Extending Regular Expressions with Homomorphic Replacement

We define H-expressions and EH-expressions as extensions of regular expressions by adding homomorphic and iterated homomorphic replacement as new operations, resp. The definition is analogous to the extension given by Gruska in order to characterize context-free languages. We compare the families of languages obtained by these extensions with the families of regular, linear context-free, context-free, and ETT0L languages. Furthermore, we present their closure properties with respect to TRIO operations and discuss the decidability status and complexity of fixed and general membership, emptiness, and the equivalence problem. Some of our proofs are only based on expressions, and thus can be used in order to get new proofs for regular sets, tcation-Status.     «

We define H-expressions and EH-expressions as extensions of regular expressions by adding homomorphic and iterated homomorphic replacement as new operations, resp. The definition is analogous to the extension given by Gruska in order to characterize context-free languages. We compare the families of languages obtained by these extensions with the families of regular, linear context-free, context-free, and ETT0L languages. Furthermore, we present their closure properties with respect to TRIO oper...     »