>>> z="this is x and this is x and also this is x and x x"
>>> m = re.search('(x+)',z)
>>> m.group(0,1)
('x', 'x')
>>> m.group(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: no such group


>>> m = re.search('(x)',z)
>>> m.group(0,1)
('x', 'x')
>>> m.group(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: no such group

>>> m = re.search(r'(x)',z)
>>> m.group(0,1)
('x', 'x')
>>> m.group(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: no such group