In-depth understanding of JS regular expression test method and its pitfalls

This article mainly introduces the test method and its traps for in-depth understanding of JS regular expressions. It has certain reference value. Now I share it with you. Friends in need can refer to it

test method

Test method introduction

RegExp.prototype.test(str)

The test method is used to test whether there is a string matching the regular expression pattern in the string parameter

Usage of the test method

let reg = /\w/

reg.test('a')       // true

reg.test('$')       // false

It can be seen from the results that if the test string parameter contains a string matching the regular expression pattern, it will return true, otherwise it will return false

The pitfalls of the test method

When the regular expression is used When global matching is disabled, the test method will have the following strange phenomenon:

let reg = /\w/g

reg.test('ab')      // true

reg.test('ab')      // true

reg.test('ab')      // false

reg.test('ab')      // true

You can see that the third matching return of each round will become true, but ab is not Are they all consistent with reg? They should all return true. The reason lies in the lastIndex attribute mentioned in the previous section.

We can try the lastIndex method that prints reg every time we run the test method:

let reg = /\w/g
reg.lastIndex       // 0

reg.test('ab')      // true
reg.lastIndex       // 1

reg.test('ab')      // true
reg.lastIndex       // 2

reg.test('ab')      // false
reg.lastIndex       // 0

reg.test('ab')      // true
reg.lastIndex       // 1

Look at the results, and then recall the definition of lastIndex, you will understand the reason.

The lastIndex attribute is the last digit of the last character matched by the current expression, and is used to specify the starting position of the next match.

When entering the regular expression global mode, each time the test method is used, it will start from lastIndex and match the substring starting from lastIndex. For example, in the example, when the test method is executed for the second time, lastIndex has changed to 2 and the substring is empty, so reg cannot match it. Since the substring matching fails, the test method returns false, sets the lastIndex attribute to 0, and restarts the loop.

Methods to avoid pitfalls in test

The first method: the test method itself is used to test whether there is a string matching the regular pattern. The purpose can be achieved without using the global mode, so the first method One way is not to use global mode.

The second method: do not store the regular object instance in the variable, and directly call the test method with the regular object instance each time. However, this method consumes memory and is not recommended in theory.

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