Native query does not allow mysql assignment operator :=

When trying to use the mysql assignment operator in a native query one gets an exception as SqlParserUtils does not qualify the character after the : as being part of a valid parameter value.

Undefined index: in vendor/doctrine/dbal/lib/Doctrine/DBAL/SQLParserUtils.php line 156 (uncaught exception)

A simple example is
$rsm = new ResultSetMapping();
$rsm->addScalarResult('rank', 'rank');
$qry = $em->createNativeQuery("
SELECT (@rank := 1) AS rank
");
$result = $qry->getResult(Query::HYDRATE_ARRAY);

Or a more complicated example is (similar to actual use):

$rsm = new ResultSetMapping();
$rsm->addScalarResult('rank', 'rank');
$qry = $em->createNativeQuery("
SELECT rank FROM
(SELECT (@rank := @rank +1) AS rank FROM (SELECT @rank :=0) rnk2) rnk1
");
$result = $qry->getResult(Query::HYDRATE_ARRAY);

I have attached quick-fix patch, but it looks like the getPlaceholderPositions method is wanting something better overall (due to the TODO comment in it).