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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?

  • 2
    Have you read (elisp) Syntax of Regexps? – npostavs Sep 30 '17 at 15:44
  • 1
    What @npostavs said. The reason your second regexp doesn't "work" is that you are expecting it to match something that it, as a regular expression, should not match. [.*\\|\n\\] matches a single character - any of these characters: .*\| (period, asterisk, backslash, vertical bar) or a newline character. – Drew Sep 30 '17 at 20:45
  • 1
    @lucky1928 Can you tell us why you think that square brackets should work that way? Like wasamasa, I've never encountered any regexp dialect where that would be true, so I'm very interested in how you came to this understanding. – phils Sep 30 '17 at 22:02
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).

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