Extended Description

#include
#include <stdio.h>

int main() {
std::regex rx("(cat)\1", std::regex::basic);
puts("Constructed without throwing: oops!");
try {
std::regex_match("(cat)", rx);
} catch (std::exception& e) {
puts("regex_match threw instead: oops!");
puts(e.what());
}
}

The output is:

Constructed without throwing: oops!
regex_match threw instead: oops!
The expression contained an invalid back reference.

I claim that the constructor should detect and throw on this pattern, rather than throwing at match time. The same issue happens with the "std::regex::grep" dialect, too.