jQuery

<script type="text/javascript">
  $(document).ready(function() {
 $('.geen-extra-spaties-toestaan').keyup(function() {
  var $th = $(this);
  $th.val($th.val().replace(/(\s{2,})|[^a-zA-Z']/g, ' '));
  $th.val($th.val().replace(/^\s*/, ''));
  });
 });
  </script>