Which, if I understand it correctly, is the approach taken by Relax NG.

A RELAX NG schema specifies a pattern for the structure and content of an XML document. A RELAX NG schema thus identifies a class of XML documents consisting of those documents that match the pattern. http://xml.coverpages.org/relax.html

(caveat: my knowledge of this is limited to light reading)

Check out:

• Relax NG home
• xml.com compact syntax tutorial - which looks like regexen