<p>Given the nature of client-server applications, most use some freeform interface, like web forms, to collect user input. The main difficulty with this approach is that all parameters obtained in this fashion need to be validated and normalized to protect the application from invalid entries. This is the problem addressed here: how to take client input and preprocess it before passing the data to a back-end, which concentrates on business logic. The method of implementation is a rule engine that uses Groovy internal domain-specific language (DSL) for specifying input requirements. We will justify why the DSL is a good fit for a validation rule engine, describe existing techniques used in this area and comprehensively address the related issues of accidental complexity, security, and user experience.</p> / Master of Science (MSc)