WordPress at a glance
filter is not described

pre_user_first_name filter-hook . WP 2.0.3

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

Usage

add_filter( 'pre_user_first_name', 'filter_function_name_3472' );
function filter_function_name_3472( $first_name ){
	// filter...

	return $first_name;
}
$first_name(string)
The user's first name.

Where the hook is called

wp_insert_user()
wp-includes/user.php 1590
$meta['first_name'] = apply_filters( 'pre_user_first_name', $first_name );

Where the hook is used (in WP core)

wp-includes/default-filters.php 17
add_filter( $filter, 'sanitize_text_field' );
wp-includes/default-filters.php 18
add_filter( $filter, 'wp_filter_kses' );
wp-includes/default-filters.php 19
add_filter( $filter, '_wp_specialchars', 30 );