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# std::regex_traits::lookup_classname

From cppreference.com
< cpp‎ | regex‎ | regex traits

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)

### [edit] 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 `ForwardIterator`.

### [edit] Return value

The bitmask representing the character classification determined by the given character class, or zero if the class is unknown.

### [edit] 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 kanji
        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

### [edit] 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) [edit]

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