NAME

std::regex_traits::value - std::regex_traits::value

Synopsis

int value( CharT ch, int radix ) const; (since C++11)

Determines the value represented by the digit ch in the numeric base radix, given
the currently imbued locale. This function is called by std::regex when processing
Quantifiers such as {1} or {2,5}, Backreferences such as \1, and hexadecimal and
Unicode character escapes.

Parameters

ch - the character that may represent a digit
radix - either 8, 10, or 16

Return value

The numeric value if ch indeed represents a digit in the currently imbued locale
that is valid for the numeric base radix, or -1 on error.

Example

// Run this code

#include <iostream>
#include <locale>
#include <regex>
#include <map>

// This custom regex traits allows japanese numerals
struct jnum_traits : std::regex_traits<wchar_t>
{
static std::map<wchar_t, int> data;
int value(wchar_t ch, int radix ) const {
wchar_t up = std::toupper(ch, getloc());
return data.count(up) ? data[up] : regex_traits::value(ch, radix);
}
};
std::map<wchar_t, int> jnum_traits::data = {{L'〇',0}, {L'一',1}, {L'二',2},
{L'三',3}, {L'四',4}, {L'五',5},
{L'六',6}, {L'七',7}, {L'八',8},
{L'九',9}, {L'Ａ',10}, {L'Ｂ',11},
{L'Ｃ',12}, {L'Ｄ',13}, {L'Ｅ',14},
{L'Ｆ',15}};
int main()
{
std::locale::global(std::locale("ja_JP.utf8"));
std::wcout.sync_with_stdio(false);
std::wcout.imbue(std::locale());

std::wstring in = L"風";

if(std::regex_match(in, std::wregex(L"\\u98a8")))
std::wcout << "\\u98a8 matched " << in << '\n';

if(std::regex_match(in, std::basic_regex<wchar_t, jnum_traits>(L"\\u九八ａ八")))
std::wcout << L"\\u九八ａ八 with custom traits matched " << in << '\n';
}

Output:

\u98a8 matched 風
\u九八ａ八 with custom traits matched 風