DB<172> $s=$s0
 => "1*((2+3)*((3+4)+5))*(6+7)+8"

  DB<173> print "$s\n" while $s =~ s# \( [^()]* \) # '.' x length($&) #gex 
1*(.....*(.....+5))*.....+8
1*(.....*.........)*.....+8
1*.................*.....+8

  DB<174> index $s, '+'
 => 25