QUERY · ISSUE

unix-ffi re module throws when looking at match containing optional groups

openby dhouckopened 2026-02-18updated 2026-02-18

Problem description

When a regex contains optional capture groups, for example (a)?b, the PCREMatch.group() method throws an overflow error for that group. I believe this is because PCRE is representing a nonexistent group as SIZE_MAX and re isnʼt checking for that.

Additional fallout

The unix-ffi json library requires the unix-ffi re library, and currently cannot parse numbers unless they have an integer part, a fractional part, and and exponential part; instead, it throws this same error.

To reproduce

>>> import re
>>> r = re.compile(r'(a)?b')
>>> m = r.match('b')
>>> (m.group(0), m.group(1))

Expected (cpython, and micropython built-in `re` library):

('b', None)

Actual (micropython `re` library)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/micropython/re.py", line 65, in group
OverflowError: overflow converting long int to machine word

CANDIDATE · PULL REQUEST

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

mergedby Josverlopened 2026-03-01updated 2026-03-19

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

5 comments

dpgeorge · 2026-03-02

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 · 2026-03-02

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 · 2026-03-02

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.

dpgeorge · 2026-03-19

@Josverl are you able to add those few tests I list above?

Josverl · 2026-03-19

sorry , got sidetracked.
You are correct that the tests are run in CI.
Added the tests as requested.