title: RegEx match if present
body: <p>Is there something in regex to match (conditionally) only if it exists ?</p>
  
  <p>e.g.</p>
  
  <p>the string maybe</p>
  
  <pre><code>question_1
  </code></pre>
  
  <p>or only</p>
  
  <pre><code>question
  </code></pre>
  
  <p>In case of the former, it should match the  integer in the end, but as in the case of the latter, it should leave it.</p>