Twitter Name Validation
I believe it's letters, numbers and underscores only, and a maximum of 15 characters.
A quick search unveiled this post (non-Twitter) covering the same topic:
http://kagan.mactane.org/blog/2009/09/22/what-characters-are-allowed-in-twitter-usernames/
The above post also contains regex examples to help you validate:
Full regex – /^[a-zA-Z0-9_]{1,15}$/
Perl-compatible regex – /^\w{1,15}$/
Check this page from Twitter for the official guidelines/rules
http://support.twitter.com/articles/101299-why-can-t-i-register-certain-usernames#
This is the final JavaScript Funcion:
function validTwitteUser(sn) {
return /^[a-zA-Z0-9_]{1,15}$/.test(sn);
}