You caught me here Richard but I think that Validation Expression is a regular expression to validate the entry, not to compare to another entry like what you are doing.
I see, perhaps it is not possible to use tokens in the expression.
I did a google but couldn't find much info on building expressions.
Would be cool if we could do it - you will remember me typing a duff email when I signed up your site :)
Good one Richard! :)
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Cheers Aderson