Regular expression (also known as regex or regexp) provides a way of specifying a pattern to be matched in a given big chunk of text data. It supports a set of characters to specify the pattern. It is widely used for a text search and string manipulation. A lot of shell commands provide an option to specify regex such as
find, and so on.
The regular expression concept is also used in other programming languages such as C++, Python, Java, Perl, and so on. Libraries are available in different languages to support regular expression's features.