The text fields in the Search and Replace and HyperSearch dialog boxes remember the last 25 entered strings.
Previous strings can be recalled by pressing Up Arrow. If you have backtracked in the history list, Down Arrow can be used to recall later strings.
Pressing Control-Up Arrow or Control-Down Arrow will search backwards or forwards, respectively, for strings beginning with the text already entered in the text field. For example, if "Th" is already entered, and the strings "Three", "Bob", "They", "Fred" and "Them" are in the history, pressing Control-Up Arrow would recall "Three", "They", and "Them".
Clicking the text field with the right mouse button will display a pop-up menu of all previously entered strings; selecting one will input it into the text field. Holding down Control while clicking will display a menu of all previously entered strings that begin with the text already entered.