Regex Tester
Test Regular Expressions Online
Build, test, and debug regular expressions in real time with live match highlighting, capture group details, and a built-in replace mode. Everything runs locally in your browser.
What Is a Regex Tester?
A regex tester lets you write a regular expression pattern and instantly see how it matches against a test string — no need to compile code, run scripts, or switch between tabs. As you type, matches are highlighted in the text, capture groups are listed with their indices and values, and execution statistics update in real time.
Our Regex Tester uses your browser's native JavaScript RegExp engine, supporting all ES2018+ features including named capture groups, lookbehind assertions, and the dotAll flag. Every evaluation is debounced for smooth performance, even with complex patterns.
The built-in replace mode shows the result of String.replace() with your pattern and replacement string, including support for back-references and named group references. Toggle it on to preview search-and-replace operations before applying them to your code or data.
A collapsible quick reference cheat sheet keeps the most common regex syntax at your fingertips — character classes, quantifiers, anchors, groups, and lookaround assertions. All processing runs 100% in your browser; no patterns or test data are ever sent to a server.
Discover Features
Live highlighting, capture groups, replace mode, cheat sheet, and full privacy — all in your browser
Live Match Highlighting
See matches highlighted directly in your test string as you type. Alternating colours make it easy to distinguish adjacent and overlapping matches at a glance — no need to click a button or refresh.
Capture Group Details
Every match lists its numbered and named capture groups as sub-items. Instantly see which parts of the string each group captured, their indices, and lengths — essential for building extraction patterns.
Replace Mode
Toggle replace mode to enter a replacement string and see the result of String.replace() in real time. Supports back-references ($1, $2, $&) and named references ($<name>) for powerful search-and-replace workflows.
Quick Reference Cheat Sheet
A collapsible panel with the most common regex patterns: character classes, quantifiers, anchors, groups, and lookahead/lookbehind. No need to leave the page to recall syntax — the reference is always one click away.
Real-Time Performance Stats
See the match count, total captured groups, and execution time in milliseconds after every evaluation. The debounced engine updates as you type with minimal latency, keeping the UI smooth even for complex patterns.
100% Private & Secure
All regex evaluation happens entirely in your browser using JavaScript's native RegExp engine. No patterns, test strings, or results are ever sent to a server. Close the tab and everything is gone.
How to Use
Four steps: enter a pattern, pick flags, provide test text, and review results
Enter a Pattern
Type your regular expression into the pattern field. The input is displayed with regex delimiters (/ ... /flags) so you can see the full expression at a glance.
Select Flags
Toggle the flags you need — global (g), case-insensitive (i), multiline (m), dotAll (s), and unicode (u). Global is enabled by default to find all matches.
Provide Test Text
Type or paste a test string into the large textarea. A sample string is pre-filled to get you started. Matches highlight in real time as you type.
Review & Replace
Inspect highlighted matches, explore capture groups, and optionally enable replace mode to preview search-and-replace results — all updating live.
Use Cases
Common scenarios where a regex tester saves time and prevents errors
Log File Parsing
Build and test regex patterns for extracting timestamps, error codes, IP addresses, and custom fields from log files before deploying them in your scripts or log management tools.
Code Refactoring
Craft search-and-replace patterns to rename variables, restructure imports, or transform code formatting across an entire codebase — preview the result before running the operation.
Email & Input Validation
Test email, phone, URL, and custom input validation patterns against a range of edge cases. See exactly which strings match and which don't, so you can fine-tune validation rules.
Data Extraction & ETL
Develop patterns to extract structured data — dates, currency values, identifiers — from unstructured text. Verify capture groups before using the regex in data pipelines or ETL scripts.
Learning & Teaching Regex
Use the live highlighting and group details to learn how regex engines work step by step. The quick reference cheat sheet keeps common syntax at your fingertips while you practice.
QA & Test Case Authoring
Validate that your regex patterns handle expected and unexpected inputs correctly. Test positive and negative cases in one place, then copy the final pattern directly into your test suite.
Free vs Paid — Regex Tester
Get started free, upgrade when you need more power.
Unlock the Full Power of Regex Tester
Remove daily limits, process larger files up to 500 MB, enable batch processing, and get priority support.
Developer Tools includes:
- 13 developer & utility tools
- Unlimited daily uses
- No file size limits
- All output formats
Also available in the All Tools Bundle
FAQ
Frequently asked questions about the Regex Tester
Q1. What regular expression engine does this tool use?
Q2. Is my data sent to any server?
Q3. What do the regex flags mean?
Q4. How does the replace mode work?
Q5. What happens if my regex causes catastrophic backtracking?
Q6. Can I test regex patterns for other languages (Python, Java, etc.)?
Q7. What are named capture groups?
Q8. Does this tool work offline?
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