Add YamlFrontMatterFilter to the PreProcessPipeline

This filter will detect YAML Front Matter and convert it to an HTML
table for prettier formatting.

2 files changed, 30 insertions, 1 deletions

diff --git a/lib/banzai/filter/yaml_front_matter_filter.rb b/lib/banzai/filter/yaml_front_matter_filter.rb
new file mode 100644
index 00000000000..e4e2f3f228d
--- /dev/null
+++ b/lib/banzai/filter/yaml_front_matter_filter.rb
@@ -0,0 +1,28 @@
+require 'html/pipeline/filter'
+require 'yaml'
+
+module Banzai
+  module Filter
+    class YamlFrontMatterFilter < HTML::Pipeline::Filter
+      DELIM = '---'.freeze
+
+      # Hat-tip to Middleman: https://git.io/v2e0z
+      PATTERN = %r{
+        \A(?:[^\r\n]*coding:[^\r\n]*\r?\n)?
+        (?<start>#{DELIM})[ ]*\r?\n
+        (?<frontmatter>.*?)[ ]*\r?\n?
+        ^(?<stop>#{DELIM})[ ]*\r?\n?
+        \r?\n?
+        (?<content>.*)
+      }mx.freeze
+
+      def call
+        match = PATTERN.match(html)
+
+        return html unless match
+
+        "```yaml\n#{match['frontmatter']}\n```\n\n#{match['content']}"
+      end
+    end
+  end
+end
diff --git a/lib/banzai/pipeline/pre_process_pipeline.rb b/lib/banzai/pipeline/pre_process_pipeline.rb
index c174f0b862d..50dc978b452 100644
--- a/lib/banzai/pipeline/pre_process_pipeline.rb
+++ b/lib/banzai/pipeline/pre_process_pipeline.rb
@@ -2,7 +2,8 @@ module Banzai
   module Pipeline
     class PreProcessPipeline < BasePipeline
       def self.filters
-        [
+        FilterArray[
+          Filter::YamlFrontMatterFilter
         ]
       end