from :https://stackoverflow.com/questions/3512471/what-is-a-non-capturing-group-what-does-a-question-mark-followed-by-a-colon

fter reading some tutorials I still don't get it.

Could someone explain how ?: is used and what it's good for?

Let me try to explain this with an example.

Consider the following text:

https://stackoverflow.com/
https://stackoverflow.com/questions/tagged/regex

Now, if I apply the regex below over it...

(http|ftp)://([^/\r\n]+)(/[^\r\n]*)?

... I would get the following result:

Match "https://stackoverflow.com/"
Group 1: "http"
Group 2: "stackoverflow.com"
Group 3: "/" Match "https://stackoverflow.com/questions/tagged/regex"
Group 1: "http"
Group 2: "stackoverflow.com"
Group 3: "/questions/tagged/regex"

But I don't care about the protocol -- I just want the host and path of the URL. So, I change the regex to include the non-capturing group (?:).

(?:http|ftp)://([^/\r\n]+)(/[^\r\n]*)?

Now, my result looks like this:

Match "https://stackoverflow.com/"
Group 1: "stackoverflow.com"
Group 2: "/" Match "https://stackoverflow.com/questions/tagged/regex"
Group 1: "stackoverflow.com"
Group 2: "/questions/tagged/regex"

See? The first group has not been captured. The parser uses it to match the text, but ignores it later, in the final result.


EDIT:

As requested, let me try to explain groups too.

Well, groups serve many purposes. They can help you to extract exact information from a bigger match (which can also be named), they let you rematch a previous matched group, and can be used for substitutions. Let's try some examples, shall we?

Ok, imagine you have some kind of XML or HTML (be aware that regex may not be the best tool for the job, but it is nice as an example). You want to parse the tags, so you could do something like this (I have added spaces to make it easier to understand):

   \<(?<TAG>.+?)\> [^<]*? \</\k<TAG>\>
or
\<(.+?)\> [^<]*? \</\1\>

The first regex has a named group (TAG), while the second one uses a common group. Both regexes do the same thing: they use the value from the first group (the name of the tag) to match the closing tag. The difference is that the first one uses the name to match the value, and the second one uses the group index (which starts at 1).

Let's try some substitutions now. Consider the following text:

Lorem ipsum dolor sit amet consectetuer feugiat fames malesuada pretium egestas.

Now, let's use the this dumb regex over it:

\b(\S)(\S)(\S)(\S*)\b

This regex matches words with at least 3 characters, and uses groups to separate the first three letters. The result is this:

Match "Lorem"
Group 1: "L"
Group 2: "o"
Group 3: "r"
Group 4: "em"
Match "ipsum"
Group 1: "i"
Group 2: "p"
Group 3: "s"
Group 4: "um"
... Match "consectetuer"
Group 1: "c"
Group 2: "o"
Group 3: "n"
Group 4: "sectetuer"
...

So, if we apply the substitution string...

$1_$3$2_$4

... over it, we are trying to use the first group, add an underscore, use the third group, then the second group, add another underscore, and then the fourth group. The resulting string would be like the one below.

L_ro_em i_sp_um d_lo_or s_ti_ a_em_t c_no_sectetuer f_ue_giat f_ma_es m_la_esuada p_er_tium e_eg_stas.

You can use named groups for substitutions too, using ${name}.

To play around with regexes, I recommend http://regex101.com/, which offers a good amount of details on how the regex works; it also offers a few regex engines to choose from.

javascript reg 不加入分组的更多相关文章

  1. JavaScript正则表达式模式匹配(2)——分组模式匹配

    var pattern=/google{4,8}$/; // {4,8}$表示匹配结尾4-8次 var str='googleeeeeeeee'; // 表示e的4-8次 alert(pattern. ...

  2. javascript正则表达式(一)

    元字符 ( [ { \ ^ $ | ) ? * + . 预定义的特殊字符 字符 正则 描述 \t /\t/ 制表符 \n /\n/ 制表符 \r /\r/ 回车符 \f /\f/ 换页符 \a /\a ...

  3. 正则表达式(javascript)学习总结

    正则表达式在jquery.linux等随处可见,已经无孔不入.因此有必要对这个工具认真的学习一番.本着认真.严谨的态度,这次总结我花了近一个月的时间.但本文无任何创新之处,属一般性学习总结. 一.思考 ...

