Tagged Deterministic Finite Automata with Lookahead

Abstract: This paper extends the work of Laurikari and Kuklewicz on tagged deterministic finite automata (TDFA) in the context of submatch extraction in regular expressions. The main goal of this work is application of TDFA to lexer generators that optimize for speed of the generated code. I suggest a number of practical improvements to Laurikari algorithm; notably, the use of one-symbol lookahead, which results in significant reduction of tag variables and operations on them. Experimental results confirm that lookahead-aware TDFA are considerably faster and usually smaller than baseline TDFA; and they are reasonably close in speed and size to ordinary DFA used for recognition of regular languages. The proposed algorithm can handle repeated submatch and therefore is applicable to full parsing. Furthermore, I examine the problem of disambiguation in the case of leftmost greedy and POSIX policies. I formalize POSIX disambiguation algorithm suggested by Kuklewicz and show that the resulting TDFA are as efficient as Laurikari TDFA or TDFA that use leftmost greedy disambiguation. All discussed algorithms are implemented in the open source lexer generator RE2C.