WordPress at a glance
filter is not described

pre_user_login filter-hook . WP 2.0.3

Filters the user_login, also known as the username, before it is added to the site.

Usage

add_filter( 'pre_user_login', 'filter_function_name_3541' );
function filter_function_name_3541( $user_login ){
	// filter...

	return $user_login;
}
$user_login(string)
The sanitized username.

Where the hook is called

In file: /wp-admin/user-new.php
pre_user_login
wp-admin/user-new.php 154
$new_user_login = apply_filters( 'pre_user_login', sanitize_user( wp_unslash( $_REQUEST['user_login'] ), true ) );
wp-includes/user.php 1481
$pre_user_login = apply_filters( 'pre_user_login', $sanitized_user_login );

Where the hook is used (in WP core)

Does not used.