how can i allow input only specific text in textbox ?

The easiest approach is to catch the key down / key press event by handling the events. You can then check for the key value passed and then decide if you want to display the value or cancel the event. This would normally be done via the client side script e.g. Javascript - example here[^].

Another simple approach would be to use a regular expression validator to control the input, example - How To: Use Regular Expressions to Constrain Input in ASP.NET[^]. If you can get this implemented, this would be a cleaner approach.

Use this Code Javascript to allow the Required String[^].

Check another condition as
if text length is equal to textbox length+1 then clear the textbox value and add the count.