Detailed explanation of the use of lastIndex in regular expressions-JS Tutorial-php.cn

Home

Web Front-end

JS Tutorial

Detailed explanation of the use of lastIndex in regular expressions

php中世界最好的语言

Mar 29, 2018 pm 04:20 PM

use Detailed explanation

This time I will bring you a detailed explanation of the use of lastIndex in regular expressions. What are the precautions for using lastIndex in regular expressions? The following is a practical case, let's take a look.

Write the following output content in sequence.

var reg1 = /a/;
var reg2 = /a/g;
console.log(reg1.test('abcabc')); // true
console.log(reg1.test('abcabc')); // true
console.log(reg1.test('abcabc')); // true
console.log(reg1.test('abcabc')); // true
console.log(reg2.test('abcabc')); // true
console.log(reg2.test('abcabc')); // true
console.log(reg2.test('abcabc')); // false
console.log(reg2.test('abcabc')); // true

Copy after login

A very simple

regular expressiontest to find whether there is a character a in the stringabcabc. But there is a special false in the result. Why?

lastIndex (for regular expression with parameter g)

In each instantiated

RegExp object , there is a lastIndex attribute whose initial value is 0.

/a/.lastIndex // 0
new RegExp('a').lastIndex // 0
lastIndex表示匹配成功时候，匹配内容最后一个字符所在原字符串中的位置 + 1，也就是匹配内容的下一个字符的index（如果匹配内容在字符串的结尾，同样返回原字符串中的位置 + 1，也就是字符串的length）。如果未带参数g，lastIndex始终为0。
var reg = /ab/g;
reg.test('123abc');
console.log(reg.lastIndex) // 5
// 匹配内容在最后
var reg = /ab/g;
reg.test('123ab');
console.log(reg.lastIndex) // 5
// 不带参数g
var reg = /ab/;
reg.test('123abc');
console.log(reg.lastIndex) // 0

Copy after login

And this lastIndex is the starting position of matching when using this regular expression for other matching operations. When the match fails, lastIndex is reset to 0.

var reg = /ab/g;
// 初始值为0，从最开始匹配 匹配成功， lastIndex为4
console.log(reg.test('12ab34ab'), reg.lastIndex); // true 4
// 从第4位字符"3"开始匹配 匹配内容为第二个ab lastIndex 为 8
console.log(reg.test('12ab34ab'), reg.lastIndex); // true 8
// 从第8位 (字符长度为8，没有第8位) 开始匹配 匹配不成功 重置lastIndex 为 0
console.log(reg.test('12ab34ab'), reg.lastIndex); // false 0
// 从头匹配 同第一步
console.log(reg.test('12ab34ab'), reg.lastIndex); // true 4

Copy after login

After seeing this, the question has been answered, and the next step is to expand.

It is easy to make mistakes with unredeclared reg.

// 测试字符串str1 和 str2 是否都含有ab字符
var reg = /ab/g;
var str1 = '123ab';
var str2 = 'ab123';
console.log(reg.test(str1)); // true
console.log(reg.test(str2)); // false

Copy after login

Obviously there is an error in judgment due to lastIndex. Here you can modify reg without parameter g or re-declare reg. Of course, you can also manually modify reg.lastIndex = 0 after the first match.

Pre-check

Then let’s talk about pre-check, which literally means to prepare the matching query, that is, to query the next content of the matching content, but only The prepared query matches and does not return.

Often we need to match certain characters in a string followed by certain characters, but we do not need to include the following characters in the matching result, for example:

Find out the following characters in the string Characters are followed by all characters of 2.

var str = 'a1b2c22d31e4fg6h2';
'a1b2c22d31e4fg6h2'.match(/[a-z]2/g); // ["b2", "c2", "h2"]

Copy after login

In this way, although the string with 2 can be matched, we do not need the number 2, only the characters are needed here. By using the pre-check:

'a1b2c22d31e4fg6h2'.match(/[a-z](?=2)/g); // ["b", "c", "h"]

Copy after login

, you can see that the conditions are completely met, but how much does the pre-check have to do with lastIndex, the subject of this article?

