Being too lazy to go ready RFCs on a Monday morning, is what a valid schema consists of spelled out? Is `isbn://0441569595` a valid URI? If so, is there a standardized list of schemas?
Read the article, someone already read the RFC for you. To your questions: Yes. Almost, urn:isbn:123-4567-89 (URNs are something I neglected to mention to keep things simple). Yes, the IANA maintains a registry.