diff --git a/app/assets/javascripts/pretty-text/engines/discourse-markdown/quotes.js.es6 b/app/assets/javascripts/pretty-text/engines/discourse-markdown/quotes.js.es6
index 27873269ee1..fd59b7328b1 100644
--- a/app/assets/javascripts/pretty-text/engines/discourse-markdown/quotes.js.es6
+++ b/app/assets/javascripts/pretty-text/engines/discourse-markdown/quotes.js.es6
@@ -129,7 +129,7 @@ const rule = {
       token = state.push('quote_header_close', 'div', -1);
     }
 
-    token        = state.push('bbcode_open', 'blockquote', 1);
+    token = state.push('bbcode_open', 'blockquote', 1);
   },
 
   after: function(state) {
@@ -145,8 +145,8 @@ export function setup(helper) {
     opts.emojiSet = siteSettings.emoji_set;
   });
 
-  helper.registerPlugin(md=>{
-     md.block.bbcode.ruler.push('quotes', rule);
+  helper.registerPlugin(md => {
+    md.block.bbcode.ruler.push('quotes', rule);
   });
 
   helper.whiteList(['img[class=avatar]']);
diff --git a/lib/cooked_post_processor.rb b/lib/cooked_post_processor.rb
index 263d139e729..3796ce79d77 100644
--- a/lib/cooked_post_processor.rb
+++ b/lib/cooked_post_processor.rb
@@ -3,6 +3,7 @@
 
 require_dependency 'url_helper'
 require_dependency 'pretty_text'
+require_dependency 'quote_comparer'
 
 class CookedPostProcessor
   include ActionView::Helpers::NumberHelper
@@ -33,6 +34,7 @@ class CookedPostProcessor
       DiscourseEvent.trigger(:before_post_process_cooked, @doc, @post)
       post_process_oneboxes
       post_process_images
+      post_process_quotes
       keep_reverse_index_up_to_date
       optimize_urls
       update_post_image
@@ -90,6 +92,24 @@ class CookedPostProcessor
     end
   end
 
+  def post_process_quotes
+    @doc.css("aside.quote").each do |q|
+      post_number = q['data-post']
+      topic_id = q['data-topic']
+      if topic_id && post_number
+        comparer = QuoteComparer.new(
+          topic_id.to_i,
+          post_number.to_i,
+          q.css('blockquote').text
+        )
+
+        if comparer.modified?
+          q['class'] = ((q['class'] || '') + " quote-modified").strip
+        end
+      end
+    end
+  end
+
   def add_large_image_placeholder!(img)
     url = img["src"]
 
diff --git a/lib/pretty_text/shims.js b/lib/pretty_text/shims.js
index aa95290e6cf..46ce2595873 100644
--- a/lib/pretty_text/shims.js
+++ b/lib/pretty_text/shims.js
@@ -73,12 +73,12 @@ function __lookupAvatar(p) {
   return __utils.avatarImg({size: "tiny", avatarTemplate: __helpers.avatar_template(p) }, __getURL);
 }
 
-function __formatUsername(u) {
-  return __helpers.format_username(u);
+function __formatUsername(username) {
+  return __helpers.format_username(username);
 }
 
-function __lookupPrimaryUserGroup(p) {
-  return __helpers.lookup_primary_user_group(p);
+function __lookupPrimaryUserGroup(username) {
+  return __helpers.lookup_primary_user_group(username);
 }
 
 function __getCurrentUser(userId) {
diff --git a/lib/quote_comparer.rb b/lib/quote_comparer.rb
new file mode 100644
index 00000000000..c466d718c65
--- /dev/null
+++ b/lib/quote_comparer.rb
@@ -0,0 +1,24 @@
+class QuoteComparer
+  def self.whitespace
+    " \t\r\n".freeze
+  end
+
+  def initialize(topic_id, post_number, text)
+    @topic_id = topic_id
+    @post_number = post_number
+    @text = text
+    @parent_post = Post.where(topic_id: @topic_id, post_number: @post_number).first
+  end
+
+  # This algorithm is far from perfect, but it follows the Discourse
+  # philosophy of "catch the obvious cases, leave moderation for the
+  # complicated ones"
+  def modified?
+    return true if @text.blank? || @parent_post.blank?
+
+    parent_text = Nokogiri::HTML::fragment(@parent_post.cooked).text.delete(QuoteComparer.whitespace)
+    text = @text.delete(QuoteComparer.whitespace)
+
+    !parent_text.include?(text)
+  end
+end
diff --git a/spec/components/cooked_post_processor_spec.rb b/spec/components/cooked_post_processor_spec.rb
index 74c2ba7f8d5..2f60a6e46f8 100644
--- a/spec/components/cooked_post_processor_spec.rb
+++ b/spec/components/cooked_post_processor_spec.rb
@@ -778,4 +778,38 @@ describe CookedPostProcessor do
 
   end
 
+  context "quote processing" do
+    let(:cpp) { CookedPostProcessor.new(cp) }
+    let(:pp) { Fabricate(:post, raw: "This post is ripe for quoting!") }
+
+    context "with an unmodified quote" do
+      let(:cp) do
+        Fabricate(
+          :post,
+          raw: "[quote=\"#{pp.user.username}, post: #{pp.post_number}, topic:#{pp.topic_id}]\nripe for quoting\n[/quote]\ntest"
+        )
+      end
+
+      it "should not be marked as modified" do
+        cpp.post_process_quotes
+        expect(cpp.doc.css('aside.quote.quote-modified')).to be_blank
+      end
+    end
+
+    context "with a modified quote" do
+      let(:cp) do
+        Fabricate(
+          :post,
+          raw: "[quote=\"#{pp.user.username}, post: #{pp.post_number}, topic:#{pp.topic_id}]\nmodified\n[/quote]\ntest"
+        )
+      end
+
+      it "should be marked as modified" do
+        cpp.post_process_quotes
+        expect(cpp.doc.css('aside.quote.quote-modified')).to be_present
+      end
+    end
+
+  end
+
 end
diff --git a/spec/components/quote_comparer_spec.rb b/spec/components/quote_comparer_spec.rb
new file mode 100644
index 00000000000..e0be873dd38
--- /dev/null
+++ b/spec/components/quote_comparer_spec.rb
@@ -0,0 +1,41 @@
+require 'rails_helper'
+require 'quote_comparer'
+
+describe QuoteComparer do
+
+  describe "#modified?" do
+    let(:post) { Fabricate(:post, raw: "This has **text** we _are_ matching") }
+
+    def qc(text)
+      QuoteComparer.new(post.topic_id, post.post_number, text)
+    end
+
+    it "returns true for no post" do
+      expect(QuoteComparer.new(nil, nil, "test")).to be_modified
+    end
+
+    it "returns true for nil text" do
+      expect(qc(nil)).to be_modified
+    end
+
+    it "returns true for empty text" do
+      expect(qc("")).to be_modified
+    end
+
+    it "returns true for modified text" do
+      expect(qc("text is modified")).to be_modified
+    end
+
+    it "return false when the text matches exactly" do
+      expect(qc("This has text we are matching")).not_to be_modified
+    end
+
+    it "return false when there's a substring" do
+      expect(qc("text we are")).not_to be_modified
+    end
+
+    it "return false when there's extra space" do
+      expect(qc("\n\ntext   we are \t")).not_to be_modified
+    end
+  end
+end