title: Python fails to compile a regex
body: <p>I'm trying to detect all <code>set</code> from a cmake file using a python regex, fo the file below:</p>
  
  <pre><code># Library to include
  set(LIB_TO_INCLUDE 
          a
          b
          c)
  
  # comon code (inclusion in source code)
  set(SHARED_TO_INCLUDE d e f)
  
  # Library to include
  set(THIRD_PARTY g h)
  </code></pre>
  
  <p>I'd like to retrieve:</p>
  
  <pre><code>LIB_TO_INCLUDE a b c
  SHARED_TO_INCLUDE d e f
  THIRD_PARTY g h
  </code></pre>
  
  <p>I tested the regex <code>set\((?s:[^)])*?\)</code> (get all but <code>)</code> items following <code>set(</code>) using regex101.com (see <a href="https://regex101.com/r/aB5tX2/1" rel="nofollow">https://regex101.com/r/aB5tX2/1</a>), it apparently does what I want.</p>
  
  <p>Now when I try to run <code>re.compile(r'set\((?s:[^)])*?\)')</code> from Python, I get the error:</p>
  
  <pre><code>  File "private\python_scripts\convert.py", line 34, in create_sde_files
      pattern = re.compile(r'set\((?s:[^)])*?\)')   File "b:\dev\vobs_ext_2015\tools_ext\python\Python34_light\lib\re.py", line 223, in compile
      return _compile(pattern, flags)   File "b:\dev\vobs_ext_2015\tools_ext\python\Python34_light\lib\re.py", line 294, in _compile
      p = sre_compile.compile(pattern, flags)   File "b:\dev\vobs_ext_2015\tools_ext\python\Python34_light\lib\sre_compile.py", line 568, in compile
      p = sre_parse.parse(p, flags)   File "b:\dev\vobs_ext_2015\tools_ext\python\Python34_light\lib\sre_parse.py", line 760, in parse
      p = _parse_sub(source, pattern, 0)   File "b:\dev\vobs_ext_2015\tools_ext\python\Python34_light\lib\sre_parse.py", line 370, in _parse_sub
      itemsappend(_parse(source, state))   File "b:\dev\vobs_ext_2015\tools_ext\python\Python34_light\lib\sre_parse.py", line 721, in _parse
      raise error("unknown extension") sre_constants.error: unknown extension
  </code></pre>
  
  <p>Is this kind of regex not supported by Python?</p>