Knockout Validation: Multiple email addresses

A simple rule for validating an observable which could contain multiple email addresses.

Assuming you have an observable like self.emailTO = ko.observable('');, you would just extend it to have self.emailTO.extend({ multiemail: true });. Use it with the required validator if you want at least one email address.

ko.validation.rules['multiemail'] = {
    validator: function (val, validate) {
        if (!validate) { return true; }

        var isValid = true;
        if (!ko.validation.utils.isEmptyVal(val)) {
            // use the required: true property if you don't want to accept empty values
            var values = val.split(';');
            $(values).each(function (index) {
                isValid = ko.validation.rules['email'].validator($.trim(this), validate);
                return isValid; // short circuit each loop if invalid
        return isValid;
    message: 'Please enter valid email addresses (separate multiple email addresses using a semicolon).'

Requires Knockout.jsKnockout Validation, and jQuery.