A data anonymization computer system selectively anonymizes data items from data structures prior to forwarding the data structures to a third-party network service. The data anonymization computer system identifies at least a respective data item of the data structure that meets a set of conditions, including at least a first condition in which at least a portion of the respective data item has a format that coincides with the predetermined format and replaces a set of characters of the respective data item having the format with a string of characters of a respective token of a pool of tokens. The data anonymization computer system forwards the data structures to the third-party network service with each of the respective data items having the string of characters of the respective token in place of the replaced set of characters.