1. A method of providing a routing key interpreter for a signalling gateway, the method comprising: generating an input file comprising a plurality of input expressions and a plurality of routing keys corresponding to the input expressions, wherein the input expressions comprise parameters of signaling messages in accordance with a first signaling protocol; supplying the input file to a lexical analyser generator to generate a lexical analyser for use in analysing messages for routing keys, said routing keys useable to forward messages by a signaling gateway from one network to another network; and providing the routing key interpreter using the lexical analyzer, wherein the lexical analyser comprises executable codes.