String.prototype.count = function(c){
var sum = 0;
for(var i = 0;i < this.length; i++){
if(this[i] == c){sum ++;}
}
return sum;
};

 $(".decimal-2-dig").keydown(function(event) {
		if(event.shiftKey || event.ctrlKey || event.altKey){return false;}
		var v = $(this).val();
		var isPass = false;
		var isDot = false;
		if(event.keyCode >= 96 && event.keyCode < 106){isPass = true;}
		if(event.keyCode >= 48 && event.keyCode < 57){isPass = true;}
		if(event.keyCode == 110 || event.keyCode == 190){isPass = true;isDot = true;}
		if(event.keyCode == 8){return true;}
		
		if(!isPass){return false;}
		if(v.count('.') > 0 && isDot){return false;}
		
        if (v.indexOf('.') != -1 && event.keyCode != 8 /*back space*/) {
            var val = $(this).val();
            $(this).val(val.substr(0, val.indexOf('.') + 2));
        }
		
		
    });


使用