std::regex_traits::value

C++

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Technical Specifications

Regular expressions library

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basic_regex (C++11)

sub_match (C++11)

match_results (C++11)

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regex_match (C++11)

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regex_iterator (C++11)

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regex_traits (C++11)

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std::regex_traits

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regex_traits::regex_traits

regex_traits::length

regex_traits::translate

regex_traits::translate_nocase

regex_traits::transform

regex_traits::transform_primary

regex_traits::lookup_collatename

regex_traits::lookup_classname

regex_traits::isctype

regex_traits::value

regex_traits::imbue

regex_traits::getloc

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'',0}, {L'',1}, {L'',2},
                                            {L'',3}, {L'',4}, {L'',5},
                                            {L'',6}, {L'',7}, {L'',8},
                                            {L'',9}, {L'',10}, {L'',11},
                                            {L'',12}, {L'',13}, {L'',14},
                                            {L'',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"";

    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")))
        std::wcout << L"\\u with custom traits matched " << in << '\n';
}

Output:

\u98a8 matched 
\u with custom traits matched