Regex for matching subdomains in document.location.hostname

getSubdomain.js

var getSubdomain = function (baseDomain) {
    if (!baseDomain) {
        return null;
    }
    
    var regSub = new RegExp('^((?!' + baseDomain + '|www)([^.]*))', 'g');
    var _subdomain = document.location.hostname.match(regSub);
    if (_subdomain && _subdomain[0]) {
        return _subdomain[0];
    } else {
        return null;
    }
};