Introduction to regular expressions

Regular expression (or in short regex) is a very useful tool that is used to describe a search pattern for matching the text. Regex is nothing but a sequence of some characters that defines a search pattern. Regex is used for parsing, filtering, validating, and extracting meaningful information from large text, such as logs and output generated from other programs.

We find regular expressions in day-to-day use on many websites. For example, while searching for your favorite recipe on search engines, while filling up forms and entering data such as username and passwords, and so on. While setting up a password on many sites, we encounter password validation errors, such as password must contain one digit or at least one uppercase letter or at least one special character, and so on. All these checks can be done using regular expressions. A few more typical examples of regular expressions are validating phone numbers or validating postal/zip/pin codes.