This time I will show you how to use regular expressions to find lookarounds, and what are the precautions for using regular expressions to find lookarounds. The following is a practical case, let's take a look.
The examples in this article describe theregular expressionsfinding lookaround before and after the tutorial. Share it with everyone for your reference, as follows:
Note: In all examples, the regular expression matching results contain [ and ]## in the source text. #, some examples will be implemented using Java. If it is the usage of regular expressions in Java itself, it will be explained in the corresponding place. All java examples are tested under JDK1.6.0_13.
1. Introduction to the problemIn an HTML page, match the text between a pair of tags, such as matching the tags of the page, that is, and :
Text:
Regular expression:
.*? [Tt][Ii][Tt][Ll][Ee ]>Result:
【
Forward search specifies a pattern that must match but is not returned in the result. Forward search is actually a subexpression, which starts with ?=, and the text to be matched follows =.
Look at an example of matching the protocol part of a URL address:
Text:
http://blog.csdn.net/mhmyqnRegular expression: .
+(?=:)Result:
[http]://blog.csdn.net/mhmyqn Analysis: The protocol part in the URL address is the part before :, the pattern .+ matches any text, the subexpression (?=:) matches:, but the matched: does not appear in the result . We use ?= to indicate to the regular expression engine that as long as: is found, it will be fine, but it will not be included in the final return result. If you do not use forward matching (?=:) here, but use (:) directly, then the matching result will be http:, which includes:, which is not what we want.
Note: The front and back in front and back search refer to the relative position between the pattern and the text to be searched. The left is the front and the right is the back. That is, the forward search is: xxx(?=xxx), and the backward search is (?3. Backward searchThe backward search operator is ?JavaScript
does not support it, and the Java language supports backward search. For example, if you want to find the price in the text (starting with $, followed by a number), the result does not contain the currency symbol:
Text:
category1:$136.25,category2:$28,category3: $88.60Regular expression:
(?Result: category1: $【136.25】,category2:$【28】,category3:$【88.60】 Analysis: (?< ;=\$) pattern matches $, \d+(\.\d+)? pattern matches integer or decimal. As can be seen from the results, the result does not include the currency symbol, but only the price. What would happen if we didn't use backward lookup? Use the pattern $\d+(\.\d+)?, which will include $ in the result. Using the pattern \d+(\.\d+)? will also match the numbers in category1(23), which is not what we want.
Note: The length of the forward search pattern is variable, they can contain metacharacters like ., *, +; while the backward search pattern can only be of fixed length, It cannot contain metacharacters such as ., *, +.
4. Combine forward search and backward search
Use forward search and backward search together to solve the previous# Problem with text between ##HTML tags :
Text:(?).*?(?= [Tt][ Ii][Tt][Ll][Ee]>)
Result:5. Negate the forward and backward searches
The forward search and backward search mentioned above are usually used to match text, and their purpose is to determine whether The position of the text of the matched result that is returned (by specifying what text must be before and after the matched result). This usage is called forward search and forward search. There is also a negative forward search and a negative backward search, which find text that does not match a given pattern.Operators for searching before and after:
Correct Forward search | |
Negative forward search | ##(? |
(? | |
I paid $30 for 10 apples , 15 oranges, and 10 pears. I saved $5 on this order.
Regular expression:(?
Result :I paid
【$30】for 10 apples, 15 oranges, and 10 pears. I saved 【$5】 on this order.
Find quantity:Text:I paid $30 for 10 apples, 15 oranges, and 10 pears. I saved $5 on this order.
Regular expression: \b(?
Result: I paid $30 for
【10】apples, 【15】 oranges, and 【10】pears. I saved $5 on this order.
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!
Recommended reading:Position matching tutorial of regular expression tutorial (with code)
Implemented with php and js Regularly matching passwords that combine numbers and letters
The above is the detailed content of How to use regular expressions to find lookarounds. For more information, please follow other related articles on the PHP Chinese website!

JavaScript is widely used in websites, mobile applications, desktop applications and server-side programming. 1) In website development, JavaScript operates DOM together with HTML and CSS to achieve dynamic effects and supports frameworks such as jQuery and React. 2) Through ReactNative and Ionic, JavaScript is used to develop cross-platform mobile applications. 3) The Electron framework enables JavaScript to build desktop applications. 4) Node.js allows JavaScript to run on the server side and supports high concurrent requests.

Python is more suitable for data science and automation, while JavaScript is more suitable for front-end and full-stack development. 1. Python performs well in data science and machine learning, using libraries such as NumPy and Pandas for data processing and modeling. 2. Python is concise and efficient in automation and scripting. 3. JavaScript is indispensable in front-end development and is used to build dynamic web pages and single-page applications. 4. JavaScript plays a role in back-end development through Node.js and supports full-stack development.

C and C play a vital role in the JavaScript engine, mainly used to implement interpreters and JIT compilers. 1) C is used to parse JavaScript source code and generate an abstract syntax tree. 2) C is responsible for generating and executing bytecode. 3) C implements the JIT compiler, optimizes and compiles hot-spot code at runtime, and significantly improves the execution efficiency of JavaScript.

JavaScript's application in the real world includes front-end and back-end development. 1) Display front-end applications by building a TODO list application, involving DOM operations and event processing. 2) Build RESTfulAPI through Node.js and Express to demonstrate back-end applications.

The main uses of JavaScript in web development include client interaction, form verification and asynchronous communication. 1) Dynamic content update and user interaction through DOM operations; 2) Client verification is carried out before the user submits data to improve the user experience; 3) Refreshless communication with the server is achieved through AJAX technology.

Understanding how JavaScript engine works internally is important to developers because it helps write more efficient code and understand performance bottlenecks and optimization strategies. 1) The engine's workflow includes three stages: parsing, compiling and execution; 2) During the execution process, the engine will perform dynamic optimization, such as inline cache and hidden classes; 3) Best practices include avoiding global variables, optimizing loops, using const and lets, and avoiding excessive use of closures.

Python is more suitable for beginners, with a smooth learning curve and concise syntax; JavaScript is suitable for front-end development, with a steep learning curve and flexible syntax. 1. Python syntax is intuitive and suitable for data science and back-end development. 2. JavaScript is flexible and widely used in front-end and server-side programming.

Python and JavaScript have their own advantages and disadvantages in terms of community, libraries and resources. 1) The Python community is friendly and suitable for beginners, but the front-end development resources are not as rich as JavaScript. 2) Python is powerful in data science and machine learning libraries, while JavaScript is better in front-end development libraries and frameworks. 3) Both have rich learning resources, but Python is suitable for starting with official documents, while JavaScript is better with MDNWebDocs. The choice should be based on project needs and personal interests.


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

Hot Tools

SecLists
SecLists is the ultimate security tester's companion. It is a collection of various types of lists that are frequently used during security assessments, all in one place. SecLists helps make security testing more efficient and productive by conveniently providing all the lists a security tester might need. List types include usernames, passwords, URLs, fuzzing payloads, sensitive data patterns, web shells, and more. The tester can simply pull this repository onto a new test machine and he will have access to every type of list he needs.

WebStorm Mac version
Useful JavaScript development tools

Atom editor mac version download
The most popular open source editor

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

DVWA
Damn Vulnerable Web App (DVWA) is a PHP/MySQL web application that is very vulnerable. Its main goals are to be an aid for security professionals to test their skills and tools in a legal environment, to help web developers better understand the process of securing web applications, and to help teachers/students teach/learn in a classroom environment Web application security. The goal of DVWA is to practice some of the most common web vulnerabilities through a simple and straightforward interface, with varying degrees of difficulty. Please note that this software