Zen Cart modification

Hi, When the customer check out the product, there is a payment information page (step 2) where the customer can type the additional comments. It's optional in zen cart. Customer can leave it blank. But I want to do it compulsory to type comments. When the customer type in any minus sign "-", or any space " ", or alphabets a to z, or totally blank, it should show the window pop up and say "invalid user input" nor "please type in the comment". And the program should not proceed to (step 3) order confirmation page if any of these condition is met. The customer must type correct, then it should allow to proceed to step 3. That's all. I believe its one simple job and want to finish it asap. If you want to clarify anything, please feel free to PM me. My budget for this is around $20 to $30. thanks. David