Regex for Twitter username

Working on a small weekend project I needed an easy way to validate that a string contained a valid Twitter username. Not necessarily an active account, but that could be an account.

Found the rules on Twitter’s site that a username can be between 1 and 15 characters and can contain only the characters A-Z, a-z, 0-9, or an underscore. Sounds like a perfect use for regex. So here it is, both in JavaScript and PHP since I needed both.