What is Regex Tester?
Regex Tester is an interactive regular expression testing workbench. Regular expressions are powerful pattern-matching rules used in input validation, search-and-replace, string parsing, and web scraping. Writing complex regex patterns without real-time visual feedback often leads to bugs, catastrophic backtracking, or missed edge cases. Our Regex Tester highlights matched substrings in real time as you type patterns and test text.
How to use Regex Tester
Enter your regular expression pattern in the pattern input bar and select desired regex flags (g: global, i: case-insensitive, m: multi-line, s: dotAll, u: unicode). Paste test text into the test area. Matching text strings are highlighted inline immediately. The match details panel displays match counts, capture group values, and match index offsets.
Key features
- Real-time instant regex match highlighting as you type
- Full flag control: global (g), case-insensitive (i), multiline (m), dotAll (s), unicode (u)
- Detailed match index offsets and capture group breakdown table
- Supports modern ES2022 regex features (named groups, lookbehinds)
- 100% private browser client-side execution
Common use cases
- Validating email, phone number, and password strength regex rules before frontend deployment
- Testing web scraping text extraction patterns
- Debugging search and replace regex rules for code refactoring
Technical details & standards
The tester uses JavaScript's native RegExp engine, supporting modern ECMAScript regex features such as named capture groups (?<name>), lookahead (?=), lookbehind (?<=), and unicode property escapes (\p{L}). All evaluation operates client-side in your browser, keeping confidential test strings and regex patterns completely private.
Frequently asked questions
What is catastrophic backtracking?
Catastrophic backtracking occurs when nested quantifiers (e.g. (a+)+) cause exponential search paths on non-matching strings, freezing the CPU. Test your regex on complex inputs to ensure safety.
Is my data safe?
All processing happens entirely in your browser. Your data never leaves your device and is never sent to or stored on a server.
Does it work offline?
Yes. Once the page has loaded, most tools keep working without an internet connection.