If you’re using such a through regex that checks for dots and allowable characters, you may wish to ditch the http:// completely. People are more likely to list websites in their .plan files without it (for example, I visit perlmonks.org and not I visit http://www.perlmonks.org)
Personally I’d feel safe putting anchor tags around anything that looks like xxx.xxx, although you could also include a list of allowable Top Level Domains, something like