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

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

If the character sequence [first, last) represents the name of a valid character class in the currently imbued locale (that is, the string between [: and :] in regular expressions), returns the implementation-defined value representing this character class. Otherwise, returns zero.

If the parameter icase is true, the character class ignores character case, e.g. the regex [:lower:] with std::regex_constants::icase generates a call to std::regex_traits<>::lookup_classname() with [first, last) indicating the string "lower" and icase == true. This call returns the same bitmask as the call generated by the regex [:alpha:] with icase == false.

The following narrow and wide character class names are always recognized by std::regex_traits<char> and std::regex_traits<wchar_t> respectively, and the classifications returned (with icase == false) correspond to the matching classifications obtained by the std::ctype facet of the imbued locale, as follows:

The classification returned for the string "w" may be exactly the same as "alnum", in which case isctype() adds '_' explicitly.

Additional classifications such as "jdigit" or "jkanji" may be provided by system-supplied locales (in which case they are also accessible through std::wctype).

The bitmask representing the character classification determined by the given character class, or char_class_type() if the class is unknown.

Demonstrates a custom regex traits implementation of lookup_classname() / isctype():

#include <cwctype>
#include <iostream>
#include <locale>
#include <regex>
 
// This custom regex traits uses wctype/iswctype to implement lookup_classname/isctype.
struct wctype_traits : std::regex_traits<wchar_t>
{
    using char_class_type = std::wctype_t;
 
    template<class It>
    char_class_type lookup_classname(It first, It last, bool = false) const
    {
        return std::wctype(std::string(first, last).c_str());
    }
 
    bool isctype(wchar_t c, char_class_type f) const
    {
        return std::iswctype(c, f);
    }
};
 
int main()
{
    std::locale::global(std::locale("ja_JP.utf8"));
    std::wcout.sync_with_stdio(false);
    std::wcout.imbue(std::locale());
 
    std::wsmatch m;
    std::wstring in = L"風の谷のナウシカ";
    // matches all characters (they are classified as alnum)
    std::regex_search(in, m, std::wregex(L"([[:alnum:]]+)"));
    std::wcout << "alnums: " << m[1] << '\n'; // prints "風の谷のナウシカ"
    // matches only the katakana
    std::regex_search(in, m,
                      std::basic_regex<wchar_t, wctype_traits>(L"([[:jkata:]]+)"));
    std::wcout << "katakana: " << m[1] << '\n'; // prints "ナウシカ"
}

Output:

alnums: 風の谷のナウシカ
katakana: ナウシカ

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