Regex Tester
Test and debug regular expressions in real-time with match highlighting and capture groups.
What is Regex Tester?
Regex Tester is a free online tool for testing and debugging regular expressions (regex) with instant visual feedback. Enter a pattern and test string to see all matches highlighted in real-time, along with detailed capture group information. It is an essential tool for developers who need to validate, refine, or learn regular expression patterns without writing test scripts.
How to Use Regex Tester
- Enter your regular expression pattern in the pattern input field
- Set the appropriate flags: g for global (find all matches), i for case-insensitive, or m for multiline mode
- Type or paste your test string in the text area below to see matches highlighted instantly
- Review the match results panel to see each match, its position, and any captured groups
- Iterate on your pattern by modifying it and observing how the matches change in real-time
Tips & Best Practices
Start Simple and Build Up
Begin with a simple pattern that matches part of what you need, then progressively add complexity. This incremental approach makes it much easier to debug when something does not match as expected.
Use Non-Capturing Groups When Possible
If you only need grouping for alternation or quantifiers but do not need to extract the match, use (?:...) instead of (...). This improves performance and keeps your capture group numbering clean.
Escape Special Characters
Characters like . * + ? ^ $ { } [ ] ( ) | \ have special meaning in regex. If you want to match them literally, you must escape them with a backslash. For example, use \. to match an actual period.
Test Edge Cases
Always test your regex against edge cases such as empty strings, strings with special characters, very long input, and input that should NOT match. A pattern that works on happy-path data may fail on real-world input.
Common Use Cases
Form Input Validation
Build and test regex patterns for validating email addresses, phone numbers, postal codes, and other user input before implementing them in your application's frontend or backend validation logic.
Log File Analysis
Create patterns to extract timestamps, error codes, IP addresses, and other structured data from server logs. Test the regex against sample log entries to ensure accurate extraction before using it in scripts.
Search and Replace in Code
Develop complex find-and-replace patterns for refactoring code across large files. Test the regex here first to verify it matches exactly what you intend before running a bulk replacement in your IDE or command line.
FAQ
What regex flavors are supported?
This tool uses JavaScript's built-in RegExp engine, which supports most common regex features.
What do the flags mean?
g = global (find all matches), i = case-insensitive, m = multiline (^ and $ match line boundaries).
Can I use lookahead and lookbehind assertions?
Yes. JavaScript supports positive lookahead (?=...), negative lookahead (?!...), positive lookbehind (?<=...), and negative lookbehind (?<!...). These are powerful features for matching patterns based on surrounding context without including that context in the match.
Why does my pattern match more than expected?
This is usually caused by greedy quantifiers like .* which match as much text as possible. Try using the lazy (non-greedy) version .*? instead, which matches as little as possible. Also check that you are using the correct anchors (^ and $) to constrain your match.
How do I match across multiple lines?
Enable the m (multiline) flag to make ^ and $ match the start and end of each line rather than the entire string. If you need the dot (.) to match newline characters as well, you can use [\s\S] as an alternative since JavaScript does not support the s (dotAll) flag in all environments.