


Detailed explanation of zero-width assertion examples of regular expressions
This article mainly introduces the zero-width assertion of regular expressions. It briefly introduces the concept, classification, PHP implementation skills and related precautions of zero-width assertion. Friends in need can refer to the example of this article.
Describes the zero-width assertion of regular expressions. Share it with everyone for your reference, the details are as follows:
Preface
I have written an article about regular expressions before (http (://www.jb51.net/article/111359.htm) Regulars are introduced in detail in this article, but the introduction of zero-width assertions is rarely mentioned. Now add this content. In this article, we mainly address the following issues:
① What is a zero-width assertion and why should we use a zero-width assertion?
② How to use a zero-width assertion
Concept
Zero-width assertion, defined in most places, is used to find things before or after certain content (but not including these content), that is, they are like \b ^ $ \ Such anchoring is used to specify a location that should meet certain conditions (ie, assertions), so they are also called zero-width assertions. My understanding is that a matching search is performed before or after a string at a limited position. Therefore, the execution process of zero-width assertions is divided into two situations. If it is a forward assertion, it should be like this. The first step is to judge whether the assertion is true (that is, whether it meets certain conditions). The second step is to proceed to the next step if the conditions are met. Find matches in one step. If it is a reverse assertion, the first step is to match in the order of regular expressions. The second step is to determine whether reverse endorsement is met when encountering reverse endorsement.
Positive lookahead assertion
#What is a positive lookahead assertion is to search and match before the corresponding position in the string, using (?=exp) matching The position before exp.
Example
$str="abcgwcab"; $parent='/bc(?=gw)/'; $str=preg_match($parent,$str,$match); var_dump($match); /** 输出结果: int 1 array (size=1) 0 => string 'bc' (length=2) */
Analysis: First find the gw position in the string "abcgwcab", assert it to be true, and then match bc. If the regular expression is written as $parent='/bc(?=gw)ca/'; the match will be unsuccessful.
Anti-lookahead assertion
What is an anti-lookahead assertion? Use (?!exp) to match something that is not followed by exp.
Example:
$str="abcgwcab"; $parent='/bc(?!ww)gw/'; $str=preg_match($parent,$str,$match); var_dump($str); var_dump($match); /** 输出: int 1 array (size=1) 0 => string 'bcgw' (length=4) */
Analysis: First determine whether the string contains bc, then determine whether it is followed by ww, and finally match gw. It can be seen that other matching conditions can be added before and after the reverse assertion.
Assertion before and after
What is assertion before and after, it is to search and match after the corresponding position in the string, (?
Example:
$str="abcgwcab"; $parent='/(?<=gw)ca/'; $str=preg_match($parent,$str,$match); var_dump($str); var_dump($match); /** 输出结果: int 1 array (size=1) 0 => string 'ca' (length=2) */
Analysis: The first step is to find whether the character "abcgwcab" contains gw , the return result is true, and then proceed to the second step to find whether there is ca after gw.
Anti-posterior assertion
What is an anti-posterior assertion, use (?
$str="abcgwcab"; $parent='/(?<!bc)gw/'; $str=preg_match($parent,$str,$match); var_dump($str); var_dump($match); /** int 0 array (size=0) empty */
Analysis: First match gw in the string, and then determine whether it is preceded by bc. It is found that it is preceded by bc, so it returns false.
Exercise questions
Example 1:\d+(?=abc)
Determine whether a string contains 'abc' and whether it contains one or more numbers in front of it
Example 2: (? <br>Determine whether there are two consecutive numbers in the string, followed by a letter
Example 3 :\d{3} (?!55)
Determine whether the string contains three consecutive numbers, and the three numbers are not followed by 55
Example 4: (? <br>Determine whether a string contains a combination of letters and numbers, and it is not preceded by ac
##Summary
What is the difference between preemptive assertions and late assertions? Preemptive assertions are regular expressions before judgment assertions, and late assertions are before judgment assertions. Regular expressions, assertions come before other regular expressions. The first and last here are the order of other matching assertions in the regular expression.The above is the detailed content of Detailed explanation of zero-width assertion examples of regular expressions. For more information, please follow other related articles on the PHP Chinese website!

