code: def format_unix_var(text):
      
      text = text.strip()
      if len(text) == 0:  # if empty string, return it
          raise ValueError("can not be empty string!")
      else:
          if text[0] in string.digits:
              raise ValueError("variable can not start with digits!")
          text = text.lower()
          # delete redundant empty space
          words = list()
          word = list()
          for char in text:
              if char in ALPHA_DIGITS:
                  word.append(char)
              else:
                  if len(word):
                      words.append("".join(word))
                      word = list()
          if len(word):
              words.append("".join(word))
  
          return "_".join(words)
docs: Example::
  
          this_is_very_good