unescaped character literals detection error

Question

When I reload my init file I get an error (or warning) saying

unescaped character literals `?(', `?)', `?[', `?]' detected!

What does this error (warning) mean and how can one fix it?

Answer 1

Section 2.3.3.1 Basic Char Syntax of the Elisp manual says:

The usual read syntax for alphanumeric characters is a question mark
followed by the character; thus, ‘?A’ for the character ‘A’, ‘?B’ for
the character ‘B’, and ‘?a’ for the character ‘a’.

   For example:

     ?Q ⇒ 81     ?q ⇒ 113

   You can use the same syntax for punctuation characters.  However, if
the punctuation character has a special syntactic meaning in Lisp, you
must quote it with a ‘\’.  For example, ‘?\(’ is the way to write the
open-paren character.  Likewise, if the character is ‘\’, you must use a
second ‘\’ to quote it: ‘?\\’.

Your init file contains ?( but Emacs prefers to see ?\(. The same goes for the other three characters mentioned in the warning.

You can load the Elisp manual inside Emacs with C-h i.