WordPress at a glance
filter is not described

validate_username filter-hook . WP 2.0.1

Filters whether the provided username is valid or not.

Usage

add_filter( 'validate_username', 'filter_function_name_9078', 10, 2 );
function filter_function_name_9078( $valid, $username ){
	// filter...

	return $valid;
}
$valid(true/false)
Whether given username is valid.
$username(string)
Username to check.

Where the hook is called

validate_username()
validate_username
wp-includes/user.php 1386
return apply_filters( 'validate_username', $valid, $username );

Where the hook is used (in WP core)

Does not used.