This is a valid "Fully Qualified Domain Name" or "FQDN" format.
You may also get requests like http://www.example.com:80/lots-of-urls-like-these/ and/or http://www.example.com.:80/lots-of-urls-like-these/
Simple enough to take care of these on most servers... What have you tried so far? Do you intend to put a fix into your .htaccess file, and if so, do you have any other working server config code in it? Does this site use (or plan to use) any other subdomains in addition to the current "www"?
This rule redirects if the hostname is not blank and if it is not *exactly* "www.example.com". It therefore catches casing errors, FQDN-format domains, and appended port numbers as previously described.
The "blank exclusion" is for HTTP/1.0 (and prior) requests. If the hostname is blank, then the request is HTTP/1.0, and trying to redirect it will result in an infinite loop, since true HTTP/1.0 requests will always have a blank hostname. Therefore, the embedded blank exclusion is used to prevent this potential loop.
Be sure your rules are in the correct order, as this is critical to success: [webmasterworld.com...]