Regex Tester

What is a regular expression (regex)?

A regular expression (regex or regexp) is a pattern used to match, search, and manipulate text. Regex is built into virtually every programming language and is used to validate inputs, extract data, find and replace strings, and parse structured text formats.

This tester lets you write a regex pattern and see matches highlighted in real time. It supports JavaScript regex syntax with all standard flags (global, case-insensitive, multiline, dotAll, unicode). Match results and capture groups are shown as you type.

Regular expressions are indispensable for tasks like validating email addresses, phone numbers, and credit card formats; extracting dates, URLs, and IP addresses from unstructured text; parsing log files and CSV data; and performing complex find-and-replace operations in code editors. Common patterns include d{4}-d{2}-d{2} for ISO dates, [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,} for email addresses, and ^s*$ for matching blank lines. This tester is useful both for learning regex interactively and for debugging production patterns before deploying them.

The tester runs your pattern in an isolated Web Worker with a 1-second hard timeout, so catastrophic backtracking from patterns like (a+)+ on long input strings will be safely terminated instead of freezing your browser tab. This safety measure is especially important when testing user-supplied patterns or when experimenting with complex lookahead and lookbehind assertions. The match result panel shows the full match text, all numbered capture groups, and named capture groups (using JavaScript's (?<name>...) syntax), with character positions for each match. For a comprehensive introduction to regex syntax and common patterns, see our regular expressions beginner's guide.

Frequently asked questions