I need to set a field to be formatted as mm/yyyy, but still allow the user to enter 99/9999 if the month and year are unknown. Now, code in Salesforce.com will allow me to use

NOT(OR(ISBLANK(B9), REGEX(B9, "[0-9]{2}/[0-9]{4}")))

But, Excel 2007 says this is a circular reference and it won't work. Does anyone have any ideas for me?