title: Regex Erasing all except numbers with limited digits
body: <p>What I want to do is erase everything except <code>\d{4,7}</code> <em>only by replacing</em>.<br>
  Any ideas to get this?  </p>
  
  <p>ex)<br>
  <code>G-A15239L → 15239</code><br>
  (<strong>G-A</strong> and <strong>L</strong> should be selected and replaced by empty strings)<br>
  <code>now200316stillcovid19asdf → 200316</code><br>
  (<strong>now</strong> and <strong>stillcovid19asdf</strong> should be selected and replaced by empty strings)</p>
  
  <p>Also, replacing text is not limited as empty string.<br>
  substitutions such as $1 are possible too.</p>
  
  <p>Using Regex in 'Kustom' apps. (including KLCK, KLWP, KWGT)<br>
  I don't know which engine it's using because there are no information about it</p>