RegExp.exec（）偶尔返回NULL [英] RegExp.exec() returns NULL sporadically

问题描述

我对此非常认真，我已经花费了不成比例的时间来试图弄清楚这里发生了什么。所以请给我一个手=）

I am seriously going crazy over this and I've already spent an unproportionate amount of time on trying to figure out what's going on here. So please give me a hand =)

我需要在JavaScript中对字符串进行一些RegExp匹配。不幸的是，它表现得非常奇怪。此代码：

I need to do some RegExp matching of strings in JavaScript. Unfortunately it behaves very strangely. This code:

var rx = /(cat|dog)/gi;
var w = new Array("I have a cat and a dog too.", "There once was a dog and a cat.", "I have a cat and a dog too.", "There once was a dog and a cat.","I have a cat and a dog too.", "There once was a dog and a cat.","I have a cat and a dog too.", "There once was a dog and a cat.","I have a cat and a dog too.", "There once was a dog and a cat.","I have a cat and a dog too.", "There once was a dog and a cat.","I have a cat and a dog too.", "There once was a dog and a cat.");

for (var i in w) {
    var m = null;
    m = rx.exec(w[i]);
    if(m){
        document.writeln("<pre>" + i + "\nINPUT: " + w[i] + "\nMATCHES: " + m.slice(1) + "</pre>");
    }else{
        document.writeln("<pre>" + i + "\n'" + w[i] + "' FAILED.</pre>");
    }
}

首先返回cat和dog两个元素，应该是，但是然后一些 exec（） -calls开始返回 null 。我不明白为什么。

Returns "cat" and "dog" for the first two elements, as it should be, but then some exec()-calls start returning null. I don't understand why.

我发了一个小提琴这里，您可以在其中运行和编辑代码。

I posted a Fiddle here, where you can run and edit the code.

到目前为止，我已经在Chrome和Firefox中尝试过此操作。

And so far I've tried this in Chrome and Firefox.

干杯！

/ Christofer

/Christofer

推荐答案

哦，就是这样。因为你正在定义你的正则表达式全局，它匹配第一个 cat ，并在循环的第二次传递 dog 。所以，基本上你只需要重置你的正则表达式（它的内部指针）。参看这个：

Oh, here it is. Because you're defining your regex global, it matches first cat, and on the second pass of the loop dog. So, basically you just need to reset your regex (it's internal pointer) as well. Cf. this:

var w = new Array("I have a cat and a dog too.", "I have a cat and a dog too.", "I have a cat and a dog too.", "I have a cat and a dog too.");

for (var i in w) {
    var rx = /(cat|dog)/gi;
    var m = null;
    m = rx.exec(w[i]);
    if(m){
        document.writeln("<p>" + i + "<br/>INPUT: " + w[i] + "<br/>MATCHES: " + w[i].length + "</p>");
    }else{
        document.writeln("<p><b>" + i + "<br/>'" + w[i] + "' FAILED.</b><br/>" + w[i].length + "</p>");
    }
    document.writeln(m);
}

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