1 files changed, 71 insertions, 0 deletions
diff --git a/lib/banzai/filter/blockquote_fence_filter.rb b/lib/banzai/filter/blockquote_fence_filter.rb
new file mode 100644
index 00000000000..d2c4b1e4d76
--- /dev/null
+++ b/lib/banzai/filter/blockquote_fence_filter.rb
@@ -0,0 +1,71 @@
+module Banzai
+  module Filter
+    class BlockquoteFenceFilter < HTML::Pipeline::TextFilter
+      REGEX = %r{
+          (?<code>
+            # Code blocks:
+            # ```
+            # Anything, including `>>>` blocks which are ignored by this filter
+            # ```
+
+            ^```
+            .+?
+            \n```$
+          )
+        |
+          (?<html>
+            # HTML block:
+            # <tag>
+            # Anything, including `>>>` blocks which are ignored by this filter
+            # </tag>
+
+            ^<[^>]+?>\n
+            .+?
+            \n<\/[^>]+?>$
+          )
+        |
+          (?:
+            # Blockquote:
+            # >>>
+            # Anything, including code and HTML blocks
+            # >>>
+
+            ^>>>\n
+            (?<quote>
+              (?:
+                  # Any character that doesn't introduce a code or HTML block
+                  (?!
+                      ^```
+                    |
+                      ^<[^>]+?>\n
+                  )
+                  .
+                |
+                  # A code block
+                  \g<code>
+                |
+                  # An HTML block
+                  \g<html>
+              )+?
+            )
+            \n>>>$
+          )
+      }mx.freeze
+
+      def initialize(text, context = nil, result = nil)
+        super text, context, result
+        @text = @text.delete("\r")
+      end
+
+      def call
+        @text.gsub(REGEX) do
+          if $~[:quote]
+            $~[:quote].gsub(/^/, "> ").gsub(/^> $/, ">")
+          else
+            $~[0]
+          end
+        end
+      end
+    end
+  end
+end