replacing forum tag text to make a link

I know the subject sounds confusing but I don't know how to put it. You know in some forums it won't allow HTML? I want to have them put a [link href="http://www.somesite.com"]SomeSite[link] and the have it replace the text with the appropriate anchor tag. I have looked all around the web but I can't find anything. By the way I am using PHP and I've tried using ereg_replace but I don't know how to use regular expressions. Thanks for any suggestions.