A method and system for ensuring system security is disclosed. The method and system split a regular expression that corresponds to a number of patterns into sub-expressions. The dependency relationships among the finite automata that correspond to the sub-expressions are maintained. Then, as data units are put through these finite automata in a sequence that is based on the dependency relationships, suspected data units are identified. The suspected data units are the ones containing content that collectively matches one or more of the aforementioned patterns. Identification of the suspected data units is based on the merged results of the finite automata. Depending on the result of identifying the suspected data units, different actions are performed.