A regular expression pattern is a pattern language used for searching, replacing, and validating text. It combines special characters (metacharacters) with literal characters to define string patterns.

In JavaScript, regex can be created using literal notation (/pattern/flags) or the constructor (new RegExp('pattern', 'flags')). Methods like test(), match(), and replace() use regex patterns. Regular expressions introduction books cover basics through advanced topics.

Basic metacharacters include . (any character), ^ (start of line), $ (end of line), and | (OR). Flags include g (global), i (case-insensitive), and m (multiline).

Regex is used in email validation, log file analysis, text editor search-and-replace, and countless other text processing tasks. Practical regex books demonstrate real-world usage patterns.