Thank you, hippo, for catching the one-line issue, and thank you graff for this enormously helpful code. I responded to your question about the specific case you raised below but I'll post it here as well.

'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.

With all that in mind, does this code still find the matches I need?

Another question: am I right to say this program will accept as input a word list with words of different lengths and has a way of handling that? Earlier I had gotten the idea that I might have to split my word list into separate lists (words of length 2, words of length 3, and so on), which the program would then handle each in turn. But does this piece of code automatically handle that? I suppose this is solved in the regex line, but I'm still learning the ins and outs of regular expressions. Any further explanations as to how that regex line works would be greatly appreciated.