ViewVC logotype

Contents of /code/trunk/doc/html/pcreperform.html

Parent Directory Parent Directory | Revision Log Revision Log

Revision 75 - (hide annotations) (download) (as text)
Sat Feb 24 21:40:37 2007 UTC (8 years, 2 months ago) by nigel
File MIME type: text/html
File size: 4013 byte(s)
Load pcre-5.0 into code/trunk.

1 nigel 63 <html>
2     <head>
3     <title>pcreperform specification</title>
4     </head>
5     <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 nigel 75 <h1>pcreperform man page</h1>
7     <p>
8     Return to the <a href="index.html">PCRE index page</a>.
9     </p>
10     <p>
11     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     <br>
15     <br><b>
17     </b><br>
18 nigel 63 <P>
19     Certain items that may appear in regular expression patterns are more efficient
20     than others. It is more efficient to use a character class like [aeiou] than a
21     set of alternatives such as (a|e|i|o|u). In general, the simplest construction
22     that provides the required behaviour is usually the most efficient. Jeffrey
23 nigel 75 Friedl's book contains a lot of useful general discussion about optimizing
24     regular expressions for efficient performance. This document contains a few
25     observations about PCRE.
26 nigel 63 </P>
27     <P>
28 nigel 75 Using Unicode character properties (the \p, \P, and \X escapes) is slow,
29     because PCRE has to scan a structure that contains data for over fifteen
30     thousand characters whenever it needs a character's property. If you can find
31     an alternative pattern that does not use character properties, it will probably
32     be faster.
33     </P>
34     <P>
35 nigel 63 When a pattern begins with .* not in parentheses, or in parentheses that are
36     not the subject of a backreference, and the PCRE_DOTALL option is set, the
37     pattern is implicitly anchored by PCRE, since it can match only at the start of
38     a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
39     optimization, because the . metacharacter does not then match a newline, and if
40     the subject string contains newlines, the pattern may match from the character
41     immediately following one of them instead of from the very start. For example,
42     the pattern
43     <pre>
44     .*second
45 nigel 75 </pre>
46 nigel 63 matches the subject "first\nand second" (where \n stands for a newline
47     character), with the match starting at the seventh character. In order to do
48     this, PCRE has to retry the match starting after every newline in the subject.
49     </P>
50     <P>
51     If you are using such a pattern with subject strings that do not contain
52     newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
53     the pattern with ^.* to indicate explicit anchoring. That saves PCRE from
54     having to scan along the subject looking for a newline to restart at.
55     </P>
56     <P>
57     Beware of patterns that contain nested indefinite repeats. These can take a
58     long time to run when applied to a string that does not match. Consider the
59     pattern fragment
60     <pre>
61     (a+)*
62 nigel 75 </pre>
63 nigel 63 This can match "aaaa" in 33 different ways, and this number increases very
64     rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
65     times, and for each of those cases other than 0, the + repeats can match
66     different numbers of times.) When the remainder of the pattern is such that the
67     entire match is going to fail, PCRE has in principle to try every possible
68     variation, and this can take an extremely long time.
69     </P>
70     <P>
71     An optimization catches some of the more simple cases such as
72     <pre>
73     (a+)*b
74 nigel 75 </pre>
75 nigel 63 where a literal character follows. Before embarking on the standard matching
76     procedure, PCRE checks that there is a "b" later in the subject string, and if
77     there is not, it fails the match immediately. However, when there is no
78     following literal this optimization cannot be used. You can see the difference
79     by comparing the behaviour of
80     <pre>
81     (a+)*\d
82 nigel 75 </pre>
83 nigel 63 with the pattern above. The former gives a failure almost instantly when
84     applied to a whole line of "a" characters, whereas the latter takes an
85     appreciable time with strings longer than about 20 characters.
86     </P>
87     <P>
88 nigel 75 In many cases, the solution to this kind of performance issue is to use an
89     atomic group or a possessive quantifier.
90     </P>
91     <P>
92     Last updated: 09 September 2004
93 nigel 63 <br>
94 nigel 75 Copyright &copy; 1997-2004 University of Cambridge.
95     <p>
96     Return to the <a href="index.html">PCRE index page</a>.
97     </p>

ViewVC Help
Powered by ViewVC 1.1.12