>>> re.findall("(.)(.)", "123456789") ## [Tuple(g(0),g(1)),…]
[('1', '2'), ('3', '4'), ('5', '6'), ('7', '8')]

>>> re.findall("((.)(.))", "123456789") ## [Tuple(g(0),g(1),g(2)),…]
[('12', '1', '2'), ('34', '3', '4'), ('56', '5', '6'), ('78', '7', '8')]

>>> re.findall("(..)", "123456789") ## [g(0),…]
['12', '34', '56', '78']

やっぱグルーピング関係あるかも orz