After digging some more, it seems the problem lies more with the domain validation, a domain like w--w.com seems to be registered (albeit by some shady domain seller), but this test seems to test that the validator does not allow such domains:

https://github.com/hibernate/hibernate-validator/blob/master/engine/src/test/java/org/hibernate/validator/test/internal/constraintvalidators/hv/URLValidatorTest.java#L374