I think I've just been caught out by the issue discussed here in 2011:

https://www.mojoportal.com/Forums/Thread.aspx?pageid=5&t=9218~1

i.e. I'm unable to make this validator work:

regexValidationExpression="VOL|NT"

... with this profile option list:

            <OptionList>
                <Option value="" TextResourceKey="Please select..."></Option>
                <Option value="VOL" TextResourceKey="the first option"></Option>
                <Option value="NT" TextResourceKey="another option"></Option>
            </OptionList>

Also, requiredForRegistration="true" does not prevent the user registering without selecting a value other than the default "Please select...". Perhaps this could be made to work when the control value is ""?

A bug? Any workarounds?

I'm no expert on regex, but I thought that "VOL|NT" would not pass an empty string, and would only pass for one of my specified values. I've also tried "^VOL|NT$" and "^.+$" but still failed to stop the user registering. Am I missing something?

this is now fixed in the source code repository. setting a dropdown to requiredForRegistration will use a required field validator and will not allow empty value.