PCREPERFORM(3)PCREPERFORM(3)NAME
PCRE - Perl-compatible regular expressions
PCRE PERFORMANCE
Two aspects of performance are discussed below: memory usage and pro‐
cessing time. The way you express your pattern as a regular expression
can affect both of them.
COMPILED PATTERN MEMORY USAGE
Patterns are compiled by PCRE into a reasonably efficient interpretive
code, so that most simple patterns do not use much memory. However,
there is one case where the memory usage of a compiled pattern can be
unexpectedly large. If 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
(abc|def){2,4}
is compiled as if it were
(abc|def)(abc|def)((abc|def)(abc|def)?)?
(Technical aside: It is done this way so that backtrack points within
each of the repetitions can be independently maintained.)
For regular expressions whose quantifiers use only small numbers, this
is not usually a problem. However, if the numbers are large, and par‐
ticularly if such repetitions are nested, the memory usage can become
an embarrassment. For example, the very simple pattern
((ab){1,1000}c){1,3}
uses 51K bytes when compiled using the 8-bit library. When PCRE is com‐
piled with its default internal pointer size of two bytes, the size
limit on a compiled pattern is 64K data units, 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 pat‐
tern to use less memory if you can.
One way of reducing the memory usage for such patterns is to make use
of PCRE's "subroutine" facility. Re-writing the above pattern as
((ab)(?2){0,999}c)(?1){0,2}
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 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.
STACK USAGE AT RUN TIME
When pcre_exec() or pcre[16|32]_exec() is used for matching, certain
kinds of pattern can cause it to use large amounts of the process
stack. In some environments the default process stack is quite small,
and if it runs out the result is often SIGSEGV. This issue is probably
the most frequently raised problem with PCRE. Rewriting your pattern
can often help. The pcrestack documentation discusses this issue in
detail.
PROCESSING TIME
Certain items in regular expression patterns are processed more effi‐
ciently 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.
Using Unicode character properties (the \p, \P, and \X escapes) is
slow, because PCRE has to use a multi-stage table lookup 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.
By default, the escape sequences \b, \d, \s, and \w, and the POSIX
character classes such as [:alpha:] do not use Unicode properties,
partly for backwards compatibility, and partly for performance reasons.
However, you can set PCRE_UCP if you want Unicode character properties
to be used. This can double the matching time for items such as \d,
when matched with a traditional matching function; the performance loss
is less with a DFA matching function, and in both cases there is not
much difference for \b.
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 new‐
lines, the pattern may match from the character immediately following
one of them instead of from the very start. For example, the pattern
.*second
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.
If you are using such a pattern with subject strings that do not con‐
tain newlines, the best performance is obtained by setting PCRE_DOTALL,
or starting the pattern with ^.* or ^.*? to indicate explicit anchor‐
ing. That saves PCRE from having to scan along the subject looking for
a newline to restart at.
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
^(a+)*
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.
An optimization catches some of the more simple cases such as
(a+)*b
where a literal character follows. Before embarking on the standard
matching procedure, PCRE checks that there is a "b" later in the sub‐
ject string, and if there is not, it fails the match immediately. How‐
ever, when there is no following literal this optimization cannot be
used. You can see the difference by comparing the behaviour of
(a+)*\d
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.
In many cases, the solution to this kind of performance issue is to use
an atomic group or a possessive quantifier.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 25 August 2012
Copyright (c) 1997-2012 University of Cambridge.
PCRE 8.30 09 January 2012 PCREPERFORM(3)
