Regex Tester
Test regular expressions online against sample text and inspect matches, capture groups and pattern behavior directly in your browser.
How to Use the Regex Tester
Debug and inspect regular expressions with ease in four simple steps:
Enter Pattern
Type your regular expression inside the top input between the slashes, or load an example via "Sample".
Select Flags
Toggle flags such as Global (g), Case-Insensitive (i), Multiline (m), or DotAll (s) as needed.
Paste Test String
Paste your multiline test text, server logs, or JSON payload into the left editor workspace.
Inspect Results
View safe match highlights in the right pane, alongside exact character indices and capture group extractions.
What is the CodeCrate Regex Tester?
The CodeCrate Regex Tester is a real-time development utility for building, testing, and debugging regular expressions against arbitrary text. Powered by the browser's native JavaScript regular expression engine (ECMA-262), it matches the exact syntax, character escaping, and flag behaviors of modern web browsers and Node.js runtimes.
Designed for security and speed, all evaluations occur client-side on your device. User text is never executed as HTML, protecting you from cross-site scripting (XSS) risks when testing untrusted strings.
Regular Expression Examples
Practical expressions with capture groups and multi-flag setups:
Pattern: \b([A-Za-z0-9._%+-]+)@([A-Za-z0-9.-]+\.[A-Za-z]{2,})\b
Flags: g, i
Sample: Contact dev@codecrate.site or team@example.org
Group 1: dev / team (Username)
Group 2: codecrate.site / example.org (Domain)Pattern: \b(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})\b
Flags: g
Sample: Release deployed on 2026-09-24 at 16:00
Groups: year: 2026, month: 09, day: 24When Developers Use Regex Testers
Essential day-to-day software engineering workflows:
Input Form Validation
Validating usernames, email addresses, postal codes, and phone numbers before dispatching API requests.
Log File Scraping
Parsing Nginx access logs, stack traces, and AWS CloudWatch events to extract IP addresses and error codes.
Data Sanitization & Masking
Locating PII, credit card numbers, and secret tokens in datasets before uploading to staging environments.
Refactoring Search & Replace
Prototyping complex search-and-replace expressions before executing bulk replacements in VS Code or JetBrains.
Key Features of CodeCrate Regex Tester
Built for precision, safety, and rapid feedback:
Real-Time Match Highlighting
Matches are highlighted dynamically as you type, with zero lag and safe character escaping to prevent XSS.
Capture Group Inspector
Examines numbered capture groups and ES2018 named capture groups with exact string start and end indices.
Graceful Syntax Validation
Unterminated parentheses, invalid range tokens, and malformed flags show immediate, clear diagnostic errors.
Instant Pattern Export
Copy your tested pattern with all active flags formatted as a ready-to-use literal with a single click.
Important Technical & Performance Notes
JavaScript regular expressions can have performance issues with certain complex patterns that involve nested quantifiers (e.g., (a+)+$ or ([a-zA-Z]+)*). When tested against non-matching inputs, the backtracking engine may perform an exponential number of states, causing the browser thread to freeze temporarily. Keep quantifiers mutually exclusive to maintain optimal performance.
All user-entered test text is strictly sanitized and escaped before rendering highlighted marks. Even if you paste raw HTML or malicious script tags into the test text box, they are rendered as inert text literals.
Frequently Asked Questions
Got questions about regular expressions? Find answers below:
Which regular expression engine is used?
CodeCrate Regex Tester uses your browser's native ECMAScript (JavaScript) RegExp engine, ensuring 100% fidelity with client-side and Node.js JavaScript code.
Which regex flags are supported?
All standard JavaScript flags are supported: g (global search), i (case insensitive), m (multiline mode), s (dotAll mode), u (unicode support), and y (sticky matching).
Is my test data sent to a remote server?
No. All regex matching and text parsing executes strictly inside your browser tab. Confidential logs and strings never leave your machine.
Does this tool display capture groups?
Yes. Each detected match displays its start/end position index and individual numbered or named capture groups.
What causes catastrophic backtracking in regex?
Nested quantifiers (e.g. (a+)+) tested against non-matching input can trigger exponential backtracks (ReDoS). Always use atomic groups or precise character classes where possible.
Can I copy the regex pattern directly to my codebase?
Yes. Clicking 'Copy Pattern' copies the regular expression with its active flags formatted for instant use in your JavaScript, Python, or Go source files.
Related Developer Tools
Helpful companion utilities for pattern matching and text parsing: