This paper examines the difficulty of the schema conformance problem for the following XML schema languages: • DTD • RELAX NG with W3C XML Schema Datatypes • Schematron • W3C XML Schema • Namespace Routing Language The schema conformance problem is defined as follows: Given a schema, is there an XML document that conforms to the schema? For these five XML schema languages, the schema conformance problem is intractable (ranging from NP-HARD to undecidable).
Robert C. Lyons