gistfile1.rb

#!/usr/bin/env ruby

require 'uri'
require 'set'

urls = Set.new()

STDIN.each_line do|line|
  begin
    url = URI.parse(line.downcase).host

    unless urls.member?(url)
      [url, "www.#{url}"].each do |url|
        urls.add(url)
        STDOUT.puts (url)
      end
    end
  rescue URI::InvalidURIError
    STDERR.puts "Invalid URI: #{line}"
  end
end