Abstract: A data processing system comprises a database configured to provide a set of internet addresses of online retailer internet pages for association with respective regular expressions each defining a code string, relative to the code representing the online retailer internet page at the respective internet address, for extracting a product price from that online retailer internet page, and a respective expected price range associated with each online retailer internet page in the set; and a testing system configured to apply the regular expression to its respective online retailer internet page to extract a product price and to detect whether the extracted product price lies within the expected price range corresponding to that online retailer internet page; the testing system being configured, in the case that the extracted product price does not lie within the expected price range, to allocate a different regular expression, from a group of candidate regular expressions, to that online retailer internet page