Regex Tester Tool
Test, Debug, and Visualize Regular Expressions
Regular Expression Tester
Match Details
What is a Regex Tester?
A Regex Tester (Regular Expression Tester) is a tool that helps developers, data analysts, and text processors test and debug regular expressions. Regular expressions are powerful pattern-matching sequences used to search, extract, and manipulate text. Our Regex Tester tool processes everything locally in your browser, ensuring your data never leaves your device, making it both secure and private.
Working with regular expressions can be challenging, especially for complex patterns. They use a dense syntax with special characters that can be difficult to understand at first glance. Our Regex Tester makes this process easier by providing real-time visual feedback, highlighting matches in your test text, and displaying detailed information about capture groups and match positions.
Why Use a Regex Tester?
Regular expressions are used across programming languages and text processing tools for everything from data validation to complex search-and-replace operations. Whether you're a developer validating form inputs, a data analyst cleaning datasets, or a content editor performing batch text operations, a Regex Tester saves time and reduces errors by allowing you to visualize and verify your patterns before implementing them in production code.
Visual Feedback
See exactly what your regular expression matches in real-time with highlighted text that makes pattern matching clear and intuitive.
Pattern Debugging
Identify issues in your regular expressions quickly by seeing exactly what text is being matched and what isn't.
Capture Groups
View and test capture groups in your regex patterns to extract specific portions of matching text for advanced text processing.
Learning Tool
Improve your regex skills by experimenting with different patterns and immediately seeing the results in a visual, interactive way.
Flag Support
Test regex with different flags (global, case-insensitive, multiline, etc.) to fine-tune pattern matching behavior for specific use cases.
Development Efficiency
Speed up development workflows by quickly testing regex patterns before implementing them in your code or text processing operations.
How to Use This Regex Tester
- Enter your regular expression pattern in the input field (without the surrounding / delimiters).
- Set any regex flags you need (g, i, m, etc.) in the flags field.
- Type or paste your test text into the test area.
- View matched portions of your text highlighted in real-time.
- Check the Match Details section to see information about each match, including indices and capture groups.
- Use the "Test Regex" button to manually run the test if real-time matching is disabled.
- Click the "Copy" button to copy your regex pattern in /pattern/flags format.
- Try preset patterns for common use cases like email validation, URLs, etc.
Regex Testing and Privacy
Our Regex Tester processes all data locally in your browser. Your patterns and test text never leave your device or get transmitted to any server, ensuring complete privacy and security. This makes our tool perfect for working with sensitive or confidential data that shouldn't be shared with third-party services.
Advantages of Our Regex Tester
100% Client-Side Processing
All regex testing happens directly in your browser - your data never leaves your device or gets sent to any server.
Real-Time Matching
See your regex matches update instantly as you type either pattern or test text, with intuitive visual highlighting.
Dark Mode Support
Choose between light and dark modes for comfortable viewing in any environment or time of day.
Capture Group Visualization
Clearly see what each capture group contains, making complex pattern extraction more manageable.
Preset Patterns
Access commonly used regex patterns for emails, URLs, dates, and more to save time and learn standard implementations.
Mobile-Friendly Design
Fully responsive interface that works well on all devices from desktops to smartphones.
Understanding Regex Syntax
Regular expressions use a special syntax to define text patterns. Here are some fundamental components:
- Literals: Characters that match themselves (e.g., "a" matches the character "a")
- Meta-characters: Characters with special meanings, such as . (any character), ^ (start of line), $ (end of line)
- Character Classes: Sets like [abc] (any of a, b, or c) or ranges like [a-z] (any lowercase letter)
- Quantifiers: Specifiers for how many times a pattern should match, such as * (0 or more), + (1 or more), ? (0 or 1), {n} (exactly n)
- Groups: Patterns enclosed in () to create capturing groups for extraction or to apply quantifiers to multiple characters
- Alternatives: The | character to specify "or" conditions (e.g., cat|dog matches "cat" or "dog")
- Escaping: Using \ to escape special characters when you want their literal meaning
- Flags: Modifiers like g (global), i (case insensitive), m (multiline), etc., that change how the pattern is applied
Common regex patterns:
- Email: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}
- URL: https?:\/\/(?:www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b(?:[-a-zA-Z0-9()@:%_\+.~#?&\/=]*)
- Phone Number: \b\d{3}[-.]?\d{3}[-.]?\d{4}\b
- Date (YYYY-MM-DD): \d{4}-\d{2}-\d{2}
- IP Address: \b(?:\d{1,3}\.){3}\d{1,3}\b
Common Regex Challenges
Greedy vs. Non-Greedy Matching
By default, quantifiers are greedy and match as much as possible. Adding a ? after a quantifier makes it non-greedy (lazy), matching as little as possible.
