Regex Tester — Free Online Regular Expression Debugger
Test regular expressions in real-time with match highlighting, group capture, pattern explanation, and replace mode. Debug your regex patterns before putting them into production code. All processing happens in your browser.
How to Use the Regex Tester
- Type your regex pattern in the pattern field — matches highlight in real-time.
- Set the flags —
gfor global,ifor case-insensitive,mfor multiline,sfor dotAll,ufor unicode. - Switch to Replace mode to test find-and-replace with group backreferences ($1, $2).
- View the Explanation tab to understand what each part of your pattern does.
- Get Code Snippets in JavaScript, Python, Go, Java, C#, or PHP.
Why Use This Regex Tester
Regular expressions are powerful but notoriously hard to read and debug. A single misplaced character can mean the difference between matching "hello@world.com" and matching nothing at all. This tester gives you instant visual feedback — you see exactly which parts of your text match, which don't, and where your capturing groups land.
The Explain tab breaks down your pattern token by token so you can understand (or debug) complex expressions. The Code tab gives you ready-to-use snippets in 6 languages. The Replace tab lets you test find-and-replace operations with group backreferences.
Frequently Asked Questions
Without the s flag, the dot . matches any character except newline. With s, the dot also matches newlines. This is useful for matching multi-line strings like .* to match everything including line breaks.
The u flag enables full Unicode support. It lets you use Unicode escapes (\u{1F600}), makes . match Unicode characters properly, and enables the \p{} property escapes for matching Unicode categories like letters, emojis, and more.
Use $1, $2, etc. to reference captured groups. For example, if your pattern is (\w+)@(\w+), then $2@$1 in the replacement swaps the username and domain. Use $& to insert the entire match.
Capturing groups are portions of your pattern wrapped in parentheses (...). They let you extract specific parts of a match. For example, in (\d{4})-(\d{2})-(\d{2}), group 1 captures the year, group 2 the month, group 3 the day.
Use Cases
Email Validation
Test email validation regex patterns to ensure proper email format checking.
URL Matching
Debug URL matching expressions for web scraping and link validation tasks.
Phone Formats
Validate phone number formats for international and local number patterns.
Password Patterns
Test password strength patterns with character requirements and length rules.
Regex Learning
Learn regex with real-time feedback, match highlighting, and pattern explanations.