title: Cannot replace single emoji from a string
body: <p>I have a string called <code>completionBar</code> which contains this:</p>
  <pre><code>let completionBar = `〚⬛⬛⬛⬛⬛〛`;
  </code></pre>
  <p>I'm trying to replace a single ⬛ with ⬜, so I tried:</p>
  <pre><code>completionBar.replace(/\U+2B1B/, 'U+2B1C');
  </code></pre>
  <p>but nothing happen, what I did wrong?</p>