Regular expression is a way to perform pattern matching on text data. It's very powerful tool to find something e.g. some character in a long string e.g. finding if a book contains some word or not. Almost all major programming language supports regular expression but Perl has been renowned for its enormous capability. Java also supports Perl like regular expression using java.util.regex package. You can use regular expression to check if a email is valid or not, if a phone number is valid, or if a zip code is valid, or even a SSN number is valid or not. One of the simplest example of regular expression is to check if a String is number or not.