WordPress at a glance
filter is not described

pre_user_login filter-hook . WP 2.0.3

Filters a username after it has been sanitized.

This filter is called before the user is created or updated.

Usage

add_filter( 'pre_user_login', 'filter_function_name_6572' );
function filter_function_name_6572( $sanitized_user_login ){
	// filter...

	return $sanitized_user_login;
}
$sanitized_user_login(string)
Username after it has been sanitized.

Where the hook is called

wp_insert_user()
pre_user_login
wp-includes/user.php 1557
$pre_user_login = apply_filters( 'pre_user_login', $sanitized_user_login );
wp-admin/user-new.php 169
$new_user_login = apply_filters( 'pre_user_login', sanitize_user( wp_unslash( $_REQUEST['user_login'] ), true ) );

Where the hook is used (in WP core)

Does not used.