You can use the function read-quoted-char to ask the user to
specify a character, and allow the user to specify a control or meta
character conveniently, either literally or as an octal character code.
The command quoted-insert uses this function.
This function is like read-char, except that if the first
character read is an octal digit (0–7), it reads any number of octal
digits (but stopping if a non-octal digit is found), and returns the
character represented by that numeric character code. If the
character that terminates the sequence of octal digits is RET,
it is discarded. Any other terminating character is used as input
after this function returns.
Quitting is suppressed when the first character is read, so that the user can enter a C-g. See Quitting.
If prompt is supplied, it specifies a string for prompting the user. The prompt string is always displayed in the echo area, followed by a single ‘-’.
In the following example, the user types in the octal number 177 (which is 127 in decimal).
(read-quoted-char "What character")
---------- Echo Area ----------
What character 1 7 7-
---------- Echo Area ----------
⇒ 127