What is Regular Expression (Regex)?

A regular expression (regex) is a sequence of characters that defines a search pattern. It is used to match, validate, search, and replace text — for example validating an email or extracting all URLs from a document.

Regex is supported in virtually every programming language and editor. The syntax (anchors, character classes, quantifiers, groups) is largely shared across "flavors" like JavaScript, PCRE, and Python.