Unfortunately, the original command fails in this case. Below is an updated version which is probably slower, but more powerful, as well as a map for my favorite way to use it. Also included is a character-switching function I developed in the process, but ended up including inline for efficiency. It turns out that directly assigning to a character in a string (let string[i] = a:new[pos]) is illegal, probably because the rhs can include more than one byte. StrSwitchPos() takes care of that, but doesn't enforce a single-byte replacement.