Regex problem

regex.rb

"EEB".gsub(/(\w)\1/,'\1X\1') # "EXEB"
"EEEB".gsub(/(\w)\1\1/,'\1X\1Z\1') # "EXEZEB" 
"EEEEB".gsub(/(\w)\1\1\1/,'\1X\1Z\1X\1') # "EXEZEXEB"

zip.rb

def mutate (str)
  def derp (str, b, c)
    return "" if str.nil?
    return str if str[0] != b[0]
    return c[0] + str[0] + derp(str[1..-1], b, c[1] + c[0])
  end

  return str[0] + derp(str[1..-1], str[0], "XZ")
end