title: Create a regular expression for Cron statement
body: <p>I need to write an regular expression that matches <a href="https://en.wikipedia.org/wiki/Cron" rel="noreferrer">Cron time expressions</a>, like these ones:</p>
  
  <ul>
  <li>28 2 7 1 1</li>
  <li>28 2 7 1 *</li>
  <li>28 2 7 * *</li>
  <li>28 2 * * *</li>
  <li>28 * * * *</li>
  <li>* * * * *</li>
  </ul>