Search / Replace keywords are re-usable from Search / Replace history menu.
To open search history menu, tap and drag the lens icon in the search window.
the words you used to search before are listed. Tap one of them to fill the search window.
Search history saves your used search keywords automatically up to 20. If you like to clear the history, tap "clear" button.
When the replace window opens, you can access replace history, too.
Tap "Replae History" button, then select one of the words from the list to reuse it without typing it again!