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How to Use
- 1Enter your regex pattern in the pattern field. The tool validates the syntax in real time and highlights errors immediately if the pattern is invalid.
- 2Set flags using the toggle buttons: g (global) to find all matches, i (case-insensitive) to ignore letter casing, and m (multiline) to make ^ and $ match line boundaries.
- 3Type or paste your test string in the text area below the pattern field. Use realistic sample data that represents the actual text your regex will process in production.
- 4Watch matches highlight in real time as you type — both in the pattern and in the test string. Every matching substring is color-coded directly in the text.
- 5Review capture groups listed below the test string. Each group shows its index, name (if using named groups like (?<name>)), and the matched content.
- 6Iterate on your pattern by adjusting quantifiers, character classes, and groups while watching the matches update instantly. Copy the final pattern when satisfied.
About Regex Tester
The Regex Tester provides a real-time environment for writing, testing, and debugging regular expressions against any sample text. Every match is highlighted directly in the test string with color-coded overlays, and capture groups are listed with their index, name, and content — giving you instant visual feedback as you build your pattern. This immediate feedback loop dramatically speeds up regex development compared to the traditional write-run-check cycle of testing patterns in application code or command-line tools like grep and sed.
Regular expressions are one of the most powerful and widely used tools in programming, with roots going back to formal language theory developed by mathematician Stephen Kleene in the 1950s. They appear everywhere in modern software: input validation (email addresses, phone numbers, postal codes, credit card numbers), search-and-replace operations in text editors and IDEs like VS Code and IntelliJ, log parsing and monitoring (extracting timestamps, error codes, IP addresses from log files), data extraction from unstructured text, web scraping with tools like BeautifulSoup and Puppeteer, URL routing in web frameworks like Express.js and Django, and configuration file processing.
The tool uses JavaScript's native RegExp engine, which implements the ECMAScript regex specification as documented on MDN Web Docs. This includes support for character classes (\d, \w, \s and their negated forms), quantifiers (*, +, ?, {n,m}), alternation (|), grouping and capturing ((...)), non-capturing groups ((?:...)), named capture groups ((?<name>...)), lookahead ((?=...) and (?!...)), lookbehind ((?<=...) and (?<!...)), Unicode property escapes (\p{...}) in modern browsers, and boundary assertions (\b for word boundaries). These features cover the vast majority of real-world pattern matching needs.
JavaScript regex syntax is highly compatible with other languages, making this tool useful even if your target environment is not JavaScript. The vast majority of patterns written for Python, Java, PHP, Ruby, Go, and .NET work identically in JavaScript. The main differences are in advanced features: JavaScript does not support possessive quantifiers (++, *+) or atomic groups ((?>...)), and some Unicode categories behave differently across engines. For everyday patterns — matching emails, extracting numbers, parsing CSV lines, validating date formats, cleaning HTML tags — a pattern that works here will work in virtually any language or tool that supports regular expressions.
A common source of regex bugs is not accounting for edge cases: anchoring patterns with ^ and $ when you mean to match the entire string, forgetting that . does not match newlines by default (requiring the [\s\S] workaround in JavaScript), using greedy quantifiers (* and +) when you need non-greedy (*? and +?), or failing to escape special characters like dots and parentheses that have regex meaning. The real-time highlighting in this tool makes these issues visible immediately — you can see exactly which parts of your test string match and which do not, catching problems before they reach production code where a faulty regex could cause incorrect data extraction or validation bypasses.
All pattern matching runs entirely in your browser using JavaScript's built-in RegExp object. Your test strings are never transmitted to any server, never stored, and never logged. This makes the tool safe for testing patterns against production data, server log samples, customer records, API payloads, or any text containing sensitive information. No account or installation is required — bookmark the page and start testing patterns instantly whenever you need to build or debug a regular expression.
Frequently Asked Questions
What regex flavor does this tool use?
JavaScript's native RegExp engine implementing the ECMAScript specification. This includes character classes, quantifiers, lookahead, lookbehind, named capture groups, non-capturing groups, and alternation. It is one of the most widely used regex engines and is highly compatible with patterns written for Python, Java, PHP, and other languages.
What flags are available?
Three flags are available: g (global) finds all matches instead of stopping at the first one, i (case-insensitive) ignores uppercase/lowercase distinctions, and m (multiline) makes ^ and $ match the start and end of each line rather than the entire string. These are the three most commonly used flags in practical regex work.
How are matches displayed?
Each match is highlighted directly in the test string with alternating colors so you can distinguish adjacent matches. Capture groups are listed below the text with their group index (or name, if using named groups), the matched substring, and the position in the test string. This makes it easy to verify that your groups capture exactly what you intend.
Can I test patterns meant for Python or PHP?
For the vast majority of patterns, yes. JavaScript regex shares the same core syntax with Python, Java, PHP, Ruby, Go, and C#. Character classes, quantifiers, groups, lookahead, and lookbehind work identically. The few differences are in advanced features like possessive quantifiers and atomic groups, which JavaScript does not support. Everyday patterns work across all these languages.
Is my test data sent to a server?
No. All pattern matching runs locally in your browser using JavaScript's built-in RegExp object. Your test strings never leave your device, are never stored, and are never logged. This makes the tool safe for testing against production data, log samples, and any text containing sensitive information.
What is the difference between greedy and non-greedy matching?
Greedy quantifiers (*, +, {n,m}) match as much text as possible, while non-greedy variants (*?, +?, {n,m}?) match as little as possible. For example, given the text '<b>hello</b>', the pattern <.*> (greedy) matches the entire string, while <.*?> (non-greedy) matches only '<b>'. The real-time highlighting makes this difference immediately visible.
How do I match across multiple lines?
Enable the m (multiline) flag to make ^ and $ match line boundaries. Note that the . character still does not match newlines by default. To match any character including newlines, use [\s\S] instead of . — this is the standard JavaScript workaround since the 's' (dotAll) flag is not available as a toggle in this tool.
Can I use this to validate email addresses or URLs?
Yes, and this is one of the most common use cases. You can test email validation patterns against a list of valid and invalid email addresses to ensure your regex catches all edge cases. However, be aware that truly comprehensive email validation with regex is extremely complex — for production systems, most developers use a simple pattern combined with server-side verification rather than trying to validate every RFC 5322 edge case.