erformance Comparison</h2>
      <p>
         The following tables provide comparisons between the following regular 
         expression libraries:</p>
      <p><a href="http://research.microsoft.com/projects/greta">GRETA</a>.</p>
      <p><a href="http://www.boost.org/">The Boost regex library</a>.</p>
      <p><a href="http://arglist.com/regex/">Henry Spencer's regular expression library</a>
         - this is provided for comparison as a typical non-backtracking implementation.</p>
      <P>Philip Hazel