206 lines
9.5 KiB
Groff
206 lines
9.5 KiB
Groff
.TH PCRE2COMPAT 3 "28 July 2018" "PCRE2 10.32"
|
|
.SH NAME
|
|
PCRE2 - Perl-compatible regular expressions (revised API)
|
|
.SH "DIFFERENCES BETWEEN PCRE2 AND PERL"
|
|
.rs
|
|
.sp
|
|
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information may sometimes be out of date.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
|
|
1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does
|
|
have are given in the
|
|
.\" HREF
|
|
\fBpcre2unicode\fP
|
|
.\"
|
|
page.
|
|
.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, \eb* (but not \eb{3}), but these do not seem to have any use.
|
|
.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
|
|
4. The following Perl escape sequences are not supported: \eF, \el, \eL, \eu,
|
|
\eU, and \eN when followed by a character name. \eN on its own, matching a
|
|
non-newline character, and \eN{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, \eU and \eu
|
|
are interpreted as ECMAScript interprets them.
|
|
.P
|
|
5. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE2 is
|
|
built with Unicode support (the default). 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&.
|
|
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
|
|
6. PCRE2 supports the \eQ...\eE 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 \eQ and \eE which, its documentation says, "may lead to
|
|
confusing results". PCRE2 treats a backslash between \eQ and \eE just like any
|
|
other character. Note the following examples:
|
|
.sp
|
|
Pattern PCRE2 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$xyz
|
|
\eQA\eB\eE A\eB A\eB
|
|
\eQ\e\eE \e \e\eE
|
|
.sp
|
|
The \eQ...\eE sequence is recognized both inside and outside character classes.
|
|
.P
|
|
7. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code})
|
|
constructions. However, PCRE2 does have a "callout" feature, which allows an
|
|
external function to be called during pattern matching. See the
|
|
.\" HREF
|
|
\fBpcre2callout\fP
|
|
.\"
|
|
documentation for details.
|
|
.P
|
|
8. Subroutine calls (whether recursive or not) were treated as atomic groups up
|
|
to PCRE2 release 10.23, but from release 10.30 this changed, and backtracking
|
|
into subroutine calls is now supported, as in Perl.
|
|
.P
|
|
9. If any of the backtracking control verbs are used in a subpattern that is
|
|
called as a subroutine (whether or not recursively), their effect is confined
|
|
to that subpattern; it does not extend to the surrounding pattern. This is not
|
|
always the case in Perl. In particular, if (*THEN) is present in a group that
|
|
is called as a subroutine, its action is limited to that group, even if the
|
|
group does not contain any | characters. Note that such subpatterns are
|
|
processed as anchored at the point where they are tested.
|
|
.P
|
|
10. If a pattern contains more than one backtracking control verb, the first
|
|
one that is backtracked onto acts. For example, in the pattern
|
|
A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
|
|
triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
|
|
same as PCRE2, but there are cases where it differs.
|
|
.P
|
|
11. Most backtracking verbs in assertions have their normal actions. They are
|
|
not confined to the assertion.
|
|
.P
|
|
12. There are some differences that are concerned with the settings of captured
|
|
strings when part of a pattern is repeated. For example, matching "aba" against
|
|
the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE2 it is set to
|
|
"b".
|
|
.P
|
|
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
|
|
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
|
|
15. Perl, when in warning mode, gives warnings for character classes such as
|
|
[A-\ed] 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
|
|
16. In PCRE2, 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 (5.24), \ep{Lu} and \ep{Ll} match all
|
|
letters, regardless of case, when case independence is specified.
|
|
.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:
|
|
.sp
|
|
(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.
|
|
.sp
|
|
(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.
|
|
.sp
|
|
(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.
|
|
.sp
|
|
(d) A backslash followed by a letter with no special meaning is faulted. (Perl
|
|
can be made to issue a warning.)
|
|
.sp
|
|
(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.
|
|
.sp
|
|
(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.
|
|
.sp
|
|
(g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTART
|
|
options have no Perl equivalents.
|
|
.sp
|
|
(h) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
|
|
by the PCRE2_BSR_ANYCRLF option.
|
|
.sp
|
|
(i) The callout facility is PCRE2-specific. Perl supports codeblocks and
|
|
variable interpolation, but not general hooks on every match.
|
|
.sp
|
|
(j) The partial matching facility is PCRE2-specific.
|
|
.sp
|
|
(k) The alternative matching function (\fBpcre2_dfa_match()\fP matches in a
|
|
different way and is not Perl-compatible.
|
|
.sp
|
|
(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
|
|
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
|
|
19. Perl has different limits than PCRE2. See the
|
|
.\" HREF
|
|
\fBpcre2limit\fP
|
|
.\"
|
|
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.
|
|
.
|
|
.
|
|
.SH AUTHOR
|
|
.rs
|
|
.sp
|
|
.nf
|
|
Philip Hazel
|
|
University Computing Service
|
|
Cambridge, England.
|
|
.fi
|
|
.
|
|
.
|
|
.SH REVISION
|
|
.rs
|
|
.sp
|
|
.nf
|
|
Last updated: 28 July 2018
|
|
Copyright (c) 1997-2018 University of Cambridge.
|
|
.fi
|