What is Regex?
Regular expressions (regex or regexp) are sequences of characters that define search patterns for matching, extracting, and replacing text in strings. Supported by virtually every programming language and text editor, regex is one of the most powerful tools in a developer's toolkit. Patterns can range from simple literal matches to complex expressions with quantifiers, character classes, lookaheads, and capture groups. Our Regex Tester provides a real-time testing environment where you can write patterns and instantly see all matches highlighted with their positions and captured groups, all running entirely in your browser.
How to Use This Tool
Enter your regex pattern in the pattern field and type or paste your test string below it. Matches appear instantly as you type, showing the matched text, its index position, and any capture groups. Toggle flags using the buttons: "g" for global (find all matches), "i" for case-insensitive, "m" for multiline (^ and $ match line boundaries), "s" for dotAll (dot matches newlines), and "u" for Unicode support. The tool uses JavaScript's native RegExp engine, so patterns behave exactly as they would in your JavaScript or TypeScript code.
Common Use Cases
- Validating user input formats like email addresses, phone numbers, postal codes, and credit card numbers
- Extracting structured data from log files, CSV rows, HTML content, or unstructured text
- Building search-and-replace patterns for code refactoring across large codebases
- Testing URL routing patterns, file path matching, and content filtering rules before deployment
Why Use a Client-Side Tool?
Test strings often contain sensitive data such as log entries with user information, API responses with personal details, or production URLs with internal paths. By running entirely in your browser, this tool ensures that your test data and patterns stay on your device. No data is sent to any server, making it safe to test patterns against real production data. The instant feedback loop also means faster iteration compared to tools that require server round-trips.
Frequently Asked Questions
Which regex engine does this tool use?
This tool uses the JavaScript RegExp engine built into your browser. Patterns and flags behave identically to how they work in JavaScript and TypeScript. If you are writing regex for Python, Java, or other languages, be aware that some advanced features like lookbehind support may differ slightly between engines.
What are capture groups and how do they work?
Capture groups are defined by parentheses in your pattern, such as (\d3)-(\d4). They let you extract specific parts of a match. This tool displays captured groups for each match, making it easy to verify that your extraction logic works correctly before using it in code.
How do I match text across multiple lines?
Enable the "m" (multiline) flag so that ^ and $ match the start and end of each line rather than the entire string. If you also need the dot character to match newline characters, enable the "s" (dotAll) flag. Combining both flags gives you full multiline pattern matching capability.