I'm Stumped!

Not sure if this is too difficult but have a requirement that a textbox can have only 9 or 11 digitvalue in length. It cannot be 10 in length. Optionally, it can multiple instances of 9 or 11 digit values that have a comma in between. No characters are allowed

for example, the followingare valid.

123456789
12345678912
123456789,12345678912,123456789

these are invalid

123,123456789
123456789,123456789123