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filter is not described

sanitize_user filter-hook . WP 2.0.1

Filters a sanitized username string.

Usage

add_filter( 'sanitize_user', 'filter_function_name_8723', 10, 3 );
function filter_function_name_8723( $username, $raw_username, $strict ){
	// filter...

	return $username;
}
$username(string)
Sanitized username.
$raw_username(string)
The username prior to sanitization.
$strict(true/false)
Whether to limit the sanitization to specific characters.
Default: false

Where the hook is called

sanitize_user()
sanitize_user
wp-includes/formatting.php 1887
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' );