Context: I've got an iOS keyboard app that's a numpad. We recently, after a lot of user feedback, added four additional keys: period, comma, space, and return. The day we released that feedback received was "what about hyphens?" and "what about dashes". Obviously this is a challenge because it's a keyboard with the main priority of making the number keys as large, and centered, as possible.
We're investigating best options for including additional keys, as well as potentially making the key selection customizable and adding secondary key functions (like the default iOS keyboard has for many characters, where you tap and hold to see the secondary functions). Priority will still have to be given to the most useful keys, as well as placement.
Do you know of any research that shows what is the most common punctuation marks used when writing numbers, as a whole? Obviously periods and commas are used for decimals and large numbers (depending on the region), but any study on the subject would be tremendously helpful in advance of just building and A/B testing different styles.
edit: for clarification this is an iOS app keyboard, you can view details here - https://itunes.apple.com/us/app/numpad-your-number-keyboard/id1072547160?mt=8