Annotation of embedaddon/pcre/doc/pcrepattern.3, revision 1.1
1.1 ! misho 1: .TH PCREPATTERN 3
! 2: .SH NAME
! 3: PCRE - Perl-compatible regular expressions
! 4: .SH "PCRE REGULAR EXPRESSION DETAILS"
! 5: .rs
! 6: .sp
! 7: The syntax and semantics of the regular expressions that are supported by PCRE
! 8: are described in detail below. There is a quick-reference syntax summary in the
! 9: .\" HREF
! 10: \fBpcresyntax\fP
! 11: .\"
! 12: page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
! 13: also supports some alternative regular expression syntax (which does not
! 14: conflict with the Perl syntax) in order to provide some compatibility with
! 15: regular expressions in Python, .NET, and Oniguruma.
! 16: .P
! 17: Perl's regular expressions are described in its own documentation, and
! 18: regular expressions in general are covered in a number of books, some of which
! 19: have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
! 20: published by O'Reilly, covers regular expressions in great detail. This
! 21: description of PCRE's regular expressions is intended as reference material.
! 22: .P
! 23: The original operation of PCRE was on strings of one-byte characters. However,
! 24: there is now also support for UTF-8 character strings. To use this,
! 25: PCRE must be built to include UTF-8 support, and you must call
! 26: \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
! 27: is also a special sequence that can be given at the start of a pattern:
! 28: .sp
! 29: (*UTF8)
! 30: .sp
! 31: Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
! 32: option. This feature is not Perl-compatible. How setting UTF-8 mode affects
! 33: pattern matching is mentioned in several places below. There is also a summary
! 34: of UTF-8 features in the
! 35: .\" HREF
! 36: \fBpcreunicode\fP
! 37: .\"
! 38: page.
! 39: .P
! 40: Another special sequence that may appear at the start of a pattern or in
! 41: combination with (*UTF8) is:
! 42: .sp
! 43: (*UCP)
! 44: .sp
! 45: This has the same effect as setting the PCRE_UCP option: it causes sequences
! 46: such as \ed and \ew to use Unicode properties to determine character types,
! 47: instead of recognizing only characters with codes less than 128 via a lookup
! 48: table.
! 49: .P
! 50: If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
! 51: PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
! 52: also some more of these special sequences that are concerned with the handling
! 53: of newlines; they are described below.
! 54: .P
! 55: The remainder of this document discusses the patterns that are supported by
! 56: PCRE when its main matching function, \fBpcre_exec()\fP, is used.
! 57: From release 6.0, PCRE offers a second matching function,
! 58: \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
! 59: Perl-compatible. Some of the features discussed below are not available when
! 60: \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
! 61: alternative function, and how it differs from the normal function, are
! 62: discussed in the
! 63: .\" HREF
! 64: \fBpcrematching\fP
! 65: .\"
! 66: page.
! 67: .
! 68: .
! 69: .\" HTML <a name="newlines"></a>
! 70: .SH "NEWLINE CONVENTIONS"
! 71: .rs
! 72: .sp
! 73: PCRE supports five different conventions for indicating line breaks in
! 74: strings: a single CR (carriage return) character, a single LF (linefeed)
! 75: character, the two-character sequence CRLF, any of the three preceding, or any
! 76: Unicode newline sequence. The
! 77: .\" HREF
! 78: \fBpcreapi\fP
! 79: .\"
! 80: page has
! 81: .\" HTML <a href="pcreapi.html#newlines">
! 82: .\" </a>
! 83: further discussion
! 84: .\"
! 85: about newlines, and shows how to set the newline convention in the
! 86: \fIoptions\fP arguments for the compiling and matching functions.
! 87: .P
! 88: It is also possible to specify a newline convention by starting a pattern
! 89: string with one of the following five sequences:
! 90: .sp
! 91: (*CR) carriage return
! 92: (*LF) linefeed
! 93: (*CRLF) carriage return, followed by linefeed
! 94: (*ANYCRLF) any of the three above
! 95: (*ANY) all Unicode newline sequences
! 96: .sp
! 97: These override the default and the options given to \fBpcre_compile()\fP or
! 98: \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
! 99: newline sequence, the pattern
! 100: .sp
! 101: (*CR)a.b
! 102: .sp
! 103: changes the convention to CR. That pattern matches "a\enb" because LF is no
! 104: longer a newline. Note that these special settings, which are not
! 105: Perl-compatible, are recognized only at the very start of a pattern, and that
! 106: they must be in upper case. If more than one of them is present, the last one
! 107: is used.
! 108: .P
! 109: The newline convention affects the interpretation of the dot metacharacter when
! 110: PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
! 111: affect what the \eR escape sequence matches. By default, this is any Unicode
! 112: newline sequence, for Perl compatibility. However, this can be changed; see the
! 113: description of \eR in the section entitled
! 114: .\" HTML <a href="#newlineseq">
! 115: .\" </a>
! 116: "Newline sequences"
! 117: .\"
! 118: below. A change of \eR setting can be combined with a change of newline
! 119: convention.
! 120: .
! 121: .
! 122: .SH "CHARACTERS AND METACHARACTERS"
! 123: .rs
! 124: .sp
! 125: A regular expression is a pattern that is matched against a subject string from
! 126: left to right. Most characters stand for themselves in a pattern, and match the
! 127: corresponding characters in the subject. As a trivial example, the pattern
! 128: .sp
! 129: The quick brown fox
! 130: .sp
! 131: matches a portion of a subject string that is identical to itself. When
! 132: caseless matching is specified (the PCRE_CASELESS option), letters are matched
! 133: independently of case. In UTF-8 mode, PCRE always understands the concept of
! 134: case for characters whose values are less than 128, so caseless matching is
! 135: always possible. For characters with higher values, the concept of case is
! 136: supported if PCRE is compiled with Unicode property support, but not otherwise.
! 137: If you want to use caseless matching for characters 128 and above, you must
! 138: ensure that PCRE is compiled with Unicode property support as well as with
! 139: UTF-8 support.
! 140: .P
! 141: The power of regular expressions comes from the ability to include alternatives
! 142: and repetitions in the pattern. These are encoded in the pattern by the use of
! 143: \fImetacharacters\fP, which do not stand for themselves but instead are
! 144: interpreted in some special way.
! 145: .P
! 146: There are two different sets of metacharacters: those that are recognized
! 147: anywhere in the pattern except within square brackets, and those that are
! 148: recognized within square brackets. Outside square brackets, the metacharacters
! 149: are as follows:
! 150: .sp
! 151: \e general escape character with several uses
! 152: ^ assert start of string (or line, in multiline mode)
! 153: $ assert end of string (or line, in multiline mode)
! 154: . match any character except newline (by default)
! 155: [ start character class definition
! 156: | start of alternative branch
! 157: ( start subpattern
! 158: ) end subpattern
! 159: ? extends the meaning of (
! 160: also 0 or 1 quantifier
! 161: also quantifier minimizer
! 162: * 0 or more quantifier
! 163: + 1 or more quantifier
! 164: also "possessive quantifier"
! 165: { start min/max quantifier
! 166: .sp
! 167: Part of a pattern that is in square brackets is called a "character class". In
! 168: a character class the only metacharacters are:
! 169: .sp
! 170: \e general escape character
! 171: ^ negate the class, but only if the first character
! 172: - indicates character range
! 173: .\" JOIN
! 174: [ POSIX character class (only if followed by POSIX
! 175: syntax)
! 176: ] terminates the character class
! 177: .sp
! 178: The following sections describe the use of each of the metacharacters.
! 179: .
! 180: .
! 181: .SH BACKSLASH
! 182: .rs
! 183: .sp
! 184: The backslash character has several uses. Firstly, if it is followed by a
! 185: character that is not a number or a letter, it takes away any special meaning
! 186: that character may have. This use of backslash as an escape character applies
! 187: both inside and outside character classes.
! 188: .P
! 189: For example, if you want to match a * character, you write \e* in the pattern.
! 190: This escaping action applies whether or not the following character would
! 191: otherwise be interpreted as a metacharacter, so it is always safe to precede a
! 192: non-alphanumeric with backslash to specify that it stands for itself. In
! 193: particular, if you want to match a backslash, you write \e\e.
! 194: .P
! 195: In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
! 196: backslash. All other characters (in particular, those whose codepoints are
! 197: greater than 127) are treated as literals.
! 198: .P
! 199: If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
! 200: pattern (other than in a character class) and characters between a # outside
! 201: a character class and the next newline are ignored. An escaping backslash can
! 202: be used to include a whitespace or # character as part of the pattern.
! 203: .P
! 204: If you want to remove the special meaning from a sequence of characters, you
! 205: can do so by putting them between \eQ and \eE. This is different from Perl in
! 206: that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
! 207: Perl, $ and @ cause variable interpolation. Note the following examples:
! 208: .sp
! 209: Pattern PCRE matches Perl matches
! 210: .sp
! 211: .\" JOIN
! 212: \eQabc$xyz\eE abc$xyz abc followed by the
! 213: contents of $xyz
! 214: \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
! 215: \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
! 216: .sp
! 217: The \eQ...\eE sequence is recognized both inside and outside character classes.
! 218: An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
! 219: by \eE later in the pattern, the literal interpretation continues to the end of
! 220: the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
! 221: a character class, this causes an error, because the character class is not
! 222: terminated.
! 223: .
! 224: .
! 225: .\" HTML <a name="digitsafterbackslash"></a>
! 226: .SS "Non-printing characters"
! 227: .rs
! 228: .sp
! 229: A second use of backslash provides a way of encoding non-printing characters
! 230: in patterns in a visible manner. There is no restriction on the appearance of
! 231: non-printing characters, apart from the binary zero that terminates a pattern,
! 232: but when a pattern is being prepared by text editing, it is often easier to use
! 233: one of the following escape sequences than the binary character it represents:
! 234: .sp
! 235: \ea alarm, that is, the BEL character (hex 07)
! 236: \ecx "control-x", where x is any ASCII character
! 237: \ee escape (hex 1B)
! 238: \ef formfeed (hex 0C)
! 239: \en linefeed (hex 0A)
! 240: \er carriage return (hex 0D)
! 241: \et tab (hex 09)
! 242: \eddd character with octal code ddd, or back reference
! 243: \exhh character with hex code hh
! 244: \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
! 245: \euhhhh character with hex code hhhh (JavaScript mode only)
! 246: .sp
! 247: The precise effect of \ecx is as follows: if x is a lower case letter, it
! 248: is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
! 249: Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
! 250: \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
! 251: than 127, a compile-time error occurs. This locks out non-ASCII characters in
! 252: both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
! 253: values are valid. A lower case letter is converted to upper case, and then the
! 254: 0xc0 bits are flipped.)
! 255: .P
! 256: By default, after \ex, from zero to two hexadecimal digits are read (letters
! 257: can be in upper or lower case). Any number of hexadecimal digits may appear
! 258: between \ex{ and }, but the value of the character code must be less than 256
! 259: in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum
! 260: value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest
! 261: Unicode code point, which is 10FFFF.
! 262: .P
! 263: If characters other than hexadecimal digits appear between \ex{ and }, or if
! 264: there is no terminating }, this form of escape is not recognized. Instead, the
! 265: initial \ex will be interpreted as a basic hexadecimal escape, with no
! 266: following digits, giving a character whose value is zero.
! 267: .P
! 268: If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
! 269: as just described only when it is followed by two hexadecimal digits.
! 270: Otherwise, it matches a literal "x" character. In JavaScript mode, support for
! 271: code points greater than 256 is provided by \eu, which must be followed by
! 272: four hexadecimal digits; otherwise it matches a literal "u" character.
! 273: .P
! 274: Characters whose value is less than 256 can be defined by either of the two
! 275: syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
! 276: way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
! 277: \eu00dc in JavaScript mode).
! 278: .P
! 279: After \e0 up to two further octal digits are read. If there are fewer than two
! 280: digits, just those that are present are used. Thus the sequence \e0\ex\e07
! 281: specifies two binary zeros followed by a BEL character (code value 7). Make
! 282: sure you supply two digits after the initial zero if the pattern character that
! 283: follows is itself an octal digit.
! 284: .P
! 285: The handling of a backslash followed by a digit other than 0 is complicated.
! 286: Outside a character class, PCRE reads it and any following digits as a decimal
! 287: number. If the number is less than 10, or if there have been at least that many
! 288: previous capturing left parentheses in the expression, the entire sequence is
! 289: taken as a \fIback reference\fP. A description of how this works is given
! 290: .\" HTML <a href="#backreferences">
! 291: .\" </a>
! 292: later,
! 293: .\"
! 294: following the discussion of
! 295: .\" HTML <a href="#subpattern">
! 296: .\" </a>
! 297: parenthesized subpatterns.
! 298: .\"
! 299: .P
! 300: Inside a character class, or if the decimal number is greater than 9 and there
! 301: have not been that many capturing subpatterns, PCRE re-reads up to three octal
! 302: digits following the backslash, and uses them to generate a data character. Any
! 303: subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
! 304: character specified in octal must be less than \e400. In UTF-8 mode, values up
! 305: to \e777 are permitted. For example:
! 306: .sp
! 307: \e040 is another way of writing a space
! 308: .\" JOIN
! 309: \e40 is the same, provided there are fewer than 40
! 310: previous capturing subpatterns
! 311: \e7 is always a back reference
! 312: .\" JOIN
! 313: \e11 might be a back reference, or another way of
! 314: writing a tab
! 315: \e011 is always a tab
! 316: \e0113 is a tab followed by the character "3"
! 317: .\" JOIN
! 318: \e113 might be a back reference, otherwise the
! 319: character with octal code 113
! 320: .\" JOIN
! 321: \e377 might be a back reference, otherwise
! 322: the byte consisting entirely of 1 bits
! 323: .\" JOIN
! 324: \e81 is either a back reference, or a binary zero
! 325: followed by the two characters "8" and "1"
! 326: .sp
! 327: Note that octal values of 100 or greater must not be introduced by a leading
! 328: zero, because no more than three octal digits are ever read.
! 329: .P
! 330: All the sequences that define a single character value can be used both inside
! 331: and outside character classes. In addition, inside a character class, \eb is
! 332: interpreted as the backspace character (hex 08).
! 333: .P
! 334: \eN is not allowed in a character class. \eB, \eR, and \eX are not special
! 335: inside a character class. Like other unrecognized escape sequences, they are
! 336: treated as the literal characters "B", "R", and "X" by default, but cause an
! 337: error if the PCRE_EXTRA option is set. Outside a character class, these
! 338: sequences have different meanings.
! 339: .
! 340: .
! 341: .SS "Unsupported escape sequences"
! 342: .rs
! 343: .sp
! 344: In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
! 345: handler and used to modify the case of following characters. By default, PCRE
! 346: does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
! 347: option is set, \eU matches a "U" character, and \eu can be used to define a
! 348: character by code point, as described in the previous section.
! 349: .
! 350: .
! 351: .SS "Absolute and relative back references"
! 352: .rs
! 353: .sp
! 354: The sequence \eg followed by an unsigned or a negative number, optionally
! 355: enclosed in braces, is an absolute or relative back reference. A named back
! 356: reference can be coded as \eg{name}. Back references are discussed
! 357: .\" HTML <a href="#backreferences">
! 358: .\" </a>
! 359: later,
! 360: .\"
! 361: following the discussion of
! 362: .\" HTML <a href="#subpattern">
! 363: .\" </a>
! 364: parenthesized subpatterns.
! 365: .\"
! 366: .
! 367: .
! 368: .SS "Absolute and relative subroutine calls"
! 369: .rs
! 370: .sp
! 371: For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
! 372: a number enclosed either in angle brackets or single quotes, is an alternative
! 373: syntax for referencing a subpattern as a "subroutine". Details are discussed
! 374: .\" HTML <a href="#onigurumasubroutines">
! 375: .\" </a>
! 376: later.
