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1.1 ! misho 1: <html> ! 2: <head> ! 3: <title>pcrecompat specification</title> ! 4: </head> ! 5: <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"> ! 6: <h1>pcrecompat man page</h1> ! 7: <p> ! 8: Return to the <a href="index.html">PCRE index page</a>. ! 9: </p> ! 10: <p> ! 11: This page is part of the PCRE HTML documentation. It was generated automatically ! 12: from the original man page. If there is any nonsense in it, please consult the ! 13: man page, in case the conversion went wrong. ! 14: <br> ! 15: <br><b> ! 16: DIFFERENCES BETWEEN PCRE AND PERL ! 17: </b><br> ! 18: <P> ! 19: This document describes the differences in the ways that PCRE and Perl handle ! 20: regular expressions. The differences described here are with respect to Perl ! 21: versions 5.10 and above. ! 22: </P> ! 23: <P> ! 24: 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what ! 25: it does have are given in the ! 26: <a href="pcreunicode.html"><b>pcreunicode</b></a> ! 27: page. ! 28: </P> ! 29: <P> ! 30: 2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do ! 31: not mean what you might think. For example, (?!a){3} does not assert that the ! 32: next three characters are not "a". It just asserts that the next character is ! 33: not "a" three times (in principle: PCRE optimizes this to run the assertion ! 34: just once). Perl allows repeat quantifiers on other assertions such as \b, but ! 35: these do not seem to have any use. ! 36: </P> ! 37: <P> ! 38: 3. Capturing subpatterns that occur inside negative lookahead assertions are ! 39: counted, but their entries in the offsets vector are never set. Perl sets its ! 40: numerical variables from any such patterns that are matched before the ! 41: assertion fails to match something (thereby succeeding), but only if the ! 42: negative lookahead assertion contains just one branch. ! 43: </P> ! 44: <P> ! 45: 4. Though binary zero characters are supported in the subject string, they are ! 46: not allowed in a pattern string because it is passed as a normal C string, ! 47: terminated by zero. The escape sequence \0 can be used in the pattern to ! 48: represent a binary zero. ! 49: </P> ! 50: <P> ! 51: 5. The following Perl escape sequences are not supported: \l, \u, \L, ! 52: \U, and \N when followed by a character name or Unicode value. (\N on its ! 53: own, matching a non-newline character, is supported.) In fact these are ! 54: implemented by Perl's general string-handling and are not part of its pattern ! 55: matching engine. If any of these are encountered by PCRE, an error is ! 56: generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, ! 57: \U and \u are interpreted as JavaScript interprets them. ! 58: </P> ! 59: <P> ! 60: 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is ! 61: built with Unicode character property support. The properties that can be ! 62: tested with \p and \P are limited to the general category properties such as ! 63: Lu and Nd, script names such as Greek or Han, and the derived properties Any ! 64: and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the ! 65: Perl documentation says "Because Perl hides the need for the user to understand ! 66: the internal representation of Unicode characters, there is no need to ! 67: implement the somewhat messy concept of surrogates." ! 68: </P> ! 69: <P> ! 70: 7. PCRE implements a simpler version of \X than Perl, which changed to make ! 71: \X match what Unicode calls an "extended grapheme cluster". This is more ! 72: complicated than an extended Unicode sequence, which is what PCRE matches. ! 73: </P> ! 74: <P> ! 75: 8. PCRE does support the \Q...\E escape for quoting substrings. Characters in ! 76: between are treated as literals. This is slightly different from Perl in that $ ! 77: and @ are also handled as literals inside the quotes. In Perl, they cause ! 78: variable interpolation (but of course PCRE does not have variables). Note the ! 79: following examples: ! 80: <pre> ! 81: Pattern PCRE matches Perl matches ! 82: ! 83: \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz ! 84: \Qabc\$xyz\E abc\$xyz abc\$xyz ! 85: \Qabc\E\$\Qxyz\E abc$xyz abc$xyz ! 86: </pre> ! 87: The \Q...\E sequence is recognized both inside and outside character classes. ! 88: </P> ! 89: <P> ! 90: 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) ! 91: constructions. However, there is support for recursive patterns. This is not ! 92: available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout" ! 93: feature allows an external function to be called during pattern matching. See ! 94: the ! 95: <a href="pcrecallout.html"><b>pcrecallout</b></a> ! 96: documentation for details. ! 97: </P> ! 98: <P> ! 