ViewVC logotype

Contents of /code/trunk/doc/pcreperform.3

Parent Directory Parent Directory | Revision Log Revision Log

Revision 859 - (show annotations)
Mon Jan 9 17:43:54 2012 UTC (8 years ago) by ph10
File size: 7283 byte(s)
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 interpretive code, so
15 that most simple patterns do not use much memory. However, there is one case
16 where 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
34 .sp
35 ((ab){1,1000}c){1,3}
36 .sp
37 uses 51K bytes when compiled using the 8-bit library. When PCRE is compiled
38 with its default internal pointer size of two bytes, the size limit on a
39 compiled pattern is 64K data units, and this is reached with the above pattern
40 if the outer repetition is increased from 3 to 4. PCRE can be compiled to use
41 larger internal pointers and thus handle larger compiled patterns, but it is
42 better to try to rewrite your 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"
48 .\"
49 facility. Re-writing the above pattern as
50 .sp
51 ((ab)(?2){0,999}c)(?1){0,2}
52 .sp
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.
66 .
67 .
69 .rs
70 .sp
71 When \fBpcre_exec()\fP or \fBpcre16_exec()\fP is used for matching, certain
72 kinds of pattern can cause it to use large amounts of the process stack. In
73 some environments the default process stack is quite small, and if it runs out
74 the result is often SIGSEGV. This issue is probably the most frequently raised
75 problem with PCRE. Rewriting your pattern can often help. The
76 .\" HREF
77 \fBpcrestack\fP
78 .\"
79 documentation discusses this issue in detail.
80 .
81 .
83 .rs
84 .sp
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 By default, the escape sequences \eb, \ed, \es, and \ew, and the POSIX
100 character classes such as [:alpha:] do not use Unicode properties, partly for
101 backwards compatibility, and partly for performance reasons. However, you can
102 set PCRE_UCP if you want Unicode character properties to be used. This can
103 double the matching time for items such as \ed, when matched with
104 a traditional matching function; the performance loss is less with
105 a DFA matching function, and in both cases there is not much difference for
106 \eb.
107 .P
108 When a pattern begins with .* not in parentheses, or in parentheses that are
109 not the subject of a backreference, and the PCRE_DOTALL option is set, the
110 pattern is implicitly anchored by PCRE, since it can match only at the start of
111 a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
112 optimization, because the . metacharacter does not then match a newline, and if
113 the subject string contains newlines, the pattern may match from the character
114 immediately following one of them instead of from the very start. For example,
115 the pattern
116 .sp
117 .*second
118 .sp
119 matches the subject "first\enand second" (where \en stands for a newline
120 character), with the match starting at the seventh character. In order to do
121 this, PCRE has to retry the match starting after every newline in the subject.
122 .P
123 If you are using such a pattern with subject strings that do not contain
124 newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
125 the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
126 from having to scan along the subject looking for a newline to restart at.
127 .P
128 Beware of patterns that contain nested indefinite repeats. These can take a
129 long time to run when applied to a string that does not match. Consider the
130 pattern fragment
131 .sp
132 ^(a+)*
133 .sp
134 This can match "aaaa" in 16 different ways, and this number increases very
135 rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
136 times, and for each of those cases other than 0 or 4, the + repeats can match
137 different numbers of times.) When the remainder of the pattern is such that the
138 entire match is going to fail, PCRE has in principle to try every possible
139 variation, and this can take an extremely long time, even for relatively short
140 strings.
141 .P
142 An optimization catches some of the more simple cases such as
143 .sp
144 (a+)*b
145 .sp
146 where a literal character follows. Before embarking on the standard matching
147 procedure, PCRE checks that there is a "b" later in the subject string, and if
148 there is not, it fails the match immediately. However, when there is no
149 following literal this optimization cannot be used. You can see the difference
150 by comparing the behaviour of
151 .sp
152 (a+)*\ed
153 .sp
154 with the pattern above. The former gives a failure almost instantly when
155 applied to a whole line of "a" characters, whereas the latter takes an
156 appreciable time with strings longer than about 20 characters.
157 .P
158 In many cases, the solution to this kind of performance issue is to use an
159 atomic group or a possessive quantifier.
160 .
161 .
163 .rs
164 .sp
165 .nf
166 Philip Hazel
167 University Computing Service
168 Cambridge CB2 3QH, England.
169 .fi
170 .
171 .
173 .rs
174 .sp
175 .nf
176 Last updated: 09 January 2012
177 Copyright (c) 1997-2012 University of Cambridge.
178 .fi


Name Value
svn:eol-style native
svn:keywords "Author Date Id Revision Url"

  ViewVC Help
Powered by ViewVC 1.1.5