Right now, the search bar is very powerful and flexible, with search possibilities, allowing the user very sophisticated filtering of results. For example, the provision of searching with regular expressions and boolean syntax.
It would be nice - especially for the categories filter panel, since the search bar does permit a filter, with respect to folders - if the search bar in the filter panel went beyond just simple one-word searches, and allowed filtering on multiple words, considered on a Regex/Boolean-expression-basis.