Don't render inline math when dollar signs are inside markup

lib/banzai/filter/math_filter.rb

@@ -5,12 +5,6 @@ module Banzai
     # HTML filter that adds class="code math" and removes the dollar sign in $`2+2`$.
     #
     class MathFilter < HTML::Pipeline::Filter
-      # This picks out <code>...</code>.
-      INLINE_MATH = 'descendant-or-self::code'.freeze
-
-      # Pick out a code block which is declared math
-      DISPLAY_MATH = "descendant-or-self::pre[contains(@class, 'math') and contains(@class, 'code')]".freeze
-
       # Attribute indicating inline or display math.
       STYLE_ATTRIBUTE = 'data-math-style'.freeze
 
@@ -22,13 +16,14 @@ module Banzai
       DOLLAR_SIGN = '$'.freeze
 
       def call
-        doc.xpath(INLINE_MATH).each do |code|
+        doc.css('code').each do |code|
           closing = code.next
           opening = code.previous
 
           # We need a sibling before and after.
           # They should end and start with $ respectively.
           if closing && opening &&
+              closing.text? && opening.text? &&
               closing.content.first == DOLLAR_SIGN &&
               opening.content.last == DOLLAR_SIGN
 
@@ -39,7 +34,7 @@ module Banzai
           end
         end
 
-        doc.xpath(DISPLAY_MATH).each do |el|
+        doc.css('pre.code.math').each do |el|
           el[STYLE_ATTRIBUTE] = 'display'
           el[:class] += " #{TAG_CLASS}"
         end

spec/lib/banzai/filter/math_filter_spec.rb

@@ -79,6 +79,13 @@ describe Banzai::Filter::MathFilter, lib: true do
     expect(doc.to_s).to eq input
   end
 
+  it 'ignores dollar signs if they are inside another element' do
+    input = '<p>We check strictly <em>$</em><code>2+2</code><em>$</em></p>'
+    doc = filter(input)
+
+    expect(doc.to_s).to eq input
+  end
+
   # Display math
 
   it 'adds data-math-style display attribute to display math' do