"[[]][]]"

%acc 0 !acc
%flag false !flag

len [
    get tochar dup
    "[" == [$acc 1 + !acc] if
    "]" == [$acc 1 - !acc] if
    $acc 0 < [true !flag] if
    ] for

print

$acc 0 # $flag or ( [" is NOT ok"] [" is OK"] ) if
print

" " input