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diff --git a/pcre/doc/pcrecompat.3 b/pcre/doc/pcrecompat.3 deleted file mode 100644 index 6156e776f53..00000000000 --- a/pcre/doc/pcrecompat.3 +++ /dev/null @@ -1,200 +0,0 @@ -.TH PCRECOMPAT 3 "10 November 2013" "PCRE 8.34" -.SH NAME -PCRE - Perl-compatible regular expressions -.SH "DIFFERENCES BETWEEN PCRE AND PERL" -.rs -.sp -This document describes the differences in the ways that PCRE and Perl handle -regular expressions. The differences described here are with respect to Perl -versions 5.10 and above. -.P -1. PCRE has only a subset of Perl's Unicode support. Details of what it does -have are given in the -.\" HREF -\fBpcreunicode\fP -.\" -page. -.P -2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do -not mean what you might think. For example, (?!a){3} does not assert that the -next three characters are not "a". It just asserts that the next character is -not "a" three times (in principle: PCRE optimizes this to run the assertion -just once). Perl allows repeat quantifiers on other assertions such as \eb, but -these do not seem to have any use. -.P -3. Capturing subpatterns that occur inside negative lookahead assertions are -counted, but their entries in the offsets vector are never set. Perl sometimes -(but not always) sets its numerical variables from inside negative assertions. -.P -4. Though binary zero characters are supported in the subject string, they are -not allowed in a pattern string because it is passed as a normal C string, -terminated by zero. The escape sequence \e0 can be used in the pattern to -represent a binary zero. -.P -5. The following Perl escape sequences are not supported: \el, \eu, \eL, -\eU, and \eN when followed by a character name or Unicode value. (\eN on its -own, matching a non-newline character, is supported.) In fact these are -implemented by Perl's general string-handling and are not part of its pattern -matching engine. If any of these are encountered by PCRE, an error is -generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, -\eU and \eu are interpreted as JavaScript interprets them. -.P -6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is -built with Unicode character property support. The properties that can be -tested with \ep and \eP are limited to the general category properties such as -Lu and Nd, script names such as Greek or Han, and the derived properties Any -and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the -Perl documentation says "Because Perl hides the need for the user to understand -the internal representation of Unicode characters, there is no need to -implement the somewhat messy concept of surrogates." -.P -7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in -between are treated as literals. This is slightly different from Perl in that $ -and @ are also handled as literals inside the quotes. In Perl, they cause -variable interpolation (but of course PCRE does not have variables). Note the -following examples: -.sp - Pattern PCRE matches Perl matches -.sp -.\" JOIN - \eQabc$xyz\eE abc$xyz abc followed by the - contents of $xyz - \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz - \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz -.sp -The \eQ...\eE sequence is recognized both inside and outside character classes. -.P -8. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) -constructions. However, there is support for recursive patterns. This is not -available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout" -feature allows an external function to be called during pattern matching. See -the -.\" HREF -\fBpcrecallout\fP -.\" -documentation for details. -.P -9. Subpatterns that are called as subroutines (whether or not recursively) are -always treated as atomic groups in PCRE. This is like Python, but unlike Perl. -Captured values that are set outside a subroutine call can be reference from -inside in PCRE, but not in Perl. There is a discussion that explains these -differences in more detail in the -.\" HTML <a href="pcrepattern.html#recursiondifference"> -.\" </a> -section on recursion differences from Perl -.\" -in the -.\" HREF -\fBpcrepattern\fP -.\" -page. -.P -10. If any of the backtracking control verbs are used in a subpattern that is -called as a subroutine (whether or not recursively), their effect is confined -to that subpattern; it does not extend to the surrounding pattern. This is not -always the case in Perl. In particular, if (*THEN) is present in a group that -is called as a subroutine, its action is limited to that group, even if the -group does not contain any | characters. Note that such subpatterns are -processed as anchored at the point where they are tested. -.P -11. If a pattern contains more than one backtracking control verb, the first -one that is backtracked onto acts. For example, in the pattern -A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C -triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the -same as PCRE, but there are examples where it differs. -.P -12. Most backtracking verbs in assertions have their normal actions. They are -not confined to the assertion. -.P -13. There are some differences that are concerned with the settings of captured -strings when part of a pattern is repeated. For example, matching "aba" against -the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b". -.P -14. PCRE's handling of duplicate subpattern numbers and duplicate subpattern -names is not as general as Perl's. This is a consequence of the fact the PCRE -works internally just with numbers, using an external table to translate -between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b>B), -where the two capturing parentheses have the same number but different names, -is not supported, and causes an error at compile time. If it were allowed, it -would not be possible to distinguish which parentheses matched, because both -names map to capturing subpattern number 1. To avoid this confusing situation, -an error is given at compile time. -.P -15. Perl recognizes comments in some places that PCRE does not, for example, -between the ( and ? at the start of a subpattern. If the /x modifier is set, -Perl allows white space between ( and ? (though current Perls warn that this is -deprecated) but PCRE never does, even if the PCRE_EXTENDED option is set. -.P -16. Perl, when in warning mode, gives warnings for character classes such as -[A-\ed] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE has no -warning features, so it gives an error in these cases because they are almost -certainly user mistakes. -.P -17. In PCRE, the upper/lower case character properties Lu and Ll are not -affected when case-independent matching is specified. For example, \ep{Lu} -always matches an upper case letter. I think Perl has changed in this respect; -in the release at the time of writing (5.16), \ep{Lu} and \ep{Ll} match all -letters, regardless of case, when case independence is specified. -.P -18. PCRE provides some extensions to the Perl regular expression facilities. -Perl 5.10 includes new features that are not in earlier versions of Perl, some -of which (such as named parentheses) have been in PCRE for some time. This list -is with respect to Perl 5.10: -.sp -(a) Although lookbehind assertions in PCRE must match fixed length strings, -each alternative branch of a lookbehind assertion can match a different length -of string. Perl requires them all to have the same length. -.sp -(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ -meta-character matches only at the very end of the string. -.sp -(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special -meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored. -(Perl can be made to issue a warning.) -.sp -(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is -inverted, that is, by default they are not greedy, but if followed by a -question mark they are. -.sp -(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried -only at the first matching position in the subject string. -.sp -(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and -PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents. -.sp -(g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF -by the PCRE_BSR_ANYCRLF option. -.sp -(h) The callout facility is PCRE-specific. -.sp -(i) The partial matching facility is PCRE-specific. -.sp -(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on -different hosts that have the other endianness. However, this does not apply to -optimized data created by the just-in-time compiler. -.sp -(k) The alternative matching functions (\fBpcre_dfa_exec()\fP, -\fBpcre16_dfa_exec()\fP and \fBpcre32_dfa_exec()\fP,) match in a different way -and are not Perl-compatible. -.sp -(l) PCRE recognizes some special sequences such as (*CR) at the start of -a pattern that set overall options that cannot be changed within the pattern. -. -. -.SH AUTHOR -.rs -.sp -.nf -Philip Hazel -University Computing Service -Cambridge CB2 3QH, England. -.fi -. -. -.SH REVISION -.rs -.sp -.nf -Last updated: 10 November 2013 -Copyright (c) 1997-2013 University of Cambridge. -.fi |