perl> $s = join'','a'..'z'

 perl> 1 while $s =~ m[((.).)(?{ print "$^N:$+\n"; })]g
ab:a
cd:c
ef:e
gh:g
ij:i
kl:k
mn:m
op:o
qr:q
st:s
uv:u
wx:w
yz:y


 perl> 1 while $s =~ m[((..).)(?{ print "$^N:$+\n"; })]g
abc:ab
def:de
ghi:gh
jkl:jk
mno:mn
pqr:pq
stu:st
vwx:vw