Simple JQuery Plugin to Format Validate a U.S Telephone number

Recently I needed a simple Validation component which would take care of auto formatting the content entered in a text box to a U.S phone number (NPA)-NXX-XXXX. As I was using JQuery, I thought of writing a simple plug-in to do the same. Sharing the code here, just in case this is useful for someone else

The plug-in does the following

   1. Formats the number to (NPA)-NXX-XXXX as it is typed
   2. It will not allow you to enter special characters or non-number in the textbox.
   3. It will validate that you can enter only 10 digits
   4. This plug-in as of now does not support chaining as I did not see a need.

 The code and the HTML Snippet are shown below. Please do add a comment for any bugs.

Plug-in code : Save this as whatever JS file you need it to be - I have used jquery.usphone.js in the HTML

(function( $ )
{
  // This function formats a text field to a US 
  // phone number as the user types the information.
 
  $.fn.usphone = function() 
  {
    this.keyup(function() 
    {
     var curchrindex = this.value.length;
     var curval = $(this).val();
     var strvalidchars = "0123456789()-";
          
     for (i =0; i < this.value.length; i++)
     {
      var curchar = curval[i];
      if (strvalidchars.indexOf(curchar) == -1) 
      {
       //delete the character typed if this is not a valid character.
       $(this).val(curval.substring(0, i) + curval.substring(i+1, this.value.length));
      }
     }
     
     // Insert formatting at the right places
     if (curchrindex == 3) 
     {
      $(this).val("(" + curval + ")" + "-");
     }
     else if (curchrindex == 9)
     {
      $(this).val(curval + "-");
     }
     
     //Do not allow more than 15 characters including the brackets and the "-"
     if (curchrindex == 15) 
     {
      //delete the last character typed 
         $(this).val(curval.substring(0, this.value.length-1 ));
     }
     
    });
  };
})( jQuery );

The HTML code to use this plug-in

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
<script type="text/javascript" src="./scripts/jquery.js"></script>
<script type="text/javascript" src="./jquery.usphone.js"></script>
</head>

<body>
    Please Enter a Valid US Phone number : 
    <input type = "text" id = "phonenumber" />
</body>

<script>
    $('#phonenumber').usphone();
</script>

</html>