In numerous apps, across multiple platforms (iOS, Android, and Windows), I have been tasked to validate user input either after the user has entered it or as it is entered. This validation can be done very easily with regular expressions; however, we do not want various regular expression strings littered throughout our code. We can solve this problem by creating different classes or structures that contain the validation code. The question is; how would we organize these types to make them easy to use and maintain? Prior to protocol extensions in Swift, I would have used protocols to define the validation requirements and would then create a type that conforms to the protocol for each validation type needed.

Prior to looking at how we would accomplish this text validation, let's take a quick look at what regular expressions are and how we would use them in Swift. A regular expression (also known as regex) is a special text string that is used to describe a search or matching...