Text::Match::FastAlternatives - efficient search for many strings
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); }
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); }
(The same applies if you want matching that is insensitive to Unicode normalization forms; see Unicode::Normalize.)
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).
Constructs a matcher that can efficiently search for all of the @keys in parallel. Throws an exception if any of the keys are undefined, or any of them contain malformed UTF-8.
Returns a boolean value indicating whether the $target string contains any of the keys in $matcher.
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.
Returns a boolean value indicating whether the $target string is exactly equal to any of the keys in $matcher.
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.
DESTROY
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:
new
match
perl
$matches++ if $tmfa->match(my $temporary_copy = $original);
Text::Match::FastAlternatives manages to be so fast by using an Aho-Corasick automaton internally. The time to search for a match (or determine that there is no match) is independent of the number of keys being sought; total worst-case search time is O(n) where n is the length of the target string.
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)) where m is the length of the longest key.
match_at
exact_match
Text::Match::FastAlternatives uses an adaptive technique to minimise memory usage; in the best case, each character in a key requires only 4 bytes of storage in the automaton. This low memory usage has the additional advantage of reducing contention for CPU cache lines, further improving performance.
http://en.wikipedia.org/wiki/Aho-Corasick_string_matching_algorithm, Regexp::Trie, Regexp::Optimizer, Regexp::Assemble, Unicode::Normalize, perl5100delta, perlunitut, perlunifaq.
Aaron Crane <arc@cpan.org>
Copyright 2006, 2007, 2008, 2010, 2012 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.
To install Text::Match::FastAlternatives, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Text::Match::FastAlternatives
CPAN shell
perl -MCPAN -e shell install Text::Match::FastAlternatives
For more information on module installation, please visit the detailed CPAN module installation guide.