WordPress at a glance
filter is not described

sanitize_user filter-hook . WP 2.0.1

Filters a sanitized username string.


add_filter( 'sanitize_user', 'filter_function_name_3398', 10, 3 );
function filter_function_name_3398( $username, $raw_username, $strict ){
	// filter...

	return $username;
Sanitized username.
The username prior to sanitization.
Whether to limit the sanitization to specific characters.
Default: false

Where the hook is called

wp-includes/formatting.php 2111
return apply_filters( 'sanitize_user', $username, $raw_username, $strict );

Where the hook is used (in WP core)

wp-includes/ms-default-filters.php 33
add_filter( 'sanitize_user', 'strtolower' );