Javascript: how to validate dates in format MM-DD-YYYY?

I saw a potential answer here but that was for YYYY-MM-DD: JavaScript date validation

I modified the code code above for MM-DD-YYYY like so but I still can't get it to work:

String.prototype.isValidDate = function() 
{
     var IsoDateRe = new RegExp("^([0-9]{2})-([0-9]{2})-([0-9]{4})$");
     var matches = IsoDateRe.exec(this);
     if (!matches) return false;
     var composedDate = new Date(matches[3], (matches[1] - 1), matches[2]);
     return ((composedDate.getMonth() == (matches[1] - 1)) &&
      (composedDate.getDate() == matches[2]) &&
      (composedDate.getFullYear() == matches[3]));
}

How can I get the above code to work for MM-DD-YYYY and better yet MM/DD/YYYY?

Thanks.