perl -le'$==1,(1x$_)=~/^(1|11)*(?{$=++})^/,print$=for 0..20' #### perl -le'$==1,(1x$_)=~/^(1|11(??{}))*(?{$=++})^/,print$=for 0..20' #### perl -le'$==0,(1x$_)=~/^(1|11(??{}))*$(?{$=++})^/,print$=for 0..20' #### 1 (1)(1)(1)(1)(1)(1) 2 (1)(1)(1)(1)(1 1) 3 (1)(1)(1)(1 1)(1) 4 (1)(1)(1 1)(1)(1) 5 (1)(1)(1 1)(1 1) 6 (1)(1 1)(1)(1)(1) 7 (1)(1 1)(1)(1 1) 8 (1)(1 1)(1 1)(1) 9 (1 1)(1)(1)(1)(1) 10 (1 1)(1)(1)(1 1) 11 (1 1)(1)(1 1)(1) 12 (1 1)(1 1)(1)(1) 13 (1 1)(1 1)(1 1) #### 1 (1)(1)(1)(1) 2 (1)(1)(1) 1 3 (1)(1)(1 1) 4 (1)(1) 1 1 5 (1)(1 1)(1) 6 (1)(1 1) 1 7 (1) 1 1 1 8 (1 1)(1)(1) 9 (1 1)(1) 1 10 (1 1)(1 1) 11 (1 1) 1 1 12 1 1 1 1