Regex Form Filter Help

I have a site where users can type whatever they want in and it posts it to a database. I'm trying to eliminate whatever types of abuse I can think of, and, since I'm using nl2br on the posts, I'm afraid a user might just hold down enter for a couple of seconds and scroll everything off the page. I'm trying to figure out a regex pattern that searches for say, more than 5 <br /> strings in a row, and anything after 5 gets stripped out. Any ideas on how to possibly do this?