A memory efficient regular expression compression algorithm is devised for deep packet inspection. First, a parameter DR (distending rate) is defined to quantify the explosive quality of regular expressions. Then based on DR, a regular expression cutting algorithm is proposed to downsize the storage needs of individual regular expression, by detecting and confining the parts which cause DFA (deterministic finite automaton) states’ exponential growth. Then according to the relation of different regular expressions, a selective grouping algorithm is introduced for regular expression sets, which could cut down the number of finite automata, and reduce the runtime memory consumption.