>perl -wMstrict -le
"my @strings = ('x', 'xa', 'x ', 'xa ',);
 ;;
 for my $s (@strings) {
   $s =~ s{ a? \s* \z }
          { print qq{\$& '$&'  pos = }, pos($s);  ''; }xmseg;
   print qq{\$s '$s' \n};
   }
"
$& ''  pos = 1
$s 'x'

$& 'a'  pos = 1
$& ''  pos = 2
$s 'x'

$& ' '  pos = 1
$& ''  pos = 2
$s 'x'

$& 'a '  pos = 1
$& ''  pos = 3
$s 'x'