public function isValid(Field $field)
 {
     return filter_var($field->getValue(), FILTER_VALIDATE_EMAIL) !== false;
 }
 public function isValid(Field $field)
 {
     return is_numeric($field->getValue());
 }
 public function isValid(Field $field)
 {
     return !empty($field->getValue());
 }
 public function isValid(Field $field)
 {
     return call_user_func($this->callable, $field->getValue());
 }
 public function isValid(Field $field)
 {
     return strlen($field->getValue()) <= $this->max;
 }
 public function isValid(Field $field)
 {
     return preg_match($this->pattern, $field->getValue());
 }
 public function isValid(Field $field)
 {
     return ctype_alpha($field->getValue());
 }