$re = '/<[^<>]*(<[^<>]*(<[^<>]*>[^<>]*)*>[^<>]*)*>/m';
$str = 'aaaaaa<bbb ccc>ddddd<dde<fff>ggg<hhh>iii>
aaaaaa<<<bb>cccc<ddd>>eeeee<ffff<gggg>>hhhh<iiii<jjjj>>jjj>';
$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