1. A computer-implemented process for performing string transformations involving lookup operations in one or more relational tables, comprising: using a computer to perform the following process actions: establishing a relational table lookup expression language comprising a set of grammar rules defining expressions; generating a synthesis procedure which learns a set of expressions in the relational table lookup expression language that produces a prescribed output from one or more input string variables using said one or more relational tables based on a set of one or more input-output examples each comprising one or more input string variables and said prescribed output; and receiving one or more input string variables of a same type found in the set of one or more input-output examples, and producing the prescribed output using the synthesis procedure.