Regular expression types for XML

We propose regular expression types as a foundation for XML processing languages. Regular expression types are a natural generalization of Document Type Definitions (DTDs), describing structures in XML documents using regular expression operators (i.e., *, ?, |, etc.) and supporting a simple but powerful notion of subtyping.The decision problem for the subtype relation is EXPTIME-hard, but it can be checked quite efficiently in many cases of practical interest. The subtyping algorithm developed here is a variant of Aiken and Murphy's set-inclusion constraint solver, to which are added several optimizations and two new properties: (1) our algorithm is provably complete, and (2) it allows a useful "subtagging" relation between nodes with different labels in XML trees.

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