Let’s use test to see. As for why we use test, we need to explain here. Match matches everything until the match is unsuccessful. When the match is unsuccessful, the lastIndex is reset to 0. .

Exec and test will return if the first match is successful or if the match fails, and will not continue to match.

var reg1 = /[a-z](?=2)/g;
var reg2 = /[a-z]2/g;
var str = 'a1b2c22d31e4fg6h2';
console.log(reg1.test(str), reg1.lastIndex); // true 3
console.log(reg1.test(str), reg1.lastIndex); // true 5
console.log(reg1.test(str), reg1.lastIndex); // true 16
console.log(reg1.test(str), reg1.lastIndex); // false 0
console.log(reg2.test(str), reg2.lastIndex); // true 4
console.log(reg2.test(str), reg2.lastIndex); // true 6
console.log(reg2.test(str), reg2.lastIndex); // true 17
console.log(reg2.test(str), reg2.lastIndex); // false 0

Copy after login

Do you see the problem? The pre-checked lastIndex does not contain the pre-checked content! This can be used to simplify many judgments.

For example, if we want to match the password, it must have at least one uppercase letter, one lowercase letter, and one number, and it must be at least 6 characters long and can only be a combination of numbers and letters.

According to the situation without pre-checking, it will be judged like this:

/[a-z]/.test(pwd) && /[A-Z]/.test(pwd) && /\d/.test(pwd) && /^[a-zA-Z0-9]{6,}$/.test(pwd);

Copy after login

But:

/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)[a-zA-Z0-9]{6,}$/.test(pwd)

Copy after login

Breaked it out:

(?=.*[ a-z]) Whether there are lowercase letters but the pre-check match fails and returns false. If the lastIndex is successful, it does not change and is still 0. In the same way, we understand the pre-check content of the two external sources. The last one is to match the combination of letters and numbers above 6, but the previous ones are all pre-checks. , lastIndex is always not 0, and each match is matched from the beginning, so the requirements are met.

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

Hot AI Tools

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress images for free

Clothoff.io

AI clothes remover

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Assassin's Creed Shadows: Seashell Riddle Solution

1 months ago By DDD

What's New in Windows 11 KB5054979 & How to Fix Update Issues

3 weeks ago By DDD

Where to find the Crane Control Keycard in Atomfall

1 months ago By DDD

How to fix KB5055523 fails to install in Windows 11?

2 weeks ago By DDD

InZoi: How To Apply To School And University

3 weeks ago By DDD

Hot Tools

Notepad++7.3.1

Easy-to-use and free code editor

SublimeText3 Chinese version

Chinese version, very easy to use

Zend Studio 13.0.1

Powerful PHP integrated development environment

Dreamweaver CS6

Visual web development tools

SublimeText3 Mac version

God-level code editing software (SublimeText3)

Hot Topics

Where is the login entrance for gmail email?

7750

Java Tutorial

1643

CakePHP Tutorial

1397

Laravel Tutorial

1293

PHP Tutorial

1234

Related knowledge

What software is crystaldiskmark? -How to use crystaldiskmark? Mar 18, 2024 pm 02:58 PM

CrystalDiskMark is a small HDD benchmark tool for hard drives that quickly measures sequential and random read/write speeds. Next, let the editor introduce CrystalDiskMark to you and how to use crystaldiskmark~ 1. Introduction to CrystalDiskMark CrystalDiskMark is a widely used disk performance testing tool used to evaluate the read and write speed and performance of mechanical hard drives and solid-state drives (SSD). Random I/O performance. It is a free Windows application and provides a user-friendly interface and various test modes to evaluate different aspects of hard drive performance and is widely used in hardware reviews

How to download foobar2000? -How to use foobar2000 Mar 18, 2024 am 10:58 AM

foobar2000 is a software that can listen to music resources at any time. It brings you all kinds of music with lossless sound quality. The enhanced version of the music player allows you to get a more comprehensive and comfortable music experience. Its design concept is to play the advanced audio on the computer The device is transplanted to mobile phones to provide a more convenient and efficient music playback experience. The interface design is simple, clear and easy to use. It adopts a minimalist design style without too many decorations and cumbersome operations to get started quickly. It also supports a variety of skins and Theme, personalize settings according to your own preferences, and create an exclusive music player that supports the playback of multiple audio formats. It also supports the audio gain function to adjust the volume according to your own hearing conditions to avoid hearing damage caused by excessive volume. Next, let me help you

