Regex Tester

Writing a regular expression is trial and error: you adjust the pattern until it matches what you want and nothing else. A live tester shortens that loop. Type a pattern and sample text, and see every match, capture group and flag effect immediately, using the same JavaScript regex engine as browsers and Node.js.

How to use it

1. Enter the regular expression without surrounding slashes.
2. Set flags such as g, i or m.
3. Paste sample text, including examples that should not match.
4. Review highlighted matches and capture groups, and refine the pattern.

Worked example

Pattern (\w+)@(\w+\.\w+) with the g flag against Contact [email protected] or [email protected] finds two matches. For the first, group 1 is ana and group 2 is example.com. Changing to named groups, (?<user>\w+)@(?<domain>\w+\.\w+), makes the code that uses the result easier to read.

Testing tips

• Test negatives. Include strings that must fail. A pattern that matches everything is easy to write by accident.
• Anchor validations. Without ^ and $, \d{4} also matches inside abc12345.
• Watch the g flag in code. A global regex object remembers lastIndex, so calling test() twice can alternate between true and false.
• Escape special characters such as . * + ? ( ) [ ] { } | \ ^ $ when you mean them literally.

Performance

Nested quantifiers like (a+)+ can cause catastrophic backtracking, freezing the page on certain inputs. This is the root of ReDoS vulnerabilities. Prefer specific character classes over .* and avoid nested repetition.