Thank you. This is extremely helpful. To answer your question about the tricky case you mention: 'cases' should NOT match 'carer', as for my purpose these words differ in two positions. (even though both positions involve an R/S swap, I still the need matches to differ in exactly one position). 'cases' SHOULD match 'caser' and 'cares'. For the same reason, 'carer' SHOULD match both of the remaining two ('caser' and 'cares') And finally, 'caser' should NOT match 'cares', again because these words differ at more than one position. Does that clarify things?