Don't match email addresses or foo@bar as user references

app/models/user.rb

@@ -369,6 +369,7 @@ def reference_prefix
     # Pattern used to extract `@user` user references from text
     def reference_pattern
       %r{
+        (?<!\w)
         #{Regexp.escape(reference_prefix)}
         (?<user>#{Gitlab::PathRegex::FULL_NAMESPACE_FORMAT_REGEX})
       }x

changelogs/unreleased/dm-emails-are-not-user-references.yml

+---
+title: Don't match email addresses or foo@bar as user references
+merge_request:
+author:

spec/lib/banzai/filter/user_reference_filter_spec.rb

@@ -16,6 +16,11 @@
     expect(reference_filter(act).to_html).to eq(exp)
   end
 
+  it 'ignores references with text before the @ sign' do
+    exp = act = "Hey foo#{reference}"
+    expect(reference_filter(act).to_html).to eq(exp)
+  end
+
   %w(pre code a style).each do |elem|
     it "ignores valid references contained inside '#{elem}' element" do
       exp = act = "<#{elem}>Hey #{reference}</#{elem}>"