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string getLongestRegex(string a, string b, string c) {
    int idx = a.size() - 1;
    for (; idx >= 0; idx--) {
        if (a[idx] == c[idx] || b[idx] == c[idx]) {
            continue;
        } else {
            break;
        }
    }
    if (idx < 0) {
        return "-1";
    }

    vector<string> arr(a.size());
    for (int k = 0; k < a.size(); k++) {
        string temp = "[";
        for (char ch = 'A'; ch <= 'Z'; ch++) {
            if (k == idx && (ch == c[idx] || ch == a[idx] || ch == b[idx])) {
                continue;
            }
            temp += ch;
        }
        temp += "]";
        arr[k] = temp;
    }

    string ans = "";
    for (auto it : arr) {
        ans += it;
    }

    return ans;
}