Input validation according a specific format

Input validation according a specific format

Hello,

I have a Single Line Text field, where users are supposed to enter numbers separated by semicolons. There is no limit on how many numbers users can enter - the only one constraint is to have numbers delimited by ";", i.e. both entries "12;6;11;64" and "1;2" are equally valid, while "20;6_4" or "20 6" are not.

How would you suggest to implement this constraint? How to take into account the fact that the length of an entry is not fixed?

Thank you for your assistance.