On the use of extended grammars

Extended context free grammars (ECFG) are context free grammars (CFG) in which the right side or each production may be an arbitrary finite state machine. There are ECFG subsets, ELR(k) and ELALR(k) which correspond to the context free subsets, LR(k) and LALR(k).Although ECFGs recognize the same set of languages as CFGs, ECFGs have some important advantages over CFGs. They tend to be smaller and more readable, containing fewer productions and non-terminals than their context free counterparts. In addition, they allow for a degree of separation of syntax and semantics which is probably impossible to obtain with CFGs.This paper briefly describes ELR parsing and then discusses PEG, a ELALR(1) parser generator currently under development at University of Alabama in Birmingham.