org-babel-read: Allow reading multi-line strings

* lisp/ob-core.el (org-babel-read): Fix regexp for detecting
string-like CELLs.  Avoid calling `read' twice.  Recover gracefully if
`read' errs.
* testing/lisp/test-ob.el (test-ob/org-babel-read): Add more tests.

Reported-by: Max Nikulin <manikulin@gmail.com>
Link: https://orgmode.org/list/v155g2$ncm$1@ciao.gmane.io

lisp/ob-core.el

@@ -3360,16 +3360,23 @@ situations in which is it not appropriate."
 		  (string= cell "*this*")))
          ;; FIXME: Arbitrary code evaluation.
 	 (eval (read cell) t))
-	((save-match-data
-           (and (string-match "^[[:space:]]*\"\\(.*\\)\"[[:space:]]*$" cell)
-                ;; CELL is a single string
-                (with-temp-buffer
-                  (insert cell)
-                  (goto-char 1)
-                  (read (current-buffer))
-                  (skip-chars-forward "[:space:]")
-                  (eobp))))
-         (read cell))
+	((let (read-val)
+           (save-match-data
+             (and (string-match
+                   (rx bos (0+ space)
+                       ?\" (0+ anychar) ?\"
+                       (0+ space) eos)
+                   cell)
+                  ;; CELL is a single string
+                  (with-temp-buffer
+                    (insert cell)
+                    (goto-char 1)
+                    (when (setq read-val
+                                (ignore-errors
+                                  (read (current-buffer))))
+                      (skip-chars-forward "[:space:]")
+                      (eobp)))
+                  read-val))))
 	(t (org-no-properties cell))))
 
 (defun org-babel--string-to-number (string)

testing/lisp/test-ob.el

@@ -2599,6 +2599,9 @@ abc
     ;; Special case: data inside quotes
     (should (equal "foo" (org-babel-read " \"foo\" " inhibit)))
     (should (equal "foo with\" inside" (org-babel-read " \"foo with\\\" inside\" " inhibit)))
+    (should (equal "abc\nsdf" (org-babel-read "\"abc\nsdf\"" inhibit)))
+    (should (equal "foo" (org-babel-read "\"foo\"" inhibit)))
+    (should (equal "\"foo\"(\"bar\"" (org-babel-read "\"foo\"(\"bar\"" inhibit)))
     ;; Unpaired quotes
     (should (equal "\"foo\"\"bar\"" (org-babel-read "\"foo\"\"bar\"" inhibit)))))