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

[dhouck]

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