the expression node supports exactly one expression returning one output value, like switch(…) in version 5.3. You cannot enter multiple expressions like switch(…)  find(…). One possible alternative is to use a chain of multiple Expression nodes. Another one would be to use, e.g., a Python Script node for more complex calculations.
In version 5.4 – to be released in the beginning of December – the node is going to support multiple expressions, each appending/replacing a column. We thought about adding support for temporary columns/variables that are not part of the output, too, but so far concluded that this is not crucial. Your feedback is welcome.