When I needed to verify something was a number(mine had 2 dashes between the numbers) I made a formula checkbox that would trigger if it was not a good number. I removed the dashes with a Part formula and then put all 3 parts back together in a variable. Then I tested to see if the number was above or below a certain number. My number was similar to a phone number so I checked if it was more than 0000000000 and less than 9999999999. If it returned true then it was a number. If it returned false it was not and I could abort the save.
The main issue with this approach is that for form rules to take effect the person needs to enter the data in the field and then click outside of the field to another one. If they do not do this then the form rule fails and the save happens.
I came to the conclusion that the best way to do this is with JavaScript. You can actually validate data with that and prevent people from entering the wrong thing. It's a nightmare doing it native in QuickBase imo.
