Need RegEx in asp.net textbox for SSN

Hi. I have a textbox control from devexpress with the following validation on an SSN value:

<ValidationSettings>
<RegularExpression ValidationExpression="\d{9}">
<ValidationSettings>

This RegEx ensures the SSN in the textbox is 9 numeric characters. I now need it to check and allow any of the following:

1) 9 numeric characters (ex: 123456789)
2) 4 numbers followed by the letter F followed by 4 numbers (ex: 1234f5678)
3) The letter F followed by 8 numbers (ex: f12345678)

I don't care about the letter F being upper or lower case. No other format is allowed.