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A distance between two languages is a useful tool to measure the language similarity, and is closely related to the intersection problem as well as the shortest string problem. A parsing expression grammar (PEG) is an unambiguous grammar such that the choice operator selects the first matching in PEG while it can be ambiguous in a context-free grammar. PEGs are also closely related to top-down parsing languages. We consider two problems on parsing expression languages (PELs). One is the r-shortest string problem that decides whether or not a given PEL contains a string of length shorter than r. The other problem is the edit-distance problem of PELs with respect to other language families such as finite languages or regular languages. We show that the r-shortest string problem and the edit-distance problem with respect to finite languages are NEXPTIME-complete, and the edit-distance problem with respect to regular languages is undecidable. In addition, we prove that it is impossible to compute a length bound \(\mathcal {B}(G)\) of a PEG G such that L(G) has a string w of length at most \(\mathcal {B}(G)\).

We study the relative edit-distance problem between two input-driven languages. The relative edit-distance is closely related to the language inclusion problem, which is a crucial problem in formal verification. Input-driven languages are a robust subclass of context-free languages that enable to model program analysis questions within tractable time complexity. For instance, the language inclusion (or equivalence) problem is undecidable for context-free languages whereas the problem is solvable in polynomial time for input-driven languages specified by deterministic input-driven pushdown automata (IDPDAs) and is EXPTIME-complete for nondeterministic IDPDAs. Our main contribution is to prove that the relative edit-distance problem for two input-driven languages is decidable by designing a polynomial time IDPDA construction, based on the edit-distance, that recognizes a neighbourhood of a given input-driven language. In fact, the relative edit-distance problem between two input-driven languages turns out to be EXPTIME-complete when the neighbourhood distance threshold is fixed as a constant.