Is there a way to test in a formula if a text field contains a valid number?

Yeah, I would have though it would work for me,but it didn't. The particular piece of text that was a problem was 'Oneil  (starting with an apostrophe). For my purposes, I am extracting FormID's which are all positive integers from 2 to 5 digits long, so I took the simple way to just test the first character, but I would like to get your more universal approach to work too, for what I'm sure will be later needs for this functionality.