std::regex_traits::lookup_classname(3)

NAME
std::regex_traits::lookup_classname - std::regex_traits::lookup_classname

Synopsis
   template< class ForwardIt >
   char_class_type lookup_classname( ForwardIt first,
   ForwardIt last,
   bool icase = false ) const;
   If the character sequence [first, last) represents the name of a valid character
   class in the currently imbued locale (that is, the string between [: and :] in
   regular expressions), returns the implementation-defined value representing this
   character class. Otherwise, returns zero.
   If the parameter icase is true, the character class ignores character case, e.g. the
   regex [:lower:] with std::regex_constants::icase generates a call to
   regex_traits<>::lookup_classname() with [first, last) indicating the string "lower"
   and icase == true. This call returns the same bitmask as the call generated by the
   regex [:alpha:] with icase == false.
   The following character classes are always recognized, in both narrow and wide
   character forms, and the classifications returned (with icase == false) correspond
   to the matching classifications obtained by the std::ctype facet of the imbued
   locale, as follows:
   character class std::ctype classification
   "alnum" std::ctype_base::alnum
   "alpha" std::ctype_base::alpha
   "blank" std::ctype_base::blank
   "cntrl" std::ctype_base::cntrl
   "digit" std::ctype_base::digit
   "graph" std::ctype_base::graph
   "lower" std::ctype_base::lower
   "print" std::ctype_base::print
   "punct" std::ctype_base::punct
   "space" std::ctype_base::space
   "upper" std::ctype_base::upper
   "xdigit" std::ctype_base::xdigit
   "d" std::ctype_base::digit
   "s" std::ctype_base::space
   "w" std::ctype_base::alnum with '_' optionally added
   The classification returned for the string "w" may be exactly the same as "alnum",
   in which case isctype() adds '_' explicitly.
   Additional classifications such as "jdigit" or "jkanji" may be provided by
   system-supplied locales (in which case they are also accessible through std::wctype)

Parameters
   first, last - a pair of iterators which determines the sequence of characters that
                 represents a name of a character class
   icase - if true, ignores the upper/lower case distinction in the character
                 classification

Type requirements
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.

Return value
   The bitmask representing the character classification determined by the given
   character class, or char_class_type() if the class is unknown.

Example
   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::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
   isctype indicates membership in a character class
           (public member function)
   wctype looks up a character classification category in the current C locale
           (function)