Regular Expression Validation

 Hello,

    I have one code field which is input field from text box,now i want  to make following kind of condition for that any one have
    idea for regular validate expression...

                code should in between Caps A TO Z.

                code should be min 4 char.

                repeated character dnt allow for example 'AACD' ar not allowed while 'ABAB' allowed.
...