perl -ne '/^(?=([abc].*){3})(?!.*([abc]).*\2).{8}k.$/&&print' /usr/share/dict/words
[me@fedora ~]$ time perl -ne '/^(?=([abc].*){3})(?!.*([abc]).*\2).{8}k.$/&&print' /usr/share/dict/words | wc -l 6 real 0m0,118s user 0m0,112s sys 0m0,007s