jQuery validate: How to add a rule for regular expression validation?

I am using the jQuery validation plugin. Great stuff! I want to migrate my existing ASP.NET solution to use jQuery instead of the ASP.NET validators. I am missing a replacement for the regular expression validator. I want to be able to do something like this:

$("Textbox").rules("add", { regularExpression: "^[a-zA-Z'.\s]{1,40}$" })

How do I add a custom rule to achieve this?