WordPress at a glance
filter is not described

pre_comment_user_agent filter-hook . WP 1.5.0

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

Usage

add_filter( 'pre_comment_user_agent', 'filter_function_name_2323' );
function filter_function_name_2323( $comment_agent ){
	// filter...

	return $comment_agent;
}
$comment_agent(string)
The comment author's browser user agent.

Where the hook is called

wp_filter_comment()
wp-includes/comment.php 1841
$commentdata['comment_agent'] = apply_filters( 'pre_comment_user_agent', ( isset( $commentdata['comment_agent'] ) ? $commentdata['comment_agent'] : '' ) );

Where the hook is used (in WP core)

Does not used.