Duh, it's actually much simpler, and this one even captures the single character(s)! (Update 2: See Variable-Width Lookbehind (hacked via recursion))

/(.)(?!((?<=(?!\1).(?=(?2)).|(?=\1)..))|.*\1)/s

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<Reveal this spoiler or all in this thread>

In reply to Re^2: Regex: matching character which happens exactly once (updated) by haukex
in thread Regex: matching character which happens exactly once by LanX

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