You can make the second argument to replace a function:
str = "testing one two three";
str = str.replace(/one/g, function(match) {
return match.toUpperCase();
});
That replaces the "one" with "ONE". The first argument to the function is the matched result from the regex. The return value of the function is what to replace the match with.
If you have any capturing groups in your regex, they'll be additional arguments to the function:
str = "testing one two three";
str = str.replace(/(on)(e)/g, function(match, group0, group1) {
return match.toUpperCase();
});
That does exactly what the first one does, but if you wanted to, you could see what was in the capturing groups. In that example, group0 would be "on" and group1 would be "e".