Prepare and process
Regex Tester compiles JavaScript regular expressions with the selected flags and highlights matches and capture groups against the supplied sample text.
- Use supported input
- Choose relevant options
- Run locally
- Review diagnostics
Regex, URL & Email Validators
Regex Tester
Test regular expressions with real-time match highlighting and capture group detection. Supports JavaScript RegExp syntax with global, case-insensitive, and multiline flag toggles. Includes a built-in cheat sheet. All evaluation is local.
Input validators
TEST STRING
MATCHES
Match results
Enter regex pattern and test string.
In brief
Regex Tester compiles JavaScript regular expressions with the selected flags and highlights matches and capture groups against the supplied sample text.
What this tool supports
- Real-time regex pattern evaluation
- Instant match highlighting with color coding
- Capture group detection and display
- Support for JavaScript regex syntax
- Regex flags (global, case-insensitive, multiline)
- Built-in regex cheat sheet reference
- One-click pattern copy to clipboard
- Works offline after initial load in your browser
- No data uploads or server dependencies
- Free to use with no registration required
How to Use Regex Tester
Write and test regular expressions with real-time match highlighting, capture group detection, and a built-in cheat sheet — all running locally in your browser.
Step 1
Enter your regex pattern
Type a JavaScript regular expression into the pattern field. The tool immediately validates the syntax using the browser's RegExp engine and reports errors with position details if the pattern is invalid.
Step 2
Paste test input text
Paste or type the string you want to match against in the test input area. The tool will highlight every match position in real time as you type or modify the pattern.
Step 3
Toggle regex flags
Enable global (g) to find all matches, case-insensitive (i) to ignore case differences, or multiline (m) to treat line breaks as pattern boundaries. These flags affect which portions of the input are matched.
Step 4
Review matches and capture groups
Inspect highlighted matches in the test string and examine each capture group's value, index, and position in the results panel below. Capture groups show exactly which part of the pattern matched which text.
Step 5
Copy your working pattern
Click the copy button to save your validated regex to the clipboard for use in JavaScript code, configuration files, text editors, or search tools. The pattern is ready to paste into your project.
Common mistakes
Using malformed or incomplete input
Consequence: The parser cannot produce a trustworthy result.
Fix: Start with a known-good vector such as valid input.
Treating transformed output as a security guarantee
Consequence: Readable or reversible data may be trusted incorrectly.
Fix: Review the result before using it in a production, security, or data-loss-sensitive workflow.
Choosing the wrong format, algorithm, unit, or variant
Consequence: Another system may reject or misinterpret the result.
Fix: Confirm the receiver follows Unicode Standard.
Skipping output verification
Consequence: Truncation, invalid syntax, or lossy changes can reach production.
Fix: Compare with reviewed local output and round-trip when possible.
Processing an unnecessarily large input at once
Consequence: The tab may become slow or run out of memory.
Fix: Very large inputs can pause the browser tab during parsing, rendering, copying, or conversion.
Copying stale output after changing input
Consequence: The saved result no longer represents the source.
Fix: Run again and review status before copying or downloading.
Forgetting to escape special regex characters
Consequence: Characters like ., *, +, ?, (, ), [, ], {, }, ^, $, and | have special meaning in regex and will not match literally.
Fix: Escape special characters with a backslash: \. to match a literal dot, \* to match a literal asterisk. Use a regex cheat sheet for reference.
Using greedy quantifiers when you need lazy matching
Consequence: Greedy quantifiers (*, +, {n,}) match as much text as possible, potentially consuming more than intended.
Fix: Use lazy quantifiers (*?, +?, {n,}?) to match as little as possible. For example, <.*?> matches HTML tags without consuming the entire line.
Not anchoring patterns with ^ and $
Consequence: Unanchored patterns match anywhere in the string, potentially producing unexpected partial matches.
Fix: Use ^ to anchor at the start and $ to anchor at the end of the string. For full-string validation, wrap the pattern with ^ and $.
Best practices
- Keep a known test vector such as valid input text to verify your regex behavior against expected results.
- Check behavior against Unicode Standard for international text, since JavaScript regex may handle Unicode differently than other languages.
- Review the result before using it in a production, security, or data-loss-sensitive workflow to confirm the pattern captures exactly what you expect.
