Shortest URL regex match in JavaScript
This one works:

var re = /(^|\s)((https?:\/\/)?[\w-]+(\.[\w-]+)+\.?(:\d+)?(\/\S*)?)/gi;

/*
(^|\s)                            : ensure that we are not matching an url 
                                    embeded in an other string
(https?:\/\/)?                    : the http or https schemes (optional)
[\w-]+(\.[\w-]+)+\.?              : domain name with at least two components;
                                    allows a trailing dot
(:\d+)?                           : the port (optional)
(\/\S*)?                          : the path (optional)
*/


Passes the tests at http://jsfiddle.net/9BYdp/1/

Also matches:


example.com. (trailing dot)
example.com:8080 (port)