! 377: .\"
! 378: Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
! 379: synonymous. The former is a back reference; the latter is a
! 380: .\" HTML <a href="#subpatternsassubroutines">
! 381: .\" </a>
! 382: subroutine
! 383: .\"
! 384: call.
! 385: .
! 386: .
! 387: .\" HTML <a name="genericchartypes"></a>
! 388: .SS "Generic character types"
! 389: .rs
! 390: .sp
! 391: Another use of backslash is for specifying generic character types:
! 392: .sp
! 393: \ed any decimal digit
! 394: \eD any character that is not a decimal digit
! 395: \eh any horizontal whitespace character
! 396: \eH any character that is not a horizontal whitespace character
! 397: \es any whitespace character
! 398: \eS any character that is not a whitespace character
! 399: \ev any vertical whitespace character
! 400: \eV any character that is not a vertical whitespace character
! 401: \ew any "word" character
! 402: \eW any "non-word" character
! 403: .sp
! 404: There is also the single sequence \eN, which matches a non-newline character.
! 405: This is the same as
! 406: .\" HTML <a href="#fullstopdot">
! 407: .\" </a>
! 408: the "." metacharacter
! 409: .\"
! 410: when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
! 411: PCRE does not support this.
! 412: .P
! 413: Each pair of lower and upper case escape sequences partitions the complete set
! 414: of characters into two disjoint sets. Any given character matches one, and only
! 415: one, of each pair. The sequences can appear both inside and outside character
! 416: classes. They each match one character of the appropriate type. If the current
! 417: matching point is at the end of the subject string, all of them fail, because
! 418: there is no character to match.
! 419: .P
! 420: For compatibility with Perl, \es does not match the VT character (code 11).
! 421: This makes it different from the the POSIX "space" class. The \es characters
! 422: are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
! 423: included in a Perl script, \es may match the VT character. In PCRE, it never
! 424: does.
! 425: .P
! 426: A "word" character is an underscore or any character that is a letter or digit.
! 427: By default, the definition of letters and digits is controlled by PCRE's
! 428: low-valued character tables, and may vary if locale-specific matching is taking
! 429: place (see
! 430: .\" HTML <a href="pcreapi.html#localesupport">
! 431: .\" </a>
! 432: "Locale support"
! 433: .\"
! 434: in the
! 435: .\" HREF
! 436: \fBpcreapi\fP
! 437: .\"
! 438: page). For example, in a French locale such as "fr_FR" in Unix-like systems,
! 439: or "french" in Windows, some character codes greater than 128 are used for
! 440: accented letters, and these are then matched by \ew. The use of locales with
! 441: Unicode is discouraged.
! 442: .P
! 443: By default, in UTF-8 mode, characters with values greater than 128 never match
! 444: \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
! 445: their original meanings from before UTF-8 support was available, mainly for
! 446: efficiency reasons. However, if PCRE is compiled with Unicode property support,
! 447: and the PCRE_UCP option is set, the behaviour is changed so that Unicode
! 448: properties are used to determine character types, as follows:
! 449: .sp
! 450: \ed any character that \ep{Nd} matches (decimal digit)
! 451: \es any character that \ep{Z} matches, plus HT, LF, FF, CR
! 452: \ew any character that \ep{L} or \ep{N} matches, plus underscore
! 453: .sp
! 454: The upper case escapes match the inverse sets of characters. Note that \ed
! 455: matches only decimal digits, whereas \ew matches any Unicode digit, as well as
! 456: any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
! 457: \eB because they are defined in terms of \ew and \eW. Matching these sequences
! 458: is noticeably slower when PCRE_UCP is set.
! 459: .P
! 460: The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
! 461: release 5.10. In contrast to the other sequences, which match only ASCII
! 462: characters by default, these always match certain high-valued codepoints in
! 463: UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
! 464: are:
! 465: .sp
! 466: U+0009 Horizontal tab
! 467: U+0020 Space
! 468: U+00A0 Non-break space
! 469: U+1680 Ogham space mark
! 470: U+180E Mongolian vowel separator
! 471: U+2000 En quad
! 472: U+2001 Em quad
! 473: U+2002 En space
! 474: U+2003 Em space
! 475: U+2004 Three-per-em space
! 476: U+2005 Four-per-em space
! 477: U+2006 Six-per-em space
! 478: U+2007 Figure space
! 479: U+2008 Punctuation space
! 480: U+2009 Thin space
! 481: U+200A Hair space
! 482: U+202F Narrow no-break space
! 483: U+205F Medium mathematical space
! 484: U+3000 Ideographic space
! 485: .sp
! 486: The vertical space characters are:
! 487: .sp
! 488: U+000A Linefeed
! 489: U+000B Vertical tab
! 490: U+000C Formfeed
! 491: U+000D Carriage return
! 492: U+0085 Next line
! 493: U+2028 Line separator
! 494: U+2029 Paragraph separator
! 495: .
! 496: .
! 497: .\" HTML <a name="newlineseq"></a>
! 498: .SS "Newline sequences"
! 499: .rs
! 500: .sp
! 501: Outside a character class, by default, the escape sequence \eR matches any
! 502: Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
! 503: .sp
! 504: (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
! 505: .sp
! 506: This is an example of an "atomic group", details of which are given
! 507: .\" HTML <a href="#atomicgroup">
! 508: .\" </a>
! 509: below.
! 510: .\"
! 511: This particular group matches either the two-character sequence CR followed by
! 512: LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
! 513: U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
! 514: line, U+0085). The two-character sequence is treated as a single unit that
! 515: cannot be split.
! 516: .P
! 517: In UTF-8 mode, two additional characters whose codepoints are greater than 255
! 518: are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
! 519: Unicode character property support is not needed for these characters to be
! 520: recognized.
! 521: .P
! 522: It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
! 523: complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
! 524: either at compile time or when the pattern is matched. (BSR is an abbrevation
! 525: for "backslash R".) This can be made the default when PCRE is built; if this is
! 526: the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
! 527: It is also possible to specify these settings by starting a pattern string with
! 528: one of the following sequences:
! 529: .sp
! 530: (*BSR_ANYCRLF) CR, LF, or CRLF only
! 531: (*BSR_UNICODE) any Unicode newline sequence
! 532: .sp
! 533: These override the default and the options given to \fBpcre_compile()\fP or
! 534: \fBpcre_compile2()\fP, but they can be overridden by options given to
! 535: \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
! 536: which are not Perl-compatible, are recognized only at the very start of a
! 537: pattern, and that they must be in upper case. If more than one of them is
! 538: present, the last one is used. They can be combined with a change of newline
! 539: convention; for example, a pattern can start with:
! 540: .sp
! 541: (*ANY)(*BSR_ANYCRLF)
! 542: .sp
! 543: They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
! 544: a character class, \eR is treated as an unrecognized escape sequence, and so
! 545: matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
! 546: .
! 547: .
! 548: .\" HTML <a name="uniextseq"></a>
! 549: .SS Unicode character properties
! 550: .rs
! 551: .sp
! 552: When PCRE is built with Unicode character property support, three additional
! 553: escape sequences that match characters with specific properties are available.
! 554: When not in UTF-8 mode, these sequences are of course limited to testing
! 555: characters whose codepoints are less than 256, but they do work in this mode.
! 556: The extra escape sequences are:
! 557: .sp
! 558: \ep{\fIxx\fP} a character with the \fIxx\fP property
! 559: \eP{\fIxx\fP} a character without the \fIxx\fP property
! 560: \eX an extended Unicode sequence
! 561: .sp
! 562: The property names represented by \fIxx\fP above are limited to the Unicode
! 563: script names, the general category properties, "Any", which matches any
! 564: character (including newline), and some special PCRE properties (described
! 565: in the
! 566: .\" HTML <a href="#extraprops">
! 567: .\" </a>
! 568: next section).
! 569: .\"
! 570: Other Perl properties such as "InMusicalSymbols" are not currently supported by
! 571: PCRE. Note that \eP{Any} does not match any characters, so always causes a
! 572: match failure.
! 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: .sp
! 578: \ep{Greek}
! 579: \eP{Han}
! 580: .sp
! 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: Arabic,
! 585: Armenian,
! 586: Avestan,
! 587: Balinese,
! 588: Bamum,
! 589: Bengali,
! 590: Bopomofo,
! 591: Braille,
! 592: Buginese,
! 593: Buhid,
! 594: Canadian_Aboriginal,
! 595: Carian,
! 596: Cham,
! 597: Cherokee,
! 598: Common,
! 599: Coptic,
! 600: Cuneiform,
! 601: Cypriot,
! 602: Cyrillic,
! 603: Deseret,
! 604: Devanagari,
! 605: Egyptian_Hieroglyphs,
! 606: Ethiopic,
! 607: Georgian,
! 608: Glagolitic,
! 609: Gothic,
! 610: Greek,
! 611: Gujarati,
! 612: Gurmukhi,
! 613: Han,
! 614: Hangul,
! 615: Hanunoo,
! 616: Hebrew,
! 617: Hiragana,
! 618: Imperial_Aramaic,
! 619: Inherited,
! 620: Inscriptional_Pahlavi,
! 621: Inscriptional_Parthian,
! 622: Javanese,
! 623: Kaithi,
! 624: Kannada,
! 625: Katakana,
! 626: Kayah_Li,
! 627: Kharoshthi,
! 628: Khmer,
! 629: Lao,
! 630: Latin,
! 631: Lepcha,
! 632: Limbu,
! 633: Linear_B,
! 634: Lisu,
! 635: Lycian,
! 636: Lydian,
! 637: Malayalam,
! 638: Meetei_Mayek,
! 639: Mongolian,
! 640: Myanmar,
! 641: New_Tai_Lue,
! 642: Nko,
! 643: Ogham,
! 644: Old_Italic,
! 645: Old_Persian,
! 646: Old_South_Arabian,
! 647: Old_Turkic,
! 648: Ol_Chiki,
! 649: Oriya,
! 650: Osmanya,
! 651: Phags_Pa,
! 652: Phoenician,
! 653: Rejang,
! 654: Runic,
! 655: Samaritan,
! 656: Saurashtra,
! 657: Shavian,
! 658: Sinhala,
! 659: Sundanese,
! 660: Syloti_Nagri,
! 661: Syriac,
! 662: Tagalog,
! 663: Tagbanwa,
! 664: Tai_Le,
! 665: Tai_Tham,
! 666: Tai_Viet,
! 667: Tamil,
! 668: Telugu,
! 669: Thaana,
! 670: Thai,
! 671: Tibetan,
! 672: Tifinagh,
! 673: Ugaritic,
! 674: Vai,
! 675: Yi.
! 676: .P
! 677: Each character has exactly one Unicode general category property, specified by
! 678: a two-letter abbreviation. For compatibility with Perl, negation can be
! 679: specified by including a circumflex between the opening brace and the property
! 680: name. For example, \ep{^Lu} is the same as \eP{Lu}.
! 681: .P
! 682: If only one letter is specified with \ep or \eP, it includes all the general
! 683: category properties that start with that letter. In this case, in the absence
! 684: of negation, the curly brackets in the escape sequence are optional; these two
! 685: examples have the same effect:
! 686: .sp
! 687: \ep{L}
! 688: \epL
! 689: .sp
! 690: The following general category property codes are supported:
! 691: .sp
! 692: C Other
! 693: Cc Control
! 694: Cf Format
! 695: Cn Unassigned
! 696: Co Private use
! 697: Cs Surrogate
! 698: .sp
! 699: L Letter
! 700: Ll Lower case letter
! 701: Lm Modifier letter
! 702: Lo Other letter
! 703: Lt Title case letter
! 704: Lu Upper case letter
! 705: .sp
! 706: M Mark
! 707: Mc Spacing mark
! 708: Me Enclosing mark
! 709: Mn Non-spacing mark
! 710: .sp
! 711: N Number
! 712: Nd Decimal number
! 713: Nl Letter number
! 714: No Other number
! 715: .sp
! 716: P Punctuation
! 717: Pc Connector punctuation
! 718: Pd Dash punctuation
! 719: Pe Close punctuation
! 720: Pf Final punctuation
! 721: Pi Initial punctuation
! 722: Po Other punctuation
! 723: Ps Open punctuation
! 724: .sp
! 725: S Symbol
! 726: Sc Currency symbol
! 727: Sk Modifier symbol
! 728: Sm Mathematical symbol
! 729: So Other symbol
! 730: .sp
! 731: Z Separator
! 732: Zl Line separator
! 733: Zp Paragraph separator
! 734: Zs Space separator
! 735: .sp
! 736: The special property L& is also supported: it matches a character that has
! 737: the Lu, Ll, or Lt property, in other words, a letter that is not classified as
! 738: a modifier or "other".
! 739: .P
! 740: The Cs (Surrogate) property applies only to characters in the range U+D800 to
! 741: U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
! 742: cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
! 743: (see the discussion of PCRE_NO_UTF8_CHECK in the
! 744: .\" HREF
! 745: \fBpcreapi\fP
! 746: .\"
! 747: page). Perl does not support the Cs property.
! 748: .P
! 749: The long synonyms for property names that Perl supports (such as \ep{Letter})
! 750: are not supported by PCRE, nor is it permitted to prefix any of these
! 751: properties with "Is".
! 752: .P
! 753: No character that is in the Unicode table has the Cn (unassigned) property.
! 754: Instead, this property is assumed for any code point that is not in the
! 755: Unicode table.
! 756: .P
! 757: Specifying caseless matching does not affect these escape sequences. For
! 758: example, \ep{Lu} always matches only upper case letters.
! 759: .P
! 760: The \eX escape matches any number of Unicode characters that form an extended
! 761: Unicode sequence. \eX is equivalent to
! 762: .sp
! 763: (?>\ePM\epM*)
! 764: .sp
! 765: That is, it matches a character without the "mark" property, followed by zero
! 766: or more characters with the "mark" property, and treats the sequence as an
! 767: atomic group
! 768: .\" HTML <a href="#atomicgroup">
! 769: .\" </a>
! 770: (see below).
! 771: .\"
! 772: Characters with the "mark" property are typically accents that affect the
! 773: preceding character. None of them have codepoints less than 256, so in
! 774: non-UTF-8 mode \eX matches any one character.
! 775: .P
! 776: Note that recent versions of Perl have changed \eX to match what Unicode calls
! 777: an "extended grapheme cluster", which has a more complicated definition.
! 778: .P
! 779: Matching characters by Unicode property is not fast, because PCRE has to search
! 780: a structure that contains data for over fifteen thousand characters. That is
! 781: why the traditional escape sequences such as \ed and \ew do not use Unicode
! 782: properties in PCRE by default, though you can make them do so by setting the
! 783: PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
! 784: (*UCP).
! 785: .
! 786: .
! 787: .\" HTML <a name="extraprops"></a>
! 788: .SS PCRE's additional properties
! 789: .rs
! 790: .sp
! 791: As well as the standard Unicode properties described in the previous
! 792: section, PCRE supports four more that make it possible to convert traditional
! 793: escape sequences such as \ew and \es and POSIX character classes to use Unicode
! 794: properties. PCRE uses these non-standard, non-Perl properties internally when
! 795: PCRE_UCP is set. They are:
! 796: .sp
! 797: Xan Any alphanumeric character
! 798: Xps Any POSIX space character
! 799: Xsp Any Perl space character
! 800: Xwd Any Perl "word" character
! 801: .sp
! 802: Xan matches characters that have either the L (letter) or the N (number)
! 803: property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
! 804: carriage return, and any other character that has the Z (separator) property.
! 805: Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
! 806: same characters as Xan, plus underscore.
! 807: .
! 808: .
