Javascript String between(left, right)

String.prototype.between = function (left, right) {
    var s = this;
    var startPos = s.indexOf(left);
    var endPos = s.indexOf(right, startPos + left.length);
    if (endPos == -1 && right != null)
        return new this.constructor('')
    else if (endPos == -1 && right == null)
        return s.substring(startPos + left.length);
    else/*from   ww w.  j  av a 2  s  .  c  om*/
        return s.slice(startPos + left.length, endPos);
};