$ perl -Mstrict -Mwarnings -E '
    my $lines = "AZ\nSX\nAZ\nDC\nAB\nYZ\n";
    say $lines;
    $lines =~ s/(?>[^A].|.[^Z])\n//gm;
    say $lines;
'
AZ
SX
AZ
DC
AB
YZ

AZ
AZ