On the bug of JS regular test method

Actually, I seldom use this , So I didn't pay attention to this problem before , I went to see this thing only after defoliation wrote a abnormal test

Actually, I seldom use this , So I didn't pay attention to this problem before , I went to see this thing only after defoliation wrote a abnormal test

The following code is all in chrome Of F12 Next debugging , You can study

Let's start with something .

      
      

       
       var re = /\d/;
       
       
console.log( re.test("1") );
       
       
console.log( re.test("1") );
       
       
console.log( re.test("1") );
       
       
console.log( re.test("1") );
      
      
      
      

       
       
1.
       
       
2.
       
       
3.
       
       
4.
       
       
5.
      
      

All is true That's all right. ..

But you put /\d/; Change to /\d/g; Try again. .

Revise again ：

      
      

       
       console.log( /\d/g.test("1") );
       
       
console.log( /\d/g.test("1") );
       
       
console.log( /\d/g.test("1") );
       
       
console.log( /\d/g.test("1") );
      
      
      
      

       
       
1.
       
       
2.
       
       
3.
       
       
4.
      
      

All is true, Why on earth is this ？

These results are quite interesting , Of course, experts naturally know why , If you know , In fact, you can skip the following without looking , It's all hydrology ..

There is one in the regular  ​​lastIndex​​  Properties of , It's the start of the next match .

      
      

       
       var re = /\d/g;
       
       
console.log( re.test("1"), re.lastIndex );
       
       
console.log( re.test("1"), re.lastIndex );
       
       
console.log( re.test("1"), re.lastIndex );
       
       
console.log( re.test("1"), re.lastIndex );
       
       

      
      
      
      

       
       
1.
       
       
2.
       
       
3.
       
       
4.
       
       
5.
      
      

You can see The first matching result is  true  Indicates that the match is successful , here  lastIndex  Record the starting position of the next match as 1.

So the second match from "1" String index 1 Location matching for , Of course, the match failed , Because this string has only one character , His index is 0.

and  /\d/g.test("1")  This is why every successful match can ?

Because it directly uses regular literals , It's equivalent to recreating one regular object at a time ,lastIndex  The initial value of the property is 0.

So you can match successfully every time .

Now do you understand , Include  exec  It's the same thing , Match one at a time ,lastIndex  Record the starting position of the next match .

If you have to use a regular object , Then only every time  test  Pre reset  lastIndex  了 , So we can make sure he doesn't have an accident .

      
      

       
       var re = /\d/g;
       
       
console.log( re.test("1") );
       
       
re.lastIndex = 0;
       
       
console.log( re.test("1") );
       
       
re.lastIndex = 0;
       
       
console.log( re.test("1") );
       
       
re.lastIndex = 0;
       
       
console.log( re.test("1") );
      
      
      
      

       
       
1.
       
       
2.
       
       
3.
       
       
4.
       
       
5.
       
       
6.
       
       
7.
       
       
8.
      
      

Okay , It took a lot of time to modify the syntax highlighting plug-in today , So water wrote an article , I hope you will have a better understanding .