CS2121 lab exercise 1 part a4 - hint

In the previous parts of this lab exercise, you just needed to match some of the characters on the lines you wanted, but here you need to match all of them - otherwise, how can you know that there is exactly one character in the word?
Try thinking about the problem again. Start by thinking about what you want to achieve and how you might go about achieving it, even if you don't know exactly how to write it down.

If you are still stuck, here are some more hints.