Hi  
I have a problem with a regular expression. I have an RegExpValidator in MXML and I want that is invalid when the source contain a or b  
My RegExpValidator is

```
<mx:RegExpValidator source="{value}"
                    property="text"
                    expression='.*[^ab].*'
                    valid="isValid(event)"
                    invalid="isInvalid(event)"/>

```

My expression is `expression='.*[^ab].*'` 
When it's just a, b or a and b (one or many times) the expression is invalid : OK  

When it's everything else the expression is valid : OK  

But when it a or/and b with other caractere, it's also valid. What do I have to change to have this invalid?