std::regex_traits<CharT>::value

#include <iostream>
#include <locale>
#include <regex>
#include <map>
 
// 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';
}