Problematic macro in the header, custom string type compatible with nothing else in C, and I have no idea where the arena type comes from.
Having it magically deallocate memory is nice, but will confuse C programmers reading the caller.
Honestly, adding -lre to the linker is just much easier, and that library comes with docs too.
They are fully aware of -lre and assume that everyone else is too. This isn't about just achieving regex somehow. It's about avoiding the crt and gc and c++ in general while using an environment that normally includes all that by default.
You don't redefine new just to get regex. Obviously there must be some larger point and this regex is just some zoomed-in detail example of existing and operating within that larger point.