Check String for values other than those allowed. Java

Question

I wrote a program that asks the use to input a string of 3 characters, which could be a combination of [a, b, v, ^];

Here's what I have so far:

do {
    System.out.print("Enter a two variable logical expression, no spaces: ");    // 'v' for or, '^' for and.
    expression = type.nextLine();
}
while ((expression.length() != 3) || !((expression.toUpperCase()).matches("[AB^V]")));

The loop is only suppose to continue when the string is not 3 characters long, or a character in the string is not allowed.

I entered a test string of: a^b, but the loop just keeps going.

How do I fix this?

Thanks.

Answer 1

The correct regular expression is [AB^V]+ . Indeed the original expression would only match strings of length 1.

((expression.toUpperCase()).matches("[AB^V]+"))

^ does not need to be escaped, since it is between [ and ] (ie in a character class).

Edit : actually ^ should be escaped if it was at the beginning of the character class (like in [\\\\^ABV]+ ), but not when it is preceded by other characters.