Message

The xlink:href MUST be expressed in a UUID syntax (RFC 4122).

Context
query:QueryResponse/rim:RegistryObjectList/rim:RegistryObject/rim:RepositoryItemRef
Test
matches(normalize-space(./@xlink:href), '.*[0-9a-fA-F]{8}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{12}.*')
Usage
/ query:QueryResponse / rim:RegistryObjectList / rim:RegistryObject / rim:RepositoryItemRef