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Sun Aug 28 15:23:03 2011 UTC (3 years ago) by ph10
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Documentation for JIT support.

1 nigel 79 .TH PCRECOMPAT 3
2 nigel 63 .SH NAME
3     PCRE - Perl-compatible regular expressions
4 nigel 75 .SH "DIFFERENCES BETWEEN PCRE AND PERL"
5 nigel 63 .rs
6     .sp
7     This document describes the differences in the ways that PCRE and Perl handle
8 ph10 456 regular expressions. The differences described here are with respect to Perl
9 ph10 562 versions 5.10 and above.
10 nigel 75 .P
11 nigel 87 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
12     it does have are given in the
13 nigel 73 .\" HREF
14 ph10 678 \fBpcreunicode\fP
15 nigel 73 .\"
16     page.
17 nigel 75 .P
18 ph10 637 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 ph10 654 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 ph10 637 these do not seem to have any use.
24 nigel 75 .P
25 nigel 73 3. Capturing subpatterns that occur inside negative lookahead assertions are
26 nigel 63 counted, but their entries in the offsets vector are never set. Perl sets its
27     numerical variables from any such patterns that are matched before the
28     assertion fails to match something (thereby succeeding), but only if the
29     negative lookahead assertion contains just one branch.
30 nigel 75 .P
31 nigel 73 4. Though binary zero characters are supported in the subject string, they are
32 nigel 63 not allowed in a pattern string because it is passed as a normal C string,
33 nigel 75 terminated by zero. The escape sequence \e0 can be used in the pattern to
34 nigel 63 represent a binary zero.
35 nigel 75 .P
36     5. The following Perl escape sequences are not supported: \el, \eu, \eL,
37 ph10 654 \eU, and \eN when followed by a character name or Unicode value. (\eN on its
38 ph10 634 own, matching a non-newline character, is supported.) In fact these are
39     implemented by Perl's general string-handling and are not part of its pattern
40     matching engine. If any of these are encountered by PCRE, an error is
41     generated.
42 nigel 75 .P
43     6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
44     built with Unicode character property support. The properties that can be
45     tested with \ep and \eP are limited to the general category properties such as
46 nigel 87 Lu and Nd, script names such as Greek or Han, and the derived properties Any
47 ph10 450 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
48     Perl documentation says "Because Perl hides the need for the user to understand
49     the internal representation of Unicode characters, there is no need to
50     implement the somewhat messy concept of surrogates."
51 nigel 75 .P
52 ph10 628 7. PCRE implements a simpler version of \eX than Perl, which changed to make
53 ph10 654 \eX match what Unicode calls an "extended grapheme cluster". This is more
54 ph10 628 complicated than an extended Unicode sequence, which is what PCRE matches.
55     .P
56     8. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
57 nigel 63 between are treated as literals. This is slightly different from Perl in that $
58     and @ are also handled as literals inside the quotes. In Perl, they cause
59     variable interpolation (but of course PCRE does not have variables). Note the
60     following examples:
61 nigel 75 .sp
62 nigel 63 Pattern PCRE matches Perl matches
63 nigel 75 .sp
64     .\" JOIN
65     \eQabc$xyz\eE abc$xyz abc followed by the
66 nigel 63 contents of $xyz
67 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
68     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
69     .sp
70     The \eQ...\eE sequence is recognized both inside and outside character classes.
71     .P
72 ph10 628 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
73 nigel 93 constructions. However, there is support for recursive patterns. This is not
74 ph10 453 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
75 nigel 93 feature allows an external function to be called during pattern matching. See
76     the
77 nigel 75 .\" HREF
78     \fBpcrecallout\fP
79     .\"
80     documentation for details.
81     .P
82 ph10 628 10. Subpatterns that are called recursively or as "subroutines" are always
83 ph10 461 treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
84 ph10 453 is a discussion of an example that explains this in more detail in the
85     .\" HTML <a href="pcrepattern.html#recursiondifference">
86     .\" </a>
87     section on recursion differences from Perl
88     .\"
89     in the
90     .\" HREF
91 ph10 456 \fBpcrepattern\fP
92 ph10 453 .\"
93     page.
94 nigel 93 .P
95 ph10 628 11. There are some differences that are concerned with the settings of captured
96 nigel 63 strings when part of a pattern is repeated. For example, matching "aba" against
97     the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
98 nigel 75 .P
99 ph10 628 12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
100 ph10 459 names is not as general as Perl's. This is a consequence of the fact the PCRE
101     works internally just with numbers, using an external table to translate
102     between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
103     where the two capturing parentheses have the same number but different names,
104     is not supported, and causes an error at compile time. If it were allowed, it
105     would not be possible to distinguish which parentheses matched, because both
106     names map to capturing subpattern number 1. To avoid this confusing situation,
107     an error is given at compile time.
108 ph10 456 .P
109 ph10 635 13. Perl recognizes comments in some places that PCRE does not, for example,
110 ph10 654 between the ( and ? at the start of a subpattern. If the /x modifier is set,
111     Perl allows whitespace between ( and ? but PCRE never does, even if the
112 ph10 635 PCRE_EXTENDED option is set.
113 ph10 562 .P
114 ph10 628 14. PCRE provides some extensions to the Perl regular expression facilities.
115 ph10 456 Perl 5.10 includes new features that are not in earlier versions of Perl, some
116     of which (such as named parentheses) have been in PCRE for some time. This list
117     is with respect to Perl 5.10:
118     .sp
119     (a) Although lookbehind assertions in PCRE must match fixed length strings,
120     each alternative branch of a lookbehind assertion can match a different length
121     of string. Perl requires them all to have the same length.
122     .sp
123 nigel 63 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
124     meta-character matches only at the very end of the string.
125 nigel 75 .sp
126 nigel 63 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
127 ph10 181 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
128     (Perl can be made to issue a warning.)
129 nigel 75 .sp
130 nigel 63 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
131     inverted, that is, by default they are not greedy, but if followed by a
132     question mark they are.
133 nigel 75 .sp
134     (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
135     only at the first matching position in the subject string.
136     .sp
137 ph10 442 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
138     PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
139 nigel 75 .sp
140 ph10 231 (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
141     by the PCRE_BSR_ANYCRLF option.
142 nigel 75 .sp
143 ph10 231 (h) The callout facility is PCRE-specific.
144 nigel 75 .sp
145 ph10 231 (i) The partial matching facility is PCRE-specific.
146     .sp
147     (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
148 nigel 75 different hosts that have the other endianness.
149 nigel 77 .sp
150 ph10 231 (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
151 nigel 77 different way and is not Perl-compatible.
152 ph10 231 .sp
153     (l) PCRE recognizes some special sequences such as (*CR) at the start of
154     a pattern that set overall options that cannot be changed within the pattern.
155 ph10 99 .
156     .
157     .SH AUTHOR
158     .rs
159     .sp
160     .nf
161     Philip Hazel
162     University Computing Service
163     Cambridge CB2 3QH, England.
164     .fi
165     .
166     .
167     .SH REVISION
168     .rs
169     .sp
170     .nf
171 ph10 678 Last updated: 24 August 2011
172 ph10 596 Copyright (c) 1997-2011 University of Cambridge.
173 ph10 99 .fi

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