[BUG] .str.findall returning incorrect results when using a quantifier with a capturing group

[JamesMaki]

Describe the bug
cuDF .str.findall returns incorrect results with regex pattern that uses quanitifier with a capturing group.

Steps/Code to reproduce bug

reg_ex = r'(\d{4}\s){4}'
test_string = 'TEST12 1111 2222 3333 4444 5555'
import cudf
s = cudf.Series([test_string])
s.str.findall(reg_ex)
# returns 
# 0    [2 , 1 , 2 , 3 , 4 ]
# dtype: list

Note: Without the quantifier, shortening the pattern to just r'(\d{4}\s)', cuDF returns the correct results of [1111 , 2222 , 3333 , 4444 ].

Expected behavior

reg_ex = r'(\d{4}\s){4}'
test_string = 'TEST12 1111 2222 3333 4444 5555'
import re
re.findall(reg_ex, test_string)
# returns
# ['4444 ']

Environment overview (please complete the following information)
Tested in latest NGC Docker image on RTX 5880 Ada and A100 SXM, also confirmed this behavior exists in the latest nightly build.

[davidwendt]

This has uncovered a couple of issues. First, there is a bug in libcudf when handling nested quantifiers which is addressed in PR #16798
Second, the rules for matching in findall do not match the python definition for re.findall():

The result depends on the number of capturing groups in the pattern. If there are no groups, return a list of strings matching the whole pattern. If there is exactly one group, return a list of strings matching that group. If multiple groups are present, return a list of tuples of strings matching the groups. Non-capturing groups do not affect the form of the result.

The current behavior does not consider the existing of capture groups and so this will need to be addressed in a separate PR.
One sticking point is the handling of multiple groups which specifies returning tuples. Since tuples are not a libcudf type, the closest result would be either a flattened list column (consecutive row elements represent the tuple) or a nested list column.