Both Nick and Adam's answers work really well.
I'd like to add a note if you want to allow latin characters like á
and ç
as I wanted to do:
jQuery.validator.addMethod('lettersonly', function(value, element) {
return this.optional(element) || /^[a-z áãâäàéêëèíîïìóõôöòúûüùçñ]+$/i.test(value);
}, "Letters and spaces only please");