String#underscore Ruby Extension

Build Status Gem Version

fast_underscore is a C extension that provides a fast implementation of ActiveSupport's String#underscore method.

Is it fast?

At last check, these were the benchmarks (obtained by running bin/bench with Rails 6.0.2):

Warming up --------------------------------------
       ActiveSupport     5.000  i/100ms
      FastUnderscore    70.000  i/100ms
Calculating -------------------------------------
       ActiveSupport     50.564  (± 9.9%) i/s -    250.000  in   5.012563s
      FastUnderscore    707.691  (± 1.3%) i/s -      3.570k in   5.045375s

      FastUnderscore:      707.7 i/s
       ActiveSupport:       50.6 i/s - 14.00x  slower


Add this line to your application's Gemfile:

gem 'fast_underscore'

And then execute:

$ bundle

Or install it yourself as:

$ gem install fast_underscore

Usage with Rails

ActiveSupport::Inflector#underscore, in addition to underscoring the input will additionally take into account known acronyms. Since this can't be done at compile time, FastUnderscore will detect when ActiveSupport is loaded and take advantage of its knowledge of acronyms while still using the native extension.

Since the #underscore method is used so much throughout the Rails boot process (for autoloading dependencies, determining table names, determining inverse associations, etc.), it's best to hook into Rails as early as possible. As such, for the best results in your Gemfile add require: false to the gem 'fast_underscore' declaration and add require 'fast_underscore' to the bottom of config/boot.rb.


After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at


The gem is available as open source under the terms of the MIT License.