>perl -wMstrict -le
"use 5.010;
 ;;
 my $regex = qr{ (?| (AAA) | (BBB) | (XXX) | (YYY) ) }xms;
 ;;
 my $s = 'AAA BBB CCC DDD AAA BBB';
 my $n_matches =()= $s =~ m{ $regex }xmsg;
 print qq{$n_matches matches};
 ;;
 $s = 'foo bar baz boff';
 $n_matches =()= $s =~ m{ $regex }xmsg;
 print qq{$n_matches matches};
"
4 matches
0 matches