Count number of times a letter appears in a string and display number in real time?

[jAQUAN]

Using the Event.CHANGE event listener, you can get notified every time the contents of a text field changes (ie. the user typed or deleted characters)

myTextField.addEventListener(Event.CHANGE, onTextChange);

Then you'll have to use String.match() to find the number of occurrences. But to count them all, you have to pass a Regular Expression to match(). Regular Expression are basically string voodoo which are fairly difficult to understand but can do amazing things. In your case a simple character match is not too difficult.

function onTextChange(event:Event):void{
  // here you can do the counting
  // first we create the expression to match.
  // The single forward slashes '/' mean "this is the beginning and end of my expression".
  // The 'g' on the end is a modifier that tells it we want a global match meaning "find all the matches in this string"
  // the open and close brackets '[]' means "here is a list of things to look for". In this example we are searching or any lower case or upper case 'S'
  // the pipe between the two s's means match a lower case OR an upper case.
  var pattern:RegExp = /[s|S]/g;
  // now we call match() on the users input and pass in the RegExp.
  // it returns an array of all matches so the length of that array is your number.
  var results:Array = myTextField.text.match(pattern);
  trace ("There are:", results.length, "S's in the text field"); // If the users in put was "See Spot Run" this would trace: "There are 2 S's in the text field"
}