>perl -wMstrict -le
"for my $s (qw(
   B111  B111B111  B111B111B111
   A111  A111A111  A111A111A111
   A111B111  A111B111A111  A111B111A111B111  A111B111A111B111A111
   B111A111  B111A111B111  B111A111B111A111  B111A111B111A111B111
   B  BB  BBB  A  AA  AAA  BA  BAB  AB  ABA
   xxx  xx  x), '',
   ) {
   local our $y_n;
   ()= $s =~ m{ ([AB]) (?{ $y_n = $^N eq 'A' ? 'yes' : 'no' }) }xmsg;
   if (defined $y_n) {
     printf qq{%3s: '%s' \n}, $y_n, $s;
     }
   else {
     warn qq{no A or B found in '$s'};
     }
   }
"
 no: 'B111'
 no: 'B111B111'
 no: 'B111B111B111'
yes: 'A111'
yes: 'A111A111'
yes: 'A111A111A111'
 no: 'A111B111'
yes: 'A111B111A111'
 no: 'A111B111A111B111'
yes: 'A111B111A111B111A111'
yes: 'B111A111'
 no: 'B111A111B111'
yes: 'B111A111B111A111'
 no: 'B111A111B111A111B111'
 no: 'B'
 no: 'BB'
 no: 'BBB'
yes: 'A'
yes: 'AA'
yes: 'AAA'
yes: 'BA'
 no: 'BAB'
 no: 'AB'
yes: 'ABA'
no A or B found in 'xxx' at -e line 1.
no A or B found in 'xx' at -e line 1.
no A or B found in 'x' at -e line 1.
no A or B found in '' at -e line 1.