`replace()` as an iterator

String.prototype.replace() with a global regular expression and a function has iterated over the interesting parts of a string since 1999, long before array extras like forEach() and filter(). It is rarely the best tool today, but it is still in a lot of code, for historical reasons and for a certain technical beauty. Let’s see how it works, what it is good for, and what it costs, so that a replace() whose result goes unused makes sense when we meet it.