Lingua::Stem - Stemming of words

    use Lingua::Stem qw(stem);
    my $stemmmed_words_anon_array   = stem(@words);

    or for the OO inclined,

    use Lingua::Stem;
    my $stemmer = Lingua::Stem->new(-locale => 'EN-UK');
    $stemmer->stem_caching({ -level => 2 });
    my $stemmmed_words_anon_array   = $stemmer->stem(@words);

This routine applies stemming algorithms to its parameters, returning the stemmed words as appropriate to the selected locale.

You can import some or all of the class methods.

use Lingua::Stem qw (stem clear_stem_cache stem_caching add_exceptions delete_exceptions get_exceptions set_locale get_locale :all :locale :exceptions :stem :caching);

 :all        - imports  stem add_exceptions delete_exceptions get_exceptions
               set_locale get_locale
 :stem       - imports  stem
 :caching    - imports  stem_caching clear_stem_cache
 :locale     - imports  set_locale get_locale
 :exceptions - imports  add_exceptions delete_exceptions get_exceptions

Currently supported locales are:

      DA          - Danish
      DE          - German
      EN          - English (also EN-US and EN-UK)
      FR          - French
      GL          - Galician
      IT          - Italian
      NO          - Norwegian
      PT          - Portuguese
      RU          - Russian (also RU-RU and RU-RU.KOI8-R)
      SV          - Swedish

If you have the memory and lots of stemming to do, I strongly suggest using cache level 2 and processing lists in 'big chunks' (long lists) for best performance.

Some benchmarks using Lingua::Stem 0.80 and Lingua::Stem::Snowball 0.7 give an idea of how batching and caching affect performance. The dataset was 3000 randomly selected words from the snowball english voc.txt list repeated 100 times (300000 words total, with 3000 unique words). The tests were performed on a Linux machine with a 1.7 Ghz Athlon processor running Perl 5.8.5. There are no tests listed for Lingua::Stem::Snowball using caching because it doesn't have a caching mode.

 Lingua::Stem::Snowball, one word at a time, no caching:    26741 words/second
 Lingua::Stem::Snowball,   3000 word batches, no caching:  169710 words/second
 Lingua::Stem::Snowball, one batch, no caching:            155797 words/second

 Lingua::Stem, one word at a time, no caching:              13509 words/second
 Lingua::Stem,   3000 word batches, no caching:             32765 words/second
 Lingua::Stem, one batch, no caching:                       34847 words/second

 Lingua::Stem, one word at a time, cache level 2:           25736 words/second
 Lingua::Stem,   3000 word batches, cache level 2:         194384 words/second
 Lingua::Stem, one batch, cache level 2:                   216602 words/second

 0.81 2004.07.26 - Minor documentation tweak. No functional change.

 0.80 2004.07.25 - Added 'RU', 'RU_RU', 'RU_RU.KOI-8' locale.
                   Added support for Lingua::Stem::Ru to
                   Makefile.PL and autoloader.

                   Added documentation stressing use of caching
                   and batches for performance. Added support
                   for '_' as a seperator in the locale strings.
                   Added example benchmark script. Expanded copyright
                   credits.

 0.70 2004.04.26 - Added FR locale and documentation fixes
                   to Lingua::Stem::Gl

 0.61 2003.09.28 - Documentation fixes. No functional changes.

 0.60 2003.04.05 - Added more locales by wrappering various stemming
                   implementations. Documented currently supported
                   list of locales.

 0.50 2000.09.14 - Fixed major implementation error. Starting with
                   version 0.30 I forgot to include rulesets 2,3 and 4
                   for Porter's algorithm. The resulting stemming results
                   were very poor. Thanks go to <csyap@netfision.com>
                   for bringing the problem to my attention.

                   Unfortunately, the fix inherently generates *different*
                   stemming results than 0.30 and 0.40 did. If you
                   need identically broken output - use locale 'en-broken'.

 0.40 2000.08.25 - Added stem caching support as an option. This
                   can provide a large speedup to the operation
                   of the stemmer. Caching is default turned off
                   to maximize compatibility with previous versions.

