Test, Debug & Validate Regular Expressions (RegEx)
Evaluate regular expression patterns with real-time match highlighting, capture group extraction, and syntax explanation directly inside your browser with complete privacy.
How to Test Regular Expressions
Evaluate regular expression patterns with real-time match highlighting, capture group extraction, and syntax explanation directly inside your browser with complete privacy.
Enter Regex Pattern
Type your regular expression pattern into the pattern input (e.g., [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}).
Select Regex Flags
Toggle desired modifier flags: g (global), i (case-insensitive), m (multiline), and s (dotAll).
Paste Test String
Enter the test text or code snippet you want to search and match against.
Review Matches & Capture Groups
Examine highlighted matches, match count, start/end indices, and captured group values instantly.
Why Test RegEx with Khalasna?
Real-Time Visual Highlighting
Matches are highlighted directly on your test text with distinct colors for each capture group.
Detailed Match Inspection
View full match listings complete with start/end character offsets and named capture groups.
Catastrophic Backtracking Guard
Protected execution loops prevent regex exponential blowup and browser tab lockups.
100% Private Client-Side Testing
Your proprietary code snippets and sensitive test strings are never sent to external servers.
Technical Architecture: JavaScript ECMAScript RegEx Engine & DFA/NFA Execution
Regular expressions in JavaScript conform to the ECMAScript RegExp specification, executed via compiled Non-deterministic Finite Automata (NFA) engines (such as V8's Irregexp engine in Chrome). Khalasna builds a reactive testing playground using native RegExp constructor instances: new RegExp(pattern, flags). The engine executes matchAll() to extract iterator results containing the full matched substring, zero-based input indices, and captured sub-groups array. To display visual highlights, the engine slices the test string into non-overlapping segments and wraps matched substrings in styled HTML span tags (<mark>). It guards against infinite loops resulting from zero-width assertions (such as /^/g or /(?=a)/g) by advancing the RegExp.lastIndex pointer manually when length === 0, ensuring smooth, non-blocking UI responsiveness.
Popular Applications for RegEx Testing
Form Validation Patterns
Test and debug regular expressions for email addresses, international phone numbers, and strong passwords.
Log Parsing & Data Extraction
Extract IP addresses, timestamps, and HTTP status codes from complex server log files.
Code Refactoring & Find-and-Replace
Verify complex search-and-replace patterns before executing them across codebase repositories.
Frequently Asked Questions
Which regex flavor is supported?
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Which regex flavor is supported?
+Our tester uses your browser's native JavaScript (ECMAScript) regular expression engine, supporting lookaheads, lookbehinds, and named groups.
Are my test strings or code snippets sent to servers?
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Are my test strings or code snippets sent to servers?
+No. All regex evaluation executes 100% locally in your web browser. No code or text is ever logged or transmitted.
What does the 'g' flag do?
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What does the 'g' flag do?
+The 'g' (global) flag finds all matches in the text rather than stopping after the first match.
What does the 'i' flag do?
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What does the 'i' flag do?
+The 'i' (case-insensitive) flag makes matching ignore differences between uppercase and lowercase letters.
Is this regex tester free to use?
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Is this regex tester free to use?
+Yes, it is completely free with no usage limits or sign-up requirements.