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 · ISSUE

docs: regex library non-capture support

closedby odewdneyopened 2026-03-07updated 2026-03-17

docs

Documentation URL

https://docs.micropython.org/en/latest/library/re.html

Description

The documentation states that non-capture groups are not supported:

non-capturing groups ((?:...))

But at https://github.com/micropython/micropython/blob/master/lib/re1.5/compilecode.c#L92 it looks like it is supported.

int capture = re[1] != '?' || re[2] != ':';

And

import re
re.match("(?:.)","1")

returns

<match num=1>

suggesting that it works.

Code of Conduct

Yes, I agree