How to distinguish escaped characters from non-escaped e.g. "\x27" from "x27" in a string in Common Lisp?

Solving Advent of Code 2015 task 8 part2 I encountered the problem to have to distinguish in a string the occurrence of "\x27" from plain "x27". But I don't see a way how I can do it. Because

(length "\x27") ;; is 3
(length "x27")  ;; is also 3
(subseq "\x27" 0 1) ;; is "x"
(subseq "x27" 0 1)  ;; is "x"

Neither print, prin1, princ made a difference.

# nor does `coerce`
(coerce "\x27" 'list)
;; (#\x #\2 #\7)

So how then to distinguish in a string when "\x27" or any of such hexadecimal representation occurs?

It turned out, one doesn't need to solve this to solve the task. However, now I still would like to know whether there is a way to distinguish "\x" from "x" in common lisp.

Solution

The string literal "\x27" is read as the same as "x27", because \ is an escape character in string literals. If you want a string with the contents \x27, you need to write the literal as "\\x27" (i. e. escape the escape character). This has nothing to do with the strings themselves. If you read a string from a file containing \x27 (e. g. with read-line), then the four-character string \x27 results.