I need help writing a REGEX Validation rule.

Condition:

                  Filed Name - TestNo

                  Field length  - 23 characters

                   No of values in the field - 4 values with 5 characters (only numbers) seperated by commas.

                  Eg: 12345,23456,12356,23456

I need a validation rule that makes sure the TestNo(s) are 5 digits and separated by commas with no spaces

Can someone help me in building a formula using REGEX or some other function.