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Documentation update.

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Philip.Hazel 2017-03-29 18:10:55 +00:00
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@ -1,4 +1,4 @@
.TH PCRE2COMPAT 3 "18 October 2016" "PCRE2 10.23"
.TH PCRE2COMPAT 3 "29 March 2017" "PCRE2 10.30"
.SH NAME
PCRE2 - Perl-compatible regular expressions (revised API)
.SH "DIFFERENCES BETWEEN PCRE2 AND PERL"
@ -6,7 +6,8 @@ PCRE2 - Perl-compatible regular expressions (revised API)
.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 5.10 and above.
versions 5.24, 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information may sometimes be out of date.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
@ -15,16 +16,17 @@ have are given in the
.\"
page.
.P
2. PCRE2 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: PCRE2 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.
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 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.
3. Capturing subpatterns that occur inside negative lookaround assertions are
counted, but their entries in the offsets vector are set only if the assertion
is a condition. Perl has changed its behaviour in this regard from time to
time.
.P
4. 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
@ -35,13 +37,13 @@ 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 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."
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 does support the \eQ...\eE escape for quoting substrings. Characters
in between are treated as literals. This is slightly different from Perl in
@ -60,29 +62,16 @@ Note the following examples:
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, there is support for recursive patterns. This is not
available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE2 "callout"
feature allows an external function to be called during pattern matching. See
the
constructions. However, there is support PCRE2's "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) are treated as atomic groups.
Atomic recursion is like Python, but unlike Perl. Captured values that are set
outside a subroutine call can be referenced from inside in PCRE2, but not in
Perl. There is a discussion that explains these differences in more detail in
the
.\" HTML <a href="pcre2pattern.html#recursiondifference">
.\" </a>
section on recursion differences from Perl
.\"
in the
.\" HREF
\fBpcre2pattern\fP
.\"
page.
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
@ -130,13 +119,13 @@ certainly user mistakes.
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 (5.16), \ep{Lu} and \ep{Ll} match all
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) have been in PCRE2 for some time. This
list is with respect to Perl 5.10:
of which (such as named parentheses) were in PCRE2 for some time before. This
list is with respect to Perl 5.24:
.sp
(a) Although lookbehind assertions in PCRE2 must match fixed length strings,
each alternative branch of a lookbehind assertion can match a different length
@ -190,6 +179,6 @@ Cambridge, England.
.rs
.sp
.nf
Last updated: 18 October 2016
Copyright (c) 1997-2016 University of Cambridge.
Last updated: 29 March 2017
Copyright (c) 1997-2017 University of Cambridge.
.fi