function validate_form(thisform)
{
for (var i=0; i < thisform.elements.length; i++) {
   var element = thisform.elements[i];
   if (element.name.length>=4) {
     if (element.name.substring(0,4)=="req_") {
       if (element.value==null||element.value=="") {
          alert("A required field ("+element.name.substring(4)+") is missing or invalid.");
          element.focus();
          return false
       }
     }
   }
}
return true
}