Expressing an algorithm at compile time is so very different from runtime when you do it in template metaprogramming. There is so much visual noise (for my personal feeling at least), writing such code is easier than reading and understanding. Nothing wrong with that special case of regular expressions here, more a general observation.

constexpr seems to try and step in, but there still is a large codebase around which is hard to maintain and extend. IMHO D and zig show that it can be better to have compile- and runtime-syntax much closer together.