javascript - V8 会自动缓存编译的正则表达式吗？

Question

所以我知道在 Javascript 中实例化一个正则表达式会导致它被自动编译，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。示例:

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

但是，重复的正则表达式是重新编译还是 V8 (Chrome/Node.js) 智能地重用它们(就像 PHP 对 preg_* 函数所做的那样)？

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

为了澄清，我知道第一个示例会导致一次正则表达式的编译。但是第二个例子呢？

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');

(打开您的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) 上:

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YES，V8 能够自动缓存正则表达式。

---

编辑:

即使 V8能够进行这种优化，它可能不会事实上选择始终使用这种优化。 V8 有许多针对不同情况的复杂启发式算法，例如何时内联函数、何时优化函数、何时取消优化函数等。除非您是 V8 开发人员，否则其中许多启发式算法看起来很奇怪或出乎意料或两者兼而有之。

我自己不是 V8 开发人员，我不知 Prop 体问题的答案，或者是否总是自动缓存正则表达式。但是，即使我知道答案，将来也可能会改变。因此，当您编写代码时，我仍然建议您存储正则表达式，以便清楚该变量的意图是什么。对于上面的示例，这意味着将正则表达式变量取出函数 isFooBar().

另一个问题是这个正则表达式是如何创建的。上面的例子和 OP 的问题是用 regular expression literal 声明的。 .但是，如果您使用 new RegExp() constructor , V8 不允许缓存正则表达式，因为它可能会在运行时发生变化。