A method and system for ensuring system security is disclosed. The method and system utilize a first processing unit to split a regular expression that corresponds to a number of patterns into sub-expressions and maintain the dependency relationships among the finite automata that correspond to the sub-expressions. Then, the method and system utilize a second processing unit to move the data units through these finite automata in a sequence that is based on the dependency relationships to identify the suspected data units. 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.