Just above the tracks window is single line window for entering a search string. As you type in this window, the tracks displayed change based on what you type and the check boxes to the right of this window. The first of these is marked ~. If it is checked, then only tracks not containing the string you type are displayed. If the next box marked A=a is checked then search (and sort) ignores case. The remaining check boxes determine which fields in the track are included in the search.
Every playlist consists of two lists. One is the result of the current search and the other is the original full list. This allows you to search a manually edited list without losing the edits you made to the full list.
However, if you edit the results of a search, you will not be able to further change the search string without loosing the edits you made on the searched list. If you attempt to change the search string or check boxes, you will be asked if you really want to discard those edits. To preserve the edits on the searched list and modify the search string first make a copy of the edited list.
You can shuffle search results without loosing manual edits on the full list. However when you sort search results the full list is also sorted. If you attempt this and the full list has been manually edited, you will be given a warning and a chance to abort the sort.
When you copy a playlist, the copy created contains only the results of the current search. When you save a playlist, you have the option of saving the full list or the search results.
Search string are associated with each playlist. Selecting a playlist will restore the search string associated with it. These strings are preserved between program executions.