In this example, we use JavaScript to write validate the expiry date in a field. We also convert the postal code to all caps
<script>
function fixPC(){
var els = document.getElementsByName("PC");
var text = els[0].value.trim();
text = text.toUpperCase();
els[0].value = text;
}
</script>
<script>
function checkForm(){
var el = document.getElementById("expiration_date");
if (checkExpire(el))
outputMsg();
return false;
}
function checkExpire(field){
var arr = field.value.split("/");
var mm=arr[0], yy=arr[1];
if ( ! (1 <= mm && 12 >= mm &&
18 <=yy && 23 >= yy)){
return formErr(field, field.value);
}
return true;
}
function formErr(entry, msg){
alert(entry.name + ": " + msg + " is invalid.");
entry.focus();
entry.select();
return false;
}
function outputMsg(){
alert("All good!");
}
</script>