Of course, if you're familiar with the syntax, you can type it in the appropriate box.
The Build Pattern button brings up a wizard, the Pattern Builder, to help users unfamiliar with regular expression syntax.
The Build Condition button brings up another wizard, the File Finder, which builds up a find command.
By the way, notice the "hyperlink style" of the words multi-selection techniques in the widget. This is a quick way to get pop-up help on what these multi-selection techniques are (Shift-click and Ctrl-click).