What is returned in std::smatch and how are you supposed to use it? : <p>string <code>"I am 5 years old"</code></p>

<p>regex <code>"(?!am )\d"</code></p>

<p>if you go to <a href="http://regexr.com/" rel="noreferrer">http://regexr.com/</a> and apply regex to the string you'll get 5.
I would like to get this result with std::regex, but I do not understand how to use match results and probably regex has to be changed as well.</p>

<pre><code>std::regex expression("(?!am )\\d");
std::smatch match;
std::string what("I am 5 years old.");
if (regex_search(what, match, expression))
{
     //???
}
</code></pre>
