Regex for empty string or some patterns

Dear Sir/Madam,

For one of my requirement I have written patterns to accept xx.yy# repeats 25 times.
How to add empty string patterns along with above support i.e How to include either empty " " or below patterns ?

leaf ABCProfile {
type string {
pattern ‘(([0-9]|1[0-8]).([0-9][0-9])#){24}([0-9]|1[0-8]).([0-9][0-9])’;
description “ABCProfile”;

Thanks and Regards