code: public static String hump2Line(String str) {
          str = lowerFirst(str);
          Matcher matcher = humpPattern.matcher(str);
          StringBuffer sb = new StringBuffer();
          while (matcher.find()) {
              matcher.appendReplacement(sb, "_" + matcher.group(0).toLowerCase());
          }
          matcher.appendTail(sb);
          return sb.toString();
      }
docs: 驼峰转蛇形
  
  @param str
  待转换字符串
  @return 转换结果