c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 my $qr_string = q((?:\G(\w)\W{2}(\w))*);
 my $qr = qr[$qr_string];
 ;;
 for my $s ('', qw(a--b  c--dE--F  g--hI--Jk--l)) {
   my @caps = $s =~ /$qr/g;
   print qq{'$s' -> }, pp \@caps;
   }
"
'' -> [undef, undef]
'a--b' -> ["a", "b", undef, undef]
'c--dE--F' -> ["c", "d", "E", "F", undef, undef]
'g--hI--Jk--l' -> ["g", "h", "I", "J", "k", "l", undef, undef]

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 my $qr_string = q((?:(\w)\W{2}(\w))*);
 my $qr = qr[$qr_string];
 ;;
 for my $s ('', qw(a--b  c--dE--F  g--hI--Jk--l)) {
   my @caps = $s =~ /$qr/g;
   print qq{'$s' -> }, pp \@caps;
   }
"
'' -> [undef, undef]
'a--b' -> ["a", "b", undef, undef]
'c--dE--F' -> ["E", "F", undef, undef]
'g--hI--Jk--l' -> ["k", "l", undef, undef]

##</code><code>##

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 my $qr_string = q((?:\G(\w)\W{2}(\w)));
 my $qr = qr[$qr_string];
 ;;
 for my $s ('', qw(a--b  c--dE--F  g--hI--Jk--l)) {
   my @caps = $s =~ /$qr/g;
   print qq{'$s' -> }, pp \@caps;
   }
"
'' -> []
'a--b' -> ["a", "b"]
'c--dE--F' -> ["c", "d", "E", "F"]
'g--hI--Jk--l' -> ["g", "h", "I", "J", "k", "l"]

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 my $qr_string = q((?:(\w)\W{2}(\w)));
 my $qr = qr[$qr_string];
 ;;
 for my $s ('', qw(a--b  c--dE--F  g--hI--Jk--l)) {
   my @caps = $s =~ /$qr/g;
   print qq{'$s' -> }, pp \@caps;
   }
"
'' -> []
'a--b' -> ["a", "b"]
'c--dE--F' -> ["c", "d", "E", "F"]
'g--hI--Jk--l' -> ["g", "h", "I", "J", "k", "l"]

##</code><code>##

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(dd);
 ;;
 my $s = 'aBcDeFg';
 ;;
 my @captures = $s =~ m{ (B) | (D) | (F) }xmsg;
 dd \@captures;
"
["B", undef, undef, undef, "D", undef, undef, undef, "F"]

##</code><code>##

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(dd);
 ;;
 my $qr_string = q((?:(\w)\W{2}(\w))*);
 my $qr = qr[$qr_string+];
 ;;
 my $s = '%%%%';
 print 'MATCH!!!' if $s =~ /$qr/g;
 dd \@-;
 ;;
 my @captures = $s =~ /$qr/g;
 dd \@captures;
"
MATCH!!!
[0]

[undef, undef, undef, undef, undef, undef, undef, undef]

##</code><code>##

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 my $qr_string = q((?:(\w)\W{2}(\w))*);
 my $qr = qr[$qr_string];
 ;;
 my $s = '%%%%';
 ;;
 print 'match @ offset ', $-[0], '  ($1, $2)==', pp $1, $2  while $s =~ /$qr/g;
 ;;
 my @captures = $s =~ /$qr/g;
 pp \@captures;
"
match @ offset 0  ($1, $2)==(undef, undef)
match @ offset 1  ($1, $2)==(undef, undef)
match @ offset 2  ($1, $2)==(undef, undef)
match @ offset 3  ($1, $2)==(undef, undef)
match @ offset 4  ($1, $2)==(undef, undef)
[undef, undef, undef, undef, undef, undef, undef, undef, undef, undef]