HTML Input - Auto Format URL's

Hi, I'm currently looking into the best way to approach this. Users will be allowed to enter HTML, but I want to add the ability to automatically format URL's and e-mail addresses. The problem here is before formatting URL's we need to ignore URL's that are already used in HTML tags and that are already formatted as hyperlinks. First I am thinking if its possible to achieve this with various regular expression patterns. Another is to do a first pass of the input and find out what to ignore, then on a second pass with format the links that were determined to be formatted. I am welcome to any suggestions about this. Thanks.