$re = '/((^)|(\<\/table\>))(.*?)((\<table\>)|($))/';
$str = 'dit is een zin<br/><table><tr><td>Dit moet niet <br/> worden afgebroken<br/></td></tr></table> Nog meer tekst<br/><table>tabel2</table>aaa';
$subst = "\4";
$result = preg_replace($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