and to squash a little further:
perl -ne '/^(?=([abc].*){3})(?!.*([abc]).*\2).{8}k.$/&&print' /usr/share/dict/words perl -ne '/^(?=([abc].*){3})(?!.*([abc]).*\2).{8}k.$/&&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