You will need to install a user script manager extension to install this script.
To open the expression editor, look for an edit icon next to the word "Selection" at the top of the RSel tab panel.
The editor is capable of parsing some natural language (e.g., 'is', 'greater than') and recognizes various abbreviations (e.g., mH for minor highway). You can also specify wildcards (e.g., Street = "St *"—note the space between St and *—which will select names such as "St Mary Rd" but not "Mark St" nor "St. Thomas Ave").
Since this was originally scripted for WME RSel Highlights, please post any problems or requests in the same thread here.