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) {
        var isDot = function() {
            return (event.keyCode == 110 || event.keyCode == 190);
        }
        var isValidKeyCode = function() {
            if (event.shiftKey || event.ctrlKey || event.altKey) { return false; }
            var isPass = false;
            if (event.keyCode >= 96 && event.keyCode < 106) { isPass = true; }
            if (event.keyCode >= 48 && event.keyCode < 58) { isPass = true; }
            if (isDot()) { isPass = true; }
            if (event.keyCode == 8) { return true; }
            return isPass;
        };

        if (!isValidKeyCode()) {
            return false;
        }

        var v = $(this).val();
        if (v.count('.') > 0 && isDot()) { return false; }

        if (v.indexOf('.') != -1 && event.keyCode != 8 /*back space*/) {
            var val = $(this).val();

            var selectStr = window.getSelection().toString();
            if (selectStr != "") {
                return isValidKeyCode();
            }
            $(this).val(val.substr(0, val.indexOf('.') + 2));
        }
        
    });

使用

<input class="decimal-2-dig" /