ia the match's <tt>value()</tt> member function. Be warned though 
  that the match's attribute may be invalid, in which case, getting the attribute 
  will result in an exception. The member function <tt>has_valid_attribute()</tt> 
  can be queried to know if it is safe to get the match's attribute. The attribute 
  may be set anytime through the member function <tt>value(v)</tt>where <tt>v</tt> 
  is the new attribute value.<br>
  <br>
  A match attribute is valid:</p>
<ul>
  <li> on a successful match</l