1. A network intrusion detection apparatus comprising: a network processor core for receiving packets from a network, and transmitting payloads of the received packets to a Deterministic Finite Automata (DFA) engine; a detection rule converter for converting a Perl Compatible Regular Expressions (PCRE)-based detection rule, preset to detect an attack packet, into a detection rule including a pattern to which only PCRE grammar corresponding to the DFA engine is applied; and the DFA engine for performing PCRE pattern matching on the payloads of the packets based on the detection rule converted by the detection rule converter.