@ OP, вот один из способов
$str = <<<A
<td>content</td>
<td *?*>content</td>
<td *specific td class*>content</td>
<td *?*> multiline
content </td>
A;
$s = explode("</td>",$str);
foreach ($s as $a=>$b){
$b=preg_replace("/.*<td.*>/","",$b);
print $b."\n";
}
выход
$ php test.php
content
content
content
multiline
content