I didn't see the replies until your comment Andrew, but now that I come back to it I don't see why it would break for long words. I think I saw indexOf and thought charAt or some such thing. I don't know, this looks like it works to me.

Also I don't get the FUD about using regex, they're not slow in general. Some regex can be slow, but one that looks for a single digit is as fast as indexOf.

In this case it not only copies the array so it isn't modified (a best practice) but also handles when n is less than 3. Without it he'd return the whole signature.

It also doesn't "iterate n times".

Too much white space, you have unnecessary indentation!