WordPress at a glance
filter is not described

pre_user_id filter-hook . WP 1.5.0

Filters the comment author's user id before it is set.

The first time this filter is evaluated, 'user_ID' is checked (for back-compat), followed by the standard 'user_id' value.

Usage

add_filter( 'pre_user_id', 'filter_function_name_3786' );
function filter_function_name_3786( $user_ID ){
	// filter...

	return $user_ID;
}
$user_ID(int)
The comment author's user ID.

Where the hook is called

wp_filter_comment()
wp-includes/comment.php 1828
$commentdata['user_id'] = apply_filters( 'pre_user_id', $commentdata['user_ID'] );
wp-includes/comment.php 1831
$commentdata['user_id'] = apply_filters( 'pre_user_id', $commentdata['user_id'] );

Where the hook is used (in WP core)

Does not used.