// A generic function for checking to see if an input element has
  // a  URL contained in it
  function checkURL( elem ) {
      // Make sure that some text was entered, and that it's
      // not the default http:// text
      return !elem.value || elem.value == 'http://' ||
          // Make sure that it looks like a valid URL
          /^https?:\/\/([a-z0-9-]+\.)+[a-z0-9]{2,4}.*$/.test(\; elem.value\; );\; \}$// Get an input element to check
  var elem = document.getElementById("url");
  
  // Check to see if the field is a valid URL
  if ( ! checkURL( elem ) ) {
      alert( "Field does not contain a URL." );
  }