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std::regex_traits<CharT>::value

From cppreference.com
< cpp‎ | regex‎ | regex traits
 
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std::regex_traits
 
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.

[edit] Parameters

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

[edit] 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.

[edit] Example

Run this code
#include <iostream>
#include <locale>
#include <map>
#include <regex>
 
/ 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
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/regex/regex_traits/value&oldid=161056"

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