What is Regex Tester?
A regex tester lets you write and test regular expressions against sample text with instant visual feedback. Regular expressions are essential for text processing, data validation, search and replace, and string parsing in every programming language.
Our tester provides live matching with highlighted results, detailed match information including capture groups and named groups, find & replace with backreferences, flag toggles (g, i, m, s, u), common pattern presets, and a quick reference cheat sheet — all running locally in your browser.
Formula
Regex syntax: . (any char), \d (digit), \w (word char), \s (whitespace), ^ (start), $ (end), * (0+), + (1+), ? (0 or 1), {n,m} (n to m), [abc] (char class), (…) (group), a|b (alternation)Formula Explained
Regular expressions are compiled into finite state machines that process input character by character. The engine tries to match the pattern at each position in the string. Quantifiers (*, +, ?) control how many times a sub-pattern can repeat. Character classes [abc] match any single character in the set. Groups (…) capture matched text for extraction or backreferencing. The JavaScript regex engine uses backtracking for complex patterns.
Example
Pattern: ([a-zA-Z]+)@([a-zA-Z]+)\.([a-z]{2,}) Flags: g Test: "Contact hello@example.com or support@company.org" Match 1: hello@example.com (index 8) $1: hello, $2: example, $3: com Match 2: support@company.org (index 28) $1: support, $2: company, $3: org Replace with "$1@newdomain.com": "Contact hello@newdomain.com or support@newdomain.com"
Tips & Best Practices
- ✓Start simple and build up complexity — test each part of your pattern separately.
- ✓Use the g flag to find all matches, not just the first one.
- ✓Escape special characters with \ when you want to match them literally.
- ✓Use non-capturing groups (?:…) when you do not need to extract the match.
- ✓Test edge cases: empty strings, strings with only whitespace, and strings with special characters.
Common Use Cases
- •Validating email addresses, phone numbers, and URLs
- •Extracting data from structured text using capture groups
- •Testing find & replace patterns before using them in code
- •Learning regular expression syntax with immediate feedback
- •Debugging complex regex patterns with detailed match information