Tree Regular Expressions for XML (TREX) is a simple schema language for XML

XML

Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....

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TREX's author, James Clark

James Clark (XML expert)

James Clark, is the author of groff and expat and has done much work with open-source software and XML. Born in London, and educated at Charterhouse and Merton College, Oxford, Clark has lived in Bangkok, Thailand since 1995, and is now a permanent resident...

, says: : A TREX pattern specifies a pattern for the structure and content of an XML document. A TREX pattern thus identifies a class of XML documents consisting of those documents that match the pattern. A TREX pattern is itself an XML document.

TREX has been merged with RELAX

Relax

Relax may refer to:In music:Albums*Relax , a 2003 album by Blank & Jones*Relax , a 2011 album by Das RacistSongs* "Relax" by Frankie Goes to Hollywood...

 to create RELAX NG

RELAX NG

In computing, RELAX NG is a schema language for XML, based on Murata Makoto's RELAX and James Clark's TREX. A RELAX NG schema specifies a pattern for the structure and content of an XML document...

. All future development of TREX will take place as part of the RELAX NG

RELAX NG

In computing, RELAX NG is a schema language for XML, based on Murata Makoto's RELAX and James Clark's TREX. A RELAX NG schema specifies a pattern for the structure and content of an XML document...

effort.