一个正则表达式，永远不会被任何东西匹配 [英] A Regex that will never be matched by anything

问题描述

这可能听起来像是一个愚蠢的问题，但是我和一些开发人员有很长的交谈，听起来好像有趣。

所以;你的想法是什么 - 正则表达式是什么样的，永远不会与任何字符串相匹配！

编辑：为什么我想要这个？那么，首先因为我觉得有趣的是想到这样的表达，其次是因为我需要一个脚本。

在这个脚本中，我将一个字典定义为字典< string，Regex> 。如您所见，这包含一个字符串和一个表达式。

根据该字典，我创建的方法都使用这个字典作为仅作为他们应该如何工作的参考，其中一个与正则表达式匹配解析的日志文件。

如果一个表达式匹配，另一个 Dictionary< string，long> 添加了表达式返回的值。所以，要捕获字典中表达式不匹配的任何日志消息，我创建了一个名为unknown的新组。

对于这个组， t匹配任何其他添加。但是为了防止未知表达式发生错误（意外）一个日志消息，我不得不创建一个绝对不能匹配的表达式，无论我提供什么字符串。

因此，你有我的理由这个不是真正的问题...

解决方案

实际上很简单，虽然这取决于实现/标志 *：

  $ a 
  

匹配一个字符 a 串。祝你好运。

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*）最初我没有多思考多行模式正则表达式，其中 $ 也匹配一行的末尾。实际上，它会匹配换行符之前的空字符串，所以一般的字符，如 a 永远不会出现在 $ 。

This might sound like a stupid question, but I had a long talk with some of my fellow developers and it sounded like a fun thing to think of.

So; what's your thought - what does a Regex look like, that will never be matched by any string, ever!

Edit: Why I want this? Well, firstly because I find it interesting to think of such an expression and secondly because I need it for a script.

In that script I define a dictionary as Dictionary<string, Regex>. This contains, as you see, a string and an expression.

Based on that dictionary I create methods that all use this dictionary as only reference on how they should do their work, one of them matches the regexes against a parsed logfile.

If an expression is matched, another Dictionary<string, long> is added a value that is returned by the expression. So, to catch any log-messages that are not matched by an expression in the dictionary I created a new group called "unknown".

To this group everything that didn't match anything other is added. But to prevent the "unknown"-expression to mismatch (by accident) a log-message, I had to create an expression that is most certainly never matched, no matter what string I give it.

Thus, there you have my reason for this "not a real question"...

解决方案

This is actually quite simple, although it depends on the implementation / flags*:

$a

Will match a character a after the end of the string. Good luck.

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*) Originally I did not give much thought on multiline-mode regexp, where $ also matches the end of a line. In fact, it would match the empty string right before the newline, so an ordinary character like a can never appear after $.

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