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 the 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...