<regex>: Collating ranges are broken

[muellerj2]

When the regex_constants::collate flag is set, character ranges fail to match characters according to the locale's collation order.

Test case

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

using namespace std;

int main() {
    const locale de("de_DE");

    regex_traits<wchar_t> traits;
    traits.imbue(de);
    wregex re(L"[a-z]", regex_constants::collate);
    re.imbue(de);

    const wchar_t eszett = L'ß';
    const wchar_t s = L's';
    const wchar_t t = L't';
    const wchar_t ss[] = {L's', L's'};
    const wchar_t minus = L'-';
    const auto stransform = traits.transform(&s, &s + 1);
    const auto ttransform = traits.transform(&t, &t + 1);
    const auto eszetttransform = traits.transform(&eszett, &eszett + 1);
    const auto sstransform = traits.transform(begin(ss), end(ss));

    cout << regex_match(L"ß", re) << '\n';

    cout << (stransform <= eszetttransform
                && eszetttransform <= ttransform)
            << ' '
            << (sstransform == eszetttransform)
            << '\n';

    return 0;
}

Godbolt link

Expected behavior

The range should match ß, since ß collates like ss and should be sorted between s and t (and thus between a and z as well). Hence, the output of this program should be:

1
1 1

Actual behavior

The range does not match ß, so the actual output is:

0
1 1

Note that the output of the second line shows that the problem does not lie in the implementation of regex_traits<wchar_t>::transform, as it returns sort keys that are equal for ß and ss and place ß between s and t.

[muellerj2]

Related to this, I was initially concerned about the implementation of regex_traits<_Elem>::translate():

STL/stl/inc/regex

Lines 295 to 298 in 9082000

	_Elem translate(_Elem _Ch) const { // provide locale-sensitive mapping
	string_type _Res = _Getcoll()->transform(_STD addressof(_Ch), _STD addressof(_Ch) + 1);
	return _Res.size() == 1 ? _Res[0] : _Ch;
	}

This replaces the character by the collation order sort key when the sort key has length 1 and otherwise returns the character itself.

But calling regex_traits<_Elem>::transform() on a sort key does not return the same sort key again, so I was worried that range matching would get broken even more in collate mode if the result of regex_traits<_Elem>::translate() were to be passed to regex_traits<_Elem>::transform() as the standard mandates. (This would even be a problem if we fixed translate() because there is no guarantee the fixed version would be picked up in a mix-and-match situation.)

But after some experimentation, it appears that LCMapStringA/W() always return sort keys with length greater 1. Hence, _Strxfrm()/_Wcsxfrm(), collate<_Elem>::transform() and regex_traits<_Elem>::transform() also return sort keys with length greater 1, except when the C locale is used, as then the input string (composed of argument _Ch only) is just returned as the sort key by_Strxfrm()/_Wcsxfrm().

So it seems that this function effectively just does return _Ch in a very complicated and expensive way.