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The invention provides a method and apparatus for advanced network intrusion detection. The system uses deep packet inspection that can recognize languages described by context-free grammars. The system combines deep packet inspection with one or more grammar parsers (409A-409M). The invention can detect token streams (408) even when polymorphic. The system looks for tokens at multiple byte alignments and is capable of detecting multiple suspicious token streams (408). The invention is capable of detecting languages expressed in LL (I) or LR (I) grammar. The result is a system that can detect attacking code wherever it is located in the data stream (408).