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string getLongestRegex(string a, string b, string c) {
  // Initialize a set to store all possible characters in the regex.
  set<char> chars;
  for (char c : a) {
    chars.insert(c);
  }
  for (char c : b) {
    chars.insert(c);
  }

  // Initialize a stack to store the characters in the regex.
  stack<char> s;

  // Iterate over all characters in the alphabet.
  for (char c = 'A'; c <= 'Z'; c++) {
    // If the character is in the set, push it onto the stack.
    if (chars.count(c)) {
      s.push(c);
    }

    // Otherwise, try to construct a regex that matches a and b but not c.
    else {
      // Create a string to store the regex.
      string regex = "[";

      // Iterate over the characters on the stack.
      while (!s.empty()) {
        regex += s.top();
        s.pop();
      }

      // Add the character c to the regex.
      regex += c;

      // Add "]" to the regex.
      regex += "]";

      // Check if the regex matches a and b.
      if (regex.find(c) == string::npos && regex.find(a) != string::npos && regex.find(b) != string::npos) {
        // The regex matches a and b but not c, so return it.
        return regex;
      }
    }
  }

  // No regex was found, so return -1.
  return "-1";
}