Regular expression to find and replace @ mentions like twitter

I am developing a PHP application which needs a regular expression to replace the @ mentions like twitter. Also the regular expression should satisfy the following needs.

1. if there is just @ and nothing before and after that then it should not be replaced.
2. @ in the emails should not be replaced. For eg. sam@example.com should not be replaced.
3. Only strings like @sam or @example should be replaced like <a href="http://twitter.com/sam">@sam</a> and <a href="http://twitter.com/example">@exa开发者_Go百科mple</a>

Please help. Thanks in advance.

Wow. I found the answer myself guys.

$tweet = preg_replace('/(^|[^a-z0-9_])@([a-z0-9_]+)/i', '$1<a href="http://twitter.com/$2">@$2</a>', $tweet);

Thanks for your help guys.

How about something like -

(?<!\w)@[\w]+

As twitter can contain up to 15 characters, you could write it like this to avoid some bugs:

$tweet = preg_replace("/(^\w)@(\w{1,15})/i", "\\1<a ref=\"http://twitter.com/\\2\">@\\2</a>", $tweet);