 0.30 1999.06.24 - Replaced core of 'En' stemmers with code from
                   Jim Richardson <jimr@maths.usyd.edu.au>
                   Aliased 'en-us' and 'en-uk' to 'en'
                   Fixed 'SYNOPSIS' to correct return value
                   type for stemmed words (SYNOPIS error spotted
                   by <Arved_37@chebucto.ns.ca>)

 0.20 1999.06.15 - Changed to '.pm' module, moved into Lingua:: namespace,
                   added OO interface, optionalized the export of routines
                   into the caller's namespace, added named parameter
                   initialization, stemming exceptions, autoloaded
                   locale support and isolated case flattening to
                   localized stemmers prevent i18n problems later.

                   Input and output text are assumed to be in UTF8
                   encoding (no operational impact right now, but
                   will be important when extending the module to
                   non-English).

new(...);

Returns a new instance of a Lingua::Stem object and, optionally, selection of the locale to be used for stemming. Examples:

  # By default the locale is en
  $us_stemmer = Lingua::Stem->new;

  # Turn on the cache
  $us_stemmer->stem_caching({ -level => 2 });

  # Overriding the default for a specific instance
  $uk_stemmer = Lingua::Stem->new({ -locale => 'en-uk' });

  # Overriding the default for a specific instance and changing the default
  $uk_stemmer = Lingua::Stem->new({ -default_locale => 'en-uk' });

set_locale($locale);

Sets the locale to one of the recognized locales. locale identifiers are converted to lowercase. Called as a class method, it changes the default locale for all subseqently generated object instances. Called as an instance method, it only changes the locale for that particular instance. 'croaks' if passed an unknown locale. Examples:

 # Change default locale
 Lingua::Stem::set_locale('en-uk'); # UK's spellings

 # Change instance locale
 $self->set_locale('en-us');  # US's spellings

get_locale;

Called as a class method, returns the current default locale. Example:

 $default_locale = Lingua::Stem::get_locale;

Called as an instance method, returns the locale for the instance

 $instance_locale = $stemmer->get_locale;

add_exceptions($exceptions_hash_ref);

Exceptions allow overriding the stemming algorithm on a case by case basis. It is done on an exact match and substitution basis: If a passed word is identical to the exception it will be replaced by the specified value. No case adjustments are performed. Called as a class method, adds exceptions to the default exceptions list used for subsequently instantations of Lingua::Stem objects. Example:

 # adding default exceptions
 Lingua::Stem::add_exceptions({ 'emily' => 'emily',
                                'driven' => 'driven',
                            });

Called as an instance method, adds exceptions only to the specific instance.

 # adding instance exceptions
 $stemmer->add_exceptions({ 'steely' => 'steely' });

The exceptions shortcut the normal stemming - if an exception matches no further stemming is performed after the substitution. Adding an exception with the same key value as an already defined exception replaces the pre-existing exception with the new value.

delete_exceptions(@exceptions_list);

The mirror of add_exceptions, this allows the _removal_ of exceptions from either the defaults for the class or from the instance.

 # Deletion of exceptions from class default exceptions
 Lingua::Stem::delete_exceptions('aragorn','frodo','samwise');

 # Deletion of exceptions from instance
 $stemmer->delete_exceptions('smaug','sauron','gollum');

 # Deletion of all class default exceptions
 delete_exceptions;

 # Deletion of all exceptions from instance
 $stemmer->delete_exceptions;

get_exceptions;

As a class method with no parameters it returns all the default exceptions as an anonymous hash of 'exception' => 'replace with' pairs. Example:

 # Returns all class default exceptions
 $exceptions = Lingua::Stem::get_exceptions;

As a class method with parameters, it returns the default exceptions listed in the parameters as an anonymous hash of 'exception' => 'replace with' pairs. If a parameter specifies an undefined 'exception', the value is set to undef.

 # Returns class default exceptions for 'emily' and 'george'
 $exceptions = Lingua::Stem::get_exceptions('emily','george');

As an instance method, with no parameters it returns the currently active exceptions for the instance.

 # Returns all instance exceptions
 $exceptions = $stemmer->get_exceptions;

As an instance method with parameters, it returns the instance exceptions listed in the parameters as an anonymous hash of 'exception' => 'replace with' pairs. If a parameter specifies an undefined 'exception', the value is set to undef.

