Building a regex-based password validator in Java is a useful way to ensure that users create passwords that meet specific security requirements. Here's one approach to building a regex-based password validator in Java:

public static boolean validatePassword(String password) {
    // The password must be at least 8 characters long and contain at least one uppercase letter, one lowercase letter, one number, and one special character
    String regex = "^(?=.*[A-Z])(?=.*[a-z])(?=.*\\d)(?=.*[!@#$%^&*()_+\\-={};':\"\\|,.<>?]).{8,}$";
    return password.matches(regex);
}

In this code, the validatePassword method takes a String argument representing a password and returns a boolean value indicating whether the password is valid or not. The regular expression used in this example matches passwords that meet the following criteria:

• At least 8 characters long
• Contains at least one uppercase letter
• Contains at least one lowercase letter
• Contains at least one number
• Contains at least one special character

The regular expression breaks down as follows:

• ^: The start of the string
• (?=.*[A-Z]): Positive lookahead for at least one uppercase letter
• (?=.*[a-z]): Positive lookahead for at least one lowercase letter
• (?=.*\d): Positive lookahead for at least one digit
• (?=.*[!@#$%^&*()_+\-={};':\"\\|,.<>?]): Positive lookahead for at least one special character
• .{8,}: Matches any character 8 or more times
• $: The end of the string

The matches method of the String class is used to compare the password with the regular expression. If the password matches the regular expression, the method returns true; otherwise, it returns false.

You can call this method from your Java code to validate passwords as needed. You can also adjust the regular expression to fit specific security requirements or password complexity guidelines.