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What Is a Regular Expression?
A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. They are used to search, match, extract, replace, and validate text. Regex patterns are supported natively in nearly every programming language and are essential tools in any developer's toolkit.
Regex can range from simple patterns like \d+ (one or more digits) to complex multi-line patterns with lookaheads, backreferences, and named groups. While powerful, complex regex can be difficult to read and maintain — the regex tester helps you build and verify patterns interactively.
How Our Regex Tester Works
The regex tester provides real-time matching with visual highlighting as you type.
- Live matching — highlights all matches in your test string as you type the pattern
- Flag support — toggle global, case-insensitive, multiline, and dotAll flags
- Group capture display — shows captured groups and named captures for complex patterns
Try it free — no signup required
Test a Regular Expression →When To Use the Regex Tester
The regex tester is useful whenever you need to develop or debug a pattern before embedding it in code.
- Form validation — develop and test patterns for email, phone, postal code, or custom input validation
- Log parsing — build regex to extract fields from log lines before processing with grep or awk
- Data extraction — test patterns for scraping or parsing structured data from text or HTML
Frequently Asked Questions
What is the difference between greedy and lazy quantifiers?
Greedy quantifiers (*, +, ?) match as much as possible. Lazy quantifiers (*?, +?, ??) match as little as possible. For example, on the string <a>text</a>, the pattern <.*> matches the entire string (greedy) while <.*?> matches only <a> (lazy). Lazy quantifiers are essential for parsing HTML or nested structures.
How do I match a literal dot or other special character in regex?
Escape the character with a backslash: \. matches a literal dot, \* matches a literal asterisk. The special characters that need escaping are: . * + ? ^ $ { } [ ] | ( ) \. If you need to match any of these literally, always prefix them with \.
Why does my regex work differently in JavaScript versus Python?
Different languages implement slightly different regex flavors. Major differences include: Python uses (?P<name>) for named groups while JavaScript uses (?<name>); lookbehind support varies; some features like \K (keep) exist in PCRE but not JavaScript. Always test your regex in the context of your target language.