Format Ruby Integers with underscore

integer_underscore.rb

# <#Integer>.underscore(n) returns a string, seperating n digits by
# an underscore. n is 3 by default. This is supposed to make
# working with large numbers easier.
# example:
#   1000000000000000.underscore  #=> "1_000_000_000_000_000"
#   100_00000_00_0000_00.underscore  #=> "1_000_000_000_000_000"
#   987654321.underscore(6)  #=> "987_654321"
class Integer
  def underscore(s=3)
    self.to_s.gsub(/(\d)(?=(\d\d\d)+(?!\d))/, "\\1_")
  end
end