! 809: .\" HTML <a name="resetmatchstart"></a>
! 810: .SS "Resetting the match start"
! 811: .rs
! 812: .sp
! 813: The escape sequence \eK causes any previously matched characters not to be
! 814: included in the final matched sequence. For example, the pattern:
! 815: .sp
! 816: foo\eKbar
! 817: .sp
! 818: matches "foobar", but reports that it has matched "bar". This feature is
! 819: similar to a lookbehind assertion
! 820: .\" HTML <a href="#lookbehind">
! 821: .\" </a>
! 822: (described below).
! 823: .\"
! 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 \eK does
! 826: not interfere with the setting of
! 827: .\" HTML <a href="#subpattern">
! 828: .\" </a>
! 829: captured substrings.
! 830: .\"
! 831: For example, when the pattern
! 832: .sp
! 833: (foo)\eKbar
! 834: .sp
! 835: matches "foobar", the first substring is still set to "foo".
! 836: .P
! 837: Perl documents that the use of \eK within assertions is "not well defined". In
! 838: PCRE, \eK is acted upon when it occurs inside positive assertions, but is
! 839: ignored in negative assertions.
! 840: .
! 841: .
! 842: .\" HTML <a name="smallassertions"></a>
! 843: .SS "Simple assertions"
! 844: .rs
! 845: .sp
! 846: The final use of backslash is for certain simple assertions. An assertion
! 847: specifies a condition that has to be met at a particular point in a match,
! 848: without consuming any characters from the subject string. The use of
! 849: subpatterns for more complicated assertions is described
! 850: .\" HTML <a href="#bigassertions">
! 851: .\" </a>
! 852: below.
! 853: .\"
! 854: The backslashed assertions are:
! 855: .sp
! 856: \eb matches at a word boundary
! 857: \eB matches when not at a word boundary
! 858: \eA matches at the start of the subject
! 859: \eZ matches at the end of the subject
! 860: also matches before a newline at the end of the subject
! 861: \ez matches only at the end of the subject
! 862: \eG matches at the first matching position in the subject
! 863: .sp
! 864: Inside a character class, \eb has a different meaning; it matches the backspace
! 865: character. If any other of these assertions appears in a character class, by
! 866: default it matches the corresponding literal character (for example, \eB
! 867: matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
! 868: escape sequence" error is generated instead.
! 869: .P
! 870: A word boundary is a position in the subject string where the current character
! 871: and the previous character do not both match \ew or \eW (i.e. one matches
! 872: \ew and the other matches \eW), or the start or end of the string if the
! 873: first or last character matches \ew, respectively. In UTF-8 mode, the meanings
! 874: of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
! 875: done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
! 876: of word" or "end of word" metasequence. However, whatever follows \eb normally
! 877: determines which it is. For example, the fragment \eba matches "a" at the start
! 878: of a word.
! 879: .P
! 880: The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
! 881: dollar (described in the next section) in that they only ever match at the very
! 882: start and end of the subject string, whatever options are set. Thus, they are
! 883: independent of multiline mode. These three assertions are not affected by the
! 884: PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
! 885: circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
! 886: argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
! 887: at a point other than the beginning of the subject, \eA can never match. The
! 888: difference between \eZ and \ez is that \eZ matches before a newline at the end
! 889: of the string as well as at the very end, whereas \ez matches only at the end.
! 890: .P
! 891: The \eG assertion is true only when the current matching position is at the
! 892: start point of the match, as specified by the \fIstartoffset\fP argument of
! 893: \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
! 894: non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
! 895: arguments, you can mimic Perl's /g option, and it is in this kind of
! 896: implementation where \eG can be useful.
! 897: .P
! 898: Note, however, that PCRE's interpretation of \eG, as the start of the current
! 899: match, is subtly different from Perl's, which defines it as the end of the
! 900: previous match. In Perl, these can be different when the previously matched
! 901: string was empty. Because PCRE does just one match at a time, it cannot
! 902: reproduce this behaviour.
! 903: .P
! 904: If all the alternatives of a pattern begin with \eG, the expression is anchored
! 905: to the starting match position, and the "anchored" flag is set in the compiled
! 906: regular expression.
! 907: .
! 908: .
! 909: .SH "CIRCUMFLEX AND DOLLAR"
! 910: .rs
! 911: .sp
! 912: Outside a character class, in the default matching mode, the circumflex
! 913: character is an assertion that is true only if the current matching point is
! 914: at the start of the subject string. If the \fIstartoffset\fP argument of
! 915: \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
! 916: option is unset. Inside a character class, circumflex has an entirely different
! 917: meaning
! 918: .\" HTML <a href="#characterclass">
! 919: .\" </a>
! 920: (see below).
! 921: .\"
! 922: .P
! 923: Circumflex need not be the first character of the pattern if a number of
! 924: alternatives are involved, but it should be the first thing in each alternative
! 925: in which it appears if the pattern is ever to match that branch. If all
! 926: possible alternatives start with a circumflex, that is, if the pattern is
! 927: constrained to match only at the start of the subject, it is said to be an
! 928: "anchored" pattern. (There are also other constructs that can cause a pattern
! 929: to be anchored.)
! 930: .P
! 931: A dollar character is an assertion that is true only if the current matching
! 932: point is at the end of the subject string, or immediately before a newline
! 933: at the end of the string (by default). Dollar need not be the last character of
! 934: the pattern if a number of alternatives are involved, but it should be the last
! 935: item in any branch in which it appears. Dollar has no special meaning in a
! 936: character class.
! 937: .P
! 938: The meaning of dollar can be changed so that it matches only at the very end of
! 939: the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
! 940: does not affect the \eZ assertion.
! 941: .P
! 942: The meanings of the circumflex and dollar characters are changed if the
! 943: PCRE_MULTILINE option is set. When this is the case, a circumflex matches
! 944: immediately after internal newlines as well as at the start of the subject
! 945: string. It does not match after a newline that ends the string. A dollar
! 946: matches before any newlines in the string, as well as at the very end, when
! 947: PCRE_MULTILINE is set. When newline is specified as the two-character
! 948: sequence CRLF, isolated CR and LF characters do not indicate newlines.
! 949: .P
! 950: For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
! 951: \en represents a newline) in multiline mode, but not otherwise. Consequently,
! 952: patterns that are anchored in single line mode because all branches start with
! 953: ^ are not anchored in multiline mode, and a match for circumflex is possible
! 954: when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
! 955: PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
! 956: .P
! 957: Note that the sequences \eA, \eZ, and \ez 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: \eA it is always anchored, whether or not PCRE_MULTILINE is set.
! 960: .
! 961: .
! 962: .\" HTML <a name="fullstopdot"></a>
! 963: .SH "FULL STOP (PERIOD, DOT) AND \eN"
! 964: .rs
! 965: .sp
! 966: Outside a character class, a dot in the pattern matches any one character in
! 967: the subject string except (by default) a character that signifies the end of a
! 968: line. In UTF-8 mode, the matched character may be more than one byte long.
! 969: .P
! 970: When a line ending is defined as a single character, dot never matches that
! 971: character; when the two-character sequence CRLF is used, dot does not match CR
! 972: if it is immediately followed by LF, but otherwise it matches all characters
! 973: (including isolated CRs and LFs). When any Unicode line endings are being
! 974: recognized, dot does not match CR or LF or any of the other line ending
! 975: characters.
! 976: .P
! 977: The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
! 978: option is set, a dot matches any one character, without exception. If the
! 979: two-character sequence CRLF is present in the subject string, it takes two dots
! 980: to match it.
! 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: The escape sequence \eN behaves like a dot, except that it is not affected by
! 987: the PCRE_DOTALL option. In other words, it matches any character except one
! 988: that signifies the end of a line. Perl also uses \eN to match characters by
! 989: name; PCRE does not support this.
! 990: .
! 991: .
! 992: .SH "MATCHING A SINGLE BYTE"
! 993: .rs
! 994: .sp
! 995: Outside a character class, the escape sequence \eC matches any one byte, both
! 996: in and out of UTF-8 mode. Unlike a dot, it always matches line-ending
! 997: characters. The feature is provided in Perl in order to match individual bytes
! 998: in UTF-8 mode, but it is unclear how it can usefully be used. Because \eC
! 999: breaks up characters into individual bytes, matching one byte with \eC in UTF-8
! 1000: mode means that the rest of the string may start with a malformed UTF-8
! 1001: character. This has undefined results, because PCRE assumes that it is dealing
! 1002: with valid UTF-8 strings (and by default it checks this at the start of
! 1003: processing unless the PCRE_NO_UTF8_CHECK option is used).
! 1004: .P
! 1005: PCRE does not allow \eC to appear in lookbehind assertions
! 1006: .\" HTML <a href="#lookbehind">
! 1007: .\" </a>
! 1008: (described below)
! 1009: .\"
! 1010: in UTF-8 mode, because this would make it impossible to calculate the length of
! 1011: the lookbehind.
! 1012: .P
! 1013: In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one
! 1014: way of using it that avoids the problem of malformed UTF-8 characters is to
! 1015: use a lookahead to check the length of the next character, as in this pattern
! 1016: (ignore white space and line breaks):
! 1017: .sp
! 1018: (?| (?=[\ex00-\ex7f])(\eC) |
! 1019: (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
! 1020: (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
! 1021: (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
! 1022: .sp
! 1023: A group that starts with (?| resets the capturing parentheses numbers in each
! 1024: alternative (see
! 1025: .\" HTML <a href="#dupsubpatternnumber">
! 1026: .\" </a>
! 1027: "Duplicate Subpattern Numbers"
! 1028: .\"
! 1029: below). The assertions at the start of each branch check the next UTF-8
! 1030: character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
! 1031: character's individual bytes are then captured by the appropriate number of
! 1032: groups.
! 1033: .
! 1034: .
! 1035: .\" HTML <a name="characterclass"></a>
! 1036: .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
! 1037: .rs
! 1038: .sp
! 1039: An opening square bracket introduces a character class, terminated by a closing
! 1040: square bracket. A closing square bracket on its own is not special by default.
! 1041: However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
! 1042: bracket causes a compile-time error. If a closing square bracket is required as
! 1043: a member of the class, it should be the first data character in the class
! 1044: (after an initial circumflex, if present) or escaped with a backslash.
! 1045: .P
! 1046: A character class matches a single character in the subject. In UTF-8 mode, the
! 1047: character may be more than one byte long. A matched character must be in the
! 1048: set of characters defined by the class, unless the first character in the class
! 1049: definition is a circumflex, in which case the subject character must not be in
! 1050: the set defined by the class. If a circumflex is actually required as a member
! 1051: of the class, ensure it is not the first character, or escape it with a
! 1052: backslash.
! 1053: .P
! 1054: For example, the character class [aeiou] matches any lower case vowel, while
! 1055: [^aeiou] matches any character that is not a lower case vowel. Note that a
! 1056: circumflex is just a convenient notation for specifying the characters that
! 1057: are in the class by enumerating those that are not. A class that starts with a
! 1058: circumflex is not an assertion; it still consumes a character from the subject
! 1059: string, and therefore it fails if the current pointer is at the end of the
! 1060: string.
! 1061: .P
! 1062: In UTF-8 mode, characters with values greater than 255 can be included in a
! 1063: class as a literal string of bytes, or by using the \ex{ escaping mechanism.
! 1064: .P
! 1065: When caseless matching is set, any letters in a class represent both their
! 1066: upper case and lower case versions, so for example, a caseless [aeiou] matches
! 1067: "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
! 1068: caseful version would. In UTF-8 mode, PCRE always understands the concept of
! 1069: case for characters whose values are less than 128, so caseless matching is
! 1070: always possible. For characters with higher values, the concept of case is
! 1071: supported if PCRE is compiled with Unicode property support, but not otherwise.
! 1072: If you want to use caseless matching in UTF8-mode for characters 128 and above,
! 1073: you must ensure that PCRE is compiled with Unicode property support as well as
! 1074: with UTF-8 support.
! 1075: .P
! 1076: Characters that might indicate line breaks are never treated in any special way
! 1077: when matching character classes, whatever line-ending sequence is in use, and
! 1078: whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
! 1079: such as [^a] always matches one of these characters.
! 1080: .P
! 1081: The minus (hyphen) character can be used to specify a range of characters in a
! 1082: character class. For example, [d-m] matches any letter between d and m,
! 1083: inclusive. If a minus character is required in a class, it must be escaped with
! 1084: a backslash or appear in a position where it cannot be interpreted as
! 1085: indicating a range, typically as the first or last character in the class.
! 1086: .P
! 1087: It is not possible to have the literal character "]" as the end character of a
! 1088: range. A pattern such as [W-]46] is interpreted as a class of two characters
! 1089: ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
! 1090: "-46]". However, if the "]" is escaped with a backslash it is interpreted as
! 1091: the end of range, so [W-\e]46] is interpreted as a class containing a range
! 1092: followed by two other characters. The octal or hexadecimal representation of
! 1093: "]" can also be used to end a range.
! 1094: .P
! 1095: Ranges operate in the collating sequence of character values. They can also be
! 1096: used for characters specified numerically, for example [\e000-\e037]. In UTF-8
! 1097: mode, ranges can include characters whose values are greater than 255, for
! 1098: example [\ex{100}-\ex{2ff}].
! 1099: .P
! 1100: If a range that includes letters is used when caseless matching is set, it
! 1101: matches the letters in either case. For example, [W-c] is equivalent to
! 1102: [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
! 1103: tables for a French locale are in use, [\exc8-\excb] matches accented E
! 1104: characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
! 1105: characters with values greater than 128 only when it is compiled with Unicode
! 1106: property support.
! 1107: .P
! 1108: The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
! 1109: \eV, \ew, and \eW may appear in a character class, and add the characters that
! 1110: they match to the class. For example, [\edABCDEF] matches any hexadecimal
! 1111: digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
! 1112: and their upper case partners, just as it does when they appear outside a
! 1113: character class, as described in the section entitled
! 1114: .\" HTML <a href="#genericchartypes">
! 1115: .\" </a>
! 1116: "Generic character types"
! 1117: .\"
! 1118: above. The escape sequence \eb has a different meaning inside a character
! 1119: class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
! 1120: are not special inside a character class. Like any other unrecognized escape
! 1121: sequences, they are treated as the literal characters "B", "N", "R", and "X" by
! 1122: default, but cause an error if the PCRE_EXTRA option is set.
! 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 [^\eW_] matches any letter or digit, but not underscore,
! 1127: whereas [\ew] 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: The only metacharacters that are recognized in character classes are backslash,
! 1132: hyphen (only where it can be interpreted as specifying a range), circumflex
! 1133: (only at the start), opening square bracket (only when it can be interpreted as
! 1134: introducing a POSIX class name - see the next section), and the terminating
! 1135: closing square bracket. However, escaping other non-alphanumeric characters
! 1136: does no harm.
! 1137: .
! 1138: .
! 1139: .SH "POSIX CHARACTER CLASSES"
! 1140: .rs
! 1141: .sp
! 1142: Perl supports the POSIX notation for character classes. This uses names
! 1143: enclosed by [: and :] within the enclosing square brackets. PCRE also supports
! 1144: this notation. For example,
! 1145: .sp
! 1146: [01[:alpha:]%]
! 1147: .sp
! 1148: matches "0", "1", any alphabetic character, or "%". The supported class names
! 1149: are:
! 1150: .sp
! 1151: alnum letters and digits
! 1152: alpha letters
! 1153: ascii character codes 0 - 127
! 1154: blank space or tab only
! 1155: cntrl control characters
! 1156: digit decimal digits (same as \ed)
! 1157: graph printing characters, excluding space
! 1158: lower lower case letters
! 1159: print printing characters, including space
! 1160: punct printing characters, excluding letters and digits and space
! 1161: space white space (not quite the same as \es)
! 1162: upper upper case letters
! 1163: word "word" characters (same as \ew)
! 1164: xdigit hexadecimal digits
! 1165: .sp
! 1166: The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
! 1167: space (32). Notice that this list includes the VT character (code 11). This
! 1168: makes "space" different to \es, which does not include VT (for Perl
! 1169: compatibility).
