🔍 Regex Tester

How to Test Regular Expressions Online

A regular expression is a pattern that describes a set of strings. Regex testers let you write and test patterns against sample text in real time — seeing which parts of the text match, which groups capture which content, and whether your pattern is syntactically valid. Our tester highlights matches as you type, making regex development fast and visual.

Regex Basics

• A period matches any single character except newline
• An asterisk matches zero or more of the preceding element
• A plus sign matches one or more of the preceding element
• A question mark makes the preceding element optional
• Caret matches the start of a line; dollar sign matches the end
• Square brackets define a character class — [abc] matches any of a, b, or c

Common Regex Patterns

• Email validation — matches the standard email address structure
• URL detection — finds http and https links in text
• Date format — matches YYYY-MM-DD format dates
• Phone numbers — matches international phone number formats
• IP addresses — matches IPv4 address format

Testing Tips

Always test your regex against edge cases — empty strings, strings with only special characters, very long strings, and strings that should not match. A regex that works on your sample might fail on real-world data that has unexpected formatting.

Regular Expressions Across Programming Languages

Regular expression syntax is largely consistent across languages but has important differences in advanced features. Python, JavaScript, Java, and PHP all support standard regex features including character classes, quantifiers, anchors, and capturing groups. However, lookahead and lookbehind assertions work differently between implementations. When copying a regex between languages, always test it thoroughly as subtle differences in how each engine handles edge cases can produce unexpected results on real-world data at scale.

Common Regex Patterns for Validation

Regular expressions are widely used for validating user input in web forms. Email address validation, phone number format checking, postcode format verification, and password strength rules are all commonly implemented with regex patterns. Keep validation patterns as simple as possible for common use cases as overly complex patterns are harder to maintain and more prone to catastrophic backtracking issues. Test all validation patterns against both valid inputs that should match and invalid inputs that should not match before deploying them to production.

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