How will we accept input in text box with validation like first two charactera shoud be number and other charactera are letters?

follow below code:

<asp:textbox id="TB1" runat="server" xmlns:asp="#unknown" />

<asp:regularexpressionvalidator id="validator" runat="server" controltovalidate="TB1"
ErrorMessage="first 2 letters should be numbers!"
ValidationExpression="^[\d]{2}\w*"/>

There is lots of way to do that .
you can use javascript,or ASP.net Regular Expresstion Validator and etc.

here i will show you sample in both

using javaScript :

<script language="javascript">
    
     function Validate() {
         var isValid = false;
         var regex = /^[0-9]{2}[A-Za-z]*$/;
         isValid = regex.test(document.getElementById("<%=txtNumbers.ClientID %>").value);
         if (isValid) {
             alert("OK");
             return true;
         }
         else {
             alert("First 2 character Should be Integer");
             return false;

         }
        
         
     }
    
    </script>

 <asp:textbox id="txtNumbers" runat="server" xmlns:asp="#unknown">
</asp:textbox>
<asp:button id="btnclick" text="Click Me" runat="server" onclientclick="return Validate()" xmlns:asp="#unknown" />

Uing RegularExpressionValidator :

  <asp:textbox id="txtNumbers" runat="server" xmlns:asp="#unknown"> 
</asp:textbox>

<asp:regularexpressionvalidator xmlns:asp="#unknown">
  id="RegularExpressionValidator1" 
  runat="server" 
  ControlToValidate="txtNumbers" 
  ErrorMessage="First 2 Character Should be Numeric" 
  ValidationExpression="^[0-9]{2}[A-Za-z]*$">
</asp:regularexpressionvalidator>