is the same, provided there are fewer than 40 previous capturing subpatterns |
| 337 | \7 is always a back reference |
| 338 | \11 might be a back reference, or another way of writing a tab |
| 339 | \011 is always a tab |
| 340 | \0113 is a tab followed by the character "3" |
| 341 | \113 might be a back reference, otherwise the character with octal code 113 |
| 342 | \377 might be a back reference, otherwise the byte consisting entirely of 1 bits |
| 343 | \81 is either a back reference, or a binary zero followed by the two characters "8" and "1" |
| 344 | </pre> |
| 345 | Note that octal values of 100 or greater must not be introduced by a leading |
| 346 | zero, because no more than three octal digits are ever read. |
| 347 | </P> |
| 348 | <P> |
| 349 | All the sequences that define a single character value can be used both inside |
| 350 | and outside character classes. In addition, inside a character class, \b is |
| 351 | interpreted as the backspace character (hex 08). |
| 352 | </P> |
| 353 | <P> |
| 354 | \N is not allowed in a character class. \B, \R, and \X are not special |
| 355 | inside a character class. Like other unrecognized escape sequences, they are |
| 356 | treated as the literal characters "B", "R", and "X" by default, but cause an |
| 357 | error if the PCRE_EXTRA option is set. Outside a character class, these |
| 358 | sequences have different meanings. |
| 359 | </P> |
| 360 | <br><b> |
| 361 | Unsupported escape sequences |
| 362 | </b><br> |
| 363 | <P> |
| 364 | In Perl, the sequences \l, \L, \u, and \U are recognized by its string |
| 365 | handler and used to modify the case of following characters. By default, PCRE |
| 366 | does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT |
| 367 | option is set, \U matches a "U" character, and \u can be used to define a |
| 368 | character by code point, as described in the previous section. |
| 369 | </P> |
| 370 | <br><b> |
| 371 | Absolute and relative back references |
| 372 | </b><br> |
| 373 | <P> |
| 374 | The sequence \g followed by an unsigned or a negative number, optionally |
| 375 | enclosed in braces, is an absolute or relative back reference. A named back |
| 376 | reference can be coded as \g{name}. Back references are discussed |
| 377 | <a href="#backreferences">later,</a> |
| 378 | following the discussion of |
| 379 | <a href="#subpattern">parenthesized subpatterns.</a> |
| 380 | </P> |
| 381 | <br><b> |
| 382 | Absolute and relative subroutine calls |
| 383 | </b><br> |
| 384 | <P> |
| 385 | For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or |
| 386 | a number enclosed either in angle brackets or single quotes, is an alternative |
| 387 | syntax for referencing a subpattern as a "subroutine". Details are discussed |
| 388 | <a href="#onigurumasubroutines">later.</a> |
| 389 | Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i> |
| 390 | synonymous. The former is a back reference; the latter is a |
| 391 | <a href="#subpatternsassubroutines">subroutine</a> |
| 392 | call. |
| 393 | <a name="genericchartypes"></a></P> |
| 394 | <br><b> |
| 395 | Generic character types |
| 396 | </b><br> |
| 397 | <P> |
| 398 | Another use of backslash is for specifying generic character types: |
| 399 | <pre> |
| 400 | \d any decimal digit |
| 401 | \D any character that is not a decimal digit |
| 402 | \h any horizontal whitespace character |
| 403 | \H any character that is not a horizontal whitespace character |
| 404 | \s any whitespace character |
| 405 | \S any character that is not a whitespace character |
| 406 | \v any vertical whitespace character |
| 407 | \V any character that is not a vertical whitespace character |
| 408 | \w any "word" character |
| 409 | \W any "non-word" character |
| 410 | </pre> |
| 411 | There is also the single sequence \N, which matches a non-newline character. |
| 412 | This is the same as |
| 413 | <a href="#fullstopdot">the "." metacharacter</a> |
| 414 | when PCRE_DOTALL is not set. Perl also uses \N to match characters by name; |
| 415 | PCRE does not support this. |
| 416 | </P> |
| 417 | <P> |
| 418 | Each pair of lower and upper case escape sequences partitions the complete set |
| 419 | of characters into two disjoint sets. Any given character matches one, and only |
| 420 | one, of each pair. The sequences can appear both inside and outside character |
| 421 | classes. They each match one character of the appropriate type. If the current |
| 422 | matching point is at the end of the subject string, all of them fail, because |
| 423 | there is no character to match. |
| 424 | </P> |
| 425 | <P> |
| 426 | For compatibility with Perl, \s does not match the VT character (code 11). |
| 427 | This makes it different from the the POSIX "space" class. The \s characters |
| 428 | are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is |
| 429 | included in a Perl script, \s may match the VT character. In PCRE, it never |
| 430 | does. |
| 431 | </P> |
| 432 | <P> |
| 433 | A "word" character is an underscore or any character that is a letter or digit. |
| 434 | By default, the definition of letters and digits is controlled by PCRE's |
| 435 | low-valued character tables, and may vary if locale-specific matching is taking |
| 436 | place (see |
| 437 | <a href="pcreapi.html#localesupport">"Locale support"</a> |
| 438 | in the |
| 439 | <a href="pcreapi.html"><b>pcreapi</b></a> |
| 440 | page). For example, in a French locale such as "fr_FR" in Unix-like systems, |
| 441 | or "french" in Windows, some character codes greater than 128 are used for |
| 442 | accented letters, and these are then matched by \w. The use of locales with |
| 443 | Unicode is discouraged. |
| 444 | </P> |
| 445 | <P> |
| 446 | By default, in UTF-8 mode, characters with values greater than 128 never match |
| 447 | \d, \s, or \w, and always match \D, \S, and \W. These sequences retain |
| 448 | their original meanings from before UTF-8 support was available, mainly for |
| 449 | efficiency reasons. However, if PCRE is compiled with Unicode property support, |
| 450 | and the PCRE_UCP option is set, the behaviour is changed so that Unicode |
| 451 | properties are used to determine character types, as follows: |
| 452 | <pre> |
| 453 | \d any character that \p{Nd} matches (decimal digit) |
| 454 | \s any character that \p{Z} matches, plus HT, LF, FF, CR |
| 455 | \w any character that \p{L} or \p{N} matches, plus underscore |
| 456 | </pre> |
| 457 | The upper case escapes match the inverse sets of characters. Note that \d |
| 458 | matches only decimal digits, whereas \w matches any Unicode digit, as well as |
| 459 | any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and |
| 460 | \B because they are defined in terms of \w and \W. Matching these sequences |
| 461 | is noticeably slower when PCRE_UCP is set. |
| 462 | </P> |
| 463 | <P> |
| 464 | The sequences \h, \H, \v, and \V are features that were added to Perl at |
| 465 | release 5.10. In contrast to the other sequences, which match only ASCII |
| 466 | characters by default, these always match certain high-valued codepoints in |
| 467 | UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters |
| 468 | are: |
| 469 | <pre> |
| 470 | U+0009 Horizontal tab |
| 471 | U+0020 Space |
| 472 | U+00A0 Non-break space |
| 473 | U+1680 Ogham space mark |
| 474 | U+180E Mongolian vowel separator |
| 475 | U+2000 En quad |
| 476 | U+2001 Em quad |
| 477 | U+2002 En space |
| 478 | U+2003 Em space |
| 479 | U+2004 Three-per-em space |
| 480 | U+2005 Four-per-em space |
| 481 | U+2006 Six-per-em space |
| 482 | U+2007 Figure space |
| 483 | U+2008 Punctuation space |
| 484 | U+2009 Thin space |
| 485 | U+200A Hair space |
| 486 | U+202F Narrow no-break space |
| 487 | U+205F Medium mathematical space |
| 488 | U+3000 Ideographic space |
| 489 | </pre> |
| 490 | The vertical space characters are: |
| 491 | <pre> |
| 492 | U+000A Linefeed |
| 493 | U+000B Vertical tab |
| 494 | U+000C Formfeed |
| 495 | U+000D Carriage return |
| 496 | U+0085 Next line |
| 497 | U+2028 Line separator |
| 498 | U+2029 Paragraph separator |
| 499 | <a name="newlineseq"></a></PRE> |
| 500 | </P> |
| 501 | <br><b> |
| 502 | Newline sequences |
| 503 | </b><br> |
| 504 | <P> |
| 505 | Outside a character class, by default, the escape sequence \R matches any |
| 506 | Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the following: |
| 507 | <pre> |
| 508 | (?>\r\n|\n|\x0b|\f|\r|\x85) |
| 509 | </pre> |
| 510 | This is an example of an "atomic group", details of which are given |
| 511 | <a href="#atomicgroup">below.</a> |
| 512 | This particular group matches either the two-character sequence CR followed by |
| 513 | LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, |
| 514 | U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next |
| 515 | line, U+0085). The two-character sequence is treated as a single unit that |
| 516 | cannot be split. |
| 517 | </P> |
| 518 | <P> |
| 519 | In UTF-8 mode, two additional characters whose codepoints are greater than 255 |
| 520 | are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). |
| 521 | Unicode character property support is not needed for these characters to be |
| 522 | recognized. |
| 523 | </P> |
| 524 | <P> |
| 525 | It is possible to restrict \R to match only CR, LF, or CRLF (instead of the |
| 526 | complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF |
| 527 | either at compile time or when the pattern is matched. (BSR is an abbrevation |
| 528 | for "backslash R".) This can be made the default when PCRE is built; if this is |
| 529 | the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option. |
| 530 | It is also possible to specify these settings by starting a pattern string with |
| 531 | one of the following sequences: |
| 532 | <pre> |
| 533 | (*BSR_ANYCRLF) CR, LF, or CRLF only |
| 534 | (*BSR_UNICODE) any Unicode newline sequence |
| 535 | </pre> |
| 536 | These override the default and the options given to <b>pcre_compile()</b> or |
| 537 | <b>pcre_compile2()</b>, but they can be overridden by options given to |
| 538 | <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. Note that these special settings, |
| 539 | which are not Perl-compatible, are recognized only at the very start of a |
| 540 | pattern, and that they must be in upper case. If more than one of them is |
| 541 | present, the last one is used. They can be combined with a change of newline |
| 542 | convention; for example, a pattern can start with: |
| 543 | <pre> |
| 544 | (*ANY)(*BSR_ANYCRLF) |
| 545 | </pre> |
| 546 | They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside |
| 547 | a character class, \R is treated as an unrecognized escape sequence, and so |
| 548 | matches the letter "R" by default, but causes an error if PCRE_EXTRA is set. |
| 549 | <a name="uniextseq"></a></P> |
| 550 | <br><b> |
| 551 | Unicode character properties |
| 552 | </b><br> |
| 553 | <P> |
| 554 | When PCRE is built with Unicode character property support, three additional |
| 555 | escape sequences that match characters with specific properties are available. |
| 556 | When not in UTF-8 mode, these sequences are of course limited to testing |
| 557 | characters whose codepoints are less than 256, but they do work in this mode. |
| 558 | The extra escape sequences are: |
| 559 | <pre> |
| 560 | \p{<i>xx</i>} a character with the <i>xx</i> property |
| 561 | \P{<i>xx</i>} a character without the <i>xx</i> property |
| 562 | \X an extended Unicode sequence |
| 563 | </pre> |
| 564 | The property names represented by <i>xx</i> above are limited to the Unicode |
| 565 | script names, the general category properties, "Any", which matches any |
| 566 | character (including newline), and some special PCRE properties (described |
| 567 | in the |
| 568 | <a href="#extraprops">next section).</a> |
| 569 | Other Perl properties such as "InMusicalSymbols" are not currently supported by |
| 570 | PCRE. Note that \P{Any} does not match any characters, so always causes a |
| 571 | match failure. |
| 572 | </P> |
| 573 | <P> |
| 574 | Sets of Unicode characters are defined as belonging to certain scripts. A |
| 575 | character from one of these sets can be matched using a script name. For |
| 576 | example: |
| 577 | <pre> |
| 578 | \p{Greek} |
| 579 | \P{Han} |
| 580 | </pre> |
| 581 | Those that are not part of an identified script are lumped together as |
| 582 | "Common". The current list of scripts is: |
| 583 | </P> |
| 584 | <P> |
| 585 | Arabic, |
| 586 | Armenian, |
| 587 | Avestan, |
| 588 | Balinese, |
| 589 | Bamum, |
| 590 | Bengali, |
| 591 | Bopomofo, |
| 592 | Braille, |
| 593 | Buginese, |
| 594 | Buhid, |
| 595 | Canadian_Aboriginal, |
| 596 | Carian, |
| 597 | Cham, |
| 598 | Cherokee, |
| 599 | Common, |
| 600 | Coptic, |
| 601 | Cuneiform, |
| 602 | Cypriot, |
| 603 | Cyrillic, |
| 604 | Deseret, |
| 605 | Devanagari, |
| 606 | Egyptian_Hieroglyphs, |
| 607 | Ethiopic, |
| 608 | Georgian, |
| 609 | Glagolitic, |
| 610 | Gothic, |
| 611 | Greek, |
| 612 | Gujarati, |
| 613 | Gurmukhi, |
| 614 | Han, |
| 615 | Hangul, |
| 616 | Hanunoo, |
| 617 | Hebrew, |
| 618 | Hiragana, |
| 619 | Imperial_Aramaic, |
| 620 | Inherited, |
| 621 | Inscriptional_Pahlavi, |
| 622 | Inscriptional_Parthian, |
| 623 | Javanese, |
| 624 | Kaithi, |
| 625 | Kannada, |
| 626 | Katakana, |
| 627 | Kayah_Li, |
| 628 | Kharoshthi, |
| 629 | Khmer, |
| 630 | Lao, |
| 631 | Latin, |
| 632 | Lepcha, |
| 633 | Limbu, |
| 634 | Linear_B, |
| 635 | Lisu, |
| 636 | Lycian, |
| 637 | Lydian, |
| 638 | Malayalam, |
| 639 | Meetei_Mayek, |
| 640 | Mongolian, |
| 641 | Myanmar, |
| 642 | New_Tai_Lue, |
| 643 | Nko, |
| 644 | Ogham, |
| 645 | Old_Italic, |
| 646 | Old_Persian, |
| 647 | Old_South_Arabian, |
| 648 | Old_Turkic, |
| 649 | Ol_Chiki, |
| 650 | Oriya, |
| 651 | Osmanya, |
| 652 | Phags_Pa, |
| 653 | Phoenician, |
| 654 | Rejang, |
| 655 | Runic, |
| 656 | Samaritan, |
| 657 | Saurashtra, |
| 658 | Shavian, |
| 659 | Sinhala, |
| 660 | Sundanese, |
| 661 | Syloti_Nagri, |
| 662 | Syriac, |
| 663 | Tagalog, |
| 664 | Tagbanwa, |
| 665 | Tai_Le, |
| 666 | Tai_Tham, |
| 667 | Tai_Viet, |
| 668 | Tamil, |
| 669 | Telugu, |
| 670 | Thaana, |
| 671 | Thai, |
| 672 | Tibetan, |
| 673 | Tifinagh, |
| 674 | Ugaritic, |
| 675 | Vai, |
| 676 | Yi. |
| 677 | </P> |
| 678 | <P> |
| 679 | Each character has exactly one Unicode general category property, specified by |
| 680 | a two-letter abbreviation. For compatibility with Perl, negation can be |
| 681 | specified by including a circumflex between the opening brace and the property |
| 682 | name. For example, \p{^Lu} is the same as \P{Lu}. |
| 683 | </P> |
| 684 | <P> |
| 685 | If only one letter is specified with \p or \P, it includes all the general |
| 686 | category properties that start with that letter. In this case, in the absence |
| 687 | of negation, the curly brackets in the escape sequence are optional; these two |
| 688 | examples have the same effect: |
| 689 | <pre> |
| 690 | \p{L} |
| 691 | \pL |
| 692 | </pre> |
| 693 | The following general category property codes are supported: |
| 694 | <pre> |
| 695 | C Other |
| 696 | Cc Control |
| 697 | Cf Format |
| 698 | Cn Unassigned |
| 699 | Co Private use |
| 700 | Cs Surrogate |
| 701 | |
| 702 | L Letter |
| 703 | Ll Lower case letter |
| 704 | Lm Modifier letter |
| 705 | Lo Other letter |
| 706 | Lt Title case letter |
| 707 | Lu Upper case letter |
| 708 | |
| 709 | M Mark |
| 710 | Mc Spacing mark |
| 711 | Me Enclosing mark |
| 712 | Mn Non-spacing mark |
| 713 | |
| 714 | N Number |
| 715 | Nd Decimal number |
| 716 | Nl Letter number |
| 717 | No Other number |
| 718 | |
| 719 | P Punctuation |
| 720 | Pc Connector punctuation |
| 721 | Pd Dash punctuation |
| 722 | Pe Close punctuation |
| 723 | Pf Final punctuation |
| 724 | Pi Initial punctuation |
| 725 | Po Other punctuation |
| 726 | Ps Open punctuation |
| 727 | |
| 728 | S Symbol |
| 729 | Sc Currency symbol |
| 730 | Sk Modifier symbol |
| 731 | Sm Mathematical symbol |
| 732 | So Other symbol |
| 733 | |
| 734 | Z Separator |
| 735 | Zl Line separator |
| 736 | Zp Paragraph separator |
| 737 | Zs Space separator |
| 738 | </pre> |
| 739 | The special property L& is also supported: it matches a character that has |
| 740 | the Lu, Ll, or Lt property, in other words, a letter that is not classified as |