! 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: .sp
! 1175: [12[:^digit:]]
! 1176: .sp
! 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: By default, in UTF-8 mode, characters with values greater than 128 do not match
! 1182: any of the POSIX character classes. However, if the PCRE_UCP option is passed
! 1183: to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
! 1184: character properties are used. This is achieved by replacing the POSIX classes
! 1185: by other sequences, as follows:
! 1186: .sp
! 1187: [:alnum:] becomes \ep{Xan}
! 1188: [:alpha:] becomes \ep{L}
! 1189: [:blank:] becomes \eh
! 1190: [:digit:] becomes \ep{Nd}
! 1191: [:lower:] becomes \ep{Ll}
! 1192: [:space:] becomes \ep{Xps}
! 1193: [:upper:] becomes \ep{Lu}
! 1194: [:word:] becomes \ep{Xwd}
! 1195: .sp
! 1196: Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
! 1197: classes are unchanged, and match only characters with code points less than
! 1198: 128.
! 1199: .
! 1200: .
! 1201: .SH "VERTICAL BAR"
! 1202: .rs
! 1203: .sp
! 1204: Vertical bar characters are used to separate alternative patterns. For example,
! 1205: the pattern
! 1206: .sp
! 1207: gilbert|sullivan
! 1208: .sp
! 1209: matches either "gilbert" or "sullivan". Any number of alternatives may appear,
! 1210: and an empty alternative is permitted (matching the empty string). The matching
! 1211: process tries each alternative in turn, from left to right, and the first one
! 1212: that succeeds is used. If the alternatives are within a subpattern
! 1213: .\" HTML <a href="#subpattern">
! 1214: .\" </a>
! 1215: (defined below),
! 1216: .\"
! 1217: "succeeds" means matching the rest of the main pattern as well as the
! 1218: alternative in the subpattern.
! 1219: .
! 1220: .
! 1221: .SH "INTERNAL OPTION SETTING"
! 1222: .rs
! 1223: .sp
! 1224: The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
! 1225: PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
! 1226: the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
! 1227: The option letters are
! 1228: .sp
! 1229: i for PCRE_CASELESS
! 1230: m for PCRE_MULTILINE
! 1231: s for PCRE_DOTALL
! 1232: x for PCRE_EXTENDED
! 1233: .sp
! 1234: For example, (?im) sets caseless, multiline matching. It is also possible to
! 1235: unset these options by preceding the letter with a hyphen, and a combined
! 1236: setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
! 1237: PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
! 1238: permitted. If a letter appears both before and after the hyphen, the option is
! 1239: unset.
! 1240: .P
! 1241: The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
! 1242: changed in the same way as the Perl-compatible options by using the characters
! 1243: J, U and X respectively.
! 1244: .P
! 1245: When one of these option changes occurs at top level (that is, not inside
! 1246: subpattern parentheses), the change applies to the remainder of the pattern
! 1247: that follows. If the change is placed right at the start of a pattern, PCRE
! 1248: extracts it into the global options (and it will therefore show up in data
! 1249: extracted by the \fBpcre_fullinfo()\fP function).
! 1250: .P
! 1251: An option change within a subpattern (see below for a description of
! 1252: subpatterns) affects only that part of the subpattern that follows it, so
! 1253: .sp
! 1254: (a(?i)b)c
! 1255: .sp
! 1256: matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
! 1257: By this means, options can be made to have different settings in different
! 1258: parts of the pattern. Any changes made in one alternative do carry on
! 1259: into subsequent branches within the same subpattern. For example,
! 1260: .sp
! 1261: (a(?i)b|c)
! 1262: .sp
! 1263: matches "ab", "aB", "c", and "C", even though when matching "C" the first
! 1264: branch is abandoned before the option setting. This is because the effects of
! 1265: option settings happen at compile time. There would be some very weird
! 1266: behaviour otherwise.
! 1267: .P
! 1268: \fBNote:\fP There are other PCRE-specific options that can be set by the
! 1269: application when the compile or match functions are called. In some cases the
! 1270: pattern can contain special leading sequences such as (*CRLF) to override what
! 1271: the application has set or what has been defaulted. Details are given in the
! 1272: section entitled
! 1273: .\" HTML <a href="#newlineseq">
! 1274: .\" </a>
! 1275: "Newline sequences"
! 1276: .\"
! 1277: above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
! 1278: to set UTF-8 and Unicode property modes; they are equivalent to setting the
! 1279: PCRE_UTF8 and the PCRE_UCP options, respectively.
! 1280: .
! 1281: .
! 1282: .\" HTML <a name="subpattern"></a>
! 1283: .SH SUBPATTERNS
! 1284: .rs
! 1285: .sp
! 1286: Subpatterns are delimited by parentheses (round brackets), which can be nested.
! 1287: Turning part of a pattern into a subpattern does two things:
! 1288: .sp
! 1289: 1. It localizes a set of alternatives. For example, the pattern
! 1290: .sp
! 1291: cat(aract|erpillar|)
! 1292: .sp
! 1293: matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
! 1294: match "cataract", "erpillar" or an empty string.
! 1295: .sp
! 1296: 2. It sets up the subpattern as a capturing subpattern. This means that, when
! 1297: the whole pattern matches, that portion of the subject string that matched the
! 1298: subpattern is passed back to the caller via the \fIovector\fP argument of
! 1299: \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
! 1300: from 1) to obtain numbers for the capturing subpatterns. For example, if the
! 1301: string "the red king" is matched against the pattern
! 1302: .sp
! 1303: the ((red|white) (king|queen))
! 1304: .sp
! 1305: the captured substrings are "red king", "red", and "king", and are numbered 1,
! 1306: 2, and 3, respectively.
! 1307: .P
! 1308: The fact that plain parentheses fulfil two functions is not always helpful.
! 1309: There are often times when a grouping subpattern is required without a
! 1310: capturing requirement. If an opening parenthesis is followed by a question mark
! 1311: and a colon, the subpattern does not do any capturing, and is not counted when
! 1312: computing the number of any subsequent capturing subpatterns. For example, if
! 1313: the string "the white queen" is matched against the pattern
! 1314: .sp
! 1315: the ((?:red|white) (king|queen))
! 1316: .sp
! 1317: the captured substrings are "white queen" and "queen", and are numbered 1 and
! 1318: 2. The maximum number of capturing subpatterns is 65535.
! 1319: .P
! 1320: As a convenient shorthand, if any option settings are required at the start of
! 1321: a non-capturing subpattern, the option letters may appear between the "?" and
! 1322: the ":". Thus the two patterns
! 1323: .sp
! 1324: (?i:saturday|sunday)
! 1325: (?:(?i)saturday|sunday)
! 1326: .sp
! 1327: match exactly the same set of strings. Because alternative branches are tried
! 1328: from left to right, and options are not reset until the end of the subpattern
! 1329: is reached, an option setting in one branch does affect subsequent branches, so
! 1330: the above patterns match "SUNDAY" as well as "Saturday".
! 1331: .
! 1332: .
! 1333: .\" HTML <a name="dupsubpatternnumber"></a>
! 1334: .SH "DUPLICATE SUBPATTERN NUMBERS"
! 1335: .rs
! 1336: .sp
! 1337: Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
! 1338: the same numbers for its capturing parentheses. Such a subpattern starts with
! 1339: (?| and is itself a non-capturing subpattern. For example, consider this
! 1340: pattern:
! 1341: .sp
! 1342: (?|(Sat)ur|(Sun))day
! 1343: .sp
! 1344: Because the two alternatives are inside a (?| group, both sets of capturing
! 1345: parentheses are numbered one. Thus, when the pattern matches, you can look
! 1346: at captured substring number one, whichever alternative matched. This construct
! 1347: is useful when you want to capture part, but not all, of one of a number of
! 1348: alternatives. Inside a (?| group, parentheses are numbered as usual, but the
! 1349: number is reset at the start of each branch. The numbers of any capturing
! 1350: parentheses that follow the subpattern start after the highest number used in
! 1351: any branch. The following example is taken from the Perl documentation. The
! 1352: numbers underneath show in which buffer the captured content will be stored.
! 1353: .sp
! 1354: # before ---------------branch-reset----------- after
! 1355: / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
! 1356: # 1 2 2 3 2 3 4
! 1357: .sp
! 1358: A back reference to a numbered subpattern uses the most recent value that is
! 1359: set for that number by any subpattern. The following pattern matches "abcabc"
! 1360: or "defdef":
! 1361: .sp
! 1362: /(?|(abc)|(def))\e1/
! 1363: .sp
! 1364: In contrast, a subroutine call to a numbered subpattern always refers to the
! 1365: first one in the pattern with the given number. The following pattern matches
! 1366: "abcabc" or "defabc":
! 1367: .sp
! 1368: /(?|(abc)|(def))(?1)/
! 1369: .sp
! 1370: If a
! 1371: .\" HTML <a href="#conditions">
! 1372: .\" </a>
! 1373: condition test
! 1374: .\"
! 1375: for a subpattern's having matched refers to a non-unique number, the test is
! 1376: true if any of the subpatterns of that number have matched.
! 1377: .P
! 1378: An alternative approach to using this "branch reset" feature is to use
! 1379: duplicate named subpatterns, as described in the next section.
! 1380: .
! 1381: .
! 1382: .SH "NAMED SUBPATTERNS"
! 1383: .rs
! 1384: .sp
! 1385: Identifying capturing parentheses by number is simple, but it can be very hard
! 1386: to keep track of the numbers in complicated regular expressions. Furthermore,
! 1387: if an expression is modified, the numbers may change. To help with this
! 1388: difficulty, PCRE supports the naming of subpatterns. This feature was not
! 1389: added to Perl until release 5.10. Python had the feature earlier, and PCRE
! 1390: introduced it at release 4.0, using the Python syntax. PCRE now supports both
! 1391: the Perl and the Python syntax. Perl allows identically numbered subpatterns to
! 1392: have different names, but PCRE does not.
! 1393: .P
! 1394: In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
! 1395: (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
! 1396: parentheses from other parts of the pattern, such as
! 1397: .\" HTML <a href="#backreferences">
! 1398: .\" </a>
! 1399: back references,
! 1400: .\"
! 1401: .\" HTML <a href="#recursion">
! 1402: .\" </a>
! 1403: recursion,
! 1404: .\"
! 1405: and
! 1406: .\" HTML <a href="#conditions">
! 1407: .\" </a>
! 1408: conditions,
! 1409: .\"
! 1410: can be made by name as well as by number.
! 1411: .P
! 1412: Names consist of up to 32 alphanumeric characters and underscores. Named
! 1413: capturing parentheses are still allocated numbers as well as names, exactly as
! 1414: if the names were not present. The PCRE API provides function calls for
! 1415: extracting the name-to-number translation table from a compiled pattern. There
! 1416: is also a convenience function for extracting a captured substring by name.
! 1417: .P
! 1418: By default, a name must be unique within a pattern, but it is possible to relax
! 1419: this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
! 1420: names are also always permitted for subpatterns with the same number, set up as
! 1421: described in the previous section.) Duplicate names can be useful for patterns
! 1422: where only one instance of the named parentheses can match. Suppose you want to
! 1423: match the name of a weekday, either as a 3-letter abbreviation or as the full
! 1424: name, and in both cases you want to extract the abbreviation. This pattern
! 1425: (ignoring the line breaks) does the job:
! 1426: .sp
! 1427: (?<DN>Mon|Fri|Sun)(?:day)?|
! 1428: (?<DN>Tue)(?:sday)?|
! 1429: (?<DN>Wed)(?:nesday)?|
! 1430: (?<DN>Thu)(?:rsday)?|
! 1431: (?<DN>Sat)(?:urday)?
! 1432: .sp
! 1433: There are five capturing substrings, but only one is ever set after a match.
! 1434: (An alternative way of solving this problem is to use a "branch reset"
! 1435: subpattern, as described in the previous section.)
! 1436: .P
! 1437: The convenience function for extracting the data by name returns the substring
! 1438: for the first (and in this example, the only) subpattern of that name that
! 1439: matched. This saves searching to find which numbered subpattern it was.
! 1440: .P
! 1441: If you make a back reference to a non-unique named subpattern from elsewhere in
! 1442: the pattern, the one that corresponds to the first occurrence of the name is
! 1443: used. In the absence of duplicate numbers (see the previous section) this is
! 1444: the one with the lowest number. If you use a named reference in a condition
! 1445: test (see the
! 1446: .\"
! 1447: .\" HTML <a href="#conditions">
! 1448: .\" </a>
! 1449: section about conditions
! 1450: .\"
! 1451: below), either to check whether a subpattern has matched, or to check for
! 1452: recursion, all subpatterns with the same name are tested. If the condition is
! 1453: true for any one of them, the overall condition is true. This is the same
! 1454: behaviour as testing by number. For further details of the interfaces for
! 1455: handling named subpatterns, see the
! 1456: .\" HREF
! 1457: \fBpcreapi\fP
! 1458: .\"
! 1459: documentation.
! 1460: .P
! 1461: \fBWarning:\fP You cannot use different names to distinguish between two
! 1462: subpatterns with the same number because PCRE uses only the numbers when
! 1463: matching. For this reason, an error is given at compile time if different names
! 1464: are given to subpatterns with the same number. However, you can give the same
! 1465: name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
! 1466: .
! 1467: .
! 1468: .SH REPETITION
! 1469: .rs
! 1470: .sp
! 1471: Repetition is specified by quantifiers, which can follow any of the following
! 1472: items:
! 1473: .sp
! 1474: a literal data character
! 1475: the dot metacharacter
! 1476: the \eC escape sequence
! 1477: the \eX escape sequence (in UTF-8 mode with Unicode properties)
! 1478: the \eR escape sequence
! 1479: an escape such as \ed or \epL that matches a single character
! 1480: a character class
! 1481: a back reference (see next section)
! 1482: a parenthesized subpattern (including assertions)
! 1483: a subroutine call to a subpattern (recursive or otherwise)
! 1484: .sp
! 1485: The general repetition quantifier specifies a minimum and maximum number of
! 1486: permitted matches, by giving the two numbers in curly brackets (braces),
! 1487: separated by a comma. The numbers must be less than 65536, and the first must
! 1488: be less than or equal to the second. For example:
! 1489: .sp
! 1490: z{2,4}
! 1491: .sp
! 1492: matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
! 1493: character. If the second number is omitted, but the comma is present, there is
! 1494: no upper limit; if the second number and the comma are both omitted, the
! 1495: quantifier specifies an exact number of required matches. Thus
! 1496: .sp
! 1497: [aeiou]{3,}
! 1498: .sp
! 1499: matches at least 3 successive vowels, but may match many more, while
! 1500: .sp
! 1501: \ed{8}
! 1502: .sp
! 1503: matches exactly 8 digits. An opening curly bracket that appears in a position
! 1504: where a quantifier is not allowed, or one that does not match the syntax of a
! 1505: quantifier, is taken as a literal character. For example, {,6} is not a
! 1506: quantifier, but a literal string of four characters.
! 1507: .P
! 1508: In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
! 1509: bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
! 1510: which is represented by a two-byte sequence. Similarly, when Unicode property
! 1511: support is available, \eX{3} matches three Unicode extended sequences, each of
! 1512: which may be several bytes long (and they may be of different lengths).
