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The task Task: Map a finite string α over an infinite alphabet onto a finite word w over a binary alphabet such that w reflects the structure of α "optimally".

Most modern implementations of regular expression engines allow the use of variables (also called backreferences). The resulting extended regular expressions (which, in the literature, are also called practical regular expressions, rewbr, or regex) are able to express non-regular languages. The present paper demonstrates that extended regular-expressions… (More)

We study the expressiveness and the complexity of static analysis of extended conjunctive regular path queries (ECRPQs), introduced by Barceló et al. (PODS '10). ECRPQs are an extension of con-junctive regular path queries (CRPQs), a well-studied language for querying graph structured databases. Our first main result shows that query containment and… (More)

We study the problem of generalizing from a finite sample to a language taken from a predefined language class. The two language classes we consider are subsets of the regular languages and have significance in the specification of XML documents (the classes corresponding to so called <i>chain regular expressions</i>, Chares, and to <i>single occurrence… (More)

A nonerasing morphism σ is said to be weakly unambiguous with respect to a word w if σ is the only nonerasing morphism that can map w to σ(w), i. e., there does not exist any other nonerasing morphism τ satisfying τ (w) = σ(w). In the present paper, we wish to characterise those words with respect to which there exists such a morphism. This question is… (More)

In the present paper, we introduce a variant of Gold-style learners that is not required to infer precise descriptions of the languages in a class, but that must find descriptive patterns, i. e., optimal generalisations within a class of pattern languages. Our first main result characterises those indexed families of recursive languages that can be inferred… (More)

We study the inclusion problems for pattern languages that are generated by patterns with a bounded number of variables. This continues the work by Frey-denberger and Reidenbach (Information and Computation 208 (2010)) by showing that restricting the inclusion problem to significantly more restricted classes of patterns preserves undecidability, at least… (More)