Check each comment for valid website

Hello,

I’ve searched around but couldn’t find anything relevant.
Is it possible to check each comment for a valid URL (if one was supplied)? On my blog, a lot of commenters don’t provide the website in valid URL format at all; they seem to fill this in because they think it’s not optinal, which is not true.

I guess there could be two ways to this hack: check for valid format, and check for valid site by pinging it. The former would be more recommended, but if anyone can point me in the right direction in terms of both of those methods, that’d be great.

Thanks.