| 741 | a modifier or "other". |
| 742 | </P> |
| 743 | <P> |
| 744 | The Cs (Surrogate) property applies only to characters in the range U+D800 to |
| 745 | U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so |
| 746 | cannot be tested by PCRE, unless UTF-8 validity checking has been turned off |
| 747 | (see the discussion of PCRE_NO_UTF8_CHECK in the |
| 748 | <a href="pcreapi.html"><b>pcreapi</b></a> |
| 749 | page). Perl does not support the Cs property. |
| 750 | </P> |
| 751 | <P> |
| 752 | The long synonyms for property names that Perl supports (such as \p{Letter}) |
| 753 | are not supported by PCRE, nor is it permitted to prefix any of these |
| 754 | properties with "Is". |
| 755 | </P> |
| 756 | <P> |
| 757 | No character that is in the Unicode table has the Cn (unassigned) property. |
| 758 | Instead, this property is assumed for any code point that is not in the |
| 759 | Unicode table. |
| 760 | </P> |
| 761 | <P> |
| 762 | Specifying caseless matching does not affect these escape sequences. For |
| 763 | example, \p{Lu} always matches only upper case letters. |
| 764 | </P> |
| 765 | <P> |
| 766 | The \X escape matches any number of Unicode characters that form an extended |
| 767 | Unicode sequence. \X is equivalent to |
| 768 | <pre> |
| 769 | (?>\PM\pM*) |
| 770 | </pre> |
| 771 | That is, it matches a character without the "mark" property, followed by zero |
| 772 | or more characters with the "mark" property, and treats the sequence as an |
| 773 | atomic group |
| 774 | <a href="#atomicgroup">(see below).</a> |
| 775 | Characters with the "mark" property are typically accents that affect the |
| 776 | preceding character. None of them have codepoints less than 256, so in |
| 777 | non-UTF-8 mode \X matches any one character. |
| 778 | </P> |
| 779 | <P> |
| 780 | Note that recent versions of Perl have changed \X to match what Unicode calls |
| 781 | an "extended grapheme cluster", which has a more complicated definition. |
| 782 | </P> |
| 783 | <P> |
| 784 | Matching characters by Unicode property is not fast, because PCRE has to search |
| 785 | a structure that contains data for over fifteen thousand characters. That is |
| 786 | why the traditional escape sequences such as \d and \w do not use Unicode |
| 787 | properties in PCRE by default, though you can make them do so by setting the |
| 788 | PCRE_UCP option for <b>pcre_compile()</b> or by starting the pattern with |
| 789 | (*UCP). |
| 790 | <a name="extraprops"></a></P> |
| 791 | <br><b> |
| 792 | PCRE's additional properties |
| 793 | </b><br> |
| 794 | <P> |
| 795 | As well as the standard Unicode properties described in the previous |
| 796 | section, PCRE supports four more that make it possible to convert traditional |
| 797 | escape sequences such as \w and \s and POSIX character classes to use Unicode |
| 798 | properties. PCRE uses these non-standard, non-Perl properties internally when |
| 799 | PCRE_UCP is set. They are: |
| 800 | <pre> |
| 801 | Xan Any alphanumeric character |
| 802 | Xps Any POSIX space character |
| 803 | Xsp Any Perl space character |
| 804 | Xwd Any Perl "word" character |
| 805 | </pre> |
| 806 | Xan matches characters that have either the L (letter) or the N (number) |
| 807 | property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or |
| 808 | carriage return, and any other character that has the Z (separator) property. |
| 809 | Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the |
| 810 | same characters as Xan, plus underscore. |
| 811 | <a name="resetmatchstart"></a></P> |
| 812 | <br><b> |
| 813 | Resetting the match start |
| 814 | </b><br> |
| 815 | <P> |
| 816 | The escape sequence \K causes any previously matched characters not to be |
| 817 | included in the final matched sequence. For example, the pattern: |
| 818 | <pre> |
| 819 | foo\Kbar |
| 820 | </pre> |
| 821 | matches "foobar", but reports that it has matched "bar". This feature is |
| 822 | similar to a lookbehind assertion |
| 823 | <a href="#lookbehind">(described below).</a> |
| 824 | However, in this case, the part of the subject before the real match does not |
| 825 | have to be of fixed length, as lookbehind assertions do. The use of \K does |
| 826 | not interfere with the setting of |
| 827 | <a href="#subpattern">captured substrings.</a> |
| 828 | For example, when the pattern |
| 829 | <pre> |
| 830 | (foo)\Kbar |
| 831 | </pre> |
| 832 | matches "foobar", the first substring is still set to "foo". |
| 833 | </P> |
| 834 | <P> |
| 835 | Perl documents that the use of \K within assertions is "not well defined". In |
| 836 | PCRE, \K is acted upon when it occurs inside positive assertions, but is |
| 837 | ignored in negative assertions. |
| 838 | <a name="smallassertions"></a></P> |
| 839 | <br><b> |
| 840 | Simple assertions |
| 841 | </b><br> |
| 842 | <P> |
| 843 | The final use of backslash is for certain simple assertions. An assertion |
| 844 | specifies a condition that has to be met at a particular point in a match, |
| 845 | without consuming any characters from the subject string. The use of |
| 846 | subpatterns for more complicated assertions is described |
| 847 | <a href="#bigassertions">below.</a> |
| 848 | The backslashed assertions are: |
| 849 | <pre> |
| 850 | \b matches at a word boundary |
| 851 | \B matches when not at a word boundary |
| 852 | \A matches at the start of the subject |
| 853 | \Z matches at the end of the subject |
| 854 | also matches before a newline at the end of the subject |
| 855 | \z matches only at the end of the subject |
| 856 | \G matches at the first matching position in the subject |
| 857 | </pre> |
| 858 | Inside a character class, \b has a different meaning; it matches the backspace |
| 859 | character. If any other of these assertions appears in a character class, by |
| 860 | default it matches the corresponding literal character (for example, \B |
| 861 | matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid |
| 862 | escape sequence" error is generated instead. |
| 863 | </P> |
| 864 | <P> |
| 865 | A word boundary is a position in the subject string where the current character |
| 866 | and the previous character do not both match \w or \W (i.e. one matches |
| 867 | \w and the other matches \W), or the start or end of the string if the |
| 868 | first or last character matches \w, respectively. In UTF-8 mode, the meanings |
| 869 | of \w and \W can be changed by setting the PCRE_UCP option. When this is |
| 870 | done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start |
| 871 | of word" or "end of word" metasequence. However, whatever follows \b normally |
| 872 | determines which it is. For example, the fragment \ba matches "a" at the start |
| 873 | of a word. |
| 874 | </P> |
| 875 | <P> |
| 876 | The \A, \Z, and \z assertions differ from the traditional circumflex and |
| 877 | dollar (described in the next section) in that they only ever match at the very |
| 878 | start and end of the subject string, whatever options are set. Thus, they are |
| 879 | independent of multiline mode. These three assertions are not affected by the |
| 880 | PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the |
| 881 | circumflex and dollar metacharacters. However, if the <i>startoffset</i> |
| 882 | argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start |
| 883 | at a point other than the beginning of the subject, \A can never match. The |
| 884 | difference between \Z and \z is that \Z matches before a newline at the end |
| 885 | of the string as well as at the very end, whereas \z matches only at the end. |
| 886 | </P> |
| 887 | <P> |
| 888 | The \G assertion is true only when the current matching position is at the |
| 889 | start point of the match, as specified by the <i>startoffset</i> argument of |
| 890 | <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is |
| 891 | non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate |
| 892 | arguments, you can mimic Perl's /g option, and it is in this kind of |
| 893 | implementation where \G can be useful. |
| 894 | </P> |
| 895 | <P> |
| 896 | Note, however, that PCRE's interpretation of \G, as the start of the current |
| 897 | match, is subtly different from Perl's, which defines it as the end of the |
| 898 | previous match. In Perl, these can be different when the previously matched |
| 899 | string was empty. Because PCRE does just one match at a time, it cannot |
| 900 | reproduce this behaviour. |
| 901 | </P> |
| 902 | <P> |
| 903 | If all the alternatives of a pattern begin with \G, the expression is anchored |
| 904 | to the starting match position, and the "anchored" flag is set in the compiled |
| 905 | regular expression. |
| 906 | </P> |
| 907 | <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br> |
| 908 | <P> |
| 909 | Outside a character class, in the default matching mode, the circumflex |
| 910 | character is an assertion that is true only if the current matching point is |
| 911 | at the start of the subject string. If the <i>startoffset</i> argument of |
| 912 | <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE |
| 913 | option is unset. Inside a character class, circumflex has an entirely different |
| 914 | meaning |
| 915 | <a href="#characterclass">(see below).</a> |
| 916 | </P> |
| 917 | <P> |
| 918 | Circumflex need not be the first character of the pattern if a number of |
| 919 | alternatives are involved, but it should be the first thing in each alternative |
| 920 | in which it appears if the pattern is ever to match that branch. If all |
| 921 | possible alternatives start with a circumflex, that is, if the pattern is |
| 922 | constrained to match only at the start of the subject, it is said to be an |
| 923 | "anchored" pattern. (There are also other constructs that can cause a pattern |
| 924 | to be anchored.) |
| 925 | </P> |
| 926 | <P> |
| 927 | A dollar character is an assertion that is true only if the current matching |
| 928 | point is at the end of the subject string, or immediately before a newline |
| 929 | at the end of the string (by default). Dollar need not be the last character of |
| 930 | the pattern if a number of alternatives are involved, but it should be the last |
| 931 | item in any branch in which it appears. Dollar has no special meaning in a |
| 932 | character class. |
| 933 | </P> |
| 934 | <P> |
| 935 | The meaning of dollar can be changed so that it matches only at the very end of |
| 936 | the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This |
| 937 | does not affect the \Z assertion. |
| 938 | </P> |
| 939 | <P> |
| 940 | The meanings of the circumflex and dollar characters are changed if the |
| 941 | PCRE_MULTILINE option is set. When this is the case, a circumflex matches |
| 942 | immediately after internal newlines as well as at the start of the subject |
| 943 | string. It does not match after a newline that ends the string. A dollar |
| 944 | matches before any newlines in the string, as well as at the very end, when |
| 945 | PCRE_MULTILINE is set. When newline is specified as the two-character |
| 946 | sequence CRLF, isolated CR and LF characters do not indicate newlines. |
| 947 | </P> |
| 948 | <P> |
| 949 | For example, the pattern /^abc$/ matches the subject string "def\nabc" (where |
| 950 | \n represents a newline) in multiline mode, but not otherwise. Consequently, |
| 951 | patterns that are anchored in single line mode because all branches start with |
| 952 | ^ are not anchored in multiline mode, and a match for circumflex is possible |
| 953 | when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The |
| 954 | PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. |
| 955 | </P> |
| 956 | <P> |
| 957 | Note that the sequences \A, \Z, and \z can be used to match the start and |
| 958 | end of the subject in both modes, and if all branches of a pattern start with |
| 959 | \A it is always anchored, whether or not PCRE_MULTILINE is set. |
| 960 | <a name="fullstopdot"></a></P> |
| 961 | <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br> |
| 962 | <P> |
| 963 | Outside a character class, a dot in the pattern matches any one character in |
| 964 | the subject string except (by default) a character that signifies the end of a |
| 965 | line. In UTF-8 mode, the matched character may be more than one byte long. |
| 966 | </P> |
| 967 | <P> |
| 968 | When a line ending is defined as a single character, dot never matches that |
| 969 | character; when the two-character sequence CRLF is used, dot does not match CR |
| 970 | if it is immediately followed by LF, but otherwise it matches all characters |
| 971 | (including isolated CRs and LFs). When any Unicode line endings are being |
| 972 | recognized, dot does not match CR or LF or any of the other line ending |
| 973 | characters. |
| 974 | </P> |
| 975 | <P> |
| 976 | The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL |
| 977 | option is set, a dot matches any one character, without exception. If the |
| 978 | two-character sequence CRLF is present in the subject string, it takes two dots |
| 979 | to match it. |
| 980 | </P> |
| 981 | <P> |
| 982 | The handling of dot is entirely independent of the handling of circumflex and |
| 983 | dollar, the only relationship being that they both involve newlines. Dot has no |
| 984 | special meaning in a character class. |
| 985 | </P> |
| 986 | <P> |
| 987 | The escape sequence \N behaves like a dot, except that it is not affected by |
| 988 | the PCRE_DOTALL option. In other words, it matches any character except one |
| 989 | that signifies the end of a line. Perl also uses \N to match characters by |
| 990 | name; PCRE does not support this. |
| 991 | </P> |
| 992 | <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br> |
| 993 | <P> |
| 994 | Outside a character class, the escape sequence \C matches any one byte, both |
| 995 | in and out of UTF-8 mode. Unlike a dot, it always matches line-ending |
| 996 | characters. The feature is provided in Perl in order to match individual bytes |
| 997 | in UTF-8 mode, but it is unclear how it can usefully be used. Because \C |
| 998 | breaks up characters into individual bytes, matching one byte with \C in UTF-8 |
| 999 | mode means that the rest of the string may start with a malformed UTF-8 |
| 1000 | character. This has undefined results, because PCRE assumes that it is dealing |
| 1001 | with valid UTF-8 strings (and by default it checks this at the start of |
| 1002 | processing unless the PCRE_NO_UTF8_CHECK option is used). |
| 1003 | </P> |
| 1004 | <P> |
| 1005 | PCRE does not allow \C to appear in lookbehind assertions |
| 1006 | <a href="#lookbehind">(described below)</a> |
| 1007 | in UTF-8 mode, because this would make it impossible to calculate the length of |
| 1008 | the lookbehind. |
| 1009 | </P> |
| 1010 | <P> |
| 1011 | In general, the \C escape sequence is best avoided in UTF-8 mode. However, one |
| 1012 | way of using it that avoids the problem of malformed UTF-8 characters is to |
| 1013 | use a lookahead to check the length of the next character, as in this pattern |
| 1014 | (ignore white space and line breaks): |
| 1015 | <pre> |
| 1016 | (?| (?=[\x00-\x7f])(\C) | |
| 1017 | (?=[\x80-\x{7ff}])(\C)(\C) | |
| 1018 | (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | |
| 1019 | (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) |
| 1020 | </pre> |
| 1021 | A group that starts with (?| resets the capturing parentheses numbers in each |
| 1022 | alternative (see |
| 1023 | <a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a> |
| 1024 | below). The assertions at the start of each branch check the next UTF-8 |
| 1025 | character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The |
| 1026 | character's individual bytes are then captured by the appropriate number of |
| 1027 | groups. |
| 1028 | <a name="characterclass"></a></P> |
| 1029 | <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br> |
| 1030 | <P> |
| 1031 | An opening square bracket introduces a character class, terminated by a closing |
| 1032 | square bracket. A closing square bracket on its own is not special by default. |
| 1033 | However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square |
| 1034 | bracket causes a compile-time error. If a closing square bracket is required as |
| 1035 | a member of the class, it should be the first data character in the class |
