Regex Tester
Test and debug regular expressions with real-time matching, syntax highlighting, and detailed explanations. Perfect for developers and data analysts.
Key Features
- Live Matching – Real-time pattern matching as you type
- Syntax Highlighting – Color-coded regex patterns
- Match Groups – Visualize capture groups
- Flags Support – Global, case-insensitive, multiline, etc.
- Pattern Explanation – Break down regex components
- Test Strings – Multiple test cases at once
- Quick Reference – Built-in regex cheat sheet
- Code Generation – Export to JavaScript, Python, etc.
- Share Patterns – Generate shareable URLs
- Browser-Based – No installation required
Common Regex Patterns
Email Validation
Pattern:^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$
Matches: [email protected], [email protected]
Use for: Form validation, email extraction
Phone Number (US)
Pattern:^\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$
Matches: (555) 123-4567, 555-123-4567, 5551234567
Use for: Phone validation, data cleaning
URL Validation
Pattern:^https?://[^\s]+$
Matches: https://example.com, http://site.org/path
Use for: Link validation, URL extraction
IPv4 Address
Pattern:^(?:[0-9]{1,3}\.){3}[0-9]{1,3}$
Matches: 192.168.1.1, 10.0.0.255
Use for: Network configuration, log parsing
Date (YYYY-MM-DD)
Pattern:^\d{4}-\d{2}-\d{2}$
Matches: 2024-01-15, 2025-12-31
Use for: Date validation, log parsing
Hex Color Code
Pattern:^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$
Matches: #FFF, #3b82f6, #000000
Use for: CSS validation, color extraction
Regex Quick Reference
Character Classes
\d– Digit (0-9)\w– Word character (A-Z, a-z, 0-9, _)\s– Whitespace (space, tab, newline).– Any character except newline[abc]– Any of a, b, or c[^abc]– Not a, b, or c
Quantifiers
*– 0 or more times+– 1 or more times?– 0 or 1 time (optional){3}– Exactly 3 times{3,}– 3 or more times{3,5}– Between 3 and 5 times
Anchors
^– Start of string/line$– End of string/line\b– Word boundary\B– Not word boundary
Groups & Lookaround
(abc)– Capture group(?:abc)– Non-capturing group(?=abc)– Positive lookahead(?!abc)– Negative lookahead
Regex Best Practices
✅ Best Practices
- Test thoroughly – Use multiple test cases
- Be specific – Avoid overly broad patterns like
.* - Use anchors –
^and$for exact matches - Escape special chars – Use
\for literals - Comment complex patterns – Use
(?#comment) - Use non-capturing groups –
(?:)when not needed
❌ Avoid These
- Don’t use regex for HTML – Use proper parsers
- Avoid catastrophic backtracking – Nested quantifiers
- Don’t trust user input – Validate before regex
- Avoid greedy quantifiers – Use lazy
*?when needed - Don’t overcomplicate – Split complex patterns
- Don’t forget to escape – Dots, brackets, etc.
Frequently Asked Questions
What are regex flags?
Flags modify how regex patterns work. Common flags: g (global – find all matches), i (case-insensitive), m (multiline – ^ and $ match line breaks), s (dotall – . matches newlines), u (unicode), y (sticky – match from lastIndex).
What is catastrophic backtracking?
A performance issue with nested quantifiers. Patterns like (a+)+ or (a|a)* can cause exponential time complexity on non-matching input. Avoid nested quantifiers and use possessive quantifiers or atomic groups when available.
How do I match literal special characters?
Escape them with backslash. Special characters: . ^ $ * + ? { } [ ] \ | ( ). To match a literal dot: \.. To match a literal backslash: \\. Inside character classes [], most characters are literal except ^, -, and ].
What’s the difference between greedy and lazy?
Greedy matches as much as possible, lazy as little as possible. .* is greedy (matches maximum), .*? is lazy (matches minimum). For input "test", pattern ".*" matches the whole string, while ".*?" matches empty string (zero-width).
Can I use regex to parse HTML/XML?
No, don’t parse HTML with regex. HTML is not a regular language and can’t be properly parsed with regex. Nested tags, attributes, and malformed markup will break regex patterns. Use a proper HTML parser (DOM, Beautiful Soup, Cheerio, etc.) instead.
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