NAME

CISTEM - Stemmer for German

SYNOPSIS

    use Lingua::Stem::Cistem;
    my $stemmed_word = Lingua::Stem::Cistem::stem($word);
    my @segments     = Lingua::Stem::Cistem::segment($word);

    use Lingua::Stem::Cistem qw(:orig);
    my $stemmed_word = stem($word);
    my @segments     = segment($word);

    use Lingua::Stem::Cistem qw(:robust);
    my $stemmed_word = stem_robust($word);
    my @segments     = segment_robust($word);

DESCRIPTION

This is the CISTEM stemmer for German based on the "OFFICIAL IMPLEMENTATION".

It targets at typical tasks like Information Retrieval, Keyword Extraction or Topic Matching.

Now (2019) CISTEM has the best f-score compared to other stemmers for German on CPAN, while being one of the fastest.

This distribution is adapted to CPAN standards, and the method "stem" is 6-9 % faster. It also provides the two methods "stem_robust" and "segment_robust" with the same logic as the official ones, but more robust against low quality input, but 40-70 % slower.

OFFICIAL IMPLEMENTATION

It is based on the paper

Leonie Weissweiler, Alexander Fraser (2017). Developing a Stemmer for German Based on a Comparative Analysis of Publicly Available Stemmers. In Proceedings of the German Society for Computational Linguistics and Language Technology (GSCL)

which can be read here:

http://www.cis.lmu.de/~weissweiler/cistem/

In the paper, the authors conducted an analysis of publicly available stemmers, developed two gold standards for German stemming and evaluated the stemmers based on the two gold standards. They then proposed the stemmer implemented here and show that it achieves slightly better f-measure than the other stemmers and is thrice as fast as the Snowball stemmer for German while being about as fast as most other stemmers.

Source repository https://github.com/LeonieWeissweiler/CISTEM

METHODS

stem

    stem($word, $case_insensitivity)

This method takes the word to be stemmed and a boolean specifiying if case-insensitive stemming should be used and returns the stemmed word. If only the word is passed to the method or the second parameter is 0, normal case-sensitive stemming is used, if the second parameter is 1, case-insensitive stemming is used.

Case sensitivity improves performance only if words in the text may be incorrectly upper case. For all-lowercase and correctly cased text, best performance is achieved by using the case-sensitive version.

stem_robust

    stem_robust($word, $case_insensitivity)

This method works like "stem" with the following differences for robustness:

- German Umlauts in decomposed normalization form (NFD) work like composed (NFC) ones. - Other characters plus combining characters as treated as graphemes, i.e. with length 1 instead of 2 or more, which has an influence on the resulting stem. - The characters $, %, & keep their value, i.e. they roundtrip.

This should not be necessary, if the input is carefully normalized, tokenized, and filtered.

segment

    segment($word, $case_insensitivity)

This method works very similarly to stem. The only difference is that in addition to returning the stem, it also returns the rest that was removed at the end. To be able to return the stem unchanged so the stem and the rest can be concatenated to form the original word, all subsitutions that altered the stem in any other way than by removing letters at the end were left out.

        my ($stem, $suffix) = segment($word);

segment_robust

    segment_robust($word, $case_insensitivity)

This method works exactly like stem_robust and returns a list of prefix, stem and suffix:

        my ($prefix, $stem, $suffix) = segment_robust($word);

SOURCE REPOSITORY

http://github.com/wollmers/Lingua-Stem-Cistem

AUTHOR

Helmut Wollmersdorfer <helmut@wollmersdorfer.at>

COPYRIGHT

Copyright 2019 Helmut Wollmersdorfer

LICENSE

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

SEE ALSO

Lingua::Stem::Snowball, Lingua::Stem::UniNE, Lingua::Stem, Lingua::Stem::Patch

cpanm Lingua::Stem::Cistem

perl -MCPAN -e shell
install Lingua::Stem::Cistem