| 1036 | (after an initial circumflex, if present) or escaped with a backslash. |
| 1037 | </P> |
| 1038 | <P> |
| 1039 | A character class matches a single character in the subject. In UTF-8 mode, the |
| 1040 | character may be more than one byte long. A matched character must be in the |
| 1041 | set of characters defined by the class, unless the first character in the class |
| 1042 | definition is a circumflex, in which case the subject character must not be in |
| 1043 | the set defined by the class. If a circumflex is actually required as a member |
| 1044 | of the class, ensure it is not the first character, or escape it with a |
| 1045 | backslash. |
| 1046 | </P> |
| 1047 | <P> |
| 1048 | For example, the character class [aeiou] matches any lower case vowel, while |
| 1049 | [^aeiou] matches any character that is not a lower case vowel. Note that a |
| 1050 | circumflex is just a convenient notation for specifying the characters that |
| 1051 | are in the class by enumerating those that are not. A class that starts with a |
| 1052 | circumflex is not an assertion; it still consumes a character from the subject |
| 1053 | string, and therefore it fails if the current pointer is at the end of the |
| 1054 | string. |
| 1055 | </P> |
| 1056 | <P> |
| 1057 | In UTF-8 mode, characters with values greater than 255 can be included in a |
| 1058 | class as a literal string of bytes, or by using the \x{ escaping mechanism. |
| 1059 | </P> |
| 1060 | <P> |
| 1061 | When caseless matching is set, any letters in a class represent both their |
| 1062 | upper case and lower case versions, so for example, a caseless [aeiou] matches |
| 1063 | "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a |
| 1064 | caseful version would. In UTF-8 mode, PCRE always understands the concept of |
| 1065 | case for characters whose values are less than 128, so caseless matching is |
| 1066 | always possible. For characters with higher values, the concept of case is |
| 1067 | supported if PCRE is compiled with Unicode property support, but not otherwise. |
| 1068 | If you want to use caseless matching in UTF8-mode for characters 128 and above, |
| 1069 | you must ensure that PCRE is compiled with Unicode property support as well as |
| 1070 | with UTF-8 support. |
| 1071 | </P> |
| 1072 | <P> |
| 1073 | Characters that might indicate line breaks are never treated in any special way |
| 1074 | when matching character classes, whatever line-ending sequence is in use, and |
| 1075 | whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class |
| 1076 | such as [^a] always matches one of these characters. |
| 1077 | </P> |
| 1078 | <P> |
| 1079 | The minus (hyphen) character can be used to specify a range of characters in a |
| 1080 | character class. For example, [d-m] matches any letter between d and m, |
| 1081 | inclusive. If a minus character is required in a class, it must be escaped with |
| 1082 | a backslash or appear in a position where it cannot be interpreted as |
| 1083 | indicating a range, typically as the first or last character in the class. |
| 1084 | </P> |
| 1085 | <P> |
| 1086 | It is not possible to have the literal character "]" as the end character of a |
| 1087 | range. A pattern such as [W-]46] is interpreted as a class of two characters |
| 1088 | ("W" and "-") followed by a literal string "46]", so it would match "W46]" or |
| 1089 | "-46]". However, if the "]" is escaped with a backslash it is interpreted as |
| 1090 | the end of range, so [W-\]46] is interpreted as a class containing a range |
| 1091 | followed by two other characters. The octal or hexadecimal representation of |
| 1092 | "]" can also be used to end a range. |
| 1093 | </P> |
| 1094 | <P> |
| 1095 | Ranges operate in the collating sequence of character values. They can also be |
| 1096 | used for characters specified numerically, for example [\000-\037]. In UTF-8 |
| 1097 | mode, ranges can include characters whose values are greater than 255, for |
| 1098 | example [\x{100}-\x{2ff}]. |
| 1099 | </P> |
| 1100 | <P> |
| 1101 | If a range that includes letters is used when caseless matching is set, it |
| 1102 | matches the letters in either case. For example, [W-c] is equivalent to |
| 1103 | [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character |
| 1104 | tables for a French locale are in use, [\xc8-\xcb] matches accented E |
| 1105 | characters in both cases. In UTF-8 mode, PCRE supports the concept of case for |
| 1106 | characters with values greater than 128 only when it is compiled with Unicode |
| 1107 | property support. |
| 1108 | </P> |
| 1109 | <P> |
| 1110 | The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, |
| 1111 | \V, \w, and \W may appear in a character class, and add the characters that |
| 1112 | they match to the class. For example, [\dABCDEF] matches any hexadecimal |
| 1113 | digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \d, \s, \w |
| 1114 | and their upper case partners, just as it does when they appear outside a |
| 1115 | character class, as described in the section entitled |
| 1116 | <a href="#genericchartypes">"Generic character types"</a> |
| 1117 | above. The escape sequence \b has a different meaning inside a character |
| 1118 | class; it matches the backspace character. The sequences \B, \N, \R, and \X |
| 1119 | are not special inside a character class. Like any other unrecognized escape |
| 1120 | sequences, they are treated as the literal characters "B", "N", "R", and "X" by |
| 1121 | default, but cause an error if the PCRE_EXTRA option is set. |
| 1122 | </P> |
| 1123 | <P> |
| 1124 | A circumflex can conveniently be used with the upper case character types to |
| 1125 | specify a more restricted set of characters than the matching lower case type. |
| 1126 | For example, the class [^\W_] matches any letter or digit, but not underscore, |
| 1127 | whereas [\w] includes underscore. A positive character class should be read as |
| 1128 | "something OR something OR ..." and a negative class as "NOT something AND NOT |
| 1129 | something AND NOT ...". |
| 1130 | </P> |
| 1131 | <P> |
| 1132 | The only metacharacters that are recognized in character classes are backslash, |
| 1133 | hyphen (only where it can be interpreted as specifying a range), circumflex |
| 1134 | (only at the start), opening square bracket (only when it can be interpreted as |
| 1135 | introducing a POSIX class name - see the next section), and the terminating |
| 1136 | closing square bracket. However, escaping other non-alphanumeric characters |
| 1137 | does no harm. |
| 1138 | </P> |
| 1139 | <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br> |
| 1140 | <P> |
| 1141 | Perl supports the POSIX notation for character classes. This uses names |
| 1142 | enclosed by [: and :] within the enclosing square brackets. PCRE also supports |
| 1143 | this notation. For example, |
| 1144 | <pre> |
| 1145 | [01[:alpha:]%] |
| 1146 | </pre> |
| 1147 | matches "0", "1", any alphabetic character, or "%". The supported class names |
| 1148 | are: |
| 1149 | <pre> |
| 1150 | alnum letters and digits |
| 1151 | alpha letters |
| 1152 | ascii character codes 0 - 127 |
| 1153 | blank space or tab only |
| 1154 | cntrl control characters |
| 1155 | digit decimal digits (same as \d) |
| 1156 | graph printing characters, excluding space |
| 1157 | lower lower case letters |
| 1158 | print printing characters, including space |
| 1159 | punct printing characters, excluding letters and digits and space |
| 1160 | space white space (not quite the same as \s) |
| 1161 | upper upper case letters |
| 1162 | word "word" characters (same as \w) |
| 1163 | xdigit hexadecimal digits |
| 1164 | </pre> |
| 1165 | The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and |
| 1166 | space (32). Notice that this list includes the VT character (code 11). This |
| 1167 | makes "space" different to \s, which does not include VT (for Perl |
| 1168 | compatibility). |
| 1169 | </P> |
| 1170 | <P> |
| 1171 | The name "word" is a Perl extension, and "blank" is a GNU extension from Perl |
| 1172 | 5.8. Another Perl extension is negation, which is indicated by a ^ character |
| 1173 | after the colon. For example, |
| 1174 | <pre> |
| 1175 | [12[:^digit:]] |
| 1176 | </pre> |
| 1177 | matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX |
| 1178 | syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not |
| 1179 | supported, and an error is given if they are encountered. |
| 1180 | </P> |
| 1181 | <P> |
| 1182 | By default, in UTF-8 mode, characters with values greater than 128 do not match |
| 1183 | any of the POSIX character classes. However, if the PCRE_UCP option is passed |
| 1184 | to <b>pcre_compile()</b>, some of the classes are changed so that Unicode |
| 1185 | character properties are used. This is achieved by replacing the POSIX classes |
| 1186 | by other sequences, as follows: |
| 1187 | <pre> |
| 1188 | [:alnum:] becomes \p{Xan} |
| 1189 | [:alpha:] becomes \p{L} |
| 1190 | [:blank:] becomes \h |
| 1191 | [:digit:] becomes \p{Nd} |
| 1192 | [:lower:] becomes \p{Ll} |
| 1193 | [:space:] becomes \p{Xps} |
| 1194 | [:upper:] becomes \p{Lu} |
| 1195 | [:word:] becomes \p{Xwd} |
| 1196 | </pre> |
| 1197 | Negated versions, such as [:^alpha:] use \P instead of \p. The other POSIX |
| 1198 | classes are unchanged, and match only characters with code points less than |
| 1199 | 128. |
| 1200 | </P> |
| 1201 | <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br> |
| 1202 | <P> |
| 1203 | Vertical bar characters are used to separate alternative patterns. For example, |
| 1204 | the pattern |
| 1205 | <pre> |
| 1206 | gilbert|sullivan |
| 1207 | </pre> |
| 1208 | matches either "gilbert" or "sullivan". Any number of alternatives may appear, |
| 1209 | and an empty alternative is permitted (matching the empty string). The matching |
| 1210 | process tries each alternative in turn, from left to right, and the first one |
| 1211 | that succeeds is used. If the alternatives are within a subpattern |
| 1212 | <a href="#subpattern">(defined below),</a> |
| 1213 | "succeeds" means matching the rest of the main pattern as well as the |
| 1214 | alternative in the subpattern. |
| 1215 | </P> |
| 1216 | <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br> |
| 1217 | <P> |
| 1218 | The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and |
| 1219 | PCRE_EXTENDED options (which are Perl-compatible) can be changed from within |
| 1220 | the pattern by a sequence of Perl option letters enclosed between "(?" and ")". |
| 1221 | The option letters are |
| 1222 | <pre> |
| 1223 | i for PCRE_CASELESS |
| 1224 | m for PCRE_MULTILINE |
| 1225 | s for PCRE_DOTALL |
| 1226 | x for PCRE_EXTENDED |
| 1227 | </pre> |
| 1228 | For example, (?im) sets caseless, multiline matching. It is also possible to |
| 1229 | unset these options by preceding the letter with a hyphen, and a combined |
| 1230 | setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and |
| 1231 | PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also |
| 1232 | permitted. If a letter appears both before and after the hyphen, the option is |
| 1233 | unset. |
| 1234 | </P> |
| 1235 | <P> |
| 1236 | The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be |
| 1237 | changed in the same way as the Perl-compatible options by using the characters |
| 1238 | J, U and X respectively. |
| 1239 | </P> |
| 1240 | <P> |
| 1241 | When one of these option changes occurs at top level (that is, not inside |
| 1242 | subpattern parentheses), the change applies to the remainder of the pattern |
| 1243 | that follows. If the change is placed right at the start of a pattern, PCRE |
| 1244 | extracts it into the global options (and it will therefore show up in data |
| 1245 | extracted by the <b>pcre_fullinfo()</b> function). |
| 1246 | </P> |
| 1247 | <P> |
| 1248 | An option change within a subpattern (see below for a description of |
| 1249 | subpatterns) affects only that part of the subpattern that follows it, so |
| 1250 | <pre> |
| 1251 | (a(?i)b)c |
| 1252 | </pre> |
| 1253 | matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). |
| 1254 | By this means, options can be made to have different settings in different |
| 1255 | parts of the pattern. Any changes made in one alternative do carry on |
| 1256 | into subsequent branches within the same subpattern. For example, |
| 1257 | <pre> |
| 1258 | (a(?i)b|c) |
| 1259 | </pre> |
| 1260 | matches "ab", "aB", "c", and "C", even though when matching "C" the first |
| 1261 | branch is abandoned before the option setting. This is because the effects of |
| 1262 | option settings happen at compile time. There would be some very weird |
| 1263 | behaviour otherwise. |
| 1264 | </P> |
| 1265 | <P> |
| 1266 | <b>Note:</b> There are other PCRE-specific options that can be set by the |
| 1267 | application when the compile or match functions are called. In some cases the |
| 1268 | pattern can contain special leading sequences such as (*CRLF) to override what |
| 1269 | the application has set or what has been defaulted. Details are given in the |
| 1270 | section entitled |
| 1271 | <a href="#newlineseq">"Newline sequences"</a> |
| 1272 | above. There are also the (*UTF8) and (*UCP) leading sequences that can be used |
| 1273 | to set UTF-8 and Unicode property modes; they are equivalent to setting the |
| 1274 | PCRE_UTF8 and the PCRE_UCP options, respectively. |
| 1275 | <a name="subpattern"></a></P> |
| 1276 | <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br> |
| 1277 | <P> |
| 1278 | Subpatterns are delimited by parentheses (round brackets), which can be nested. |
| 1279 | Turning part of a pattern into a subpattern does two things: |
| 1280 | <br> |
| 1281 | <br> |
| 1282 | 1. It localizes a set of alternatives. For example, the pattern |
| 1283 | <pre> |
| 1284 | cat(aract|erpillar|) |
| 1285 | </pre> |
| 1286 | matches "cataract", "caterpillar", or "cat". Without the parentheses, it would |
| 1287 | match "cataract", "erpillar" or an empty string. |
| 1288 | <br> |
| 1289 | <br> |
| 1290 | 2. It sets up the subpattern as a capturing subpattern. This means that, when |
| 1291 | the whole pattern matches, that portion of the subject string that matched the |
| 1292 | subpattern is passed back to the caller via the <i>ovector</i> argument of |
| 1293 | <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting |
| 1294 | from 1) to obtain numbers for the capturing subpatterns. For example, if the |
| 1295 | string "the red king" is matched against the pattern |
| 1296 | <pre> |
| 1297 | the ((red|white) (king|queen)) |
| 1298 | </pre> |
| 1299 | the captured substrings are "red king", "red", and "king", and are numbered 1, |
| 1300 | 2, and 3, respectively. |
| 1301 | </P> |
| 1302 | <P> |
| 1303 | The fact that plain parentheses fulfil two functions is not always helpful. |
| 1304 | There are often times when a grouping subpattern is required without a |
| 1305 | capturing requirement. If an opening parenthesis is followed by a question mark |
| 1306 | and a colon, the subpattern does not do any capturing, and is not counted when |
| 1307 | computing the number of any subsequent capturing subpatterns. For example, if |
| 1308 | the string "the white queen" is matched against the pattern |
| 1309 | <pre> |
| 1310 | the ((?:red|white) (king|queen)) |
| 1311 | </pre> |
| 1312 | the captured substrings are "white queen" and "queen", and are numbered 1 and |
| 1313 | 2. The maximum number of capturing subpatterns is 65535. |
| 1314 | </P> |
| 1315 | <P> |
| 1316 | As a convenient shorthand, if any option settings are required at the start of |
| 1317 | a non-capturing subpattern, the option letters may appear between the "?" and |
| 1318 | the ":". Thus the two patterns |
| 1319 | <pre> |
| 1320 | (?i:saturday|sunday) |
| 1321 | (?:(?i)saturday|sunday) |
| 1322 | </pre> |
| 1323 | match exactly the same set of strings. Because alternative branches are tried |
| 1324 | from left to right, and options are not reset until the end of the subpattern |
| 1325 | is reached, an option setting in one branch does affect subsequent branches, so |
| 1326 | the above patterns match "SUNDAY" as well as "Saturday". |
| 1327 | <a name="dupsubpatternnumber"></a></P> |
| 1328 | <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br> |
