regular expression

String technology : Regular expressions belong to string related operations

There are many formats of strings , It can be constrained according to some rules

Regular expressions yes js A kind of object of , There are two ways to create ：

1、 Direct measurement

It is itself an object , The meaning of expression is a rule , In two slashes （/.../） Write rules between

2、new RegExp()

RegExp() Is its construction method , Code example ：

<script>
    var re = new RegExp("aaa")// Rules passed in brackets , At this time re amount to /aaa/
</script>

Three properties of regular expressions i,g,m Write after regular expression , Separate role

i: Case insensitive

g: The global matching （ Find all matches instead of stopping after finding the first one ）

m: Perform multi line matching

Specific code display ：

Which USES test() and match(),

test(): Check whether there are characters in the string that conform to the rules

match(): Match the string according to the rules and return

<script>
            var reg=/l/i// Add i, Case insensitive at this time   
			var str="hjsLddhhy7hL"
			var res=reg.test(str)
			console.log(res)//true

            var reg=/l/ig  Plus g Indicates matching full text    
			var str="hdLDIhhglBHdL"
			var arr=reg.match(str)
			console.log(arr) //["L","l","L"]
    
            var str="abcdea"
            reg=/^a/gm // Caret ^ Means in letters a Start with 
            str.match(reg);//["a","a"]
</script>

square brackets

Square brackets are used to find characters in a range , One brackets On behalf of a , The content in brackets represents the range of this bit

Caret ^ Put it in [] Inside indicates " Not " It means , stay Brackets Add in "|" Express " or " It means ,"|" Put matching rules on both sides of the operator

Specify ：

<script>
                var reg=/[ Zhao qiansun ][ Cloud crane ][abc]/g; // Three brackets represent three , Each item in the array consists of three digits , The scope of each item is specified in brackets 
				var str="a Qian He b Zhao cd";
				var arr=str.match(reg);
				console.log(arr) //[" Qian He b"," zhaoyun c"]

                var reg=/\.(jpg|png)$/ //\. Escape character  $ Said to .jpg  perhaps  .png ending 
			    var str="qabAc89d84efAg6acbb66a.jpg"
			    var re=reg.test(str)
			    console.log(re) //true

                var reg=/^[ Zhao qiansun ][ Cloud crane ][^(a-y)]/g;
                 //^[ Zhao qiansun ] Start with Zhao qiansun 、 [^(a-y)] Express non (a-y)  
				var str=" Qian He z zhaoyun zd";
				var arr=str.match(reg);
				console.log(arr) //[" Qian He z"]
</script>

Metacharacters

Metacharacters are characters with special meanings , Metacharacters can also be combined and put into brackets , A metacharacter represents a bit

Metacharacters	describe
\w --word	Find word characters ( Letter + Numbers + Underline )
\W	Find non word characters ==[^\w]
\d --data	Find number
\D	Find non numeric characters
\s --space	Look for blank characters
\S	Find non white space characters
\b --border	Match word boundaries "today is friday"
\B	Match non word boundaries
\t	Look for tabs
\n	Find line breaks
\f	Look for page breaks
\v	Find vertical tabs
\uXXXX	Find the specified in hexadecimal Unicode character
. -- Must remember	（ Order number ） Find single character , Except for line breaks and line terminators

quantifiers

quantifiers , A word for quantity （ The following expression n Represents a matching rule ,n The rules of quantifiers are defined by the symbols of the following symbols ）.

quantifiers	describe
n+	Match any containing at least one n String
n*	Match any containing zero or more n String
n？	Match any containing zero or one n String
n{X}	Matching inclusion X individual n The sequence of strings /\w{10}/
n{X,Y}	Match any containing X One to one Y individual n The sequence of strings /\w{6,16}/
n{X,}	The match contains at least X individual n The sequence of strings
n$	Match any ending with n String
^n	Match any start with n String
S(?=n)	Matches any string immediately following the specified string n String S abc(?!d)
S(?!n)	Matches any string that is not immediately followed by the specified string n String S

Two methods of regularizing objects ：test() and exec() Requirements

test(): Detect whether the string contains detected characters . return ：boolean.

exec(): Find whether the string contains detected characters . return ： Return the found characters as an array .

exec Use ：

<script>
    		var reg=/[a-z]{3}[1-7]/g
			var str="aaa5ABC3ccc9bbb6qwetret"
			var arr=reg.exec(str)		
			var arr2=reg.exec(str)			
			console.log(arr,arr2)//["aaa5"] ["bbb6"]	
            // Despite having global attribute , Each use still returns only one matching value 
</script>

Small exercise ：

<script>
    	// Remove the space at the beginning of the string 
        	var reg=/^\s/
			var str="  abc"
			var re=str.replace(reg,"")	
        // matching 11 Digit number 
        //1
         var reg=/[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]/ 
		//2
        var reg=/[0-9]{11}/
        
        // The regularity of mobile phone number 
         var reg=/^1[3|5|7|8|9][0-9]{9}/
</script>

Instance properties of regular expressions

1、ignoreCase Is it set i

2、global Is it set g

3、multiline Is it set m

4、source String corresponding to literal form

5、lastIndex Start searching for the character position of the next match , Default 0

Instructions ：

<script>
    var reg=/^\d[\s\S]*\d$/g;
    reg.ignoreCase;//false, Test for the presence of i  attribute 
    reg.global;//true, Test for the presence of g  attribute 
    reg.multiline;//false, Test for the presence of m  attribute 
    reg.source;//"^\d[\s\S]*\d$", Match the content body 
    
    var str="3h4jfdidf"
    reg.lastIndex//0
    lastIndex Often with exec Use with return cursor position 
</script>

Add ："\num" backreferences , It refers to the... To be quoted num The expression value in brackets

Code demonstration ：

<script>
    	      var str="aaaabbbb"
   			 var reg=/(\w)\1\1/g // There are two \1 It also refers to the value in the first bracket 2 Time 
    		console.log(str.match(reg))//["aaa","bbb"]
</script>

Note that the quoted values are those in parentheses , Otherwise, the value obtained is null, Such as ： var reg=/a\1\1/g

Regular expression support String Object method

Method	describe
search	Retrieve the value that matches the regular expression , Returns the position of the matching string
match	Find a match for one or more regular expressions
replace	Replace strings that match regular expressions
split	Split a string into an array of strings

Greedy matching and non greedy matching

Greedy matching is shining " quantifiers " The rules require more cases to match

Non greedy matching , stay " quantifiers " Add a question mark after the rule "?"

When two question marks are together , Such as ：/a??/g, The first question mark represents 0~1 individual , The second question mark means you can take 0 Don't take 1 To do the matching .

Appear in the written examination questions ：

<script>
    //1、 Remove the spaces at the beginning and end of the string 
    var str="   yyds   "
    // The first method , Using string method trim()
    var re= str.trim()
    // The second method runs regular expressions 
    var reg=/(^/s+)|(\s+$)/
    var re=str.match(reg)

    //2、 Determine whether the substring exists or the number of repetitions 
   
     var str="hello world!"
    // Whether there is ：
    var reg=/wor/
    var re=reg.test(str)//true There is   false There is no 

    // Repeat the number 
    // Method 1
   var count=0;
    var ins=0;
    while(str.indexOf("l",ins)!=-1){
        count++;
        ins=str.indexOf("l",ins)+1;
    }
    console.log(count)//3
    
    // Method 2( Regular expressions )
    var reg=/l/g
    var re=str.match(reg)
    var re1=re.length
</script>