This operation moves the scrollback view backwards (older) so the top of the window is the next older line that matches the regex <regex>. If there are no matches going backwards, it will beep. If you omit <regex> it will re-use the last <regex> you provided to either search_back or search_forward.
This operation may put you in scrollback view, which means you need to use window scroll_end to go back to the normal scrolling view.
You can read the window help file for more information about views.