Triggered event when any key is pressed?

Hello,

Is there a wild card available so that when any key is pressed I can trigger an event?  Assuming I have an input text field, I'd like to have a triggered event, such as changing the color of the background, when the user presses any key.  

I'm currently using the top 20 most popular letters but now I'm realizing I have to create a new triggered event for them as capital letters too... and then I have the 10 digits to work with.

It would be super nice if there was a wildcard option though I can't find any solid reference of this online.  Even if there was the ability to use an 'onfocus' feature, but I don't think that's possible yet.

Thank you for any insight!