Regex Tester
Test and debug regular expressions with live matching
The Regex Tester runs a JavaScript regular expression against your test text and highlights every match directly in the string. It also lists capture groups, match indexes, and lengths so you can debug both validation patterns and extraction patterns without leaving the browser.
All processing runs locally in your browser. Your files and text are not sent to Tool-web's server.
How to Use
- Enter your regular expression pattern
- Choose flags (g, i, m, etc.)
- Enter test text to match against
- View matches and capture groups highlighted
Features
- Live pattern matching as you type
- Highlighted matches inside the test string
- Capture-group output with match index and length
- Multiple JavaScript regex flags including g, i, m, s, u, and d
- Runs entirely in the browser
Tips for getting Regex Tester right
- Anchor the pattern when you need a full-string match rather than a substring match
- Escape literal characters that are also regex metacharacters, like . and +
- Test the pattern against both matching and non-matching inputs
- Beware of nested quantifiers that can slow matching on long inputs
Worked example: validate an invoice code
Enter `^INV-(\d{4})-(\d{2})$` with the test string `INV-2026-08` to confirm the entire value matches your expected format. The results list shows the full match plus the captured year and month groups, which is useful when you are debugging extraction as well as validation.
What the common flags change
| Flag | Effect |
|---|---|
| g | Find every match instead of just the first one |
| i | Ignore case while matching |
| m | Make ^ and $ work line by line |
| s | Let . match newline characters too |
| u | Enable Unicode-aware matching rules |
Real-World Use Cases
- Debugging regular expressions during development
- Testing pattern matching on sample data
- Learning regex with real-time match highlighting
- Validating regex patterns before deploying to production
- Building complex patterns with group capture visualization
Best Practices
- Start with simple patterns and add complexity gradually
- Test with multiple sample strings including edge cases
- Use named capture groups for readability in complex patterns
- Check for anchoring (^ and $) when matching full strings
- Document complex regex patterns with comments
Common Mistakes to Avoid
- Not escaping special characters when they should be literal
- Forgetting that . matches any character except newlines by default
- Using greedy quantifiers when lazy quantifiers are needed
- Not testing with edge cases and boundary conditions
- Overcomplicating patterns when simpler alternatives exist
Troubleshooting
- If the pattern doesn't match, try breaking it into smaller parts
- For performance issues, avoid catastrophic backtracking with nested quantifiers
- Check if the regex flavor matches your target environment
- Use the match highlighting to understand where the pattern succeeds or fails
- Test with both matching and non-matching strings
Frequently Asked Questions
What regex syntax is supported?
Can I use regex flags?
Why do I only see one match sometimes?
Privacy & Security
Regular expression testing happens entirely in your browser. Your test data and patterns are never transmitted to any server.
Tips & Best Practices
- Anchor the pattern when you need a full-string match rather than a substring match
- Escape literal characters that are also regex metacharacters, like . and +
- Test the pattern against both matching and non-matching inputs
- Beware of nested quantifiers that can slow matching on long inputs
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