V8会自动缓存编译好的正则表达式吗?

好吧，我们来看看。

// http://stackoverflow.com/questions/3466850/complex-password-regular-expression
function isFooBar(str) {
  return 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}
console.time('1');isFooBar("Foo");console.timeEnd('1');
console.time('2');isFooBar("Bar");console.timeEnd('2');

(打开控制台并运行上面的代码片段，看看结果是否适合您的浏览器。)

在Node.js 0.10.36 (V8 3.14.5.9):

1: 65ms
2: 0ms

在io.js 1.6.3 (V8 4.1.0.27)上:

1: 32ms
2: 0ms

所以，我猜你的问题的答案是是的， V8能够自动缓存正则表达式。

即使V8 能够进行这种优化，它实际上可能不会总是选择使用这种优化。V8针对不同的情况有许多复杂的启发式方法，比如何时内联函数，何时优化函数，何时反优化函数，等等。除非你是一名V8开发人员，否则这些启发式方法中的许多可能看起来很奇怪或出乎意料，或者两者兼而有之。

我自己不是V8开发人员，我不知道具体问题的答案，也不知道正则表达式是否总是自动缓存。然而，即使我知道答案，将来也可能会改变。因此，当您编写代码时，我仍然建议以这样一种方式存储正则表达式，即清楚该变量的意图是什么。对于上面的示例，这意味着将regex变量从函数isFooBar()中取出。

另一个问题是如何创建这个正则表达式。上面的例子和OP的问题都是用正则表达式literal声明的。但是，如果您使用的是new RegExp()构造函数，V8 将不允许缓存正则表达式，因为它可能在运行时更改。

http://code.google.com/p/v8/issues/detail?id=933#c3

在Chrome 9中，我们已经删除了RegExp结果的基本缓存。这意味着我们不再需要花费不切实际的低时间来一次又一次地对相同的输入重复相同的操作。最初，添加缓存是为了指出过于简单(当时包括我们自己的)的基准测试太容易通过添加这样的缓存来"游戏"(这在实际场景中没有什么区别)。此后，大多数基准测试都有了很大的改进，因此我们决定删除缓存(及其不可避免的开销)。该缓存在v8版本4083 (Chrome 5发布)中添加，并在版本5755(将在Chrome 9中)中删除