code: StringBuilder subBuf(int start, int end) {
  		if (start > end) {
  			return null;
  		}
  		StringBuilder ret = new StringBuilder(end - start + 1);
  		for (int i=start; i<=end; i++) {
  			ret.append(buf[i]);
  		}
  		return ret;
  	}
docs: scanPara 与 scanNoParse 存在 start > end 的情况