[libc++] `<regex>`: `\00` should be rejected

[Alcaro]

#include <regex>
#include <stdio.h>

int main() {
    try {
        std::regex r{"\\00"};
        puts("valid");
    } catch (const std::exception& e) {
        printf("not valid: %s\n", e.what());
    }
    try {
        std::regex r{"\\01"};
        puts("valid");
    } catch (const std::exception& e) {
        printf("not valid: %s\n", e.what());
    }
}

Expected: Reject them. 00 and 01 do not match DecimalIntegerLiteral in the JS spec, and lookahead can't be a digit either.
Actual: Both are valid. (Can't find what they're actually parsed as, though.)

https://godbolt.org/z/heM1o1aGe (MS-STL is busy fixing it microsoft/STL#5380)

[pinskia]

Note libstdc++ used to reject \0 and then it was changed in GCC 12 (and backported to 11/10). See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84110 .