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8.33

    1: .TH PCRECOMPAT 3 "19 March 2013" "PCRE 8.33"
    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 sometimes
   27: (but not always) sets its numerical variables from inside negative assertions.
   28: .P
   29: 4. Though binary zero characters are supported in the subject string, they are
   30: not allowed in a pattern string because it is passed as a normal C string,
   31: terminated by zero. The escape sequence \e0 can be used in the pattern to
   32: represent a binary zero.
   33: .P
   34: 5. The following Perl escape sequences are not supported: \el, \eu, \eL,
   35: \eU, and \eN when followed by a character name or Unicode value. (\eN on its
   36: own, matching a non-newline character, is supported.) In fact these are
   37: implemented by Perl's general string-handling and are not part of its pattern
   38: matching engine. If any of these are encountered by PCRE, an error is
   39: generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
   40: \eU and \eu are interpreted as JavaScript interprets them.
   41: .P
   42: 6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
   43: built with Unicode character property support. The properties that can be
   44: tested with \ep and \eP are limited to the general category properties such as
   45: Lu and Nd, script names such as Greek or Han, and the derived properties Any
   46: and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
   47: Perl documentation says "Because Perl hides the need for the user to understand
   48: the internal representation of Unicode characters, there is no need to
   49: implement the somewhat messy concept of surrogates."
   50: .P
   51: 7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
   52: between are treated as literals. This is slightly different from Perl in that $
   53: and @ are also handled as literals inside the quotes. In Perl, they cause
   54: variable interpolation (but of course PCRE does not have variables). Note the
   55: following examples:
   56: .sp
   57:     Pattern            PCRE matches      Perl matches
   58: .sp
   59: .\" JOIN
   60:     \eQabc$xyz\eE        abc$xyz           abc followed by the
   61:                                            contents of $xyz
   62:     \eQabc\e$xyz\eE       abc\e$xyz          abc\e$xyz
   63:     \eQabc\eE\e$\eQxyz\eE   abc$xyz           abc$xyz
   64: .sp
   65: The \eQ...\eE sequence is recognized both inside and outside character classes.
   66: .P
   67: 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
   68: constructions. However, there is support for recursive patterns. This is not
   69: available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
   70: feature allows an external function to be called during pattern matching. See
   71: the
   72: .\" HREF
   73: \fBpcrecallout\fP
   74: .\"
   75: documentation for details.
   76: .P
   77: 9. Subpatterns that are called as subroutines (whether or not recursively) are
   78: always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
   79: Captured values that are set outside a subroutine call can be reference from
   80: inside in PCRE, but not in Perl. There is a discussion that explains these
   81: differences in more detail in the
   82: .\" HTML <a href="pcrepattern.html#recursiondifference">
   83: .\" </a>
   84: section on recursion differences from Perl
   85: .\"
   86: in the
   87: .\" HREF
   88: \fBpcrepattern\fP
   89: .\"
   90: page.
   91: .P
   92: 10. If any of the backtracking control verbs are used in a subpattern that is
   93: called as a subroutine (whether or not recursively), their effect is confined
   94: to that subpattern; it does not extend to the surrounding pattern. This is not
   95: always the case in Perl. In particular, if (*THEN) is present in a group that
   96: is called as a subroutine, its action is limited to that group, even if the
   97: group does not contain any | characters. Note that such subpatterns are
   98: processed as anchored at the point where they are tested.
   99: .P
  100: 11. If a pattern contains more than one backtracking control verb, the first
  101: one that is backtracked onto acts. For example, in the pattern
  102: A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
  103: triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
  104: same as PCRE, but there are examples where it differs.
  105: .P
  106: 12. Most backtracking verbs in assertions have their normal actions. They are
  107: not confined to the assertion.
  108: .P
  109: 13. 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: 14. 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: 15. 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: 16. In PCRE, the upper/lower case character properties Lu and Ll are not
  129: affected when case-independent matching is specified. For example, \ep{Lu}
  130: always matches an upper case letter. I think Perl has changed in this respect;
  131: in the release at the time of writing (5.16), \ep{Lu} and \ep{Ll} match all
  132: letters, regardless of case, when case independence is specified.
  133: .P
  134: 17. PCRE provides some extensions to the Perl regular expression facilities.
  135: Perl 5.10 includes new features that are not in earlier versions of Perl, some
  136: of which (such as named parentheses) have been in PCRE for some time. This list
  137: is with respect to Perl 5.10:
  138: .sp
  139: (a) Although lookbehind assertions in PCRE must match fixed length strings,
  140: each alternative branch of a lookbehind assertion can match a different length
  141: of string. Perl requires them all to have the same length.
  142: .sp
  143: (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
  144: meta-character matches only at the very end of the string.
  145: .sp
  146: (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
  147: meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
  148: (Perl can be made to issue a warning.)
  149: .sp
  150: (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
  151: inverted, that is, by default they are not greedy, but if followed by a
  152: question mark they are.
  153: .sp
  154: (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
  155: only at the first matching position in the subject string.
  156: .sp
  157: (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
  158: PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
  159: .sp
  160: (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
  161: by the PCRE_BSR_ANYCRLF option.
  162: .sp
  163: (h) The callout facility is PCRE-specific.
  164: .sp
  165: (i) The partial matching facility is PCRE-specific.
  166: .sp
  167: (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
  168: different hosts that have the other endianness. However, this does not apply to
  169: optimized data created by the just-in-time compiler.
  170: .sp
  171: (k) The alternative matching functions (\fBpcre_dfa_exec()\fP,
  172: \fBpcre16_dfa_exec()\fP and \fBpcre32_dfa_exec()\fP,) match in a different way
  173: and are not Perl-compatible.
  174: .sp
  175: (l) PCRE recognizes some special sequences such as (*CR) at the start of
  176: a pattern that set overall options that cannot be changed within the pattern.
  177: .
  178: .
  179: .SH AUTHOR
  180: .rs
  181: .sp
  182: .nf
  183: Philip Hazel
  184: University Computing Service
  185: Cambridge CB2 3QH, England.
  186: .fi
  187: .
  188: .
  189: .SH REVISION
  190: .rs
  191: .sp
  192: .nf
  193: Last updated: 19 March 2013
  194: Copyright (c) 1997-2013 University of Cambridge.
  195: .fi