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pcre

    1: .TH PCRECOMPAT 3 "08 January 2012" "PCRE 8.30"
    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 Unicode support. Details of what it does
   12: 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 any of the backtracking control verbs are used in an assertion or in a
   99: subpattern that is called as a subroutine (whether or not recursively), their
  100: effect is confined to that subpattern; it does not extend to the surrounding
  101: pattern. This is not always the case in Perl. In particular, if (*THEN) is
  102: present in a group that is called as a subroutine, its action is limited to
  103: that group, even if the group does not contain any | characters. There is one
  104: exception to this: the name from a *(MARK), (*PRUNE), or (*THEN) that is
  105: encountered in a successful positive assertion \fIis\fP passed back when a
  106: match succeeds (compare capturing parentheses in assertions). Note that such
  107: subpatterns are processed as anchored at the point where they are tested.
  108: .P
  109: 12. There are some differences that are concerned with the settings of captured
  110: strings when part of a pattern is repeated. For example, matching "aba" against
  111: the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
  112: .P
  113: 13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
  114: names is not as general as Perl's. This is a consequence of the fact the PCRE
  115: works internally just with numbers, using an external table to translate
  116: between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
  117: where the two capturing parentheses have the same number but different names,
  118: is not supported, and causes an error at compile time. If it were allowed, it
  119: would not be possible to distinguish which parentheses matched, because both
  120: names map to capturing subpattern number 1. To avoid this confusing situation,
  121: an error is given at compile time.
  122: .P
  123: 14. Perl recognizes comments in some places that PCRE does not, for example,
  124: between the ( and ? at the start of a subpattern. If the /x modifier is set,
  125: Perl allows white space between ( and ? but PCRE never does, even if the
  126: PCRE_EXTENDED option is set.
  127: .P
  128: 15. PCRE provides some extensions to the Perl regular expression facilities.
  129: Perl 5.10 includes new features that are not in earlier versions of Perl, some
  130: of which (such as named parentheses) have been in PCRE for some time. This list
  131: is with respect to Perl 5.10:
  132: .sp
  133: (a) Although lookbehind assertions in PCRE must match fixed length strings,
  134: each alternative branch of a lookbehind assertion can match a different length
  135: of string. Perl requires them all to have the same length.
  136: .sp
  137: (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
  138: meta-character matches only at the very end of the string.
  139: .sp
  140: (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
  141: meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
  142: (Perl can be made to issue a warning.)
  143: .sp
  144: (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
  145: inverted, that is, by default they are not greedy, but if followed by a
  146: question mark they are.
  147: .sp
  148: (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
  149: only at the first matching position in the subject string.
  150: .sp
  151: (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
  152: PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
  153: .sp
  154: (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
  155: by the PCRE_BSR_ANYCRLF option.
  156: .sp
  157: (h) The callout facility is PCRE-specific.
  158: .sp
  159: (i) The partial matching facility is PCRE-specific.
  160: .sp
  161: (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
  162: different hosts that have the other endianness. However, this does not apply to
  163: optimized data created by the just-in-time compiler.
  164: .sp
  165: (k) The alternative matching functions (\fBpcre_dfa_exec()\fP and
  166: \fBpcre16_dfa_exec()\fP) match in a different way and are not Perl-compatible.
  167: .sp
  168: (l) PCRE recognizes some special sequences such as (*CR) at the start of
  169: a pattern that set overall options that cannot be changed within the pattern.
  170: .
  171: .
  172: .SH AUTHOR
  173: .rs
  174: .sp
  175: .nf
  176: Philip Hazel
  177: University Computing Service
  178: Cambridge CB2 3QH, England.
  179: .fi
  180: .
  181: .
  182: .SH REVISION
  183: .rs
  184: .sp
  185: .nf
  186: Last updated: 01 June 2012
  187: Copyright (c) 1997-2012 University of Cambridge.
  188: .fi