Prevent double-prefixing of help page paths

Prior, because the link "api/README.md" was matched twice, the first
link became "help/help/api/README.md". Now we do a negative lookahead to
make sure the link doesn't start with `help/`.

This fix is still not ideal, see TODO note.

app/controllers/help_controller.rb

@@ -4,6 +4,8 @@ class HelpController < ApplicationController
   layout 'help'
 
   def index
+    @help_index = File.read(Rails.root.join('doc', 'README.md'))
+    prefix_help_links!(@help_index)
   end
 
   def show
@@ -57,6 +59,22 @@ class HelpController < ApplicationController
     params
   end
 
+  # Prefix links in a Markdown document with `help/` unless they already have
+  # been
+  #
+  # TODO (rspeicher): This should be a pipeline filter that only gets included
+  # for help pages, and it should operate on the Nokogiri doc to be more robust.
+  #
+  # text - Markdown String
+  #
+  # Modifies `text` in-place
+  def prefix_help_links!(text)
+    # Match text inside a Markdown link unless it already starts with `help/`
+    #
+    # See http://rubular.com/r/nwwhzH6Z8X
+    text.gsub!(%r{(\]\()(?!help\/)([^\)\(]+)(\))}x, '\1help/\2\3')
+  end
+
   PATH_SEPS = Regexp.union(*[::File::SEPARATOR, ::File::ALT_SEPARATOR].compact)
 
   # Taken from ActionDispatch::FileHandler

app/views/help/index.html.haml

@@ -27,11 +27,7 @@
   .col-md-8
     .documentation-index
       = preserve do
-        - readme_text = File.read(Rails.root.join("doc", "README.md"))
-        - text = readme_text.dup
-        - readme_text.scan(/\]\(([^(]+)\)/) { |match| text.gsub!(match.first, "help/#{match.first}") }
-        = markdown text
-
+        = markdown(@help_index)
   .col-md-4
     .panel.panel-default
       .panel-heading