Annotation of embedaddon/pcre/doc/html/pcrepattern.html, revision 1.1
1.1 ! misho 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>
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