Why is the order of : and - important in a regexp character class

Question

Defining the following function

(defun windows-path-backwards ()
  (interactive)
  (skip-chars-backward "a-zA-Z0-9\\\\_-:"))

and running it while standing at | in the below text

C:\this\path|

moves it to

C:|\this\path

Redefining the function as

(defun windows-path-backwards ()
  (interactive)
  (skip-chars-backward "a-zA-Z0-9\\\\_:-"))

and repeating the same procedure moves the pointer from

C:\this\path|

to

|C:\this\path

Why is there difference between these two functions when the only change is the order of the : and - characters in the character class skip-chars-backward takes as an argument.

Answer 1

It is because - is used to denote ranges of characters. One will have to escape - for it to not have that meaning when placed between two other characters. See the function definition below:

(defun windows-path-backwards ()
  (interactive)
  (skip-chars-backward "a-zA-Z0-9\\\\_\\-:")) 

In other words, : is not important in itself, it just happened to be one of two characters surrounding a - in the nonfunctioning example given in the question.