Javascript to validate HKID number

When i looking for script to validate HKID, I find that most of the available script did not cater for two character prefix case and thus is not really valid to use. Therefore I have write my own which also allow to handle the bracket for the check digit.


function IsHKID(str) {
    var strValidChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

    // basic check length
    if (str.length < 8)
        return false;
 
    // handling bracket
    if (str.charAt(str.length-3) == '(' && str.charAt(str.length-1) == ')')
        str = str.substring(0, str.length - 3) + str.charAt(str.length -2);

    // convert to upper case
    str = str.toUpperCase();

    // regular expression to check pattern and split
    var hkidPat = /^([A-Z]{1,2})([0-9]{6})([A0-9])$/;
    var matchArray = str.match(hkidPat);

    // not match, return false
    if (matchArray == null)
        return false;

    // the character part, numeric part and check digit part
    var charPart = matchArray[1];
    var numPart = matchArray[2];
    var checkDigit = matchArray[3];

    // calculate the checksum for character part
    var checkSum = 0;
    if (charPart.length == 2) {
        checkSum += 9 * (10 + strValidChars.indexOf(charPart.charAt(0)));
        checkSum += 8 * (10 + strValidChars.indexOf(charPart.charAt(1)));
    } else {
        checkSum += 9 * 36;
        checkSum += 8 * (10 + strValidChars.indexOf(charPart));
    }

    // calculate the checksum for numeric part
    for (var i = 0, j = 7; i < numPart.length; i++, j--)
        checkSum += j * numPart.charAt(i);

    // verify the check digit
    var remaining = checkSum % 11;
    var verify = remaining == 0 ? 0 : 11 - remaining;

    return verify == checkDigit || (verify == 10 && checkDigit == 'A');
}