NAME
Text::Match::FastAlternatives - efficient search for many strings
SYNOPSIS
use Text::Match::FastAlternatives;
my $expletives = Text::Match::FastAlternatives->new(@naughty);
while (my $line = <>) {
print "Do you email your mother with that keyboard?\n"
if $expletives->match($line);
}
DESCRIPTION
This module allows you to search for any of a list of substrings
("keys") in a larger string. It is particularly efficient when the set
of keys is large.
This efficiency comes at the cost of some flexibility: if you want
case-insensitive matching, you have to fold case yourself:
my $expletives = Text::Match::FastAlternatives->new(
map { lc } @naughty);
while (my $line = <>) {
print "Do you email your mother with that keyboard?\n"
if $expletives->match(lc $line);
}
This module is designed as a drop-in replacement for Perl code of the
following form:
my $expletives_regex = join '|', map { quotemeta } @naughty;
$expletives_regex = qr/$expletives_regex/;
while (my $line = <>) {
print "Do you email your mother with that keyboard?\n"
if $line =~ $expletives_regex;
}
Text::Match::FastAlternatives can easily perform this test a hundred
times faster than the equivalent regex, if you have enough keys. The
more keys it searches for, the faster it gets compared to the regex.
Modules like Regexp::Trie can build an optimised version of such a
regex, designed to take advantage of the niceties of perl's regex
engine. With a large number of keys, this module will substantially
outperform even an optimised regex like that. In one real-world
situation with 339 keys, running on Perl 5.8, Regexp::Trie produced a
regex that ran 857% faster than the naive regex (according to
Benchmark), but using Text::Match::FastAlternatives ran 18275% faster
than the naive regex, or twenty times faster than Regexp::Trie's
optimised regex.
The enhancements to the regex engine in Perl 5.10 include algorithms
similar to those in Text::Match::FastAlternatives. However, even with
very small sets of keys, Perl has to do extra work to be fully general,
so Text::Match::FastAlternatives is still faster. The difference is
greater for larger sets of keys. For one test with only 5 keys,
Text::Match::FastAlternatives was 21% faster than perl-5.10.0; with 339
keys (as before), the difference was 111% (that is, slightly over twice
as fast).
METHODS
Text::Match::FastAlternatives->new(@keys)
Constructs a matcher that can efficiently search for all of the
@keys in parallel. Throws an exception if any of the keys are
undefined.
$matcher->match($target)
Returns a boolean value indicating whether the $target string
contains any of the keys in $matcher.
$matcher->match_at($target, $pos)
Returns a boolean value indicating whether the $target string
contains any of the keys in $matcher at position $pos. Returns false
(without emitting any warning) if $pos is larger than the length of
$string.
$matcher->exact_match($target)
Returns a boolean value indicating whether the $target string is
exactly equal to any of the keys in $matcher.
CAVEATS
Subclassing
Text::Match::FastAlternatives has a "DESTROY" method implemented in XS.
If you write a subclass with its own destructor, you will need to invoke
the base destructor, or you will leak memory.
Interaction with Perl internals
Text::Match::FastAlternatives may change the Perl-internal encoding of
strings passed to "new" or to its "match" methods. This is not
considered a bug, as the Perl-internal encoding of a string is not
normally of interest to Perl code (as opposed to Perl internals).
However, you may encounter situations where preserving a string's
existing encoding is important (perhaps to work around a bug in some
other module). If so, you may need to copy scalar variables before
matching them:
$matches++ if $tmfa->match(my $temporary_copy = $original);
IMPLEMENTATION
Text::Match::FastAlternatives manages to be so fast by using a trie
internally. The time to find a match at a given position in the string
(or determine that there is no match) is independent of the number of
keys being sought; worst-case match time is linear in the length of the
longest key. Since a match must be attempted at each position in the
target string, total worst-case search time is O(*mn*) where *m* is the
length of the target string and *n* is the length of the longest key.
The "match_at" and "exact_match" methods only need to find a match at
one position, so they have worst-case running time of O(min(*n*, *m*)).
SEE ALSO
<http://en.wikipedia.org/wiki/Trie>, Regexp::Trie, Regexp::Optimizer,
Regexp::Assemble, perl5100delta, perlunitut, perlunifaq.
AUTHOR
Aaron Crane <arc@cpan.org>
COPYRIGHT
Copyright 2006, 2007, 2008 Aaron Crane.
This library is free software; you can redistribute it and/or modify it
under the terms of the Artistic License, or (at your option) under the
terms of the GNU General Public License version 2.