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Showing content from https://en.cppreference.com/w/cpp/language/../keyword/../../cpp/regex/regex_traits/value.html below:

std::regex_traits<CharT>::value - cppreference.com

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 <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'ã',0}, {L'ä¸',1}, {L'äº',2},
                                            {L'ä¸',3}, {L'å',4}, {L'äº',5},
                                            {L'å',6}, {L'ä¸',7}, {L'å«',8},
                                            {L'ä¹',9}, {L'ï¼¡',10}, {L'ï¼¢',11},
                                            {L'ï¼£',12}, {L'ï¼¤',13}, {L'ï¼¥',14},
                                            {L'ï¼¦',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"é¢¨";
 
    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"\\uä¹å«ï½å«")))
        std::wcout << L"\\uä¹å«ï½å« with custom traits matched " << in << '\n';
}

Output:

\u98a8 matched é¢¨
\uä¹å«ï½å« with custom traits matched é¢¨

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