!](https://img-blog.csdnimg.cn/direct/bf9a3d781a8043c997593260c0a8306f.png)

第一部分的字符可以少于…

const str = "5F3Z-2e-9w";
const str1 = "2-5g-3-J";
function solution(num, str) {
  const arr = str.split("-");
  const head = arr[0];
  arr.shift();
  const tailUpperArr = arr.join("").toUpperCase().split("");
  const tail = tailUpperArr
    .map((char, index) => {
      if ((index + 1) % num === 0 && index + 1 !== tailUpperArr.length) {
        return char + "-";
      } else {
        return char;
      }
    })
    .join("");
  return `${head}-${tail}`;
}

console.log(solution(2, str1));
/* 
   4
   5F3Z-2e-9w  =>  5F3Z-2E9W 
   2
   2-5g-3-J => 2-5G-3J
 */