| 1329 | <P> |
| 1330 | Perl 5.10 introduced a feature whereby each alternative in a subpattern uses |
| 1331 | the same numbers for its capturing parentheses. Such a subpattern starts with |
| 1332 | (?| and is itself a non-capturing subpattern. For example, consider this |
| 1333 | pattern: |
| 1334 | <pre> |
| 1335 | (?|(Sat)ur|(Sun))day |
| 1336 | </pre> |
| 1337 | Because the two alternatives are inside a (?| group, both sets of capturing |
| 1338 | parentheses are numbered one. Thus, when the pattern matches, you can look |
| 1339 | at captured substring number one, whichever alternative matched. This construct |
| 1340 | is useful when you want to capture part, but not all, of one of a number of |
| 1341 | alternatives. Inside a (?| group, parentheses are numbered as usual, but the |
| 1342 | number is reset at the start of each branch. The numbers of any capturing |
| 1343 | parentheses that follow the subpattern start after the highest number used in |
| 1344 | any branch. The following example is taken from the Perl documentation. The |
| 1345 | numbers underneath show in which buffer the captured content will be stored. |
| 1346 | <pre> |
| 1347 | # before ---------------branch-reset----------- after |
| 1348 | / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x |
| 1349 | # 1 2 2 3 2 3 4 |
| 1350 | </pre> |
| 1351 | A back reference to a numbered subpattern uses the most recent value that is |
| 1352 | set for that number by any subpattern. The following pattern matches "abcabc" |
| 1353 | or "defdef": |
| 1354 | <pre> |
| 1355 | /(?|(abc)|(def))\1/ |
| 1356 | </pre> |
| 1357 | In contrast, a subroutine call to a numbered subpattern always refers to the |
| 1358 | first one in the pattern with the given number. The following pattern matches |
| 1359 | "abcabc" or "defabc": |
| 1360 | <pre> |
| 1361 | /(?|(abc)|(def))(?1)/ |
| 1362 | </pre> |
| 1363 | If a |
| 1364 | <a href="#conditions">condition test</a> |
| 1365 | for a subpattern's having matched refers to a non-unique number, the test is |
| 1366 | true if any of the subpatterns of that number have matched. |
| 1367 | </P> |
| 1368 | <P> |
| 1369 | An alternative approach to using this "branch reset" feature is to use |
| 1370 | duplicate named subpatterns, as described in the next section. |
| 1371 | </P> |
| 1372 | <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br> |
| 1373 | <P> |
| 1374 | Identifying capturing parentheses by number is simple, but it can be very hard |
| 1375 | to keep track of the numbers in complicated regular expressions. Furthermore, |
| 1376 | if an expression is modified, the numbers may change. To help with this |
| 1377 | difficulty, PCRE supports the naming of subpatterns. This feature was not |
| 1378 | added to Perl until release 5.10. Python had the feature earlier, and PCRE |
| 1379 | introduced it at release 4.0, using the Python syntax. PCRE now supports both |
| 1380 | the Perl and the Python syntax. Perl allows identically numbered subpatterns to |
| 1381 | have different names, but PCRE does not. |
| 1382 | </P> |
| 1383 | <P> |
| 1384 | In PCRE, a subpattern can be named in one of three ways: (?<name>...) or |
| 1385 | (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing |
| 1386 | parentheses from other parts of the pattern, such as |
| 1387 | <a href="#backreferences">back references,</a> |
| 1388 | <a href="#recursion">recursion,</a> |
| 1389 | and |
| 1390 | <a href="#conditions">conditions,</a> |
| 1391 | can be made by name as well as by number. |
| 1392 | </P> |
| 1393 | <P> |
| 1394 | Names consist of up to 32 alphanumeric characters and underscores. Named |
| 1395 | capturing parentheses are still allocated numbers as well as names, exactly as |
| 1396 | if the names were not present. The PCRE API provides function calls for |
| 1397 | extracting the name-to-number translation table from a compiled pattern. There |
| 1398 | is also a convenience function for extracting a captured substring by name. |
| 1399 | </P> |
| 1400 | <P> |
| 1401 | By default, a name must be unique within a pattern, but it is possible to relax |
| 1402 | this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate |
| 1403 | names are also always permitted for subpatterns with the same number, set up as |
| 1404 | described in the previous section.) Duplicate names can be useful for patterns |
| 1405 | where only one instance of the named parentheses can match. Suppose you want to |
| 1406 | match the name of a weekday, either as a 3-letter abbreviation or as the full |
| 1407 | name, and in both cases you want to extract the abbreviation. This pattern |
| 1408 | (ignoring the line breaks) does the job: |
| 1409 | <pre> |
| 1410 | (?<DN>Mon|Fri|Sun)(?:day)?| |
| 1411 | (?<DN>Tue)(?:sday)?| |
| 1412 | (?<DN>Wed)(?:nesday)?| |
| 1413 | (?<DN>Thu)(?:rsday)?| |
| 1414 | (?<DN>Sat)(?:urday)? |
| 1415 | </pre> |
| 1416 | There are five capturing substrings, but only one is ever set after a match. |
| 1417 | (An alternative way of solving this problem is to use a "branch reset" |
| 1418 | subpattern, as described in the previous section.) |
| 1419 | </P> |
| 1420 | <P> |
| 1421 | The convenience function for extracting the data by name returns the substring |
| 1422 | for the first (and in this example, the only) subpattern of that name that |
| 1423 | matched. This saves searching to find which numbered subpattern it was. |
| 1424 | </P> |
| 1425 | <P> |
| 1426 | If you make a back reference to a non-unique named subpattern from elsewhere in |
| 1427 | the pattern, the one that corresponds to the first occurrence of the name is |
| 1428 | used. In the absence of duplicate numbers (see the previous section) this is |
| 1429 | the one with the lowest number. If you use a named reference in a condition |
| 1430 | test (see the |
| 1431 | <a href="#conditions">section about conditions</a> |
| 1432 | below), either to check whether a subpattern has matched, or to check for |
| 1433 | recursion, all subpatterns with the same name are tested. If the condition is |
| 1434 | true for any one of them, the overall condition is true. This is the same |
| 1435 | behaviour as testing by number. For further details of the interfaces for |
| 1436 | handling named subpatterns, see the |
| 1437 | <a href="pcreapi.html"><b>pcreapi</b></a> |
| 1438 | documentation. |
| 1439 | </P> |
| 1440 | <P> |
| 1441 | <b>Warning:</b> You cannot use different names to distinguish between two |
| 1442 | subpatterns with the same number because PCRE uses only the numbers when |
| 1443 | matching. For this reason, an error is given at compile time if different names |
| 1444 | are given to subpatterns with the same number. However, you can give the same |
| 1445 | name to subpatterns with the same number, even when PCRE_DUPNAMES is not set. |
| 1446 | </P> |
| 1447 | <br><a name="SEC15" href="#TOC1">REPETITION</a><br> |
| 1448 | <P> |
| 1449 | Repetition is specified by quantifiers, which can follow any of the following |
| 1450 | items: |
| 1451 | <pre> |
| 1452 | a literal data character |
| 1453 | the dot metacharacter |
| 1454 | the \C escape sequence |
| 1455 | the \X escape sequence (in UTF-8 mode with Unicode properties) |
| 1456 | the \R escape sequence |
| 1457 | an escape such as \d or \pL that matches a single character |
| 1458 | a character class |
| 1459 | a back reference (see next section) |
| 1460 | a parenthesized subpattern (including assertions) |
| 1461 | a subroutine call to a subpattern (recursive or otherwise) |
| 1462 | </pre> |
| 1463 | The general repetition quantifier specifies a minimum and maximum number of |
| 1464 | permitted matches, by giving the two numbers in curly brackets (braces), |
| 1465 | separated by a comma. The numbers must be less than 65536, and the first must |
| 1466 | be less than or equal to the second. For example: |
| 1467 | <pre> |
| 1468 | z{2,4} |
| 1469 | </pre> |
| 1470 | matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special |
| 1471 | character. If the second number is omitted, but the comma is present, there is |
| 1472 | no upper limit; if the second number and the comma are both omitted, the |
| 1473 | quantifier specifies an exact number of required matches. Thus |
| 1474 | <pre> |
| 1475 | [aeiou]{3,} |
| 1476 | </pre> |
| 1477 | matches at least 3 successive vowels, but may match many more, while |
| 1478 | <pre> |
| 1479 | \d{8} |
| 1480 | </pre> |
| 1481 | matches exactly 8 digits. An opening curly bracket that appears in a position |
| 1482 | where a quantifier is not allowed, or one that does not match the syntax of a |
| 1483 | quantifier, is taken as a literal character. For example, {,6} is not a |
| 1484 | quantifier, but a literal string of four characters. |
| 1485 | </P> |
| 1486 | <P> |
| 1487 | In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual |
| 1488 | bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of |
| 1489 | which is represented by a two-byte sequence. Similarly, when Unicode property |
| 1490 | support is available, \X{3} matches three Unicode extended sequences, each of |
| 1491 | which may be several bytes long (and they may be of different lengths). |
| 1492 | </P> |
| 1493 | <P> |
| 1494 | The quantifier {0} is permitted, causing the expression to behave as if the |
| 1495 | previous item and the quantifier were not present. This may be useful for |
| 1496 | subpatterns that are referenced as |
| 1497 | <a href="#subpatternsassubroutines">subroutines</a> |
| 1498 | from elsewhere in the pattern (but see also the section entitled |
| 1499 | <a href="#subdefine">"Defining subpatterns for use by reference only"</a> |
| 1500 | below). Items other than subpatterns that have a {0} quantifier are omitted |
| 1501 | from the compiled pattern. |
| 1502 | </P> |
| 1503 | <P> |
| 1504 | For convenience, the three most common quantifiers have single-character |
| 1505 | abbreviations: |
| 1506 | <pre> |
| 1507 | * is equivalent to {0,} |
| 1508 | + is equivalent to {1,} |
| 1509 | ? is equivalent to {0,1} |
| 1510 | </pre> |
| 1511 | It is possible to construct infinite loops by following a subpattern that can |
| 1512 | match no characters with a quantifier that has no upper limit, for example: |
| 1513 | <pre> |
| 1514 | (a?)* |
| 1515 | </pre> |
| 1516 | Earlier versions of Perl and PCRE used to give an error at compile time for |
| 1517 | such patterns. However, because there are cases where this can be useful, such |
| 1518 | patterns are now accepted, but if any repetition of the subpattern does in fact |
| 1519 | match no characters, the loop is forcibly broken. |
| 1520 | </P> |
| 1521 | <P> |
| 1522 | By default, the quantifiers are "greedy", that is, they match as much as |
| 1523 | possible (up to the maximum number of permitted times), without causing the |
| 1524 | rest of the pattern to fail. The classic example of where this gives problems |
| 1525 | is in trying to match comments in C programs. These appear between /* and */ |
| 1526 | and within the comment, individual * and / characters may appear. An attempt to |
| 1527 | match C comments by applying the pattern |
| 1528 | <pre> |
| 1529 | /\*.*\*/ |
| 1530 | </pre> |
| 1531 | to the string |
| 1532 | <pre> |
| 1533 | /* first comment */ not comment /* second comment */ |
| 1534 | </pre> |
| 1535 | fails, because it matches the entire string owing to the greediness of the .* |
| 1536 | item. |
| 1537 | </P> |
| 1538 | <P> |
| 1539 | However, if a quantifier is followed by a question mark, it ceases to be |
| 1540 | greedy, and instead matches the minimum number of times possible, so the |
| 1541 | pattern |
| 1542 | <pre> |
| 1543 | /\*.*?\*/ |
| 1544 | </pre> |
| 1545 | does the right thing with the C comments. The meaning of the various |
| 1546 | quantifiers is not otherwise changed, just the preferred number of matches. |
| 1547 | Do not confuse this use of question mark with its use as a quantifier in its |
| 1548 | own right. Because it has two uses, it can sometimes appear doubled, as in |
| 1549 | <pre> |
| 1550 | \d??\d |
| 1551 | </pre> |
| 1552 | which matches one digit by preference, but can match two if that is the only |
| 1553 | way the rest of the pattern matches. |
| 1554 | </P> |
| 1555 | <P> |
| 1556 | If the PCRE_UNGREEDY option is set (an option that is not available in Perl), |
| 1557 | the quantifiers are not greedy by default, but individual ones can be made |
| 1558 | greedy by following them with a question mark. In other words, it inverts the |
| 1559 | default behaviour. |
| 1560 | </P> |
| 1561 | <P> |
| 1562 | When a parenthesized subpattern is quantified with a minimum repeat count that |
| 1563 | is greater than 1 or with a limited maximum, more memory is required for the |
| 1564 | compiled pattern, in proportion to the size of the minimum or maximum. |
| 1565 | </P> |
| 1566 | <P> |
| 1567 | If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent |
| 1568 | to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is |
| 1569 | implicitly anchored, because whatever follows will be tried against every |
| 1570 | character position in the subject string, so there is no point in retrying the |
| 1571 | overall match at any position after the first. PCRE normally treats such a |
| 1572 | pattern as though it were preceded by \A. |
| 1573 | </P> |
| 1574 | <P> |
| 1575 | In cases where it is known that the subject string contains no newlines, it is |
| 1576 | worth setting PCRE_DOTALL in order to obtain this optimization, or |
| 1577 | alternatively using ^ to indicate anchoring explicitly. |
| 1578 | </P> |
| 1579 | <P> |
| 1580 | However, there is one situation where the optimization cannot be used. When .* |
| 1581 | is inside capturing parentheses that are the subject of a back reference |
| 1582 | elsewhere in the pattern, a match at the start may fail where a later one |
| 1583 | succeeds. Consider, for example: |
| 1584 | <pre> |
| 1585 | (.*)abc\1 |
| 1586 | </pre> |
| 1587 | If the subject is "xyz123abc123" the match point is the fourth character. For |
| 1588 | this reason, such a pattern is not implicitly anchored. |
| 1589 | </P> |
| 1590 | <P> |
| 1591 | When a capturing subpattern is repeated, the value captured is the substring |
| 1592 | that matched the final iteration. For example, after |
| 1593 | <pre> |
| 1594 | (tweedle[dume]{3}\s*)+ |
| 1595 | </pre> |
| 1596 | has matched "tweedledum tweedledee" the value of the captured substring is |
| 1597 | "tweedledee". However, if there are nested capturing subpatterns, the |
| 1598 | corresponding captured values may have been set in previous iterations. For |
| 1599 | example, after |
| 1600 | <pre> |
| 1601 | /(a|(b))+/ |
| 1602 | </pre> |
| 1603 | matches "aba" the value of the second captured substring is "b". |
| 1604 | <a name="atomicgroup"></a></P> |
| 1605 | <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br> |
| 1606 | <P> |
| 1607 | With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") |
| 1608 | repetition, failure of what follows normally causes the repeated item to be |
| 1609 | re-evaluated to see if a different number of repeats allows the rest of the |
| 1610 | pattern to match. Sometimes it is useful to prevent this, either to change the |
| 1611 | nature of the match, or to cause it fail earlier than it otherwise might, when |
| 1612 | the author of the pattern knows there is no point in carrying on. |
| 1613 | </P> |
| 1614 | <P> |
| 1615 | Consider, for example, the pattern \d+foo when applied to the subject line |
| 1616 | <pre> |
| 1617 | 123456bar |
| 1618 | </pre> |
| 1619 | After matching all 6 digits and then failing to match "foo", the normal |
| 1620 | action of the matcher is to try again with only 5 digits matching the \d+ |
| 1621 | item, and then with 4, and so on, before ultimately failing. "Atomic grouping" |
| 1622 | (a term taken from Jeffrey Friedl's book) provides the means for specifying |
| 1623 | that once a subpattern has matched, it is not to be re-evaluated in this way. |
| 1624 | </P> |
| 1625 | <P> |
| 1626 | If we use atomic grouping for the previous example, the matcher gives up |
| 1627 | immediately on failing to match "foo" the first time. The notation is a kind of |
| 1628 | special parenthesis, starting with (?> as in this example: |
| 1629 | <pre> |
| 1630 | (?>\d+)foo |
| 1631 | </pre> |
