/\B@[a-z0-9_-]+/g

The regex matches all twitter usernames, the @ is not a word boundary, 
so the @ is required to be matched before the username. 
The character class [a-z0-9_-] matches all letters, numbers and underscores.
The + is a quantifier which means "one or more of the preceding character".
The g modifier means "global", so the regex will find all matches.