Unlike \1..\99 notation, .group() is not constrained by 99.
It was constrained up to Python 3.4 (AssertionError: sorry, but this version only supports 100 named groups)
but not since 3.5 (9baa5b2, https://bugs.python.org/issue22437):

>>> re.search('()'*999 + '(y)', 'xy').group(99, 999, 1000)
('', '', 'y')
>>> _sre.MAXGROUPS
1073741823