| 1632 | This kind of parenthesis "locks up" the part of the pattern it contains once |
| 1633 | it has matched, and a failure further into the pattern is prevented from |
| 1634 | backtracking into it. Backtracking past it to previous items, however, works as |
| 1635 | normal. |
| 1636 | </P> |
| 1637 | <P> |
| 1638 | An alternative description is that a subpattern of this type matches the string |
| 1639 | of characters that an identical standalone pattern would match, if anchored at |
| 1640 | the current point in the subject string. |
| 1641 | </P> |
| 1642 | <P> |
| 1643 | Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as |
| 1644 | the above example can be thought of as a maximizing repeat that must swallow |
| 1645 | everything it can. So, while both \d+ and \d+? are prepared to adjust the |
| 1646 | number of digits they match in order to make the rest of the pattern match, |
| 1647 | (?>\d+) can only match an entire sequence of digits. |
| 1648 | </P> |
| 1649 | <P> |
| 1650 | Atomic groups in general can of course contain arbitrarily complicated |
| 1651 | subpatterns, and can be nested. However, when the subpattern for an atomic |
| 1652 | group is just a single repeated item, as in the example above, a simpler |
| 1653 | notation, called a "possessive quantifier" can be used. This consists of an |
| 1654 | additional + character following a quantifier. Using this notation, the |
| 1655 | previous example can be rewritten as |
| 1656 | <pre> |
| 1657 | \d++foo |
| 1658 | </pre> |
| 1659 | Note that a possessive quantifier can be used with an entire group, for |
| 1660 | example: |
| 1661 | <pre> |
| 1662 | (abc|xyz){2,3}+ |
| 1663 | </pre> |
| 1664 | Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY |
| 1665 | option is ignored. They are a convenient notation for the simpler forms of |
| 1666 | atomic group. However, there is no difference in the meaning of a possessive |
| 1667 | quantifier and the equivalent atomic group, though there may be a performance |
| 1668 | difference; possessive quantifiers should be slightly faster. |
| 1669 | </P> |
| 1670 | <P> |
| 1671 | The possessive quantifier syntax is an extension to the Perl 5.8 syntax. |
| 1672 | Jeffrey Friedl originated the idea (and the name) in the first edition of his |
| 1673 | book. Mike McCloskey liked it, so implemented it when he built Sun's Java |
| 1674 | package, and PCRE copied it from there. It ultimately found its way into Perl |
| 1675 | at release 5.10. |
| 1676 | </P> |
| 1677 | <P> |
| 1678 | PCRE has an optimization that automatically "possessifies" certain simple |
| 1679 | pattern constructs. For example, the sequence A+B is treated as A++B because |
| 1680 | there is no point in backtracking into a sequence of A's when B must follow. |
| 1681 | </P> |
| 1682 | <P> |
| 1683 | When a pattern contains an unlimited repeat inside a subpattern that can itself |
| 1684 | be repeated an unlimited number of times, the use of an atomic group is the |
| 1685 | only way to avoid some failing matches taking a very long time indeed. The |
| 1686 | pattern |
| 1687 | <pre> |
| 1688 | (\D+|<\d+>)*[!?] |
| 1689 | </pre> |
| 1690 | matches an unlimited number of substrings that either consist of non-digits, or |
| 1691 | digits enclosed in <>, followed by either ! or ?. When it matches, it runs |
| 1692 | quickly. However, if it is applied to |
| 1693 | <pre> |
| 1694 | aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa |
| 1695 | </pre> |
| 1696 | it takes a long time before reporting failure. This is because the string can |
| 1697 | be divided between the internal \D+ repeat and the external * repeat in a |
| 1698 | large number of ways, and all have to be tried. (The example uses [!?] rather |
| 1699 | than a single character at the end, because both PCRE and Perl have an |
| 1700 | optimization that allows for fast failure when a single character is used. They |
| 1701 | remember the last single character that is required for a match, and fail early |
| 1702 | if it is not present in the string.) If the pattern is changed so that it uses |
| 1703 | an atomic group, like this: |
| 1704 | <pre> |
| 1705 | ((?>\D+)|<\d+>)*[!?] |
| 1706 | </pre> |
| 1707 | sequences of non-digits cannot be broken, and failure happens quickly. |
| 1708 | <a name="backreferences"></a></P> |
| 1709 | <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br> |
| 1710 | <P> |
| 1711 | Outside a character class, a backslash followed by a digit greater than 0 (and |
| 1712 | possibly further digits) is a back reference to a capturing subpattern earlier |
| 1713 | (that is, to its left) in the pattern, provided there have been that many |
| 1714 | previous capturing left parentheses. |
| 1715 | </P> |
| 1716 | <P> |
| 1717 | However, if the decimal number following the backslash is less than 10, it is |
| 1718 | always taken as a back reference, and causes an error only if there are not |
| 1719 | that many capturing left parentheses in the entire pattern. In other words, the |
| 1720 | parentheses that are referenced need not be to the left of the reference for |
| 1721 | numbers less than 10. A "forward back reference" of this type can make sense |
| 1722 | when a repetition is involved and the subpattern to the right has participated |
| 1723 | in an earlier iteration. |
| 1724 | </P> |
| 1725 | <P> |
| 1726 | It is not possible to have a numerical "forward back reference" to a subpattern |
| 1727 | whose number is 10 or more using this syntax because a sequence such as \50 is |
| 1728 | interpreted as a character defined in octal. See the subsection entitled |
| 1729 | "Non-printing characters" |
| 1730 | <a href="#digitsafterbackslash">above</a> |
| 1731 | for further details of the handling of digits following a backslash. There is |
| 1732 | no such problem when named parentheses are used. A back reference to any |
| 1733 | subpattern is possible using named parentheses (see below). |
| 1734 | </P> |
| 1735 | <P> |
| 1736 | Another way of avoiding the ambiguity inherent in the use of digits following a |
| 1737 | backslash is to use the \g escape sequence. This escape must be followed by an |
| 1738 | unsigned number or a negative number, optionally enclosed in braces. These |
| 1739 | examples are all identical: |
| 1740 | <pre> |
| 1741 | (ring), \1 |
| 1742 | (ring), \g1 |
| 1743 | (ring), \g{1} |
| 1744 | </pre> |
| 1745 | An unsigned number specifies an absolute reference without the ambiguity that |
| 1746 | is present in the older syntax. It is also useful when literal digits follow |
| 1747 | the reference. A negative number is a relative reference. Consider this |
| 1748 | example: |
| 1749 | <pre> |
| 1750 | (abc(def)ghi)\g{-1} |
| 1751 | </pre> |
| 1752 | The sequence \g{-1} is a reference to the most recently started capturing |
| 1753 | subpattern before \g, that is, is it equivalent to \2 in this example. |
| 1754 | Similarly, \g{-2} would be equivalent to \1. The use of relative references |
| 1755 | can be helpful in long patterns, and also in patterns that are created by |
| 1756 | joining together fragments that contain references within themselves. |
| 1757 | </P> |
| 1758 | <P> |
| 1759 | A back reference matches whatever actually matched the capturing subpattern in |
| 1760 | the current subject string, rather than anything matching the subpattern |
| 1761 | itself (see |
| 1762 | <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a> |
| 1763 | below for a way of doing that). So the pattern |
| 1764 | <pre> |
| 1765 | (sens|respons)e and \1ibility |
| 1766 | </pre> |
| 1767 | matches "sense and sensibility" and "response and responsibility", but not |
| 1768 | "sense and responsibility". If caseful matching is in force at the time of the |
| 1769 | back reference, the case of letters is relevant. For example, |
| 1770 | <pre> |
| 1771 | ((?i)rah)\s+\1 |
| 1772 | </pre> |
| 1773 | matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original |
| 1774 | capturing subpattern is matched caselessly. |
| 1775 | </P> |
| 1776 | <P> |
| 1777 | There are several different ways of writing back references to named |
| 1778 | subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or |
| 1779 | \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified |
| 1780 | back reference syntax, in which \g can be used for both numeric and named |
| 1781 | references, is also supported. We could rewrite the above example in any of |
| 1782 | the following ways: |
| 1783 | <pre> |
| 1784 | (?<p1>(?i)rah)\s+\k<p1> |
| 1785 | (?'p1'(?i)rah)\s+\k{p1} |
| 1786 | (?P<p1>(?i)rah)\s+(?P=p1) |
| 1787 | (?<p1>(?i)rah)\s+\g{p1} |
| 1788 | </pre> |
| 1789 | A subpattern that is referenced by name may appear in the pattern before or |
| 1790 | after the reference. |
| 1791 | </P> |
| 1792 | <P> |
| 1793 | There may be more than one back reference to the same subpattern. If a |
| 1794 | subpattern has not actually been used in a particular match, any back |
| 1795 | references to it always fail by default. For example, the pattern |
| 1796 | <pre> |
| 1797 | (a|(bc))\2 |
| 1798 | </pre> |
| 1799 | always fails if it starts to match "a" rather than "bc". However, if the |
| 1800 | PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an |
| 1801 | unset value matches an empty string. |
| 1802 | </P> |
| 1803 | <P> |
| 1804 | Because there may be many capturing parentheses in a pattern, all digits |
| 1805 | following a backslash are taken as part of a potential back reference number. |
| 1806 | If the pattern continues with a digit character, some delimiter must be used to |
| 1807 | terminate the back reference. If the PCRE_EXTENDED option is set, this can be |
| 1808 | whitespace. Otherwise, the \g{ syntax or an empty comment (see |
| 1809 | <a href="#comments">"Comments"</a> |
| 1810 | below) can be used. |
| 1811 | </P> |
| 1812 | <br><b> |
| 1813 | Recursive back references |
| 1814 | </b><br> |
| 1815 | <P> |
| 1816 | A back reference that occurs inside the parentheses to which it refers fails |
| 1817 | when the subpattern is first used, so, for example, (a\1) never matches. |
| 1818 | However, such references can be useful inside repeated subpatterns. For |
| 1819 | example, the pattern |
| 1820 | <pre> |
| 1821 | (a|b\1)+ |
| 1822 | </pre> |
| 1823 | matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of |
| 1824 | the subpattern, the back reference matches the character string corresponding |
| 1825 | to the previous iteration. In order for this to work, the pattern must be such |
| 1826 | that the first iteration does not need to match the back reference. This can be |
| 1827 | done using alternation, as in the example above, or by a quantifier with a |
| 1828 | minimum of zero. |
| 1829 | </P> |
| 1830 | <P> |
| 1831 | Back references of this type cause the group that they reference to be treated |
| 1832 | as an |
| 1833 | <a href="#atomicgroup">atomic group.</a> |
| 1834 | Once the whole group has been matched, a subsequent matching failure cannot |
| 1835 | cause backtracking into the middle of the group. |
| 1836 | <a name="bigassertions"></a></P> |
| 1837 | <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br> |
| 1838 | <P> |
| 1839 | An assertion is a test on the characters following or preceding the current |
| 1840 | matching point that does not actually consume any characters. The simple |
| 1841 | assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described |
| 1842 | <a href="#smallassertions">above.</a> |
| 1843 | </P> |
| 1844 | <P> |
| 1845 | More complicated assertions are coded as subpatterns. There are two kinds: |
| 1846 | those that look ahead of the current position in the subject string, and those |
| 1847 | that look behind it. An assertion subpattern is matched in the normal way, |
| 1848 | except that it does not cause the current matching position to be changed. |
| 1849 | </P> |
| 1850 | <P> |
| 1851 | Assertion subpatterns are not capturing subpatterns. If such an assertion |
| 1852 | contains capturing subpatterns within it, these are counted for the purposes of |
| 1853 | numbering the capturing subpatterns in the whole pattern. However, substring |
| 1854 | capturing is carried out only for positive assertions, because it does not make |
| 1855 | sense for negative assertions. |
| 1856 | </P> |
| 1857 | <P> |
| 1858 | For compatibility with Perl, assertion subpatterns may be repeated; though |
| 1859 | it makes no sense to assert the same thing several times, the side effect of |
| 1860 | capturing parentheses may occasionally be useful. In practice, there only three |
| 1861 | cases: |
| 1862 | <br> |
| 1863 | <br> |
| 1864 | (1) If the quantifier is {0}, the assertion is never obeyed during matching. |
| 1865 | However, it may contain internal capturing parenthesized groups that are called |
| 1866 | from elsewhere via the |
| 1867 | <a href="#subpatternsassubroutines">subroutine mechanism.</a> |
| 1868 | <br> |
| 1869 | <br> |
| 1870 | (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it |
| 1871 | were {0,1}. At run time, the rest of the pattern match is tried with and |
| 1872 | without the assertion, the order depending on the greediness of the quantifier. |
| 1873 | <br> |
| 1874 | <br> |
| 1875 | (3) If the minimum repetition is greater than zero, the quantifier is ignored. |
| 1876 | The assertion is obeyed just once when encountered during matching. |
| 1877 | </P> |
| 1878 | <br><b> |
| 1879 | Lookahead assertions |
| 1880 | </b><br> |
| 1881 | <P> |
| 1882 | Lookahead assertions start with (?= for positive assertions and (?! for |
| 1883 | negative assertions. For example, |
| 1884 | <pre> |
| 1885 | \w+(?=;) |
| 1886 | </pre> |
| 1887 | matches a word followed by a semicolon, but does not include the semicolon in |
| 1888 | the match, and |
| 1889 | <pre> |
| 1890 | foo(?!bar) |
| 1891 | </pre> |
| 1892 | matches any occurrence of "foo" that is not followed by "bar". Note that the |
| 1893 | apparently similar pattern |
| 1894 | <pre> |
| 1895 | (?!foo)bar |
| 1896 | </pre> |
| 1897 | does not find an occurrence of "bar" that is preceded by something other than |
| 1898 | "foo"; it finds any occurrence of "bar" whatsoever, because the assertion |
| 1899 | (?!foo) is always true when the next three characters are "bar". A |
| 1900 | lookbehind assertion is needed to achieve the other effect. |
| 1901 | </P> |
| 1902 | <P> |
| 1903 | If you want to force a matching failure at some point in a pattern, the most |
| 1904 | convenient way to do it is with (?!) because an empty string always matches, so |
| 1905 | an assertion that requires there not to be an empty string must always fail. |
| 1906 | The backtracking control verb (*FAIL) or (*F) is a synonym for (?!). |
| 1907 | <a name="lookbehind"></a></P> |
| 1908 | <br><b> |
| 1909 | Lookbehind assertions |
| 1910 | </b><br> |
| 1911 | <P> |
| 1912 | Lookbehind assertions start with (?<= for positive assertions and (?<! for |
| 1913 | negative assertions. For example, |
| 1914 | <pre> |
| 1915 | (?<!foo)bar |
| 1916 | </pre> |
| 1917 | does find an occurrence of "bar" that is not preceded by "foo". The contents of |
| 1918 | a lookbehind assertion are restricted such that all the strings it matches must |
| 1919 | have a fixed length. However, if there are several top-level alternatives, they |
| 1920 | do not all have to have the same fixed length. Thus |
| 1921 | <pre> |
| 1922 | (?<=bullock|donkey) |
| 1923 | </pre> |
| 1924 | is permitted, but |
| 1925 | <pre> |
| 1926 | (?<!dogs?|cats?) |
| 1927 | </pre> |
| 1928 | causes an error at compile time. Branches that match different length strings |
| 1929 | are permitted only at the top level of a lookbehind assertion. This is an |
| 1930 | extension compared with Perl, which requires all branches to match the same |
| 1931 | length of string. An assertion such as |
| 1932 | <pre> |
| 1933 | (?<=ab(c|de)) |
| 1934 | </pre> |
| 1935 | is not permitted, because its single top-level branch can match two different |
| 1936 | lengths, but it is acceptable to PCRE if rewritten to use two top-level |
| 1937 | branches: |
| 1938 | <pre> |
| 1939 | (?<=abc|abde) |
| 1940 | </pre> |
| 1941 | In some cases, the escape sequence \K |
| 1942 | <a href="#resetmatchstart">(see above)</a> |
| 1943 | can be used instead of a lookbehind assertion to get round the fixed-length |
| 1944 | restriction. |
| 1945 | </P> |
| 1946 | <P> |
