how can I allow epmty value in matches[] rule in validation?

[eluser]suba[/eluser]
Hi .

CASE 1
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I used form validation class.

I want to check only value should be either A, B , C , D or empty values.

How can i use in form validation rule in form validation.?

please advise me.

CASE 2
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I want to allow empty value and number only.

How can i use this case?