Regular Expression to allow an empty string or a string with specific length

Question

Hi I am having trouble trying to create a regex that will allow me to have the field empty or a string with length 9 if populated.

The current regex im using is: ^[0-9]{9}

Which works if the field is mandatory, however we have a new requirement that this can also be empty. I've tried ^[0-9]{0|9} but obviously that doesnt work.

Thanks in advance

Answer 1

^(|[0-9]{9})$

Should do the trick.

Edit: But seriously; do what @Felix Kling suggests in a comment: test the string via a .length property, .equals("") or the like.