TREX

TREX (Tree Regular Expressions for XML) is a simple schema language for validating XML documents developed by James Clark.  According to Clark calling it a schema language is "perhaps misleading.  It's goal is purely validation. It doesn't aim to assist in interpreting or processing the document. The post-validation infoset is exactly the same as the pre-validation infoset.

TREX is basically the type system with an XML syntax and  additional features (like support for attributes and namespaces) needed to make it a practical language for structure validation. Of existing Schema languages, it's closest to RELAX. TREX is not tied to any particular datatyping language.  The idea is to enable you to use  whatever datatyping language you want.  XML Schema datatyping could be used, for example.

The primary concept in TREX is the  pattern. An unordered collection of attributes and an ordered sequence of elements and characters are matched jointly against a pattern with respect to an environment. An environment is a mapping from names to patterns together with a possibly null reference to a parent environment. The result of matching is true or false. XML document is valid with respect to a TREX pattern if an empty collection of attributes and a sequence containing just the document element of the document matches the pattern with respect to an empty environment.

You can learn more about TREX by reading the specification at http://www.thaiopensource.com/trex/.  James Clark has also provided a tutorial.