Regular Expression Tester
Test and debug regular expressions with instant pattern matching and replacement.
About Regular Expression Tester
Our free regex tester and debugger is a powerful online tool designed for developers, data analysts, and programmers who work with regular expressions daily. This comprehensive regular expression validator helps you test patterns, debug complex regex, perform text replacements, and validate pattern matching instantly in your browser. With complete privacy and no data sent to any server, you can confidently test your regex patterns without any concerns.
Key Features
- ✓ Real-Time Pattern Matching: Test your regex patterns instantly against sample text and view all matches in real-time
- ✓ Regex Flags Support: Full support for global (g), case-insensitive (i), multiline (m), and dotall (s) flags
- ✓ Text Replacement: Test regex replacements with support for capture groups and backreferences
- ✓ Match Highlighting: Visual highlighting of all matches and groups in the test string
- ✓ Detailed Match Information: View index, length, and content of each match with group information
- ✓ Error Detection: Instant error reporting for invalid regex patterns with helpful error messages
- ✓ Copy & Download: Easily copy results to clipboard or download test results for documentation
How to Use the Regex Tester
- Enter Pattern: Enter your regular expression pattern in the first field (without slashes unless needed)
- Paste Test String: Enter or paste the text you want to test your regex against
- Select Flags: Choose appropriate flags (global, case-insensitive, multiline, dotall) for your pattern
- Test Pattern: Click "Test" to see all matches highlighted in the test string
- Optional Replace: Enter replacement text and click "Replace" to test substitutions with capture groups
- Review Results: View match details, indices, and replacement output in the results panel
Regular Expression Basics
Common Metacharacters
.- Matches any single character (except newline)*- 0 or more of preceding element+- 1 or more of preceding element?- 0 or 1 of preceding element^- Start of string/line$- End of string/line
Character Classes
\d- Any digit (0-9)\w- Any word character (a-z, A-Z, 0-9, _)\s- Any whitespace character[...]- Character class (e.g., [a-z])()- Capture group|- Alternation (OR)
Common Use Cases
Form Validation
Validate email addresses, phone numbers, URLs, and other user input with regex patterns
Text Search & Replace
Find and replace patterns in code, documents, and bulk text processing tasks
Data Extraction
Extract specific patterns from logs, emails, documents, and unstructured text
Code Cleanup
Refactor code, reformat files, and perform bulk edits using advanced regex patterns
Pattern Testing
Debug and test complex regex patterns before implementing them in your applications
Log Analysis
Parse and extract information from log files, error messages, and system outputs
Why Choose Our Regex Tester?
Understanding Regex Flags
🔍 Global Flag (g)
Matches all occurrences in the string instead of stopping at the first match. Essential for find-all operations.
Aa Case Insensitive Flag (i)
Matches without distinguishing between uppercase and lowercase letters. Useful for case-agnostic searches.
⬇️ Multiline Flag (m)
Treats ^ and $ as line boundaries instead of string boundaries. Important for multi-line text processing.
⊙ Dotall Flag (s)
Makes . match newline characters in addition to any other character. Used for matching across lines.
Advanced Regex Techniques
Capture Groups & Backreferences
Use parentheses to create capture groups: (\d{3})-(\d{3})-(\d{4}). Reference them in replacements with $1, $2, $3
Quantifiers & Ranges
Specify how many times to match: a{3} (exactly 3), a{2,4} (2 to 4), a{2,} (2 or more)
Lookahead & Lookbehind
Use assertions to match without consuming characters: (?=pattern) positive lookahead, (?!pattern) negative lookahead
Non-Capturing Groups
Create groups that don't capture: (?:pattern). Useful for grouping without creating backreferences
Common Regex Patterns
Email Validation
/^[^\s@]+@[^\s@]+\.[^\s@]+$/
URL Validation
/^https?:\/\/.+/i
Phone Number (US)
/^\(?\d{3}\)?[\s.-]?\d{3}[\s.-]?\d{4}$/
IP Address
/^(\d{1,3}\.){3}\d{1,3}$/
Frequently Asked Questions
Is my test data safe when using this regex tester?
Yes, completely safe. All processing happens locally in your browser. Your test strings and regex patterns are never transmitted to any server.
What regex flavor does this tool support?
Our regex tester uses JavaScript regex engine (ECMAScript), which supports most common regex features including character classes, quantifiers, groups, and lookarounds.
How do I use capture groups in replacements?
Use parentheses in your pattern to create capture groups, then reference them in replacements with $1, $2, etc. For example: pattern (\w+) (\w+) with replacement $2 $1
Can I test regex replacements?
Yes! Enter your replacement text in the "Replacement" field and click "Replace" to test substitutions. You can use capture groups in your replacement text.
How do I match newlines?
Use the multiline flag (m) to match line boundaries with ^ and $, or use the dotall flag (s) to make . match newline characters.
What should I do if my regex isn't working?
Use this tester to debug! Start simple, test individual components, review the match results, and gradually build complexity. Check for syntax errors in the error messages.
Regex Best Practices
\d, \w, \s over broader patterns for better performance.
Start Testing Regex Patterns Today
Whether you're validating form input, extracting data from text, refactoring code, or debugging complex patterns, our free regex tester is the perfect companion for your development work. Test patterns instantly, experiment with flags and capture groups, and perfect your regex before implementation—all without leaving your browser!