$re = '/<(th|td)>.*?<\/\1>(*SKIP)(*F)|\n/s';
$str = '<table>
<tr><th>HEader 1</th><th> header
2</th>
</tr>
<tr><td>cell
content</td><td>cell 2</td></tr>
</table>';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php