In this case, you could add an additional criteria to compare the length of the strings. The one with the length closest to your target string would be 'source1'. I'm not sure if the idea will apply generally, but it's worth investigation.

Update: just noticed you mentioned the same exact thing in your question. Silly me for not reading carefully enough. Sorry about that. :-)

I don't think it would be too particularly "gross", though, to have two passes. After all, this is a heuristic, right? Heuristics are usually messy.