A reprogrammable packet processing system for processing a stream of data is disclosed herein. A reprogrammable data processor is implemented with a programmable logic device (PLD), such as a field programmable gate array (FPGA), that is programmed to determine whether a stream of data applied thereto includes a string that matches a redefinable data pattern. If a matching string is found, the data processor performs a specified action in response thereto. The data processor is reprogrammable to search packets for the presence of different data patterns and/or perform different actions when a matching string is detected. A reconfiguration device receives input from a user specifying the data pattern and action, processes the input to generate the configuration information necessary to reprogram the PLD, and transmits the configuration information to the packet processor for reprogramming thereof.