You can use the ValidationRule property to specify requirements for data entered into a record, field, or control. When data is entered that violates the ValidationRule setting, you can use the ValidationText property to specify the message to be displayed to the user.
The following table contains expression examples for the ValidationRule and ValidationText properties.

ValidationRule property

<> 0
> 1000 Or Is Null
Like "A????"
>= #1/1/96# And <#1/1/97#
DLookup("CustomerID", "Customers", "CustomerID = Forms!Customers!CustomerID") Is Null

ValidationText property

Entry must be a nonzero value.
Entry must be blank or greater than 1000.
Entry must be 5 characters and begin with the letter "A".
Entry must be a date in 1996.
Entry must be a unique CustomerID (domain aggregate functions are allowed only for form-level validation).