Fixed regex to match the dkim_slector RFC compliant #6459

The dkim_select can start with a number, but has to contain atleast
1 letter and has to be between 1 and 63 characters long.

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

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