Regex Tester

Regex Tester is a free, browser-based tool built for one job: test and debug regular expressions with live match highlighting, capture groups, replace and example patterns. It is part of ToolsMonk's developer tools collection, so you can finish the work without downloading software, creating an account, or jumping between multiple websites.

Typical jobs it handles include building and debugging a regex for form validation before shipping it, checking why a pattern isn't matching the strings you expect, and extracting and verifying capture groups for parsing tasks. The same page works on desktop, tablet, and mobile, so it is equally quick for a single urgent task and for work you repeat every week.

Everything about Regex Tester is tuned for speed to first result, with privacy-friendly processing and the rest of ToolsMonk's tools nearby when the next task comes up.

Regex Tester lets you write a regular expression and instantly see what it matches in sample text, with live highlighting and a breakdown of captured groups. It turns the usually opaque process of crafting a regex into immediate visual feedback.

It uses the JavaScript (ECMAScript) regex engine, the one in every browser and Node.js, and exposes the common flags (global, case-insensitive, multiline, dotall) so the behaviour matches how your code will actually run the pattern. Toggling flags and watching the highlights update is the fastest way to understand why a pattern does or doesn't fire.

Everything runs locally, so your patterns and test strings, which often contain real data, never leave your browser. That makes it safe to paste production samples while debugging.

The most reliable way to build a complex pattern is incrementally: match a small fragment, confirm it highlights, then extend it, leaning on capture groups to extract exactly the substrings you need. For converting or generating expressions, pair this with the other ToolsMonk developer utilities.