Suppose you are working with the BNC (XML edition) and you want to find all the comparative adjectives. They are marked up (as the BNC list shows) with AJC.
Put the search word like this:
(Why the * at left and right? Well, the tag will be right next to a word on its right in all BNC editions and might be next to one on the left too.)
and after you have selected the texts, you should be able to get something like this:
With the undefined tags visible:
See also: hide or show the mark-up