< cpp‎ | regex‎ | regex traits
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.

[edit] Parameters

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

[edit] 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.

[edit] Example

#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},
int main()
    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';


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