| 1947 | The implementation of lookbehind assertions is, for each alternative, to |
| 1948 | temporarily move the current position back by the fixed length and then try to |
| 1949 | match. If there are insufficient characters before the current position, the |
| 1950 | assertion fails. |
| 1951 | </P> |
| 1952 | <P> |
| 1953 | In UTF-8 mode, PCRE does not allow the \C escape (which matches a single byte, |
| 1954 | even in UTF-8 mode) to appear in lookbehind assertions, because it makes it |
| 1955 | impossible to calculate the length of the lookbehind. The \X and \R escapes, |
| 1956 | which can match different numbers of bytes, are also not permitted. |
| 1957 | </P> |
| 1958 | <P> |
| 1959 | <a href="#subpatternsassubroutines">"Subroutine"</a> |
| 1960 | calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long |
| 1961 | as the subpattern matches a fixed-length string. |
| 1962 | <a href="#recursion">Recursion,</a> |
| 1963 | however, is not supported. |
| 1964 | </P> |
| 1965 | <P> |
| 1966 | Possessive quantifiers can be used in conjunction with lookbehind assertions to |
| 1967 | specify efficient matching of fixed-length strings at the end of subject |
| 1968 | strings. Consider a simple pattern such as |
| 1969 | <pre> |
| 1970 | abcd$ |
| 1971 | </pre> |
| 1972 | when applied to a long string that does not match. Because matching proceeds |
| 1973 | from left to right, PCRE will look for each "a" in the subject and then see if |
| 1974 | what follows matches the rest of the pattern. If the pattern is specified as |
| 1975 | <pre> |
| 1976 | ^.*abcd$ |
| 1977 | </pre> |
| 1978 | the initial .* matches the entire string at first, but when this fails (because |
| 1979 | there is no following "a"), it backtracks to match all but the last character, |
| 1980 | then all but the last two characters, and so on. Once again the search for "a" |
| 1981 | covers the entire string, from right to left, so we are no better off. However, |
| 1982 | if the pattern is written as |
| 1983 | <pre> |
| 1984 | ^.*+(?<=abcd) |
| 1985 | </pre> |
| 1986 | there can be no backtracking for the .*+ item; it can match only the entire |
| 1987 | string. The subsequent lookbehind assertion does a single test on the last four |
| 1988 | characters. If it fails, the match fails immediately. For long strings, this |
| 1989 | approach makes a significant difference to the processing time. |
| 1990 | </P> |
| 1991 | <br><b> |
| 1992 | Using multiple assertions |
| 1993 | </b><br> |
| 1994 | <P> |
| 1995 | Several assertions (of any sort) may occur in succession. For example, |
| 1996 | <pre> |
| 1997 | (?<=\d{3})(?<!999)foo |
| 1998 | </pre> |
| 1999 | matches "foo" preceded by three digits that are not "999". Notice that each of |
| 2000 | the assertions is applied independently at the same point in the subject |
| 2001 | string. First there is a check that the previous three characters are all |
| 2002 | digits, and then there is a check that the same three characters are not "999". |
| 2003 | This pattern does <i>not</i> match "foo" preceded by six characters, the first |
| 2004 | of which are digits and the last three of which are not "999". For example, it |
| 2005 | doesn't match "123abcfoo". A pattern to do that is |
| 2006 | <pre> |
| 2007 | (?<=\d{3}...)(?<!999)foo |
| 2008 | </pre> |
| 2009 | This time the first assertion looks at the preceding six characters, checking |
| 2010 | that the first three are digits, and then the second assertion checks that the |
| 2011 | preceding three characters are not "999". |
| 2012 | </P> |
| 2013 | <P> |
| 2014 | Assertions can be nested in any combination. For example, |
| 2015 | <pre> |
| 2016 | (?<=(?<!foo)bar)baz |
| 2017 | </pre> |
| 2018 | matches an occurrence of "baz" that is preceded by "bar" which in turn is not |
| 2019 | preceded by "foo", while |
| 2020 | <pre> |
| 2021 | (?<=\d{3}(?!999)...)foo |
| 2022 | </pre> |
| 2023 | is another pattern that matches "foo" preceded by three digits and any three |
| 2024 | characters that are not "999". |
| 2025 | <a name="conditions"></a></P> |
| 2026 | <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br> |
| 2027 | <P> |
| 2028 | It is possible to cause the matching process to obey a subpattern |
| 2029 | conditionally or to choose between two alternative subpatterns, depending on |
| 2030 | the result of an assertion, or whether a specific capturing subpattern has |
| 2031 | already been matched. The two possible forms of conditional subpattern are: |
| 2032 | <pre> |
| 2033 | (?(condition)yes-pattern) |
| 2034 | (?(condition)yes-pattern|no-pattern) |
| 2035 | </pre> |
| 2036 | If the condition is satisfied, the yes-pattern is used; otherwise the |
| 2037 | no-pattern (if present) is used. If there are more than two alternatives in the |
| 2038 | subpattern, a compile-time error occurs. Each of the two alternatives may |
| 2039 | itself contain nested subpatterns of any form, including conditional |
| 2040 | subpatterns; the restriction to two alternatives applies only at the level of |
| 2041 | the condition. This pattern fragment is an example where the alternatives are |
| 2042 | complex: |
| 2043 | <pre> |
| 2044 | (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) |
| 2045 | |
| 2046 | </PRE> |
| 2047 | </P> |
| 2048 | <P> |
| 2049 | There are four kinds of condition: references to subpatterns, references to |
| 2050 | recursion, a pseudo-condition called DEFINE, and assertions. |
| 2051 | </P> |
| 2052 | <br><b> |
| 2053 | Checking for a used subpattern by number |
| 2054 | </b><br> |
| 2055 | <P> |
| 2056 | If the text between the parentheses consists of a sequence of digits, the |
| 2057 | condition is true if a capturing subpattern of that number has previously |
| 2058 | matched. If there is more than one capturing subpattern with the same number |
| 2059 | (see the earlier |
| 2060 | <a href="#recursion">section about duplicate subpattern numbers),</a> |
| 2061 | the condition is true if any of them have matched. An alternative notation is |
| 2062 | to precede the digits with a plus or minus sign. In this case, the subpattern |
| 2063 | number is relative rather than absolute. The most recently opened parentheses |
| 2064 | can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside |
| 2065 | loops it can also make sense to refer to subsequent groups. The next |
| 2066 | parentheses to be opened can be referenced as (?(+1), and so on. (The value |
| 2067 | zero in any of these forms is not used; it provokes a compile-time error.) |
| 2068 | </P> |
| 2069 | <P> |
| 2070 | Consider the following pattern, which contains non-significant white space to |
| 2071 | make it more readable (assume the PCRE_EXTENDED option) and to divide it into |
| 2072 | three parts for ease of discussion: |
| 2073 | <pre> |
| 2074 | ( \( )? [^()]+ (?(1) \) ) |
| 2075 | </pre> |
| 2076 | The first part matches an optional opening parenthesis, and if that |
| 2077 | character is present, sets it as the first captured substring. The second part |
| 2078 | matches one or more characters that are not parentheses. The third part is a |
| 2079 | conditional subpattern that tests whether or not the first set of parentheses |
| 2080 | matched. If they did, that is, if subject started with an opening parenthesis, |
| 2081 | the condition is true, and so the yes-pattern is executed and a closing |
| 2082 | parenthesis is required. Otherwise, since no-pattern is not present, the |
| 2083 | subpattern matches nothing. In other words, this pattern matches a sequence of |
| 2084 | non-parentheses, optionally enclosed in parentheses. |
| 2085 | </P> |
| 2086 | <P> |
| 2087 | If you were embedding this pattern in a larger one, you could use a relative |
| 2088 | reference: |
| 2089 | <pre> |
| 2090 | ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... |
| 2091 | </pre> |
| 2092 | This makes the fragment independent of the parentheses in the larger pattern. |
| 2093 | </P> |
| 2094 | <br><b> |
| 2095 | Checking for a used subpattern by name |
| 2096 | </b><br> |
| 2097 | <P> |
| 2098 | Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used |
| 2099 | subpattern by name. For compatibility with earlier versions of PCRE, which had |
| 2100 | this facility before Perl, the syntax (?(name)...) is also recognized. However, |
| 2101 | there is a possible ambiguity with this syntax, because subpattern names may |
| 2102 | consist entirely of digits. PCRE looks first for a named subpattern; if it |
| 2103 | cannot find one and the name consists entirely of digits, PCRE looks for a |
| 2104 | subpattern of that number, which must be greater than zero. Using subpattern |
| 2105 | names that consist entirely of digits is not recommended. |
| 2106 | </P> |
| 2107 | <P> |
| 2108 | Rewriting the above example to use a named subpattern gives this: |
| 2109 | <pre> |
| 2110 | (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) |
| 2111 | </pre> |
| 2112 | If the name used in a condition of this kind is a duplicate, the test is |
| 2113 | applied to all subpatterns of the same name, and is true if any one of them has |
| 2114 | matched. |
| 2115 | </P> |
| 2116 | <br><b> |
| 2117 | Checking for pattern recursion |
| 2118 | </b><br> |
| 2119 | <P> |
| 2120 | If the condition is the string (R), and there is no subpattern with the name R, |
| 2121 | the condition is true if a recursive call to the whole pattern or any |
| 2122 | subpattern has been made. If digits or a name preceded by ampersand follow the |
| 2123 | letter R, for example: |
| 2124 | <pre> |
| 2125 | (?(R3)...) or (?(R&name)...) |
| 2126 | </pre> |
| 2127 | the condition is true if the most recent recursion is into a subpattern whose |
| 2128 | number or name is given. This condition does not check the entire recursion |
| 2129 | stack. If the name used in a condition of this kind is a duplicate, the test is |
| 2130 | applied to all subpatterns of the same name, and is true if any one of them is |
| 2131 | the most recent recursion. |
| 2132 | </P> |
| 2133 | <P> |
| 2134 | At "top level", all these recursion test conditions are false. |
| 2135 | <a href="#recursion">The syntax for recursive patterns</a> |
| 2136 | is described below. |
| 2137 | <a name="subdefine"></a></P> |
| 2138 | <br><b> |
| 2139 | Defining subpatterns for use by reference only |
| 2140 | </b><br> |
| 2141 | <P> |
| 2142 | If the condition is the string (DEFINE), and there is no subpattern with the |
| 2143 | name DEFINE, the condition is always false. In this case, there may be only one |
| 2144 | alternative in the subpattern. It is always skipped if control reaches this |
| 2145 | point in the pattern; the idea of DEFINE is that it can be used to define |
| 2146 | subroutines that can be referenced from elsewhere. (The use of |
| 2147 | <a href="#subpatternsassubroutines">subroutines</a> |
| 2148 | is described below.) For example, a pattern to match an IPv4 address such as |
| 2149 | "192.168.23.245" could be written like this (ignore whitespace and line |
| 2150 | breaks): |
| 2151 | <pre> |
| 2152 | (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) |
| 2153 | \b (?&byte) (\.(?&byte)){3} \b |
| 2154 | </pre> |
| 2155 | The first part of the pattern is a DEFINE group inside which a another group |
| 2156 | named "byte" is defined. This matches an individual component of an IPv4 |
| 2157 | address (a number less than 256). When matching takes place, this part of the |
| 2158 | pattern is skipped because DEFINE acts like a false condition. The rest of the |
| 2159 | pattern uses references to the named group to match the four dot-separated |
| 2160 | components of an IPv4 address, insisting on a word boundary at each end. |
| 2161 | </P> |
| 2162 | <br><b> |
| 2163 | Assertion conditions |
| 2164 | </b><br> |
| 2165 | <P> |
| 2166 | If the condition is not in any of the above formats, it must be an assertion. |
| 2167 | This may be a positive or negative lookahead or lookbehind assertion. Consider |
| 2168 | this pattern, again containing non-significant white space, and with the two |
| 2169 | alternatives on the second line: |
| 2170 | <pre> |
| 2171 | (?(?=[^a-z]*[a-z]) |
| 2172 | \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) |
| 2173 | </pre> |
| 2174 | The condition is a positive lookahead assertion that matches an optional |
| 2175 | sequence of non-letters followed by a letter. In other words, it tests for the |
| 2176 | presence of at least one letter in the subject. If a letter is found, the |
| 2177 | subject is matched against the first alternative; otherwise it is matched |
| 2178 | against the second. This pattern matches strings in one of the two forms |
| 2179 | dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. |
| 2180 | <a name="comments"></a></P> |
| 2181 | <br><a name="SEC20" href="#TOC1">COMMENTS</a><br> |
| 2182 | <P> |
| 2183 | There are two ways of including comments in patterns that are processed by |
| 2184 | PCRE. In both cases, the start of the comment must not be in a character class, |
| 2185 | nor in the middle of any other sequence of related characters such as (?: or a |
| 2186 | subpattern name or number. The characters that make up a comment play no part |
| 2187 | in the pattern matching. |
| 2188 | </P> |
| 2189 | <P> |
| 2190 | The sequence (?# marks the start of a comment that continues up to the next |
| 2191 | closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED |
| 2192 | option is set, an unescaped # character also introduces a comment, which in |
| 2193 | this case continues to immediately after the next newline character or |
| 2194 | character sequence in the pattern. Which characters are interpreted as newlines |
| 2195 | is controlled by the options passed to <b>pcre_compile()</b> or by a special |
| 2196 | sequence at the start of the pattern, as described in the section entitled |
| 2197 | <a href="#newlines">"Newline conventions"</a> |
| 2198 | above. Note that the end of this type of comment is a literal newline sequence |
| 2199 | in the pattern; escape sequences that happen to represent a newline do not |
| 2200 | count. For example, consider this pattern when PCRE_EXTENDED is set, and the |
| 2201 | default newline convention is in force: |
| 2202 | <pre> |
| 2203 | abc #comment \n still comment |
| 2204 | </pre> |
| 2205 | On encountering the # character, <b>pcre_compile()</b> skips along, looking for |
| 2206 | a newline in the pattern. The sequence \n is still literal at this stage, so |
| 2207 | it does not terminate the comment. Only an actual character with the code value |
| 2208 | 0x0a (the default newline) does so. |
| 2209 | <a name="recursion"></a></P> |
| 2210 | <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br> |
| 2211 | <P> |
| 2212 | Consider the problem of matching a string in parentheses, allowing for |
| 2213 | unlimited nested parentheses. Without the use of recursion, the best that can |
| 2214 | be done is to use a pattern that matches up to some fixed depth of nesting. It |
| 2215 | is not possible to handle an arbitrary nesting depth. |
| 2216 | </P> |
| 2217 | <P> |
| 2218 | For some time, Perl has provided a facility that allows regular expressions to |
| 2219 | recurse (amongst other things). It does this by interpolating Perl code in the |
| 2220 | expression at run time, and the code can refer to the expression itself. A Perl |
| 2221 | pattern using code interpolation to solve the parentheses problem can be |
| 2222 | created like this: |
| 2223 | <pre> |
| 2224 | $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; |
| 2225 | </pre> |
| 2226 | The (?p{...}) item interpolates Perl code at run time, and in this case refers |
| 2227 | recursively to the pattern in which it appears. |
| 2228 | </P> |
| 2229 | <P> |
| 2230 | Obviously, PCRE cannot support the interpolation of Perl code. Instead, it |
| 2231 | supports special syntax for recursion of the entire pattern, and also for |
| 2232 | individual subpattern recursion. After its introduction in PCRE and Python, |
| 2233 | this kind of recursion was subsequently introduced into Perl at release 5.10. |
| 2234 | </P> |
| 2235 | <P> |
| 2236 | A special item that consists of (? followed by a number greater than zero and a |
| 2237 | closing parenthesis is a recursive subroutine call of the subpattern of the |
| 2238 | given number, provided that it occurs inside that subpattern. (If not, it is a |
