Regex Tester

Test and debug regular expressions with real-time pattern matching, capture group visualization, match highlighting, and replace functionality. Includes comprehensive regex cheat sheet and library of common patterns.

Client-Side Processing

All regex testing happens in your browser. Your patterns and test strings never leave your device.

//
Enter test string...

Real-Time Testing

See matches instantly as you type with live highlighting and match counts.

Capture Groups

View and reference capture groups with detailed match information.

Pattern Library

Common patterns for email, URL, phone, dates, and more built-in.

Replace Function

Test replacements with capture group references in real-time.

Professional Regular Expression Testing

The Regex Tester is a powerful tool for developers, data analysts, and anyone working with text patterns. Test regular expressions in real-time with instant visual feedback showing all matches highlighted in your test string. View detailed match information including index positions, lengths, and capture groups. Test regex flags (global, case-insensitive, multiline, dotall, unicode, sticky), use the replace function with capture group references, save frequently used patterns, and access a comprehensive cheat sheet and library of common regex patterns for emails, URLs, phone numbers, dates, and more.

Why Use a Regex Tester?

Debug Complex Patterns

Regular expressions can be tricky to get right. See exactly what your pattern matches in real-time with highlighted results and detailed match information.

Test Before Deploying

Validate your regex patterns with real test data before using them in production code. Catch edge cases and errors early in development.

Learn Regex Syntax

Experiment with different patterns and see results instantly. Use the built-in cheat sheet and common patterns library to learn regex syntax.

Visualize Capture Groups

See exactly what each capture group matches with clear visual display. Perfect for complex patterns with multiple groups.

Key Features

  • ✓Real-Time Matching: See matches instantly as you type your pattern or test string with no delay or refresh needed.
  • ✓Match Highlighting: All matches are highlighted with yellow background in the test string for easy visual identification.
  • ✓Capture Group Display: View all capture groups for each match with $1, $2, $3 notation and group content.
  • ✓Match Details: See match index position, length, and full match text for every match found.
  • ✓Regex Flags: Toggle global (g), case-insensitive (i), multiline (m), dotall (s), unicode (u), and sticky (y) flags.
  • ✓Replace Functionality: Test regex replace operations with capture group references ($1, $2, etc.) in real-time.
  • ✓Save Patterns: Save up to 5 frequently used regex patterns (free) with name and description for quick access.
  • ✓Common Patterns Library: Built-in library with 12+ common patterns for email, URL, phone, IP, date, time, and more.
  • ✓Regex Cheat Sheet: Comprehensive reference covering character classes, quantifiers, anchors, groups, and flags.
  • ✓Share Patterns: Generate shareable URLs with your regex pattern and flags encoded for collaboration.
  • ✓Error Detection: Clear error messages when regex pattern has syntax errors with helpful descriptions.

Common Use Cases

Email Validation: Test patterns for validating email addresses in registration forms, contact forms, and user input validation.

URL Extraction: Extract URLs from text content for link validation, web scraping, or content analysis.

Phone Number Formatting: Validate and extract phone numbers in various formats (US, international) from text data.

Data Validation: Validate dates, times, credit cards, postal codes, and other structured data formats in forms and APIs.

Log File Parsing: Extract specific information from server logs, application logs, or system logs using regex patterns.

Text Transformation: Test find-and-replace operations for bulk text processing, data cleaning, or content migration.

Web Scraping: Design patterns to extract structured data from HTML, XML, or text content during web scraping.

How to Use the Regex Tester

  1. 1.Enter your regular expression pattern in the pattern field (without the surrounding slashes). The pattern is automatically tested in real-time.
  2. 2.Select regex flags by clicking the flag buttons (g, i, m, s, u, y). Blue indicates active, gray indicates inactive.
  3. 3.Paste or type your test string in the test area. All matches are highlighted instantly with match count displayed.
  4. 4.Review match details below showing index, length, full match, and capture groups for each match found.
  5. 5.Optionally test replace functionality by entering replacement text (use $1, $2 for capture group references).
  6. 6.Save useful patterns with a name and description, or load patterns from the common patterns library.

Understanding Regex Flags

g (Global): Find all matches in the string instead of stopping after the first match. Essential for counting or replacing all occurrences.

i (Case Insensitive): Ignore case when matching. "ABC" will match "abc", "Abc", "aBc", etc.

m (Multiline): Make ^ and $ match the start/end of each line instead of the entire string.

s (Dotall): Make the dot (.) match newline characters. Without this flag, dot matches everything except newlines.

u (Unicode): Enable full Unicode matching support for emoji and special characters.

y (Sticky): Match only from the lastIndex position (advanced use case for sequential matching).

