Regex Tester & Builder
Test and debug regular expressions with live match highlighting, capture group inspector, replace preview, and instant code snippets in JavaScript, Python, Go, Java & PHP. Includes a pattern library for common DevOps patterns.
Click any pattern to load it into the tester
How to Use the Regex Tester
Enter your pattern
Type a regular expression in the pattern bar at the top. Use the g, i, m, s flag buttons to control matching behaviour.
Paste test string
Paste the text you want to match against. Matches are highlighted in green in real time as you type.
Inspect results
Switch between the Matches, Groups, Replace, Code Snippet, and Cheatsheet tabs to explore your results.
Use the Pattern Builder
Click any pattern in the right panel (Email, IPv4, Semver, Docker image, etc.) to instantly load it into the tester.
Test replacements
Go to the Replace tab, enter a substitution string with $1, $2 for capture groups, and see the live preview.
Copy code snippet
Switch to Code Snippet, select your language (JS, Python, Go, Java, PHP), and copy the ready-to-use code.
Frequently Asked Questions
What is a regex tester?
A regex tester is an online tool that lets you type a regular expression and instantly see which parts of your test string match. It highlights matches in real time so you can debug and refine your pattern without running code.
How do I test a regular expression in JavaScript?
In JavaScript you can use const matches = text.match(/your-pattern/g) or the newer [...text.matchAll(/pattern/g)]. This tool generates the exact JavaScript snippet for your regex with one click — just switch to the "Code Snippet" tab.
What are regex capture groups?
Capture groups let you extract parts of a match. Wrap part of your pattern in parentheses: (\w+)@(\w+\.\w+). The first group $1 captures the username and $2 captures the domain. Use named groups (?<name>pattern) for readability.
What does the "g" flag mean in regex?
The "g" (global) flag tells the regex engine to find ALL matches in the string rather than stopping after the first. Without "g", most regex functions return only the first match.
How do I use regex in Python?
Import the re module and use re.findall(r"pattern", text) for all matches, re.search(r"pattern", text) for the first match, or re.sub(r"pattern", "replacement", text) to replace. This tool generates the Python snippet automatically from your pattern.
What regex pattern matches an email address?
A common email regex is: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}. You can load this instantly from the "Pattern Builder" panel under the Contact category. Note: fully RFC-5321 compliant email validation requires a much more complex pattern.
How do I match an IPv4 address with regex?
A precise IPv4 regex is: \b(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b — this validates that each octet is 0-255. Find it in the Network category of the Pattern Builder.
What is the difference between greedy and lazy quantifiers?
Greedy quantifiers (*, +, {n,m}) match as much text as possible. Lazy quantifiers (*?, +?, {n,m}?) match as little as possible. For example, <.+> on "<b>bold</b>" matches the entire string greedily, while <.+?> matches just "<b>".
Learn Bash & Linux: Step by Step
Regex powers Bash scripting, log analysis, and CI/CD pipelines. Master it with our free Linux tutorials.