Fix unix-ffi/re: Fix OverflowError in re.groups().

[Josverl]

Adjust the re.groups() methods to properly handle the PCRE2_UNSET value for unmatched optional groups.
This change prevents OverflowError when calling groups() on a match with no content.

The return matches CPython's.

A test case for an empty string match has been added to verify expected behavior.

Fixes micropython/micropython#18877

[dpgeorge]

Thanks for this, the fix looks good!

Can I suggest adding more tests, ie:

m = re.match(r"(.)?", "")
assert m.group() == ""
assert m.group(0, 1) == ("", None)
assert m.groups() == (None,)
assert m.groups("default") == ("default",)

That will test group() behaviour and default value.

[Josverl]

Sure, happy to add.

I noticed that the readme mentions 'unsupported' for the unix-ffi folder, and I do not think these modules are included in CI testing either.

So AFIKT the tests need to be run manually, so still useful for validating.

[dpgeorge]

I do not think these modules are included in CI testing either.

I think it is run under CI, see tools/ci.sh:ci_package_tests_run.