This is ugly and untested, but it's what came to mind. 
/^([a-z])(?!.*\1)  # letter, never followed by the same letter
  ([a-z])(?!.*\2)  # another
  ([a-z])(?=(?!\3).(?!\3).\3(?!.*\3))    # letter followed by two othe
+rs, then itself and never again
  ([a-z])(?!.*\4)  # another of the first type
  ([a-z])(?!.*\5)  # another
  \3               # the repeated char
  ([a-z])(?!\6)    # another unique char
  ([a-z])\z        # final char
/x;

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(update: changed ?: to ?=)
Caution: Contents may have been coded under pressure.

In reply to Re: Regex help by Roy Johnson
in thread Regex help by Anonymous Monk

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