validate

Client requirements can be complex and those complex requirements often require custom code to be written. This includes making sure that the editing process conforms to certain validations.

Drupal can easily handle simple validation like having a value in the field or making sure an email is valid, but with more complex validations usually require custom code.

Whilst it is possible to inject custom validators into form submissions, I find using validation constraint classes makes the whole process much more predicable. Also, validation constraints are applied at a lower level than form validations, which means we can validate the data is correct even if we are creating the entity from an API. 

I wanted to impart a piece of advice to do with validation and formatting of user input, although I've never seen anyone suggest it. I guess it would come under the single responsibility principle so it might seem obvious to some people. There can be reasons why this might at least seem like a good idea at the time.

Essentially, if you want to validate that something is correct, don't format it at the same time. These two actions should be done in separate functions or even classes. I hope to demonstrate that using a single function validate and format anything is a bad idea. I'll mainly be using PHP to demonstrate this, but the principle should be pretty much the same in any language.

Take the following function called isValid(). This is an arbitrary and simple example but shows validation and formatting in use in a single function.

Creating a function to validate UK postcodes would seem like a simple task, but there is a little more to it than checking the number of characters. In fact there are several different variants of UK postal codes, especially if you include BFPO and overseas addresses. The official postcode specification details the structure of the postcode, and there is also a list of BFPO numbers from mod.uk.