--- /dev/null
+<html>
+<head>
+<title>pcre2compat specification</title>
+</head>
+<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
+<h1>pcre2compat man page</h1>
+<p>
+Return to the <a href="index.html">PCRE2 index page</a>.
+</p>
+<p>
+This page is part of the PCRE2 HTML documentation. It was generated
+automatically from the original man page. If there is any nonsense in it,
+please consult the man page, in case the conversion went wrong.
+<br>
+<br><b>
+DIFFERENCES BETWEEN PCRE2 AND PERL
+</b><br>
+<P>
+This document describes the differences in the ways that PCRE2 and Perl handle
+regular expressions. The differences described here are with respect to Perl
+versions 5.26, but as both Perl and PCRE2 are continually changing, the
+information may sometimes be out of date.
+</P>
+<P>
+1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does
+have are given in the
+<a href="pcre2unicode.html"><b>pcre2unicode</b></a>
+page.
+</P>
+<P>
+2. Like Perl, PCRE2 allows repeat quantifiers 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; PCRE2 optimizes this to run the
+assertion just once). Perl allows some repeat quantifiers on other assertions,
+for example, \b* (but not \b{3}), but these do not seem to have any use.
+</P>
+<P>
+3. Capturing subpatterns that occur inside negative lookaround assertions are
+counted, but their entries in the offsets vector are set only when a negative
+assertion is a condition that has a matching branch (that is, the condition is
+false).
+</P>
+<P>
+4. The following Perl escape sequences are not supported: \F, \l, \L, \u,
+\U, and \N when followed by a character name. \N on its own, matching a
+non-newline character, and \N{U+dd..}, matching a Unicode code point, are
+supported. The escapes that modify the case of following letters are
+implemented by Perl's general string-handling and are not part of its pattern
+matching engine. If any of these are encountered by PCRE2, an error is
+generated by default. However, if the PCRE2_ALT_BSUX option is set, \U and \u
+are interpreted as ECMAScript interprets them.
+</P>
+<P>
+5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2 is
+built with Unicode support (the default). The properties that can be tested
+with \p and \P 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&.
+PCRE2 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>
+<P>
+6. PCRE2 supports the \Q...\E escape for quoting substrings. Characters
+in between are treated as literals. However, 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 PCRE2 does not have
+variables). Also, Perl does "double-quotish backslash interpolation" on any
+backslashes between \Q and \E which, its documentation says, "may lead to
+confusing results". PCRE2 treats a backslash between \Q and \E just like any
+other character. Note the following examples:
+<pre>
+ Pattern PCRE2 matches Perl matches
+
+ \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
+ \Qabc\$xyz\E abc\$xyz abc\$xyz
+ \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
+ \QA\B\E A\B A\B
+ \Q\\E \ \\E
+</pre>
+The \Q...\E sequence is recognized both inside and outside character classes.
+</P>
+<P>
+7. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code})
+constructions. However, PCRE2 does have a "callout" feature, which allows an
+external function to be called during pattern matching. See the
+<a href="pcre2callout.html"><b>pcre2callout</b></a>
+documentation for details.
+</P>
+<P>
+8. Subroutine calls (whether recursive or not) were treated as atomic groups up
+to PCRE2 release 10.23, but from release 10.30 this changed, and backtracking
+into subroutine calls is now supported, as in Perl.
+</P>
+<P>
+9. If any of the backtracking control verbs are used in a subpattern that is
+called as a subroutine (whether or not recursively), their effect is confined
+to that subpattern; it does not extend to the surrounding pattern. This is not
+always the case in Perl. In particular, if (*THEN) is present in a group that
+is called as a subroutine, its action is limited to that group, even if the
+group does not contain any | characters. Note that such subpatterns are
+processed as anchored at the point where they are tested.
+</P>
+<P>
+10. If a pattern contains more than one backtracking control verb, the first
+one that is backtracked onto acts. For example, in the pattern
+A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
+triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
+same as PCRE2, but there are cases where it differs.
