The 'pattern' attribute allows specifying a regular expression pattern for input validation. Key points: 1) Works with text, search, url, tel, email, and password inputs, 2) Pattern must match the entire value, not just a portion, 3) Can be accessed via JavaScript through input.pattern property, 4) Integrates with built-in form validation, 5) Supports :valid/:invalid CSS pseudo-classes, 6) Should be combined with title attribute to explain the required format to users.