Untitled

<body onload="replaceLinks()">

<script>
function replaceLinks() {
    var links = document.querySelectorAll('.restore a:link');
    for (var i = 0; i < links.length; i++) {
        if (/.(jpg|gif|png|jpe?g)$/.test(links[i].href)) continue;
        links[i].innerHTML = '<a href="/register.php" class="guestbutton">DOWNLOAD VIDEO</a>';
        links[i].href = 'register.php';
    }
}
</script>

<html>
  <head>
    <script>
        function replaceLinks() {
          var links = document.querySelectorAll('.restore a:link');
          for (var i = 0; i < links.length; i++) {
            if (/.(jpg|gif|png|jpe?g)$/.test(links[i].href)) continue;
              links[i].innerHTML = '<a href="/register.php" class="guestbutton">DOWNLOAD VIDEO</a>';
              links[i].href = 'register.php';
            }
          }
          replaceLinks(); // run the replace function
    </script>
  </head>
  <body>
    <a class="restore" href="#">A link</a>
  </body>
</html>

<html>
  <head>
  </head>
  <body>
    <a class="restore" href="#">A link</a>
    <script>
        function replaceLinks() {
          var links = document.querySelectorAll('.restore a:link');
          for (var i = 0; i < links.length; i++) {
            if (/.(jpg|gif|png|jpe?g)$/.test(links[i].href)) continue;
              links[i].innerHTML = '<a href="/register.php" class="guestbutton">DOWNLOAD VIDEO</a>';
              links[i].href = 'register.php';
            }
          }
          replaceLinks(); // run the replace function
    </script>
  </body>
</html>