Flex: Restrict data input

I have a TextField as:

 <s:TextInput restrict="A-Z0-9_" maxChars="30"/>

And I am restricting the input to:

• Capital letters
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• Numbers
• _
• no spaces
• max chars 30

How can I restrict the input more to allow only letters at the begging and end?

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The restrict-field validates immediately when the user types in something.
You need an RegExp-Validator to validate after the user wants to submit the data, so you can't use the restrict-field.