POSIX Regular Expression Parsing with Derivatives

We adapt the POSIX policy to the setting of regular expression parsing. POSIX favors longest left-most parse trees. Compared to other policies such as greedy left-most, the POSIX policy is more intuitive but much harder to implement. Almost all POSIX implementations are buggy as observed by Kuklewicz. We show how to obtain a POSIX algorithm for the general parsing problem based on Brzozowski’s regular expression derivatives. Correctness is fairly straightforward to establish and our benchmark results show that our approach is promising.