Let's admit it. The girl's in great shape. Now how did she get there?
I am looking for a complete exercise regimen and diet of Britney
Spears. Does she do a specific type of exercise? How many times per
week? How long? What strength training does she do? Is she on a
specific diet (Low Carb, Low Fat, Vegetarian, etc)? Basically, how
does she keep fit and healthy?

You're going to hate this, I know you will.  But this is how to get
that body you want.

First go out and buy Arnold Schwarzenneger's Encyclopedia of body
building.  Body building is Arnold passion!  If you read the book
cover to cover you'll see that.  Do exactly want he tells you to do. 
Use heavy weights, that is a must. No little girl 2 pounds weights No
little 5 pound weights etc.  Your muscles will hurt, enjoy the pain. 
That's a hard thing to do, but it is possible. When the muscle pain
hits think, THINK about what is happening to your body!! Turn the pain
into a positive feeling.  And if you can, you will be stronger and
MOST IMPORTANTLY, more shapely.

YOU WILL NOT BUILD BIG MUSCLE - NEVER EVER NEVER EVER NEVER EVER.

THE WOMEN YOU SEE WITH BIG MUSCLES USE DRUGS.

WOMEN WITH GREAT BODIES DO NOT TAKE DRUGS BUT THEY WORK OUT WITH HEAVY WEIGHTS.

Most importantly, don't workout at a pretty boy gym.  Find the gym in
town where the bodybuilders workout.  It's not a pretty gym, but you
will find motivation there.  Try not to use machines, use barbells.
Get a trainer.  Not a pretty boy trainer, a bodybuilder trainer that
will use heavy weights with you and no matter how you complain he just
puts more weight on.  Get the really thing, you will find HIM at this
type of gym.

Diet, eat meat! After ever workout drink milk.  You can buy the
protein shakes if you want to, but milk is just as good, maybe add a
scoop of protein powder. And they are cheap.

Workout twice a day for one hour. Follow Arnold's plan. Workout hard.
Keep records and charts. Take before and after pictures.  YOU WILL BE
VERY HAPPY. And guess what, you will be so strong and have the hottest
body that Britney Spears will be asking you how you did it!!

Set a some kind of goal.  A goal were you showoff your new body.  I'm
thinkin' wet tee shirt contest ---- but I digress.  :-)

Keep your body fat at around 10 to 15 percent, maybe a little more. It
will depend on your jaw line.  Women may like you skin and bones and
tell you so, but guys don't.  Believe the guys,(after all we are the
experts on good lookin') don't believe the girls. You want to look
like a strong shapely women - not a 12 year old girl.

And then send Arnold a thank you note.  

I hope this helps.