Question

What I’m trying to achieve is a code checker. Only the first 4 numbers are important, the other numbers can be any number. The form will be used for users to put in productcodes.

The problem is that if the variable changes to say, 5 numbers the variable is false.

See below example:

http://jsfiddle.net/MZfxs/3/

If the user puts in the numbers 3541 the box changes color, but if the user put in the remaining numbers the value is set to false.

Additionally I’m trying to make the box only change color when 13 numbers are inserted AND the first 4 numbers are matching, in that order.

Solved!
Working Example:
http://jsfiddle.net/MZfxs/8/

Answer 1

If I understood correctly, you need a field value validation and the requirement is the value should start from 4 numbers like 7514 or 9268. Here you can use a regular expression to validate input value like:

// 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...
    }
}

Also if your requirement is strict to length of 13 than you can modify the regular epression to

var re = /^(\d{4})(\d{9})$/;

and retrieve first 4 numbers in first group and rest 9 in second group:

var matches = re.exec(value);
if(!!matches) {
    var first4Digits = matches[1],
        rest9Digits = matches[2];
    // ...
    // Also in this way you'll not need to check value.length to be 13.
}