- Round-trip reversible transformations by encoding and decoding to verify the regex does not corrupt data.
- Keep the original source text until review is complete, so you can re-test if edge cases are discovered.
- Use the smallest practical input and close unused large-file tabs to avoid browser memory pressure during testing.
Connected workflows
Verify before integration
Validate reviewed local output before another application uses it.
- Run locally
- Verify the result
- Copy, download, or continue
Continue through the tool family
Move the reviewed result into a related local workflow.
- Verify the result
- Copy, download, or continue
Build an email validation pattern
Create and test a regex pattern for validating email addresses in form inputs.
- Start with a basic email pattern
- Test against valid and invalid emails
- Refine for edge cases
- Copy the final pattern
Extract data from log files
Build a regex pattern to extract specific fields from structured log output.
- Paste sample log lines
- Build capture groups for each field
- Test against multiple log formats
- Copy the extraction pattern
Browser limitations
- Very large inputs can pause the browser tab during parsing, rendering, copying, or conversion.
- Clipboard, camera, downloads, and local files can require a secure context or explicit browser permission.
- Available memory varies by device, and mobile systems may reclaim background tabs.
Authoritative resources
Concrete examples
- Input
- valid input
- Result
- reviewed local output
This example verifies Regex Tester's primary behavior without uploading data.
Frequently asked questions
What is the difference between regex and glob patterns?
Regex (regular expressions) is a powerful pattern syntax for complex text matching with quantifiers, alternation, and lookahead. Glob patterns are simpler wildcards used for filename matching (*.txt). Regex is more flexible but harder to read. Use regex for text validation and extraction; use globs for file path matching.
What is a regular expression?
A regular expression (regex) is a pattern-matching syntax used to find, validate, or replace text. Regex uses special characters like dots, asterisks, and brackets to define search patterns. For example, the pattern ^[\w.-]+@[\w.-]+\.\w{2,}$ matches email addresses. Regex is supported in virtually every programming language and text editor.
What is Regex Tester?
Regex Tester uses the browser's JavaScript RegExp engine to compile the entered pattern and flags, then marks matches and capture groups in the supplied sample text. Invalid regex syntax is reported by that engine with position and error details for quick debugging.
Does Regex Tester upload my data?
No. All regex testing and evaluation happens entirely within your browser sandbox. Your patterns and test strings are never transmitted to any external server, ensuring complete privacy and confidentiality of your data, including sensitive log files or API payloads.
How do I test a regular expression?
Type your regex pattern in the pattern field and enter your test string in the input area. The tool instantly highlights all matches with colored overlays and displays detailed information about capture groups, match positions, and match counts in the results panel.
What regex flavors are supported?
The tool supports JavaScript-style regular expressions, which include most common features like character classes, quantifiers, lookaheads, lookbehinds, backreferences, and named groups. This covers the vast majority of real-world regex use cases for web development.
Does it show capture groups?
Yes. The tool displays all captured groups with their corresponding values and positions, making it easy to understand which parts of your pattern are capturing specific portions of the input text for downstream processing.
Can I use regex flags?
Yes. You can toggle common regex flags such as global (g), case-insensitive (i), and multiline (m) directly in the interface. These flags modify how the pattern is applied to your test input and affect the number and type of matches found.
Does it highlight matches in real time?
Yes. As you type your pattern or modify your test string, matches are highlighted instantly in the input area with colored overlays, providing immediate visual feedback on what your regex captures without requiring a separate compile step.
Is there a cheat sheet available?
Yes. The tool includes a built-in regex cheat sheet that lists common patterns, character classes, quantifiers, anchors, and modifiers for quick reference while building and debugging your expressions.
Does it work offline?
Yes. Once the page loads, all regex evaluation happens locally in your browser without requiring any internet connection. You can use it in offline after initial load or restricted network environments after initial asset caching.
Can I copy the regex pattern?
Yes. The tool provides a convenient copy button to place your validated regex pattern on your clipboard, making it easy to paste into your code, configuration files, text editors, or search tools.
Privacy and offline behavior
Regex Tester processes input and produces output inside this browser. The tool does not upload tool data. Browser capabilities such as files, clipboard, camera, Canvas, Web Crypto, workers, or downloads are used only after the workflow or an explicit user action requires them.
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