Regex Tester
Test and debug regular expressions with live match highlighting and capture group display.
About the Regex Tester
Regular expressions (regex or regexp) are patterns used to match, search, and manipulate text. They are supported in virtually every programming language and are a fundamental tool for tasks like validation, parsing, search-and-replace, and data extraction. This tester lets you build and test JavaScript-compatible regular expressions with live match highlighting.
Regex flags
- g (global) — Find all matches, not just the first one.
- i (case-insensitive) — Treat uppercase and lowercase as equivalent.
- m (multiline) —
^and$match the start and end of each line, not just the whole string. - s (dotAll) — The
.metacharacter matches newline characters as well.
Common regex patterns
- Email address (simplified):
[\w.+-]+@[\w-]+\.[a-z]{2,} - IPv4 address:
(\d{1,3}\.){3}\d{1,3} - UK postcode:
[A-Z]{1,2}\d[A-Z\d]? ?\d[A-Z]{2} - URL with protocol:
https?://[^\s/$.?#].[^\s]* - Digits only:
^\d+$ - Capture groups:
(\d{4})-(\d{2})-(\d{2})to extract date components
Performance considerations
Complex regular expressions with nested quantifiers (like (a+)+) can cause catastrophic backtracking — exponential time complexity that may freeze or crash an application when processing long inputs. Test your regex with adversarial inputs, and prefer atomic groups or possessive quantifiers where available. For production use, validate input length before applying regex patterns.