An explanation of regular laziness and greed

This article brings an explanation of regular laziness and greed. I hope it will be helpful to everyone.

exec - > Regular capture

Every time you capture, the default match is performed first. If there is no successful match, The captured result is null; we can capture only if there is matching content;

Captured content format

1. Captured content Is an array. The first item in the array is the content captured by the current regular expression.

Index: The index position where the captured content starts in the string.

Input: The captured original string

  reg = /\d+/ str = 'woshi2016ni2017' res =

var res = reg.exec(str);
console.log(res) // ['2016',index:5,input:'woshi2016ni2017']

2. Characteristics of regular capture

1) Lazy Properties -> Each time exec is executed, only the first matching content is captured. Without any processing, even if multiple captures are performed, the first matching content is still captured.

lastIndex: is the position where the search starts in the string for each regular capture. The default value is 0

2) How to solve laziness Sexuality? Add a modifier "g"

at the end of the regular expression Modifiers: g, i, m

global(g): Global matching

##　ignoreCase(i): Ignore case matching

multiline(m):Multiline matching　

var reg = /\d/g;

var str = 'woshi2016ni2017';

console.log(reg.lastIndex)

console.log(reg.exec(str))

Principle: After adding the global modifier g, after each capture, the value of our lastIndex becomes the latest value, and the next capture Start searching from the latest location, so that you can get all the content that needs to be captured

　3) Write your own program to get all the content captured by the regular expression (don’t forget to add g)

##　

var reg = /\d+/g;var str = 'aswofde2015xsewde2016awdefer2017';var ary = [];var res = reg.exec(str);while(res){
            ary.push(res[0])
            res = reg.exec(str);
        }
        console.log(ary)//[2015,2016,2017]

　

4), Greedy Every capture of the regular pattern is based on the longest match The results are captured, for example: 2 conforms to the regular pattern, 2015 also conforms to the regular pattern, and what we capture by default is 2015

　5) How to solve the greedy nature of the regular pattern -> after the quantifier metacharacter add one? Just

　

var reg = /\d+?/g;var str = 'aswofde2015xsewde2016awdefer2017';
        console.log(reg.exec(str));

　

?has many functions in regular expressions:

　Place A

ordinary

metacharacter followed by represents 0-1 occurrences of /\d?/ -> The number may or may not appear Putting it after the

metacharacter of a

quantifier is greedy when canceling the capture 3. The match method in the string -> Match everything with the regular expression The characters of

are obtained

var reg = /\d+?/g;var str = 'aswofde2015xsewde2016awdefer2017';var ary = str.match(reg);//[2,0,1,5,2,0,1,6,2,0,1,7]

Although match is simpler than our exec in the current situation, there are There are some problems that cannot be solved by oneself. In the case of grouped capture, match can only capture the content matched by large regular expressions, but the content captured by small regular expressions cannot be obtained.

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