But Δ has no latin-1 representation. So Δ currently cannot be used as a group name in bytes regex, although it is a valid Python identifier. So that's a bug.

I mean, if you insist of having group names as strings even for bytes regexes, then it is not reasonable to prevent them from going _in_.

b"(??<\xce\x94>)" is a valid utf-8-encoded bytestring, why wouldn't you accept it as a valid re pattern?

IMHO, either

- group names from byte regexes should be returned as bytes
- or any utf-8-encoded representation of a valid Python identifier should be accepted as a group name of a bytes regex pattern.