isdigit in elisp

Question

I wanted to check that char is a digit by comparing its code with codes of 0 and 9, presuming that the digits codes are contiguous in code table:

(let ((char ?\e))
  (if (and (>= char ?\0) (<= char ?\9))
      (message "aaa")
    (message "bbb")))

But it doesn't work: this code returns "aaa", even though e is not a digit.

What is wrong?

Answer 1

Remove the backslashes before e, 0 and 9.

(let ((char ?e))
  (if (<= ?0 char ?9)
      (message "It's a digit")
    (message "It's not a digit")))

With the backslashes:

• \e is the escape char - ASCII 27.
• \0 is the null char - ASCII 0.
• \9 isn't special, so it's the same as 9 - ASCII 57.
• 27 is >= 0 and <= 57.