Changing the Link code of a URL

[Gian04]

I have a site that accepts user comment that allows them to submit even an HTML code.

Now I just want to programmatically change their post if they submitted a link

example:
if they submitted http://www.example.com

it will become
[mydomain.com?redirect=http:...]

[FourDegreez]

preg_replace('/http:\/\/([a-z0-9\.\/,_-]+)/i', '<a href="http://www.mydomain.com?redirect=$0">$0</a>', $text);

where $text is the body of the post.