how to use .contains to select certain letters of the alphabet

Question

I need the user input to:
- have no numbers
- be 4 characters long
- only use certain letters of the alphabet [R, B, G, P, Y, O]
I have figured out how to do no numbers and only 4 character length, however, I can't seem to figure out how to restrict certain letters of the alphabet (everything other than R, B, G, P, Y, O.)

        guess = input.nextLine();
        guess = guess.toUpperCase();
        while (guess.length() != 4 || guess.contains("[0-9]") || guess.contains("[ACDEFHIJKLMNQSTUVWXZ]")) {
            System.out.println("Bad input! Try again");
            System.out.println("Use the form \"BGRY\"");
            guess = input.nextLine();
        }

This is the code I have so far, it doesn't seem to work

Answer 1

Do it as follows:

while(!guess.matches("[RBGPYO]{4}")) {
    // ...
}

Demo:

public class Main {
    public static void main(String s[]) {
        // Tests
        System.out.println(matches("RBGPYO"));
        System.out.println(matches("RBGP"));
        System.out.println(matches("R1BGP"));
        System.out.println(matches("ABCD"));
        System.out.println(matches("1234"));
        System.out.println(matches("BGPY"));
        System.out.println(matches("BYPG"));
    }

    static boolean matches(String input) {
        return input.matches("[RBGPYO]{4}");
    }
}

Output:

false
true
false
false
false
true
true