doc/html/pcreperform.html

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<head>
<title>pcreperform specification</title>
</head>
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
<h1>pcreperform man page</h1>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
<p>
This page is part of the PCRE HTML documentation. It was generated automatically
from the original man page. If there is any nonsense in it, please consult the
man page, in case the conversion went wrong.
<br>
<br><b>
PCRE PERFORMANCE
</b><br>
<P>
Certain items that may appear in regular expression patterns are more efficient
than others. It is more efficient to use a character class like [aeiou] than a
set of alternatives such as (a|e|i|o|u). In general, the simplest construction
that provides the required behaviour is usually the most efficient. Jeffrey
Friedl's book contains a lot of useful general discussion about optimizing
regular expressions for efficient performance. This document contains a few
observations about PCRE.
</P>
<P>
Using Unicode character properties (the \p, \P, and \X escapes) is slow,
because PCRE has to scan a structure that contains data for over fifteen
thousand characters whenever it needs a character's property. If you can find
an alternative pattern that does not use character properties, it will probably
be faster.
</P>
<P>
When a pattern begins with .* not in parentheses, or in parentheses that are
not the subject of a backreference, and the PCRE_DOTALL option is set, the
pattern is implicitly anchored by PCRE, since it can match only at the start of
a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
optimization, because the . metacharacter does not then match a newline, and if
the subject string contains newlines, the pattern may match from the character
immediately following one of them instead of from the very start. For example,
the pattern
<pre>
  .*second
</pre>
matches the subject "first\nand second" (where \n stands for a newline
character), with the match starting at the seventh character. In order to do
this, PCRE has to retry the match starting after every newline in the subject.
</P>
<P>
If you are using such a pattern with subject strings that do not contain
newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
from having to scan along the subject looking for a newline to restart at.
</P>
<P>
Beware of patterns that contain nested indefinite repeats. These can take a
long time to run when applied to a string that does not match. Consider the
pattern fragment
<pre>
  (a+)*
</pre>
This can match "aaaa" in 33 different ways, and this number increases very
rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
times, and for each of those cases other than 0, the + repeats can match
different numbers of times.) When the remainder of the pattern is such that the
entire match is going to fail, PCRE has in principle to try every possible
variation, and this can take an extremely long time.
</P>
<P>
An optimization catches some of the more simple cases such as
<pre>
  (a+)*b
</pre>
where a literal character follows. Before embarking on the standard matching
procedure, PCRE checks that there is a "b" later in the subject string, and if
there is not, it fails the match immediately. However, when there is no
following literal this optimization cannot be used. You can see the difference
by comparing the behaviour of
<pre>
  (a+)*\d
</pre>
with the pattern above. The former gives a failure almost instantly when
applied to a whole line of "a" characters, whereas the latter takes an
appreciable time with strings longer than about 20 characters.
</P>
<P>
In many cases, the solution to this kind of performance issue is to use an
atomic group or a possessive quantifier.
</P>
<P>
Last updated: 28 February 2005
<br>
Copyright &copy; 1997-2005 University of Cambridge.
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
1	<html>
2	<head>
3	<title>pcreperform specification</title>
4	</head>
5	<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6	<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>
16	PCRE PERFORMANCE
17	</b><br>
18	<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	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	</P>
27	<P>
28	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	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	</pre>
46	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 ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
54	from 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	</pre>
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	</pre>
75	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	</pre>
83	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	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: 28 February 2005
93	<br>
94	Copyright © 1997-2005 University of Cambridge.
95	<p>
96	Return to the <a href="index.html">PCRE index page</a>.
97	</p>