ier">awk</span></code>.
        You can construct POSIX extended regular expressions in Boost.Regex by passing
        the flag <code class="computeroutput"><span class="identifier">extended</span></code> to the
        regex constructor, for example:
      </p>
<pre class="programlisting"><span class="comment">// e1 is a case sensitive POSIX-Extended expression:</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e1</span