[xml] Validating a single xml node with a schema definition

[dhkuhl optonline net]

How can an xml node be validated against a schema definition before it is added to the xml document?

I would like to validate user input from a form field before going ontothe next field.  By doing this I won't have to rewrite the rulesdefined in the schema and if the rules change it would just be a schemachange and not a code change or both.

Thanks,

--dhk