Yes, thanks, I’m starting to see some light. :-) But:

In the first case, the engine starts from the left and finds a match, repeating (\w\w\w) 3 times:

Makes sense, but then, if the $1 match effectively ends up as (\w\w\w){3}, shouldn’t $1 contain uvXYZdabc? Or, conversely, if the ? quantifier following the * causes it to find the minimum string in uvXYZdabc matching (\w\w\w)*, shouldn’t that be "", the null string? Why does $1 match abc here?