Allow lowercase prefix for Youtrack issue ids

Relates to #42595.
Fixes #62661.

app/models/project_services/youtrack_service.rb

@@ -5,12 +5,12 @@ class YoutrackService < IssueTrackerService
 
   prop_accessor :description, :project_url, :issues_url
 
-  # {PROJECT-KEY}-{NUMBER} Examples: YT-1, PRJ-1
+  # {PROJECT-KEY}-{NUMBER} Examples: YT-1, PRJ-1, gl-030
   def self.reference_pattern(only_long: false)
     if only_long
-      /(?<issue>\b[A-Z][A-Za-z0-9_]*-\d+)/
+      /(?<issue>\b[A-Za-z][A-Za-z0-9_]*-\d+)/
     else
-      /(?<issue>\b[A-Z][A-Za-z0-9_]*-\d+)|(#{Issue.reference_prefix}(?<issue>\d+))/
+      /(?<issue>\b[A-Za-z][A-Za-z0-9_]*-\d+)|(#{Issue.reference_prefix}(?<issue>\d+))/
     end
   end
 

doc/user/project/integrations/youtrack.md

@@ -31,8 +31,8 @@ To disable the internal issue tracker in a project:
 ## Referencing YouTrack issues in GitLab
 
 Issues in YouTrack can be referenced as `<PROJECT>-<ID>`. `<PROJECT>`
-must start with a capital letter and can then be followed by capital or lower case
-letters, numbers or underscores. `<ID>` is a number. An example reference is `YT-101` or `Api_32-143`.
+must start with a letter and can then be followed by capital or lower case
+letters, numbers or underscores. `<ID>` is a number. An example reference is `YT-101`, `Api_32-143` or `gl-030`.
 
 References to `<PROJECT>-<ID>` in merge requests, commits, or comments are automatically linked to the YouTrack issue URL.
 For more information, see the [External Issue Tracker](../../../integration/external-issue-tracker.md) documentation.

spec/lib/banzai/filter/external_issue_reference_filter_spec.rb

@@ -155,6 +155,13 @@ describe Banzai::Filter::ExternalIssueReferenceFilter do
 
       it_behaves_like "external issue tracker"
     end
+
+    context "with a lowercase prefix" do
+      let(:issue) { ExternalIssue.new("gl-030", project) }
+      let(:reference) { issue.to_reference }
+
+      it_behaves_like "external issue tracker"
+    end
   end
 
   context "jira project" do