std::regex_traits<CharT>::isctype (3) - Linux Manuals

std::regex_traits<CharT>::isctype: std::regex_traits<CharT>::isctype


std::regex_traits<CharT>::isctype - std::regex_traits<CharT>::isctype


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 some temporary value m of type std::ctype_base::mask in implementation-defined manner
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 returned true, true is returned by isctype().
3) Otherwise, checks whether c equals '_' and the bitmask f includes the result of calling lookup_classname() for the character class [:w:], in which case true is returned.
4) Otherwise, false is returned.


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.


The description above summarizes C++14; the C++11 phrasing required this function to return true for '_' in all cases (LWG_issue_2018)


// Run this code

  #include <iostream>
  #include <string>
  #include <regex>

  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';


  true true
  true false
  false false

demonstraits a custom regex_traits implementation of lookup_classname/isctype
// Run this code

  #include <iostream>
  #include <locale>
  #include <regex>
  #include <cwctype>

  // 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::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"


  alnums: 風の谷のナウシカ
  katakana: ナウシカ

See also

                 gets a character class by name
lookup_classname (public member function)

do_is classifies a character or a character sequence
                 (virtual protected member function of std::ctype<CharT>)
                 classifies a wide character according to the specified LC_CTYPE category
iswctype (function)