! 1513: .P
! 1514: The quantifier {0} is permitted, causing the expression to behave as if the
! 1515: previous item and the quantifier were not present. This may be useful for
! 1516: subpatterns that are referenced as
! 1517: .\" HTML <a href="#subpatternsassubroutines">
! 1518: .\" </a>
! 1519: subroutines
! 1520: .\"
! 1521: from elsewhere in the pattern (but see also the section entitled
! 1522: .\" HTML <a href="#subdefine">
! 1523: .\" </a>
! 1524: "Defining subpatterns for use by reference only"
! 1525: .\"
! 1526: below). Items other than subpatterns that have a {0} quantifier are omitted
! 1527: from the compiled pattern.
! 1528: .P
! 1529: For convenience, the three most common quantifiers have single-character
! 1530: abbreviations:
! 1531: .sp
! 1532: * is equivalent to {0,}
! 1533: + is equivalent to {1,}
! 1534: ? is equivalent to {0,1}
! 1535: .sp
! 1536: It is possible to construct infinite loops by following a subpattern that can
! 1537: match no characters with a quantifier that has no upper limit, for example:
! 1538: .sp
! 1539: (a?)*
! 1540: .sp
! 1541: Earlier versions of Perl and PCRE used to give an error at compile time for
! 1542: such patterns. However, because there are cases where this can be useful, such
! 1543: patterns are now accepted, but if any repetition of the subpattern does in fact
! 1544: match no characters, the loop is forcibly broken.
! 1545: .P
! 1546: By default, the quantifiers are "greedy", that is, they match as much as
! 1547: possible (up to the maximum number of permitted times), without causing the
! 1548: rest of the pattern to fail. The classic example of where this gives problems
! 1549: is in trying to match comments in C programs. These appear between /* and */
! 1550: and within the comment, individual * and / characters may appear. An attempt to
! 1551: match C comments by applying the pattern
! 1552: .sp
! 1553: /\e*.*\e*/
! 1554: .sp
! 1555: to the string
! 1556: .sp
! 1557: /* first comment */ not comment /* second comment */
! 1558: .sp
! 1559: fails, because it matches the entire string owing to the greediness of the .*
! 1560: item.
! 1561: .P
! 1562: However, if a quantifier is followed by a question mark, it ceases to be
! 1563: greedy, and instead matches the minimum number of times possible, so the
! 1564: pattern
! 1565: .sp
! 1566: /\e*.*?\e*/
! 1567: .sp
! 1568: does the right thing with the C comments. The meaning of the various
! 1569: quantifiers is not otherwise changed, just the preferred number of matches.
! 1570: Do not confuse this use of question mark with its use as a quantifier in its
! 1571: own right. Because it has two uses, it can sometimes appear doubled, as in
! 1572: .sp
! 1573: \ed??\ed
! 1574: .sp
! 1575: which matches one digit by preference, but can match two if that is the only
! 1576: way the rest of the pattern matches.
! 1577: .P
! 1578: If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
! 1579: the quantifiers are not greedy by default, but individual ones can be made
! 1580: greedy by following them with a question mark. In other words, it inverts the
! 1581: default behaviour.
! 1582: .P
! 1583: When a parenthesized subpattern is quantified with a minimum repeat count that
! 1584: is greater than 1 or with a limited maximum, more memory is required for the
! 1585: compiled pattern, in proportion to the size of the minimum or maximum.
! 1586: .P
! 1587: If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
! 1588: to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
! 1589: implicitly anchored, because whatever follows will be tried against every
! 1590: character position in the subject string, so there is no point in retrying the
! 1591: overall match at any position after the first. PCRE normally treats such a
! 1592: pattern as though it were preceded by \eA.
! 1593: .P
! 1594: In cases where it is known that the subject string contains no newlines, it is
! 1595: worth setting PCRE_DOTALL in order to obtain this optimization, or
! 1596: alternatively using ^ to indicate anchoring explicitly.
! 1597: .P
! 1598: However, there is one situation where the optimization cannot be used. When .*
! 1599: is inside capturing parentheses that are the subject of a back reference
! 1600: elsewhere in the pattern, a match at the start may fail where a later one
! 1601: succeeds. Consider, for example:
! 1602: .sp
! 1603: (.*)abc\e1
! 1604: .sp
! 1605: If the subject is "xyz123abc123" the match point is the fourth character. For
! 1606: this reason, such a pattern is not implicitly anchored.
! 1607: .P
! 1608: When a capturing subpattern is repeated, the value captured is the substring
! 1609: that matched the final iteration. For example, after
! 1610: .sp
! 1611: (tweedle[dume]{3}\es*)+
! 1612: .sp
! 1613: has matched "tweedledum tweedledee" the value of the captured substring is
! 1614: "tweedledee". However, if there are nested capturing subpatterns, the
! 1615: corresponding captured values may have been set in previous iterations. For
! 1616: example, after
! 1617: .sp
! 1618: /(a|(b))+/
! 1619: .sp
! 1620: matches "aba" the value of the second captured substring is "b".
! 1621: .
! 1622: .
! 1623: .\" HTML <a name="atomicgroup"></a>
! 1624: .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
! 1625: .rs
! 1626: .sp
! 1627: With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
! 1628: repetition, failure of what follows normally causes the repeated item to be
! 1629: re-evaluated to see if a different number of repeats allows the rest of the
! 1630: pattern to match. Sometimes it is useful to prevent this, either to change the
! 1631: nature of the match, or to cause it fail earlier than it otherwise might, when
! 1632: the author of the pattern knows there is no point in carrying on.
! 1633: .P
! 1634: Consider, for example, the pattern \ed+foo when applied to the subject line
! 1635: .sp
! 1636: 123456bar
! 1637: .sp
! 1638: After matching all 6 digits and then failing to match "foo", the normal
! 1639: action of the matcher is to try again with only 5 digits matching the \ed+
! 1640: item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
! 1641: (a term taken from Jeffrey Friedl's book) provides the means for specifying
! 1642: that once a subpattern has matched, it is not to be re-evaluated in this way.
! 1643: .P
! 1644: If we use atomic grouping for the previous example, the matcher gives up
! 1645: immediately on failing to match "foo" the first time. The notation is a kind of
! 1646: special parenthesis, starting with (?> as in this example:
! 1647: .sp
! 1648: (?>\ed+)foo
! 1649: .sp
! 1650: This kind of parenthesis "locks up" the part of the pattern it contains once
! 1651: it has matched, and a failure further into the pattern is prevented from
! 1652: backtracking into it. Backtracking past it to previous items, however, works as
! 1653: normal.
! 1654: .P
! 1655: An alternative description is that a subpattern of this type matches the string
! 1656: of characters that an identical standalone pattern would match, if anchored at
! 1657: the current point in the subject string.
! 1658: .P
! 1659: Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
! 1660: the above example can be thought of as a maximizing repeat that must swallow
! 1661: everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
! 1662: number of digits they match in order to make the rest of the pattern match,
! 1663: (?>\ed+) can only match an entire sequence of digits.
! 1664: .P
! 1665: Atomic groups in general can of course contain arbitrarily complicated
! 1666: subpatterns, and can be nested. However, when the subpattern for an atomic
! 1667: group is just a single repeated item, as in the example above, a simpler
! 1668: notation, called a "possessive quantifier" can be used. This consists of an
! 1669: additional + character following a quantifier. Using this notation, the
! 1670: previous example can be rewritten as
! 1671: .sp
! 1672: \ed++foo
! 1673: .sp
! 1674: Note that a possessive quantifier can be used with an entire group, for
! 1675: example:
! 1676: .sp
! 1677: (abc|xyz){2,3}+
! 1678: .sp
! 1679: Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
! 1680: option is ignored. They are a convenient notation for the simpler forms of
! 1681: atomic group. However, there is no difference in the meaning of a possessive
! 1682: quantifier and the equivalent atomic group, though there may be a performance
! 1683: difference; possessive quantifiers should be slightly faster.
! 1684: .P
! 1685: The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
! 1686: Jeffrey Friedl originated the idea (and the name) in the first edition of his
! 1687: book. Mike McCloskey liked it, so implemented it when he built Sun's Java
! 1688: package, and PCRE copied it from there. It ultimately found its way into Perl
! 1689: at release 5.10.
! 1690: .P
! 1691: PCRE has an optimization that automatically "possessifies" certain simple
! 1692: pattern constructs. For example, the sequence A+B is treated as A++B because
! 1693: there is no point in backtracking into a sequence of A's when B must follow.
! 1694: .P
! 1695: When a pattern contains an unlimited repeat inside a subpattern that can itself
! 1696: be repeated an unlimited number of times, the use of an atomic group is the
! 1697: only way to avoid some failing matches taking a very long time indeed. The
! 1698: pattern
! 1699: .sp
! 1700: (\eD+|<\ed+>)*[!?]
! 1701: .sp
! 1702: matches an unlimited number of substrings that either consist of non-digits, or
! 1703: digits enclosed in <>, followed by either ! or ?. When it matches, it runs
! 1704: quickly. However, if it is applied to
! 1705: .sp
! 1706: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
! 1707: .sp
! 1708: it takes a long time before reporting failure. This is because the string can
! 1709: be divided between the internal \eD+ repeat and the external * repeat in a
! 1710: large number of ways, and all have to be tried. (The example uses [!?] rather
! 1711: than a single character at the end, because both PCRE and Perl have an
! 1712: optimization that allows for fast failure when a single character is used. They
! 1713: remember the last single character that is required for a match, and fail early
! 1714: if it is not present in the string.) If the pattern is changed so that it uses
! 1715: an atomic group, like this:
! 1716: .sp
! 1717: ((?>\eD+)|<\ed+>)*[!?]
! 1718: .sp
! 1719: sequences of non-digits cannot be broken, and failure happens quickly.
! 1720: .
! 1721: .
! 1722: .\" HTML <a name="backreferences"></a>
! 1723: .SH "BACK REFERENCES"
! 1724: .rs
! 1725: .sp
! 1726: Outside a character class, a backslash followed by a digit greater than 0 (and
! 1727: possibly further digits) is a back reference to a capturing subpattern earlier
! 1728: (that is, to its left) in the pattern, provided there have been that many
! 1729: previous capturing left parentheses.
! 1730: .P
! 1731: However, if the decimal number following the backslash is less than 10, it is
! 1732: always taken as a back reference, and causes an error only if there are not
! 1733: that many capturing left parentheses in the entire pattern. In other words, the
! 1734: parentheses that are referenced need not be to the left of the reference for
! 1735: numbers less than 10. A "forward back reference" of this type can make sense
! 1736: when a repetition is involved and the subpattern to the right has participated
! 1737: in an earlier iteration.
! 1738: .P
! 1739: It is not possible to have a numerical "forward back reference" to a subpattern
! 1740: whose number is 10 or more using this syntax because a sequence such as \e50 is
! 1741: interpreted as a character defined in octal. See the subsection entitled
! 1742: "Non-printing characters"
! 1743: .\" HTML <a href="#digitsafterbackslash">
! 1744: .\" </a>
! 1745: above
! 1746: .\"
! 1747: for further details of the handling of digits following a backslash. There is
! 1748: no such problem when named parentheses are used. A back reference to any
! 1749: subpattern is possible using named parentheses (see below).
! 1750: .P
! 1751: Another way of avoiding the ambiguity inherent in the use of digits following a
! 1752: backslash is to use the \eg escape sequence. This escape must be followed by an
! 1753: unsigned number or a negative number, optionally enclosed in braces. These
! 1754: examples are all identical:
! 1755: .sp
! 1756: (ring), \e1
! 1757: (ring), \eg1
! 1758: (ring), \eg{1}
! 1759: .sp
! 1760: An unsigned number specifies an absolute reference without the ambiguity that
! 1761: is present in the older syntax. It is also useful when literal digits follow
! 1762: the reference. A negative number is a relative reference. Consider this
! 1763: example:
! 1764: .sp
! 1765: (abc(def)ghi)\eg{-1}
! 1766: .sp
! 1767: The sequence \eg{-1} is a reference to the most recently started capturing
! 1768: subpattern before \eg, that is, is it equivalent to \e2 in this example.
! 1769: Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
! 1770: can be helpful in long patterns, and also in patterns that are created by
! 1771: joining together fragments that contain references within themselves.
! 1772: .P
! 1773: A back reference matches whatever actually matched the capturing subpattern in
! 1774: the current subject string, rather than anything matching the subpattern
! 1775: itself (see
! 1776: .\" HTML <a href="#subpatternsassubroutines">
! 1777: .\" </a>
! 1778: "Subpatterns as subroutines"
! 1779: .\"
! 1780: below for a way of doing that). So the pattern
! 1781: .sp
! 1782: (sens|respons)e and \e1ibility
! 1783: .sp
! 1784: matches "sense and sensibility" and "response and responsibility", but not
! 1785: "sense and responsibility". If caseful matching is in force at the time of the
! 1786: back reference, the case of letters is relevant. For example,
! 1787: .sp
! 1788: ((?i)rah)\es+\e1
! 1789: .sp
! 1790: matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
! 1791: capturing subpattern is matched caselessly.
! 1792: .P
! 1793: There are several different ways of writing back references to named
! 1794: subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
! 1795: \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
! 1796: back reference syntax, in which \eg can be used for both numeric and named
! 1797: references, is also supported. We could rewrite the above example in any of
! 1798: the following ways:
! 1799: .sp
! 1800: (?<p1>(?i)rah)\es+\ek<p1>
! 1801: (?'p1'(?i)rah)\es+\ek{p1}
! 1802: (?P<p1>(?i)rah)\es+(?P=p1)
! 1803: (?<p1>(?i)rah)\es+\eg{p1}
! 1804: .sp
! 1805: A subpattern that is referenced by name may appear in the pattern before or
! 1806: after the reference.
! 1807: .P
! 1808: There may be more than one back reference to the same subpattern. If a
! 1809: subpattern has not actually been used in a particular match, any back
! 1810: references to it always fail by default. For example, the pattern
! 1811: .sp
! 1812: (a|(bc))\e2
! 1813: .sp
! 1814: always fails if it starts to match "a" rather than "bc". However, if the
! 1815: PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
! 1816: unset value matches an empty string.
! 1817: .P
! 1818: Because there may be many capturing parentheses in a pattern, all digits
! 1819: following a backslash are taken as part of a potential back reference number.
! 1820: If the pattern continues with a digit character, some delimiter must be used to
! 1821: terminate the back reference. If the PCRE_EXTENDED option is set, this can be
! 1822: whitespace. Otherwise, the \eg{ syntax or an empty comment (see
! 1823: .\" HTML <a href="#comments">
! 1824: .\" </a>
! 1825: "Comments"
! 1826: .\"
! 1827: below) can be used.
! 1828: .
! 1829: .SS "Recursive back references"
! 1830: .rs
! 1831: .sp
! 1832: A back reference that occurs inside the parentheses to which it refers fails
! 1833: when the subpattern is first used, so, for example, (a\e1) never matches.
! 1834: However, such references can be useful inside repeated subpatterns. For
! 1835: example, the pattern
! 1836: .sp
! 1837: (a|b\e1)+
! 1838: .sp
! 1839: matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
! 1840: the subpattern, the back reference matches the character string corresponding
! 1841: to the previous iteration. In order for this to work, the pattern must be such
! 1842: that the first iteration does not need to match the back reference. This can be
! 1843: done using alternation, as in the example above, or by a quantifier with a
! 1844: minimum of zero.
! 1845: .P
! 1846: Back references of this type cause the group that they reference to be treated
! 1847: as an
! 1848: .\" HTML <a href="#atomicgroup">
! 1849: .\" </a>
! 1850: atomic group.
! 1851: .\"
! 1852: Once the whole group has been matched, a subsequent matching failure cannot
! 1853: cause backtracking into the middle of the group.
! 1854: .
! 1855: .
