Question

How can I construct a regular expression such that a password field I am making contains at least one number, at least one lowercase letter and at least one uppercase letter where order DOES NOT matter and anything can be in-between these characters?

I have tried this and got something similar: ".*\\d.*[a-z].*[A-Z].*"
but that requires that the number comes first, the lowercase second and the uppercase last.

Answer 1

Yours is not one of the use case regular expressions are particularly good at – if you’re stuck with regular expressions, I guess you have no choice but to enumerate the 6 cases:

.*[0-9].*[a-z].*[A-Z].*|.*[0-9].*[A-Z].*[a-z].*|.*[a-z] ... | ...

Check with actual java code would be easier (and more readable):

String  password = "";
boolean complies = password.matches(".*[0-9].*")
                && password.matches(".*[a-z].*")
                && password.matches(".*[A-Z].*");

or (uglier, but possibly faster):

boolean complies = false;
{
    boolean hasDigit     = false;
    boolean hasLowercase = false;
    boolean hasUppercase = false;
    for (int i = 0; i < password.length(); i++) {
        char c = password.charAt(i);
        hasDigit     |= '0' <= c && c <= '9';
        hasLowercase |= 'a' <= c && c <= 'z';
        hasUppercase |= 'A' <= c && c <= 'Z';
        if (hasDigit && hasLowercase && hasUppercase) {
            complies = true;
            break;
        }
    }
}