title: Meaning of question mark and x in a regex group
body: <p>I'm trying to learn Atom's syntax highlighting/grammar rules, which heavily use JS regular expressions, and came across an unfamiliar pattern in the <a href="https://github.com/atom/language-python/blob/master/grammars/python.cson#L657" rel="noreferrer">python grammar file</a>.</p>
  
  <p>The pattern starts with a <code>(?x)</code> which is an unfamiliar regex to me. I looked it up in an <a href="https://www.debuggex.com/r/G_diWHHYSnLypxML" rel="noreferrer">online regex tester</a>, which seems to say that it's invalid. My initial thought was it represents an optional left paren, but I believe the paren should be escaped here.</p>
  
  <p>Does this only have meaning in the Atom's coffeescript grammar, or am I overlooking a regex meaning?</p>
  
  <p>(This pattern also appear in the <a href="https://github.com/textmate/python.tmbundle/blob/master/Syntaxes/Python.tmLanguage#L1169" rel="noreferrer">textmate language</a> file that I believe Atom's came from).</p>