! 1856: .\" HTML <a name="bigassertions"></a>
! 1857: .SH ASSERTIONS
! 1858: .rs
! 1859: .sp
! 1860: An assertion is a test on the characters following or preceding the current
! 1861: matching point that does not actually consume any characters. The simple
! 1862: assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
! 1863: .\" HTML <a href="#smallassertions">
! 1864: .\" </a>
! 1865: above.
! 1866: .\"
! 1867: .P
! 1868: More complicated assertions are coded as subpatterns. There are two kinds:
! 1869: those that look ahead of the current position in the subject string, and those
! 1870: that look behind it. An assertion subpattern is matched in the normal way,
! 1871: except that it does not cause the current matching position to be changed.
! 1872: .P
! 1873: Assertion subpatterns are not capturing subpatterns. If such an assertion
! 1874: contains capturing subpatterns within it, these are counted for the purposes of
! 1875: numbering the capturing subpatterns in the whole pattern. However, substring
! 1876: capturing is carried out only for positive assertions, because it does not make
! 1877: sense for negative assertions.
! 1878: .P
! 1879: For compatibility with Perl, assertion subpatterns may be repeated; though
! 1880: it makes no sense to assert the same thing several times, the side effect of
! 1881: capturing parentheses may occasionally be useful. In practice, there only three
! 1882: cases:
! 1883: .sp
! 1884: (1) If the quantifier is {0}, the assertion is never obeyed during matching.
! 1885: However, it may contain internal capturing parenthesized groups that are called
! 1886: from elsewhere via the
! 1887: .\" HTML <a href="#subpatternsassubroutines">
! 1888: .\" </a>
! 1889: subroutine mechanism.
! 1890: .\"
! 1891: .sp
! 1892: (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
! 1893: were {0,1}. At run time, the rest of the pattern match is tried with and
! 1894: without the assertion, the order depending on the greediness of the quantifier.
! 1895: .sp
! 1896: (3) If the minimum repetition is greater than zero, the quantifier is ignored.
! 1897: The assertion is obeyed just once when encountered during matching.
! 1898: .
! 1899: .
! 1900: .SS "Lookahead assertions"
! 1901: .rs
! 1902: .sp
! 1903: Lookahead assertions start with (?= for positive assertions and (?! for
! 1904: negative assertions. For example,
! 1905: .sp
! 1906: \ew+(?=;)
! 1907: .sp
! 1908: matches a word followed by a semicolon, but does not include the semicolon in
! 1909: the match, and
! 1910: .sp
! 1911: foo(?!bar)
! 1912: .sp
! 1913: matches any occurrence of "foo" that is not followed by "bar". Note that the
! 1914: apparently similar pattern
! 1915: .sp
! 1916: (?!foo)bar
! 1917: .sp
! 1918: does not find an occurrence of "bar" that is preceded by something other than
! 1919: "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
! 1920: (?!foo) is always true when the next three characters are "bar". A
! 1921: lookbehind assertion is needed to achieve the other effect.
! 1922: .P
! 1923: If you want to force a matching failure at some point in a pattern, the most
! 1924: convenient way to do it is with (?!) because an empty string always matches, so
! 1925: an assertion that requires there not to be an empty string must always fail.
! 1926: The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
! 1927: .
! 1928: .
! 1929: .\" HTML <a name="lookbehind"></a>
! 1930: .SS "Lookbehind assertions"
! 1931: .rs
! 1932: .sp
! 1933: Lookbehind assertions start with (?<= for positive assertions and (?<! for
! 1934: negative assertions. For example,
! 1935: .sp
! 1936: (?<!foo)bar
! 1937: .sp
! 1938: does find an occurrence of "bar" that is not preceded by "foo". The contents of
! 1939: a lookbehind assertion are restricted such that all the strings it matches must
! 1940: have a fixed length. However, if there are several top-level alternatives, they
! 1941: do not all have to have the same fixed length. Thus
! 1942: .sp
! 1943: (?<=bullock|donkey)
! 1944: .sp
! 1945: is permitted, but
! 1946: .sp
! 1947: (?<!dogs?|cats?)
! 1948: .sp
! 1949: causes an error at compile time. Branches that match different length strings
! 1950: are permitted only at the top level of a lookbehind assertion. This is an
! 1951: extension compared with Perl, which requires all branches to match the same
! 1952: length of string. An assertion such as
! 1953: .sp
! 1954: (?<=ab(c|de))
! 1955: .sp
! 1956: is not permitted, because its single top-level branch can match two different
! 1957: lengths, but it is acceptable to PCRE if rewritten to use two top-level
! 1958: branches:
! 1959: .sp
! 1960: (?<=abc|abde)
! 1961: .sp
! 1962: In some cases, the escape sequence \eK
! 1963: .\" HTML <a href="#resetmatchstart">
! 1964: .\" </a>
! 1965: (see above)
! 1966: .\"
! 1967: can be used instead of a lookbehind assertion to get round the fixed-length
! 1968: restriction.
! 1969: .P
! 1970: The implementation of lookbehind assertions is, for each alternative, to
! 1971: temporarily move the current position back by the fixed length and then try to
! 1972: match. If there are insufficient characters before the current position, the
! 1973: assertion fails.
! 1974: .P
! 1975: In UTF-8 mode, PCRE does not allow the \eC escape (which matches a single byte,
! 1976: even in UTF-8 mode) to appear in lookbehind assertions, because it makes it
! 1977: impossible to calculate the length of the lookbehind. The \eX and \eR escapes,
! 1978: which can match different numbers of bytes, are also not permitted.
! 1979: .P
! 1980: .\" HTML <a href="#subpatternsassubroutines">
! 1981: .\" </a>
! 1982: "Subroutine"
! 1983: .\"
! 1984: calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
! 1985: as the subpattern matches a fixed-length string.
! 1986: .\" HTML <a href="#recursion">
! 1987: .\" </a>
! 1988: Recursion,
! 1989: .\"
! 1990: however, is not supported.
! 1991: .P
! 1992: Possessive quantifiers can be used in conjunction with lookbehind assertions to
! 1993: specify efficient matching of fixed-length strings at the end of subject
! 1994: strings. Consider a simple pattern such as
! 1995: .sp
! 1996: abcd$
! 1997: .sp
! 1998: when applied to a long string that does not match. Because matching proceeds
! 1999: from left to right, PCRE will look for each "a" in the subject and then see if
! 2000: what follows matches the rest of the pattern. If the pattern is specified as
! 2001: .sp
! 2002: ^.*abcd$
! 2003: .sp
! 2004: the initial .* matches the entire string at first, but when this fails (because
! 2005: there is no following "a"), it backtracks to match all but the last character,
! 2006: then all but the last two characters, and so on. Once again the search for "a"
! 2007: covers the entire string, from right to left, so we are no better off. However,
! 2008: if the pattern is written as
! 2009: .sp
! 2010: ^.*+(?<=abcd)
! 2011: .sp
! 2012: there can be no backtracking for the .*+ item; it can match only the entire
! 2013: string. The subsequent lookbehind assertion does a single test on the last four
! 2014: characters. If it fails, the match fails immediately. For long strings, this
! 2015: approach makes a significant difference to the processing time.
! 2016: .
! 2017: .
! 2018: .SS "Using multiple assertions"
! 2019: .rs
! 2020: .sp
! 2021: Several assertions (of any sort) may occur in succession. For example,
! 2022: .sp
! 2023: (?<=\ed{3})(?<!999)foo
! 2024: .sp
! 2025: matches "foo" preceded by three digits that are not "999". Notice that each of
! 2026: the assertions is applied independently at the same point in the subject
! 2027: string. First there is a check that the previous three characters are all
! 2028: digits, and then there is a check that the same three characters are not "999".
! 2029: This pattern does \fInot\fP match "foo" preceded by six characters, the first
! 2030: of which are digits and the last three of which are not "999". For example, it
! 2031: doesn't match "123abcfoo". A pattern to do that is
! 2032: .sp
! 2033: (?<=\ed{3}...)(?<!999)foo
! 2034: .sp
! 2035: This time the first assertion looks at the preceding six characters, checking
! 2036: that the first three are digits, and then the second assertion checks that the
! 2037: preceding three characters are not "999".
! 2038: .P
! 2039: Assertions can be nested in any combination. For example,
! 2040: .sp
! 2041: (?<=(?<!foo)bar)baz
! 2042: .sp
! 2043: matches an occurrence of "baz" that is preceded by "bar" which in turn is not
! 2044: preceded by "foo", while
! 2045: .sp
! 2046: (?<=\ed{3}(?!999)...)foo
! 2047: .sp
! 2048: is another pattern that matches "foo" preceded by three digits and any three
! 2049: characters that are not "999".
! 2050: .
! 2051: .
! 2052: .\" HTML <a name="conditions"></a>
! 2053: .SH "CONDITIONAL SUBPATTERNS"
! 2054: .rs
! 2055: .sp
! 2056: It is possible to cause the matching process to obey a subpattern
! 2057: conditionally or to choose between two alternative subpatterns, depending on
! 2058: the result of an assertion, or whether a specific capturing subpattern has
! 2059: already been matched. The two possible forms of conditional subpattern are:
! 2060: .sp
! 2061: (?(condition)yes-pattern)
! 2062: (?(condition)yes-pattern|no-pattern)
! 2063: .sp
! 2064: If the condition is satisfied, the yes-pattern is used; otherwise the
! 2065: no-pattern (if present) is used. If there are more than two alternatives in the
! 2066: subpattern, a compile-time error occurs. Each of the two alternatives may
! 2067: itself contain nested subpatterns of any form, including conditional
! 2068: subpatterns; the restriction to two alternatives applies only at the level of
! 2069: the condition. This pattern fragment is an example where the alternatives are
! 2070: complex:
! 2071: .sp
! 2072: (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
! 2073: .sp
! 2074: .P
! 2075: There are four kinds of condition: references to subpatterns, references to
! 2076: recursion, a pseudo-condition called DEFINE, and assertions.
! 2077: .
! 2078: .SS "Checking for a used subpattern by number"
! 2079: .rs
! 2080: .sp
! 2081: If the text between the parentheses consists of a sequence of digits, the
! 2082: condition is true if a capturing subpattern of that number has previously
! 2083: matched. If there is more than one capturing subpattern with the same number
! 2084: (see the earlier
! 2085: .\"
! 2086: .\" HTML <a href="#recursion">
! 2087: .\" </a>
! 2088: section about duplicate subpattern numbers),
! 2089: .\"
! 2090: the condition is true if any of them have matched. An alternative notation is
! 2091: to precede the digits with a plus or minus sign. In this case, the subpattern
! 2092: number is relative rather than absolute. The most recently opened parentheses
! 2093: can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
! 2094: loops it can also make sense to refer to subsequent groups. The next
! 2095: parentheses to be opened can be referenced as (?(+1), and so on. (The value
! 2096: zero in any of these forms is not used; it provokes a compile-time error.)
! 2097: .P
! 2098: Consider the following pattern, which contains non-significant white space to
! 2099: make it more readable (assume the PCRE_EXTENDED option) and to divide it into
! 2100: three parts for ease of discussion:
! 2101: .sp
! 2102: ( \e( )? [^()]+ (?(1) \e) )
! 2103: .sp
! 2104: The first part matches an optional opening parenthesis, and if that
! 2105: character is present, sets it as the first captured substring. The second part
! 2106: matches one or more characters that are not parentheses. The third part is a
! 2107: conditional subpattern that tests whether or not the first set of parentheses
! 2108: matched. If they did, that is, if subject started with an opening parenthesis,
! 2109: the condition is true, and so the yes-pattern is executed and a closing
! 2110: parenthesis is required. Otherwise, since no-pattern is not present, the
! 2111: subpattern matches nothing. In other words, this pattern matches a sequence of
! 2112: non-parentheses, optionally enclosed in parentheses.
! 2113: .P
! 2114: If you were embedding this pattern in a larger one, you could use a relative
! 2115: reference:
! 2116: .sp
! 2117: ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
! 2118: .sp
! 2119: This makes the fragment independent of the parentheses in the larger pattern.
! 2120: .
! 2121: .SS "Checking for a used subpattern by name"
! 2122: .rs
! 2123: .sp
! 2124: Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
! 2125: subpattern by name. For compatibility with earlier versions of PCRE, which had
! 2126: this facility before Perl, the syntax (?(name)...) is also recognized. However,
! 2127: there is a possible ambiguity with this syntax, because subpattern names may
! 2128: consist entirely of digits. PCRE looks first for a named subpattern; if it
! 2129: cannot find one and the name consists entirely of digits, PCRE looks for a
! 2130: subpattern of that number, which must be greater than zero. Using subpattern
! 2131: names that consist entirely of digits is not recommended.
! 2132: .P
! 2133: Rewriting the above example to use a named subpattern gives this:
! 2134: .sp
! 2135: (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
! 2136: .sp
! 2137: If the name used in a condition of this kind is a duplicate, the test is
! 2138: applied to all subpatterns of the same name, and is true if any one of them has
! 2139: matched.
! 2140: .
! 2141: .SS "Checking for pattern recursion"
! 2142: .rs
! 2143: .sp
! 2144: If the condition is the string (R), and there is no subpattern with the name R,
! 2145: the condition is true if a recursive call to the whole pattern or any
! 2146: subpattern has been made. If digits or a name preceded by ampersand follow the
! 2147: letter R, for example:
! 2148: .sp
! 2149: (?(R3)...) or (?(R&name)...)
! 2150: .sp
! 2151: the condition is true if the most recent recursion is into a subpattern whose
! 2152: number or name is given. This condition does not check the entire recursion
! 2153: stack. If the name used in a condition of this kind is a duplicate, the test is
! 2154: applied to all subpatterns of the same name, and is true if any one of them is
! 2155: the most recent recursion.
! 2156: .P
! 2157: At "top level", all these recursion test conditions are false.
! 2158: .\" HTML <a href="#recursion">
! 2159: .\" </a>
! 2160: The syntax for recursive patterns
! 2161: .\"
! 2162: is described below.
! 2163: .
! 2164: .\" HTML <a name="subdefine"></a>
! 2165: .SS "Defining subpatterns for use by reference only"
! 2166: .rs
! 2167: .sp
! 2168: If the condition is the string (DEFINE), and there is no subpattern with the
! 2169: name DEFINE, the condition is always false. In this case, there may be only one
! 2170: alternative in the subpattern. It is always skipped if control reaches this
! 2171: point in the pattern; the idea of DEFINE is that it can be used to define
! 2172: subroutines that can be referenced from elsewhere. (The use of
! 2173: .\" HTML <a href="#subpatternsassubroutines">
! 2174: .\" </a>
! 2175: subroutines
! 2176: .\"
! 2177: is described below.) For example, a pattern to match an IPv4 address such as
! 2178: "192.168.23.245" could be written like this (ignore whitespace and line
! 2179: breaks):
! 2180: .sp
! 2181: (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
! 2182: \eb (?&byte) (\e.(?&byte)){3} \eb
! 2183: .sp
! 2184: The first part of the pattern is a DEFINE group inside which a another group
! 2185: named "byte" is defined. This matches an individual component of an IPv4
! 2186: address (a number less than 256). When matching takes place, this part of the
! 2187: pattern is skipped because DEFINE acts like a false condition. The rest of the
! 2188: pattern uses references to the named group to match the four dot-separated
! 2189: components of an IPv4 address, insisting on a word boundary at each end.
! 2190: .
! 2191: .SS "Assertion conditions"
! 2192: .rs
! 2193: .sp
! 2194: If the condition is not in any of the above formats, it must be an assertion.
