std::regex_traits<CharT>::isctype

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std::regex_traits
 
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 unspecified matches lookup_classname()'s minimal support

See also

 gets a character class by name
(public member function)
[virtual]
 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
(function)
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