Ghost/core/client/app/validators/nav-item.js

import BaseValidator from './base';

export default BaseValidator.create({
    properties: ['label', 'url'],

    label: function (model) {
        var label = model.get('label'),
            hasValidated = model.get('hasValidated');

        if (this.canBeIgnored(model)) { return; }

        if (validator.empty(label)) {
            model.get('errors').add('label', 'You must specify a label');
            this.invalidate();
        }

        hasValidated.addObject('label');
    },

    url: function (model) {
        var url = model.get('url'),
            hasValidated = model.get('hasValidated'),
            validatorOptions = {require_protocol: true},
            urlRegex = new RegExp(/^(\/|#|[a-zA-Z0-9\-]+:)/);

        if (this.canBeIgnored(model)) { return; }

        if (validator.empty(url)) {
            model.get('errors').add('url', 'You must specify a URL or relative path');
            this.invalidate();
        } else if (url.match(/\s/) || (!validator.isURL(url, validatorOptions) && !url.match(urlRegex))) {
            model.get('errors').add('url', 'You must specify a valid URL or relative path');
            this.invalidate();
        }

        hasValidated.addObject('url');
    },

    canBeIgnored: function (model) {
        var label = model.get('label'),
            url = model.get('url'),
            isLast = model.get('last');

        // if nav item is last and completely blank, mark it valid and skip
        if (isLast && (validator.empty(url) || url === '/') && validator.empty(label)) {
            model.get('errors').clear();
            return true;
        }

        return false;
    }
});