How to distinguish escaped characters from non-escaped e.g. "\x27" from "x27" i-CodePudding

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.

CodePudding user response：

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.

CodePudding user response：

By the time that the Lisp reader gets to work, \x is the same as x. There may be some way to turn this off - I wouldn't be surprised - but the original text talks about Santa's file.

So, I created my own file, like this:

x27
\x27

And I read the data into special variables like this:

 (defun read-line-crlf (stream)
    (string-right-trim '(#\Return) (read-line stream nil)))

 (defun read-lines (filename)
    (with-open-file (stream filename)
      (setf x (read-line-crlf stream))
      (setf x-esc (read-line-crlf stream))
    ))

The length of x is then 3, and the length of x-esc is 4. The returned string must be trimmed on Windows, or an external format declared, because otherwise SBCL will leave half of the CR-LF on the end of the read strings.