std::ranges::mismatch

cppreference.com

Example

Run on Compiler Explorer

mirror_ends: (str: std::string_view) -> std::string_view == {
    end:= std::ranges::mismatch(str, str | std::views::reverse).in1;
    return (str.cbegin(), end);
    _ = end;
}

main: () = {
    std::cout << mirror_ends("abXYZba") << '\n'
              << mirror_ends("abca") << '\n'
              << mirror_ends("ABBA") << '\n'
              << mirror_ends("level") << '\n';

    using std::literals::string_view_literals::_;

    static_assert("123"sv == mirror_ends("123!@#321"));
    static_assert("radar"sv == mirror_ends("radar"));
}

Output

ab
a
ABBA
level