sed one-liner to delete any line that does not contain lowercase letters

Question

So, basically

THIS LINE WOULD BE DELETED

and

(THIS LINE WOULD ALSO BE DELETED)

but

Indeed, THIS LINE WOULD NOT

Answer 1

Quite a few ways. Think negatively:

sed '/[a-z]/!d'    # !x runs x if the pattern doesn't match
grep -v '[a-z]'    # -v means print if the regexp doesn't match
awk '!/[a-z]/'     # !expr negates expr

Answer 2

Try this:

sed '/[a-z]/!d' file