You can search post by typing the keyword on the search bar. Alternatively, when you enter the Forum tool, you will be able to see the Search & Export option.
You can filter the search result by user name, email or id, user group name, headers, categories, discussion threads and also by a date range. When you click on Search button, you will see all the posts by all users according to your search result.
There is also Export To Excel button in this search page that exports the data to excel file of your search result.