isnumeric - checking if text variable is convertable to numeric

I am writing a pgsql function and I would need to create a condition
based on the value of a text variable. I would need to check if
text_var is convertable to numeric type and if yes do the conversion,
if no do something else. I couldn't find out how to do that.

Something like:
IF isnumeric(text_var) Then
 num_var:=Cast (text_var AS numeric);
ELSE
....
END IF;

Thanks for the help.
Balázs