perl -e 'print map {++$count," : ",$_,"\n"}split /(a|b)+/,"a12cdabab"'
1 : 
2 : a
3 : 12cd
4 : b                   

##</code><code>##

perl -e 'print map {++$count," : ",$_,"\n"}split /((?:a|b)+)/,"a12cdabab"'
1 : 
2 : a
3 : 12cd
4 : abab

##</code><code>##

perl -e 'print map {++$count," : ",$_,"\n"}split /(?:a|b)+/,"a12cdabab"'
1 : 
2 : 12cd