c:\@Work\Perl>perl -wMstrict -le
"my $maybe = qr{ cd? }xms;
 ;;
 my $start    = qr{ A | BC | DE?F }xms;
 my $end      = qr{ U | VW | XY?Z }xms;
 my $excluded = qr{ (?! $end | $maybe) . }xms;
 ;;
 for my $s ('AxxcdxxVWABCDEFUVWXYZ', 'BCxxxUABCDEFUVWXYZ', @ARGV) {
   my $t = $s;
   print qq{'$t'};
   $t =~ s{ $start $excluded* ($maybe?) .*? $end }{=$1=}xms;
   print qq{'$t' \n};
   }
"  BCxxcxxXZABCDEFUVWXYZ
'AxxcdxxVWABCDEFUVWXYZ'
'=cd=ABCDEFUVWXYZ'

'BCxxxUABCDEFUVWXYZ'
'==ABCDEFUVWXYZ'

'BCxxcxxXZABCDEFUVWXYZ'
'=c=ABCDEFUVWXYZ'