Patterns with bounded treewidth

A pattern is a string consisting of variables and terminal symbols, and its language is the set of all words that can be obtained by substituting arbitrary words for the variables. The membership problem for pattern languages, i.e., deciding on whether or not a given word is in the pattern language of a given pattern is NP-complete. We show that any parameter of patterns that is an upper bound for the treewidth of appropriate encodings of patterns as relational structures, if restricted, allows the membership problem for pattern languages to be solved in polynomial time. Furthermore, we identify new such parameters.