std::regex_traits::isctype

1	`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.

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.

Notes

The description above summarizes C++14; the C++11 phrasing required this function to return true for '_' in all cases (LWG issue 2018).

Example

#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';
}

Output:

true true
true false
false false

demonstraits a custom regex_traits implementation of lookup_classname/isctype.

#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::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 kanji
    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"
}