<regex>: integer overflow on large backreference value

[pauldreik]

The following simple program leads to a signed integer overflow inside _Do_digits

#include <regex>

int main() {
  std::regex regex{ "\\3333333334", std::regex_constants::ECMAScript };
}

Compiling with clang, using undefined behaviour sanitizer gives the following report:

C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\regex:3965:14: runtime error: signed integer overflow: 333333333 * 10 cannot be represented in type 'int'

I verified this by using msvc and stopping in the debugger at the mentioned place in regex.

Expected behavior
I expected a std::regex_error exception to be thrown.

STL version
MSVC 16.11.2. I copied the latest regex from this repo into a local file, the same thing happened so it applies to the latest version as well.

Additional context
Both gcc 11 (with libstdc++) and clang 12 (with either libc++ or libstdc++) on linux throw a std::regex_error exception for this particular program (tested on godbolt). Here is a link: https://godbolt.org/z/obKn549da