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 terminators

I tried to convert it to unix style using the dos2unix. The below is the output

$ dos2unix COPYING 
dos2unix: Skipping binary file COPYING

I was surprised to find that dos2unix program reports it as an binary file. Then using some other editor(not emacs) i found that the file contains a control character. I am interested to find all the invisible characters in the file using emacs.

By Googling i have found the following solution which uses tr

tr -cd '\11\12\40-\176' < file_name

How to do the same in emacs way. I tried the hexl-mode. The hexl-mode shows text and their corresponding ascii values in a single buffer which is great. How to find the characters which has ASCII values other than 11-12, 40-176(i.e tab, space and visible characters). I tried to create a regular expression for that search, but it is quite complicated.

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.