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(elisp)Nonprinting Characters

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Nonprinting Characters in Strings
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   You can use the same backslash escape-sequences in a string constant
as in character literals (but do not use the question mark that begins a
character constant).  For example, you can write a string containing the
nonprinting characters tab and `C-a', with commas and spaces between
them, like this: `"\t, \C-a"'.  Note: Character Type, for a
description of the read syntax for characters.

   However, not all of the characters you can write with backslash
escape-sequences are valid in strings.  The only control characters that
a string can hold are the ASCII control characters.  Strings do not
distinguish case in ASCII control characters.

   Properly speaking, strings cannot hold meta characters; but when a
string is to be used as a key sequence, there is a special convention
that provides a way to represent meta versions of ASCII characters in a
string.  If you use the `\M-' syntax to indicate a meta character in a
string constant, this sets the 2**7 bit of the character in the string.
If the string is used in `define-key' or `lookup-key', this numeric
code is translated into the equivalent meta character.  Note: Character
Type.

   Strings cannot hold characters that have the hyper, super, or alt
modifiers.