About backslash in regexp

[Xue Fuqiao]

I understand that the usual regexp special characters are not special
inside a character alternative.  But in the definition of the function
`command-line-normalize-file-name':

(defun command-line-normalize-file-name (file)
[...]
  (string-match "^[A-Za-z]:\\(\\\\[\\\\/]\\)" file) ; C:\/ or C:\\
[...]

There four backslashes in the `[ ... ]' construct here, why?  Aren't
they duplicated?

-- 
Xue Fuqiao
http://www.gnu.org/software/emacs/