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
std::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 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).

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¶

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

// Run this code

#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::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)