Shorten Text in a Note Callout

How would I shorten the text displayed for a parameter in a note. For our system, our File Names are stored as: for example: XYY4512999-1_x_TM_CP. And this is what is displayed when I use the parameter &model_name on a multi-sheet drawing, which defines the components of an assembly. However, all I really want to display is XYY4512999-1 (everything up until the first underscore). Is there a switch or modifier I can apply along with the parameter callout which will cause the note to only display the text up until the underscore character?