I have a very simple test program using the latest 1.33 version of regex library. As soon as I run it, it assert (crash) at the line where std::string s = (*m1)[0]. I am doing something wrong?

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#include " stdafx.h"
#include <boost/regex.hpp>

const char matchpattern[] = "\\s+kitty this";
const char astring[] = "hello Kitty this is a test";

using namespace boost;

int main(int argc, char* argv[])
{
    regex header_expr(matchpattern, regex_constants::basic | regex_constants::icase);

    cregex_iterator m1(astring, astring + strlen(astring), header_expr);

    std::string s = (*m1)[0];   <==== crash here
    return 0;
}