$hydphb='GAFIVL';
$polar='DEHAW';
$charged='DGYAH';

$_ =~ /[$polar.$charged]([$hydphb]|[$polar]|
     H[$hydphb][$polar][$hydphb]W){18-24}[$polar.$charged]/;

##</code><code>##

$_ =~ /[$polar.$charged]
       ([$hydphb]|[$polar]){18-24}
      [$polar.$charged]/;
$_ =~ /H[$hydphb][$polar][$hydphb]W/;