Regex Tester

About Regular Expression Testing

Regular expression testing is essential for developing, debugging, and validating regex patterns. Our regex tester provides real-time matching, detailed explanations, and comprehensive testing capabilities to ensure your patterns work correctly.

Real-time pattern matching and validation
Detailed match results with capture groups
Pattern explanation and breakdown
Multiple test strings and batch testing
Support for different regex flavors and flags

Testing Features

Match Analysis

Live match highlighting
Capture group extraction
Match count and positions
Non-matching text identification
Performance timing information

Pattern Validation

Syntax error detection
Pattern explanation
Optimization suggestions
Common pitfall warnings
Cross-platform compatibility

Frequently Asked Questions

How to test regex online?

Paste your regular expression pattern into the pattern field, enter your test string, and view matches highlighted in real time. utilAZ provides a free online regex tester that shows match results, capture groups, and pattern explanations instantly in your browser.

What is a regex tester tool?

A regex tester tool lets you write, test, and debug regular expressions against sample text without running code. It highlights matches, displays capture groups, and reports errors so you can refine patterns quickly. utilAZ offers this functionality entirely in the browser with no sign-up required.

How to use a regex tester?

Enter your regex pattern in the pattern input, type or paste the text you want to test against, and select the appropriate flags such as global or case-insensitive. The tool will instantly highlight all matches and display capture group details so you can verify your pattern works correctly.

Best free regex tester?

utilAZ offers a free regex tester with live matching, syntax highlighting, capture group extraction, and support for multiple flags. It runs entirely in the browser, requires no account, and never stores your data, making it a reliable choice for developers and learners alike.

How do I use a Regex Tester?

Type your regular expression into the pattern field and paste your sample text into the test string area. Choose the flags you need (global, case-insensitive, multiline) and the tool will highlight matches in real time. You can then review capture groups and match positions to confirm your pattern behaves as expected.

Testing Examples

Email Validation Test:

Pattern: /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/

Test Strings:

john@example.com ✓ Match

user.name+tag@domain.co.uk ✓ Match

invalid.email ✗ No match

@domain.com ✗ No match

Phone Number Extraction:

Pattern: /$?([0-9]{3})$?[-. ]?([0-9]{3})[-. ]?([0-9]{4})/g

Test String:

Call us at (555) 123-4567 or 555.987.6543 for support.

Matches found:

1. (555) 123-4567 - Groups: 555, 123, 4567

2. 555.987.6543 - Groups: 555, 987, 6543

Regex Flags and Modifiers

Common Flags

g - Global

Find all matches, not just the first one

/pattern/g

i - Ignore Case

Case-insensitive matching

/pattern/i

m - Multiline

^ and $ match line boundaries

/pattern/m

Advanced Flags

s - Dot All

. matches newline characters

/pattern/s

u - Unicode

Enable Unicode features

/pattern/u

y - Sticky

Match only at lastIndex position

/pattern/y

Testing Best Practices

Test Edge Cases: Empty strings, very long strings, special characters
Use Multiple Examples: Test both matching and non-matching cases
Check Performance: Test with large inputs to identify bottlenecks
Validate Capture Groups: Ensure groups capture the expected content
Cross-Platform Testing: Test patterns in target environments
Document Test Cases: Keep examples for future reference
Gradual Building: Start simple and add complexity incrementally

Using Regex in Code

Once you have built and tested your pattern with our tool, integrating it into your codebase is straightforward. Here is the core API in each major language.

Quick Reference by Language:

JavaScript: const m = str.match(new RegExp(pattern, 'gi'));

Python: m = re.findall(pattern, text, re.IGNORECASE)

Java: Pattern.compile(pattern).matcher(text).find()

CLI (grep): grep -E 'pattern' file.txt

Testing Tips:

Always test with both matching and non-matching inputs
Include edge cases: empty strings, special chars
Verify capture groups return expected values
Benchmark with large inputs for performance

Common Pitfalls:

Forgetting to escape backslashes in strings
Greedy vs. lazy quantifiers (.* vs. .*?)
Anchors (^ $) behaving differently in multiline
Character class differences across engines

Build and validate your pattern here first, then copy the tested regex directly into your code with confidence.

Common Use Cases

Form validation pattern development
Data extraction and parsing
Input sanitization testing
Log file analysis patterns
Search and replace validation
API response parsing
Configuration file processing
Text processing automation