Как то так ...

gistfile1.txt

old_string = "312211"

counter = 1
new_string = ""

for index_of_char in 0..old_string.size-1 
  char = old_string[index_of_char]
   
  if old_string[index_of_char] == old_string[index_of_char+1]
   counter += 1
   next   
  else
   new_string << counter.to_s
   new_string << char.to_s
   counter = 1      
  end  
end

puts new_string