Filter Hook: pre_user_last_name

<?php

add_action( 'register_form', 'additional_profile_fields' );
function additional_profile_fields() { ?>
    <p>
        <label><?php _e('First Name') ?><br />
        <input type="text" name="first_name" id="first_name" class="input" size="25" tabindex="20" /></label>
    </p>
    <p>
        <label><?php _e('Last Name') ?><br />
        <input type="text" name="last_name" id="last_name" class="input" size="25" tabindex="20" /></label>
    </p>
<?php }
 
add_action( 'register_post', 'add_register_field_validate_first_name', 10, 3 );
function add_register_field_validate_first_name( $sanitized_user_login, $user_email, $errors) {
    if (!isset($_POST[ 'first_name' ]) || empty($_POST[ 'first_name' ])) {
    	return $errors->add( 'firstnameempty', '<strong>ERROR</strong>: Please provide a first name.' );
    }    
}
add_action( 'register_post', 'add_register_field_validate_last_name', 10, 3 );
function add_register_field_validate_last_name( $sanitized_user_login, $user_email, $errors) {
    if (!isset($_POST[ 'last_name' ]) || empty($_POST[ 'last_name' ])) {
    	return $errors->add( 'lastnameempty', '<strong>ERROR</strong>: Please provide a last name.' );
    }    
}

add_action( 'user_register', 'insert_register_fields' );
function insert_register_fields( $user_id ) {
	
	$first_name = apply_filters('pre_user_first_name', $_POST['first_name']);
	$last_name = apply_filters('pre_user_last_name', $_POST['last_name']);
	
	update_user_meta( $user_id, 'first_name', $first_name );
	update_user_meta( $user_id, 'last_name', $last_name );
}