You can also do:

    sed -e '/^........k.$/!d; /a/!d; /b/!d; /c/!d; /a.*a/d; /b.*b/d; /c.*c/d' /usr/share/dict/words 

I wouldn't expect it to be much faster than AWK, but not much slower either.

Interesting that the pipeline is the fastest! I would expect the sheer number of separate processes to slow things down considerably, but apparently it doesn't really matter. Probably because the first `grep` already filters out most of the dictionary, so there isn't a lot of I/O through the pipes.