increment left col ("carry") <- '0' -> '1' -> .. '9' (digits)
                                 ^                |
                                 |                |
                                 +----------------+

increment left col ("carry") <- 'a' -> 'b' -> .. 'z' (lower case)
                                 ^                |
                                 |                |
                                 +----------------+

increment left col ("carry") <- 'A' -> 'B' -> .. 'Z' (upper case)
                                 ^                |
                                 |                |
                                 +----------------+

##</code><code>##

c:\@Work\Perl\monks>perl -e "my $s = 'zZ9';  print qq{'$s' -> };  ++$s;  print qq{'$s'};"
'zZ9' -> 'aaA0'
c:\@Work\Perl\monks>perl -e "my $s = 'Zz9';  print qq{'$s' -> };  ++$s;  print qq{'$s'};"
'Zz9' -> 'AAa0'
c:\@Work\Perl\monks>perl -e "my $s = 'Zzzz9';  print qq{'$s' -> };  ++$s;  print qq{'$s'};"
'Zzzz9' -> 'AAaaa0'
c:\@Work\Perl\monks>perl -e "my $s = 'zzzz9';  print qq{'$s' -> };  ++$s;  print qq{'$s'};"
'zzzz9' -> 'aaaaa0'
c:\@Work\Perl\monks>perl -e "my $s = 'ZZZZ9';  print qq{'$s' -> };  ++$s;  print qq{'$s'};"
'ZZZZ9' -> 'AAAAA0'