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Documentation for JIT support.

1 nigel 63 <html>
2     <head>
3     <title>pcrecompat specification</title>
4     </head>
5     <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 nigel 75 <h1>pcrecompat man page</h1>
7     <p>
8     Return to the <a href="index.html">PCRE index page</a>.
9     </p>
10 ph10 111 <p>
11 nigel 75 This page is part of the PCRE HTML documentation. It was generated automatically
12     from the original man page. If there is any nonsense in it, please consult the
13     man page, in case the conversion went wrong.
14 ph10 111 <br>
15 nigel 75 <br><b>
16     DIFFERENCES BETWEEN PCRE AND PERL
17     </b><br>
18 nigel 63 <P>
19     This document describes the differences in the ways that PCRE and Perl handle
20 ph10 461 regular expressions. The differences described here are with respect to Perl
21 ph10 567 versions 5.10 and above.
22 nigel 63 </P>
23     <P>
24 nigel 87 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
25     it does have are given in the
26 ph10 678 <a href="pcreunicode.html"><b>pcreunicode</b></a>
27 nigel 73 page.
28     </P>
29     <P>
30 ph10 654 2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
31     not mean what you might think. For example, (?!a){3} does not assert that the
32     next three characters are not "a". It just asserts that the next character is
33     not "a" three times (in principle: PCRE optimizes this to run the assertion
34     just once). Perl allows repeat quantifiers on other assertions such as \b, but
35     these do not seem to have any use.
36 nigel 63 </P>
37     <P>
38 nigel 73 3. Capturing subpatterns that occur inside negative lookahead assertions are
39 nigel 63 counted, but their entries in the offsets vector are never set. Perl sets its
40     numerical variables from any such patterns that are matched before the
41     assertion fails to match something (thereby succeeding), but only if the
42     negative lookahead assertion contains just one branch.
43     </P>
44     <P>
45 nigel 73 4. Though binary zero characters are supported in the subject string, they are
46 nigel 63 not allowed in a pattern string because it is passed as a normal C string,
47 nigel 75 terminated by zero. The escape sequence \0 can be used in the pattern to
48 nigel 63 represent a binary zero.
49     </P>
50     <P>
51 nigel 73 5. The following Perl escape sequences are not supported: \l, \u, \L,
52 ph10 654 \U, and \N when followed by a character name or Unicode value. (\N on its
53     own, matching a non-newline character, is supported.) In fact these are
54     implemented by Perl's general string-handling and are not part of its pattern
55     matching engine. If any of these are encountered by PCRE, an error is
56     generated.
57 nigel 63 </P>
58     <P>
59 nigel 75 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
60     built with Unicode character property support. The properties that can be
61     tested with \p and \P are limited to the general category properties such as
62 nigel 87 Lu and Nd, script names such as Greek or Han, and the derived properties Any
63 ph10 453 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
64     Perl documentation says "Because Perl hides the need for the user to understand
65     the internal representation of Unicode characters, there is no need to
66     implement the somewhat messy concept of surrogates."
67 nigel 75 </P>
68     <P>
69 ph10 654 7. PCRE implements a simpler version of \X than Perl, which changed to make
70     \X match what Unicode calls an "extended grapheme cluster". This is more
71     complicated than an extended Unicode sequence, which is what PCRE matches.
72     </P>
73     <P>
74     8. PCRE does support the \Q...\E escape for quoting substrings. Characters in
75 nigel 63 between are treated as literals. This is slightly different from Perl in that $
76     and @ are also handled as literals inside the quotes. In Perl, they cause
77     variable interpolation (but of course PCRE does not have variables). Note the
78     following examples:
79     <pre>
80     Pattern PCRE matches Perl matches
81 nigel 75
82     \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
83 nigel 63 \Qabc\$xyz\E abc\$xyz abc\$xyz
84     \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
85 nigel 75 </pre>
86 nigel 73 The \Q...\E sequence is recognized both inside and outside character classes.
87 nigel 63 </P>
88     <P>
89 ph10 654 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
90 nigel 93 constructions. However, there is support for recursive patterns. This is not
91 ph10 453 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
92 nigel 93 feature allows an external function to be called during pattern matching. See
93     the
94 nigel 75 <a href="pcrecallout.html"><b>pcrecallout</b></a>
95     documentation for details.
