1. A method of operating a computer system for determining whether a computer-storable expression matches a filter, the method comprising the steps of: evaluating a first code structure representing the expression for determining a value of said expression; analyzing a second code structure representing the filter for determining the characteristics of the filter; and filtering said value according to the filter characteristics; wherein said first code structure is constructed from a plurality of first programming language code structure elements and said second code structure is constructed from a plurality of second programming language code structure elements; each second structure element corresponding to one of said first structure elements.