Converting camel case to underscore case in ruby

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"

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"

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.