16. The computer-readable storage medium as recited in claim 15 , further comprising computer-executable instructions that, when executed, direct the computing system to derive a final result of the input message processing from at least one result of the sub-expression processing.
15. The computer-readable storage medium as recited in claim 11 , further comprising computer-executable instructions that, when executed, direct the computing system to perform acts comprising: prior to determining which filter sub-engine will process the input message, parse the input message into two or more sub-expressions; for each of the two or more sub-expressions, determine an appropriate filter sub-engine that can process the sub-expression; and direct each of the two or more sub-expressions to the appropriate filter sub-engine for processing.