reverse file character by character using tac

I want to use the tacto reverse a text file character by character. On the info page for coreutils I found an example saying: #Reverse a file character by character tac -r -s 'x\|[^x]' However running tac -r -s seems to open standard input instead of printing the file. What does 'x\|[^x]' mean and what should I be doing? I also noted that the output for tac [file] and tac -r [file] are same and they're the same as cat [file]. Still can't figure out char by char reverse.