I’ve tried to use Activity Diagram to depict this [pseudo-]code:
get expression
while match '('
get expression
if !match ')'
error
get term
see:

Is there a way to depict that code without repeat get expression node twice?
Or I’m using a wrong diagram do depict that?
The diagram is wrong. You’re traversing
get_expressiontwice in each iteration. Add a node below the firstget_expressionand the bottom [yes] would go to that new node.