Javascript to validate HKID number

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');
}