Awhile ago I noticed a small difference in behavior between mw.ustring.gsub and string.gsub. When the pattern (argument 2) has no captures, string.gsub treats the whole match as capture 1, but mw.ustring.gsub does not. For instance, string.gsub('abc', '%a', '%1!') returns "a!b!c!", 3, but mw.ustring.gsub('abc', '%a', '%1!') throws an error: "invalid capture index %1 in replacement string".

This behavior of string.gsub isn't officially documented in the Lua 5.1 manual (here is its implementation in the source code), but maybe it is best for the two functions to be compatible in this way. The officially documented way to get the whole match is by capture 0 (which also works when there are captures): string.gsub('abc', '%a', '%0!') and mw.ustring.gsub('abc', '%a', '%0!') → "a!b!c!", 3.