How about allowing a special character to be replaced by more than 1 special character?

I am looking at the checkboxes for the contol characters to search for. Each of the checkboxes has a caption to the right of it. At the end of the caption is the escape sequence for the contol character that the caption describes.

If you recognize the escape sequences in the Replacement field, then you will not have to change the User Interface. Just the processing of the Replacement field.