Query with special chars throws sre_constants.error exception

Queries like "C++" are not working:

<pre>
  File "/home/mirec/.virtualenvs/django/lib/python2.7/site-packages/xapian_backend.py", line 647, in _do_highlight
    match_re = re.compile(match, re.I)
  File "/usr/lib64/python2.7/re.py", line 190, in compile
    return _compile(pattern, flags)
  File "/usr/lib64/python2.7/re.py", line 242, in _compile
    raise error, v # invalid expression
error: multiple repeat
</pre>