[Bug libstdc++/69794] New: [5 Regression] std::regex_search match failure with regex object with flags grep|icase

["andrey.vul at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69794

            Bug ID: 69794
           Summary: [5 Regression] std::regex_search match failure with
                    regex object with flags grep|icase
           Product: gcc
           Version: 5.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andrey.vul at gmail dot com
  Target Milestone: ---

I have observed a regression in std::regex_search when the regex object has
flags grep | icase and is multi-line for grep semantics.

The following testcase works as expected in GCC 4.9.3 and clang 3.7 with
libc++.
The failing behavior is observed in gcc 5.3.0 and is reproducible when gcc 5.3
stdlib is used with clang 3.7.

Gcc 5.1 and 5.2 were not tested.

TC:
// begin re.cc
#include <regex>
#include <string>

int main() {
        std::string re_str
        {       "/abcd" "\n"
                "/aecf" "\n"
                "/ghci"
        };
        auto rx = std::regex(re_str, std::regex_constants::grep |
std::regex_constants::icase);
        std::cmatch result;
        return !std::regex_search("/abcd", result, rx);
}
// end re.cc

INVOCATION:
gcc -std=c++11 re.cc && ./a.out; echo $?

OUTPUT:
EXPECTED: 0
ACTUAL: 1

GCC VERSION:
gcc -v:
Using built-in specs.
COLLECT_GCC=/usr/x86_64-pc-linux-gnu/gcc-bin/5.3.0/gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/5.3.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /var/tmp/portage/sys-devel/gcc-5.3.0/work/gcc-5.3.0/configure
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr
--bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/5.3.0
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/5.3.0/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/5.3.0
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/5.3.0/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/5.3.0/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/5.3.0/include/g++-v5
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/5.3.0/python
--enable-languages=c,c++,fortran --enable-obsolete --enable-secureplt
--disable-werror --with-system-zlib --enable-nls --without-included-gettext
--enable-checking=release --with-bugurl=https://bugs.gentoo.org/
--with-pkgversion='Gentoo 5.3.0 p1.0, pie-0.6.5' --enable-libstdcxx-time
--enable-shared --enable-threads=posix --enable-__cxa_atexit
--enable-clocale=gnu --enable-multilib --with-multilib-list=m32,m64
--disable-altivec --disable-fixed-point --enable-targets=all --disable-libgcj
--enable-libgomp --disable-libmudflap --disable-libssp --enable-libcilkrts
--enable-lto --with-isl --disable-isl-version-check --enable-libsanitizer
Thread model: posix
gcc version 5.3.0 (Gentoo 5.3.0 p1.0, pie-0.6.5)