That doesn't solve the any number
of no more than two different characters
solutions posted so far will match up to 2 non-atcg characters,
but I read the spec as allowing for, say 50 Rs and 100 Ws to
be mixed in, but not one each of Y, H, and N.
I could be wrong, but I have a feeling that this one can't
be solved by a single regex. (Or at least not one written
by a mere mortal - there are some regex deities floating
around here who might prove me wrong...)
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