I don't know about golfing it down, but in terms of presenting a human-readable regex, it would somehow seem more intelligible to me if the whitespace were arranged differently: 
/^
  (.)(?!\1)  # any character not followed by itself
  (.)(?!\1|\2)  # any character not followed by itself or previous
  (.)(?!\1|\2|\3)   # likewise unique...
  (.)(?!\1|\2|\3|\4)
  (.)
  \3 (?!\1|\2|\3|\4|\5) # 6th character must be same as 3rd
  (.)(?!\1|\2|\3|\4|\5|\6)  # 7th and 8th must be unique
   .
 $/x

[download]
(Admittedly, it's still a bit of a mind-bender.)

Of course, having that handy regex generator makes the regex formatting and commentary a moot point -- so much nicer to allow people to use the simple "rhyme-scheme" alphabetics for specifying the target pattern, and keep the actual regex syntax a purely "script-internal" detail, hidden from human eyes.

In reply to Re^2: Regex help by graff
in thread Regex help by Anonymous Monk

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