| 2239 | <a href="#subpatternsassubroutines">non-recursive subroutine</a> |
| 2240 | call, which is described in the next section.) The special item (?R) or (?0) is |
| 2241 | a recursive call of the entire regular expression. |
| 2242 | </P> |
| 2243 | <P> |
| 2244 | This PCRE pattern solves the nested parentheses problem (assume the |
| 2245 | PCRE_EXTENDED option is set so that white space is ignored): |
| 2246 | <pre> |
| 2247 | \( ( [^()]++ | (?R) )* \) |
| 2248 | </pre> |
| 2249 | First it matches an opening parenthesis. Then it matches any number of |
| 2250 | substrings which can either be a sequence of non-parentheses, or a recursive |
| 2251 | match of the pattern itself (that is, a correctly parenthesized substring). |
| 2252 | Finally there is a closing parenthesis. Note the use of a possessive quantifier |
| 2253 | to avoid backtracking into sequences of non-parentheses. |
| 2254 | </P> |
| 2255 | <P> |
| 2256 | If this were part of a larger pattern, you would not want to recurse the entire |
| 2257 | pattern, so instead you could use this: |
| 2258 | <pre> |
| 2259 | ( \( ( [^()]++ | (?1) )* \) ) |
| 2260 | </pre> |
| 2261 | We have put the pattern into parentheses, and caused the recursion to refer to |
| 2262 | them instead of the whole pattern. |
| 2263 | </P> |
| 2264 | <P> |
| 2265 | In a larger pattern, keeping track of parenthesis numbers can be tricky. This |
| 2266 | is made easier by the use of relative references. Instead of (?1) in the |
| 2267 | pattern above you can write (?-2) to refer to the second most recently opened |
| 2268 | parentheses preceding the recursion. In other words, a negative number counts |
| 2269 | capturing parentheses leftwards from the point at which it is encountered. |
| 2270 | </P> |
| 2271 | <P> |
| 2272 | It is also possible to refer to subsequently opened parentheses, by writing |
| 2273 | references such as (?+2). However, these cannot be recursive because the |
| 2274 | reference is not inside the parentheses that are referenced. They are always |
| 2275 | <a href="#subpatternsassubroutines">non-recursive subroutine</a> |
| 2276 | calls, as described in the next section. |
| 2277 | </P> |
| 2278 | <P> |
| 2279 | An alternative approach is to use named parentheses instead. The Perl syntax |
| 2280 | for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We |
| 2281 | could rewrite the above example as follows: |
| 2282 | <pre> |
| 2283 | (?<pn> \( ( [^()]++ | (?&pn) )* \) ) |
| 2284 | </pre> |
| 2285 | If there is more than one subpattern with the same name, the earliest one is |
| 2286 | used. |
| 2287 | </P> |
| 2288 | <P> |
| 2289 | This particular example pattern that we have been looking at contains nested |
| 2290 | unlimited repeats, and so the use of a possessive quantifier for matching |
| 2291 | strings of non-parentheses is important when applying the pattern to strings |
| 2292 | that do not match. For example, when this pattern is applied to |
| 2293 | <pre> |
| 2294 | (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() |
| 2295 | </pre> |
| 2296 | it yields "no match" quickly. However, if a possessive quantifier is not used, |
| 2297 | the match runs for a very long time indeed because there are so many different |
| 2298 | ways the + and * repeats can carve up the subject, and all have to be tested |
| 2299 | before failure can be reported. |
| 2300 | </P> |
| 2301 | <P> |
| 2302 | At the end of a match, the values of capturing parentheses are those from |
| 2303 | the outermost level. If you want to obtain intermediate values, a callout |
| 2304 | function can be used (see below and the |
| 2305 | <a href="pcrecallout.html"><b>pcrecallout</b></a> |
| 2306 | documentation). If the pattern above is matched against |
| 2307 | <pre> |
| 2308 | (ab(cd)ef) |
| 2309 | </pre> |
| 2310 | the value for the inner capturing parentheses (numbered 2) is "ef", which is |
| 2311 | the last value taken on at the top level. If a capturing subpattern is not |
| 2312 | matched at the top level, its final captured value is unset, even if it was |
| 2313 | (temporarily) set at a deeper level during the matching process. |
| 2314 | </P> |
| 2315 | <P> |
| 2316 | If there are more than 15 capturing parentheses in a pattern, PCRE has to |
| 2317 | obtain extra memory to store data during a recursion, which it does by using |
| 2318 | <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can |
| 2319 | be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. |
| 2320 | </P> |
| 2321 | <P> |
| 2322 | Do not confuse the (?R) item with the condition (R), which tests for recursion. |
| 2323 | Consider this pattern, which matches text in angle brackets, allowing for |
| 2324 | arbitrary nesting. Only digits are allowed in nested brackets (that is, when |
| 2325 | recursing), whereas any characters are permitted at the outer level. |
| 2326 | <pre> |
| 2327 | < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > |
| 2328 | </pre> |
| 2329 | In this pattern, (?(R) is the start of a conditional subpattern, with two |
| 2330 | different alternatives for the recursive and non-recursive cases. The (?R) item |
| 2331 | is the actual recursive call. |
| 2332 | <a name="recursiondifference"></a></P> |
| 2333 | <br><b> |
| 2334 | Differences in recursion processing between PCRE and Perl |
| 2335 | </b><br> |
| 2336 | <P> |
| 2337 | Recursion processing in PCRE differs from Perl in two important ways. In PCRE |
| 2338 | (like Python, but unlike Perl), a recursive subpattern call is always treated |
| 2339 | as an atomic group. That is, once it has matched some of the subject string, it |
| 2340 | is never re-entered, even if it contains untried alternatives and there is a |
| 2341 | subsequent matching failure. This can be illustrated by the following pattern, |
| 2342 | which purports to match a palindromic string that contains an odd number of |
| 2343 | characters (for example, "a", "aba", "abcba", "abcdcba"): |
| 2344 | <pre> |
| 2345 | ^(.|(.)(?1)\2)$ |
| 2346 | </pre> |
| 2347 | The idea is that it either matches a single character, or two identical |
| 2348 | characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE |
| 2349 | it does not if the pattern is longer than three characters. Consider the |
| 2350 | subject string "abcba": |
| 2351 | </P> |
| 2352 | <P> |
| 2353 | At the top level, the first character is matched, but as it is not at the end |
| 2354 | of the string, the first alternative fails; the second alternative is taken |
| 2355 | and the recursion kicks in. The recursive call to subpattern 1 successfully |
| 2356 | matches the next character ("b"). (Note that the beginning and end of line |
| 2357 | tests are not part of the recursion). |
| 2358 | </P> |
| 2359 | <P> |
| 2360 | Back at the top level, the next character ("c") is compared with what |
| 2361 | subpattern 2 matched, which was "a". This fails. Because the recursion is |
| 2362 | treated as an atomic group, there are now no backtracking points, and so the |
| 2363 | entire match fails. (Perl is able, at this point, to re-enter the recursion and |
| 2364 | try the second alternative.) However, if the pattern is written with the |
| 2365 | alternatives in the other order, things are different: |
| 2366 | <pre> |
| 2367 | ^((.)(?1)\2|.)$ |
| 2368 | </pre> |
| 2369 | This time, the recursing alternative is tried first, and continues to recurse |
| 2370 | until it runs out of characters, at which point the recursion fails. But this |
| 2371 | time we do have another alternative to try at the higher level. That is the big |
| 2372 | difference: in the previous case the remaining alternative is at a deeper |
| 2373 | recursion level, which PCRE cannot use. |
| 2374 | </P> |
| 2375 | <P> |
| 2376 | To change the pattern so that it matches all palindromic strings, not just |
| 2377 | those with an odd number of characters, it is tempting to change the pattern to |
| 2378 | this: |
| 2379 | <pre> |
| 2380 | ^((.)(?1)\2|.?)$ |
| 2381 | </pre> |
| 2382 | Again, this works in Perl, but not in PCRE, and for the same reason. When a |
| 2383 | deeper recursion has matched a single character, it cannot be entered again in |
| 2384 | order to match an empty string. The solution is to separate the two cases, and |
| 2385 | write out the odd and even cases as alternatives at the higher level: |
| 2386 | <pre> |
| 2387 | ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) |
| 2388 | </pre> |
| 2389 | If you want to match typical palindromic phrases, the pattern has to ignore all |
| 2390 | non-word characters, which can be done like this: |
| 2391 | <pre> |
| 2392 | ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ |
| 2393 | </pre> |
| 2394 | If run with the PCRE_CASELESS option, this pattern matches phrases such as "A |
| 2395 | man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note |
| 2396 | the use of the possessive quantifier *+ to avoid backtracking into sequences of |
| 2397 | non-word characters. Without this, PCRE takes a great deal longer (ten times or |
| 2398 | more) to match typical phrases, and Perl takes so long that you think it has |
| 2399 | gone into a loop. |
| 2400 | </P> |
| 2401 | <P> |
| 2402 | <b>WARNING</b>: The palindrome-matching patterns above work only if the subject |
| 2403 | string does not start with a palindrome that is shorter than the entire string. |
| 2404 | For example, although "abcba" is correctly matched, if the subject is "ababa", |
| 2405 | PCRE finds the palindrome "aba" at the start, then fails at top level because |
| 2406 | the end of the string does not follow. Once again, it cannot jump back into the |
| 2407 | recursion to try other alternatives, so the entire match fails. |
| 2408 | </P> |
| 2409 | <P> |
| 2410 | The second way in which PCRE and Perl differ in their recursion processing is |
| 2411 | in the handling of captured values. In Perl, when a subpattern is called |
| 2412 | recursively or as a subpattern (see the next section), it has no access to any |
| 2413 | values that were captured outside the recursion, whereas in PCRE these values |
| 2414 | can be referenced. Consider this pattern: |
| 2415 | <pre> |
| 2416 | ^(.)(\1|a(?2)) |
| 2417 | </pre> |
| 2418 | In PCRE, this pattern matches "bab". The first capturing parentheses match "b", |
| 2419 | then in the second group, when the back reference \1 fails to match "b", the |
| 2420 | second alternative matches "a" and then recurses. In the recursion, \1 does |
| 2421 | now match "b" and so the whole match succeeds. In Perl, the pattern fails to |
| 2422 | match because inside the recursive call \1 cannot access the externally set |
| 2423 | value. |
| 2424 | <a name="subpatternsassubroutines"></a></P> |
| 2425 | <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br> |
| 2426 | <P> |
| 2427 | If the syntax for a recursive subpattern call (either by number or by |
| 2428 | name) is used outside the parentheses to which it refers, it operates like a |
| 2429 | subroutine in a programming language. The called subpattern may be defined |
| 2430 | before or after the reference. A numbered reference can be absolute or |
| 2431 | relative, as in these examples: |
| 2432 | <pre> |
| 2433 | (...(absolute)...)...(?2)... |
| 2434 | (...(relative)...)...(?-1)... |
| 2435 | (...(?+1)...(relative)... |
| 2436 | </pre> |
| 2437 | An earlier example pointed out that the pattern |
| 2438 | <pre> |
| 2439 | (sens|respons)e and \1ibility |
| 2440 | </pre> |
| 2441 | matches "sense and sensibility" and "response and responsibility", but not |
| 2442 | "sense and responsibility". If instead the pattern |
| 2443 | <pre> |
| 2444 | (sens|respons)e and (?1)ibility |
| 2445 | </pre> |
| 2446 | is used, it does match "sense and responsibility" as well as the other two |
| 2447 | strings. Another example is given in the discussion of DEFINE above. |
| 2448 | </P> |
| 2449 | <P> |
| 2450 | All subroutine calls, whether recursive or not, are always treated as atomic |
| 2451 | groups. That is, once a subroutine has matched some of the subject string, it |
| 2452 | is never re-entered, even if it contains untried alternatives and there is a |
| 2453 | subsequent matching failure. Any capturing parentheses that are set during the |
| 2454 | subroutine call revert to their previous values afterwards. |
| 2455 | </P> |
| 2456 | <P> |
| 2457 | Processing options such as case-independence are fixed when a subpattern is |
| 2458 | defined, so if it is used as a subroutine, such options cannot be changed for |
| 2459 | different calls. For example, consider this pattern: |
| 2460 | <pre> |
| 2461 | (abc)(?i:(?-1)) |
| 2462 | </pre> |
| 2463 | It matches "abcabc". It does not match "abcABC" because the change of |
| 2464 | processing option does not affect the called subpattern. |
| 2465 | <a name="onigurumasubroutines"></a></P> |
| 2466 | <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br> |
| 2467 | <P> |
| 2468 | For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or |
| 2469 | a number enclosed either in angle brackets or single quotes, is an alternative |
| 2470 | syntax for referencing a subpattern as a subroutine, possibly recursively. Here |
| 2471 | are two of the examples used above, rewritten using this syntax: |
| 2472 | <pre> |
| 2473 | (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) |
| 2474 | (sens|respons)e and \g'1'ibility |
| 2475 | </pre> |
| 2476 | PCRE supports an extension to Oniguruma: if a number is preceded by a |
| 2477 | plus or a minus sign it is taken as a relative reference. For example: |
| 2478 | <pre> |
| 2479 | (abc)(?i:\g<-1>) |
| 2480 | </pre> |
| 2481 | Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i> |
| 2482 | synonymous. The former is a back reference; the latter is a subroutine call. |
| 2483 | </P> |
| 2484 | <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br> |
| 2485 | <P> |
| 2486 | Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl |
| 2487 | code to be obeyed in the middle of matching a regular expression. This makes it |
| 2488 | possible, amongst other things, to extract different substrings that match the |
| 2489 | same pair of parentheses when there is a repetition. |
| 2490 | </P> |
| 2491 | <P> |
| 2492 | PCRE provides a similar feature, but of course it cannot obey arbitrary Perl |
| 2493 | code. The feature is called "callout". The caller of PCRE provides an external |
| 2494 | function by putting its entry point in the global variable <i>pcre_callout</i>. |
| 2495 | By default, this variable contains NULL, which disables all calling out. |
| 2496 | </P> |
| 2497 | <P> |
| 2498 | Within a regular expression, (?C) indicates the points at which the external |
| 2499 | function is to be called. If you want to identify different callout points, you |
| 2500 | can put a number less than 256 after the letter C. The default value is zero. |
| 2501 | For example, this pattern has two callout points: |
| 2502 | <pre> |
| 2503 | (?C1)abc(?C2)def |
| 2504 | </pre> |
| 2505 | If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are |
| 2506 | automatically installed before each item in the pattern. They are all numbered |
| 2507 | 255. |
| 2508 | </P> |
| 2509 | <P> |
| 2510 | During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is |
| 2511 | set), the external function is called. It is provided with the number of the |
| 2512 | callout, the position in the pattern, and, optionally, one item of data |
| 2513 | originally supplied by the caller of <b>pcre_exec()</b>. The callout function |
| 2514 | may cause matching to proceed, to backtrack, or to fail altogether. A complete |
| 2515 | description of the interface to the callout function is given in the |
| 2516 | <a href="pcrecallout.html"><b>pcrecallout</b></a> |
| 2517 | documentation. |
| 2518 | <a name="backtrackcontrol"></a></P> |
| 2519 | <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br> |
| 2520 | <P> |
| 2521 | Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which |
| 2522 | are described in the Perl documentation as "experimental and subject to change |
| 2523 | or removal in a future version of Perl". It goes on to say: "Their usage in |
| 2524 | production code should be noted to avoid problems during upgrades." The same |
| 2525 | remarks apply to the PCRE features described in this section. |
| 2526 | </P> |
| 2527 | <P> |
