Fix markdown rendering for label references that begin with a digit

app/models/label.rb

@@ -52,11 +52,14 @@ class Label < ActiveRecord::Base
   # This pattern supports cross-project references.
   #
   def self.reference_pattern
+    # NOTE: The id pattern only matches when all characters on the expression
+    # are digits, so it will match ~2 but not ~2fa because that's probably a
+    # label name and we want it to be matched as such.
     @reference_pattern ||= %r{
       (#{Project.reference_pattern})?
       #{Regexp.escape(reference_prefix)}
       (?:
-        (?<label_id>\d+) | # Integer-based label ID, or
+        (?<label_id>\d+(?!\S\w)\b) | # Integer-based label ID, or
         (?<label_name>
           [A-Za-z0-9_\-\?&]+ | # String-based single-word label title, or
           "([^"]*)"            # String-based multi-word label surrounded in quotes

spec/lib/banzai/filter/label_reference_filter_spec.rb

@@ -104,6 +104,30 @@ describe Banzai::Filter::LabelReferenceFilter, lib: true do
     end
   end
 
+  context 'String-based single-word references that begin with a digit' do
+    let(:label)     { create(:label, name: '2fa', project: project) }
+    let(:reference) { "#{Label.reference_prefix}#{label.name}" }
+
+    it 'links to a valid reference' do
+      doc = reference_filter("See #{reference}")
+
+      expect(doc.css('a').first.attr('href')).to eq urls.
+        namespace_project_issues_url(project.namespace, project, label_name: label.name)
+      expect(doc.text).to eq 'See 2fa'
+    end
+
+    it 'links with adjacent text' do
+      doc = reference_filter("Label (#{reference}.)")
+      expect(doc.to_html).to match(%r(\(<a.+><span.+>#{label.name}</span></a>\.\)))
+    end
+
+    it 'ignores invalid label names' do
+      exp = act = "Label #{Label.reference_prefix}#{label.id}#{label.name.reverse}"
+
+      expect(reference_filter(act).to_html).to eq exp
+    end
+  end
+
   context 'String-based single-word references with special characters' do
     let(:label)     { create(:label, name: '?gfm&', project: project) }
     let(:reference) { "#{Label.reference_prefix}#{label.name}" }
@@ -153,6 +177,30 @@ describe Banzai::Filter::LabelReferenceFilter, lib: true do
     end
   end
 
+  context 'String-based multi-word references that begin with a digit' do
+    let(:label)     { create(:label, name: '2 factor authentication', project: project) }
+    let(:reference) { label.to_reference(format: :name) }
+
+    it 'links to a valid reference' do
+      doc = reference_filter("See #{reference}")
+
+      expect(doc.css('a').first.attr('href')).to eq urls.
+        namespace_project_issues_url(project.namespace, project, label_name: label.name)
+      expect(doc.text).to eq 'See 2 factor authentication'
+    end
+
+    it 'links with adjacent text' do
+      doc = reference_filter("Label (#{reference}.)")
+      expect(doc.to_html).to match(%r(\(<a.+><span.+>#{label.name}</span></a>\.\)))
+    end
+
+    it 'ignores invalid label names' do
+      exp = act = "Label #{Label.reference_prefix}#{label.id}#{label.name.reverse}"
+
+      expect(reference_filter(act).to_html).to eq exp
+    end
+  end
+
   context 'String-based multi-word references with special characters in quotes' do
     let(:label)     { create(:label, name: 'gfm & references?', project: project) }
     let(:reference) { label.to_reference(format: :name) }