void main()

{

    import std.stdio : writefln;

    int[char[2]] aa;

    auto arr = "ABBBA";

​

    // Iterate over all pairs in the string and observe each pair

    // ('A', 'B'), ('B', 'B'), ('B', 'A'), ...

    // String slicing doesn't allocate a copy

    foreach (i; 0 .. arr.length - 1)

        aa[arr[i .. $][0 .. 2]]++;

​

    foreach (key, value; aa)

        writefln("key: %s, value: %d", key, value);

}

​

​