std::regex_traits::value(3)

NAME
std::regex_traits::value - std::regex_traits::value

Synopsis
   int value( CharT ch, int radix ) const; (since C++11)
   Determines the value represented by the digit ch in the numeric base radix, given
   the currently imbued locale. This function is called by std::regex when processing
   Quantifiers such as {1} or {2,5}, Backreferences such as \1, and hexadecimal and
   Unicode character escapes.

Parameters
   ch - the character that may represent a digit
   radix - either 8, 10, or 16

Return value
   The numeric value if ch indeed represents a digit in the currently imbued locale
   that is valid for the numeric base radix, or -1 on error.

Example
// Run this code
 #include <iostream>
 #include <locale>
 #include <map>
 #include <regex>
 // This custom regex traits allows japanese numerals
 struct jnum_traits : std::regex_traits<wchar_t>
 {
     static std::map<wchar_t, int> data;
     int value(wchar_t ch, int radix) const
     {
         wchar_t up = std::toupper(ch, getloc());
         return data.count(up) ? data[up] : regex_traits::value(ch, radix);
     }
 };
 std::map<wchar_t, int> jnum_traits::data = {{L'〇',0}, {L'一',1}, {L'二',2},
                        {L'三',3}, {L'四',4}, {L'五',5},
                        {L'六',6}, {L'七',7}, {L'八',8},
                        {L'九',9}, {L'Ａ',10}, {L'Ｂ',11},
                        {L'Ｃ',12}, {L'Ｄ',13}, {L'Ｅ',14},
                        {L'Ｆ',15}};
 int main()
 {
     std::locale::global(std::locale("ja_JP.utf8"));
     std::wcout.sync_with_stdio(false);
     std::wcout.imbue(std::locale());
     std::wstring in = L"風";
     if (std::regex_match(in, std::wregex(L"\\u98a8")))
         std::wcout << "\\u98a8 matched " << in << '\n';
     if (std::regex_match(in, std::basic_regex<wchar_t, jnum_traits>(L"\\u九八ａ八")))
         std::wcout << L"\\u九八ａ八 with custom traits matched " << in << '\n';
 }

Output:
 \u98a8 matched 風
 \u九八ａ八 with custom traits matched 風