code: def is_valid_delta_name(file):
          
          filename = basename(file)
          pattern = re.compile(Delta.FILENAME_PATTERN)
          if re.match(pattern, filename):
              return True
          return False
docs: Return if a file has a valid name
  
          A delta file name can be:
          - pre-all.py
          - pre-all.sql
          - delta_x.x.x_ddmmyyyy.pre.py
          - delta_x.x.x_ddmmyyyy.pre.sql
          - delta_x.x.x_ddmmyyyy.py
          - delta_x.x.x_ddmmyyyy.sql
          - delta_x.x.x_ddmmyyyy.post.py
          - delta_x.x.x_ddmmyyyy.post.sql
          - post-all.py
          - post-all.sql
  
          where x.x.x is the version number and _ddmmyyyy is an optional
          description, usually representing the date of the delta file