Lingua::Stem::En

NAME

Lingua::Stem::En - Porter’s stemming algorithm for ’generic’ English

SYNOPSIS

use Lingua::Stem::En;
my $stems = Lingua::Stem::En::stem({ -words => $word_list_reference,
-locale => 'en',
-exceptions => $exceptions_hash,
});

DESCRIPTION

This routine applies the Porter Stemming Algorithm to its parameters, returning the stemmed words.

It is derived from the C program "stemmer.c" as found in freewais and elsewhere, which contains these notes:

Purpose: Implementation of the Porter stemming algorithm documented
in: Porter, M.F., "An Algorithm For Suffix Stripping,"
Program 14 (3), July 1980, pp. 130-137.
Provenance: Written by B. Frakes and C. Cox, 1986.

I have re-interpreted areas that use Frakes and Cox’s "WordSize" function. My version may misbehave on short words starting with "y", but I can’t think of any examples.

The step numbers correspond to Frakes and Cox, and are probably in Porter’s article (which I’ve not seen). Porter’s algorithm still has rough spots (e.g current/currency, -ings words), which I’ve not attempted to cure, although I have added support for the British -ise suffix.

CHANGES

1999.06.15 - Changed to '.pm' module, moved into Lingua::Stem namespace,
optionalized the export of the 'stem' routine
into the caller's namespace, added named parameters
1999.06.24 - Switch core implementation of the Porter stemmer to
the one written by Jim Richardson <jimr AT maths DOT usyd DOT edu DOT au>
2000.08.25 - 2.11 Added stemming cache
2000.09.14 - 2.12 Fixed *major* :( implementation error of Porter's algorithm
Error was entirely my fault - I completely forgot to include
rule sets 2,3, and 4 starting with Lingua::Stem 0.30.
-- Jerilyn Franz
2003.09.28 - 2.13 Corrected documentation error pointed out by Simon Cozens.
2005.11.20 - 2.14 Changed rule declarations to conform to Perl style convention
for 'private' subroutines. Changed Exporter invokation to more
portable 'require' vice 'use'.
2006.02.14 - 2.15 Added ability to pass word list by 'handle' for in-place stemming.
2009.07.27 - 2.16 Documentation Fix
2020.06.20 - 2.30 Version renumber for module consistency.
2020.09.26 - 2.31 Fix for Latin1/UTF8 issue in documentation

METHODS

stem({ -words => \@words, -locale => ’en’, -exceptions => \%exceptions
});

Stems a list of passed words using the rules of US English. Returns an anonymous array reference to the stemmed words.

Example:

my @words = ( 'wordy', 'another' );
my $stemmed_words = Lingua::Stem::En::stem({ -words => \@words,
-locale => 'en',
-exceptions => \%exceptions,
});

If the first element of @words is a list reference, then the stemming is performed ’in place’ on that list (modifying the passed list directly instead of copying it to a new array).

This is only useful if you do not need to keep the original list. If you do need to keep the original list, use the normal semantic of having ’stem’ return a new list instead - that is faster than making your own copy and using the ’in place’ semantics since the primary difference between ’in place’ and ’by value’ stemming is the creation of a copy of the original list. If you don’t need the original list, then the ’in place’ stemming is about 60% faster.

Example of ’in place’ stemming:

my $words = [ 'wordy', 'another' ];
my $stemmed_words = Lingua::Stem::En::stem({ -words => [$words],
-locale => 'en',
-exceptions => \%exceptions,
});

The ’in place’ mode returns a reference to the original list with the words stemmed.

stem_caching({ -level => 0|1|2 });

Sets the level of stem caching.

’0’ means ’no caching’. This is the default level.

’1’ means ’cache per run’. This caches stemming results during a single
call to ’stem’.

’2’ means ’cache indefinitely’. This caches stemming results until
either the process exits or the ’clear_stem_cache’ method is called.

clear_stem_cache;

Clears the cache of stemmed words

NOTES

This code is almost entirely derived from the Porter 2.1 module written by Jim Richardson.

SEE ALSO

Lingua::Stem

AUTHOR

Jim Richardson, University of Sydney
jimr AT maths DOT usyd DOT edu DOT au or http://www.maths.usyd.edu.au:8000/jimr.html
Integration in Lingua::Stem by
Jerilyn Franz, FreeRun Technologies,
<cpan AT jerilyn DOT info>

COPYRIGHT

Jim Richardson, University of Sydney Jerilyn Franz, FreeRun Technologies

This code is freely available under the same terms as Perl.

BUGS

TODO