! 2195: This may be a positive or negative lookahead or lookbehind assertion. Consider
! 2196: this pattern, again containing non-significant white space, and with the two
! 2197: alternatives on the second line:
! 2198: .sp
! 2199: (?(?=[^a-z]*[a-z])
! 2200: \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
! 2201: .sp
! 2202: The condition is a positive lookahead assertion that matches an optional
! 2203: sequence of non-letters followed by a letter. In other words, it tests for the
! 2204: presence of at least one letter in the subject. If a letter is found, the
! 2205: subject is matched against the first alternative; otherwise it is matched
! 2206: against the second. This pattern matches strings in one of the two forms
! 2207: dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
! 2208: .
! 2209: .
! 2210: .\" HTML <a name="comments"></a>
! 2211: .SH COMMENTS
! 2212: .rs
! 2213: .sp
! 2214: There are two ways of including comments in patterns that are processed by
! 2215: PCRE. In both cases, the start of the comment must not be in a character class,
! 2216: nor in the middle of any other sequence of related characters such as (?: or a
! 2217: subpattern name or number. The characters that make up a comment play no part
! 2218: in the pattern matching.
! 2219: .P
! 2220: The sequence (?# marks the start of a comment that continues up to the next
! 2221: closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
! 2222: option is set, an unescaped # character also introduces a comment, which in
! 2223: this case continues to immediately after the next newline character or
! 2224: character sequence in the pattern. Which characters are interpreted as newlines
! 2225: is controlled by the options passed to \fBpcre_compile()\fP or by a special
! 2226: sequence at the start of the pattern, as described in the section entitled
! 2227: .\" HTML <a href="#newlines">
! 2228: .\" </a>
! 2229: "Newline conventions"
! 2230: .\"
! 2231: above. Note that the end of this type of comment is a literal newline sequence
! 2232: in the pattern; escape sequences that happen to represent a newline do not
! 2233: count. For example, consider this pattern when PCRE_EXTENDED is set, and the
! 2234: default newline convention is in force:
! 2235: .sp
! 2236: abc #comment \en still comment
! 2237: .sp
! 2238: On encountering the # character, \fBpcre_compile()\fP skips along, looking for
! 2239: a newline in the pattern. The sequence \en is still literal at this stage, so
! 2240: it does not terminate the comment. Only an actual character with the code value
! 2241: 0x0a (the default newline) does so.
! 2242: .
! 2243: .
! 2244: .\" HTML <a name="recursion"></a>
! 2245: .SH "RECURSIVE PATTERNS"
! 2246: .rs
! 2247: .sp
! 2248: Consider the problem of matching a string in parentheses, allowing for
! 2249: unlimited nested parentheses. Without the use of recursion, the best that can
! 2250: be done is to use a pattern that matches up to some fixed depth of nesting. It
! 2251: is not possible to handle an arbitrary nesting depth.
! 2252: .P
! 2253: For some time, Perl has provided a facility that allows regular expressions to
! 2254: recurse (amongst other things). It does this by interpolating Perl code in the
! 2255: expression at run time, and the code can refer to the expression itself. A Perl
! 2256: pattern using code interpolation to solve the parentheses problem can be
! 2257: created like this:
! 2258: .sp
! 2259: $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
! 2260: .sp
! 2261: The (?p{...}) item interpolates Perl code at run time, and in this case refers
! 2262: recursively to the pattern in which it appears.
! 2263: .P
! 2264: Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
! 2265: supports special syntax for recursion of the entire pattern, and also for
! 2266: individual subpattern recursion. After its introduction in PCRE and Python,
! 2267: this kind of recursion was subsequently introduced into Perl at release 5.10.
! 2268: .P
! 2269: A special item that consists of (? followed by a number greater than zero and a
! 2270: closing parenthesis is a recursive subroutine call of the subpattern of the
! 2271: given number, provided that it occurs inside that subpattern. (If not, it is a
! 2272: .\" HTML <a href="#subpatternsassubroutines">
! 2273: .\" </a>
! 2274: non-recursive subroutine
! 2275: .\"
! 2276: call, which is described in the next section.) The special item (?R) or (?0) is
! 2277: a recursive call of the entire regular expression.
! 2278: .P
! 2279: This PCRE pattern solves the nested parentheses problem (assume the
! 2280: PCRE_EXTENDED option is set so that white space is ignored):
! 2281: .sp
! 2282: \e( ( [^()]++ | (?R) )* \e)
! 2283: .sp
! 2284: First it matches an opening parenthesis. Then it matches any number of
! 2285: substrings which can either be a sequence of non-parentheses, or a recursive
! 2286: match of the pattern itself (that is, a correctly parenthesized substring).
! 2287: Finally there is a closing parenthesis. Note the use of a possessive quantifier
! 2288: to avoid backtracking into sequences of non-parentheses.
! 2289: .P
! 2290: If this were part of a larger pattern, you would not want to recurse the entire
! 2291: pattern, so instead you could use this:
! 2292: .sp
! 2293: ( \e( ( [^()]++ | (?1) )* \e) )
! 2294: .sp
! 2295: We have put the pattern into parentheses, and caused the recursion to refer to
! 2296: them instead of the whole pattern.
! 2297: .P
! 2298: In a larger pattern, keeping track of parenthesis numbers can be tricky. This
! 2299: is made easier by the use of relative references. Instead of (?1) in the
! 2300: pattern above you can write (?-2) to refer to the second most recently opened
! 2301: parentheses preceding the recursion. In other words, a negative number counts
! 2302: capturing parentheses leftwards from the point at which it is encountered.
! 2303: .P
! 2304: It is also possible to refer to subsequently opened parentheses, by writing
! 2305: references such as (?+2). However, these cannot be recursive because the
! 2306: reference is not inside the parentheses that are referenced. They are always
! 2307: .\" HTML <a href="#subpatternsassubroutines">
! 2308: .\" </a>
! 2309: non-recursive subroutine
! 2310: .\"
! 2311: calls, as described in the next section.
! 2312: .P
! 2313: An alternative approach is to use named parentheses instead. The Perl syntax
! 2314: for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
! 2315: could rewrite the above example as follows:
! 2316: .sp
! 2317: (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
! 2318: .sp
! 2319: If there is more than one subpattern with the same name, the earliest one is
! 2320: used.
! 2321: .P
! 2322: This particular example pattern that we have been looking at contains nested
! 2323: unlimited repeats, and so the use of a possessive quantifier for matching
! 2324: strings of non-parentheses is important when applying the pattern to strings
! 2325: that do not match. For example, when this pattern is applied to
! 2326: .sp
! 2327: (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
! 2328: .sp
! 2329: it yields "no match" quickly. However, if a possessive quantifier is not used,
! 2330: the match runs for a very long time indeed because there are so many different
! 2331: ways the + and * repeats can carve up the subject, and all have to be tested
! 2332: before failure can be reported.
! 2333: .P
! 2334: At the end of a match, the values of capturing parentheses are those from
! 2335: the outermost level. If you want to obtain intermediate values, a callout
! 2336: function can be used (see below and the
! 2337: .\" HREF
! 2338: \fBpcrecallout\fP
! 2339: .\"
! 2340: documentation). If the pattern above is matched against
! 2341: .sp
! 2342: (ab(cd)ef)
! 2343: .sp
! 2344: the value for the inner capturing parentheses (numbered 2) is "ef", which is
! 2345: the last value taken on at the top level. If a capturing subpattern is not
! 2346: matched at the top level, its final captured value is unset, even if it was
! 2347: (temporarily) set at a deeper level during the matching process.
! 2348: .P
! 2349: If there are more than 15 capturing parentheses in a pattern, PCRE has to
! 2350: obtain extra memory to store data during a recursion, which it does by using
! 2351: \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
! 2352: be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
! 2353: .P
! 2354: Do not confuse the (?R) item with the condition (R), which tests for recursion.
! 2355: Consider this pattern, which matches text in angle brackets, allowing for
! 2356: arbitrary nesting. Only digits are allowed in nested brackets (that is, when
! 2357: recursing), whereas any characters are permitted at the outer level.
! 2358: .sp
! 2359: < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
! 2360: .sp
! 2361: In this pattern, (?(R) is the start of a conditional subpattern, with two
! 2362: different alternatives for the recursive and non-recursive cases. The (?R) item
! 2363: is the actual recursive call.
! 2364: .
! 2365: .
! 2366: .\" HTML <a name="recursiondifference"></a>
! 2367: .SS "Differences in recursion processing between PCRE and Perl"
! 2368: .rs
! 2369: .sp
! 2370: Recursion processing in PCRE differs from Perl in two important ways. In PCRE
! 2371: (like Python, but unlike Perl), a recursive subpattern call is always treated
! 2372: as an atomic group. That is, once it has matched some of the subject string, it
! 2373: is never re-entered, even if it contains untried alternatives and there is a
! 2374: subsequent matching failure. This can be illustrated by the following pattern,
! 2375: which purports to match a palindromic string that contains an odd number of
! 2376: characters (for example, "a", "aba", "abcba", "abcdcba"):
! 2377: .sp
! 2378: ^(.|(.)(?1)\e2)$
! 2379: .sp
! 2380: The idea is that it either matches a single character, or two identical
! 2381: characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
! 2382: it does not if the pattern is longer than three characters. Consider the
! 2383: subject string "abcba":
! 2384: .P
! 2385: At the top level, the first character is matched, but as it is not at the end
! 2386: of the string, the first alternative fails; the second alternative is taken
! 2387: and the recursion kicks in. The recursive call to subpattern 1 successfully
! 2388: matches the next character ("b"). (Note that the beginning and end of line
! 2389: tests are not part of the recursion).
! 2390: .P
! 2391: Back at the top level, the next character ("c") is compared with what
! 2392: subpattern 2 matched, which was "a". This fails. Because the recursion is
! 2393: treated as an atomic group, there are now no backtracking points, and so the
! 2394: entire match fails. (Perl is able, at this point, to re-enter the recursion and
! 2395: try the second alternative.) However, if the pattern is written with the
! 2396: alternatives in the other order, things are different:
! 2397: .sp
! 2398: ^((.)(?1)\e2|.)$
! 2399: .sp
! 2400: This time, the recursing alternative is tried first, and continues to recurse
! 2401: until it runs out of characters, at which point the recursion fails. But this
! 2402: time we do have another alternative to try at the higher level. That is the big
! 2403: difference: in the previous case the remaining alternative is at a deeper
! 2404: recursion level, which PCRE cannot use.
! 2405: .P
! 2406: To change the pattern so that it matches all palindromic strings, not just
! 2407: those with an odd number of characters, it is tempting to change the pattern to
! 2408: this:
! 2409: .sp
! 2410: ^((.)(?1)\e2|.?)$
! 2411: .sp
! 2412: Again, this works in Perl, but not in PCRE, and for the same reason. When a
! 2413: deeper recursion has matched a single character, it cannot be entered again in
! 2414: order to match an empty string. The solution is to separate the two cases, and
! 2415: write out the odd and even cases as alternatives at the higher level:
! 2416: .sp
! 2417: ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
! 2418: .sp
! 2419: If you want to match typical palindromic phrases, the pattern has to ignore all
! 2420: non-word characters, which can be done like this:
! 2421: .sp
! 2422: ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
! 2423: .sp
! 2424: If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
! 2425: man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
! 2426: the use of the possessive quantifier *+ to avoid backtracking into sequences of
! 2427: non-word characters. Without this, PCRE takes a great deal longer (ten times or
! 2428: more) to match typical phrases, and Perl takes so long that you think it has
! 2429: gone into a loop.
! 2430: .P
! 2431: \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
! 2432: string does not start with a palindrome that is shorter than the entire string.
! 2433: For example, although "abcba" is correctly matched, if the subject is "ababa",
! 2434: PCRE finds the palindrome "aba" at the start, then fails at top level because
! 2435: the end of the string does not follow. Once again, it cannot jump back into the
! 2436: recursion to try other alternatives, so the entire match fails.
! 2437: .P
! 2438: The second way in which PCRE and Perl differ in their recursion processing is
! 2439: in the handling of captured values. In Perl, when a subpattern is called
! 2440: recursively or as a subpattern (see the next section), it has no access to any
! 2441: values that were captured outside the recursion, whereas in PCRE these values
! 2442: can be referenced. Consider this pattern:
! 2443: .sp
! 2444: ^(.)(\e1|a(?2))
! 2445: .sp
! 2446: In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
! 2447: then in the second group, when the back reference \e1 fails to match "b", the
! 2448: second alternative matches "a" and then recurses. In the recursion, \e1 does
! 2449: now match "b" and so the whole match succeeds. In Perl, the pattern fails to
! 2450: match because inside the recursive call \e1 cannot access the externally set
! 2451: value.
! 2452: .
! 2453: .
! 2454: .\" HTML <a name="subpatternsassubroutines"></a>
! 2455: .SH "SUBPATTERNS AS SUBROUTINES"
! 2456: .rs
! 2457: .sp
! 2458: If the syntax for a recursive subpattern call (either by number or by
! 2459: name) is used outside the parentheses to which it refers, it operates like a
! 2460: subroutine in a programming language. The called subpattern may be defined
! 2461: before or after the reference. A numbered reference can be absolute or
! 2462: relative, as in these examples:
! 2463: .sp
! 2464: (...(absolute)...)...(?2)...
! 2465: (...(relative)...)...(?-1)...
! 2466: (...(?+1)...(relative)...
! 2467: .sp
! 2468: An earlier example pointed out that the pattern
! 2469: .sp
! 2470: (sens|respons)e and \e1ibility
! 2471: .sp
! 2472: matches "sense and sensibility" and "response and responsibility", but not
! 2473: "sense and responsibility". If instead the pattern
! 2474: .sp
! 2475: (sens|respons)e and (?1)ibility
! 2476: .sp
! 2477: is used, it does match "sense and responsibility" as well as the other two
! 2478: strings. Another example is given in the discussion of DEFINE above.
! 2479: .P
! 2480: All subroutine calls, whether recursive or not, are always treated as atomic
! 2481: groups. That is, once a subroutine has matched some of the subject string, it
! 2482: is never re-entered, even if it contains untried alternatives and there is a
! 2483: subsequent matching failure. Any capturing parentheses that are set during the
! 2484: subroutine call revert to their previous values afterwards.
! 2485: .P
! 2486: Processing options such as case-independence are fixed when a subpattern is
! 2487: defined, so if it is used as a subroutine, such options cannot be changed for
! 2488: different calls. For example, consider this pattern:
! 2489: .sp
! 2490: (abc)(?i:(?-1))
! 2491: .sp
! 2492: It matches "abcabc". It does not match "abcABC" because the change of
! 2493: processing option does not affect the called subpattern.
! 2494: .
! 2495: .
! 2496: .\" HTML <a name="onigurumasubroutines"></a>
! 2497: .SH "ONIGURUMA SUBROUTINE SYNTAX"
! 2498: .rs
! 2499: .sp
! 2500: For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
! 2501: a number enclosed either in angle brackets or single quotes, is an alternative
! 2502: syntax for referencing a subpattern as a subroutine, possibly recursively. Here
! 2503: are two of the examples used above, rewritten using this syntax:
! 2504: .sp
! 2505: (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
! 2506: (sens|respons)e and \eg'1'ibility
! 2507: .sp
! 2508: PCRE supports an extension to Oniguruma: if a number is preceded by a
! 2509: plus or a minus sign it is taken as a relative reference. For example:
! 2510: .sp
! 2511: (abc)(?i:\eg<-1>)
! 2512: .sp
! 2513: Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
! 2514: synonymous. The former is a back reference; the latter is a subroutine call.
! 2515: .
! 2516: .
! 2517: .SH CALLOUTS
! 2518: .rs
! 2519: .sp
! 2520: Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
! 2521: code to be obeyed in the middle of matching a regular expression. This makes it
! 2522: possible, amongst other things, to extract different substrings that match the
! 2523: same pair of parentheses when there is a repetition.
