1. A deterministic finite state machine organised for the detection of positionally significant matches of characters in a string of characters, and comprising a multiplicity of states including an initial state, wherein the machine examines each character in turn to determine a exit transition for a current state of the machine to another state and wherein the machine: responds to an examination of the string of characters by executing in response to a first character at the commencement of the string a transition from the initial state to another state; has at least one state for every character position; includes a exit transition from each state for each character to another state; and possesses only forward exit transitions each from any of the states whereby the current state of the machine unambiguously represents a count of the number of characters from the commencement of the string.