 # Returns instance exceptions for 'lisa' and 'bart'
 $exceptions = $stemmer->get_exceptions('lisa','bart');

stem(@list);

Called as a class method, it applies the default settings and stems the list of passed words, returning an anonymous array with the stemmed words in the same order as the passed list of words. Example:

    # Default settings applied
    my $anon_array_of_stemmed_words = Lingua::Stem::stem(@words);

Called as an instance method, it applies the instance's settings and stems the list of passed words, returning an anonymous array with the stemmed words in the same order as the passed list of words.

   # Instance's settings applied
   my $stemmed_words = $stemmer->stem(@words);

The stemmer performs best when handed long lists of words rather than one word at a time. The cache also provides a huge speed up if you are processing lots of text.

clear_stem_cache;

Clears the stemming cache for the current locale. Can be called as either a class method or an instance method.

    $stemmer->clear_stem_cache;

    clear_stem_cache;

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

Sets stemming cache level for the current locale. Can be called as either a class method or an instance method.

    $stemmer->stem_caching({ -level => 1 });

    stem_caching({ -level => 1 });

For the sake of maximum compatibility with previous versions, stem caching is set to '-level => 0' by default. '-level' definitions

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

 '1' means 'cache per run'. This caches stemming results during each
    call to 'stem'.

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

stem caching is global to the locale. If you turn on stem caching for one instance of a locale stemmer, all instances using the same locale will have it turned on as well. I STRONGLY suggest turning caching on if you have enough memory and are processing a lot of data.

 0.81 2004.07.26

It started with the 'Text::Stem' module which has been adapted into a more general framework and moved into the more language oriented 'Lingua' namespace and re-organized to support a OOP interface as well as switch core 'En' locale stemmers.

Version 0.40 added a cache for stemmed words. This can provide up to a several fold performance improvement.

Organization is such that extending this module to any number of languages should be direct and simple.

Case flattening is a function of the language, so the 'exceptions' methods have to be used appropriately to the language. For 'En' family stemming, use lower case words, only, for exceptions.

 Benjamin Franz <snowhare@nihongo.org>
 Jim Richardson  <imr@maths.usyd.edu.au>

 Jim Richardson             <imr@maths.usyd.edu.au>
 Ulrich Pfeifer             <pfeifer@ls6.informatik.uni-dortmund.de>
 Aldo Calpini               <dada@perl.it>
 xern                       <xern@cpan.org>
 Ask Solem Hoel             <ask@unixmonks.net>
 Dennis Haney               <davh@davh.dk>
 Sébastien Darribere-Pleyt  <sebastien.darribere@lefute.com>
 Aleksandr Guidrevitch      <pillgrim@mail.ru>

 Lingua::Stem::En            Lingua::Stem::En            Lingua::Stem::Da
 Lingua::Stem::De            Lingua::Stem::Gl            Lingua::Stem::No
 Lingua::Stem::Pt            Lingua::Stem::Sv            Lingua::Stem::It
 Lingua::Stem::Fr            Lingua::Stem::Ru            Text::German
 Lingua::PT::Stemmer         Lingua::GL::Stemmer         Lingua::Stem::Snowball::No
 Lingua::Stem::Snowball::Se  Lingua::Stem::Snowball::Da  Lingua::Stem::Snowball::Sv
 Lingua::Stemmer::GL         Lingua::Stem::Snowball

 http://snowball.tartarus.org

Copyright 1999-2004

Freerun Technologies, Inc (Freerun), Jim Richardson, University of Sydney <imr@maths.usyd.edu.au> and Benjamin Franz <snowhare@nihongo.org>. All rights reserved.

Text::German was written and is copyrighted by Ulrich Pfeifer.

Lingua::Stem::Snowball::Da was written and is copyrighted by Dennis Haney and Ask Solem Hoel.

Lingua::Stem::It was written and is copyrighted by Aldo Calpini.

Lingua::Stem::Snowball::No, Lingua::Stem::Snowball::Se, Lingua::Stem::Snowball::Sv were written and are copyrighted by Ask Solem Hoel.

Lingua::Stemmer::GL and Lingua::PT::Stemmer were written and are copyrighted by Xern.

Lingua::Stem::Fr was written and is copyrighted by Aldo Calpini and Sébastien Darribere-Pley.

Lingua::Stem::Ru was written and is copyrighted by Aleksandr Guidrevitch.

This software may be freely copied and distributed under the same terms and conditions as Perl.

None known.

Add more languages. Extend regression tests. Add support for the Lingua::Stem::Snowball family of stemmers as an alternative core stemming engine.