Regex for an empty field

[sarahk]

I've created a custom field for registration only and it is supposed to be left blank.

I'd like to use the regex field to test that the field is still blank and use the internal vB checking and error reporting system to reject the registration if the user has entered anything at all in the field.

Regex seems to be great for testing patterns, and the inclusion of characters. How do I get it to test for the emptiness?