How do I process the input of a textbox, by keystroke or other entry method (swipe)?

I have a text box on a form that, when data is entered, by keystroke, or by swiping a card through a card reader that populates the text box with the data read from the card, and I'd like to know how I would go about doing this. I'm guessing that in either case I could / would somehow use the length of the string in the text box to be a determining factor but how? There is a limit to the number of characters that will be input so I do know that before hand. Ultimately what I'm trying to achieve is that when the user is done entering data, I manipulate the string and replace the text (in the same textbox) with the new "processed" / "manipulated" string. How would I go about doin this? Code examples or links to examples would be appreciated...(in the markup, codebehind and or javascript if needed). Thanks