What Are Regular Expressions?
Regular expressions (regex) are sequences of characters that define a search pattern. They are used for string matching, validation, and text manipulation across virtually every programming language.
Basic Syntax
A regex like /d+/g matches one or more digits. The d is a character class meaning any digit, the + is a quantifier meaning "one or more", and g is a flag meaning "global" (find all matches, not just the first).
Common Patterns
/^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}$/— Email validation/^(?=.*[A-Z])(?=.*d).{8,}$/— Password with uppercase and digit/https?://[^s]+/g— URL extraction
Testing Your Regex
Always test regex patterns before deploying them. Our Regex Tester provides live match highlighting as you type, making it easy to verify your pattern handles all edge cases.
Performance Considerations
Complex regex patterns can cause catastrophic backtracking. Avoid nested quantifiers like (a+)+ on untrusted input. Keep patterns as specific as possible.
