A method and system that ensures system security is disclosed. Specifically, the method and system formulate a finite automaton that corresponds to a number of patterns. Then, as data units are put through the finite automaton, suspected data units are identified. The suspected data units are the ones containing content that collectively matches one or more of the aforementioned patterns. To identify the suspected data units, the dependency relationships among various states in the finite automaton that have been compressed are relied upon. Depending on the result of identifying the suspected data units, different actions are performed.