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in reply to Re: Regular expression for a comma separated string
in thread Regular expression for a comma separated string

It doesn't have to be. I didn't mention earlier, I did try to break it down but with no success. 
/^(?=[ACGT]{1,4},[ACGT]{1,4}$).*$/ &&
/^(?!.*(.).*\1.*,).*$/ &&
/^(?!,.*(.).*\1).*$/

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