[Bug libstdc++/77492] std regex icase doesn't seem to work correctly.

["flashmozzg at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77492

--- Comment #2 from Danila <flashmozzg at gmail dot com> ---
(In reply to Tim Shen from comment #1)
> Seems to be the same issue as 71500.

Even though I assume that that bug (71500) was fixed (even though the status is
still UNCONFIRMED) it haven't fixed the issue.
I tried with both latest gcc (7.2) and clang (5.0) on Ubuntu 16.04 and in both
cases the result of this simple code was the same:

#include <iostream>
#include <regex>

using namespace std;

void check(const string& s, regex re) {
    cout << s << " : " << (regex_match(s, re) ? "Match" : "Nope") << endl;
}

int main() {
    regex re1 = regex("([a-z]+) \\1", regex::icase);
    check("abc abc", re1);
    check("Abc abc", re1);
    check("abc Abc", re1);
}

output:
abc abc : Match
Abc abc : Nope
abc Abc : Nope

It looks like both compilers don't ignore the case of the backreferences which
looks like a bug. I haven't found any exceptions to the icase flag in case of
backrefs and this how it works in most languages. Also, it looks like Visual
C++ compiler works with this case correctly (i.e. it matches the string in all
3 cases).