PS Validations - Using a subset of a field not allowed?

Dear All,

We are in ECC 6.0 and I am trying to create a PS Validation to check between

Project Profile vs PROJ-PSPID :1-1: (1st character of the Project Number)

However I am getting GGB520 error : "Conditions cannot be edited with the formula editor". This leads me to OSS Note 167634. However It is not clear how to achieve a validation by using subset of a field. The corrections are not valid for ECC 6.0 and earlier in ECC 5.0, I have done such validations successfully. 

Can you  please clarify, what should I do? We don't want to go to user exit validation route.

Thanks,

Regards,

Davit