A hierarchy of uniquely parsable grammar classes and deterministic acceptors

Abstract.

We introduce a new class of grammars called uniquely parsable grammars (UPGs). A UPG is a kind of phrase structure grammar having a restricted type of rewriting rules, where parsing can be performed without backtracking. We show that, in spite of such restriction to the rules, UPGs are universal in their generating ability. We then define three subclasses of UPGs. They are M-UPGs (monotonic UPGs), RC-UPGs (UPGs with right-terminating and context-free-like rules), and REG-UPGs (regular UPGs). It is proved that the generating abilities of the classes of M-UPGs, RC-UPGs, and REG-UPGs are exactly characterized by the classes of deterministic linear-bounded automata, deterministic pushdown automata, and deterministic finite automata, respectively. Especially, the class of RC-UPGs gives a very simple grammatical characterization of the class of deterministic context-free languages. Thus, these four classes form a deterministic counterpart of the classical Chomsky hierarchy.