All I wanted was to extract only the 4 letters words from /usr/share/dict/words. It tooks me a full 10-minutes to figure out the regex for it to work with egrep.
$ egrep '^[A-Za-z]{4}$' /usr/share/dict/words Note to self: Must practice regex syntax more often.
Update (a slightly cleaner way, from @gabegundy):
$ egrep '^[a-Z]{4}$' /usr/share/dict/word