Here you go, this passes all of my test cases:

/^(?(?=((?(3)(?(?!.*\3)(?(?!((?<=(?!\3).(?=(?2)).
|(?=\3)..)))(*F))))(?(?=.)(?=(.)(?1)))))(*F))/sx

[download]

Simple!*

<Reveal this spoiler or all in this thread>

In reply to Re: Regex: matching character which happens exactly once by haukex
in thread Regex: matching character which happens exactly once by LanX

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