heh. I had a capture and $1 in there orginally, and then to be 'clever' changed it to $& to save a couple characters :)

As far as the positive look-behind, y, it's not necessary -- just (since it's zero-width) saves having to reference a second capture in the replacement: 
s/([a-z])([12])/ $1 . ($2 == 1 ? 2 : 1) /eig;

[download]
(i think the /i is less efficient, too.. too many different ways to write this one)

In reply to Re^3: replace a digit with another digit by davidrw
in thread replace a digit with another digit by kevyt

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