Help me, please. Check my code.Did I do everything right?

There is an assignment:

Implement the function to find the combination of 4 digits in a row which gives the max multiplication. If object is not a string or there are no combinations found – return nil. If combination is found – return it’s multiplication result.

E.g.

max_multiplication('abc12345def') => 120 # 2*3*4*5  max_multiplication('a1b2c3d4e') => nil 

I did this:

DIGITS = 4  class String   def numeric?     !(self =~ /[[:digit:]]/).nil?   end end  def multiply(row, start_index)   result = 1   start_index.upto(start_index + DIGITS - 1) do |i|       result *= row[i].to_i   end   result end  def max_multiplication(row)   current_position = 0   result = nil   while current_position < row.length - (DIGITS - 1) do     should_continue = true     1.upto(DIGITS) do |i|       symbol = row[current_position - 1 + i]       unless symbol.numeric?         current_position += i         should_continue = false         break       end     end     next unless should_continue     current_result = multiply(row, current_position)     result = current_result if result.nil? || current_result > result     current_position += 1   end   result end  puts max_multiplication('abc12345def') puts max_multiplication('a1b2c3d4e').nil?