replace() can take a function

I discovered today that String.replace() can take a function as the second argument. I checked the ECMAScript 5.1 reference and it looks like it matches the MDN docs (though it is much less readable, as any good spec should be ;-).

The arguments passed to the function begin with match (the matched substring), followed by one argument for each regex capture group, followed by offset (the offset of the matched substring), and finally string (the entire string).

So you can do something like this:

" test ".replace(/t(e)s(t)/, function(match, grp1, grp2, offset, str) {
return 't' + grp2 + 's' + grp1;
});

And it will swap the two capture groups, resulting in “ttse”. Of course, this example would be better written as " test ".replace(/t(e)s(t)/, "t$2s$1"); but I’m sure I’ll find a situation someday that calls for the added flexibility.