The disclosed embodiments relate to validation of prior system functionality subsequent to modification using parallel regression testing of the old, pre-modified system and new, modified system, based on the processing of a set of previously processed inputs known to have produced valid results. More particularly, the disclosed embodiments provide a system/method for validating a modified transaction processing system to determine whether the modification thereto altered unmodified and previously operational functionality by selecting a set of transactions, previously processed by the unmodified system and known to have produced acceptable results, process them using test instances of both the unmodified and modified versions of the system, and compare the results to determine any differences, aside from those which are expected as a result of the modification. Unexpected differences may be indicative of the modification compromising previously operational functionality of the system which was not intended to be modified.
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