title: Enforcing a regex to be wrapped with quotation marks from both sides or not at all without duplicating the expression
body: <p>I have an expression that should either be wrapped with quotation marks from both sides or not at all, and I want to create a regex to capture this expression.<br>
  The point is, I don't want to duplicate the expression.<br>
  For example, if the expression is any single letter in english then the duplicated regex would be</p>
  <pre><code>^[a-z]|'[a-z]'$
  </code></pre>
  <p>Now both <code>a</code> and <code>'a'</code> would be accepted.</p>
  <p>Is there a way to define this regex without duplicating the expression?</p>