new_contents: import re
  
  
  def dna(dna: str) -> str:
  
      """
      https://en.wikipedia.org/wiki/DNA
      Returns the second side of a DNA strand
  
      >>> dna("GCTA")
      'CGAT'
      >>> dna("ATGC")
      'TACG'
      >>> dna("CTGA")
      'GACT'
      >>> dna("GFGG")
      'Invalid Strand'
      """
  
      r = len(re.findall("[ATCG]", dna)) != len(dna)
      val = dna.translate(dna.maketrans("ATCG", "TAGC"))
      return "Invalid Strand" if r else val
  
  
  if __name__ == "__main__":
      __import__("doctest").testmod()
  
subject: add the dna algorithm (#6323)