+</P>
+<P>
+11. Most backtracking verbs in assertions have their normal actions. They are
+not confined to the assertion.
+</P>
+<P>
+12. There are some differences that are concerned with the settings of captured
+strings when part of a pattern is repeated. For example, matching "aba" against
+the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE2 it is set to
+"b".
+</P>
+<P>
+13. PCRE2'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 PCRE2
+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>
+<P>
+14. Perl used to recognize comments in some places that PCRE2 does not, for
+example, between the ( and ? at the start of a subpattern. If the /x modifier
+is set, Perl allowed white space between ( and ? though the latest Perls give
+an error (for a while it was just deprecated). There may still be some cases
+where Perl behaves differently.
+</P>
+<P>
+15. Perl, when in warning mode, gives warnings for character classes such as
+[A-\d] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE2 has no
+warning features, so it gives an error in these cases because they are almost
+certainly user mistakes.
+</P>
+<P>
+16. In PCRE2, the upper/lower case character properties Lu and Ll are not
+affected when case-independent matching is specified. For example, \p{Lu}
+always matches an upper case letter. I think Perl has changed in this respect;
+in the release at the time of writing (5.24), \p{Lu} and \p{Ll} match all
+letters, regardless of case, when case independence is specified.
+</P>
+<P>
+17. PCRE2 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) were in PCRE2 for some time before. This
+list is with respect to Perl 5.26:
+<br>
+<br>
+(a) Although lookbehind assertions in PCRE2 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.
+<br>
+<br>
+(b) From PCRE2 10.23, backreferences to groups of fixed length are supported
+in lookbehinds, provided that there is no possibility of referencing a
+non-unique number or name. Perl does not support backreferences in lookbehinds.
+<br>
+<br>
+(c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $
+meta-character matches only at the very end of the string.
+<br>
+<br>
+(d) A backslash followed by a letter with no special meaning is faulted. (Perl
+can be made to issue a warning.)
+<br>
+<br>
+(e) If PCRE2_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.
+<br>
+<br>
+(f) PCRE2_ANCHORED can be used at matching time to force a pattern to be tried
+only at the first matching position in the subject string.
+<br>
+<br>
+(g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTART
+options have no Perl equivalents.
+<br>
+<br>
+(h) The \R escape sequence can be restricted to match only CR, LF, or CRLF
+by the PCRE2_BSR_ANYCRLF option.
+<br>
+<br>
+(i) The callout facility is PCRE2-specific. Perl supports codeblocks and
+variable interpolation, but not general hooks on every match.
+<br>
+<br>
+(j) The partial matching facility is PCRE2-specific.
+<br>
+<br>
+(k) The alternative matching function (<b>pcre2_dfa_match()</b> matches in a
+different way and is not Perl-compatible.
+<br>
+<br>
+(l) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT) at
+the start of a pattern that set overall options that cannot be changed within
+the pattern.
+</P>
+<P>
+18. The Perl /a modifier restricts /d numbers to pure ascii, and the /aa
+modifier restricts /i case-insensitive matching to pure ascii, ignoring Unicode
+rules. This separation cannot be represented with PCRE2_UCP.
+</P>
+<P>
+19. Perl has different limits than PCRE2. See the
+<a href="pcre2limit.html"><b>pcre2limit</b></a>
+documentation for details. Perl went with 5.10 from recursion to iteration
+keeping the intermediate matches on the heap, which is ~10% slower but does not
+fall into any stack-overflow limit. PCRE2 made a similar change at release
+10.30, and also has many build-time and run-time customizable limits.
+</P>
+<br><b>
+AUTHOR
+</b><br>
+<P>
+Philip Hazel
+<br>
+University Computing Service
+<br>
+Cambridge, England.
+<br>
+</P>
+<br><b>
+REVISION
+</b><br>
+<P>
+Last updated: 28 July 2018
+<br>
+Copyright © 1997-2018 University of Cambridge.
+<br>
+<p>
+Return to the <a href="index.html">PCRE2 index page</a>.
+</p>
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