Why did my pattern time out?

Some patterns, particularly ones with nested quantifiers, can exhibit catastrophic backtracking on certain inputs. The tester enforces a timeout and terminates the worker rather than locking up your tab. If you hit it, the pattern likely needs restructuring rather than a longer timeout.

Does it support every regex flavor?

Matching uses the JavaScript RegExp engine, so flavor-specific features from PCRE, Python's re, or other engines may not behave identically here.

Regex Tester

Test regular expressions with real-time highlighting, capture groups, and replace functionality. Debug patterns instantly.

Common Patterns

Test String

Results

john.doe@example.com
invalid-email
user@domain.co.uk
test@subdomain.example.org

About this tool

Tests a regular expression against sample text with matches highlighted live. Matching runs inside a Web Worker with a hard timeout, so a runaway pattern can't freeze the page.

When you’d use it

Debugging why a pattern isn't matching the input you expect.
Checking a regex against edge cases before it goes into validation code.

Frequently asked questions

Why did my pattern time out?: Some patterns, particularly ones with nested quantifiers, can exhibit catastrophic backtracking on certain inputs. The tester enforces a timeout and terminates the worker rather than locking up your tab. If you hit it, the pattern likely needs restructuring rather than a longer timeout.
Does it support every regex flavor?: Matching uses the JavaScript RegExp engine, so flavor-specific features from PCRE, Python's re, or other engines may not behave identically here.

Free developer tools by Clocktower and Associates. All processing happens in your browser: no data is collected or transmitted.