96 nigel 63 </P>
97     <P>
98 ph10 654 10. Subpatterns that are called recursively or as "subroutines" are always
99 ph10 461 treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
100 ph10 453 is a discussion of an example that explains this in more detail in the
101     <a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
102     in the
103 ph10 461 <a href="pcrepattern.html"><b>pcrepattern</b></a>
104 ph10 453 page.
105 nigel 93 </P>
106     <P>
107 ph10 654 11. There are some differences that are concerned with the settings of captured
108 nigel 63 strings when part of a pattern is repeated. For example, matching "aba" against
109     the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
110     </P>
111     <P>
112 ph10 654 12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
113 ph10 461 names is not as general as Perl's. This is a consequence of the fact the PCRE
114     works internally just with numbers, using an external table to translate
115     between numbers and names. In particular, a pattern such as (?|(?&#60;a&#62;A)|(?&#60;b)B),
116     where the two capturing parentheses have the same number but different names,
117     is not supported, and causes an error at compile time. If it were allowed, it
118     would not be possible to distinguish which parentheses matched, because both
119     names map to capturing subpattern number 1. To avoid this confusing situation,
120     an error is given at compile time.
121     </P>
122     <P>
123 ph10 654 13. Perl recognizes comments in some places that PCRE does not, for example,
124     between the ( and ? at the start of a subpattern. If the /x modifier is set,
125     Perl allows whitespace between ( and ? but PCRE never does, even if the
126     PCRE_EXTENDED option is set.
127 ph10 567 </P>
128     <P>
129 ph10 654 14. PCRE provides some extensions to the Perl regular expression facilities.
130 ph10 461 Perl 5.10 includes new features that are not in earlier versions of Perl, some
131     of which (such as named parentheses) have been in PCRE for some time. This list
132     is with respect to Perl 5.10:
133 nigel 75 <br>
134     <br>
135 ph10 461 (a) Although lookbehind assertions in PCRE must match fixed length strings,
136     each alternative branch of a lookbehind assertion can match a different length
137     of string. Perl requires them all to have the same length.
138 nigel 75 <br>
139     <br>
140 nigel 63 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
141     meta-character matches only at the very end of the string.
142 nigel 75 <br>
143     <br>
144     (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
145 ph10 182 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
146     (Perl can be made to issue a warning.)
147 nigel 75 <br>
148     <br>
149 nigel 63 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
150     inverted, that is, by default they are not greedy, but if followed by a
151     question mark they are.
152 nigel 75 <br>
153     <br>
154     (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
155     only at the first matching position in the subject string.
156     <br>
157     <br>
158 ph10 453 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
159     PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
160 nigel 75 <br>
161     <br>
162 ph10 231 (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
163     by the PCRE_BSR_ANYCRLF option.
164 nigel 75 <br>
165     <br>
166 ph10 231 (h) The callout facility is PCRE-specific.
167 nigel 75 <br>
168     <br>
169 ph10 231 (i) The partial matching facility is PCRE-specific.
170     <br>
171     <br>
172     (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
173 nigel 75 different hosts that have the other endianness.
174 nigel 77 <br>
175     <br>
176 ph10 231 (k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
177 nigel 77 different way and is not Perl-compatible.
178 ph10 231 <br>
179     <br>
180     (l) PCRE recognizes some special sequences such as (*CR) at the start of
181     a pattern that set overall options that cannot be changed within the pattern.
182 nigel 63 </P>
183 ph10 99 <br><b>
184     AUTHOR
185     </b><br>
186 nigel 63 <P>
187 ph10 99 Philip Hazel
188 nigel 63 <br>
189 ph10 99 University Computing Service
190     <br>
191     Cambridge CB2 3QH, England.
192     <br>
193     </P>
194     <br><b>
195     REVISION
196     </b><br>
197     <P>
198 ph10 678 Last updated: 24 August 2011
199 ph10 99 <br>
200 ph10 654 Copyright &copy; 1997-2011 University of Cambridge.
201 ph10 99 <br>
202 nigel 75 <p>
203     Return to the <a href="index.html">PCRE index page</a>.
204     </p>

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