In a Text Search mode, the text to search for is entered in this field. Otherwise this field is disabled.
There is a Find Match/Find RegExp pushbutton to the immediate left of this field. This pushbutton is a toggle for the text search moding between a search for a text string (Find Match) or a regular expression (Find RegExp) text search.The label of the pushbutton indicates the current mode. Press the pushbutton to toggle the mode to the alternate state.
See the file Express.txt in the TXT directory for the rules of a regular expression search.