Validating URLs is hard! On the face of it, you could use a RegEx to match on the URL structure, but that is likely only going to match a narrow subset of URLs, whether intentionally or accidentally.
Beyond having a "valid" URL, an application might be applying extra conditions on top. For example, accepting only http: or https: URLs, but not file: and others. This might be ok depending on the application, but knowing that takes some active design work to figure out. For the purpose of handling user errors, and offering suggestions, it also helps to know in what manner the URL failed validation. For example, we might be able to offer precise suggestions for our application-specific errors, while general URL parsing issues can be out of scope.
To make life easier, let's separate the term "valid" and "invalid" into a few different ones:
- Valid URL: a URL that can be parsed successfully according to the standard (which, uhm, I am reluctant to dive into, or rather I feel unqualified to give you an overvi