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Sun Mar 7 12:05:20 2010 UTC (10 years, 8 months ago) by ph10
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Add crossreference from pcreperform to pcrestack.
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 .
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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 the memory usage of a compiled pattern can be unexpectedly large. If a
17 parenthesized subpattern has a quantifier with a minimum greater than 1 and/or
18 a limited maximum, the whole subpattern is repeated in the compiled code. For
19 example, the pattern
20 .sp
21 (abc|def){2,4}
22 .sp
23 is compiled as if it were
24 .sp
25 (abc|def)(abc|def)((abc|def)(abc|def)?)?
26 .sp
27 (Technical aside: It is done this way so that backtrack points within each of
28 the repetitions can be independently maintained.)
29 .P
30 For regular expressions whose quantifiers use only small numbers, this is not
31 usually a problem. However, if the numbers are large, and particularly if such
32 repetitions are nested, the memory usage can become an embarrassment. For
33 example, the very simple pattern
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35 ((ab){1,1000}c){1,3}
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37 uses 51K bytes when compiled. When PCRE is compiled with its default internal
38 pointer size of two bytes, the size limit on a compiled pattern is 64K, and
39 this is reached with the above pattern if the outer repetition is increased
40 from 3 to 4. PCRE can be compiled to use larger internal pointers and thus
41 handle larger compiled patterns, but it is better to try to rewrite your
42 pattern to use less memory if you can.
43 .P
44 One way of reducing the memory usage for such patterns is to make use of PCRE's
45 .\" HTML <a href="pcrepattern.html#subpatternsassubroutines">
46 .\" </a>
47 "subroutine"
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49 facility. Re-writing the above pattern as
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51 ((ab)(?2){0,999}c)(?1){0,2}
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53 reduces the memory requirements to 18K, and indeed it remains under 20K even
54 with the outer repetition increased to 100. However, this pattern is not
55 exactly equivalent, because the "subroutine" calls are treated as
56 .\" HTML <a href="pcrepattern.html#atomicgroup">
57 .\" </a>
58 atomic groups
59 .\"
60 into which there can be no backtracking if there is a subsequent matching
61 failure. Therefore, PCRE cannot do this kind of rewriting automatically.
62 Furthermore, there is a noticeable loss of speed when executing the modified
63 pattern. Nevertheless, if the atomic grouping is not a problem and the loss of
64 speed is acceptable, this kind of rewriting will allow you to process patterns
65 that PCRE cannot otherwise handle.
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67 .
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71 When \fBpcre_exec()\fP is used for matching, certain kinds of pattern can cause
72 it to use large amounts of the process stack. In some environments the default
73 process stack is quite small, and if it runs out the result is often SIGSEGV.
74 This issue is probably the most frequently raised problem with PCRE. Rewriting
75 your pattern can often help. The
76 .\" HREF
77 \fBpcrestack\fP
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79 documentation discusses this issue in detail.
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85 Certain items in regular expression patterns are processed more efficiently
86 than others. It is more efficient to use a character class like [aeiou] than a
87 set of single-character alternatives such as (a|e|i|o|u). In general, the
88 simplest construction that provides the required behaviour is usually the most
89 efficient. Jeffrey Friedl's book contains a lot of useful general discussion
90 about optimizing regular expressions for efficient performance. This document
91 contains a few observations about PCRE.
92 .P
93 Using Unicode character properties (the \ep, \eP, and \eX escapes) is slow,
94 because PCRE has to scan a structure that contains data for over fifteen
95 thousand characters whenever it needs a character's property. If you can find
96 an alternative pattern that does not use character properties, it will probably
97 be faster.
98 .P
99 When a pattern begins with .* not in parentheses, or in parentheses that are
100 not the subject of a backreference, and the PCRE_DOTALL option is set, the
101 pattern is implicitly anchored by PCRE, since it can match only at the start of
102 a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
103 optimization, because the . metacharacter does not then match a newline, and if
104 the subject string contains newlines, the pattern may match from the character
105 immediately following one of them instead of from the very start. For example,
106 the pattern
107 .sp
108 .*second
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110 matches the subject "first\enand second" (where \en stands for a newline
111 character), with the match starting at the seventh character. In order to do
112 this, PCRE has to retry the match starting after every newline in the subject.
113 .P
114 If you are using such a pattern with subject strings that do not contain
115 newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
116 the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
117 from having to scan along the subject looking for a newline to restart at.
118 .P
119 Beware of patterns that contain nested indefinite repeats. These can take a
120 long time to run when applied to a string that does not match. Consider the
121 pattern fragment
122 .sp
123 ^(a+)*
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125 This can match "aaaa" in 16 different ways, and this number increases very
126 rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
127 times, and for each of those cases other than 0 or 4, the + repeats can match
128 different numbers of times.) When the remainder of the pattern is such that the
129 entire match is going to fail, PCRE has in principle to try every possible
130 variation, and this can take an extremely long time, even for relatively short
131 strings.
132 .P
133 An optimization catches some of the more simple cases such as
134 .sp
135 (a+)*b
136 .sp
137 where a literal character follows. Before embarking on the standard matching
138 procedure, PCRE checks that there is a "b" later in the subject string, and if
139 there is not, it fails the match immediately. However, when there is no
140 following literal this optimization cannot be used. You can see the difference
141 by comparing the behaviour of
142 .sp
143 (a+)*\ed
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145 with the pattern above. The former gives a failure almost instantly when
146 applied to a whole line of "a" characters, whereas the latter takes an
147 appreciable time with strings longer than about 20 characters.
148 .P
149 In many cases, the solution to this kind of performance issue is to use an
150 atomic group or a possessive quantifier.
151 .
152 .
154 .rs
155 .sp
156 .nf
157 Philip Hazel
158 University Computing Service
159 Cambridge CB2 3QH, England.
160 .fi
161 .
162 .
164 .rs
165 .sp
166 .nf
167 Last updated: 07 March 2010
168 Copyright (c) 1997-2010 University of Cambridge.
169 .fi


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