Reg: regular expression should be alpha numeric
2.min 5 characters and max 15
3."two alphabets followed by a number" format not allowed in the
beginning .if any one have solution.pls send me

Completely untested. (Straight from brain to keyboard, so you will have to debug.) Not sure I understand #3 correctly, but if you *want* to match two letters followed by one or more alpha-numerics, something like this should work, but again, untested..

   validates_length_of :whatever, :within => 5..15, :on => :create, :message => "must be between 5 and 15 characters long"
   validates_format_of :whatever, :with => %r{\A[a-zA-Z]{2}\w+\Z}, :message => "must be blah blah blah."

Good luck.