my $rx = qr/
    ([abcd])           # a, b, c, or d -- stored in \1
    .*                 # well, whatever
    (?!\1)             # the next character can't be \1
    ([abcd])           # a, b, c, or d -- stored in \2
    .*                 # y'know, stuff
    (?!\1|\2)          # next thing not \1 or \2
    ([abcd])           # a, b, c, or d -- stored in \3
    .*
    (?!\1|\2|\3)       # next thing not \1, \2, or \3
    [abcd]             # you get the idea
    /x;