/
    \x26\x67     # Starting sequence (let's call it AB)
    (?!\x26\x67) # Exclude double starting seqeuence, e.g. ABAB
    (?:          # non-capture
      .(?!\x26\x67) # Any character that's not followed by the sequence
    ){6}         # Need N-2 of these
    .{2}         # And the last two characters can be anything.
                 # note we know they're not AB because otherwise the
                 # (N-2)th item in the previous part wouldn't have matched
  /sx            # Enable comments, and make . mean everything