At the moment, _sre is a modified version of _sre.py from an old version of PyPy: https://github.com/nikhaldi/_sre.py.

This is s l o w. If you ctrl-C rustpython while it's running a decently complex script, there's a good chance that the traceback from the KeyboardInterrupt will have originated somewhere in _sre.py. While it's definitely feasible to implement _sre in Rust -- I was able to modify the constant-generation script to output Rust instead of C code -- both the C and Python versions of _sre don't really lend well to translation to Rust. The C version (_sre.c/sre_lib.h) uses lots of pointer arithmetic and a kind of "as long as we own an active reference to the str object, we can send pointers anywhere we want" approach that doesn't jive well with Rust's memory-safety rules. The Python version (Lib/_sre.py in the RustPython source tree) uses enough nested references (context.x_stack[n].context is context), generator functions, and dynamic typing that it's also tricky to translate to Rust.

If anyone wants to take a stab at this, I've left a scaffold of the implementation in the _sre-wip branch, that has the SRE constants available to Rust, and some of my sketches for how it might work, but no real functionality. Be warned, either route you go to emulate is tricky, so feel free to try whichever approach you think would be easier.