RegExp.exec（）偶尔返回NULL

我为此感到非常疯狂，并且我已经花费了不成比例的时间来试图弄清这里正在发生什么。所以请帮我=）

我需要对JavaScript中的字符串进行RegExp匹配。不幸的是，它的行为非常奇怪。这段代码：

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>");
    }
}

按原样返回前两个元素的“ cat”和“ dog”，但随后一些exec()调用开始返回null。我不明白为什么。

我在这里发布了一个小提琴，您可以在其中运行和编辑代码。

到目前为止，我已经在Chrome和Firefox中进行了尝试。

干杯!

/克里斯托弗

哦，在这里。因为要定义全局正则表达式，所以它首先匹配cat，然后在循环的第二遍匹配dog。因此，基本上，您只需要重置您的正则表达式（它是内部指针）即可。cf. 这个：

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);
}

regex对象具有一个lastIndex在运行时会更新的属性exec。因此，当您在例如“我也有猫和狗。”上执行正则表达式时，lastIndex设置为12。下一次exec在同一个正则表达式对象上运行时，它将从索引12开始查找。因此，您必须重置lastIndex属性每次运行之间。

两件事情：

1. 当使用（全局）标志时，提到需要重置g。为了解决这个问题，我建议您简单地分配0给lastIndex该RegExp对象的成员。这比销毁并重新创建具有更好的性能。
2. 在使用in关键字行走Array对象时要小心，因为使用某些库可能导致意外结果。有时，您应该检查一些类似的东西isNaN(i)，或者，如果您知道它没有孔，请使用经典的for循环。

代码可以是：

var rx = /(cat|dog)/gi;
w = ["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)
 if(!isNaN(i))        // Optional, check it is an element if Array could have some odd members.
  {
   var m = null;
   m = rx.exec(w[i]); // Run
   rx.lastIndex = 0;  // Reset
   if(m)
    {
     document.writeln("<pre>" + i + "\nINPUT: " + w[i] + "\nMATCHES: " + m.slice(1) + "</pre>");
    } else {
     document.writeln("<pre>" + i + "\n'" + w[i] + "' FAILED.</pre>");
    }
  }

我仅使用/ g时遇到了类似的问题，在FireFox 3.6.8中，这里提出的解决方案对我不起作用。我的脚本正在与

var myRegex = new RegExp("my string", "g");

如果有人遇到与上述解决方案相同的问题，则添加此内容。