UserNameConstraint.php

<?php

namespace Drupal\user\Plugin\Validation\Constraint;

use Symfony\Component\Validator\Constraint;

/**
 * Checks if a value is a valid user name.
 *
 * @Constraint(
 *   id = "UserName",
 *   label = @Translation("User name", context = "Validation"),
 * )
 */
class UserNameConstraint extends Constraint {
    public $emptyMessage = 'You must enter a username.';
    public $spaceBeginMessage = 'The username cannot begin with a space.';
    public $spaceEndMessage = 'The username cannot end with a space.';
    public $multipleSpacesMessage = 'The username cannot contain multiple spaces in a row.';
    public $illegalMessage = 'The username contains an illegal character.';
    public $tooLongMessage = 'The username %name is too long: it must be %max characters or less.';

}