Replace nth regex match in C

I've found this topic which only explains how to replace the first regex match. What about some n'th match?

Say I have a string and a regex like these:

std::string str  = "fast cast dust crust must last rust";
std::regex  expr = " .(a|u)st";

How can I replace, say, the third match with some other sequence? std::regex_replace allows to replace either all matches or the first one. I'm also thinking about std::regex_iterator, but still can't figure out how to apply it in this task. What are the most convenient ways to solve the problem?