# works with both a keyed number and a track data
    my $card_num = $tender->ccnumber;
    $card_num =~ s/^.?         # Start sentinel
                    [bB]?      # Format code="B"
                    (          # Capture to $1
                        [^^]+  #     Primary account number
                    )          # End of card
                    .*         # Slurp up the rest
                    /$1/x;
    $card_num =~ s/^(.+)(.{4})$/('*'x(length $1)).$2/e;