>perl -wMstrict -le
"my $s = 'aaabacadaabbbccccd';
 print qq{'$1'} while $s =~ m[ (?= ((.) (?! \2) . ) ) ]xmsg;;
"
'ab'
'ba'
'ac'
'ca'
'ad'
'da'
'ab'
'bc'
'cd'