Regular expressions (regex) are sequences of characters that define a search pattern. They're supported natively in every major programming language — JavaScript, Python, Go, Ruby, Java — and in text editors, terminal tools like grep, and database query engines.
The three most important concepts: Literals match exactly (the pattern cat matches "cat"). Metacharacters have special meaning (. matches any character, * means zero or more). Anchors match positions (^ is start of string, $ is end).
Testing regex interactively is the fastest way to learn and debug. The real-time highlighting in this tool shows you exactly which parts of your test string match — and which don't — so you can iterate quickly without running code.
