This function returns the YouTube ID if the input was any form of valid YouTube link or the ID itself:

private function extractID(input:String):String
{
  if (input.length == 11) return input;
  var reg:RegExp = /http:\/\/(?:youtu\.be\/|(?:[a-z]{2,3}\.)?youtube\.com\/watch(?:\?|#\!)v=)([\w_-]{11}).*/gi;
  var res:Object = reg.exec(input);
  if (res != null) return res[1];
  return "";
}

Code