std::regex_traits<CharT>::lookup_classname (3) - Linux Manuals

std::regex_traits<CharT>::lookup_classname: std::regex_traits<CharT>::lookup_classname

NAME

std::regex_traits<CharT>::lookup_classname - std::regex_traits<CharT>::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