| 2528 | Since these verbs are specifically related to backtracking, most of them can be |
| 2529 | used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses |
| 2530 | a backtracking algorithm. With the exception of (*FAIL), which behaves like a |
| 2531 | failing negative assertion, they cause an error if encountered by |
| 2532 | <b>pcre_dfa_exec()</b>. |
| 2533 | </P> |
| 2534 | <P> |
| 2535 | If any of these verbs are used in an assertion or in a subpattern that is |
| 2536 | called as a subroutine (whether or not recursively), their effect is confined |
| 2537 | to that subpattern; it does not extend to the surrounding pattern, with one |
| 2538 | exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in |
| 2539 | a successful positive assertion <i>is</i> passed back when a match succeeds |
| 2540 | (compare capturing parentheses in assertions). Note that such subpatterns are |
| 2541 | processed as anchored at the point where they are tested. Note also that Perl's |
| 2542 | treatment of subroutines is different in some cases. |
| 2543 | </P> |
| 2544 | <P> |
| 2545 | The new verbs make use of what was previously invalid syntax: an opening |
| 2546 | parenthesis followed by an asterisk. They are generally of the form |
| 2547 | (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour, |
| 2548 | depending on whether or not an argument is present. A name is any sequence of |
| 2549 | characters that does not include a closing parenthesis. If the name is empty, |
| 2550 | that is, if the closing parenthesis immediately follows the colon, the effect |
| 2551 | is as if the colon were not there. Any number of these verbs may occur in a |
| 2552 | pattern. |
| 2553 | </P> |
| 2554 | <P> |
| 2555 | PCRE contains some optimizations that are used to speed up matching by running |
| 2556 | some checks at the start of each match attempt. For example, it may know the |
| 2557 | minimum length of matching subject, or that a particular character must be |
| 2558 | present. When one of these optimizations suppresses the running of a match, any |
| 2559 | included backtracking verbs will not, of course, be processed. You can suppress |
| 2560 | the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option |
| 2561 | when calling <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the |
| 2562 | pattern with (*NO_START_OPT). |
| 2563 | </P> |
| 2564 | <P> |
| 2565 | Experiments with Perl suggest that it too has similar optimizations, sometimes |
| 2566 | leading to anomalous results. |
| 2567 | </P> |
| 2568 | <br><b> |
| 2569 | Verbs that act immediately |
| 2570 | </b><br> |
| 2571 | <P> |
| 2572 | The following verbs act as soon as they are encountered. They may not be |
| 2573 | followed by a name. |
| 2574 | <pre> |
| 2575 | (*ACCEPT) |
| 2576 | </pre> |
| 2577 | This verb causes the match to end successfully, skipping the remainder of the |
| 2578 | pattern. However, when it is inside a subpattern that is called as a |
| 2579 | subroutine, only that subpattern is ended successfully. Matching then continues |
| 2580 | at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so |
| 2581 | far is captured. For example: |
| 2582 | <pre> |
| 2583 | A((?:A|B(*ACCEPT)|C)D) |
| 2584 | </pre> |
| 2585 | This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by |
| 2586 | the outer parentheses. |
| 2587 | <pre> |
| 2588 | (*FAIL) or (*F) |
| 2589 | </pre> |
| 2590 | This verb causes a matching failure, forcing backtracking to occur. It is |
| 2591 | equivalent to (?!) but easier to read. The Perl documentation notes that it is |
| 2592 | probably useful only when combined with (?{}) or (??{}). Those are, of course, |
| 2593 | Perl features that are not present in PCRE. The nearest equivalent is the |
| 2594 | callout feature, as for example in this pattern: |
| 2595 | <pre> |
| 2596 | a+(?C)(*FAIL) |
| 2597 | </pre> |
| 2598 | A match with the string "aaaa" always fails, but the callout is taken before |
| 2599 | each backtrack happens (in this example, 10 times). |
| 2600 | </P> |
| 2601 | <br><b> |
| 2602 | Recording which path was taken |
| 2603 | </b><br> |
| 2604 | <P> |
| 2605 | There is one verb whose main purpose is to track how a match was arrived at, |
| 2606 | though it also has a secondary use in conjunction with advancing the match |
| 2607 | starting point (see (*SKIP) below). |
| 2608 | <pre> |
| 2609 | (*MARK:NAME) or (*:NAME) |
| 2610 | </pre> |
| 2611 | A name is always required with this verb. There may be as many instances of |
| 2612 | (*MARK) as you like in a pattern, and their names do not have to be unique. |
| 2613 | </P> |
| 2614 | <P> |
| 2615 | When a match succeeds, the name of the last-encountered (*MARK) on the matching |
| 2616 | path is passed back to the caller via the <i>pcre_extra</i> data structure, as |
| 2617 | described in the |
| 2618 | <a href="pcreapi.html#extradata">section on <i>pcre_extra</i></a> |
| 2619 | in the |
| 2620 | <a href="pcreapi.html"><b>pcreapi</b></a> |
| 2621 | documentation. Here is an example of <b>pcretest</b> output, where the /K |
| 2622 | modifier requests the retrieval and outputting of (*MARK) data: |
| 2623 | <pre> |
| 2624 | re> /X(*MARK:A)Y|X(*MARK:B)Z/K |
| 2625 | data> XY |
| 2626 | 0: XY |
| 2627 | MK: A |
| 2628 | XZ |
| 2629 | 0: XZ |
| 2630 | MK: B |
| 2631 | </pre> |
| 2632 | The (*MARK) name is tagged with "MK:" in this output, and in this example it |
| 2633 | indicates which of the two alternatives matched. This is a more efficient way |
| 2634 | of obtaining this information than putting each alternative in its own |
| 2635 | capturing parentheses. |
| 2636 | </P> |
| 2637 | <P> |
| 2638 | If (*MARK) is encountered in a positive assertion, its name is recorded and |
| 2639 | passed back if it is the last-encountered. This does not happen for negative |
| 2640 | assertions. |
| 2641 | </P> |
| 2642 | <P> |
| 2643 | After a partial match or a failed match, the name of the last encountered |
| 2644 | (*MARK) in the entire match process is returned. For example: |
| 2645 | <pre> |
| 2646 | re> /X(*MARK:A)Y|X(*MARK:B)Z/K |
| 2647 | data> XP |
| 2648 | No match, mark = B |
| 2649 | </pre> |
| 2650 | Note that in this unanchored example the mark is retained from the match |
| 2651 | attempt that started at the letter "X". Subsequent match attempts starting at |
| 2652 | "P" and then with an empty string do not get as far as the (*MARK) item, but |
| 2653 | nevertheless do not reset it. |
| 2654 | </P> |
| 2655 | <br><b> |
| 2656 | Verbs that act after backtracking |
| 2657 | </b><br> |
| 2658 | <P> |
| 2659 | The following verbs do nothing when they are encountered. Matching continues |
| 2660 | with what follows, but if there is no subsequent match, causing a backtrack to |
| 2661 | the verb, a failure is forced. That is, backtracking cannot pass to the left of |
| 2662 | the verb. However, when one of these verbs appears inside an atomic group, its |
| 2663 | effect is confined to that group, because once the group has been matched, |
| 2664 | there is never any backtracking into it. In this situation, backtracking can |
| 2665 | "jump back" to the left of the entire atomic group. (Remember also, as stated |
| 2666 | above, that this localization also applies in subroutine calls and assertions.) |
| 2667 | </P> |
| 2668 | <P> |
| 2669 | These verbs differ in exactly what kind of failure occurs when backtracking |
| 2670 | reaches them. |
| 2671 | <pre> |
| 2672 | (*COMMIT) |
| 2673 | </pre> |
| 2674 | This verb, which may not be followed by a name, causes the whole match to fail |
| 2675 | outright if the rest of the pattern does not match. Even if the pattern is |
| 2676 | unanchored, no further attempts to find a match by advancing the starting point |
| 2677 | take place. Once (*COMMIT) has been passed, <b>pcre_exec()</b> is committed to |
| 2678 | finding a match at the current starting point, or not at all. For example: |
| 2679 | <pre> |
| 2680 | a+(*COMMIT)b |
| 2681 | </pre> |
| 2682 | This matches "xxaab" but not "aacaab". It can be thought of as a kind of |
| 2683 | dynamic anchor, or "I've started, so I must finish." The name of the most |
| 2684 | recently passed (*MARK) in the path is passed back when (*COMMIT) forces a |
| 2685 | match failure. |
| 2686 | </P> |
| 2687 | <P> |
| 2688 | Note that (*COMMIT) at the start of a pattern is not the same as an anchor, |
| 2689 | unless PCRE's start-of-match optimizations are turned off, as shown in this |
| 2690 | <b>pcretest</b> example: |
| 2691 | <pre> |
| 2692 | re> /(*COMMIT)abc/ |
| 2693 | data> xyzabc |
| 2694 | 0: abc |
| 2695 | xyzabc\Y |
| 2696 | No match |
| 2697 | </pre> |
| 2698 | PCRE knows that any match must start with "a", so the optimization skips along |
| 2699 | the subject to "a" before running the first match attempt, which succeeds. When |
| 2700 | the optimization is disabled by the \Y escape in the second subject, the match |
| 2701 | starts at "x" and so the (*COMMIT) causes it to fail without trying any other |
| 2702 | starting points. |
| 2703 | <pre> |
| 2704 | (*PRUNE) or (*PRUNE:NAME) |
| 2705 | </pre> |
| 2706 | This verb causes the match to fail at the current starting position in the |
| 2707 | subject if the rest of the pattern does not match. If the pattern is |
| 2708 | unanchored, the normal "bumpalong" advance to the next starting character then |
| 2709 | happens. Backtracking can occur as usual to the left of (*PRUNE), before it is |
| 2710 | reached, or when matching to the right of (*PRUNE), but if there is no match to |
| 2711 | the right, backtracking cannot cross (*PRUNE). In simple cases, the use of |
| 2712 | (*PRUNE) is just an alternative to an atomic group or possessive quantifier, |
| 2713 | but there are some uses of (*PRUNE) that cannot be expressed in any other way. |
| 2714 | The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an |
| 2715 | anchored pattern (*PRUNE) has the same effect as (*COMMIT). |
| 2716 | <pre> |
| 2717 | (*SKIP) |
| 2718 | </pre> |
| 2719 | This verb, when given without a name, is like (*PRUNE), except that if the |
| 2720 | pattern is unanchored, the "bumpalong" advance is not to the next character, |
| 2721 | but to the position in the subject where (*SKIP) was encountered. (*SKIP) |
| 2722 | signifies that whatever text was matched leading up to it cannot be part of a |
| 2723 | successful match. Consider: |
| 2724 | <pre> |
| 2725 | a+(*SKIP)b |
| 2726 | </pre> |
| 2727 | If the subject is "aaaac...", after the first match attempt fails (starting at |
| 2728 | the first character in the string), the starting point skips on to start the |
| 2729 | next attempt at "c". Note that a possessive quantifer does not have the same |
| 2730 | effect as this example; although it would suppress backtracking during the |
| 2731 | first match attempt, the second attempt would start at the second character |
| 2732 | instead of skipping on to "c". |
| 2733 | <pre> |
| 2734 | (*SKIP:NAME) |
| 2735 | </pre> |
| 2736 | When (*SKIP) has an associated name, its behaviour is modified. If the |
| 2737 | following pattern fails to match, the previous path through the pattern is |
| 2738 | searched for the most recent (*MARK) that has the same name. If one is found, |
| 2739 | the "bumpalong" advance is to the subject position that corresponds to that |
| 2740 | (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a |
| 2741 | matching name is found, the (*SKIP) is ignored. |
| 2742 | <pre> |
| 2743 | (*THEN) or (*THEN:NAME) |
| 2744 | </pre> |
| 2745 | This verb causes a skip to the next innermost alternative if the rest of the |
| 2746 | pattern does not match. That is, it cancels pending backtracking, but only |
| 2747 | within the current alternative. Its name comes from the observation that it can |
| 2748 | be used for a pattern-based if-then-else block: |
| 2749 | <pre> |
| 2750 | ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... |
| 2751 | </pre> |
| 2752 | If the COND1 pattern matches, FOO is tried (and possibly further items after |
| 2753 | the end of the group if FOO succeeds); on failure, the matcher skips to the |
| 2754 | second alternative and tries COND2, without backtracking into COND1. The |
| 2755 | behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN). |
| 2756 | If (*THEN) is not inside an alternation, it acts like (*PRUNE). |
| 2757 | </P> |
| 2758 | <P> |
| 2759 | Note that a subpattern that does not contain a | character is just a part of |
| 2760 | the enclosing alternative; it is not a nested alternation with only one |
| 2761 | alternative. The effect of (*THEN) extends beyond such a subpattern to the |
| 2762 | enclosing alternative. Consider this pattern, where A, B, etc. are complex |
| 2763 | pattern fragments that do not contain any | characters at this level: |
| 2764 | <pre> |
| 2765 | A (B(*THEN)C) | D |
| 2766 | </pre> |
| 2767 | If A and B are matched, but there is a failure in C, matching does not |
| 2768 | backtrack into A; instead it moves to the next alternative, that is, D. |
| 2769 | However, if the subpattern containing (*THEN) is given an alternative, it |
| 2770 | behaves differently: |
| 2771 | <pre> |
| 2772 | A (B(*THEN)C | (*FAIL)) | D |
| 2773 | </pre> |
| 2774 | The effect of (*THEN) is now confined to the inner subpattern. After a failure |
| 2775 | in C, matching moves to (*FAIL), which causes the whole subpattern to fail |
| 2776 | because there are no more alternatives to try. In this case, matching does now |
| 2777 | backtrack into A. |
| 2778 | </P> |
| 2779 | <P> |
| 2780 | Note also that a conditional subpattern is not considered as having two |
| 2781 | alternatives, because only one is ever used. In other words, the | character in |
| 2782 | a conditional subpattern has a different meaning. Ignoring white space, |
| 2783 | consider: |
| 2784 | <pre> |
| 2785 | ^.*? (?(?=a) a | b(*THEN)c ) |
| 2786 | </pre> |
| 2787 | If the subject is "ba", this pattern does not match. Because .*? is ungreedy, |
| 2788 | it initially matches zero characters. The condition (?=a) then fails, the |
| 2789 | character "b" is matched, but "c" is not. At this point, matching does not |
| 2790 | backtrack to .*? as might perhaps be expected from the presence of the | |
| 2791 | character. The conditional subpattern is part of the single alternative that |
| 2792 | comprises the whole pattern, and so the match fails. (If there was a backtrack |
| 2793 | into .*?, allowing it to match "b", the match would succeed.) |
| 2794 | </P> |
| 2795 | <P> |
| 2796 | The verbs just described provide four different "strengths" of control when |
| 2797 | subsequent matching fails. (*THEN) is the weakest, carrying on the match at the |
| 2798 | next alternative. (*PRUNE) comes next, failing the match at the current |
| 2799 | starting position, but allowing an advance to the next character (for an |
| 2800 | unanchored pattern). (*SKIP) is similar, except that the advance may be more |
| 2801 | than one character. (*COMMIT) is the strongest, causing the entire match to |
| 2802 | fail. |
| 2803 | </P> |
| 2804 | <P> |
| 2805 | If more than one such verb is present in a pattern, the "strongest" one wins. |
| 2806 | For example, consider this pattern, where A, B, etc. are complex pattern |
| 2807 | fragments: |
| 2808 | <pre> |
| 2809 | (A(*COMMIT)B(*THEN)C|D) |
| 2810 | </pre> |
| 2811 | Once A has matched, PCRE is committed to this match, at the current starting |
| 2812 | position. If subsequently B matches, but C does not, the normal (*THEN) action |
| 2813 | of trying the next alternative (that is, D) does not happen because (*COMMIT) |
| 2814 | overrides. |
| 2815 | </P> |
| 2816 | <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br> |
| 2817 | <P> |
| 2818 | <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), |
| 2819 | <b>pcresyntax</b>(3), <b>pcre</b>(3). |
| 2820 | </P> |
| 2821 | <br><a name="SEC27" href="#TOC1">AUTHOR</a><br> |
| 2822 | <P> |
| 2823 | Philip Hazel |
| 2824 | <br> |
| 2825 | University Computing Service |
| 2826 | <br> |
| 2827 | Cambridge CB2 3QH, England. |
| 2828 | <br> |
| 2829 | </P> |
| 2830 | <br><a name="SEC28" href="#TOC1">REVISION</a><br> |
| 2831 | <P> |
| 2832 | Last updated: 29 November 2011 |
| 2833 | <br> |
| 2834 | Copyright © 1997-2011 University of Cambridge. |
| 2835 | <br> |
| 2836 | <p> |
| 2837 | Return to the <a href="index.html">PCRE index page</a>. |
| 2838 | </p> |