/^                              ## With the $, exactly 8 chars only
    (.)                         ## Any char as \1
    (?!\1)(.)                   ## Any char as \2 except \1
    (?!\1|\2)(.)                ## Any char as \3 except \1 or \2
    (?!\1|\2|\3)(.)             ## Any char as \4 except ...
    (?!\1|\2|\3|\4)(.)          ## Any char as \5 except ...
    \3                          ## Only whatever is in \3
    (?!\1|\2|\3|\4|\5)(.)       ## Any char as \6 except ...
    (?!\1|\2|\3|\4|\5|\6).      ## Any char except any char we've already seen
$/x