< cpp‎ | regex‎ | regex traits
template< class ForwardIt >

char_class_type lookup_classname( ForwardIt first,
                                  ForwardIt last,

                                  bool icase = false ) const;

If the character sequence [firstlast) 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 std::regex_traits<>::lookup_classname() with [firstlast) 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 narrow and wide character class names are always recognized by std::regex_traits<char> and std::regex_traits<wchar_t> respectively, 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 name     std::ctype classification  
Narrow Wide
"alnum" L"alnum" std::ctype_base::alnum
"alpha" L"alpha" std::ctype_base::alpha
"blank" L"blank" std::ctype_base::blank
"cntrl" L"cntrl" std::ctype_base::cntrl
"digit" L"digit" std::ctype_base::digit
"graph" L"graph" std::ctype_base::graph
"lower" L"lower" std::ctype_base::lower
"print" L"print" std::ctype_base::print
"punct" L"punct" std::ctype_base::punct
"space" L"space" std::ctype_base::space
"upper" L"upper" std::ctype_base::upper
"xdigit" L"xdigit" std::ctype_base::xdigit
"d" L"d" std::ctype_base::digit
"s" L"s" std::ctype_base::space
"w" L"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 LegacyForwardIterator.

[edit] Return value

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

[edit] Example

Demonstrates a custom regex traits implementation of lookup_classname() / isctype():

#include <cwctype>
#include <iostream>
#include <locale>
#include <regex>
// 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::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"


alnums: 風の谷のナウシカ
katakana: ナウシカ

[edit] See also

indicates membership in a character class
(public member function)
looks up a character classification category in the current C locale
(function) [edit]