code: def subn(pattern, repl, string, count=0, flags=0):
      
      return _compile(pattern, flags).subn(repl, string, count)
docs: Return a 2-tuple containing (new_string, number).
      new_string is the string obtained by replacing the leftmost
      non-overlapping occurrences of the pattern in the source
      string by the replacement repl.  number is the number of
      substitutions that were made. 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.