How to hook into user registration process Before user registers

Question

I need to add a field to the user registration form and then validate it before I add a new user to my WordPress

I know how to hook into the registration form via add_filter('register_form', 'my_function');

But how do I hook into the registration process before the user registers? Hence I do not want add_action('user_register','my_other_function');.

Answer 1

You need to create your own hook for registration_errors filter:

add_filter( 'registration_errors', 'wpse8170_registration_errors', 10, 3 );
function wpse8170_registration_errors( $errors, $sanitized_user_login, $user_email ) {
    if ( /* something happens */ ) {
        $errors->add( 'myexception_code', 'This is my message' );
    }

    return $errors;
}

Answer 2

Take a look at the WordPress Codex: Customizing The Registration Form

Theme and plugin developers can customize WordPress's built-in user registration page through the use of hooks.

Customizing the registration form involves utilizing the following three hooks:

1. register_form
   Allows rendering of new HTML form elements.
2. registration_errors
   Perform validation on form registration fields.
3. user_register
   Save custom form data.