Annotation of embedaddon/pcre/doc/html/pcrecompat.html, revision 1.1
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>
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