Check the docs for Data::FormValidator
(ValidateRM uses D::FV profiles). You'll want to write your own constraint, it should be fairly straightforward once you decipher the docs.
To clarify the other recommendation, don't rely on client-side code, as the client can disable any such code. You can include it to avoid server work, but you'll want the code on the server just in case anyway.
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Want more info? How to link or
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