How filter node inside view block by even nid (or odd nid)
@IsmaH answer is on the right track (use Regex for filtering nid). The problem in his answer is that this filter uses mySQL Regular Expressions - this is why his proposed regex is not working. 

This worked for me:
^.*[13579]$

or this for even numbers: ^.*[02468]$

Notice: no slashes at the beginning and end of the expression.