In command mode (where you are when you hit : in normal mode), you can do ctrl-R /, which will expand to your last search term (other ctrl-R favorites are ” for your yank buffer, or % for the full path of the current window)
You actually don’t need to do that though. If you leave out the search term for :s, it will assume you want to use the last thing you searched for. so you can /searchregex, and then right after do :%s//replaceregex/ and it will use search regex to do the replace.