.TH PCREPERFORM 3

.SH NAME

PCRE - Perl-compatible regular expressions

.SH "PCRE PERFORMANCE"

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Two aspects of performance are discussed below: memory usage and processing

time. The way you express your pattern as a regular expression can affect both

of them.

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.SH "MEMORY USAGE"

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Patterns are compiled by PCRE into a reasonably efficient byte code, so that

most simple patterns do not use much memory. However, there is one case where

memory usage can be unexpectedly large. When a parenthesized subpattern has a

quantifier with a minimum greater than 1 and/or a limited maximum, the whole

subpattern is repeated in the compiled code. For example, the pattern

.sp

  (abc|def){2,4}

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is compiled as if it were

.sp

  (abc|def)(abc|def)((abc|def)(abc|def)?)?

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(Technical aside: It is done this way so that backtrack points within each of

the repetitions can be independently maintained.)

.P

For regular expressions whose quantifiers use only small numbers, this is not

usually a problem. However, if the numbers are large, and particularly if such

repetitions are nested, the memory usage can become an embarrassment. For

example, the very simple pattern

.sp

  ((ab){1,1000}c){1,3}

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uses 51K bytes when compiled. When PCRE is compiled with its default internal

pointer size of two bytes, the size limit on a compiled pattern is 64K, and

this is reached with the above pattern if the outer repetition is increased

from 3 to 4. PCRE can be compiled to use larger internal pointers and thus

handle larger compiled patterns, but it is better to try to rewrite your

pattern to use less memory if you can.

.P

One way of reducing the memory usage for such patterns is to make use of PCRE's

.\" HTML <a href="pcrepattern.html#subpatternsassubroutines">

.\" </a>

"subroutine"

.\"

facility. Re-writing the above pattern as

.sp

  ((ab)(?2){0,999}c)(?1){0,2}

.sp

reduces the memory requirements to 18K, and indeed it remains under 20K even

with the outer repetition increased to 100. However, this pattern is not

exactly equivalent, because the "subroutine" calls are treated as

.\" HTML <a href="pcrepattern.html#atomicgroup">

.\" </a>

atomic groups

.\"

into which there can be no backtracking if there is a subsequent matching

failure. Therefore, PCRE cannot do this kind of rewriting automatically.

Furthermore, there is a noticeable loss of speed when executing the modified

pattern. Nevertheless, if the atomic grouping is not a problem and the loss of

speed is acceptable, this kind of rewriting will allow you to process patterns

that PCRE cannot otherwise handle.

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.SH "PROCESSING TIME"

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Certain items in regular expression patterns are processed more efficiently

than others. It is more efficient to use a character class like [aeiou] than a

set of single-character 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

Using Unicode character properties (the \ep, \eP, and \eX 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

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

.sp

  .*second

.sp

matches the subject "first\enand second" (where \en 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

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

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

.sp

  ^(a+)*

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This can match "aaaa" in 16 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 or 4, 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, even for relatively short

strings.

.P

An optimization catches some of the more simple cases such as

.sp

  (a+)*b

.sp

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

.sp

  (a+)*\ed

.sp

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

In many cases, the solution to this kind of performance issue is to use an

atomic group or a possessive quantifier.

.

.

.SH AUTHOR

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Philip Hazel

University Computing Service

Cambridge CB2 3QH, England.

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.SH REVISION

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Last updated: 06 March 2007

Copyright (c) 1997-2007 University of Cambridge.

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