Hi, (I also posted this basic message in the online forums) I'm trying to use an XPath 2.0 (regular expression) function within Schematron, and I'm seeming to have a problem with the supposedly optional namespace "fn" when I try to validate. The error message I get is: The URI http://www.w3.org/2003/11/xpath-functions does not identify an external Java class I've tried it with and without the namespaces, whether in <schema>, <ns>, or within assert@test... I've also specified in the options that I want XPath 2.0, and it's showing up correctly as 2.0 in the toolbar. I'm using the trial edition of Oxygen Home edition, though from the feature comparison chart at http://www.oxygenxml.com/feature_matrix.html , I don't see any feature missing there that I need for this... Any ideas of what I need to do? <?xml version="1.0" encoding="UTF-8"?> <schema xmlns="http://purl.oclc.org/dsdl/schematron" xmlns:tei="http://www.tei-c.org/ns/1.0" xmlns:fn="http://www.w3.org/2003/11/xpath-functions"> <ns prefix="tei" uri="http://www.tei-c.org/ns/1.0" /> <ns prefix="fn" uri="http://www.w3.org/2003/11/xpath-functions" /> <pattern id="Bahai_Date"> <rule context="tei:date/@calendar"> <assert test="fn:matches(., 'Bahai')">The date's calendar should be set to "Baha'i"</assert> </rule> </pattern> </schema> sincerely, Brett