1 files changed, 49 insertions, 0 deletions
diff --git a/lib/banzai/filter/dollar_math_pre_filter.rb b/lib/banzai/filter/dollar_math_pre_filter.rb
new file mode 100644
index 00000000000..aaa186f87a6
--- /dev/null
+++ b/lib/banzai/filter/dollar_math_pre_filter.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+# Generated HTML is transformed back to GFM by:
+# - app/assets/javascripts/behaviors/markdown/marks/math.js
+# - app/assets/javascripts/behaviors/markdown/nodes/code_block.js
+module Banzai
+  module Filter
+    # HTML filter that implements our dollar math syntax, one of three filters:
+    # DollarMathPreFilter, DollarMathPostFilter, and MathFilter
+    #
+    class DollarMathPreFilter < HTML::Pipeline::TextFilter
+      # Based on the Pandoc heuristics,
+      # https://pandoc.org/MANUAL.html#extension-tex_math_dollars
+      #
+      # Handle the $$\n...\n$$ syntax in this filter, before markdown processing,
+      # by converting it into the ```math syntax. In this way, we can ensure
+      # that it's considered a code block and will not have any markdown processed inside it.
+
+      # Corresponds to the "$$\n...\n$$" syntax
+      REGEX = %r{
+          #{::Gitlab::Regex.markdown_code_or_html_blocks}
+        |
+          (?=(?<=^\n|\A)\$\$\ *\n.*\n\$\$\ *(?=\n$|\z))(?:
+            # Display math block:
+            # $$
+            # latex math
+            # $$
+
+            (?<=^\n|\A)\$\$\ *\n
+            (?<display_math>
+              (?:.)+?
+            )
+            \n\$\$\ *(?=\n$|\z)
+          )
+      }mx.freeze
+
+      def call
+        @text.gsub(REGEX) do
+          if $~[:display_math]
+            # change from $$ to ```math
+            "```math\n#{$~[:display_math]}\n```"
+          else
+            $~[0]
+          end
+        end
+      end
+    end
+  end
+end