Lowercase letters support and additional tests for YouTrack integration service

app/models/project_services/youtrack_service.rb

@@ -8,9 +8,9 @@ class YoutrackService < IssueTrackerService
   # {PROJECT-KEY}-{NUMBER} Examples: YT-1, PRJ-1
   def self.reference_pattern(only_long: false)
     if only_long
-      /(?<issue>\b[A-Z][A-Z0-9_]*-\d+)/
+      /(?<issue>\b[A-Z][A-Za-z0-9_]*-\d+)/
     else
-      /(?<issue>\b[A-Z][A-Z0-9_]*-\d+)|(#{Issue.reference_prefix}(?<issue>\d+))/
+      /(?<issue>\b[A-Z][A-Za-z0-9_]*-\d+)|(#{Issue.reference_prefix}(?<issue>\d+))/
     end
   end
 

spec/features/projects/services/user_activates_issue_tracker_spec.rb

@@ -87,6 +87,7 @@ describe 'User activates issue tracker', :js do
   end
 
   it_behaves_like 'external issue tracker activation', tracker: 'Redmine'
+  it_behaves_like 'external issue tracker activation', tracker: 'YouTrack'
   it_behaves_like 'external issue tracker activation', tracker: 'Bugzilla'
   it_behaves_like 'external issue tracker activation', tracker: 'Custom Issue Tracker'
 end

spec/lib/banzai/filter/external_issue_reference_filter_spec.rb

@@ -135,6 +135,20 @@ describe Banzai::Filter::ExternalIssueReferenceFilter do
       it_behaves_like "external issue tracker"
     end
 
+    context "with underscores in the prefix" do
+      let(:issue) { ExternalIssue.new("PRJ_1-123", project) }
+      let(:reference) { issue.to_reference }
+
+      it_behaves_like "external issue tracker"
+    end
+
+    context "with lowercase letters in the prefix" do
+      let(:issue) { ExternalIssue.new("YTkPrj-123", project) }
+      let(:reference) { issue.to_reference }
+
+      it_behaves_like "external issue tracker"
+    end
+
     context "with a single-letter prefix" do
       let(:issue) { ExternalIssue.new("T-123", project) }
       let(:reference) { issue.to_reference }