Describe the bug

The regex [\d-e] (character class containing the range \d to e) is accepted (treated as \d, and the literal characters - and e), contrary to the ECMA-262 spec (\d isn't a single character, so it can't be used like that)

Command-line test case

#include <regex>

int main()
{
    try {
        std::regex r("[\\d-e]");
        puts("it's legal");
    } catch (std::exception& e) {
        puts(e.what());
    }
    try {
        std::regex r("[b-a]");
        puts("it's legal");
    } catch (std::exception& e) {
        puts(e.what());
    }
}

https://godbolt.org/z/oMvEr5YTs

Expected behavior

Both should be illegal (currently, only the latter is rejected)

STL version

Ask Godbolt

Additional context

Feel free to close this one as wontfix, if you feel it's ossified into a vendor extension. As long as it's a conscious choice, I'm fine with whichever outcome.