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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _split_string_to_tokens(text):
 """Splits text to a list of string tokens."""

  if not text:
    return []
  ret = []
  token_start = 0
  # Classify each character in the input string
  is_alnum = [c in _ALPHANUMERIC_CHAR_SET for c in text]
  for pos in xrange(1, len(text)):
    if is_alnum[pos] != is_alnum[pos - 1]:
      token = text[token_start:pos]
      if token != u" " or token_start == 0:
        ret.append(token)
      token_start = pos
  final_token = text[token_start:]
  ret.append(final_token)
  return ret