问题描述：

I have the following RegEx ^http:\\/\\/(?!www\\.)(.*)$

Expected behavior:

http://example.com - Match
http://www.example.com - Does not match

It looks like golang does not support negative lookahead. How can I rewrite this RegEx to work on golang?

UPDATE

I\'m not coding using golang, I\'m using Traefik that accepts a Regex (golang flavor) as a config value, so basically I have this:

regex = \"^https://(.*)$\"
replacement = \"https://www.$1\"

What I want is to always add www. to the URL, but NOT if the URL has it already, otherwise it would become www.www.*

图片转代码服务由CSDN问答提供

感谢您的意见，我们尽快改进~

功能建议

我有以下RegEx ^ http：\\ / \\ /（?! www \\。）（。 *）$ </ code> </ p>

预期的行为：</ p>

  http://example.com-匹配
http：// www  .example.com-不匹配
</ code> </ pre> 
  golang </ code>似乎不支持否定超前。 我如何重写此RegEx以在 golang </ code>上工作？</ p> 
 更新</ strong> </ p> 
我是 我没有使用golang进行编码，因此我使用的是 Traefik ，它接受Regex（golang风格）作为配置值，因此基本上 我有这个：</ p> 
  regex =“ ^ https：//（。*）$” 
replacement =“ https：//www.$1” 
</ code>  </ pre> 
我想要的是始终在URL中添加 www。</ strong>，但是如果URL中已经包含 NOT </ strong>，则为 NOT </ strong>，否则 成为 www.www。* </ strong> </ p> 
</ div> 
网友观点：
If you\'re really bent on creating a negative lookahead manually, you will need to exclude all possible w in the regexp:
^https?://(([^w].+|w(|[^w].*)|ww(|[^w].+)|www.+)\\.)?example\\.com$
This regexp allows any word with a dot before example.com, unless that word is just www. It does so by allowing any word that does not start with w, or, if it starts with w it is either just that w or followed by a non-w and other stuff. If it starts with two w, then it must be either just that or followed by a non-w. If it starts with www, it must be followed by something.
Demo
The clarification makes this much much easier. The approach is to always (optionally) match www. and then to put that back in the replacement always:
Search:
^http://(?:www\\.)?(.*)\\b$
Replace:
http://www.$1
Demo 2
    
Golang uses the RE2 regex engine, which doesn\'t support look arounds of any kind.
Since you are dealing with URLs, you can simply parse them and inspect the host part:
package main
import (
\\\"net/url\\\"
\\\"strings\\\"
\\\"testing\\\"
)
func Match(s string) bool {
u, err := url.Parse(s)
switch {
case err != nil:
return false
case u.Scheme != \\\"http\\\":
return false
case u.User != nil:
return false
}
return !strings.HasPrefix(u.Host, \\\"www.\\\")
}
func TestMatch(t *testing.T) {
testCases := []struct {
URL  string
Want bool
}{
{\\\"http://example.com\\\", true},
{\\\"http://wwwexample.com\\\", true},
{\\\"http://www.example.com\\\", false},
{\\\"http://user@example.com\\\", false},
{\\\"http://user@www.example.com\\\", false},
{\\\"www.example.com\\\", false},
{\\\"example.com\\\", false},
}
for _, tc := range testCases {
if m := Match(tc.URL); m != tc.Want {
t.Errorf(\\\"Match(%q) = %v; want %v\\\", tc.URL, m, tc.Want)
}
}
}
    
一个正则表达式引发的血案，让线上CPU100%异常！
作者：陈树义 | 该文来自腾讯云+社区 陈树义的专栏
前几天线上一个项目监控信息突然报告异常，上到机器上后查看相关资源的使用情况，发现 CPU 利用率将近 100%。通过 Java 自带的线程 Dump 工具，我们导出了出问题的堆栈信息。


我们可以看到所有的堆栈都指向了一个名为 validateUrl 的方法，这样的报错信息在堆栈中一共超过 100 处。通过排查代码，我们知道这个方法的主要功能是校验 URL 是否合法。
很奇怪，一个正则表达式怎么会导致 CPU 利用率居高不下。为了弄清楚复现问题，我们将其中的关键代码摘抄出来，做了个简单的单元测试。