Improve GistTag (#555)

app/liquid_tags/gist_tag.rb

@@ -32,12 +32,14 @@ class GistTag < LiquidTagBase
     if valid_link?(link)
       input_no_space
     else
-      raise StandardError, "Invalid Gist link"
+      raise StandardError,
+        "Invalid Gist link: #{link} Links must follow this format: https://gist.github.com/username/gist_id"
     end
   end
 
   def valid_link?(link)
-    (link =~ /^(http(s)?:)?\/\/(gist.github.com)\/.*/)&.zero?
+    (link =~ /^https\:\/\/gist\.github\.com\/([a-zA-Z0-9\-]){1,39}\/([a-zA-Z0-9]){32}\s/)&.
+      zero?
   end
 end
 

spec/liquid_tags/gist_tag_spec.rb

@@ -8,6 +8,7 @@ RSpec.describe GistTag, type: :liquid_template do
         "//pastebin.com/raw/b77FrXUA#gist.github.com",
         "https://gist.github.com@evil.com",
         "https://gist.github.com.evil.com",
+        "https://gist.github.com/string/string/raw/string/file",
       ]
     end