Question

所以我知道在Javascript实例化一个正则表达式会导致它自动编译，如果该正则表达式存储在一个变量中，它将被重用而无需重新编译。例如：

var re = /[Ff]oo|[Bb]ar/;
"Foo".match(re); // ["Foo"]
"Baz".match(re); // null

但是，是否重新编译了重复的正则表达式，或者V8（Chrome / Node.js）是否会智能地重用它们（就像PHP与preg_ *函数一样）？

function isFooBar(str) {
  return str.match(/[Ff]oo|[Bb]ar/);
}
isFooBar("Foo"); // ["Foo"]
isFooBar("Baz"); // null

因此，为了澄清，我知道第一个例子是一个正则表达式汇编的结果。但第二个例子怎么样？

Answer 1

好吧，让我们找出答案。

// 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');

（打开你的控制台并运行上面的代码片段，看看你的浏览器是否有结果。）

在Node.js 0.10.36（V8 3.14.5.9）上：

1: 65ms
2: 0ms

on io.js 1.6.3（V8 4.1.0.27）：

1: 32ms
2: 0ms

所以，我猜你问题的答案是是，V8能够自动缓存正则表达式。

编辑：

即使V8 能够进行此类优化，可能实际上也不会选择始终使用此优化。 V8针对不同的情况提供了许多复杂的启发式方法，例如何时内联函数，何时优化函数，何时对函数进行去优化等。除非您是V8开发人员，否则许多这些启发式方法似乎都是奇异或意外的或两者兼而有之。

我自己不是V8开发人员，我不知道是否总是自动缓存正则表达式的具体问题的答案。但是，即使我知道答案，也可能在将来发生变化。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，即清楚该变量的用途是什么。对于上面的示例，这意味着将正则表达式变量提升函数isFooBar()。

另一个问题是如何创建此正则表达式。上面的示例和OP的问题用regular expression literal声明。但是，如果您使用new RegExp() constructor，则不允许使用V8 来缓存正则表达式，因为它可能会在运行时更改。

Answer 2

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

在Chrome 9中，我们删除了RegExp结果的原始缓存。这意味着我们不再采取不切实际的低时间一遍又一遍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化的基准（我们自己的，当时包括在内）通过添加这样的缓存太容易“游戏”（这在现实场景中没有任何区别）。大多数基准测试已经改进了很多，我们决定删除缓存（及其不可避免的开销）。缓存已添加到v8版本4083（在Chrome 5中发布）中，并在版本5755中删除（将在Chrome 9中删除）

V8是否自动缓存编译的正则表达式？

2 个答案: