$ perl -ne 'print if /^(?=.{8}k.$)(?=[^a]*a[^a]*$)(?=[^b]*b[^b]*$)(?=[^c]*c[^c]*$)/' /usr/share/dict/words [me@fedora ~]$ time perl -ne 'print if /^(?=.{8}k.$)(?=[^a]*a[^a]*$)(?=[^b]*b[^b]*$)(?=[^c]*c[^c]*$)/' /usr/share/dict/words | wc -l
17
real 0m0,250s
user 0m0,245s
sys 0m0,006s 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