I'm trying to validate VAT Codes entered into a form.

Whats the best way of checking the contents of a form input? For example, Austria's VAT code has the standard template of "U12345678" (a "U" followed by 8 numbers) and I want to check that the first character of the input value is "U"

Has anyone got any ideas?

My current Javascript is:

Code:
 var co = document.frmPayment.country.value;
 var vatcode = document.frmPayment.VATCode.value;
 var vatfirst = left(vatcode, 1);
 var vatlen = vatcode.length;
 
 alert("Country = '" + co + "'\nVAT Code = '" + vatcode + "'\nVAT Length = '"+ vatlen + "'\nFirst Character = " + firstchar + "'");
 
 if (co=='040' || co=='056' || co=='620' || co=='208' || co=='246' || co=='442' || co=='250' || co=='276' || co=='300' || co=='372' || co=='380' || co=='528' || co=='724' || co=='752') {
 // if any of the VAT code countries are selected
 
 
  if (vatlen==0) {
   alert("Please enter your VAT code");
   document.frmPayment.VATCode.focus();
   return false;
   
  } else {
   if (co == '040') {  // if the country is austria
	
	if (vatfirst != 'U') {
	 alert ("Please enter a valid VAT code.\n\n Eg. U12345678");
	 document.frmPayment.VATCode.focus();
	 return false;
	} 
   }
  }

 }
BTW, the country selection works fine and the length is detected fine.

Cheers,