If you don’t want to use my plugin, just hook your logic into one of the following validate filters – acf/validate_value, acf/validate_value/type={$field['type']}, acf/validate_value/name={$field['name']}, or acf/validate_value/key={$field['key']}. These filters take the arguments $valid, $value, $field, $input and return true for success or a string if there was an error.
Also let me know if you would like to try out the new version of my validation plugin – it might contain a few bugs still but has a lot of new features.
