Yeah, I failed to make the pattern case insensitive. Here's a fixed version that also handles S/5: sed -E -e '/^[A-FIOSa-fios]{6}$/!d; y/abcdefiosIOS/ABCD

Yeah, I failed to make the pattern case insensitive. Here's a fixed version that also handles S/5: sed -E -e '/^[A-FIOSa-fios]{6}$/!d; y/abcdefiosIOS/ABCDEF105105/; s/^/#/' /usr/share/dict/words