我喝了一罐过期半年的汽水 还有救吗

Aspartame in diet sodas will naturally break down over time, especially if stored in higher temperatures, and in both diet and regular sodas the carbon dioxide will produce further carbonic acid (incidentally making itself flat), but aside from a change in flavour, the beverage is likely to be completely safe to drink -- and will probably still taste good.
汽水中的天冬甜素会随着时间自然削弱，尤其是被储存在高温条件下的时候，无论是饮用汽水还是普通苏打水中，二氧化碳都会产生更多的碳酸（可能会使它变瘪），但是除了味道上有所变化，饮料还是完全可以放心饮用的-而且味道可能依然很好。

It would take a very, very long time for the flavour to be affected enough to be completely unacceptable -- and even then it would still be safe.
让味道变得完全不可接受需要很长，很长的时间-但即使到那时汽水还是可以被放心饮用的。

Even if all of the remaining carbon dioxide has broken down into sour acid (which would take years in an unopened can), it's still far more dilute and less acidic than your stomach acid, assuming you can handle the terrible flavour.
就算所有剩下的二氧化碳都弱化成酸（在没打开的罐子里应该需要好几年的时间），和你的胃酸相比它还是更淡的，没有那么酸，只要你能忍受它糟糕的味道的话。

Soda is usually consumed well before this becomes a problem, of course.
当然，在这成为一个问题之前，汽水一般已经被卖得差不多了。

One caution should be made: any sodas that contain real fruit juice, like orange soda, grape soda, etc., should be viewed as potentially unsafe after their expiration. They can mould or ferment rapidly once the container is opened, in the case of aluminum cans, and glass or plastic bottles which have less perfect seals should be viewed as unsafe even before opening.
值得注意的是：任何含有纯果汁的碳酸饮料，比如橙子味汽水、葡萄味汽水等等，过期之后都被认为是无法安全饮用的。一旦密闭容器被打开，它们可能会很快地发霉或者发酵，铝罐、玻璃或者塑料罐都可能密封得没有那么好，甚至往往在被打开之前就是不安全的。

PS: The date on the packaging is the "Optimum Taste Date." This is the date by which companies recommend you to consume the beverage in order to enjoy the optimum taste.
另：包装上的日期是“最佳饮用期”。这是食品公司的建议，为了品尝到最佳的味道，最好在这个日期之前喝完饮料。