XML + RegExp = Relax NG

Although it’s an “old” definition for validation of XML, now Relax NG is being considered to integrated inside Sun development mentioned by: Tim Bray. With this has a new revival interest on it.
We can see the beauty of simplicity on this project:

In science, strong theories tend to be simple, yet have almost infinite potential for complexity in application. RELAX NG is, because of its simplicity, one of those theories that is easy to explain, easy to implement, and generic and flexible enough to meet the most stringent requirements.

Eric van der Vlist, RELAX NG, O’Reilly ,2003.
more information can be found at: http://relaxng.org and its most important implementations at the moment in Java are Jing and The Sun Multi-Schema XML Validator (MSV).