[:xdigit:] does not work with std::wstring in a Cygwin environment

[Gans, Markus]

This seems to be an internal Cygwin error:

https://www.reddit.com/r/cpp_questions/comments/sp52gq/xdigit_does_not_work_with_stdwstring_in_a_cygwin/

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I have an unexpected behavior with Cygwin for the character class [:xdigit:]. The pattern matching for [:xdigit:] behaves like the pattern matching of [:digit:] when using a wide string. With `std::string` everything works fine.

Example:

    #include <iostream>
    #include <string>
    #include <regex>
    
    int main ()
    {
      std::cout << "Wide character string\n";
      std::wstring w_character = L"a";
    
      if ( regex_match(w_character, std::wregex(L"[[:xdigit:]]")) )
        std::cout << "'" << char(w_character[0]) << "' is a hex digit\n";
      else
        std::cout << "'" << char(w_character[0]) << "' is not a hex digit\n";
    
      std::cout << "----------------------\n"
                << "String with 1 byte character\n";
      std::string character = "a";
    
      if ( regex_match(character, std::regex("[[:xdigit:]]")) )
        std::cout << "'" << char(w_character[0]) << "' is a hex digit\n";
      else
        std::cout << "'" << char(w_character[0]) << "' is not a hex digit\n";
    
      return 0;
    }

Output in a Cygwin environment:

    Wide character string
    'a' is not a hex digit
    ----------------------
    Character string
    'a' is a hex digit

Output on Linux:

    Wide character string
    'a' is a hex digit
    ----------------------
    String with 1 byte character
    'a' is a hex digit

Question: Why does Cygwin not detect the letters a, b, c, d, e, and f as hexadecimal digits in a wide string?
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