Validate user input to start with 2 letters and follow with 4 numbers

[Master]

I want to validate user input to start with 2 letters and follow with 4 numbers when user tries to add data to a submission form

[MayMusic]

You can have a JS like the example below in the Footer

 

<script>
var inputfield= document.getElementsByName('InsertRecordFIELDNAME')[0];
inputfield.onchange= function (){
var str=inputfield.value; 
if(str.length > 0 && str.length <7) {
var res = str.substring(0, 2);
var res2 = str.substring(2, 6);
if (!/^[a-zA-Z]+$/.test(res)) alert('It should start with 2 letters'); 
if (isNaN(res2)) alert('It should end with 4 numbers'); 
} else {
alert('the length is exceeding');
}
}
</script>