Below regex in c++ is not working. Can you please explain why?

Below is the C++ program which uses regex to match a string, but is not working. My OS is Ubuntu Linux and compiler is standard C++ compiler which comes with ubuntu.

#include <iostream>
#include <regex>

using namespace std;

int main() {
    const char s[] = "This is a target string";
    regex e ("^([0-9a-zA-Z]*).*?");
    cmatch matches;
    if(regex_match(s, matches, e)) {
        cout << "Match found" << endl;
        for (int i = 0; i < matches.size(); ++i) {
            cout << matches[i] << endl;
        }
    }
    return 0;
}

On compiling using g++ like below

g++ -o test test.cpp -std=c++11

And running the program is failing with the below output

terminate called after throwing an instance of 'std::regex_error'
  what():  regex_error
[1]    24898 abort (core dumped)  ./test

The pattern is working fine and I tried it in rubular.com.I am expecting it to print

Match Found
This is a target string
This

I am new to using regular expressions in C++. Please point me in the right direction.