using SQL to evaluate arbitrary expressions?

Hi,

we need to process user-defined integrity conditions, which are stored as 
strings at application level, e. g. "person.age - 2 > 18". We would like to 
substitute the variable names within these expressions by their values at 
trigger time. So we have expressions containing constants only, e. g. "29 - 2 
> 18". We are glad to have found out that we can evaluate such expressions 
simply by calling them with SELECT, e. g. "SELECT 29 - 2 > 18" returns true. 
We think that it would be very nice to benefit from the pg database system 
parser. But can we rely on this functionality in the future? Is this standard 
SQL?

Thank you,
Markus