$re = '/(?:(?<=[^-<]|^)--(?=[^->])|-{4,}).*|(?<!-)-->>>.*/m';
$str = '-- test
test1 -- test
--->>> test
<<<--- test
--->>>->>> test
---->>> test
test --->>> test
test --->>> test <<<---
-->>> test';
$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