title: Show/Hide button if iframe "src" contains certain text
body: <p>I am trying to display a button if the src attribute of an iframe on the same page contains a certain text.</p>
  
  <p>See <a href="https://jsfiddle.net/hkh82p69/" rel="nofollow noreferrer">jsFiddle</a> for example.</p>
  
  <p>I am basically trying to only show the "download MP3" button if the iframe has a valid soundcloud url as src attribute.</p>
  
  <p>The one thing all valid soundcloud iframes have in common is: all src urls start with </p>
  
  <pre><code>//w.soundcloud.com/player/?url=https%3A%2F%2Fsoundcloud.com%2F
  </code></pre>