What is Regular expression?

Regex combines literals with metacharacters: . matches any character, */+/? are quantifiers, [...] is a character class, ( ) groups, and ^/$ anchor to start/end. For example ^\d{3}-\d{4}$ matches a 7-digit phone number.

Regexes power form validation (emails, URLs), search-and-replace, log parsing and tokenizing — but complex patterns can be hard to read, so keep them documented.