V8是否自动缓存编译的正则表达式？ [英] Does V8 cache compiled regular expressions automatically?
本文介绍了V8是否自动缓存编译的正则表达式？的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
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。示例：
var re = / [Ff] oo | [Bb] ar /;
Foo。匹配（重新）; // [Foo]
Baz。匹配（重新）; // null
然而，是重新编译重复的正则表达式还是智能地执行V8（Chrome / Node.js）重用它们（就像PHP用preg_ *函数）？
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？
解决方案
好吧，让我们来看看。
< pre class =snippet-code-js lang-js prettyprint-override>
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[@＃$％^&安培; + =]）* $）（^（= * [AZ]）（= * [AZ。？。？。 ]）（= * [@＃$％^&安培;。+ =]）* $）$ /）;}？console.time（ '1'）; isFooBar（ 富）; console.timeEnd（ 1' ），C onsole.time（ '2'）; isFooBar（ 酒吧）; 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。）
在节点上。 js 0.10.36（V8 3.14.5.9）：
1：65ms
2：0ms
在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，其实可能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。
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。以上和OP的问题中的示例使用正则表达式声明文字。但是，如果您使用的是 新的RegExp（）构造函数，V8 不允许缓存正则表达式，因为它可能在运行时更改。
So I know that in Javascript instantiating a regex causes it to be compiled automatically and if that regex is stored in a variable it will be reused without recompiling. Example:
var re = /[Ff]oo|[Bb]ar/;
"Foo".match(re); // ["Foo"]
"Baz".match(re); // null
However, are duplicated regular expressions recompiled or does V8 (Chrome/Node.js) intelligently re-use them (like PHP does with preg_* functions)?
function isFooBar(str) {
return str.match(/[Ff]oo|[Bb]ar/);
}
isFooBar("Foo"); // ["Foo"]
isFooBar("Baz"); // null
So to clarify, I know the first example results in one compilation of the regex. But what about the second example?
解决方案
Well, let's find out.
// 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');
(Open your console and run the code snippet above to see if the results for your browser.)
On Node.js 0.10.36 (V8 3.14.5.9):
1: 65ms
2: 0ms
On io.js 1.6.3 (V8 4.1.0.27):
1: 32ms
2: 0ms
So, I guess the answer to your question is YES, V8 is capable of automatically caching the regex.
EDIT:
Even though V8 is capable of such optimization, it might not in fact choose to always use this optimization. V8 has many complex heuristics for different cases, like when to inline a function, when to optimize a function, when to de-optimize a function, etc. Unless you are a V8 developer, many of these heuristics would appear to be bizarre or unexpected or both.
Not being a V8 developer myself, I do not know the answer to the specific question of if regexes are always automatically cached. However, even if I do know the answer, it may change in the future. So when you are writing code, I would still recommend storing the regex in such a way that it is clear what the intention for this variable is. And for the example above, this means lifting the regex variable out of the function isFooBar().
Another issue is how this regex is created. The example above and in OP's question are declared with a regular expression literal. However, if you are using the new RegExp() constructor, V8 would not be allowed to cache the regex since it might change during runtime.
这篇关于V8是否自动缓存编译的正则表达式？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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