>perl -wMstrict -le
"my $s = 'Fu Feet Fie Foe Fum';
 my $f = qr{ F \w* }xms;
 for my $rx (
   qr{ (X)                             }xms,
   qr{ ($f) \s* ($f)                   }xms,
   qr{ ($f) \s* ($f) \s* ($f)          }xms,
   qr{ ($f) \s* ($f) \s* ($f) \s* ($f) }xms,
   ) {
   $s =~ $rx;
   print qq{highest capture group is $#-, captures are};
   printf qq{\$$_->[0] eq '$_->[1]'  }
     for map [ $_, eval qq{\$$_} ], 1 .. $#-;
   print qq{\n};
   }
"
highest capture group is -1, captures are


highest capture group is 2, captures are
$1 eq 'Fu'  $2 eq 'Feet'

highest capture group is 3, captures are
$1 eq 'Fu'  $2 eq 'Feet'  $3 eq 'Fie'

highest capture group is 4, captures are
$1 eq 'Fu'  $2 eq 'Feet'  $3 eq 'Fie'  $4 eq 'Foe'