DB<83> $re= qr/(?:^|(.)(?!\1)) ([[:alpha:]]){2} (?!\2)/x

  DB<84> say join "\t", $_, $_ =~ $re for <{,x}AA{,A}{,y}>
AA              A
AAy             A
AAA
AAAy
xAA     x       A
xAAy    x       A
xAAA
xAAAy

  DB<85>
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Edit

Just realized, in the end it's very similar to the last solution in Re: regex to match double letters but not triple or more.

But somehow it works for me...???

Updates

improved character class and tests