print length pack 'H4C3H4b12H4'; # prints 11

    H4 :  4 nybbles =  2 bytes
    C3 :  3 chars   =  3 bytes
    H4 :  4 nybbles =  2 bytes
    b12: 12 bits!!  =  2 bytes
    H4 :  4 nybbles =  2 bytes
                      --
                      11 bytes