field_ui.schema.yml

# Schema for configuration files of the Field UI module.

field_ui.settings:
  type: config_object
  label: 'Field UI settings'
  constraints:
    FullyValidatable: ~
  mapping:
    field_prefix:
      type: string
      label: 'The prefix for new fields created via Field UI'
      constraints:
        Regex:
          # @see \Drupal\field\Entity\FieldStorageConfig::__construct()
          pattern: '/^[_a-z]+[_a-z0-9]*$/'
          message: "The %value prefix is not valid."
        Length:
          # The maximum length of the field name is 32 characters. This maximum
          # length also encompasses the length of the field_prefix, which is
          # capped at 30 characters here. By setting the field_ui.settings:field_prefix config
          # to its maximum length, only two characters will remain available
          # for the actual field name based on the above specified pattern.
          # @see \Drupal\field\Entity\FieldStorageConfig::NAME_MAX_LENGTH
          # @see \Drupal\field\Entity\FieldStorageConfig::preSaveNew()
          max: 30