custom Input Mask on a textbox

Hi

I have a form with multiple fields from Table Projects

In the textbox Project_Code, I inserted an Input Mask that goes like this

L00LLL00

L are for letters, 0 for numbers

example: 'A24DEX77'

the 3 middle letters, 'DEX' in this case, are exactly the same as an existing field called City_Code in the same form

So I'd like to create an Input Mask like L00 me.City_Code 00

Obviously that line isn't good

Is it possible to create that Input Mask?

<input onkeypress="return Mask(this, '@##DEX##');">
<script>
function Mask(e, m){
if(event.keyCode>0){
e.focus();
var sel = document.selection;
var rng = sel.createRange();
rng.colapse;
if((sel.type == "Text" || sel.type == "None") && rng != null){
rng.text = String.fromCharCode(event.keyCode);
}
}
var ev = e.value;
if (event.keyCode>0){
e.value = '';
e.select();
}
var cIndx = -1;
var Out = '';
var cChar = '';
var mChar = ''
var nChar = '';
var Hold = '';
var rx;

for(var i=0; i<m.length; i++){
mChar = m.substr(i, 1)
if(mChar=='#' || mChar=='@'){
if(mChar=='#'){rx = /\d/;} else if(mChar=='@'){rx = /[a-zA-Z]/;}
cChar = ev.substr(++cIndx, 1);
if(m.substr(i+1,1)!='*')
cChar.toString();
while(cIndx < ev.length && !rx.test(cChar))
cChar = ev.substr(++cIndx, 1);
if (cIndx < ev.length && rx.test(cChar)){
Out += Hold + ev.substr(cIndx,1);
Hold = '';
if(m.substr(i+1,1)=='*' || m.substr(i+1,1)=='+'){
nChar = ev.substr(cIndx+1, 1);
while(cIndx < ev.length && rx.test(nChar)){
Out += nChar;
nChar = ev.substr(++cIndx+1, 1);
}
}
} else if (cIndx < ev.length && m.substr(i+1,1)=='*'){
--cIndx;
}
} else if(mChar!='*' && mChar!='+') {
if(ev.substr(cIndx+1, 1).toLowerCase()==mChar.toLowerCase()){
Out+=mChar;
cIndx++;
} else {
Hold += mChar;
}
}
if(cIndx >= ev.length){
e.value = Out;
return false;
}
}
e.value = Out;
return false;
}
</script>