Hi Zachary, Hi Dale,
Thanks for answering. I did what both of you suggested, problem is that what I want is a more complex case.
I mean that when I put in the expression a single field without manipulations, it works good ( such as the case in what dale did, and example 3 in the help). But when I put in the expression a field with manipulations (such as str or zfill), the "simple user" running the model from the toolbox can change the full expression but not (as I would have wanted it) the field name only. If only there would be a modelbuilder tool that would let me take apart the expression, maybe I could have feeded the fieldname as a parameter for the expression.
Now I'm confused. I thought the point was to have the user enter in a field that they wanted a predefined expression to be applied to?