@usernames: Twitter Style Mentions

[eluser]nitrammit[/eluser]
I'm trying to to build a Twitter style notification system that alerts users when their username has been mentioned on the site by another user.

For example, user1 wants to draw user2's attention a comment of his. He would post:

"You should ask @user2, he knows how to do it!"

I've worked out how to convert these mentions to links through the following function:

Code:

if ( ! function_exists('at_usernames'))

{

function at_usernames($content)

{

    return preg_replace("#(^|[\n ])@([^ \"\t\n\r<]*)#ise", "'\\1<a >@\\2</a>'", $content);
    
    }

}

Which basically just turns the mention into a link, linking to the user's profile page.

But I'm not sure how to take the ACTUAL USERNAME mentioned and use it to add mentions into my database so I can then notify the mentioned user. I understand the CI/DB side of things, but how would I strip the username out? With regex?

[EDIT] I can't seem to post my actual code, it is being altered. Here's a screenshot instead:

http://cl.ly/3z1z3e0Z0x282E2Y1y2o