Significant progress has been made on the specification for 'RELAX NG' since the April 2001 announcement by the TREX and RELAX design teams declaring their intent to unify the two similar structure-validation languages. The OASIS Technical Committee originally chartered under the name TREX has been named RELAX NG, and key draft documents have been published as sketches for the new validation language. These include a RELAX NG Tutorial, a RELAX NG Formal Semantics specification, and a draft RELAX NG schema for RELAX NG. The goals for RELAX NG are summarized in a recent announcement from the TC: "Members of the OASIS TREX Technical Committee announced their decision to integrate TREX (Tree Regular Expressions for XML) and RELAX (REgular LAnguage description for XML) in order to collaborate on a unified lightweight specification for validating XML-based languages. They renamed their work RELAX NG. RELAX was initially developed at the Information Technology Research and Standardization Centre (INSTAC) in Japan, which advances Japanese national standards for XML under the auspices of the Japanese Standard Association (JSA). TREX was created by James Clark, widely regarded as one of the most prolific contributors to the field of structured information standards. Clark decided to continue development of his schema language at the OASIS XML interoperability consortium in March 2001. 'RELAX and TREX both focus on simplicity,' said James Clark, chair of what is now the OASIS RELAX NG Technical Committee. 'RELAX NG will remain straightforward and easy to use, incorporating the best of TREX and RELAX.' Said Murata Makoto, one of the original developers of RELAX: 'It is important to note that RELAX NG is not intended to replace the W3C XML Schema Recommendation. Instead, it represents a lightweight alternative to Schema. We believe that users are likely to adopt multiple schema languages, and many will find RELAX NG fills a very important need.' According to the OASIS technical committee, the specification offers a middle ground that will make RELAX NG a useful tool for many developers. The team is interested in facilitating conversion among DTDs, XML Schema and RELAX NG. 'RELAX NG fits in well with the W3C XML Schema Formal Description,' added Clark. 'Our hope is that RELAX NG will be a constructive influence on the future development of XML Schema'."

TREX description from James Clark's recent DDJ article: "...You can think of it [TREX] as DTDs in XML syntax minus some things and plus some others. TREX just does validation. DTDs mush together both validation and interpretation of the documents, providing various things like entities and notations. Mushing them together is problematic because often you want one thing but not the other. My work with XML and SGML has convinced me that what you need is good separation between these different things. I wanted to remove from DTDs the things that augment the information in the XML document. And I wanted to add in some of the things that I think XML DTDs have always been missing. One of the things XML DTDs removed from SGML DTDs was 'AND' groups, which allow you to have unordered content. The SGML AND groups had a bad reputation, and don't have quite the right semantics. TREX adds them back and tries to do them right. XML also radically simplified the kinds of mixed content that you're allowed because there's a problem with the way SGML does it. Instead of restricting it, TREX solves the problem..." See "A Triumph of Simplicity: James Clark on Markup Languages and XML. Markup Languages, the Standardization Process, and the Importance of Simplicity. [DDJ Interviews James Clark. Feature.]" By Eugene Eric Kim and James Clark. In Dr. Dobb's Journal Issue 326 (July 2001), pages 56-60.