Accept only numeric values with a decimal precision of 2 in TextBox using JavaScript in ASP.Net

I'm attempting to trigger a pop-up warning to the user when any characters that is not an integer or decimal point is entered.

HTML:

<asp:TextBox ID="TextBox1" runat="server" onkeyup="checkTB(this.value)"/>

JS:

function checkTB(val) {

    var allowedString = new RegExp("^[\d.]+$");//only allow integers & decimal places
    if (allowedString.test(val) == false) {
        alert("FORBIDDEN"+val);
    }

}

However the alert triggers regardless of whether the RegExp test is true or false as it triggers regarless of whether I enter integers or alphabets. Why is this happening?