Brzozowski introduced in 1964 a beautifully simple algorithm for regular expression matching based on the notion of derivatives of regular expressions. In 2014, Sulzmann and Lu extended this algorithm to not just give an answer whether or not a regular expression matches a string, but in case it matches also how it matches the string. This is important for applications such as lexing (tokenising a string). The problem is to make the algorithm by Sulzmann and Lu fast on all input without breaking its correctness.