Regex Tester
Regex Tester lets you create, test, and debug regular expressions directly in your browser using the native JavaScript RegExp engine. All processing happens client-side, ensuring your data remains private with no server interaction.
Regex Tester is a browser-based tool that enables you to validate and debug regular expressions instantly using JavaScript's native RegExp engine. It supports common flags like g, i, m, and s, providing detailed match indices, capture groups, and named groups information. Since all processing runs client-side, no data is uploaded to any server, preserving your privacy. This makes it ideal for developers, designers, and SEO specialists looking to refine patterns quickly. For additional text comparison or data validation needs, consider our Diff Checker and JSON Validator tools.
How to Use Regex Tester
- Enter your regular expression pattern in the input field, using JavaScript RegExp syntax.
- Type or paste the test string to evaluate against your pattern.
- Select the desired flags:
gfor global,ifor case-insensitive,mfor multiline, andsfor dotAll mode. - Choose the mode of operation: match to find matches, replace to substitute matched substrings, or split to divide the input string by the pattern.
- View the results instantly, including match indices, capture groups, and named groups.
How It Works
This tool leverages the browser's native JavaScript RegExp engine, which adheres to the ECMAScript specification for regular expressions. It processes patterns client-side, avoiding latency and privacy concerns linked to server-side processing. The engine supports standard flags such as g (global search), i (case-insensitive matching), m (multiline anchors), and s (dotAll allowing . to match newline characters). The tool parses the pattern and applies it to the input string, extracting detailed data about each match including start and end indices, numbered capture groups, and named capture groups defined with (?. In replace mode, it uses JavaScript's String.prototype.replace() to substitute matches, while split mode utilizes String.prototype.split() with the regex as a separator.
Example
Pattern: (\w+)@(\w+).com
Flags: gi
Input: Contact us at [email protected] or [email protected].
Match Mode Output:
- Match 1: '[email protected]' (indices 13-32)
- Group 1: 'support'
- Group 2: 'example'
- Match 2: '[email protected]' (indices 36-53)
- Group 1: 'sales'
- Group 2: 'domain'When to Use Regex Tester
- As a developer, quickly validate complex regex patterns before integrating them into JavaScript code to avoid runtime errors.
- For designers working with web forms, test input validation patterns ensuring proper user data formats.
- SEO specialists can extract or validate URLs, metadata, or keywords from bulk text or logs.
- Students learning regular expressions benefit from immediate visual feedback on matches and capture groups.
- When debugging replacement logic in text processing, use the replace mode to preview output.
For additional tasks involving text comparison or data integrity, combine Regex Tester with tools like Diff Checker for comparing results or Hash Generator for verifying content signatures.
Frequently Asked Questions
Does Regex Tester support lookahead and lookbehind assertions?
Yes, Regex Tester uses the browser's native JavaScript RegExp engine, which supports lookahead assertions (?= and ?!) and lookbehind assertions (?<= and ?<!) in modern browsers following ECMAScript 2018 and later.
How does Regex Tester handle multiline input with the <code>m</code> flag?
With the m flag enabled, the ^ and $ anchors match the start and end of each line within the input string, instead of just the start and end of the entire string. This allows you to test patterns that operate on individual lines in multiline text.
Is my data uploaded to a server when using Regex Tester?
No, all pattern matching and processing occur entirely within your browser using JavaScript's native RegExp engine. This client-side execution ensures that none of your input data or regex patterns are transmitted to any external server, maintaining full privacy.
Can I test replacement patterns with backreferences in Regex Tester?
Yes, in replace mode you can use JavaScript replacement patterns with backreferences such as $1, $2, etc., to insert captured groups into the replacement string. The tool applies String.prototype.replace() semantics for these substitutions.
What happens if my regex pattern has a syntax error?
If the pattern is invalid according to JavaScript RegExp syntax, Regex Tester will display an error message indicating the issue. This immediate feedback helps you correct syntax errors before applying the pattern to your input.