Annotation of embedaddon/pcre/doc/pcrecompat.3, revision 1.1
1.1 ! misho 1: .TH PCRECOMPAT 3
! 2: .SH NAME
! 3: PCRE - Perl-compatible regular expressions
! 4: .SH "DIFFERENCES BETWEEN PCRE AND PERL"
! 5: .rs
! 6: .sp
! 7: This document describes the differences in the ways that PCRE and Perl handle
! 8: regular expressions. The differences described here are with respect to Perl
! 9: versions 5.10 and above.
! 10: .P
! 11: 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
! 12: it does have are given in the
! 13: .\" HREF
! 14: \fBpcreunicode\fP
! 15: .\"
! 16: page.
! 17: .P
! 18: 2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
! 19: not mean what you might think. For example, (?!a){3} does not assert that the
! 20: next three characters are not "a". It just asserts that the next character is
! 21: not "a" three times (in principle: PCRE optimizes this to run the assertion
! 22: just once). Perl allows repeat quantifiers on other assertions such as \eb, but
! 23: these do not seem to have any use.
! 24: .P
! 25: 3. Capturing subpatterns that occur inside negative lookahead assertions are
! 26: counted, but their entries in the offsets vector are never set. Perl sets its
! 27: numerical variables from any such patterns that are matched before the
! 28: assertion fails to match something (thereby succeeding), but only if the
! 29: negative lookahead assertion contains just one branch.
! 30: .P
! 31: 4. Though binary zero characters are supported in the subject string, they are
! 32: not allowed in a pattern string because it is passed as a normal C string,
! 33: terminated by zero. The escape sequence \e0 can be used in the pattern to
! 34: represent a binary zero.
! 35: .P
! 36: 5. The following Perl escape sequences are not supported: \el, \eu, \eL,
! 37: \eU, and \eN when followed by a character name or Unicode value. (\eN on its
! 38: own, matching a non-newline character, is supported.) In fact these are
! 39: implemented by Perl's general string-handling and are not part of its pattern
! 40: matching engine. If any of these are encountered by PCRE, an error is
! 41: generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
! 42: \eU and \eu are interpreted as JavaScript interprets them.
! 43: .P
! 44: 6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
! 45: built with Unicode character property support. The properties that can be
! 46: tested with \ep and \eP are limited to the general category properties such as
! 47: Lu and Nd, script names such as Greek or Han, and the derived properties Any
! 48: and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
! 49: Perl documentation says "Because Perl hides the need for the user to understand
! 50: the internal representation of Unicode characters, there is no need to
! 51: implement the somewhat messy concept of surrogates."
! 52: .P
! 53: 7. PCRE implements a simpler version of \eX than Perl, which changed to make
! 54: \eX match what Unicode calls an "extended grapheme cluster". This is more
! 55: complicated than an extended Unicode sequence, which is what PCRE matches.
! 56: .P
! 57: 8. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
! 58: between are treated as literals. This is slightly different from Perl in that $
! 59: and @ are also handled as literals inside the quotes. In Perl, they cause
! 60: variable interpolation (but of course PCRE does not have variables). Note the
! 61: following examples:
! 62: .sp
! 63: Pattern PCRE matches Perl matches
! 64: .sp
! 65: .\" JOIN
! 66: \eQabc$xyz\eE abc$xyz abc followed by the
! 67: contents of $xyz
! 68: \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
! 69: \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
! 70: .sp
! 71: The \eQ...\eE sequence is recognized both inside and outside character classes.
! 72: .P
! 73: 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
! 74: constructions. However, there is support for recursive patterns. This is not
! 75: available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
! 76: feature allows an external function to be called during pattern matching. See
! 77: the
! 78: .\" HREF
! 79: \fBpcrecallout\fP
! 80: .\"
! 81: documentation for details.
! 82: .P
! 83: 10. Subpatterns that are called as subroutines (whether or not recursively) are
! 84: always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
! 85: Captured values that are set outside a subroutine call can be reference from
! 86: inside in PCRE, but not in Perl. There is a discussion that explains these
! 87: differences in more detail in the
! 88: .\" HTML <a href="pcrepattern.html#recursiondifference">
! 89: .\" </a>
! 90: section on recursion differences from Perl
! 91: .\"
! 92: in the
! 93: .\" HREF
! 94: \fBpcrepattern\fP
! 95: .\"
! 96: page.
! 97: .P
! 98: 11. If (*THEN) is present in a group that is called as a subroutine, its action
! 99: is limited to that group, even if the group does not contain any | characters.
! 100: .P
! 101: 12. There are some differences that are concerned with the settings of captured
! 102: strings when part of a pattern is repeated. For example, matching "aba" against
! 103: the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
! 104: .P
! 105: 13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
! 106: names is not as general as Perl's. This is a consequence of the fact the PCRE
! 107: works internally just with numbers, using an external table to translate
! 108: between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
! 109: where the two capturing parentheses have the same number but different names,
! 110: is not supported, and causes an error at compile time. If it were allowed, it
! 111: would not be possible to distinguish which parentheses matched, because both
! 112: names map to capturing subpattern number 1. To avoid this confusing situation,
! 113: an error is given at compile time.
! 114: .P
! 115: 14. Perl recognizes comments in some places that PCRE does not, for example,
! 116: between the ( and ? at the start of a subpattern. If the /x modifier is set,
! 117: Perl allows whitespace between ( and ? but PCRE never does, even if the
! 118: PCRE_EXTENDED option is set.
! 119: .P
! 120: 15. PCRE provides some extensions to the Perl regular expression facilities.
! 121: Perl 5.10 includes new features that are not in earlier versions of Perl, some
! 122: of which (such as named parentheses) have been in PCRE for some time. This list
! 123: is with respect to Perl 5.10:
! 124: .sp
! 125: (a) Although lookbehind assertions in PCRE must match fixed length strings,
! 126: each alternative branch of a lookbehind assertion can match a different length
! 127: of string. Perl requires them all to have the same length.
! 128: .sp
! 129: (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
! 130: meta-character matches only at the very end of the string.
! 131: .sp
! 132: (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
! 133: meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
! 134: (Perl can be made to issue a warning.)
! 135: .sp
! 136: (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
! 137: inverted, that is, by default they are not greedy, but if followed by a
! 138: question mark they are.
! 139: .sp
! 140: (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
! 141: only at the first matching position in the subject string.
! 142: .sp
! 143: (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
! 144: PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
! 145: .sp
! 146: (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
! 147: by the PCRE_BSR_ANYCRLF option.
! 148: .sp
! 149: (h) The callout facility is PCRE-specific.
! 150: .sp
! 151: (i) The partial matching facility is PCRE-specific.
! 152: .sp
! 153: (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
! 154: different hosts that have the other endianness. However, this does not apply to
! 155: optimized data created by the just-in-time compiler.
! 156: .sp
! 157: (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
! 158: different way and is not Perl-compatible.
! 159: .sp
! 160: (l) PCRE recognizes some special sequences such as (*CR) at the start of
! 161: a pattern that set overall options that cannot be changed within the pattern.
! 162: .
! 163: .
! 164: .SH AUTHOR
! 165: .rs
! 166: .sp
! 167: .nf
! 168: Philip Hazel
! 169: University Computing Service
! 170: Cambridge CB2 3QH, England.
! 171: .fi
! 172: .
! 173: .
! 174: .SH REVISION
! 175: .rs
! 176: .sp
! 177: .nf
! 178: Last updated: 14 November 2011
! 179: Copyright (c) 1997-2011 University of Cambridge.
! 180: .fi
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