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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../../cpp/locale/isxdigit.html below:

std::isxdigit(std::locale) - cppreference.com

template< class CharT >
bool isxdigit( CharT ch, const locale& loc );

Checks if the given character is classified as a hexadecimal digit by the given locale's std::ctype facet.

Returns true if the character is classified as a hexadecimal digit, false otherwise.

#include <iostream>
#include <locale>
#include <string>
#include <unordered_set>
 
struct gxdigit_ctype : std::ctype<wchar_t>
{
    std::unordered_set<wchar_t> greek_digits{L'α', L'β', L'γ', L'δ', L'ε', L'ζ'};
 
    bool do_is(mask m, char_type c) const override
    {
        return (m & xdigit) && greek_digits.contains(c)
            ? true // 6 first Greek small letters will be classified as digits
            : ctype::do_is(m, c); // leave the rest to the parent class
    }
};
 
int main()
{
    std::wstring text = L"0123456789abcdefABCDEFαβγδεζηθικλμ";
    std::locale loc(std::locale(""), new gxdigit_ctype);
 
    std::locale::global(std::locale("en_US.utf8"));
    std::wcout.imbue(std::locale());
 
    std::wcout << "Hexadecimal digits in text: ";
    for (const wchar_t c : text)
        if (std::isxdigit(c, loc))
            std::wcout << c << L' ';
    std::wcout << L'\n';
 
    std::wcout << "Not hexadecimal digits in text: ";
    for (const wchar_t c : text)
        if (not std::isxdigit(c, loc))
            std::wcout << c << L' ';
    std::wcout << L'\n';
}

Output:

Hexadecimal digits in text: 0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F α β γ δ ε ζ
Not hexadecimal digits in text: η θ ι κ λ μ

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