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

; TREX s author, James Clark, 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 to create RELAX NG. All future development of TREX will take place as part of the RELAX NG effort.

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