code: def checksum_from_sha1(value):
      
      # More constrained regex at lexer level
      CHECKSUM_RE = re.compile('SHA1:\s*([\S]+)', re.UNICODE)
      match = CHECKSUM_RE.match(value)
      if match:
          return checksum.Algorithm(identifier='SHA1', value=match.group(1))
      else:
          return None
docs: Return an spdx.checksum.Algorithm instance representing the SHA1
      checksum or None if does not match CHECKSUM_RE.