为什么" $ 1 QUOT;在我Regex.Replace（）的结果结束了？ [英] Why is "$1" ending up in my Regex.Replace() result?

问题描述

我想写一个正则表达式来重写URL指向一个代理服务器。

I am trying to write a regular expression to rewrite URLs to point to a proxy server.

bodystring = Regex.Replace(bodystring, "(src='/+)", "$1" + proxyStr);

这表达的想法很简单，基本上找到src =/或实例 SRC =//和插入在这一点上一个代理URL。这在一般的，但偶尔我已经发现一些文字$ 1将在结果字符串结束。

The idea of this expression is pretty simple, basically find instances of "src='/" or "src='//" and insert a PROXY url at that point. This works in general but occasionally I have found cases where a literal "$1" will end up in the result string.

这是没有意义的我，因为如果没有比赛，那么为什么它在所有替换什么？

This makes no sense to me because if there was no match, then why would it replace anything at all?

可惜我不能在它只有非常大的绳子，会发生至今对此进行一个简单的例子，但我。倒是想从概念上知道什么可以做这样的事情发生。

Unfortunately I can't give a simple example of this at it only happens with very large strings so far, but I'd like to know conceptually what could make this sort of thing happen.

顺便说一句，我试图用一个积极的后向如下重写这个表达式：

As an aside, I tried rewriting this expression using a positive lookbehind as follows:

bodystring = Regex.Replace(bodystring, "(?<=src='/+)", proxyStr);

但如果输入字符串包含SRC ='这两次结束了与proxyStr输出// 。这也没有多大意义，因为我认为SRC =必须是存在于输入为了得到proxyStr在输出端两次提高一倍。

But this ends up with proxyStr TWICE in the output if the input string contains "src='//". This also doesn't make much sense to me because I thought that "src=" would have to be present in the input twice in order to get proxyStr to end up twice in the output.

推荐答案

在 proxyStr =10.15.15.15:8008/proxy?url=http://，更换字符串变成$ 110.15.15.15：8008 /代理URL = HTTP：//。它包含组号码110，这肯定是不存在的参考。

When proxyStr = "10.15.15.15:8008/proxy?url=http://", the replacement string becomes "$110.15.15.15:8008/proxy?url=http://". It contains a reference to group number 110, which certainly does not exist.

您需要确保您的代理字符串不以数字开始。

You need to make sure that your proxy string does not start in a digit. In your case you can do it by not capturing the last slash, and changing the replacement string to "$1/"+proxyStr, like this:

bodystring = Regex.Replace(bodystring, "(src='/*)/", "$1/" + proxyStr);

编辑：

Rawling的指出是.NET的正则表达式库解决了这个问题：你可以附上 1 在大括号，以避免假走样，是这样的：

Rawling pointed out that .NET's regexp library addresses this issue: you can enclose 1 in curly braces to avoid false aliasing, like this:

bodystring = Regex.Replace(bodystring, "(src='/+)", "${1}" + proxyStr);

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