download_youtube_video.rb

 def parse_youtube(url)
     youtube = "http://www.youtube.com/"
     # url =~ /(?:http\:\/\/.*youtube.com\/(?:watch\?v=|v\/))?(.*)$/
     url =~ /watch\?v=(.*)/ 
     video_id = $1
     video_id = video_id.split("&")[0]
     flv_url = nil
     open("#{youtube}watch\?v=#{video_id}") do |f|
       f.each_line do |line|
         if line =~ /watch_fullscreen\?(.*?)video_id=([\w-]+)&(.*?)&t=([\w-]+)&/
           # p line
           flv_url = "#{youtube}get_video?video_id=#{$2}&t=#{$4};auto"
           break
         end
       end
     end
     flv_url
  end

USER_AGENT = %{Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-US; rv:1.8.1.11) 
Gecko/20071231 Firefox/2.0.0.11 Flock/1.0.5}

IO.popen("curl -o \"#{file_name}\" -L -A \"#{USER_AGENT}\" \"#{parse_youtube(url)}\"  2>&1")