title: Error when trying to use pymysql with sqlalchemy sre_constants.error: nothing to repeat
body: <p>I tried to use pymsql with sqlalchemy using this code : 
      <pre>
      from sqlalchemy import create_engine
      engine = create_engine("mysql+pymsql://root:@localhost/pydb")
      conn = engine.connect()
      </pre></p>
  
  <p>and this exception is raised here is the full stack trace : 
      <pre>
      Traceback (most recent call last):
        File "D:\Parser\dal__init__.py", line 3, in 
          engine = create_engine("mysql+pymsql://root:@localhost/pydb")
        File "C:\Python33\lib\site-packages\sqlalchemy-0.9.2-py3.3.egg\sqlalchemy\engine__init__.py", line 344, in create_engine
        File "C:\Python33\lib\site-packages\sqlalchemy-0.9.2-py3.3.egg\sqlalchemy\engine\strategies.py", line 48, in create
        File "C:\Python33\lib\site-packages\sqlalchemy-0.9.2-py3.3.egg\sqlalchemy\engine\url.py", line 163, in make_url
        File "C:\Python33\lib\site-packages\sqlalchemy-0.9.2-py3.3.egg\sqlalchemy\engine\url.py", line 183, in _parse_rfc1738_args
        File "C:\Python33\lib\re.py", line 214, in compile
          return _compile(pattern, flags)
        File "C:\Python33\lib\re.py", line 281, in _compile
          p = sre_compile.compile(pattern, flags)
        File "C:\Python33\lib\sre_compile.py", line 498, in compile
          code = _code(p, flags)
        File "C:\Python33\lib\sre_compile.py", line 483, in _code
          _compile(code, p.data, flags)
        File "C:\Python33\lib\sre_compile.py", line 75, in _compile
          elif _simple(av) and op is not REPEAT:
        File "C:\Python33\lib\sre_compile.py", line 362, in _simple
          raise error("nothing to repeat")
      sre_constants.error: nothing to repeat
      </pre></p>