Detailed explanation of the use of regular lazy matching pattern (?)

This time I will bring you a detailed explanation of the use of the regular lazy matching mode (?). What are the precautions when using the regular lazy matching mode (?). The following is a practical case, let's take a look.

Regular expressionLazy matching mode:
In the chapter on greedy matching mode, it has been said that human nature is greedy, hoping to get more money, status and even beautiful women, but there are also Many stoic people only need to meet their basic life needs. There is also such a matching principle in regular expressions. Let’s introduce it below.

1. The concept of lazy mode:

This mode is just the opposite of greedy mode. It matches as few characters as possible to satisfy the regular expression, for example:

var str="axxyyzbdkb"; 
console.log(str.match(/a.*b/));

The above code is greedy mode, so it can match the entire string. Let’s modify it into lazy matching mode:

var str="axxyyzbdkb"; 
console.log(str.match(/a.*?b/));

The above code is lazy matching. The method is to add a question mark (?) after the repeated quantifier.
The lazy matching mode is to match as few characters as possible, but must meet the matching rules of the regular expression. For example, in the above code, * can repeatedly match 0 or more previous characters or subexpressions, but the regular expression The end of the formula must be b, so the regular expression can match axxyyzb in the above string.

The summary is as follows:

1. Add a question mark (?) after the repeated quantifier to form a lazy match.
2. Lazy matching will match as few characters as possible, but the entire matching pattern must be satisfied.

2. LazyQualifierList:

##Grammar structureSemantic explanation *?Can be repeated any number of times, but as few times as possible. +?Can be repeated once or as many times as you like, but as few times as possible, but the minimum number is 1. ??Can be repeated 0 or 1 times, but as few times as possible. {n,m}?You can repeat n to m, but repeat as little as possible. The minimum number of matches is n. {n,}? can be repeated more than n times, but it should be repeated as little as possible, and at least n should be matched.

首先引入一个介绍比较详细的网站

http://www.jb51.net/article/31491.htm

接下来是本人的简介

其实贪婪和惰性很容易理解，从字面意思我们就可以知道，所谓的"贪婪"的意思就是，如果符合要求就一直往后匹配，一直到无法匹配为止，这就是贪婪模式。所谓的惰性模式就是一旦匹配到合适的就结束，不在继续匹配下去了，下面我介绍几个例子来主要讲述一下。

首先讲述一下贪婪模式的标示符：+，？，*，{n},{n,},{n,m}.惰性模式：+？，？？，*？？，{n}?，{n,}?,{n,m}?;

例子一

var pattern=/8[a-zA-Z0-9]*7/;贪婪模式
var string="abc8defghij7klngon8qrstwxy7";

这时使用了贪婪模式*，表示8与8之间可有有任意多个字母,那这个正则先匹配第一个8，如果匹配到了后，就无限制的匹配后面的内容，只要后面的内容都满足[a-zA-Z0-9]就可以。一直匹配，匹配到不能再匹配为止，看紧接着后面的一个是不是7，如果不是那他就往前进一个(吐出一个看是不是7)，如果不是再继续吐直到吐出7为止，然后匹配到的就是这之间的内容。所以结果匹配到的内容就是整条字符串。

var pattern=/8[a-zA-Z0-9]*?7/ig;惰性模式
var string="abc8defghij7klngon8qrstwxy7";

上面正则使用了惰性模式*?,此时匹配方式是这样的,先匹配一个8，然后在往后匹配一个字符看是不是符合[a-zA-Z0-9]，如果符合，再去看紧接着后面的一个字符是不是7，如果是7就结束，如果不是就再往后匹配一个字符，看是不是符合[a-zA-Z0-9]，如果符合，就再看紧接着后面的一个字符是不是7，如果是7就结束，否则，按照上面的方式依次循环下去,指导符合为止。

(2).贪婪和惰性模式还可以用另一种方式来表达。

例子二

var test="<img src="aaa/111.jpg"/><img src="aaa/112.jpg"/><img src="aaa/113.jpg"/>";
var pattern=/<img [^>]*\/>/ig;

这样也可以实现惰性模式,[^>]这个表示的就是在之间不能出现>,所以结果可以找寻每个标签。

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