198 lines
8.3 KiB
HTML
198 lines
8.3 KiB
HTML
<html>
|
|
<head>
|
|
<title>pcre2perform specification</title>
|
|
</head>
|
|
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
|
|
<h1>pcre2perform man page</h1>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE2 index page</a>.
|
|
</p>
|
|
<p>
|
|
This page is part of the PCRE2 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>
|
|
<ul>
|
|
<li><a name="TOC1" href="#SEC1">PCRE2 PERFORMANCE</a>
|
|
<li><a name="TOC2" href="#SEC2">COMPILED PATTERN MEMORY USAGE</a>
|
|
<li><a name="TOC3" href="#SEC3">STACK USAGE AT RUN TIME</a>
|
|
<li><a name="TOC4" href="#SEC4">PROCESSING TIME</a>
|
|
<li><a name="TOC5" href="#SEC5">AUTHOR</a>
|
|
<li><a name="TOC6" href="#SEC6">REVISION</a>
|
|
</ul>
|
|
<br><a name="SEC1" href="#TOC1">PCRE2 PERFORMANCE</a><br>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<br><a name="SEC2" href="#TOC1">COMPILED PATTERN MEMORY USAGE</a><br>
|
|
<P>
|
|
Patterns are compiled by PCRE2 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
|
|
<pre>
|
|
(abc|def){2,4}
|
|
</pre>
|
|
is compiled as if it were
|
|
<pre>
|
|
(abc|def)(abc|def)((abc|def)(abc|def)?)?
|
|
</pre>
|
|
(Technical aside: It is done this way so that backtrack points within each of
|
|
the repetitions can be independently maintained.)
|
|
</P>
|
|
<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
|
|
<pre>
|
|
((ab){1,1000}c){1,3}
|
|
</pre>
|
|
uses 51K bytes when compiled using the 8-bit library. When PCRE2 is compiled
|
|
with its default internal pointer size of two bytes, the size limit on a
|
|
compiled pattern is 64K code units in the 8-bit and 16-bit libraries, and this
|
|
is reached with the above pattern if the outer repetition is increased from 3
|
|
to 4. PCRE2 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>
|
|
<P>
|
|
One way of reducing the memory usage for such patterns is to make use of
|
|
PCRE2's
|
|
<a href="pcre2pattern.html#subpatternsassubroutines">"subroutine"</a>
|
|
facility. Re-writing the above pattern as
|
|
<pre>
|
|
((ab)(?2){0,999}c)(?1){0,2}
|
|
</pre>
|
|
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
|
|
<a href="pcre2pattern.html#atomicgroup">atomic groups</a>
|
|
into which there can be no backtracking if there is a subsequent matching
|
|
failure. Therefore, PCRE2 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 PCRE2 cannot otherwise handle.
|
|
</P>
|
|
<br><a name="SEC3" href="#TOC1">STACK USAGE AT RUN TIME</a><br>
|
|
<P>
|
|
When <b>pcre2_match()</b> 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. Rewriting your pattern can often help. The
|
|
<a href="pcre2stack.html"><b>pcre2stack</b></a>
|
|
documentation discusses this issue in detail.
|
|
</P>
|
|
<br><a name="SEC4" href="#TOC1">PROCESSING TIME</a><br>
|
|
<P>
|
|
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 PCRE2.
|
|
</P>
|
|
<P>
|
|
Using Unicode character properties (the \p, \P, and \X escapes) is slow,
|
|
because PCRE2 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.
|
|
</P>
|
|
<P>
|
|
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 the PCRE2_UCP option or start the pattern with (*UCP) if you want Unicode
|
|
character properties to be used. This can double the matching time for items
|
|
such as \d, when matched with <b>pcre2_match()</b>; the performance loss is
|
|
less with a DFA matching function, and in both cases there is not much
|
|
difference for \b.
|
|
</P>
|
|
<P>
|
|
When a pattern begins with .* not in atomic parentheses, nor in parentheses
|
|
that are the subject of a backreference, and the PCRE2_DOTALL option is set,
|
|
the pattern is implicitly anchored by PCRE2, since it can match only at the
|
|
start of a subject string. If the pattern has multiple top-level branches, they
|
|
must all be anchorable. The optimization can be disabled by the
|
|
PCRE2_NO_DOTSTAR_ANCHOR option, and is automatically disabled if the pattern
|
|
contains (*PRUNE) or (*SKIP).
|
|
</P>
|
|
<P>
|
|
If PCRE2_DOTALL is not set, PCRE2 cannot make this optimization, because the
|
|
dot 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, PCRE2 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 PCRE2_DOTALL, or starting
|
|
the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE2
|
|
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 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, PCRE2 has in principle to try every possible
|
|
variation, and this can take an extremely long time, even for relatively short
|
|
strings.
|
|
</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, PCRE2 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>
|
|
<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
|
|
<P>
|
|
Philip Hazel
|
|
<br>
|
|
University Computing Service
|
|
<br>
|
|
Cambridge, England.
|
|
<br>
|
|
</P>
|
|
<br><a name="SEC6" href="#TOC1">REVISION</a><br>
|
|
<P>
|
|
Last updated: 02 January 2015
|
|
<br>
|
|
Copyright © 1997-2015 University of Cambridge.
|
|
<br>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE2 index page</a>.
|
|
</p>
|