A Regex or Regular Expression is a sequence of characters that define a search pattern. It’s mainly used for pattern matching. Each character in a regular expression is either understood to be a metacharacter -with it’s special meaning, or a regular character with it’s listeral meaning.
A basic understanding of regular expressions is recommended. For more information about the expressions and metacharacters which can be used, please refer to a regex cheat sheet. Various tools are available on the internet to test your regular expressions. For example: RegExPal.