code: def sub(pattern, repl, string, count=0, flags=0):
      
      return _compile(pattern, flags).sub(repl, string, count)
docs: Return the string obtained by replacing the leftmost
      non-overlapping occurrences of the pattern in string by the
      replacement repl.  repl can be either a string or a callable;
      if a string, backslash escapes in it are processed.  If it is
      a callable, it's passed the match object and must return
      a replacement string to be used.