DocBook RELAX Schema - An experimental RELAX Schema version of DocBook, with no official version. www.oasis-open.org/committees/docbook/relax/index.shtml
RELAX Core (ISO/IEC TR 22250-1) - Document Description and Processing Languages, Regular Language Description for XML (RELAX), Part 1: RELAX Core, 2000 October www.xml.gr.jp/relax
RELAX NG specification - Specification of RELAX NG, an alternative to XML Schema for validating XML documents. www.oasis-open.org/committees/relax-ng/spec-20011203.html
TREX - Tree Regular Expressions for XML - A language for validating XML documents. Site includes tutorial, specification and sample implementation in Java. www.thaiopensource.com/trex
