$re = '/^=separator=.*(?:\R(?!(?:=separator=|.*?\bunwantedKeyword\b)).*)*\R.*?\bunwantedKeyword\b.*\R*/m';
$str = '=separator=
data
unwantedKeyword
data
=separator=
data
wantedKeyword
data
=separator=
data
unwantedKeyword
data
=separator=
data
wantedKeyword
data
=separator=';
$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