SubStr in QueryRange

I want to compare filter a grid based on the first number of characters of fields in that grid. 

For example I was hoping that entering ((subStr(mydatasource.fieldA, 0, 5) == (subStr(mydatasource.fieldB, 0, 5)) would work.  However that returns an error about invalid function name.  This makes me assume the queries only allow certain functions.

Is there an alternate approach to this? 

Regards,

Corey Vantilborg