A simple atoi function using ruby 2.0 refinements.

atoi_refined.rb

module AtoiString
  refine String do
    def atoi
      fixed_self = fix self
      
      result = 0
      fixed_self.each_char do |c|
        result = result * 10 + c.to_i
      end
  
      fixed_self[0] == '-' ? -result : result
    end

    private
    def fix string
      minus = false
  
      string.split('.').first.each_char do |c|
        minus = true if c == '-'
        break if c >= '0' && c <= '9'
      end

      string.each_char do |c|
        string.delete! c if c < '0' || c > '9' 
      end

      minus ? "-#{string}" : string
    end
  end
end

using AtoiString

'123'.atoi
#=> 123

'-af+240.sf'.atoi
#=> -240

';sf2.34-34'.atoi
#=> 2