allow one dot or comma to be enter in javascript function

hi

i have a javascript function for asp.net during keypress event for a custom textbox which only allow numbers to be enter.

this textbox have a property of decimalplace. which indicating how many decimal places the text is.

in my function

var key = window.event ? e.keyCode : e.which;
     var keychar = String.fromCharCode(key);
     var reg = null
     var input_value = input_value;
     var objdecimal = decimal;
     if (objdecimal > 0) {
       reg = new RegExp("[0-9,.]");
     }
     else {
         reg = new RegExp("[0-9]");
     }
     //if (key == 8) {
     //    keychar = String.fromCharCode(key);
     //}
     //if (key == 13) {
     //    key = 8;,
     //    keychar = String.fromCharCode(key);
     //}
     return reg.test(keychar);

allow user to enter number if decimal =0 if >0 then he or she will be allow to enter comma or dot. but the problem is

if there is a . or , i wan to stop user from keying in . or , again if there is already contain both of this.

example

accepted value

1.00

1,00

1,000.00

1.000,00

not accepted

1.000.00

1,000,00

or

1,,000..

1..00.00