Filter empty cells with row filter node and regular expression ^$ ?

I want to keep all rows that have e.g. empty cells in column x. I tried this with a row filter node and an regular expression like ^$, but that does not work.

 

I do not know why? Can anyone explain this?

 

To generate missing value is not an option, because the input table was generated via a cell splitter node and this node cannot generate missing values for empty cells. 

 

A workaround is to use a java row filter with return $column_x$.equals("");

 

This is currently not an easy task, I would recommend using the Java Snippet Row Filter using return $column$.isEmpty(); in the Method Body.