How about this?
/^http:\/\/\w+(\.\w+)*(:[0-9]+)?\/?(\/[.\w]*)*$/
Will match: http://domain.com:port/path or just http://domain or http://domain:port
/^http:\/\/\w+(\.\w+)*(:[0-9]+)?\/?$/
match URLs without path
Some explanations of regex blocks:
Domain: \w+(\.\w+)* to match text with dots: localhost or www.yahoo.com (could be as long as Path or Port section begins)
Port: (:[0-9]+)? to match or to not match a number starting with semicolon: :8000 (and it could be only one)
Path: \/?(\/[.\w]*)* to match any alphanums with slashes and dots: /user/images/0001.jpg (until the end of the line)
(path is very interesting part, now I did it to allow lone or adjacent dots, i.e. such expressions could be possible: /. or /./ or /.../ and etc. If you'd like to have dots in path like in domain section - without border or adjacent dots, then use \/?(\/\w+(.\w+)*)* regexp, similar to domain part.)
* UPDATED *
Also, if you would like to have (it is valid) - characters in your URL (or any other), you should simply expand character class for "URL text matching", i.e. \w+ should become [\-\w]+ and so on.