javascriptでサブドメインに使える文字列か判定する

幾つかの条件をjsで判定

function validSubDomainP(subdomain) {
    if (subdomain.length < 3 || subdomain.length > 63) {
        return false;           // too long or too short
    }
    else if (!subdomain.match(/^[a-zA-Z0-9\-]+$/)) {
        return false;           // container illegal characters
    }
    else if (subdomain.match(/(^-|-$)/)) {
        return false;           // begins with - or ends with -
    }
    else if (subdomain.match(/^[a-zA-Z][a-zA-Z]--/)) {
        return false;           // xx-xxxx is not permitted.
    }
    else {
        return true;
    }
};