terminate called after throwing an instance of 'std::regex_error' what(): regex_error Aborted (core dumped)

I see somebody asked this question too,but I don't find the answer of this question.This is my code :

    // regex_match example
#include <iostream>
#include <string>
#include <regex>

int main ()
{
    std::string s ("12345678");
    std::regex e ("^\\d{8}$");
    if(std::regex_match (s,e))
        std::cout << "string match" << std::endl;
    return 0;
}

I make it by : g++ reg.cpp -o reg, then it show the error "terminate called after throwing an instance of 'std::regex_error' what(): regex_error Aborted (core dumped) [closed]".

This is my g++ version :

bob@work03:~$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.2-19ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.8 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap --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-4.8-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-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 --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 4.8.2 (Ubuntu 4.8.2-19ubuntu1) 

But if I use this code, it can work.

// regex_match example
#include <iostream>
#include <string>
#include <regex>

int main ()
{
    std::string s ("subject");
    std::regex e ("(sub)(.*)");
    if (std::regex_match (s,e))
        std::cout << "string object matched\n";
    return 0;
}

I can't understand where I was wrong.