Regular expression for twitter username

I need a javascript regular expression to match twitter usernames. The username is entered by the user while signing up, so I don't want to distract them with too many error notifications. Because of that, I need the expression to match valid usernames regardles if they have the @ before the username or not. Twitter usernames can contain latin characters, underscores and numbers, and the only limitation is the can be up to 15 characters long. ( but I need the regex to match 16 characters as well, in case someone enters the @ before the username ).