https://github.com/xogroup/felicity/issues/116#issuecomment-308537870

felicityFail.js

'use strict';

const Urls = ['www.google.es','www.google.com','www.google.it','www.google.de','www.google.no'];

const extension = (joi) => ({
    base: joi.string(),
    name: 'url',
    language: {
        metrics: 'needs to be one of {{urls}}'
    },
    pre(value, state, options) {

        return value.trim(); // remove whitespaces
    },
    describe(description) {

        if ( this._flags.metrics ) {
            description.valids = Urls;
        }
        return description;
    },
    rules: [
        {
            name: 'metrics',
            setup( params ) {

                this._flags.metrics = true;
            },
            validate(params, value, state, options) {

                if (Urls.indexOf(value) === -1) {
                    // Generate an error, state and options need to be passed
                    return this.createError('url.metrics', { urls: Urls }, state, options);
                }

                return value; // Everything is OK
            }
        }
    ]
});

const Joi = require('joi');
const Felicity = require('felicity');
const joi = Joi.extend(extension);
Felicity.example(joi.object().keys({url: joi.url()}));