title: What is returned in std::smatch and how are you supposed to use it?
body: <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>