Converting camel case to underscore case in ruby

Question

Rails' ActiveSupport adds underscore to the String using the following:

class String
  def underscore
    self.gsub(/::/, '/').
    gsub(/([A-Z]+)([A-Z][a-z])/,'\1_\2').
    gsub(/([a-z\d])([A-Z])/,'\1_\2').
    tr("-", "_").
    downcase
  end
end

Then you can do fun stuff:

"CamelCase".underscore
=> "camel_case"

You can use

"CamelCasedName".tableize.singularize

Or just

"CamelCasedName".underscore

Both options ways will yield "camel_cased_name". You can check more details it here.

One-liner Ruby implementation:

class String
   # ruby mutation methods have the expectation to return self if a mutation occurred, nil otherwise. (see http://www.ruby-doc.org/core-1.9.3/String.html#method-i-gsub-21)
   def to_underscore!
     gsub!(/(.)([A-Z])/,'\1_\2')
     downcase!
   end

   def to_underscore
     dup.tap { |s| s.to_underscore! }
   end
end

So "SomeCamelCase".to_underscore # =>"some_camel_case"

There is a Rails inbuilt method called 'underscore' that you can use for this purpose

"CamelCaseString".underscore #=> "camel_case_string" 

The 'underscore' method can typically be considered as inverse of 'camelize'

Here's how Rails does it:

   def underscore(camel_cased_word)
     camel_cased_word.to_s.gsub(/::/, '/').
       gsub(/([A-Z]+)([A-Z][a-z])/,'\1_\2').
       gsub(/([a-z\d])([A-Z])/,'\1_\2').
       tr("-", "_").
       downcase
   end

In case someone looking for case when he need to apply underscore to string with spaces and want to convert them to underscores as well you can use something like this

'your String will be converted To underscore'.parameterize.underscore
#your_string_will_be_converted_to_underscore

Or just use .parameterize('_') but keep in mind that this one is deprecated

'your String will be converted To underscore'.parameterize('_')
#your_string_will_be_converted_to_underscore