Textbox validation for one number and two decimal values in ASP.NET

See more:

C#

i am unsure how to do it as i do not know how to start
It allows only two decimal places but i want to allow only a digit before decimal place also

What I have tried:

<asp:TextBox ID="txtrate" runat="server" >
function validateFloatKeyPress(el, evt) {
var charCode = (evt.which) ? evt.which : event.keyCode;
var number = el.value.split('.');
if (charCode != 46 && charCode > 31 && (charCode < 48 || charCode > 57)) {
return false;
}
//just one dot
if (number.length > 1 && charCode == 46) {
return false;
}
//get the carat position
var caratPos = getSelectionStart(el);
var dotPos = el.value.indexOf(".");
if (caratPos > dotPos && dotPos > -1 && (number[1].length > 1)) {
return false;
}
return true;
}
function getSelectionStart(o) {
if (o.createTextRange) {
var r = document.selection.createRange().duplicate()
r.moveEnd('character', o.value.length)
if (r.text == '') return o.value.length
return o.value.lastIndexOf(r.text)
} else return o.selectionStart
}