The Porter 2 stemmer

This is the Porter 2 stemming algorithm, as described at 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.

The gb_english flag to the various procedures allows the stemmer to treat the British English '-ise' the same as the American English '-ize'.

Longest suffixes

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"


The Porter 2 stemming algorithm was developed by Martin Porter. This implementation is by Neil Smith.

Contributing to porter2stemmer

  • Check out the latest master to make sure the feature hasn't been implemented or the bug hasn't been fixed yet

  • Check out the issue tracker to make sure someone already hasn't requested it and/or contributed it

  • Fork the project

  • Start a feature/bugfix branch

  • Commit and push until you are happy with your contribution

  • Make sure to add tests for it. This is important so I don't break it in a future version unintentionally.

  • Please try not to mess with the Rakefile, version, or history. If you want to have your own version, or is otherwise necessary, that is fine, but please isolate to its own commit so I can cherry-pick around it.


Copyright © 2011 Neil Smith. See LICENSE.txt for further details.