javascript regExp check last character

javascript regExp check last character

I'm using this RexExp code var userIdPattern = new
RegExp('^([A-z0-9_.]{4,15})$');, I want to check if last character was dot
(.) then .test() returns me false:
var userIdPattern = new RegExp('^([A-z0-9_.]{4,15})$');
console.log(userIdPattern.test('Omid.my.')); // -> I need this to be false
and in this case return me true:
userIdPattern.test('Omid.my'); //-> true