class Object
  def commify(sep=',')
      if self.class == Fixnum || self.class == Float
          numstr = self.to_s
      else
          numstr = self.dup
      end
      numstr.gsub(/(\d)(?=\d{3}+(?:\.|$))(\d{3}\..*)?/, "\\1#{sep}\\2")
  end
end

# 12345.commify --> "12,345"
# "12345".commify --> "12,345"
# 45623.commify('_') --> 45_623