I did something similar to this with C++ templates - it's Parsing Expression Grammar based, so not full regex, but enough for a lot of tasks:

  using sign      = Atoms<'+', '-'>;
  using digit     = Range<'0', '9'>;
  using onenine   = Range<'1', '9'>;
  using digits    = Some<digit>;
  using integer   = Seq<Opt<Atom<'-'>>, Oneof<Seq<onenine, digits>, digit>>;
  using fraction  = Seq<Atom<'.'>, digits>;
  using exponent  = Seq<Atoms<'e', 'E'>, Opt<sign>, digits>;
  using number    = Seq<integer, Opt<fraction>, Opt<exponent>>;

and I've confirmed that it does all get inlined and optimized on -O3.

JSON parser example here - https://github.com/aappleby/matcheroni/blob/main/examples/js...