Regex Tester
A Regex Tester allows you to write a regular expression pattern and test it against sample text in real-time. It highlights all matches, shows capture groups, and indicates global, case-insensitive, and multiline flags.
What is Regex Tester?
Regular expressions are patterns used to match character combinations in strings. They are fundamental to input validation, text parsing, search-and-replace operations, and log analysis. This tester runs JavaScript's built-in RegExp engine in the browser—the same engine used in Node.js, Chrome, Firefox, and Safari—showing matches highlighted inline, capture group values, and match metadata.
Use Cases
- Validating email addresses, phone numbers, and postcodes.
- Parsing log files for error patterns.
- Extracting structured data from unstructured text.
- Building search-and-replace scripts.
How to Use It
Enter your regex pattern in the Pattern field (without slashes).
Set flags: g (global), i (ignore case), m (multiline), s (dot-all).
Type or paste your test string below.
Matches are highlighted in real-time. Capture groups are listed below.
Pros
- Real-time match highlighting.
- Shows capture group values.
- Uses the native JS regex engine.
Limitations
- Only tests JavaScript regex syntax—may differ from Python, PCRE, etc.
Best Practices
- Always test with g (global) flag to find all occurrences.
- Use non-capturing groups (?:...) when you don't need to reference a group.
- Benchmark complex patterns on large text to avoid catastrophic backtracking.
Common Mistakes to Avoid
- Forgetting the g flag and only matching the first occurrence.
- Not escaping special characters like . * + ? which have regex meaning.
- Writing patterns vulnerable to ReDoS (catastrophic backtracking).
FAQs
Does this support PCRE features like lookbehind?
This tests the JavaScript (ES2018+) regex engine which supports lookbehind assertions, named capture groups, and Unicode property escapes.
Why is my regex matching nothing?
Common causes: missing g flag, wrong anchors (^ and $), unescaped special characters, or character class issues.