check first two number jquery
// Will work for " 123433 " or "12345634 ", etc.
var value = $(this).val(),
re = /^\s*(\d{4})(\d+)\s*$/, // better to initialize only once somewhere in parent scope
matches = re.exec(value),
expectedCode = 3541,
expectedLength = 13;
if(!!matches) {
var code = matches[1]; // first group is exactly first 4 digits
// in matches[2] you'll find the rest numbers.
if(value.length == expectedLength && code == expectedCode) {
// Change the color...
}
}