URL valid characters. java to validate

a string like: 'www.test.com' is good. a string like: 'www.888.com' is good. a string like: 'stackoverflow.com' is good. a string like: 'GOoGle.Com' is good.

why ? because those are valid urls. it does not necessarely matter if they have been registered or not.

now bad strings are:

'goog*d\x' 'manydots...com'

why because you can't register those urls.

if I have a string in java which is supposed to be a good url what's the best way to validate it ?

thanks a lot