<?xml version="1.0" encoding="utf-8"?>
  <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
     backgroundColor="#FFFFFF" width="300" height="150">
     <mx:Script>
             <![CDATA[
              private function validate():void{
         // check for length of 10
         if(myText.text.length < 10){
           // assign errorString to TextInput
           myText.errorString = "This field requires at least 10 characters";
         } else {
           // remove errorString
           myText.errorString = null;
         }
       }
       ]]>
     </mx:Script>
     <mx:Panel title="Simple AS Validate" width="210" height="75">
       <mx:HBox width="100%" height="100%" paddingLeft="3" paddingTop="3">
         <mx:TextInput id="myText" width="100"/>
         <mx:Button label="Validate" click="validate()" />
       </mx:HBox>
     </mx:Panel>
  </mx:Application>