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The damerau-levenshtein gem allows to find edit distance between two UTF-8 or ASCII encoded strings with O(N*M) efficiency.

This gem implements pure Levenshtein algorithm, Damerau modification of it (where 2 character transposition counts as 1 edit distance). It also includes Boehmer & Rees 2008 modification of Damerau algorithm, where transposition of bigger than 1 character blocks is taken in account as well (Rees 2014).

require "damerau-levenshtein"
DamerauLevenshtein.distance("Something", "Smoething") #returns 1

It also returns a diff between two strings according to Levenshtein alrorithm. The diff is expressed by tags <ins>, <del>, and <subst>

require "damerau-levenshtein"
differ = DamerauLevenshtein::Differ.new
differ.run("corn", "cron")
# output: ["c<subst>or</subst>n", "c<subst>ro</subst>n"]

Gem damerau-levenshtein is compatible with ruby versions 1.8.7 and 1.9.2 and higher, as well as 2.0.0 and higher


sudo apt-get install build-essential libgmp3-dev


gem install damerau-levenshtein


require "damerau-levenshtein"
dl = DamerauLevenshtein
  • compare using Damerau Levenshtein algorithm
dl.distance("Something", "Smoething") #returns 1
  • compare using Levensthein algorithm
dl.distance("Something", "Smoething", 0) #returns 2
  • compare using Boehmer & Rees modification
dl.distance("Something", "meSothing", 2) #returns 2 instead of 4
  • comparison of words with UTF-8 characters should work fine:
dl.distance("Sjöstedt", "Sjostedt") #returns 1
  • compare two arrays
dl.array_distance([1,2,3,5], [1,2,3,4]) #returns 1
  • return diff between two strings
differ = DamerauLevenshtein::Differ.new
differ.run("Something", "smthg")
  • return diff between two strings in raw format
differ = DamerauLevenshtein::Differ.new
differ.format = :raw
differ.run("Something", "smthg")

API Description



#returns version number of the gem


DamerauLevenshtein.distance(string1, string2, block_size, max_distance)
#returns edit distance between 2 strings

DamerauLevenshtein.string_distance(string1, string2, block_size, max_distance)
# an alias for .distance

DamerauLevenshtein.array_distance(array1, array2, block_size, max_distance)
# returns edit distance between 2 arrays of integers

DamerauLevenshtein.distance and .array_distance take 4 arguments:

  • string1 (array1 for .array_distance)
  • string2 (array2 for .array_distance)
  • block_size (default is 1)
  • max_distance (default is 10)

block_size determines maximum number of characters in a transposition block:

block_size = 0
(transposition does not count -- it is a pure Levenshtein algorithm)

block_size = 1
(transposition between 2 adjustent characters --
it is pure Damerau-Levenshtein algorithm)

block_size = 2
(transposition between blocks as big as 2 characters -- so abcd and cdab
counts as edit distance 2, not 4)

block_size = 3
(transposition between blocks as big as 3 characters --
so abcdef and defabc counts as edit distance 3, not 6)


max_distance -- is a threshold after which algorithm gives up and returns max_distance instead of real edit distance.

Levenshtein algorithm is expensive, so it makes sense to give up when edit distance is becoming too big. The argument max_distance does just that.

DamerauLevenshtein.distance("abcdefg", "1234567", 0, 3)
# output: 4 -- it gave up when edit distance exceeded 3


differ = DamerauLevenshtein::Differ.new creates an instance of new differ class to return difference between two strings

differ.format shows current format for diff. Default is :tag format

differ.format = :raw changes current format for diffs. Possible values are :tag and :raw

differ.run("String1", "String2") returns difference between two strings.

For example:

differ = DamerauLevenshtein::Differ.new
differ.run("Something", "smthng")
# output: ["<ins>S</ins><subst>o</subst>m<ins>e</ins>th<ins>i</ins>ng",
#          "<del>S</del><subst>s</subst>m<del>e</del>th<del>i</del>ng"]

Or with parsing:

require "damerau-levenshtein"
require "nokogiri"

differ = DamerauLevenshtein::Differ.new
res = differ.run("Something", "Smothing!")
nodes = Nokogiri::XML("<root>#{res.first}</root>")

markup = nodes.root.children.map do |n|
  case n.name
  when "text"
  when "del"
  when "ins"
  when "subst"

puts markup

Contributing to damerau-levenshtein

  • 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.


This gem is following practices of Semantic Versioning


Dmitry Mozzherin


lazylester, Ran Xie, Alexey Zapparov, azhi


Copyright (c) 2011-2017 Dmitry Mozzherin. See LICENSE.txt for further details.