PHP type prompts to improve code quality and readability. 1) Scalar type tips: Since PHP7.0, basic data types are allowed to be specified in function parameters, such as int, float, etc. 2) Return type prompt: Ensure the consistency of the function return value type. 3) Union type prompt: Since PHP8.0, multiple types are allowed to be specified in function parameters or return values. 4) Nullable type prompt: Allows to include null values and handle functions that may return null values.

In PHP, use the clone keyword to create a copy of the object and customize the cloning behavior through the \_\_clone magic method. 1. Use the clone keyword to make a shallow copy, cloning the object's properties but not the object's properties. 2. The \_\_clone method can deeply copy nested objects to avoid shallow copying problems. 3. Pay attention to avoid circular references and performance problems in cloning, and optimize cloning operations to improve efficiency.

PHP is suitable for web development and content management systems, and Python is suitable for data science, machine learning and automation scripts. 1.PHP performs well in building fast and scalable websites and applications and is commonly used in CMS such as WordPress. 2. Python has performed outstandingly in the fields of data science and machine learning, with rich libraries such as NumPy and TensorFlow.

Key players in HTTP cache headers include Cache-Control, ETag, and Last-Modified. 1.Cache-Control is used to control caching policies. Example: Cache-Control:max-age=3600,public. 2. ETag verifies resource changes through unique identifiers, example: ETag: "686897696a7c876b7e". 3.Last-Modified indicates the resource's last modification time, example: Last-Modified:Wed,21Oct201507:28:00GMT.

In PHP, password_hash and password_verify functions should be used to implement secure password hashing, and MD5 or SHA1 should not be used. 1) password_hash generates a hash containing salt values to enhance security. 2) Password_verify verify password and ensure security by comparing hash values. 3) MD5 and SHA1 are vulnerable and lack salt values, and are not suitable for modern password security.

PHP is a server-side scripting language used for dynamic web development and server-side applications. 1.PHP is an interpreted language that does not require compilation and is suitable for rapid development. 2. PHP code is embedded in HTML, making it easy to develop web pages. 3. PHP processes server-side logic, generates HTML output, and supports user interaction and data processing. 4. PHP can interact with the database, process form submission, and execute server-side tasks.

PHP has shaped the network over the past few decades and will continue to play an important role in web development. 1) PHP originated in 1994 and has become the first choice for developers due to its ease of use and seamless integration with MySQL. 2) Its core functions include generating dynamic content and integrating with the database, allowing the website to be updated in real time and displayed in personalized manner. 3) The wide application and ecosystem of PHP have driven its long-term impact, but it also faces version updates and security challenges. 4) Performance improvements in recent years, such as the release of PHP7, enable it to compete with modern languages. 5) In the future, PHP needs to deal with new challenges such as containerization and microservices, but its flexibility and active community make it adaptable.

The core benefits of PHP include ease of learning, strong web development support, rich libraries and frameworks, high performance and scalability, cross-platform compatibility, and cost-effectiveness. 1) Easy to learn and use, suitable for beginners; 2) Good integration with web servers and supports multiple databases; 3) Have powerful frameworks such as Laravel; 4) High performance can be achieved through optimization; 5) Support multiple operating systems; 6) Open source to reduce development costs.


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

AI Hentai Generator
Generate AI Hentai for free.

Hot Article

Hot Tools

MinGW - Minimalist GNU for Windows
This project is in the process of being migrated to osdn.net/projects/mingw, you can continue to follow us there. MinGW: A native Windows port of the GNU Compiler Collection (GCC), freely distributable import libraries and header files for building native Windows applications; includes extensions to the MSVC runtime to support C99 functionality. All MinGW software can run on 64-bit Windows platforms.

EditPlus Chinese cracked version
Small size, syntax highlighting, does not support code prompt function

SublimeText3 Chinese version
Chinese version, very easy to use

SublimeText3 Linux new version
SublimeText3 Linux latest version

Zend Studio 13.0.1
Powerful PHP integrated development environment