WordPress at a glance
filter is not described

pre_user_nicename filter-hook . WP 2.0.3

Filters a user's nicename before the user is created or updated.

Usage

add_filter( 'pre_user_nicename', 'filter_function_name_6013' );
function filter_function_name_6013( $user_nicename ){
	// filter...

	return $user_nicename;
}
$user_nicename(string)
The user's nicename.

Where the hook is called

wp_insert_user()
wp-includes/user.php 1535
$user_nicename = apply_filters( 'pre_user_nicename', $user_nicename );

Where the hook is used (in WP core)

Does not used.