Change WordPress default registration error text [Error: This username is invalid because it uses illegal characters. Please enter a valid username.]

You can achieve this via ‘registration_errors‘ filter hook. The hook filters the WP_Error object that holds all the current errors. The code used will be something like this:

add_filter( 'registration_errors', function( $errors ){
    if( $errors->get_error_messages( 'invalid_username' ) ) {
        $errors->remove('invalid_username');
        $errors->add('invalid_username', '<strong>Error</strong>: My custom error message');
    }
    return $errors;
});

Please note that we checked the presence of the target error message by its code “invalid_username”, but this code may contain a different message for the username if it is found in the array of ‘illegal_user_logins’ which may contain a list of disallowed usernames, so if you need a different message for the disallowed username error you may use the second parameter “$sanitized_user_login” to check if it’s in the disallowed list and change the error message if so. Your code may be something like this:

add_filter( 'registration_errors', function( $errors, $sanitized_user_login ){
    
    if( $errors->get_error_messages( 'invalid_username' ) ) {
        
        // Get the list of disallowed usernames
        $illegal_user_logins = array_map('strtolower', (array) apply_filters( 'illegal_user_logins', array() ));
        
        // Set our default message
        $message="<strong>Error</strong>: My custom error message";
        
        // Change the message if the current username is one of the disallowed usernames
        if( in_array( strtolower( $sanitized_user_login ), $illegal_user_logins, true) ) {
            $message="<strong>Error</strong>: My custom error message 2";
        }
        
        $errors->remove('invalid_username');
        $errors->add('invalid_username', $message);
    }
    
    return $errors;
}, 10, 2);