How to restrict a text input to letters?

How can I restrict a text input to uppercase and lowercase letters? I have this : new_user.username = input('Username: ', 's');

Answer 1

[the cyclist]

Here's one way, taking advantage of the isletter command:

new_user.username = 0;
while ~all(isletter(new_user.username))
    new_user.username = input('Username: ', 's');
end