<regex>: c++ regex character class case insensitive search problem

[AlexGuteniev]

Describe the bug
regex::icase is not handled correctly for some input.

Command-line test case

d:\Temp2>type repro.cpp
#include <iostream>
#include <regex>

using namespace std;

int main()
{
        wstring target(L" Copyright");
        wsmatch match;
        wregex regexp(L"[a-z][a-z]", regex::icase);
        if (regex_search(target.cbegin(), target.cend(), match, regexp)) {
                wcout << L"Matched: \"" << match.str() << L"\"" << endl;
        }
        return (0);
}
d:\Temp2>cl /EHsc /W4 /WX .\repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.27.29009.1 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

repro.cpp
Microsoft (R) Incremental Linker Version 14.27.29009.1
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:repro.exe
repro.obj

d:\Temp2>.\repro.exe
Matched: "op"

Expected behavior

Matched: "Co"

Command-line test case 2

d:\Temp2>type repro.cpp
#include <regex>
#include <iostream>

int main()
{
        const wchar_t* test_raw_string = L"blahZblah";
        const wchar_t* test_patterns[] = { L"[Z]", L"[z]" };

        for (const auto* test_pattern : test_patterns)
        {
                const std::wstring test_string = test_raw_string;
                std::wregex case_regex(test_pattern, std::regex_constants::ECMAScript);
                std::wcmatch match1;

                std::wcout << test_pattern << L" search " << test_string << L" case sensitive = " << std::regex_search(test_string, case_regex) << L'\n';
                std::wcout << test_pattern << L" search " << test_string << L" case sensitive with match = " << std::regex_search(test_raw_string, match1, case_regex) << L'\n';

                std::wregex icase_regex(test_pattern, std::regex_constants::icase | std::regex_constants::ECMAScript);
                std::wcmatch match2;

                std::wcout << test_pattern << L" search " << test_string << L" case insensitive = " << std::regex_search(test_string, icase_regex) << L'\n';
                std::wcout << test_pattern << L" search " << test_string << L" case insensitive with match = " << std::regex_search(test_raw_string, match2, icase_regex) << L'\n';

                std::wcout << L'\n';
        }
    return 0;
}


d:\Temp2>cl /EHsc /W4 /WX .\repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.27.29009.1 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

repro.cpp
Microsoft (R) Incremental Linker Version 14.27.29009.1
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:repro.exe
repro.obj

d:\Temp2>.\repro.exe
[Z] search blahZblah case sensitive = 1
[Z] search blahZblah case sensitive with match = 1
[Z] search blahZblah case insensitive = 0
[Z] search blahZblah case insensitive with match = 0

[z] search blahZblah case sensitive = 0
[z] search blahZblah case sensitive with match = 0
[z] search blahZblah case insensitive = 0
[z] search blahZblah case insensitive with match = 0

Expected behavior 2

[Z] search blahZblah case sensitive = 1
[Z] search blahZblah case sensitive with match = 1
[Z] search blahZblah case insensitive = 1
[Z] search blahZblah case insensitive with match = 1

[z] search blahZblah case sensitive = 0
[z] search blahZblah case sensitive with match = 0
[z] search blahZblah case insensitive = 1
[z] search blahZblah case insensitive with match = 1

STL version

Microsoft Visual Studio Professional 2019 Preview
Version 16.7.0 Preview 3.1

Additional context
This item is also tracked on Developer Community as DevCom-230267 and DevCom-246258 and by Microsoft-internal VSO-287844 / AB#287844.

See also #405