The invention provides a computer-implemented method of analyzing symbols in a computer system, the symbols conforming to a specification for the symbols, in which the specification has been codified into a set of computer-readable rules; and, the symbols analyzed using the computer-readable rules to obtain patterns of the symbols by determining the path that is taken by the symbols through the rules that successfully terminates, and grouping the symbols according to said paths, the method comprising; upon receipt of a message at a computer, performing a lexical analysis of the message; and, in dependence on lexical analysis of the message assigning the message to one of the groups identified according to said paths. The invention also provides a computer programmed to perform the method and a computer program comprising program instructions for causing a computer to perform the method.