This isn't a bug, this is a limitation of the implementation. Regular expression parser is recursive and Python has a limit for recursion depth. You can increase this limit if needed.

>>> import sys, re
>>> sys.setrecursionlimit(2000)
>>> re.compile('(' * 500)
Traceback (most recent call last):
  File "/home/serhiy/py/cpython/Lib/sre_parse.py", line 437, in _parse_sub
    itemsappend(_parse(source, state))
  File "/home/serhiy/py/cpython/Lib/sre_parse.py", line 778, in _parse
    p = _parse_sub(source, state)
  File "/home/serhiy/py/cpython/Lib/sre_parse.py", line 437, in _parse_sub
    itemsappend(_parse(source, state))
...
  File "/home/serhiy/py/cpython/Lib/sre_parse.py", line 778, in _parse
    p = _parse_sub(source, state)
  File "/home/serhiy/py/cpython/Lib/sre_parse.py", line 437, in _parse_sub
    itemsappend(_parse(source, state))
  File "/home/serhiy/py/cpython/Lib/sre_parse.py", line 781, in _parse
    source.tell() - start)
sre_constants.error: missing ), unterminated subpattern at position 499