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.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 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
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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