When using regex_replace(), what't the correct way to back-reference a submatch when it's directly followed by another digit in the format string?

In the following code, I tried to use $1 to refer to the first submatch:

#include <iostream>
#include <string>
#include <regex>

using namespace std;

int main()
{
    string str {"1-2-3 4-5-6 7-8-9"};
    int r = 1;
    str = regex_replace(str, regex{R"((\d*\-\d*\-)\d*)"}, "$1" + to_string(r));
    cout << str << "\n";
    return 0;
}

What I expect is:

1-2-1 4-5-1 7-8-1

But it doesn't work because the actual format string passed to regex_replace() is $11 as if I were trying to refer to the 11th submatch.

So when using regex_replace(), what is the correct way to back-reference a submatch which is followed directly by another digit in the format string?

I tried using ${1} but it didn't work for every mainstream implementation that I tried.