Using StringValidator to check if the character count is within range
<?xml version="1.0" encoding="utf-8"?> <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"> <mx:StringValidator source="{username}" property="text" minLength="3" maxLength="20" trigger="{submitButton}" triggerEvent="click" tooShortError="at least 3 characters" tooLongError="too long" /> <mx:Text text="Enter your username:" /> <mx:TextInput id="username" /> <mx:Button label="Submit" id="submitButton" /> </mx:Application>