I have a list of permutations of
ABCabc and I want to remove any permutations with two adjacent characters with different cases (uppercase and lowercase).
ABCacbmust be removed because
Cacontains two adjacent characters with different cases.
AbBcaCmust be removed as well.
Here is my attempt but without filtering.
Select[StringJoin /@ Permutations[Characters@"ABCabc"],....]