Std::regex_traits::lookup_classname (3) Linux Manual Page

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

std::regex_traits::lookup_classname (3) Linux Manual Page