Merge branch '6459-fix-dkim-selector-regex' into 'develop'

Fixed regex for dkim_selector to match only valid entires #6459

Closes #6459

See merge request ispconfig/ispconfig3!1692

Author: Till Brehm

interface/web/mail/form/mail_domain.tform.php

@@ -132,7 +132,7 @@
 			'width'  => '20',
 			'maxlength' => '63',
 			'validators' => array (  0 => array (   'type' => 'REGEX',
-					'regex' => '/^[a-z0-9]{0,63}$/',
+					'regex' => '/^(?=.*[a-z])[a-z0-9]{1,63}$/',
 					'errmsg'=> 'dkim_selector_error'),
 			),
 		),