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std::regex_traits::isctype(3) C++ Standard Libary std::regex_traits::isctype(3)


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

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