> It's theoretically possible to build a regex parser in C++ templates, right?

You'd probably do large portions of it in "constexpr" types and functions rather than relying on recursive metaprogramming techniques.

They are also planning on adding overloading based on whether a function is constexpr, which is important if you want the same library to support both compile-time and run-time matchers.

So it's all theoretically possible already. But D has the advantage here in that it's not just possible but usable.