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Contents of /code/tags/pcre-7.4/doc/pcreperform.3

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Fri Sep 21 08:41:43 2007 UTC (13 years, 7 months ago) by ph10
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Tag 7.4
3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 Two aspects of performance are discussed below: memory usage and processing
8 time. The way you express your pattern as a regular expression can affect both
9 of them.
10 .
12 .rs
13 .sp
14 Patterns are compiled by PCRE into a reasonably efficient byte code, so that
15 most simple patterns do not use much memory. However, there is one case where
16 memory usage can be unexpectedly large. When a parenthesized subpattern has a
17 quantifier with a minimum greater than 1 and/or a limited maximum, the whole
18 subpattern is repeated in the compiled code. For example, the pattern
19 .sp
20 (abc|def){2,4}
21 .sp
22 is compiled as if it were
23 .sp
24 (abc|def)(abc|def)((abc|def)(abc|def)?)?
25 .sp
26 (Technical aside: It is done this way so that backtrack points within each of
27 the repetitions can be independently maintained.)
28 .P
29 For regular expressions whose quantifiers use only small numbers, this is not
30 usually a problem. However, if the numbers are large, and particularly if such
31 repetitions are nested, the memory usage can become an embarrassment. For
32 example, the very simple pattern
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34 ((ab){1,1000}c){1,3}
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36 uses 51K bytes when compiled. When PCRE is compiled with its default internal
37 pointer size of two bytes, the size limit on a compiled pattern is 64K, and
38 this is reached with the above pattern if the outer repetition is increased
39 from 3 to 4. PCRE can be compiled to use larger internal pointers and thus
40 handle larger compiled patterns, but it is better to try to rewrite your
41 pattern to use less memory if you can.
42 .P
43 One way of reducing the memory usage for such patterns is to make use of PCRE's
44 .\" HTML <a href="pcrepattern.html#subpatternsassubroutines">
45 .\" </a>
46 "subroutine"
47 .\"
48 facility. Re-writing the above pattern as
49 .sp
50 ((ab)(?2){0,999}c)(?1){0,2}
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52 reduces the memory requirements to 18K, and indeed it remains under 20K even
53 with the outer repetition increased to 100. However, this pattern is not
54 exactly equivalent, because the "subroutine" calls are treated as
55 .\" HTML <a href="pcrepattern.html#atomicgroup">
56 .\" </a>
57 atomic groups
58 .\"
59 into which there can be no backtracking if there is a subsequent matching
60 failure. Therefore, PCRE cannot do this kind of rewriting automatically.
61 Furthermore, there is a noticeable loss of speed when executing the modified
62 pattern. Nevertheless, if the atomic grouping is not a problem and the loss of
63 speed is acceptable, this kind of rewriting will allow you to process patterns
64 that PCRE cannot otherwise handle.
65 .
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69 Certain items in regular expression patterns are processed more efficiently
70 than others. It is more efficient to use a character class like [aeiou] than a
71 set of single-character alternatives such as (a|e|i|o|u). In general, the
72 simplest construction that provides the required behaviour is usually the most
73 efficient. Jeffrey Friedl's book contains a lot of useful general discussion
74 about optimizing regular expressions for efficient performance. This document
75 contains a few observations about PCRE.
76 .P
77 Using Unicode character properties (the \ep, \eP, and \eX escapes) is slow,
78 because PCRE has to scan a structure that contains data for over fifteen
79 thousand characters whenever it needs a character's property. If you can find
80 an alternative pattern that does not use character properties, it will probably
81 be faster.
82 .P
83 When a pattern begins with .* not in parentheses, or in parentheses that are
84 not the subject of a backreference, and the PCRE_DOTALL option is set, the
85 pattern is implicitly anchored by PCRE, since it can match only at the start of
86 a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
87 optimization, because the . metacharacter does not then match a newline, and if
88 the subject string contains newlines, the pattern may match from the character
89 immediately following one of them instead of from the very start. For example,
90 the pattern
91 .sp
92 .*second
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94 matches the subject "first\enand second" (where \en stands for a newline
95 character), with the match starting at the seventh character. In order to do
96 this, PCRE has to retry the match starting after every newline in the subject.
97 .P
98 If you are using such a pattern with subject strings that do not contain
99 newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
100 the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
101 from having to scan along the subject looking for a newline to restart at.
102 .P
103 Beware of patterns that contain nested indefinite repeats. These can take a
104 long time to run when applied to a string that does not match. Consider the
105 pattern fragment
106 .sp
107 ^(a+)*
108 .sp
109 This can match "aaaa" in 16 different ways, and this number increases very
110 rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
111 times, and for each of those cases other than 0 or 4, the + repeats can match
112 different numbers of times.) When the remainder of the pattern is such that the
113 entire match is going to fail, PCRE has in principle to try every possible
114 variation, and this can take an extremely long time, even for relatively short
115 strings.
116 .P
117 An optimization catches some of the more simple cases such as
118 .sp
119 (a+)*b
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121 where a literal character follows. Before embarking on the standard matching
122 procedure, PCRE checks that there is a "b" later in the subject string, and if
123 there is not, it fails the match immediately. However, when there is no
124 following literal this optimization cannot be used. You can see the difference
125 by comparing the behaviour of
126 .sp
127 (a+)*\ed
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129 with the pattern above. The former gives a failure almost instantly when
130 applied to a whole line of "a" characters, whereas the latter takes an
131 appreciable time with strings longer than about 20 characters.
132 .P
133 In many cases, the solution to this kind of performance issue is to use an
134 atomic group or a possessive quantifier.
135 .
136 .
138 .rs
139 .sp
140 .nf
141 Philip Hazel
142 University Computing Service
143 Cambridge CB2 3QH, England.
144 .fi
145 .
146 .
148 .rs
149 .sp
150 .nf
151 Last updated: 06 March 2007
152 Copyright (c) 1997-2007 University of Cambridge.
153 .fi


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