The Porter 2 stemmer
This is the Porter 2 stemming algorithm, as described at http://snowball.tartarus.org/algorithms/english/stemmer.html The original paper is:
Porter, 1980, "An algorithm for suffix stripping", Program, Vol. 14, no. 3, pp 130-137
Features of this implementation
This stemmer is written in pure Ruby, making it easy to modify for language variants. For instance, the original Porter stemmer only works for American English and does not recognise British English's '-ise' as an alternate spelling of '-ize'. This implementation has been extended to handle correctly British English.
This stemmer also features a comprehensive test set of over 29,000 words, taken from the Porter 2 stemmer website.
Constants for the stemmer are in the Porter2 module.
Procedures that implement the stemmer are added to the String class.
The stemmer algorithm is implemented in the String#porter2_stem procedure.
There isn't much, as this is a stemmer that only works for English.
gb_english flag to the various procedures allows the stemmer to treat the British
English '-ise' the same as the American English '-ize'.
Several places in the algorithm require matching the longest suffix of a word. The regexp engine in Ruby 1.9 seems to handle alterntives in regexps by finding the alternative that matches at the first position in the string. As we're only talking about suffixes, that first match is also the longest suffix. If the regexp engine changes, this behaviour may change and break the stemmer.
Call the String#porter2_stem or String#stem methods on a string to return its stem "consistency".stem # => "consist" "knitting".stem # => "knit" "articulated".stem # => "articul" "nationalize".stem # => "nation" "nationalise".stem # => "nationalis" "nationalise".stem(true) # => "nation"