RegExp.exec() 偶尔返回 NULL

Question

我真的为此发疯了，而且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。所以请帮我一把=)

我需要对 JavaScript 中的字符串进行一些正则表达式匹配。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>");
    }
}

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

我发布了一个 Fiddle here，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 中尝试过。

Answer 1

哦，来了。因为您定义的是全局正则表达式，所以它首先匹配cat，然后在循环的第二遍dog 上匹配。所以，基本上你只需要重置你的正则表达式（它是内部指针）。参照。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);
}

Answer 2

正则表达式对象有一个属性lastIndex，当您运行exec 时它会更新。因此，当您在例如上执行正则表达式时“我也有猫和狗。”，lastIndex 设置为 12。下次您在同一个正则表达式对象上运行 exec 时，它会从索引 12 开始查找。所以您必须重置 lastIndex每次运行之间的属性。

Answer 3

我只使用 /g 时遇到了类似的问题，这里建议的解决方案在 FireFox 3.6.8 中对我不起作用。我的脚本正在使用

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

我添加这个以防其他人遇到与上述解决方案相同的问题。

Answer 4

两件事：

1. 使用g（全局）标志时提到的reset需要。为了解决这个问题，我建议简单地将0 分配给RegExp 对象的lastIndex 成员。这比销毁和重建具有更好的性能。
2. 使用in 关键字来遍历Array 对象时要小心，因为某些库可能会导致意外结果。有时您应该使用 isNaN(i) 之类的东西进行检查，或者如果您知道它没有漏洞，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>");
    }
  }