Set the selection state of all documents in an index that match search request.

public long SelectItemsBySearch(long whichIndex, ref String searchRequest, bool isSelected);

Public Function SelectItemsBySearch(whichIndex As long, ByRef searchRequest As String, isSelected As bool) As long

You can also use SearchJob.WantResultsAsFilter to create a SearchFilter from the results of a search. Using SearchJob.WantResultsAsFilter is more flexible than SelectItemsBySearch because you can use any of the properties of the SearchJob (search flags, FileConditions, etc.) rather than just a simple search request.