Abstract: A user interface displays: a first column comprising non-editable input strings retrieved from a data field; a second column comprising editable output strings initialized from the data field; and an expression window displaying a transformation function ƒ. The computer iteratively processes user inputs, each user input i providing a sample row transformation to edit an ith output string ti. Some user inputs i designate a contiguous substring ssi of the corresponding input string si. The contiguous substring expresses a causal basis for transforming the input string si. into the output string ti. The computer updates the transformation function ƒ according to the provided sample row transformations so that: ƒ(s1)=t1, . . . , ƒ(si)=ti; the transformation function ƒ specifies text or string position of at least one contiguous substring; and ƒ has minimal branching among possible transformation functions that satisfy the samples.