Generalize display math regular expression to cover \\[..\\]

Also add a test for double backslash math.

Closes GH-270

CHANGES.md

@@ -26,6 +26,7 @@
         Safronov for a patch.  ([GH-248][])
     -   New user option `markdown-translate-filename-function` to translate
         filenames when following file links.  ([GH-268][], [GH-277][])
+    -   Support double-backslash math delimiters.  ([GH-270][])
 
 *   Bug fixes:
 
@@ -65,6 +66,7 @@
   [gh-263]: https://github.com/jrblevin/markdown-mode/pull/263
   [gh-266]: https://github.com/jrblevin/markdown-mode/issues/266
   [gh-268]: https://github.com/jrblevin/markdown-mode/issues/268
+  [gh-270]: https://github.com/jrblevin/markdown-mode/issues/270
   [gh-272]: https://github.com/jrblevin/markdown-mode/issues/272
   [gh-273]: https://github.com/jrblevin/markdown-mode/issues/273
   [gh-274]: https://github.com/jrblevin/markdown-mode/pull/274

markdown-mode.el

@@ -1750,10 +1750,16 @@ Groups 1 and 3 match opening and closing dollar signs.
 Group 2 matches the mathematical expression contained within.")
 
 (defconst markdown-regex-math-display
-  "^\\(\\\\\\[\\)\\(\\(?:.\\|\n\\)*?\\)?\\(\\\\\\]\\)$"
-  "Regular expression for itex \[..\] display mode expressions.
-Groups 1 and 3 match the opening and closing delimiters.
-Group 2 matches the mathematical expression contained within.")
+  (rx line-start
+      (group (group (repeat 1 2 "\\")) "[")
+      (group (*? anything))
+      (group (backref 2) "]")
+      line-end)
+  "Regular expression for \[..\] or \\[..\\] display math.
+Groups 1 and 4 match the opening and closing markup.
+Group 3 matches the mathematical expression contained within.
+Group 2 matches the opening slashes, and is used internally to
+match the closing slashes.")
 
 (defsubst markdown-make-tilde-fence-regex (num-tildes &optional end-of-line)
   "Return regexp matching a tilde code fence at least NUM-TILDES long.
@@ -2886,10 +2892,10 @@ Depending on your font, some reasonable choices are:
     (markdown-match-math-double . ((1 markdown-markup-face prepend)
                                    (2 markdown-math-face append)
                                    (3 markdown-markup-face prepend)))
-    ;; Math mode \[..\]
+    ;; Math mode \[..\] and \\[..\\]
     (markdown-match-math-display . ((1 markdown-markup-face prepend)
-                                    (2 markdown-math-face append)
-                                    (3 markdown-markup-face prepend)))
+                                    (3 markdown-math-face append)
+                                    (4 markdown-markup-face prepend)))
     (markdown-match-bold . ((1 markdown-markup-properties prepend)
                             (2 markdown-bold-face append)
                             (3 markdown-markup-properties prepend)))
@@ -3780,7 +3786,7 @@ $..$ or `markdown-regex-math-inline-double' for matching $$..$$."
   (markdown-match-math-generic markdown-regex-math-inline-double last))
 
 (defun markdown-match-math-display (last)
-  "Match single quoted \[..\] math from point to LAST."
+  "Match bracketed display math \[..\] and \\[..\\] from point to LAST."
   (markdown-match-math-generic markdown-regex-math-display last))
 
 (defun markdown-match-propertized-text (property last)

tests/markdown-test.el

@@ -5019,6 +5019,15 @@ This includes preserving whitespace after the pipe."
       (markdown-test-range-has-face 394 398 markdown-math-face)
       (markdown-test-range-has-face 399 400 markdown-markup-face))))
 
+(ert-deftest test-markdown-math/double-slash-display-math ()
+  "Test double slash display math font lock."
+  (let ((markdown-enable-math t))
+    (markdown-test-file "math.text"
+      (markdown-test-range-has-face 403 474 nil)
+      (markdown-test-range-has-face 475 477 markdown-markup-face)
+      (markdown-test-range-has-face 478 543 markdown-math-face)
+      (markdown-test-range-has-face 544 546 markdown-markup-face))))
+
 (ert-deftest test-markdown-math/font-lock-italics ()
   "Test markdown math mode with underscores."
   (let ((markdown-enable-math t))

tests/math.text

@@ -24,6 +24,12 @@ $**η = (-1)^{k(n-k)}sη$, where $**η$, is the Hodge star applied twice.
 This is neither math nor italic
 \[ a_2 \]
 
+Support brackets with two backslashes, as do MultiMarkdown and Pandoc:
+
+\\[
+\Gamma(x) = \int_0^\infty t^{x-1}{\mathrm{e}}^{-t}\,{\mathrm{d}}
+\\]
+
 <!-- Local Variables: -->
 <!-- markdown-enable-math: t -->
 <!-- End: -->