Source of: /manual/en/regexp.reference.performances.php
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<div id="regexp.reference.performances" class="section">
<h2 class="title">Performances</h2>
<p class="para">
Certain items that may appear in 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 discussion about optimizing
regular expressions for efficient performance.
</p>
<p class="para">
When a pattern begins with .* and the <a href="reference.pcre.pattern.modifiers.php" class="link">PCRE_DOTALL</a> option is
set, the pattern is implicitly anchored by PCRE, since it
can match only at the start of a subject string. However, if
<a href="reference.pcre.pattern.modifiers.php" class="link">PCRE_DOTALL</a>
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
<i>(.*) second</i>
matches the subject "first\nand second" (where \n stands for
a newline character) with the first captured substring being
"and". In order to do this, PCRE has to retry the match
starting after every newline in the subject.
</p>
<p class="para">
If you are using such a pattern with subject strings that do
not contain newlines, the best performance is obtained by
setting <a href="reference.pcre.pattern.modifiers.php" class="link">PCRE_DOTALL</a>,
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 class="para">
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
<i>(a+)*</i>
</p>
<p class="para">
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 class="para">
An optimization catches some of the more simple cases such
as
<i>(a+)*b</i>
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
<i>(a+)*\d</i>
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>
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