title: Regex exactly n OR m times
body: <p>Consider the following regular expression, where <code>X</code> is <em>any</em> regex.</p>
  
  <pre><code>X{n}|X{m}
  </code></pre>
  
  <p>This regex would test for <code>X</code> occurring <em>exactly</em> <code>n</code> or <code>m</code> times.</p>
  
  <p>Is there a regex quantifier that can test for an occurrence <code>X</code> exactly <code>n</code> or <code>m</code> times?</p>