And if you're just interested in the number of overlaps, here's a way based on an approach covered in the recent node already linked (with the actual overlaps thrown in as a convincer):

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = '1234';
 ;;
 my $n =()= $s =~ m{ (?= (\d\d)) }xmsg;
 print qq{total overlaps: $n};
 ;;
 my @overlaps = $s =~ m{ (?= (\d\d)) }xmsg;
 printf 'overlaps:';
 printf qq{ '$_'} for @overlaps;
"
total overlaps: 3
overlaps: '12' '23' '34'
[download]