RegEx for twitter usernames

Question

I need some help with regex for usernames which start with "@" and following rules:

1. Username includes only \w symbols.

2. Match if string has any non word character except "@", e.g.

   • Match @username in @username!&?()*^
   • But don't match in @username@username, @username!@username or @username!%^@ (if string has at least one more "@").

3. Don't match if string has any symbols before "@", e.g.

   • Don't match @username in not@username or !@username.

For now i have:

(?<!\w)@(\w{4,15})\b(?!@)

Which excludes only \w symbols before and "@" if it stands only after username.