Field API - which hook to use? I have successfully created a field with my module and managed to show it on user registration form. The next step would be validating my field. There are two hooks for this, and I'm confused as to which one to use. One is hook_field_validate and the other is hook_field_attach_validate. I'm also confused as to how to refer to the field. $instance['user']['field_tckn_no']? (This is what dpm(field_info_instances('user') shows me. Also, one thing that confuses me is that hook_field_validate iterates over the fields but all I have is one field. Is it still the one to use?

Regards,
K.
Well, at last I was able to figure it out. Since the field was part of the form after creating the instance, I thought I could use hook_form_alter, and I was right!