ABSTRACT

A regular expression is called unambiguous if every tape in the event can be generated from the expression in one way only. The flow-graph technique for constructing an expression is shown to preserve ambiguities of the graph, and thus, if the graph is that of a deterministic automaton, the expression is unambiguous. A procedure for generating a nondeterministic automaton which preserves the ambiguities of the given regular expression is described. Finally, a procedure for testing whether a given expression is ambiguous is given.

INDEX TERMS

Ambiguity of regular expressions, decidability of ambiguity, flow-graph algorithm, preservation of ambiguity of graphs, preservation of ambiguity of regular expressions.

CITATION

S. Even, S. Greibach, R. Book and G. Ott, "Ambiguity in Graphs and Expressions," in

*IEEE Transactions on Computers*, vol. 20, no. , pp. 149-153, 1971.

doi:10.1109/T-C.1971.223204

CITATIONS