irb(main):035:0*  str="[1,2][2,1][3,1][4,1]"
=> "[1,2][2,1][3,1][4,1]"
irb(main):036:0> str.gsub '][', '],['
=> "[1,2],[2,1],[3,1],[4,1]"


irb(main):037:0> str += ','
=> "[1,2][2,1][3,1][4,1],"
irb(main):038:0> str.chomp ','
=> "[1,2][2,1][3,1][4,1]"

Jim