While I agree that overly-aggressive validation can be a problem I think validating too late is a worse solution. In this case I think the best solution would be to present an error tip to the user if they typed in an invalid character. You could have an error tip fade in for a few seconds and then fade out immediately when a valid character is entered. Of course giving the user some sort of hint as to what characters are valid is probably a good thing to do.
Re: Flex Examples : Validating user input in a TextInput ...
While I agree that overly-aggressive validation can be a problem I think validating too late is a worse solution. In this case I think the best solution would be to present an error tip to the user if they typed in an invalid character. You could have an error tip fade in for a few seconds and then fade out immediately when a valid character is entered. Of course giving the user some sort of hint as to what characters are valid is probably a good thing to do.
For your example with the social security field I use this component http://www.adobe.com/cfusion/exchange/index.cfm?event=extensionDetail&ex.... It extends TextInput to allow the developer to specify a mask for the input.