referring to http://daringfireball.net/misc/2010/07/url-matching-regex-test-data.text http://daringfireball.net/misc/2010/07/url-matching-regex-te... you can

referring to http://daringfireball.net/misc/2010/07/url-matching-regex-test-data.text http://daringfireball.net/misc/2010/07/url-matching-regex-te... you can see he wants to match things like www.example.com, but not filename.txt So www. can introduce a url also.