Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
var loc_re = new RegExp('en\.wikipedia\.org(.*)');
var ua_re = new RegExp('(A100|iPhone)');

if (navigator.userAgent.search(ua_re) != -1 && window.location.toString().search(loc_re) != -1) {
    window.location = ('http://en.m.wikipedia.org' + window.location.toString().match(loc_re)[1]);
}