只有当该模式恰好出现一次时，正则表达式才能匹配模式吗？(Can regex match a pattern only if that pattern appears exactly one time?

只有当该模式恰好出现一次时，正则表达式才能匹配模式吗？(Can regex match a pattern only if that pattern appears exactly one time?)

举个例子：

string1 = "aaxadfxasdf" string2 = "asdfxadf" string3 = "sadfas"

我需要一个匹配x的模式，使得它仅为string2返回x的匹配。

注意：出于这个问题的目的，我必须使用java正则表达式。 此外，我无法访问代码，因此我不能简单地检查java regex对象返回的匹配数。

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大家好，

感谢您的回复，其中一些确实回答了我的问题。 我的例子并不像我需要的那样具体。 所以让我把这个怪癖添加到混合中：

如果我匹配多个字符怎么办？

string4 = "This is a sentence!" string5 = "This is just another sentence with some repetition in it. This is a sentence" string6 = "this is"

在这种情况下，我只想在字符串中出现一次时才匹配“句子”。

此外，虽然未在示例中表示，但我需要能够跨行搜索。

Take this example:

string1 = "aaxadfxasdf" string2 = "asdfxadf" string3 = "sadfas"

i need a pattern that matches x such that it returns a match for x only for string2.

Note: for the purpose of this question, i must use java regex. Also, i don't have access to the code, so i can't simply check the # of matches returned by the java regex object.

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Hey All,

Thanks for the responses, some of which do answer my question. My example wasn't as specific as i needed it to be. So let me add this quirk to the mix:

what if i'm matching more than a single character?

string4 = "This is a sentence!" string5 = "This is just another sentence with some repetition in it. This is a sentence" string6 = "this is"

In this case, i want to match "sentence" only if it appears in the string only once.

Also, though not represented in the example, i need to be able to search across lines.

最满意答案

^[a-wy-z]+x[a-wy-z]+$

我不确定你是否只是期待字母，或者你想要匹配的x之前和之后是否包含符号和数字。 如果是这样，请使用[^x]而不是[a-wy-z] ，如下所示：

^[^x]+x[^x]+$

如果您还希望自己匹配x ，请将+符号替换为*如下所示：

^[a-wy-z]*x[a-wy-z]*$ ^[a-wy-z]+x[a-wy-z]+$

I'm not sure if you are only expecting letters, or if symbols and numbers might be included before and after the x you want to match. If so, use [^x] instead of [a-wy-z], like so:

^[^x]+x[^x]+$

If you are also wanting to match x by itself, replace the + symbols with * like so:

^[a-wy-z]*x[a-wy-z]*$