a.rb

def parse(str)
  tokens = str.scan(/(.+?)([,=]|\z)/).flatten

  pairs = []
  stack = []

  while token = tokens.shift
    case token
    when "="
      stack << token
    when ","
      stack << token
      
      if tokens.first != ',' && 2 <= stack.size
        pairs << stack.dup
        stack.clear
      end
    else
      stack << token
    end
  end
  pairs << stack.dup unless stack.empty?

  pairs = pairs.inject([]) { |r, pair|
    if !pair.find{ |_| _ == '='.freeze } && r.last
      r.last.push(*pair) 
      r
    else
      r << pair
    end
  }
  pairs.each(&:pop)
  Hash[pairs.map{ |_| _.join.split(/=/,2) }]
end

p parse("a=b,c=d") # {"a"=>"b", "c"=>"d"}
p parse("aa=b=,,c=d,e=f,g,h,i=j") # {"aa"=>"b=,", "c"=>"d", "e"=>"f,g,h", "i"=>"j"}