In this paper we describe a scalable service for customized data validation and content assistance by means of domain-independent, user-provided sets of complex data constraints. We present an integrated architecture and a particular implementation that combines the use of existing grammar- and rule-based schema languages that allows providers to specify rules in a declarative manner. The integrated architecture provides a way to semi-automatically fill in form fields by calling the proposed service, which enumerates domains from the previously stored data validation constraints. Additionally, better error reporting can be achieved by leveraging structure from the rules' definitions. The proposed architecture has been shown to be practical and is in production use by a large organization, successfully fulfilling its role. © 2012 IEEE.