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". See Character Type, and its
subsections for a description of the various kinds of 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. See Character Type.
Strings cannot hold characters that have the hyper, super, or alt modifiers.