  4. JS正则表达式---分组

    JS正则表达式---分组 之前写了一篇关于正则新手入门的文章,本以为对正则表达式相对比较了解 但是今天我又遇到了一个坑,可能是自己不够细心的原因吧,今天就着重和大家分享一下javascript正则表达 ...

  5. javascript的正则表达式总结

    网上正则表达式的教程够多了,但由于javascript的历史比较悠久,也比较古老,因此有许多特性是不支持的.我们先从最简单地说起,文章所演示的正则基本都是perl方式. 元字符 ( [ { \ ^ $ ...

  6. javascript:正则大全

    :replace函数,为写自己的js模板做准备 待完善 function 1,声明&用法 //数组: var arr=[];//字面量 var arr=new Array();//构造函数 / ...

  7. JavaScript探秘系列

    此文章所在专题列表如下: 我们应该如何去了解JavaScript引擎的工作原理 JavaScript探秘:编写可维护的代码的重要性 JavaScript探秘:谨慎使用全局变量 JavaScript探秘 ...

  8. 系列文章--JavaScript教程文章

    JavaScript教程文章专题列表如下: 我们应该如何去了解JavaScript引擎的工作原理 JavaScript探秘:编写可维护的代码的重要性 JavaScript探秘:谨慎使用全局变量 Jav ...

  9. 温故知新 javascript 正则表达式

    很长时间没看 正则表达式了,碰巧今天用到,温故知新了一把 看书学习吧 50% 的举一反三练习中的原创.   一 javascript正则表达式的基本知识 1     javascript 正则对象创建 ...

随机推荐

  1. TFS2015源代码管理器无法建立团队项目的问题

    最近在服务器安装了微软最新版的TFS2015  正版要钱,网络上还没有能找到可用的key,因此我只能使用试用版. 安装完成后,使用我本地的vs2013  vs2012  vs2010  vs2014 ...

  2. 中文乱码—Servlet—SpringMVC

    一.SpringMVC中的中文乱码问题 a:处理全局请求的中文乱码(配置Web.xml的字符编码过滤器) <filter> <filter-name>encodingFilte ...

  3. Atcoder CADDi 2018 Solution

    C - Product and GCD Solved. 题意: 给出$n个数$的乘积,求$这n个数$的最大的可能是GCD 思路: 分解质因子,那么$每个质因子的贡献就是其质因子个数/ n的乘积$ #i ...

  4. 详细解析Linux /etc/passwd文件

    本文转自:http://os.51cto.com/art/201003/187533.htm 在Linux /etc/passwd文件中每个用户都有一个对应的记录行,它记录了这个用户的一些基本属性.系 ...

  5. java -- JVM的符号引用和直接引用

    在JVM中类加载过程中,在解析阶段,Java虚拟机会把类的二级制数据中的符号引用替换为直接引用. 1.符号引用(Symbolic References): 符号引用以一组符号来描述所引用的目标,符号可 ...

  6. linux下 ip指令

    目录 Network ip command Command :ip 简介 内容 Network ip command Command :ip 简介 ip 是個指令喔!並不是那個 TCP/IP 的 IP ...

  7. [java]String和Date、Timestamp之间的转换

    一.String与Date(java.util.Date)互转  1.1 String -> Date Date date = DateFormat.parse(String  str); St ...

  8. ElasticSearch安装和head插件安装

    本文主要介绍elasticsearch5.0安装及head插件安装.确保系统已经安装好jdk1.8以上,操作系统CentOS6以上. 一.elasticsearch安装配置 1.官网下载源码包 下载不 ...

  9. Android 之WebView实现下拉刷新和其他相关刷新功能

    最近项目中需要用到WebView下拉刷新的功能,经过查找资料终于完成了此功能,现在拿出来和大家分享一下.希望对大家有所帮助. 效果如下图:   代码: activity.xml <?xml ve ...

  10. bzoj1211: [HNOI2004]树的计数 prufer序列裸题

    一个有n个结点的树,设它的结点分别为v1, v2, …, vn,已知第i个结点vi的度数为di,问满足这样的条件的不同的树有多少棵.给定n,d1, d2, …, dn,编程需要输出满足d(vi)=di ...