This may be implementation dependent as to whether an IRI will match the format=uri rule or not. Example from @tkanai: https://github.com/w3c/web-annotation/issues/183#issuecomment-198304922 If it _is_ implementation dependent, we could either: 1. be less restrictive and just check for strings 2. devise a slower regular expression to test for IRIs 3. implement custom format checkers for format=iri