Ruby Standard Library: Abbrev

July 9, 2021   • ruby

This post explores the Abbrev module that ships with the Ruby standard library. We take a look at what this module does and how it works. Then we take a look behind the hood and see how it’s implemented.

What is Abbrev?

This module helps you find out all the possible abbreviations for one or more strings so that there are no duplicate abbreviations.

Input: A set of words, such as [ruby, rust]

Output: A hash consisting of unambiguous abbreviations for the above string.

irb(main):001:0> require 'abbrev'
=> true
irb(main):002:0> Abbrev.abbrev(['ruby', 'rust'])
=> {"ruby"=>"ruby", "rub"=>"ruby", "rust"=>"rust", "rus"=>"rust"}

Note the resulting hash omitted ru as a key as it would be confusing to find which word it refers to, ruby or rust.

It also works as an array extension, i.e. it adds an abbrev method to the Array class. This lets you call the abbrev method directly on an array.

irb(main):003:0> %w(ruby rust).abbrev
=> {"ruby"=>"ruby", "rub"=>"ruby", "rust"=>"rust", "rus"=>"rust"}

When do I use it?

Let’s say your program has a set of commands. But you also want to allow the users of your program to type as few keys as possible when entering the commands. For example, to execute a generate command, the users could type gen or even g, and your program should work as expected.

However, let’s say you have a generic command, then you shouldn’t allow the users to type gen, as that would be ambiguous. In this case, use the Abbrev module to get the non-conflicting abbreviations for your commands.

Here is a code example from the book Programming Ruby that demonstrates this feature.

require 'abbrev'

COMMANDS = %w{ sample send start status stp }.abbrev

while line = gets
  line = line.chomp

  case COMMANDS[line]
  when "sample" then
    puts "Executing sample command"
  when "send" then
    puts "Executing send command"
  when "start" then
    puts "Executing start command"
  when "status" then
    puts "Executing status command"
  when "stp" then
    puts "Executing stp command"
    STDERR.puts "Unknown command: #{line}"

# Output
Executing sample command
Unknown command: st
Unknown command: sta
Executing start command

How does it work?

Here is a simple version of the code implemented in the standard library, in the lib/abbrev.rb directory. It ignores the optional pattern parameter, which can be a string or a regex. It includes only strings that match the pattern or start with the string.

module Abbr

  def Abbr.abbr(words)
    table = {}
    seen =

    words.each do |word|
      next if word.empty?
      word.size.downto(1) do |len|
        abbrev = word[0...len]

        case seen[abbrev] += 1
        when 1
          table[abbrev] = word
        when 2  # found duplicate abbreviation
        else  # no need to go further

    words.each do |word|
      table[word] = word


Hope this helps.