se ti vuoi allenare con le espressioni regolari prova anche questa
codice:
//------------------------------------------------
//valida la stringa dateStr
//------------------------------------------------
function IsDate(dateStr)
{
	var datePat = /^((3[01]|[12]\d|0?[1-9])\/(0?[13578]|10|12)\/(\d\d)?\d\d|(30|[12]\d|0?[1-9])\/(0?[469]|11)\/(\d\d)?\d\d|(2[0-8]|[01]\d|0?[1-9])\/(0?2)\/(\d\d)?\d\d|29\/(0?2)\/(1200|1600|2000|2400|2800|00)|29\/(0?2)\/(\d\d)?(0[48]|[2468][048]|[13579][26]))$/;
    var matchArray = dateStr.match(datePat); 
	return !(matchArray == null);
	
}