elisp multiple line match string regexp

Question

To match with multiple line regexp, I can use \\(.*\\|\n\\), but I think it should be [.*\\|\n\\] which actually doesn't work as expected.

#+NAME: exam1
#+BEGIN_SRC elisp :results output
(let ((str "")
      (start 0))
  (setq str "this is\na test and this is a test")
  (while (string-match "this is\\(.*\\|\n\\)a test" str start)
    (setq start (match-end 0))
    (print (match-string 0 str))))

#+END_SRC

#+RESULTS:
: 
: "this is
: a test"
: 
: "this is a test"

#+NAME: exam2
#+BEGIN_SRC elisp :results output
(let ((str "")
      (start 0))
  (setq str "this is\na test and this is a test")
  (while (string-match "this is[.*\\|\n]a test" str start)
    (setq start (match-end 0))
    (print (match-string 0 str))))

#+END_SRC

#+RESULTS: exam2
: 
: "this is
: a test"

What's the reason exam1 works but exam2 doesn't?

Answer 1

I haven't heard of any implementation of regular expression where character alternatives ([...]) would permit alternation (|) as that construct only makes sense inside groups ((...)). Remember that character alternatives are interpreted as one of the included characters matching a character ([abc] matches a, b or c), whereas alternation means that a sequence must match any of the enclosed sequences (\\(foo\\|bar\\) matches foo or bar).