Date: Jan. 10, 2001
Link: http://www.thaiopensource.com/trex/
Source Author or Organization: James Clark, Thai Open Source Software Center Ltd

This is the home site for James Clark's Tree Regular Expressions for XML (TREX) language for validating XML documents. TREX is described as a Schema Language, but its purpose is "purely validation".

This site includes links to a TREX tutorial, the TREX specification, related topics including Xduce ("transduce") and REgular LAnguage description for XML (RELAX), and related resources such as a sample implementation in Java written on top of Simple Application Programming Interface for XML 2 (SAX2) and an Extensible Style Language Transformations (XSLT) stylesheet that converts RELAX Core schemas into TREX schemas.

TREX does not describe data types, although it is expected in the future that it might partner with a datatyping language. TREX also does not interpret the document or change the infoset of the document (pre-validation infoset is exactly the same as the post-validation infoset).

As Clark himself explains: "It's basically the type system of XDuce with an XML syntax and with a bunch of 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. It's not tied to any particular datatyping language; rather, the idea is that you can plug whatever datatyping language you want (eg XML Schemas Part 2)."

The primary concept in TREX is the pattern. A TREX pattern specifies a pattern for the structure and content of an XML document, identifying a class of XML documents consisting of those documents that match the pattern. A TREX pattern is itself an XML document. The language also has the concept of a name-class, which is a set of expanded-names.