Corrigendum: Inside the class of REGEX languages [6]

This corrigendum identifies an issue with a proof of a lemma found in the paper Inside the Class of REGEX Languages [M. L. Schmid, Int. J. Found. Comput. Sci. 24, 07 (2013)], and proposes a method to bridge the gap. The relevant part of the original paper aims to show that a certain variant of pattern expressions (where variables are substituted uniformly) is equivalent to an interesting variant of extended regular expressions, which treat undefined variable references as reference definitions (as opposed to the empty symbol or set, as is done usually). Using a new concept of necessarily initialised regex automata systems we prove this equivalence.