std::regex_traits::value (3) Linux Manual Page

std::regex_traits<CharT>::value – std::regex_traits<CharT>::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 <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';
}

Output:

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