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1 .TH PCRE 3
2 .SH NAME
3 PCRE - Perl-compatible regular expressions
4 .SH PCRE PERFORMANCE
5 .rs
6 .sp
7 Certain items that may appear in regular expression patterns are more efficient
8 than others. It is more efficient to use a character class like [aeiou] than a
9 set of alternatives such as (a|e|i|o|u). In general, the simplest construction
10 that provides the required behaviour is usually the most efficient. Jeffrey
11 Friedl's book contains a lot of discussion about optimizing regular expressions
12 for efficient performance.
13
14 When a pattern begins with .* not in parentheses, or in parentheses that are
15 not the subject of a backreference, and the PCRE_DOTALL option is set, the
16 pattern is implicitly anchored by PCRE, since it can match only at the start of
17 a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
18 optimization, because the . metacharacter does not then match a newline, and if
19 the subject string contains newlines, the pattern may match from the character
20 immediately following one of them instead of from the very start. For example,
21 the pattern
22
23 .*second
24
25 matches the subject "first\\nand second" (where \\n stands for a newline
26 character), with the match starting at the seventh character. In order to do
27 this, PCRE has to retry the match starting after every newline in the subject.
28
29 If you are using such a pattern with subject strings that do not contain
30 newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
31 the pattern with ^.* to indicate explicit anchoring. That saves PCRE from
32 having to scan along the subject looking for a newline to restart at.
33
34 Beware of patterns that contain nested indefinite repeats. These can take a
35 long time to run when applied to a string that does not match. Consider the
36 pattern fragment
37
38 (a+)*
39
40 This can match "aaaa" in 33 different ways, and this number increases very
41 rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
42 times, and for each of those cases other than 0, the + repeats can match
43 different numbers of times.) When the remainder of the pattern is such that the
44 entire match is going to fail, PCRE has in principle to try every possible
45 variation, and this can take an extremely long time.
46
47 An optimization catches some of the more simple cases such as
48
49 (a+)*b
50
51 where a literal character follows. Before embarking on the standard matching
52 procedure, PCRE checks that there is a "b" later in the subject string, and if
53 there is not, it fails the match immediately. However, when there is no
54 following literal this optimization cannot be used. You can see the difference
55 by comparing the behaviour of
56
57 (a+)*\\d
58
59 with the pattern above. The former gives a failure almost instantly when
60 applied to a whole line of "a" characters, whereas the latter takes an
61 appreciable time with strings longer than about 20 characters.
62
63 .in 0
64 Last updated: 03 February 2003
65 .br
66 Copyright (c) 1997-2003 University of Cambridge.

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