How to find and remove the invisible characters in text file using emacs

Question

I have a txt file named COPYING which is edited on windows. It contains windows style eol

$ file COPYING 
COPYING: ASCII English text, with CRLF line ter         


        

Answer 1

To see invisible characters, you can try whitespace-mode. Spaces and tabs will be displayed with a symbol in a different face. If the coding system is automatically being detected as dos (showing (DOS) on the status bar), carriage returns at the end of a line will be hidden as well. Run revert-buffer-with-coding-system to switch it to Unix or binary (e.g. C-x RET r unix) and they'll always show up as ^M. The binary coding system will display any non-ASCII characters as control characters as well.