No I mean a formatted error, not a server error. When your column is set to null, if a value is entered and you want, say all digits, your regex would be:

^\d+$

...however if I left the field blank I'd get an error saying "The column 'blah' cannot contain the value ''" ..but really it can because the column is null.

I can get around it by doing:

^(?:|\d+)$

...but that's ugly