string getLongestRegex(string a, string b, string c)
{
  const size_t n = a.size();
  int idx = -1;
  for (int i = 0; i < n; i++) {
    if (c[i] != a[i] && c[i] != b[i]) { idx = i; }
  }
  if (idx == -1) return "-1";
  string res;
  for (int i = 0; i < n; i++) {
   
    if (i == idx) {
      string cur = "[";
      for (int j = 'A'; j <= 'Z'; j++)  if (j != c[i]) cur += j;
      cur += "]";
      res += cur;
    } else {
      res += "[ABCDEFGHIJKLMNOPQRSTUVWXYZ]";
    }
  }
  return res;
}

C++
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