Common Regex Patterns Explained

Email Address

[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}

Matches standard email format: username@domain.extension. Allows letters, numbers, dots, underscores, percent, plus, and hyphens in username.

URL (HTTP/HTTPS)

https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}...

Matches web URLs starting with http or https, with optional www subdomain, and supports paths, query strings, and fragments.

US Phone Number

\(?\d{3}\)?[-.\\s]?\d{3}[-.\\s]?\d{4}

Matches US phone numbers in various formats: (123) 456-7890, 123-456-7890, 123.456.7890, or 1234567890.

Strong Password

^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])...

Validates strong passwords requiring at least one lowercase letter, one uppercase letter, one digit, one special character, and minimum 8 characters.

Using Capture Groups

Capture groups allow you to extract specific parts of a match. Wrap parts of your pattern in parentheses to create groups:

Pattern: (\w+)@(\w+\.\w+)

Test: "contact@example.com"

$1 = "contact" (username)
$2 = "example.com" (domain)

Replace: $2 < $1

Result: "example.com < contact"

Advanced Features (Premium)

Premium users get access to advanced regex tools and AI-powered features:

  • •Unlimited Saved Patterns: Save unlimited regex patterns with full metadata, tags, and categories for easy organization.
  • •AI Pattern Explanation: Get AI-powered plain English explanations of complex regex patterns with component breakdown.
  • •Pattern Generation: Describe what you want to match in plain English and get AI-generated regex patterns.
  • •Performance Analysis: Get performance warnings for slow patterns and suggestions for optimization.
  • •Pattern History: Access full history of tested patterns with timestamps and test strings for future reference.
  • •Bulk Testing: Test one pattern against multiple test strings simultaneously with comparison view.

Regex Best Practices

Be Specific: Narrow patterns are more efficient and accurate than overly broad patterns. Use character classes and quantifiers precisely.

Use Non-Capturing Groups: When you don't need to reference a group, use (?:...) instead of (...) for better performance.

Avoid Catastrophic Backtracking: Be careful with nested quantifiers like (a+)+ which can cause exponential performance issues.

Test Edge Cases: Test with empty strings, very long strings, special characters, and unexpected input formats.

Document Complex Patterns: Save patterns with descriptive names and comments explaining what each part does.

Privacy and Security

All regex testing happens entirely in your browser using JavaScript. Your patterns and test strings never leave your device, are never uploaded to our servers, and are never stored in our databases. Saved patterns are stored only in your browser's local storage on your device. This ensures complete privacy and security for sensitive data, proprietary patterns, or confidential text content.

Browser Compatibility

The Regex Tester works in all modern browsers including Chrome, Firefox, Safari, Edge, and Opera. The tool uses standard JavaScript RegExp which is supported universally. Some advanced features like unicode flag (u) and sticky flag (y) require modern browser versions.

Frequently Asked Questions

What regex flavor does this tool use?

The tool uses JavaScript regex (ECMAScript). This is the same regex engine used in Node.js, browsers, and many JavaScript frameworks. It supports most common regex features.

How do I test the global flag?

Click the "g" button to enable global matching. With global enabled, the tool finds all matches in the test string. Without it, only the first match is found.

What are capture groups and how do I use them?

Wrap parts of your pattern in parentheses like (pattern) to create capture groups. These groups can be referenced as $1, $2, etc. in replacements and are shown separately in match details.

How many patterns can I save?

Free users can save up to 5 patterns in browser local storage. Premium users get unlimited cloud-synced pattern storage with search and organization features.

Can I share my regex pattern with others?

Yes! Click the "Share" button to generate a URL with your pattern and flags encoded. Anyone with the URL can load your pattern into the tester.

What does the "Explain" button do?

The Explain feature is a premium tool that uses AI to provide plain English explanations of regex patterns, breaking down each component and explaining what it matches.

Why isn't my pattern matching?

Check for syntax errors in the error message. Common issues: unescaped special characters, unclosed groups, invalid quantifiers. Use the cheat sheet for syntax reference.

How do I match special characters literally?

Escape special regex characters with a backslash. For example, to match a literal dot, use \. instead of just . (which matches any character).

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