xml.el doesn't handle empty attributes correctly

[Rob Austein]

Problem:

  XML attribute values can be empty, but xml.el doesn't allow that.
  The regexps in xml.el are wrong.

Example:

  bash$ cat xml-bug.el
  (message "Emacs version: %s" emacs-version)
  (defun test ()
    (prin1-to-string
     (let ((buf (get-buffer-create "xml-bug")))
       (princ "<foo bar=''/>" buf)
       (set-buffer buf)
       (message "Test buffer: %s" (buffer-string))
       (unwind-protect
           (let (cc)
             (condition-case cc
                 (xml-parse-region (point-min) (point-max))
               (error cc)))
         (kill-buffer buf)))))
  (load "/usr/local/share/emacs/21.3/lisp/xml.el")
  (message "Parse result: %s" (test))
  (load "/tmp/xml.el")
  (message "Parse result: %s" (test))

  bash$ emacs -batch -l xml-bug.el
  Emacs version: 21.3.1
  Loading /usr/local/share/emacs/21.3/lisp/xml.el (source)...
  Test buffer: <foo bar=''/>
  Parse result: (error "XML: Attribute values must be given between quotes")
  Loading /tmp/xml.el (source)...
  Test buffer: <foo bar=''/>
  Parse result: ((foo ((bar . "")) ""))

Patch:

--- /usr/local/share/emacs/21.3/lisp/xml.el     Thu Oct 18 20:19:51 2001
+++ /tmp/xml.el Sat Sep 24 18:39:53 2005
@@ -289,8 +289,8 @@
 
       ;; Do we have a string between quotes (or double-quotes),
       ;;  or a simple word ?
-      (unless (looking-at "\"\\([^\"]+\\)\"")
-       (unless (looking-at "'\\([^']+\\)'")
+      (unless (looking-at "\"\\([^\"]*\\)\"")
+       (unless (looking-at "'\\([^']*\\)'")
          (error "XML: Attribute values must be given between quotes")))
 
       ;; Each attribute must be unique within a given element

--Rob