Elisp Meta Characters in String

The setup of outshine package for emacs requires the user to set the variable outline-minor-mode-prefix to "\M-#" before the outline-mode package (that ships with emacs) is loaded.

The best way to do this is via the Customize interface in emacs.

So I did M-x customize and set that variable’s value to "\M-#" and restarted emacs and that worked.

But then I was surprised to see that value being saved in the custom.el as '(outline-minor-mode-prefix "\243").

After some digging, I came across this elisp meta-character syntax reference,

In a string, the 2**7 bit attached to an ASCII character indicates a meta character; thus, the meta characters that can fit in a string have codes in the range from 128 to 255, and are the meta versions of the ordinary ASCII characters.

Thus if the decimal ASCII value of the # character is 35, the decimal value of \M-# will be “2**7 bit attached to #” i.e. 2**7 + 35 = 128 + 35 = 163. But the value of \M-# got stored in custom.el as \243.

From the elisp general escape syntax reference,

An octal escape sequence consists of a backslash followed by up to three octal digits; thus, ‘?\101’ for the character A

.. and doing decimal to octal conversion of 163 gives 243!