WordPress at a glance
filter is not described

register_url filter-hook . WP 3.6.0

Filters the user registration URL.

Usage

add_filter( 'register_url', 'filter_function_name_2454' );
function filter_function_name_2454( $register ){
	// filter...

	return $register;
}
$register(string)
The user registration URL.

Where the hook is called

wp_registration_url()
register_url
wp-includes/general-template.php 356
return apply_filters( 'register_url', site_url( 'wp-login.php?action=register', 'login' ) );

Where the hook is used (in WP core)

Does not used.