BibDesk supports smart groups that dynamically change their content based on a list of user-specified criteria. To add a smart group, use the command "New Smart Group" (⌥⌘G) from the "File" menu. Additional search criteria can be added using the round "+" button, and removed using the round "-" button, while the popup menu at the top of the panel can be used to specify whether "all" or "any" of the additional criteria are matched. This feature offers more flexible searching than the standard Boolean quicksearch, and is always available.
The search criteria can later edit using thecommand "Get Info" (⌘I) in the "File" menu, or in the contextual menhu for the group. Its name can be changed after doubleclicking the group or choosing the appropriate contextual menu item.
Smart groups are stored in the `@comment' field of your BibTeX file. The format is that of a Mac OS X XML property list, but you should not modify the comment with a text editor, or rely on the format remaining consistent across versions of BibDesk. Some BibTeX editors may not preserve the contents of the `@comment' field, unfortunately (early versions of BibDesk didn't preserve it, either).
Text-based fields are compared case-insensitively, for the criteria specified. The BibDesk `Date-Modified' and `Date-Created' fields are handled specially. You can enter date ranges in various convenient ways. Date comparisons are automatically updated several times per day, so the relative comparison should always be true.