How to Restrict Format of Username

You can write in custom validation for any field with the wpmem_pre_register_data action. That allows you to halt registration with an error message.

Thank you for your quick assistance and the suggestion.

I’ve used the example code on the link you provided, pasting it into the function.php file in a child theme – and as anticipated, the registration process is halted, with the entered credentials being displayed.

Unfortunately, adding custom validation to this example is a step a little too far for me – a non-developer!

I’ll bear this in mind though – and if my skills improve in future, I’ll it a go.

Thank you!