Visual Regex builder · Built-in cheat sheet and complexity analysis

Regex regular expression generator Builder & Cheat Sheet

Automatically generate regular expressions by checking conditions, and built-in commonly used template library and syntax cheat sheet. Supports Regex Explain parsing and Complexity Meter complexity analysis to help you learn and create efficient regular expressions.

Condition setting

Live Preview

Regex has not been generated yet, please check the conditions and click "Generate Regex"

complexity
☆☆☆☆☆
performance risk
Low
nested quantifier
None

Waiting for analysis⋯

0 matches

Frequently Asked Questions

What are Regexes? Why do you need to learn it?
Regular Expression (Regex for short) Is a language used to describe string patterns. Through special syntax, you can compare, search, and replace text that meets specific rules.

For example,^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$ This seemingly complex string of symbols actually describes the format rules of "Email address".

Learning Regex can greatly improve your efficiency in scenarios such as data verification, word processing, log analysis, and program code reconstruction. While it may seem intimidating at first, once you master the core concepts, you'll find it to be one of the most powerful weapons in a developer's toolbox.
What is the difference between Regex Tester and Regex Builder?
The two are complementary tools:

Regex Tester Focus on "testing and debugging" - you enter a set of Regex and test strings, and it can instantly display matching results, capture groups, and mark highlights to help you verify whether the Regex is correct.

Regex Builder Then focus on "combination and learning" - automatically generate Regex code by checking conditions and selecting templates. For those who are learning Regex or don’t want to write syntax from scratch, Builder can significantly lower the barrier to use.

It is recommended to use it together: first use Builder to quickly generate basic Regex, and then copy it to Regex Tester for detailed adjustment and verification.
What is Catastrophic Backtracking?
Catastrophic backtracking is a problem with regex engines because of "nested quantifiers" (e.g. (a+)+$) resulting in an exponential operation explosion.

When there is a nested Regex +、* or {n,m}, and when the input string "almost matches but not completely matches", the engine will try all possible combinations, causing the calculation time to increase exponentially.

Classic example: (a+)+b Match input aaaaaaaaaaaaaX(15 a), the engine needs to try 2^15 = 32,768 paths; with 30 a, the number jumps to more than 1 billion.

This tool's Complexity Meter automatically detects potential nested quantifier risks to help you avoid writing dangerous regular expressions.
How to improve the readability of Regex?
The following four tips can greatly improve the readability of Regex:

1. Use named capture groups
Instead of writing (\d{4})-(\d{2})-(\d{2}), it is better to write (?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2}), clear at a glance.

2. use /x Add annotations to the pattern
In some Regex engines (such as PCRE, Perl), you can use verbose mode to add whitespace and comments to Regex.

3. Decompose complex Regex into variable combinations
Assign complex parts to variables first and then combine them, for example:const email = /^[\w.-]+@[\w.-]+\.\w+$/。

4. Build step by step
Don’t write complex Regexes all at once. Write the simplest part first, and then gradually add conditions after confirming it is correct - this is exactly the design concept of Regex Builder.
What are Lookahead and Lookbehind in Regex?
Lookahead and Lookbehind are collectively called Lookaround Assertions, they check the "conditions before and after the position" but do not consume characters or include matches in the results.

Lookahead (?=...)： Check whether the front is the specified mode.
Example:\d(?=px) Can match 16px in 6(but will not match 16em in 6）。

Deny look-ahead (?!...)： Check that the preceding "is not" specified mode.

Lookbehind (?<=...)： Check whether the rear is in the specified mode.
Example:(?<=\$)100 Can match $100 in 100。

Note: Lookbehind has limited support in some browsers or older versions of JavaScript. It is recommended to confirm the target environment before using it.
What is the difference between greedy and lazy quantifiers in Regex?
Greedy is the default behavior of quantifiers:*、+、{n,m} By default it will match "as many characters as possible".

Lazy/Non-greedy is added after the quantifier ? to match "as few characters as possible".

Example description:
pair string <div><p>text</p></div>：
• <.+>(greedy) will match the entire string
• <.+?>(lazy) will only match <div>

Choosing greed or laziness depends on your needs. Excessive use of greedy quantifiers may lead to performance problems (especially in nested structures). Appropriate use of lazy quantifiers can improve matching efficiency and accuracy.

A complete introduction to regular expressions: building your first Regex from scratch

Regular expressions are one of the most powerful and most underestimated tools in program development. Whether you are just starting to learn programming or an experienced engineer, Regex can help you handle more complex text logic with less code.

The core concepts of Regex

The essence of regular expressions is "a language for describing string patterns." It consists of ordinary characters (such as letters, numbers) and special characters (called metacharacters). For example,[0-9] stands for "any number", and + It means "the previous character can appear one or more times".

The shortcut to learning Regex is not to memorize the syntax, but to understand three basic elements:Character Classes Tell the engine what type of characters to match;Quantifiers Tell the engine how many to match;Anchors Tell the engine where in the string to match.

Common application scenarios

Form validation: Validation of email, phone number, password strength, and URL format almost all rely on Regex. A good Regex can complete format checking before the user submits the form, improving the user experience.

Text search and replace: The "Find in Files" function in the IDE, the sed command, and VS Code's search replacement all support Regex. Learning Regex allows you to accurately locate targets among tens of thousands of lines of code.

Log analysis: Server logs (such as Nginx, Apache, Docker logs) usually use Regex to retrieve specific timestamps, IP addresses, error codes and other information.

Data cleaning: Extract structured data from cluttered text—such as extracting all links from HTML or filtering specifically formatted rows of data from CSV.

Learning path suggestions

If you are new to Regex, it is recommended to learn in the following order: first master single character matching (.、\d、\w、\s), and then understand the quantifier (*、+、?、{n,m}), then the anchor point (^、$), and finally group and lookaround assertions. Using this tool's Pattern Builder and Cheat Sheet to learn together can make the learning curve smoother.

Operation successful