my Hashie Mash basic implementation

mash.rb

class Mash
  def initialize
    @mash = {}
  end
  
  def method_missing(method_id, *args)
    string_method = method_id.to_s

    if is_predicate?(method_id)
      key = string_method[0...-1]
      @mash.include?(key)
    else
      if args.empty?
        @mash[string_method]
      else
        key = string_method[0...-1]
        @mash[key] = args.join
        @mash[key]
      end
    end
  end

  def is_predicate?(method)
    return true if method =~ /[a-zA-z]\?\z/
    false
  end
end

mash = Mash.new
p mash.hello?
mash.hello = "let me say"

p mash.hello
p mash.hello?

p mash.alex?
mash.alex = "Ruby Programming Language"
p mash.alex?
p mash.alex