Another approach: Schematron

Built on XSLT, XPath Schematron emerged from the realization that XSLT could be used to test document structures, and developed into a simple language that relies on XPath.
Lots of little pieces Schematron schemas are collections of tests, not sets of rules. This has massive advantages for testing complex circumstances, but is really tedious for definining complete schemas.
Designed for combination Schematron was built to be used as a supplement to other schema languages, not just as a schema language of its own.
Human-intelligible messages Instead of generic "Line 47 of this document does not conform to the definition of the sprocket element" messages, you can create messages like "This book has fewer than three chapters. Please add additional chapters before submitting it to production." or "You've specified a price for this, but not a currency. Please add a currency if you're going to specify a price."