Allow single letter external issue IDs

app/models/project_services/issue_tracker_service.rb

@@ -12,9 +12,9 @@ class IssueTrackerService < Service
   # overriden patterns. See ReferenceRegexes::EXTERNAL_PATTERN
   def self.reference_pattern(only_long: false)
     if only_long
-      /(\b[A-Z][A-Z0-9_]+-)(?<issue>\d+)/
+      /(\b[A-Z][A-Z0-9_]*-)(?<issue>\d+)/
     else
-      /(\b[A-Z][A-Z0-9_]+-|#{Issue.reference_prefix})(?<issue>\d+)/
+      /(\b[A-Z][A-Z0-9_]*-|#{Issue.reference_prefix})(?<issue>\d+)/
     end
   end