1. A method of automatically generating transcoding instructions to locate and extract a subset of data from a selected web page of a web site, the method comprising: receiving an input describing the subset of data, said input comprising one or more data fields and, for each data field, respective field values from at least two sample web pages of a web page family for the web site; identifying a common pattern among the respective field values for the at least two sample web pages to define a pattern match between the respective data field values, said matching pattern defining a signature for the data field; and generating the transcoding instructions in accordance with the identified common pattern, the transcoding instructions comprising location information of field values for extracting the subset of data within a web page of the web page family, the transcoding instructions for expressing the extracted subset of data in a target format thereby to transcode the web page, wherein identifying the commonality among the respective field values comprises locating the respective field values in the respective web page code.