! 2524: .P
! 2525: PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
! 2526: code. The feature is called "callout". The caller of PCRE provides an external
! 2527: function by putting its entry point in the global variable \fIpcre_callout\fP.
! 2528: By default, this variable contains NULL, which disables all calling out.
! 2529: .P
! 2530: Within a regular expression, (?C) indicates the points at which the external
! 2531: function is to be called. If you want to identify different callout points, you
! 2532: can put a number less than 256 after the letter C. The default value is zero.
! 2533: For example, this pattern has two callout points:
! 2534: .sp
! 2535: (?C1)abc(?C2)def
! 2536: .sp
! 2537: If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
! 2538: automatically installed before each item in the pattern. They are all numbered
! 2539: 255.
! 2540: .P
! 2541: During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
! 2542: set), the external function is called. It is provided with the number of the
! 2543: callout, the position in the pattern, and, optionally, one item of data
! 2544: originally supplied by the caller of \fBpcre_exec()\fP. The callout function
! 2545: may cause matching to proceed, to backtrack, or to fail altogether. A complete
! 2546: description of the interface to the callout function is given in the
! 2547: .\" HREF
! 2548: \fBpcrecallout\fP
! 2549: .\"
! 2550: documentation.
! 2551: .
! 2552: .
! 2553: .\" HTML <a name="backtrackcontrol"></a>
! 2554: .SH "BACKTRACKING CONTROL"
! 2555: .rs
! 2556: .sp
! 2557: Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
! 2558: are described in the Perl documentation as "experimental and subject to change
! 2559: or removal in a future version of Perl". It goes on to say: "Their usage in
! 2560: production code should be noted to avoid problems during upgrades." The same
! 2561: remarks apply to the PCRE features described in this section.
! 2562: .P
! 2563: Since these verbs are specifically related to backtracking, most of them can be
! 2564: used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
! 2565: a backtracking algorithm. With the exception of (*FAIL), which behaves like a
! 2566: failing negative assertion, they cause an error if encountered by
! 2567: \fBpcre_dfa_exec()\fP.
! 2568: .P
! 2569: If any of these verbs are used in an assertion or in a subpattern that is
! 2570: called as a subroutine (whether or not recursively), their effect is confined
! 2571: to that subpattern; it does not extend to the surrounding pattern, with one
! 2572: exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
! 2573: a successful positive assertion \fIis\fP passed back when a match succeeds
! 2574: (compare capturing parentheses in assertions). Note that such subpatterns are
! 2575: processed as anchored at the point where they are tested. Note also that Perl's
! 2576: treatment of subroutines is different in some cases.
! 2577: .P
! 2578: The new verbs make use of what was previously invalid syntax: an opening
! 2579: parenthesis followed by an asterisk. They are generally of the form
! 2580: (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
! 2581: depending on whether or not an argument is present. A name is any sequence of
! 2582: characters that does not include a closing parenthesis. If the name is empty,
! 2583: that is, if the closing parenthesis immediately follows the colon, the effect
! 2584: is as if the colon were not there. Any number of these verbs may occur in a
! 2585: pattern.
! 2586: .P
! 2587: PCRE contains some optimizations that are used to speed up matching by running
! 2588: some checks at the start of each match attempt. For example, it may know the
! 2589: minimum length of matching subject, or that a particular character must be
! 2590: present. When one of these optimizations suppresses the running of a match, any
! 2591: included backtracking verbs will not, of course, be processed. You can suppress
! 2592: the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
! 2593: when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
! 2594: pattern with (*NO_START_OPT).
! 2595: .P
! 2596: Experiments with Perl suggest that it too has similar optimizations, sometimes
! 2597: leading to anomalous results.
! 2598: .
! 2599: .
! 2600: .SS "Verbs that act immediately"
! 2601: .rs
! 2602: .sp
! 2603: The following verbs act as soon as they are encountered. They may not be
! 2604: followed by a name.
! 2605: .sp
! 2606: (*ACCEPT)
! 2607: .sp
! 2608: This verb causes the match to end successfully, skipping the remainder of the
! 2609: pattern. However, when it is inside a subpattern that is called as a
! 2610: subroutine, only that subpattern is ended successfully. Matching then continues
! 2611: at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
! 2612: far is captured. For example:
! 2613: .sp
! 2614: A((?:A|B(*ACCEPT)|C)D)
! 2615: .sp
! 2616: This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
! 2617: the outer parentheses.
! 2618: .sp
! 2619: (*FAIL) or (*F)
! 2620: .sp
! 2621: This verb causes a matching failure, forcing backtracking to occur. It is
! 2622: equivalent to (?!) but easier to read. The Perl documentation notes that it is
! 2623: probably useful only when combined with (?{}) or (??{}). Those are, of course,
! 2624: Perl features that are not present in PCRE. The nearest equivalent is the
! 2625: callout feature, as for example in this pattern:
! 2626: .sp
! 2627: a+(?C)(*FAIL)
! 2628: .sp
! 2629: A match with the string "aaaa" always fails, but the callout is taken before
! 2630: each backtrack happens (in this example, 10 times).
! 2631: .
! 2632: .
! 2633: .SS "Recording which path was taken"
! 2634: .rs
! 2635: .sp
! 2636: There is one verb whose main purpose is to track how a match was arrived at,
! 2637: though it also has a secondary use in conjunction with advancing the match
! 2638: starting point (see (*SKIP) below).
! 2639: .sp
! 2640: (*MARK:NAME) or (*:NAME)
! 2641: .sp
! 2642: A name is always required with this verb. There may be as many instances of
! 2643: (*MARK) as you like in a pattern, and their names do not have to be unique.
! 2644: .P
! 2645: When a match succeeds, the name of the last-encountered (*MARK) on the matching
! 2646: path is passed back to the caller via the \fIpcre_extra\fP data structure, as
! 2647: described in the
! 2648: .\" HTML <a href="pcreapi.html#extradata">
! 2649: .\" </a>
! 2650: section on \fIpcre_extra\fP
! 2651: .\"
! 2652: in the
! 2653: .\" HREF
! 2654: \fBpcreapi\fP
! 2655: .\"
! 2656: documentation. Here is an example of \fBpcretest\fP output, where the /K
! 2657: modifier requests the retrieval and outputting of (*MARK) data:
! 2658: .sp
! 2659: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
! 2660: data> XY
! 2661: 0: XY
! 2662: MK: A
! 2663: XZ
! 2664: 0: XZ
! 2665: MK: B
! 2666: .sp
! 2667: The (*MARK) name is tagged with "MK:" in this output, and in this example it
! 2668: indicates which of the two alternatives matched. This is a more efficient way
! 2669: of obtaining this information than putting each alternative in its own
! 2670: capturing parentheses.
! 2671: .P
! 2672: If (*MARK) is encountered in a positive assertion, its name is recorded and
! 2673: passed back if it is the last-encountered. This does not happen for negative
! 2674: assertions.
! 2675: .P
! 2676: After a partial match or a failed match, the name of the last encountered
! 2677: (*MARK) in the entire match process is returned. For example:
! 2678: .sp
! 2679: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
! 2680: data> XP
! 2681: No match, mark = B
! 2682: .sp
! 2683: Note that in this unanchored example the mark is retained from the match
! 2684: attempt that started at the letter "X". Subsequent match attempts starting at
! 2685: "P" and then with an empty string do not get as far as the (*MARK) item, but
! 2686: nevertheless do not reset it.
! 2687: .
! 2688: .
! 2689: .SS "Verbs that act after backtracking"
! 2690: .rs
! 2691: .sp
! 2692: The following verbs do nothing when they are encountered. Matching continues
! 2693: with what follows, but if there is no subsequent match, causing a backtrack to
! 2694: the verb, a failure is forced. That is, backtracking cannot pass to the left of
! 2695: the verb. However, when one of these verbs appears inside an atomic group, its
! 2696: effect is confined to that group, because once the group has been matched,
! 2697: there is never any backtracking into it. In this situation, backtracking can
! 2698: "jump back" to the left of the entire atomic group. (Remember also, as stated
! 2699: above, that this localization also applies in subroutine calls and assertions.)
! 2700: .P
! 2701: These verbs differ in exactly what kind of failure occurs when backtracking
! 2702: reaches them.
! 2703: .sp
! 2704: (*COMMIT)
! 2705: .sp
! 2706: This verb, which may not be followed by a name, causes the whole match to fail
! 2707: outright if the rest of the pattern does not match. Even if the pattern is
! 2708: unanchored, no further attempts to find a match by advancing the starting point
! 2709: take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
! 2710: finding a match at the current starting point, or not at all. For example:
! 2711: .sp
! 2712: a+(*COMMIT)b
! 2713: .sp
! 2714: This matches "xxaab" but not "aacaab". It can be thought of as a kind of
! 2715: dynamic anchor, or "I've started, so I must finish." The name of the most
! 2716: recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
! 2717: match failure.
! 2718: .P
! 2719: Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
! 2720: unless PCRE's start-of-match optimizations are turned off, as shown in this
! 2721: \fBpcretest\fP example:
! 2722: .sp
! 2723: re> /(*COMMIT)abc/
! 2724: data> xyzabc
! 2725: 0: abc
! 2726: xyzabc\eY
! 2727: No match
! 2728: .sp
! 2729: PCRE knows that any match must start with "a", so the optimization skips along
! 2730: the subject to "a" before running the first match attempt, which succeeds. When
! 2731: the optimization is disabled by the \eY escape in the second subject, the match
! 2732: starts at "x" and so the (*COMMIT) causes it to fail without trying any other
! 2733: starting points.
! 2734: .sp
! 2735: (*PRUNE) or (*PRUNE:NAME)
! 2736: .sp
! 2737: This verb causes the match to fail at the current starting position in the
! 2738: subject if the rest of the pattern does not match. If the pattern is
! 2739: unanchored, the normal "bumpalong" advance to the next starting character then
! 2740: happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
! 2741: reached, or when matching to the right of (*PRUNE), but if there is no match to
! 2742: the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
! 2743: (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
! 2744: but there are some uses of (*PRUNE) that cannot be expressed in any other way.
! 2745: The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
! 2746: anchored pattern (*PRUNE) has the same effect as (*COMMIT).
! 2747: .sp
! 2748: (*SKIP)
! 2749: .sp
! 2750: This verb, when given without a name, is like (*PRUNE), except that if the
! 2751: pattern is unanchored, the "bumpalong" advance is not to the next character,
! 2752: but to the position in the subject where (*SKIP) was encountered. (*SKIP)
! 2753: signifies that whatever text was matched leading up to it cannot be part of a
! 2754: successful match. Consider:
! 2755: .sp
! 2756: a+(*SKIP)b
! 2757: .sp
! 2758: If the subject is "aaaac...", after the first match attempt fails (starting at
! 2759: the first character in the string), the starting point skips on to start the
! 2760: next attempt at "c". Note that a possessive quantifer does not have the same
! 2761: effect as this example; although it would suppress backtracking during the
! 2762: first match attempt, the second attempt would start at the second character
! 2763: instead of skipping on to "c".
! 2764: .sp
! 2765: (*SKIP:NAME)
! 2766: .sp
! 2767: When (*SKIP) has an associated name, its behaviour is modified. If the
! 2768: following pattern fails to match, the previous path through the pattern is
! 2769: searched for the most recent (*MARK) that has the same name. If one is found,
! 2770: the "bumpalong" advance is to the subject position that corresponds to that
! 2771: (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
! 2772: matching name is found, the (*SKIP) is ignored.
! 2773: .sp
! 2774: (*THEN) or (*THEN:NAME)
! 2775: .sp
! 2776: This verb causes a skip to the next innermost alternative if the rest of the
! 2777: pattern does not match. That is, it cancels pending backtracking, but only
! 2778: within the current alternative. Its name comes from the observation that it can
! 2779: be used for a pattern-based if-then-else block:
! 2780: .sp
! 2781: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
! 2782: .sp
! 2783: If the COND1 pattern matches, FOO is tried (and possibly further items after
! 2784: the end of the group if FOO succeeds); on failure, the matcher skips to the
! 2785: second alternative and tries COND2, without backtracking into COND1. The
! 2786: behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
! 2787: If (*THEN) is not inside an alternation, it acts like (*PRUNE).
! 2788: .P
! 2789: Note that a subpattern that does not contain a | character is just a part of
! 2790: the enclosing alternative; it is not a nested alternation with only one
! 2791: alternative. The effect of (*THEN) extends beyond such a subpattern to the
! 2792: enclosing alternative. Consider this pattern, where A, B, etc. are complex
! 2793: pattern fragments that do not contain any | characters at this level:
! 2794: .sp
! 2795: A (B(*THEN)C) | D
! 2796: .sp
! 2797: If A and B are matched, but there is a failure in C, matching does not
! 2798: backtrack into A; instead it moves to the next alternative, that is, D.
! 2799: However, if the subpattern containing (*THEN) is given an alternative, it
! 2800: behaves differently:
! 2801: .sp
! 2802: A (B(*THEN)C | (*FAIL)) | D
! 2803: .sp
! 2804: The effect of (*THEN) is now confined to the inner subpattern. After a failure
! 2805: in C, matching moves to (*FAIL), which causes the whole subpattern to fail
! 2806: because there are no more alternatives to try. In this case, matching does now
! 2807: backtrack into A.
! 2808: .P
! 2809: Note also that a conditional subpattern is not considered as having two
! 2810: alternatives, because only one is ever used. In other words, the | character in
! 2811: a conditional subpattern has a different meaning. Ignoring white space,
! 2812: consider:
! 2813: .sp
! 2814: ^.*? (?(?=a) a | b(*THEN)c )
! 2815: .sp
! 2816: If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
! 2817: it initially matches zero characters. The condition (?=a) then fails, the
! 2818: character "b" is matched, but "c" is not. At this point, matching does not
! 2819: backtrack to .*? as might perhaps be expected from the presence of the |
! 2820: character. The conditional subpattern is part of the single alternative that
! 2821: comprises the whole pattern, and so the match fails. (If there was a backtrack
! 2822: into .*?, allowing it to match "b", the match would succeed.)
! 2823: .P
! 2824: The verbs just described provide four different "strengths" of control when
! 2825: subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
! 2826: next alternative. (*PRUNE) comes next, failing the match at the current
! 2827: starting position, but allowing an advance to the next character (for an
! 2828: unanchored pattern). (*SKIP) is similar, except that the advance may be more
! 2829: than one character. (*COMMIT) is the strongest, causing the entire match to
! 2830: fail.
! 2831: .P
! 2832: If more than one such verb is present in a pattern, the "strongest" one wins.
! 2833: For example, consider this pattern, where A, B, etc. are complex pattern
! 2834: fragments:
! 2835: .sp
! 2836: (A(*COMMIT)B(*THEN)C|D)
! 2837: .sp
! 2838: Once A has matched, PCRE is committed to this match, at the current starting
! 2839: position. If subsequently B matches, but C does not, the normal (*THEN) action
! 2840: of trying the next alternative (that is, D) does not happen because (*COMMIT)
! 2841: overrides.
! 2842: .
! 2843: .
! 2844: .SH "SEE ALSO"
! 2845: .rs
! 2846: .sp
! 2847: \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
! 2848: \fBpcresyntax\fP(3), \fBpcre\fP(3).
! 2849: .
! 2850: .
! 2851: .SH AUTHOR
! 2852: .rs
! 2853: .sp
! 2854: .nf
! 2855: Philip Hazel
! 2856: University Computing Service
! 2857: Cambridge CB2 3QH, England.
! 2858: .fi
! 2859: .
! 2860: .
! 2861: .SH REVISION
! 2862: .rs
! 2863: .sp
! 2864: .nf
! 2865: Last updated: 29 November 2011
! 2866: Copyright (c) 1997-2011 University of Cambridge.
! 2867: .fi
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