Custom validation (callback) for blank field value

[eluser]Dan Dar3[/eluser]
Hi,

I'd like to have a custom validation message for a particular field when left blank, and for that I've tried defining a custom callback function instead of the "required" rule.

Now, it looks like the custom validation function is not called when the field is blank, only when there is a value in the field! If I attach the "required" next to my custom callback function, I get the validation behaviour and the standard "required" message.

Any thoughts, is this by design or some sort of a bug?

Thanks,
Dan