Help: Unique meta key value & custom validate

Hello,

Is there a way to add extra validation to “Meta Key Unique Value” in the registration form?

I’m trying to couple meta key Unique Value validation with another custom validation, but I can’t find the solution.

I need users to register with their DNI (Identity Document in Spain) number only once, but also verify that this DNI is valid with the following custom validation.

/**
 * Validar campo DNI
 * @param string $key
 * @param attay  $array
 * @param array  $args
 */
function um_custom_validate_dni($key, $array, $args){
  $letra = substr($args[$key], -1);
  $numeros = substr($args[$key], 0, -1);
  if ( isset( $args[$key]) && substr("TRWAGMYFPDXBNJZSQVHLCKE", $numeros%23, 1) == $letra && strlen($letra) == 1 && strlen ($numeros) == 8 ){
    //UM()->form()->add_error( $key, __( 'DNI VALIDO.', 'ultimate-member' ) );
  }else{
    UM()->form()->add_error( $key, __( 'DNI INCORRECTO, utilice formato: Ocho números y letra mayúscula.', 'ultimate-member' ) );
  }
}

Would both things be possible? I’m sorry if I don’t explain myself well.

Thank you very much.

All the best.