Formatting string in textbox on asp.net

for this too many external libraries are available you can use Telrik ajax lib you can follow following method to make that First, include the jQuery and masked input javascript files.

<script src="jquery.js" type="text/javascript"></script>
<script src="jquery.maskedinput.js" type="text/javascript"></script>

Next, call the mask function for those items you wish to have masked.

jQuery(function($){
$("#phone").mask("999-9999999");
});

Optionally, if you are not satisfied with the underscore ('_') character as a placeholder, you may pass an optional argument to the maskedinput method.

jQuery(function($){
$("#phone").mask("999-9999999",{placeholder:" "});
});

Optionally, if you would like to execute a function once the mask has been completed, you can specify that function as an optional argument to the maskedinput method.

jQuery(function($){
 $("#phone").mask("999-9999999",{completed:function(){alert("You typed the 
  following: "+this.val());}});
 });

You can have part of your mask be optional. Anything listed after '?' within the mask is considered optional user input. The common example for this is phone number + optional extension.

jQuery(function($){
  $("#phone").mask("(999) 999-9999? x99999");
});

If your requirements aren't met by the predefined placeholders, you can always add your own. For example, maybe you need a mask to only allow hexadecimal characters. You can add your own definition for a placeholder, say 'h', like so: $.mask.definitions['h'] = "[A-Fa-f0-9]"; Then you can use that to mask for something like css colors.

jQuery(function($){
 $("#phone").mask("#hhhhhh");
});

By design, this plugin will reject input which doesn't complete the mask. You can bypass this by using a '?' character at the position where you would like to consider input optional. For example, a mask of "(999) 999-9999? x99999" would require only the first 10 digits of a phone number with extension being optional.