How to use Baidu Netdisk app Mar 27, 2024 pm 06:46 PM

Cloud storage has become an indispensable part of our daily life and work nowadays. As one of the leading cloud storage services in China, Baidu Netdisk has won the favor of a large number of users with its powerful storage functions, efficient transmission speed and convenient operation experience. And whether you want to back up important files, share information, watch videos online, or listen to music, Baidu Cloud Disk can meet your needs. However, many users may not understand the specific use method of Baidu Netdisk app, so this tutorial will introduce in detail how to use Baidu Netdisk app. Users who are still confused can follow this article to learn more. ! How to use Baidu Cloud Network Disk: 1. Installation First, when downloading and installing Baidu Cloud software, please select the custom installation option.

How to use NetEase Mailbox Master Mar 27, 2024 pm 05:32 PM

NetEase Mailbox, as an email address widely used by Chinese netizens, has always won the trust of users with its stable and efficient services. NetEase Mailbox Master is an email software specially created for mobile phone users. It greatly simplifies the process of sending and receiving emails and makes our email processing more convenient. So how to use NetEase Mailbox Master, and what specific functions it has. Below, the editor of this site will give you a detailed introduction, hoping to help you! First, you can search and download the NetEase Mailbox Master app in the mobile app store. Search for "NetEase Mailbox Master" in App Store or Baidu Mobile Assistant, and then follow the prompts to install it. After the download and installation is completed, we open the NetEase email account and log in. The login interface is as shown below

BTCC tutorial: How to bind and use MetaMask wallet on BTCC exchange? Apr 26, 2024 am 09:40 AM

MetaMask (also called Little Fox Wallet in Chinese) is a free and well-received encryption wallet software. Currently, BTCC supports binding to the MetaMask wallet. After binding, you can use the MetaMask wallet to quickly log in, store value, buy coins, etc., and you can also get 20 USDT trial bonus for the first time binding. In the BTCCMetaMask wallet tutorial, we will introduce in detail how to register and use MetaMask, and how to bind and use the Little Fox wallet in BTCC. What is MetaMask wallet? With over 30 million users, MetaMask Little Fox Wallet is one of the most popular cryptocurrency wallets today. It is free to use and can be installed on the network as an extension

Detailed explanation of obtaining administrator rights in Win11 Mar 08, 2024 pm 03:06 PM

Windows operating system is one of the most popular operating systems in the world, and its new version Win11 has attracted much attention. In the Win11 system, obtaining administrator rights is an important operation. Administrator rights allow users to perform more operations and settings on the system. This article will introduce in detail how to obtain administrator permissions in Win11 system and how to effectively manage permissions. In the Win11 system, administrator rights are divided into two types: local administrator and domain administrator. A local administrator has full administrative rights to the local computer

Detailed explanation of division operation in Oracle SQL Mar 10, 2024 am 09:51 AM

Detailed explanation of division operation in OracleSQL In OracleSQL, division operation is a common and important mathematical operation, used to calculate the result of dividing two numbers. Division is often used in database queries, so understanding the division operation and its usage in OracleSQL is one of the essential skills for database developers. This article will discuss the relevant knowledge of division operations in OracleSQL in detail and provide specific code examples for readers' reference. 1. Division operation in OracleSQL

Teach you how to use the new advanced features of iOS 17.4 'Stolen Device Protection' Mar 10, 2024 pm 04:34 PM

Apple rolled out the iOS 17.4 update on Tuesday, bringing a slew of new features and fixes to iPhones. The update includes new emojis, and EU users will also be able to download them from other app stores. In addition, the update also strengthens the control of iPhone security and introduces more "Stolen Device Protection" setting options to provide users with more choices and protection. "iOS17.3 introduces the "Stolen Device Protection" function for the first time, adding extra security to users' sensitive information. When the user is away from home and other familiar places, this function requires the user to enter biometric information for the first time, and after one hour You must enter information again to access and change certain data, such as changing your Apple ID password or turning off stolen device protection.

See all articles