doc/html/pcreperform.html

<html>
<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 ^.* 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: 09 September 2004
<br>
Copyright &copy; 1997-2004 University of Cambridge.
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
1	nigel	63	<html>
2			<head>
3			<title>pcreperform specification</title>
4			</head>
5			<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6	nigel	75	<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	nigel	63	<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	nigel	75	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	nigel	63	</P>
27			<P>
28	nigel	75	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	nigel	63	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	nigel	75	</pre>
46	nigel	63	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 ^.* to indicate explicit anchoring. That saves PCRE from
54			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	nigel	75	</pre>
63	nigel	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	nigel	75	</pre>
75	nigel	63	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	nigel	75	</pre>
83	nigel	63	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	nigel	75	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: 09 September 2004
93	nigel	63	<br>
94	nigel	75	Copyright © 1997-2004 University of Cambridge.
95			<p>
96			Return to the <a href="index.html">PCRE index page</a>.
97			</p>