Formal Grammars for Linguistic Treebank Queries

There has been recent interest in looking at what is required for a tree query language for linguistic corpora. One approach is to start from existing formal machinery, such as tree grammars and automata, to see what kind of machine is an appropriate underlying one for the query language. The goal of the paper is then to examine what is an appropriate machine for a linguistic tree query language, with a view to future work defining a query language based on it. In this paper we review work relating XPath to regular tree grammars, and as the paper’s first contribution show how regular tree grammars can also be a basis for extensions proposed for XPath for common linguistic corpus querying. As the paper’s second contribution we demonstrate that, on the other hand, regular tree grammars cannot describe a number of structures of interest; we then show that, instead, a slightly more powerful machine is appropriate, and indicate how linguistic tree query languages might be augmented to include this extra power.