string expression

Question

In mapping data flow, how is it possible to execute an expression which is retrieved from a config file?
For example, in value expression, I can run expressions like lower($$) which is fine but I would like to know how to know how to run this expression if this is passed in?
Example:
lower($$) --> works as expected which is that it converts values into lower case
expr('lower($$)') --> works as expected which is that it converts values into lower case
Config#lookup($0).Expression --> shows as just the text for lower($$)
expr(toString(Config#lookup($0).Expression)) --> it gives error java.lang.NullPointerException

Thank you