Complex languages built on XML require support for validation that goes far beyond what can be provided using traditional schema languages like XML Schema. In this paper we look at the experience we made as part of the FpML Validation Working Group, which is specifying standard validation rules for the Financial Products Markup Language (FpML), a cross-industry standard for financial derivatives trades.

The issues surfaced in here are likely to be of interest to any organisation or standards body dealing with data that exhibits complex structure or semantics. We discuss the process for validation rule gathering that the working group followed, and the issues it has been facing. We then look at the two reference implementation languages, Schematron and CLIX, that we used to formally express the validation rules, and some of the requirements we found: validation of the documents against external data sources like relational databases, expressiveness requirements, and complex data type support.