>perl -wMstrict -le
"my $gotA    = qr{ (?= .*? A) }xms;
 my $gotB    = qr{ (?= .*? B) }xms;
 my $gotC    = qr{ (?= .*? C) }xms;
 my $onlyABC = qr{ (?! .*? [^ABC]) }xms;
 for my $s (@ARGV) {
   my $match = $s =~ m{ \A $gotA $gotB $gotC $onlyABC }xms;
   print qq{'$s' }, $match ? 'matches' : 'no match';
   }
"
"" A B C AB AC BC ABC CAB ABABCBCBCACA xABC ABCx xA xB xC Ax
'' no match
'A' no match
'B' no match
'C' no match
'AB' no match
'AC' no match
'BC' no match
'ABC' matches
'CAB' matches
'ABABCBCBCACA' matches
'xABC' no match
'ABCx' no match
'xA' no match
'xB' no match
'xC' no match
'Ax' no match