99: 10. Subpatterns that are called as subroutines (whether or not recursively) are ! 100: always treated as atomic groups in PCRE. This is like Python, but unlike Perl. ! 101: Captured values that are set outside a subroutine call can be reference from ! 102: inside in PCRE, but not in Perl. There is a discussion that explains these ! 103: differences in more detail in the ! 104: <a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a> ! 105: in the ! 106: <a href="pcrepattern.html"><b>pcrepattern</b></a> ! 107: page. ! 108: </P> ! 109: <P> ! 110: 11. If (*THEN) is present in a group that is called as a subroutine, its action ! 111: is limited to that group, even if the group does not contain any | characters. ! 112: </P> ! 113: <P> ! 114: 12. There are some differences that are concerned with the settings of captured ! 115: strings when part of a pattern is repeated. For example, matching "aba" against ! 116: the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b". ! 117: </P> ! 118: <P> ! 119: 13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern ! 120: names is not as general as Perl's. This is a consequence of the fact the PCRE ! 121: works internally just with numbers, using an external table to translate ! 122: between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B), ! 123: where the two capturing parentheses have the same number but different names, ! 124: is not supported, and causes an error at compile time. If it were allowed, it ! 125: would not be possible to distinguish which parentheses matched, because both ! 126: names map to capturing subpattern number 1. To avoid this confusing situation, ! 127: an error is given at compile time. ! 128: </P> ! 129: <P> ! 130: 14. Perl recognizes comments in some places that PCRE does not, for example, ! 131: between the ( and ? at the start of a subpattern. If the /x modifier is set, ! 132: Perl allows whitespace between ( and ? but PCRE never does, even if the ! 133: PCRE_EXTENDED option is set. ! 134: </P> ! 135: <P> ! 136: 15. PCRE provides some extensions to the Perl regular expression facilities. ! 137: Perl 5.10 includes new features that are not in earlier versions of Perl, some ! 138: of which (such as named parentheses) have been in PCRE for some time. This list ! 139: is with respect to Perl 5.10: ! 140: <br> ! 141: <br> ! 142: (a) Although lookbehind assertions in PCRE must match fixed length strings, ! 143: each alternative branch of a lookbehind assertion can match a different length ! 144: of string. Perl requires them all to have the same length. ! 145: <br> ! 146: <br> ! 147: (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ ! 148: meta-character matches only at the very end of the string. ! 149: <br> ! 150: <br> ! 151: (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special ! 152: meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored. ! 153: (Perl can be made to issue a warning.) ! 154: <br> ! 155: <br> ! 156: (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is ! 157: inverted, that is, by default they are not greedy, but if followed by a ! 158: question mark they are. ! 159: <br> ! 160: <br> ! 161: (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried ! 162: only at the first matching position in the subject string. ! 163: <br> ! 164: <br> ! 165: (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and ! 166: PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents. ! 167: <br> ! 168: <br> ! 169: (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF ! 170: by the PCRE_BSR_ANYCRLF option. ! 171: <br> ! 172: <br> ! 173: (h) The callout facility is PCRE-specific. ! 174: <br> ! 175: <br> ! 176: (i) The partial matching facility is PCRE-specific. ! 177: <br> ! 178: <br> ! 179: (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on ! 180: different hosts that have the other endianness. However, this does not apply to ! 181: optimized data created by the just-in-time compiler. ! 182: <br> ! 183: <br> ! 184: (k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a ! 185: different way and is not Perl-compatible. ! 186: <br> ! 187: <br> ! 188: (l) PCRE recognizes some special sequences such as (*CR) at the start of ! 189: a pattern that set overall options that cannot be changed within the pattern. ! 190: </P> ! 191: <br><b> ! 192: AUTHOR ! 193: </b><br> ! 194: <P> ! 195: Philip Hazel ! 196: <br> ! 197: University Computing Service ! 198: <br> ! 199: Cambridge CB2 3QH, England. ! 200: <br> ! 201: </P> ! 202: <br><b> ! 203: REVISION ! 204: </b><br> ! 205: <P> ! 206: Last updated: 14 November 2011 ! 207: <br> ! 208: Copyright © 1997-2011 University of Cambridge. ! 209: <br> ! 210: <p> ! 211: Return to the <a href="index.html">PCRE index page</a>. ! 212: </p>