1. Point of proposal is Lex it self. This is one of possible implementation.
2. Open question: is there regex_search complexity guarantee?
3. Next step should be something like `yacc`. Covering Type-2 or Type-3
grammars will be very helpful.

And as usual allow small inconsistency and incompleteness.

Document: https://github.com/dmilos/cpp_proposal/blob/master/lex.md