I rather recommend you to create Variable instead of storing expressions in a .txt file that's a wrong approach.
Create variable once and enter your expression value there. This is one time activity.
After creating variable for a particular expression call that expression eg. vLastQuarterSales in your Bar Chart, Straight Table, Text Box, anywhere. It will just work fine and easier for you.
It's the Best Practice. Also your Application Performance will surely be increased.