hm, on second thought: phpcr implementations may consider additional characters invalid. so maybe the only thing we want to do here is to prevent the `/` for a nodename in the ParentIdGenerator? for other illegal characters, the phpcr implementation will throw an exception - and throwing a doctrine exception if the character is `*` but a phpcr exception if its `ä` (more realistic: some weird unicode character) and the implementation does not support this character is weird.