Validating RDF with Shape Expressions

We propose shape expression schema (ShEx), a novel schema formalism for describing the topology of an RDF graph that uses regular bag expressions (RBEs) to define constraints on the admissible neighborhood for the nodes of a given type. We provide two alternative semantics, multi- and single-type, depending on whether or not a node may have more than one type. We study the expressive power of ShEx and study the complexity of the validation problem. We show that the single-type semantics is strictly more expressive than the multi-type semantics, single-type validation is generally intractable and multi-type validation is feasible for a small class of RBEs. To further curb the high computational complexity of validation, we propose a natural notion of determinism and show that multi-type validation for the class of deterministic schemas using single-occurrence regular bag expressions (SORBEs) is tractable. Finally, we consider the problem of validating only a fragment of a graph with preassigned types for some of its nodes, and argue that for deterministic ShEx using SORBEs, multi-type validation can be performed efficiently and single-type validation can be performed with a single pass over the graph.