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How do XML schema validators work?

How XML schema validators work? If you know the architecture of any of them, can you describe it briefly? Do DOM parsers depend on SAX?

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Schema validators work by using the schema to build an internal representation of the grammar described in the schema. Typically, a state machine is created from the grammer and then the document is tested against the state machine.

One way to think of it is that it is a bit like a generalized programming language. Programming languages are typically described formally by grammars. Tools such as YACC allow programmers to describe grammers and then generate code to validate programs against the grammar. An XML validator is a bit like that except that the YACC part of the process happens "on the fly".

The best place to look if you are interested in XML validators is the branch of computer science that deals with parsing. The classic text is the "dragon book" by Aho and Ullman (

This was first published in January 2004

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