Regex Tester

What this tool does

The Regex Tester is an interactive tool for testing and debugging regular expressions (regex). Regular expressions are patterns used to match character combinations in strings, and they're essential for text processing, validation, search operations, and data extraction.

This tool allows you to test regex patterns against sample text, view matches in real-time, and understand how different regex flags affect matching behavior. It highlights matches, shows match positions, displays captured groups, and provides clear error messages for invalid regex patterns.

The tool supports standard JavaScript regex syntax and flags (global, case-insensitive, multiline, dotall, unicode, sticky), making it perfect for web developers and anyone working with text processing in JavaScript or similar languages.

When to use it

Pattern Development: Develop and refine regex patterns before implementing them in your code.

Debugging: Debug regex patterns that aren't working as expected in your applications.

Learning: Understand how regex patterns work and experiment with different syntax and flags.

Validation: Test regex patterns for input validation, email validation, phone number formats, etc.

Text Processing: Develop regex patterns for text extraction, replacement, or transformation operations.

Code Review: Verify regex patterns during code reviews to ensure they work correctly and efficiently.

Documentation: Test regex examples for documentation to ensure they're accurate and work as described.

How to use it

1. Enter Regex Pattern: Type your regular expression pattern in the regex input field. Include flags if needed, or set them separately.

2. Set Flags: Configure regex flags (g for global, i for case-insensitive, m for multiline, etc.) in the flags field.

3. Enter Test String: Paste or type the text you want to test against the regex pattern.

4. View Matches: The tool automatically shows all matches, their positions, and captured groups when you finish editing.

5. Review Errors: If the regex is invalid, the tool displays an error message explaining what's wrong.

6. Reset: Use the reset button to clear all fields and start fresh.

7. Load Example: Click the example button to see a sample regex test.

Example

Regex Pattern: \d+ Flags: g Test String: I have 5 apples and 10 oranges.

Matches:

• Match: 5 at index 7
• Match: 10 at index 19

Common mistakes

Escaping Special Characters: Special regex characters (., *, +, ?, ^, $, etc.) must be escaped with backslash when matching literally.

Greedy vs Lazy: Quantifiers (*, +, ?) are greedy by default. Use ? after quantifiers for lazy matching.

Anchors: ^ matches start of string, $ matches end. Use \A and \Z for start/end of line in multiline mode.

Character Classes: Use square brackets [] for character classes. Remember that some characters have special meaning inside brackets.

Grouping: Use parentheses () for capturing groups. Use (?:) for non-capturing groups when you don't need the captured value.

Flag Confusion: Different flags affect matching behavior. Understand what each flag does before using it.

Unicode: For Unicode characters, use the u flag and appropriate Unicode character classes or escapes.

Tips for better results

Start Simple: Begin with simple patterns and gradually add complexity. Test each addition to ensure it works as expected.

Use Anchors: Use ^ and $ anchors when you want to match the entire string, not just a substring.

Test Edge Cases: Test your regex with edge cases like empty strings, very long strings, and strings with special characters.

Understand Flags: Learn what each flag does. Global (g) finds all matches, case-insensitive (i) ignores case, multiline (m) affects ^ and $ behavior.

Use Non-Capturing Groups: When you don't need captured groups, use (?:) for better performance.

Escape Properly: Remember to escape special characters when matching them literally. Use \ for a literal backslash.

Test Performance: Very complex regex patterns can be slow. Test with realistic data sizes to ensure acceptable performance.

Document Patterns: Document complex regex patterns with comments explaining what they match.

FAQ

What is regex? Regular expressions (regex) are patterns used to match character combinations in strings. They're powerful tools for text processing and validation.

What regex flags are supported? Standard JavaScript regex flags: g (global), i (case-insensitive), m (multiline), s (dotall), u (unicode), y (sticky).

Can I test complex regex patterns? Yes, the tool supports all standard JavaScript regex syntax including lookaheads, lookbehinds, and complex patterns.

What happens if my regex is invalid? The tool displays an error message indicating what's wrong with your regex pattern, helping you fix it.

Can I see captured groups? Yes, the tool displays all captured groups for each match, showing what each group captured.

Is there a limit to test string length? While there's no strict limit, very long strings may take longer to process. For extremely large texts, consider using specialized tools.

Can I use this offline? Yes, once the page is loaded, regex testing happens entirely in your browser and works offline.

Does the tool support all regex features? The tool supports standard JavaScript regex features. Some advanced features may vary depending on browser support.

Can I save my regex patterns? The tool doesn't save patterns, but you can copy them for use in your code or documentation.

What's the difference between global and non-global matching? Global (g) flag finds all matches in the string. Without it, only the first match is found.