The regular expression language is documented here: http://www.w3.org/TR/xmlschema-2/#regexs
The documentation is not very easy to understand but you might be able to spot the problem with your syntax.
I have included a detailed RegularExpression tutorial in my XSD book which will be out in a few days. Keep a watch at http://www.sqlservercentral.com/articles/books/65843/
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