C:\Daten\perl>perl
$data = qq{<font color="#000000"><pre>
<br>"Hallo"
<br>1234<br>214214<br>346<br>
<br>
<br>();
<br>("Test! 2134",12421<br>
);</pre></font>};
$data =~ s~(\d+)(?=[^>]*(?:<|$))~###$1###~g;
print $data;
^Z
<font color="#000000"><pre>
<br>"Hallo"
<br>###1234###<br>###214214###<br>###346###<br>
<br>
<br>();
<br>("Test! ###2134###",###12421###<br>
);</pre></font>