std::regex_traits::isctype(3)

NAME
std::regex_traits::isctype - std::regex_traits::isctype

Synopsis
   bool isctype( CharT c, char_class_type f ) const;
   Determines whether the character c belongs to the character class identified by f,
   which, in turn, is a value returned by lookup_classname() or a bitwise OR of several
   such values.
   The version of this function provided in the standard library specializations of
   std::regex_traits does the following:
   1) First converts f to a value m of type std::ctype_base::mask.
   For each std::ctype category listed in the table in the page lookup_classname(), if
   the bits in f corresponding to the category are set, the corresponding bits in m
   will also be set.
   2) Then attempts to classify the character in the imbued locale by calling
   std::use_facet<std::ctype<CharT>>(getloc()).is(m, c).
     * If that returns true, isctype() will also return true.
     * Otherwise, if c equals '_', and f includes the result of calling
       lookup_classname() for the character class [:w:], true is returned, otherwise
       false is returned.

Parameters
   c - the character to classify
   f - the bitmask obtained from one or several calls to lookup_classname()

Return value
   true if c is classified by f, false otherwise.

Example
// Run this code
 #include <iostream>
 #include <regex>
 #include <string>
 int main()
 {
     std::regex_traits<char> t;
     std::string str_alnum = "alnum";
     auto a = t.lookup_classname(str_alnum.begin(), str_alnum.end());
     std::string str_w = "w"; // [:w:] is [:alnum:] plus '_'
     auto w = t.lookup_classname(str_w.begin(), str_w.end());
     std::cout << std::boolalpha
               << t.isctype('A', w) << ' ' << t.isctype('A', a) << '\n'
               << t.isctype('_', w) << ' ' << t.isctype('_', a) << '\n'
               << t.isctype(' ', w) << ' ' << t.isctype(' ', a) << '\n';
 }

Output:
 true true
 true false
 false false
   Demonstrates a custom regex traits implementation of lookup_classname() / isctype(): // Run this code
 #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
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 2018 C++11 the value of m was matches lookup_classname()'s minimal
                       unspecified support

See also
   lookup_classname gets a character class by name
                    (public member function)
   do_is classifies a character or a character sequence
   [virtual] (virtual protected member function of std::ctype<CharT>)
                    classifies a wide character according to the specified LC_CTYPE
   iswctype category
                    (function)