The Data Validator Snap will fail if the incoming data does not match the defined constraints with the regex.
You can try with:
If DOB field is optional, and is expected to be null, then you can check if that field(“DOB”) is null/or not right before the data validator snap.
If it is null, then process it further(to the snaps after the Data Validator) without calling the Data Validator Snap, containing the regex.
If it is not null, then go through the Data Validator Snap and perform validations.
Also, you can achieve the desired format of the date field by using the proposed Date Functions.
Hope this helps.