Regex Tester:
Debug with Confidence
Write, test, and debug regular expressions in real-time. Whether you're validating emails or scraping complex data, see exactly what your pattern matches—before you deploy code.
Regex Pattern & Test String
Type your expression below. We'll highlight matches as you type, so you can catch mistakes instantly.
Regex Recipe Library
Proven patterns for production systems.
Validation
Network & Web
Data Engineering
Match Results & Substitution
Review every capture group and test dynamic replacements in real-time
Built for Clarity
Live Evaluation
No "Run" button needed. Your pattern is evaluated on every keystroke, giving you immediate feedback on what works and what breaks.
Visual Match Highlights
We color-code the exact text your pattern captures. It helps you instantly spot if your regex is being too greedy or missing data.
Deep Capture Inspection
See the breakdown of every match, including index positions and specific capture groups ($1, $2, etc.) for granular debugging.
Flag Control
Easily toggle powerful modifiers like Global (g), Case-Insensitive (i), and Multiline (m) without memorizing syntax.
Substitution Testing
Validate your "Find & Replace" logic safely. Preview how your capture groups will transform the final text output.
Pattern Library
Stuck? Access our built-in cheat sheet of common, verified patterns for emails, passwords, dates, and more.
Workflow: Writing & Testing Regex
Define Your Pattern
Start typing your Regular Expression in the top input. If you are new to regex, try pulling a preset from the "Common Patterns" menu.
Input Test Data
Paste the actual text you want to search through. This could be a log file, a block of code, or a list of user inputs.
Adjust Flags
Use the flag toggles to refine behavior. Need to ignore capitalization? Turn on "i". Need to match multiple lines? Turn on "m".
Inspect Matches
Look at the "Highlighted Matches" area. If sections are glowing, you have a match. Check the details panel for group breakdowns.
Test Replacements (Optional)
Want to reformat the data? Enter a replacement string using standard syntax (e.g., $1-$2) to preview the transformation.
Export Results
Once you are confident, copy the pattern to your clipboard for use in your code, or download the cleaning results.
Mastering Regular Expressions
What is a Regex?
Regular Expressions (Regex) are sequences of characters that define a search pattern. Think of them as "wildcards on steroids." Instead of searching for a specific word, you search for a specific structure—like "any sequence of 3 digits" or "text between brackets." They are fundamental in almost every programming language for validation and data processing.
Essential Syntax Cheatsheet
.: Matches any single character\d: Matches any digit (0-9)\w: Matches any word char (Alphanumeric + _)[abc]: Look for any of these characters^...$: Anchors. Start and End of string*vs+: Zero-or-more vs One-or-more
Understanding Capture Groups
Parentheses () do two things: they group parts of your pattern together, and they "capture" the match relative to that group. This is incredibly powerful for extraction. For example, in a date string like 2023-12-25, you can use groups to extract just the "2023" or just the "12".
Why Test Before Deploying?
Regex is notorious for "catastrophic backtracking"—where a poorly written pattern can freeze a server by trying millions of combinations. Always validate your expressions against realistic test data to ensure they are both accurate and performant before putting them into production code.
Technical FAQ
Expert answers to common questions about our file and folder diff engine, security, and document comparison features.
This tester uses the JavaScript RegExp engine (ECMAScript standard). It is standard for web development and Node.js environments. While most basic syntax is universal (like PCRE), some advanced lookbehind features may be specific to JS.
By default, quantifiers like `*` and `+` are "greedy"—they will match as much text as possible. To make them "lazy" (match as little as possible), simply append a `?`. Example: `.*?` instead of `.*`.
Yes! Modern browsers support both Lookaheads `(?=...)` and Lookbehinds `(?<=...)`. You can use these to match a pattern only if it is (or isn't) preceded/followed by another pattern, without including that check in the match result.
Use standard dollar-sign syntax. `$1` refers to the first capture group, `$2` to the second. `$0` or `$&` often refers to the entire match. This allows you to reorder or reformat data dynamically.
100%. This tool runs entirely on the "Client Side" (in your specific browser window). No text is sent to our servers. feel free to debug sensitive logs or config files safely.
Without the Global `g` flag, a regex engine stops after the first successful match. If you are cleaning a document or validating multiple entries, you almost always want `g` enabled to process everything.
The sticky `y` flag forces the match to start exactly at the current `lastIndex` position. It's an advanced feature mostly used when building parsers or compilers, where you need strict consecutive matching.
Currently, due to our strict privacy commitments (no server storage), we don't create share links. However, you can simply copy the pattern text to share it with your team.