Greedy: ".+" in "<div>Hello</div><div>World</div>"
Matches: <div>Hello</div><div>World</div>
Non-greedy: ".+?" in the same text
Matches: <div>Hello</div>
Escaping Special Characters
Special characters like ., +, *, ?, ^, $, (, ), [, ], {, }, |, \ must be escaped with a backslash (\) to match their literal characters.
To match example.com literally: example\.com
To match 5+10=15 literally: 5\+10=15
Capturing vs. Non-Capturing Groups
Regular groups (parentheses) capture matched text for later use, while non-capturing groups (?:) group elements without capturing.
Capturing: (\w+)@(\w+\.\w+) in user@example.com
Group 1: user, Group 2: example.com
Non-capturing: (?:\w+)@(\w+\.\w+)
Only captures example.com
Lookahead and Lookbehind
Lookahead and lookbehind are zero-width assertions that don't consume characters but check what comes before or after the current position.
Positive lookahead: \d+(?=px) in 12px
Matches: 12 (the number before "px")
Lookbehind examples need special handling in JavaScript
Frequently Asked Questions About Regular Expressions
What are regular expressions and why are they useful?
Regular expressions (regex) are special text strings that describe search patterns. They're extremely powerful for text processing tasks like validation, extraction, search, and replacement. Regex allows you to define complex patterns with relatively concise syntax, making them invaluable for data cleaning, form validation, syntax highlighting, and many other applications.
Is my data secure when using this regex tester?
Yes, your data is completely secure. Our regex tester processes all data locally in your browser - nothing is ever sent to our servers or stored anywhere. This means you can safely test patterns against sensitive or confidential text without any privacy concerns.
What's the difference between various regex flags?
Regex flags modify how pattern matching works. Common flags include: g (global, finds all matches instead of just the first), i (case-insensitive matching), m (multiline, makes ^ and $ match start/end of lines, not just the whole string), s (dot-all, makes . match newlines too), and u (Unicode, for proper handling of Unicode characters).
How do capture groups work in regular expressions?
Capture groups are portions of a pattern enclosed in parentheses (). They "capture" the matched text for later use or extraction. For example, in the pattern (\w+)@(\w+\.\w+) applied to "user@example.com", the first group captures "user" and the second captures "example.com". You can reference these groups in replacement patterns or in your code.
Why is my regular expression not matching as expected?
Common causes of unexpected regex behavior include: not escaping special characters, confusion between greedy and non-greedy quantifiers, boundary issues (word boundaries, line start/end), character class misunderstandings, and overlooking multiline or Unicode considerations. Our tester helps visualize matches to identify these issues.
Are regular expressions the same in all programming languages?
While the core syntax is similar across most languages, there are differences in advanced features, available flags, and specific behaviors. JavaScript, Python, Java, PHP, Ruby, and others all implement regex with slight variations. Our tester uses JavaScript's implementation since it runs in the browser, which is largely compatible with most modern regex dialects.
What is the meaning of "index" in the match results?
The index represents the starting position (zero-based) of the match in the test string. For example, if your regex matches a word that begins at the 5th character of your test string, the index will be 4 (since counting starts from 0).
Can regular expressions match nested structures like HTML?
Regular expressions aren't well-suited for parsing nested structures like HTML or programming languages. While they can handle simple cases, they can't properly validate or parse arbitrarily nested content. For HTML parsing, specialized parsers like DOM methods are more appropriate. Regex is best for pattern matching in flat text structures.
Regex Tester - Privacy Commitment
We're committed to providing a regex testing tool that respects your privacy completely. Our tool processes all data locally in your browser - nothing is ever sent to our servers, stored, or shared with any third parties. You can use this tool with complete confidence, knowing your patterns and test data remain private and secure on your own device.