Case statement allow only alphabetic characters?

case "$1" in all) echo "$1" ;; [a-z][a-z][a-z][a-z][a-z][a-z]) echo "$1" ;; *) printf 'Invalid: %s\n' "$3" exit 1 ;; esac With this the only input accepted is all, and 6 characters. It won't accept 4 characters or more than 6. What I want to do here is to only allow characters, not digits or symbols, but of unlimited length. What is the correct syntax? Thanks