javascript reg 不加入分组
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 ${name}
.
To play around with regexes, I recommend http://regex101.com/, which offers a good amount of details on how the regex works; it also offers a few regex engines to choose from.
javascript reg 不加入分组的更多相关文章
- JavaScript正则表达式模式匹配(2)——分组模式匹配
var pattern=/google{4,8}$/; // {4,8}$表示匹配结尾4-8次 var str='googleeeeeeeee'; // 表示e的4-8次 alert(pattern. ...
- 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 ...
- 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 ...
- 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 ...
- 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 ( [ { \ ^ $ ...
- 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 / ...
- 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 ...
- 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 ...
- 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
- (转) bicabo Visual Studio 2012自动添加注释(如版权信息等)
如何使用Visual Studio 2012给程序文件的头部自动添加如下的注释? /********************************************************** ...
- [LeetCode]160.Intersection of Two Linked Lists(2个链表的公共节点)
Intersection of Two Linked Lists Write a program to find the node at which the intersection of two s ...
- Qml应用程序的性能考虑与建议
本文翻译自Qt官网文档: http://doc.qt.io/qt-5/qtquick-performance.html QtQml应用程序的性能考虑与建议 1.时间考虑 作为一名程序开发者,应该努力使 ...
- bzoj1617 / P2904 [USACO08MAR]跨河River Crossing
P2904 [USACO08MAR]跨河River Crossing 显然的dp 设$f[i]$表示运走$i$头奶牛,木筏停在未过河奶牛一侧所用的最小代价 $s[i]$表示一次运$i$头奶牛到对面的代 ...
- Java实现获取属性文件的参数值
Java实现获取属性文件的参数值 1,属性文件内容(analysis.properties),路径必须在:src根目录下: #client data path analysis.client.data ...
- JAVA面试题整理(7)-Redis
Redis面试题汇总 1.Redis用过哪些类型数据,以及Redis底层怎么实现 分析:是不是觉得这个问题很基础,其实我也这么觉得.然而根据面试经验发现,至少百分八十的人答不上这个问题.建议,在项目中 ...
- 配置zbar识别二维码(转载)
原文地址:http://blog.csdn.net/dcrmg/article/details/52108258 二维码解码器Zbar+VS2012开发环境配置 Zbar条码解码器是一个开源的二维码 ...
- [AHOI2008]上学路线
题意:给定一个无向图,删除某些边有一定的代价,要求删掉使得最短路径减小,求最小代价. 首先要spfa求出起点到各个点的最短距离.对于一条权值为w,起点为i,终点为j的边,设dis[k]为起点到k点的距 ...
- POJ 2337 Catenyms
http://poj.org/problem?id=2337 题意: 判断给出的单词能否首尾相连,输出字典序最小的欧拉路径. 思路: 因为要按字典序大小输出路径,所以先将字符串排序,这样加边的时候就会 ...
- JDK_源码
1.http://hg.openjdk.java.net/ (ZC:这个貌似像官网的样子,不知道 到底是不是...) 1.1.jdk8u_jdk8u_jdk_ 5b86f66575b7 _src_.h ...