null in preg_replace replacement attribute
<p>I built a google currency converter,but i didn't understand this part of code</p>

<pre><code>$converted_amount = preg_replace("/[^0-9\.]/", null, $get[0]);
</code></pre>

<p>what is the job of this pattern,and what is null means here?</p>

<p>full code:</p>

<pre><code>$amount = urlencode($_POST['amount']);
            $from_Currency = urlencode($_POST['from']);
            $to_Currency = urlencode($_POST['to']);
            $get = file_get_contents("https://www.google.com/finance/converter?a=$amount&amp;from=$from_Currency&amp;to=$to_Currency");
            $get = explode("&lt;span class=bld&gt;",$get);
            $get = explode("&lt;/span&gt;",$get[1]);
            print_r($get);
            $converted_amount = preg_replace("/[^0-9\.]/", null, $get[0]);
</code></pre>