Why does my regex match only the first occurrence in the text?

Without the global flag (g), regular expression engines stop searching after finding the first match. Enabling the g flag instructs the engine to scan the entire string and return every matching instance across your test data.

How do capture groups work in replacement strings?

Parentheses in regex define capture groups numbered from $1 upward. In replace mode, referencing $1, $2, etc., substitutes the text captured by the corresponding set of parentheses, allowing flexible text reformatting and data restructuring.

What is the difference between multiline (m) and dotAll (s) flags?

The multiline flag (m) makes ^ and $ match the start and end of individual lines rather than the whole string. The dotAll flag (s) allows the dot operator (.) to match newline characters (\n), enabling cross-line pattern matching.

Does this regex tester support PCRE or Python regex syntax?

This tool uses your web browser's native JavaScript ECMAScript RegExp engine. While most core regex syntax is identical across dialects, advanced features like named lookbehinds or possessive quantifiers follow JavaScript specifications rather than Python or PCRE rules.

How does catastrophic backtracking protection work?

Nested quantifiers can cause regex engines to evaluate exponentially many execution paths on non-matching strings, freezing browsers. Our evaluator time-boxes evaluation so complex or runaway patterns fail gracefully with an informative performance warning instead of hanging your tab.

Is my test code or log data sent to any server?

No data is ever sent to an external server. The entire regular expression compilation, pattern matching, and substitution rendering process occurs locally inside your web browser's JavaScript engine, ensuring complete privacy for sensitive logs.

How do I match special characters like dots, brackets, or slashes?

Special metacharacters such as ., *, +, ?, [, ], (, ), {, }, ^, $, and \ must be escaped with a preceding backslash (\) when you want to match their literal string characters in text.

Can I save or copy my regex pattern and test results?

Yes, you can copy your regex pattern, target test strings, and formatted replacement output directly to your clipboard using one-click copy buttons, making it seamless to transfer verified patterns straight into your codebase.

Regex Tester

Test regular expressions live against your own text: highlighted matches, capture groups, flags, and replace mode. Runs in your browser.

100% Private — Runs entirely in browserInstant calculation & resultFree, no sign-up

Why use a live regex tester for pattern debugging?

Regular expressions are fundamental tools for search, data validation, and text transformation in software development. However, crafting complex regex patterns without immediate visual feedback often leads to subtle bugs, unmatched edge cases, and unexpected performance bottlenecks. A live regular expression tester eliminates guessing by evaluating patterns against sample text in real time, highlighting every matching substring and capture group instantly as you type.

Whether you are parsing server log files, validating input form fields, extracting data attributes from HTML markup, or executing complex bulk string replacements, instant feedback accelerates development and ensures pattern accuracy across diverse inputs.

Live Match Highlighting: Instant visual feedback on matched strings as you type your pattern, highlighting full matches and individual capture groups without manual form submissions or page refreshes.
Capture Group Extraction: Detailed breakdown of numbered and named capture groups, displaying exact character offsets and extracted sub-components for intricate nested patterns.
Interactive Flag Toggles: Seamless toggling between global (g), case-insensitive (i), multiline (m), dotAll (s), and unicode (u) flags to test different evaluation rules.
Substitution & Replace Preview: Real-time simulation of string replacement operations using backreferences ($1, $2, $&), allowing developers to test complex refactoring rules before applying them to code.

JavaScript RegExp execution engine architecture

This testing tool relies directly on the native V8 or JavaScriptCore ECMAScript regular expression engine built into your modern web browser. When you input a pattern, it is compiled using native JavaScript RegExp constructors, guaranteeing 100% fidelity with the regular expression behavior you experience in Node.js, Deno, Bun, and browser-based web applications.

To maintain browser responsiveness when dealing with complex patterns, evaluation is executed within a time-boxed execution sandbox. Regular expressions containing nested quantifiers (such as (a+)+$) can trigger catastrophic backtracking when evaluated against non-matching strings, causing CPU spikes. Our engine monitors execution time and safely interrupts runaway evaluations, presenting a helpful warning diagnostic rather than causing browser tab unresponsiveness.

Client-side security for private test data

Your sensitive test strings, API logs, source code snippets, and customer data never leave your local machine during evaluation. All regex matching, group extraction, and replacement operations execute entirely within your browser's local JavaScript runtime without sending telemetry or payload data to remote servers.

Step-by-step guide to testing regular expressions

Enter Your Expression and Select Flags: Type your regular expression into the pattern field and toggle the required modifiers such as global matching (g) or case insensitivity (i).
Paste Target Test Text: Input your sample strings, log contents, or mock data into the test panel to see matching segments highlighted dynamically in real time.
Inspect Capture Groups or Test Replacements: Review the capture groups breakdown panel to inspect sub-match extractions, or switch to Replace mode to test dynamic string substitution rules.

Explore related developer utilities

Enhance your text processing and software development workflow with our suite of free online developer utilities:Regex Generator for pre-built pattern templates,Diff Checker for comparing text and code differences,Case Converter for string casing transformations, andEmail Extractor for extracting contact addresses from raw text.

Frequently Asked Questions

Why does my regex match only the first occurrence in the text?
Without the global flag (g), regular expression engines stop searching after finding the first match. Enabling the g flag instructs the engine to scan the entire string and return every matching instance across your test data.
How do capture groups work in replacement strings?
Parentheses in regex define capture groups numbered from $1 upward. In replace mode, referencing $1, $2, etc., substitutes the text captured by the corresponding set of parentheses, allowing flexible text reformatting and data restructuring.
What is the difference between multiline (m) and dotAll (s) flags?
The multiline flag (m) makes ^ and $ match the start and end of individual lines rather than the whole string. The dotAll flag (s) allows the dot operator (.) to match newline characters (\n), enabling cross-line pattern matching.
Does this regex tester support PCRE or Python regex syntax?
This tool uses your web browser's native JavaScript ECMAScript RegExp engine. While most core regex syntax is identical across dialects, advanced features like named lookbehinds or possessive quantifiers follow JavaScript specifications rather than Python or PCRE rules.
How does catastrophic backtracking protection work?
Nested quantifiers can cause regex engines to evaluate exponentially many execution paths on non-matching strings, freezing browsers. Our evaluator time-boxes evaluation so complex or runaway patterns fail gracefully with an informative performance warning instead of hanging your tab.
Is my test code or log data sent to any server?
No data is ever sent to an external server. The entire regular expression compilation, pattern matching, and substitution rendering process occurs locally inside your web browser's JavaScript engine, ensuring complete privacy for sensitive logs.
How do I match special characters like dots, brackets, or slashes?
Special metacharacters such as ., *, +, ?, [, ], (, ), {, }, ^, $, and \ must be escaped with a preceding backslash (\) when you want to match their literal string characters in text.
Can I save or copy my regex pattern and test results?
Yes, you can copy your regex pattern, target test strings, and formatted replacement output directly to your clipboard using one-click copy buttons, making it seamless to transfer verified patterns straight into your codebase.