From c7b537b26a08fa818bf04e592ef38856f2681e76 Mon Sep 17 00:00:00 2001
From: Michael Howell <michael@notriddle.com>
Date: Fri, 27 Mar 2026 14:35:39 -0700
Subject: [PATCH] More sophisticated flanking rules for math

With this change, `$` that's preceded by word chars can't open,
and followed by word chars can't close. This reduces "false positives"
on existing markdown documents like this:

    vim /etc/systemd/system/$UNIT.$TYPE
                            ^-----^ currently parsed as latex math

I've checked a few other commonmark-based markdown flavors that accept
LaTeX math, to see what they did. The rule I added is supposed to be the
same as the one used in VSCode's markdown-it plugin and Discourse's math plugin:

- https://github.com/microsoft/vscode-markdown-it-katex/blob/efd01d8/src/index.ts#L22-L32
- https://github.com/discourse/discourse/blob/41f62aa/plugins/discourse-math/assets/javascripts/lib/discourse-markdown/discourse-math.js#L46

Full disclosure: I know of a few implementations that don't do this:

- https://github.com/kivikakk/comrak/blob/0d4a9ca/src/parser/inlines.rs#L2104 (narrow special case for digits after the closer)
- https://github.com/classeur/markdown-it-mathjax/blob/c1e34d4/markdown-it-mathjax.js#L80 (same narrow special case for digits)
- https://github.com/jupyterlab/jupyterlab/blob/61a2db4/packages/rendermime/src/latex.ts#L17 (very permissive; even `1 $ 2 $ 3` thinks the `2` is in a math span)
---
 .../src/Commonmark/Extensions/Math.hs             | 12 ++++++++++--
 commonmark-extensions/test/math.md                | 15 +++++++++++++++
 commonmark/src/Commonmark/Inlines.hs              |  1 +
 3 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/commonmark-extensions/src/Commonmark/Extensions/Math.hs b/commonmark-extensions/src/Commonmark/Extensions/Math.hs
index a5a7c7d..ca94d9d 100644
--- a/commonmark-extensions/src/Commonmark/Extensions/Math.hs
+++ b/commonmark-extensions/src/Commonmark/Extensions/Math.hs
@@ -15,11 +15,16 @@ import Commonmark.Html
 import Text.Parsec
 import Data.Text (Text)
 import qualified Data.Text as T
+import qualified Data.Map.Strict as M
 
 mathSpec :: (Monad m, IsBlock il bl, IsInline il, HasMath il)
          => SyntaxSpec m il bl
 mathSpec = mempty
   { syntaxInlineParsers = [withAttributes parseMath]
+  -- A very odd workaround. Is there a better way to:
+  -- 1. Get the precedingTokTypes map populated with info about '$'
+  -- 2. Have the math spans otherwise follow the same precedence as code spans, not emphasis
+  , syntaxFormattingSpecs = [FormattingSpec '$' False False Nothing Nothing '$']
   }
 
 class HasMath a where
@@ -38,13 +43,16 @@ instance (HasMath i, Monoid i) => HasMath (WithSourceMap i) where
 
 parseMath :: (Monad m, HasMath a) => InlineParser m a
 parseMath = try $ do
-  symbol '$'
+  Tok _ pos _ <- symbol '$'
   display <- (True <$ symbol '$') <|> (False <$ notFollowedBy whitespace)
   contents <- try $ untokenize <$> pDollarsMath 0
+  st <- getState
+  let isPrecededByWordChars = M.lookup pos (precedingTokTypes st) == Just WordChars
+  isFollowedByWordChars <- (==) WordChars . maybe LineEnd tokType <$> optionMaybe (lookAhead anyTok)
   let isWs c = c == ' ' || c == '\t' || c == '\r' || c == '\n'
   if display
      then displayMath contents <$ symbol '$'
-     else if T.null contents || isWs (T.last contents)
+     else if T.null contents || isWs (T.last contents) || isPrecededByWordChars || isFollowedByWordChars
              -- don't allow math to end with SPACE + $
              then mzero
              else return $ inlineMath contents
diff --git a/commonmark-extensions/test/math.md b/commonmark-extensions/test/math.md
index f8d6dc3..6ba5d05 100644
--- a/commonmark-extensions/test/math.md
+++ b/commonmark-extensions/test/math.md
@@ -87,3 +87,18 @@ $b<a>c$
 .
 <p><span class="math inline">\(b&lt;a&gt;c\)</span></p>
 ````````````````````````````````
+
+Inline math openers can't be immediately preceded by
+alphanumeric characters:
+```````````````````````````````` example
+Expect$Nothing$
+.
+<p>Expect$Nothing$</p>
+````````````````````````````````
+
+Or immediately followed by them:
+```````````````````````````````` example
+./install.sh $PLATFORM-$VERSION
+.
+<p>./install.sh $PLATFORM-$VERSION</p>
+````````````````````````````````
diff --git a/commonmark/src/Commonmark/Inlines.hs b/commonmark/src/Commonmark/Inlines.hs
index 1988666..59d58d3 100644
--- a/commonmark/src/Commonmark/Inlines.hs
+++ b/commonmark/src/Commonmark/Inlines.hs
@@ -26,6 +26,7 @@ module Commonmark.Inlines
   , pLinkTitle
   , pEscaped
   , pEscapedSymbol
+  , precedingTokTypes
   , processEmphasis
   , processBrackets
   , pBacktickSpan