Michael Kay wrote:

Unfortunately schema-aware processing in XQuery and XSLT doesn't
give you this capability.

Is there a workaround?

Is there is no way for an XSLT program to express: "Hey, validate xyz
against the XML Schema and if xyz is not schema-valid then discard
it." No way to express that?

Can't you just not validate the input, then test within the stylesheet
as you are testing an atomic/simple type.

<xsl:if test=". instance of bk:ISBN-type">

If the input hasn't been validated you can just get the text content of
the node and check

Or of course you could more simply in this case not import the schema
and just check the regular expression directly in the stylesheet (then
it will work with non schema aware systems too)

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