Regular Expressions

The string methods find(), index(), and the membership operator in let us find out whether a string is a substring of another. However, we often need a more sophisticated pattern-matching ability. This is facilitated through regular expressions (also called REs or regex).

Regular expressions are often used for validating input. For example, when we fill out a form online, the name, age, email address, mobile number, etc., are matched against acceptable patterns. The input is considered valid if the values we enter match these patterns.

For example, a mobile number is considered invalid if its length is ...