to_underscore implementation

string.rb

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!
     g = gsub!(/(.)([A-Z])/,'\1_\2'); d = downcase!
     g || d
   end

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