Ways of allowing only alpha-numeric characters in Textbox.

<asp:TextBox runat="server" Id="txt_policy_code" />
<asp:RegularExpressionValidator runat="server" id="rex_policy_code" ControlToValidate="txt_policy_code" ValidationExpression="^[a-zA-Z0-9]$" ErrorMessage="Only Alpha-Numeric Allowed" />

function checkAlphaNumeric(e) 
{ 
   if ((e.keyCode >= 48 && e.keyCode <= 57) ||
      (e.keyCode >= 65 && e.keyCode <= 90) ||
      (e.keyCode >= 97 && e.keyCode <= 122))
   {
      return true;
   }
   
   return false;
}
 
<asp:Textbox Id="txt_policy_code" runat="server" onKeyPress="return checkAlphaNumeric();"></asp:Textbox>