Bug in std::regex related to repeat patterns? [duplicate]

I think there's a bug related to non-capture/capture groups in combination. I changed the regex to (\s-?\d+){2}, bringing the space inside the capture group and I still only obtain the last match as a capture. Anyone see anything obviously wrong? I ultimately need to match the spaces with the numbers, but only need the numbers. This is a simplified example, it is part of a larger regex where I have to match 12 of these patterns

#include <regex>
#include <string>
#include <iostream>

int main()
{

  std::string pattern=R"((?:\s(-?\d+)){2})";
  
  std::regex RegexPattern(pattern);

  const std::string matchString = " 1 0";
  
  std::smatch matches;

  if (std::regex_search(matchString, matches, RegexPattern))
  {
    for (auto match: matches)
    {
      std::cout << match << std::endl;
    }

    // matches is 1 indexed, matches[0] is the part of the matchString that matched the pattern (so the whole thing)
  }
}

produces

 1 0
0

instead of

 1 0
1
0

g++ -v

g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 5.4.0-6ubuntu1~16.04.12' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-5 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.12)