Two-Pass Greedy Regular Expression Parsing

Niels Bjørn Bugge Grathwohl, Fritz Henglein, Lasse Nielsen, Ulrik Terp Rasmussen
July, 2013
Proceedings 18th International Conference on Implementation and Application of Automata (CIAA)


We present new algorithms for producing greedy parses for regular expressions (REs) in a semi-streaming fashion. Our lean-log algorithm executes in time O(mn) for REs of size m and input strings of size n and outputs a compact bit-coded parse tree representation. It improves on previous algorithms by: operating in only 2 passes; using only O(m) words of random-access memory (independent of n); requiring only kn bits of sequentially written and read log storage, where k < 1/3 m is the number of alternatives and Kleene stars in the RE; processing the input string as a symbol stream and not requiring it to be stored at all. Previous RE parsing algorithms do not scale linearly with input size, or require substantially more log storage and employ 3 passes where the first consists of reversing the input, or do not or are not known to produce a greedy parse. The performance of our unoptimized C-based prototype indicates that the superior performance of our lean-log algorithm can also be observed in practice; it is also surprisingly competitive with RE tools not performing full parsing, such as Grep.