V8是否自动缓存编译的正则表达式？

Question

所以我知道在Javascript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.例:

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

但是,重新编译重复的正则表达式还是V8(Chrome/Node.js)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

所以为了澄清,我知道第一个例子是一个正则表达式编译的结果.但是第二个例子怎么样？

Answer 1

好吧,让我们来看看.

// 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,看看您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

在io.js 1.6.3(V8 4.1.0.27)上:

1: 32ms
2: 0ms

所以,我想你的问题的答案是肯定的,V8能够自动缓存正则表达式.

编辑:

即使V8 能够进行这样的优化,实际上也可能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怪的或意外的或两者.

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,以便明确该变量的用途是什么.对于上面的例子,这意味着将正则表达式变量从函数中提取出来isFooBar().

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