A particularly useful method of the flash.text.TextField class is the setTextFormat() method. Using setTextFormat(), you can assign specific properties to part of a text field's contents to respond to user input, such as forms that need to remind users that certain entries are required or to change the emphasis of a subsection of a passage of text within a text field as a user selects parts of the text.

The following example uses TextField.setTextFormat() on a range of characters to change the appearance of part of the content of myTextField when the user clicks on the text field:

var myTextField:TextField = new TextField();
myTextField.text = "No matter where you click on this text field the TEXT IN ALL CAPS changes format.";
myTextField.autoSize = TextFieldAutoSize.LEFT;
addChild(myTextField);
addEventListener(MouseEvent.CLICK, changeText);

var myformat:TextFormat = new TextFormat();
myformat.color = 0xFF0000;
myformat.size = 18;
myformat.underline = true;

function changeText(event:MouseEvent):void
{
    myTextField.setTextFormat(myformat, 49, 65);
}