You won't regain all your weight if you reintroduce carbs; as in you won't gain 3 stone overnight after a weekend off. What you'll find though is your weight will fluctuate, so for me it's about 5lbs. Target isn't static, you will bob about and yes if you have a week away and eat and drink whatever you want, you'll gain some weight back. But then you get back on plan and lose it again. Repeatedly eating carbs and not getting back on plan will result in weight gain. Let's be realistic here, for most of us that's what has caused our weight gain in the first place.
I've maintained, not intentionally but fairly effectively, for 18 months. I do it by not eating carbs in the week, but I drink at weekends and eat occasional carbs. There's a lot of foods that I almost never eat now: potato, cereal, bread, bananas, pasta, rice, beer/cider/full sugar drinks, fruit juice or smoothies (even on a night off I would never have rice for example with a takeaway). I eat more sugar than I think is really healthy for me at Times (but I also think I am very sugar sensitive and possibly pre-diabetic) . To lose and then maintain it's more than simply calorie counting. For me it requires regular exercise, making this food a way of life and not a diet, and really tackling emotional and compulsive eating behaviour. It's the emotional eating which is preventing me from losing that last stone and it's something I always fall back quite heavily on when life is tough but I do work at it.
To be brutal and honest, "if you do what you have always done, you get what you've always got " - I would love to get to a place where, not so much that I can have pizza and cake and maintain, but actually achieve that elusive holy grail of not even wanting the pizza and cake in the first place. I want it a hell of a lot less when I'm on plan properly, and considerably less than I used to, but that takes work and mental strength that sometimes I don't have. However, for me, I know that if I go back to eating how I used to I will (and do, and have) regain some weight. Similarly I know if if I just stop exercising regularly, I'd regain some weight.