Extract regex into a common method

The regex (?<issue>\d+\b) is used in several places and here we are
abstracting into a common reference.

app/models/issue.rb

@@ -128,12 +128,12 @@ class Issue < ApplicationRecord
   def self.reference_pattern
     @reference_pattern ||= %r{
       (#{Project.reference_pattern})?
-      #{Regexp.escape(reference_prefix)}(?<issue>\d+)
+      #{Regexp.escape(reference_prefix)}#{Gitlab::Regex.issue}
     }x
   end
 
   def self.link_reference_pattern
-    @link_reference_pattern ||= super("issues", /(?<issue>\d+)/)
+    @link_reference_pattern ||= super("issues", Gitlab::Regex.issue)
   end
 
   def self.reference_valid?(reference)

app/models/project_services/issue_tracker_service.rb

@@ -19,9 +19,9 @@ class IssueTrackerService < Service
   # overridden patterns. See ReferenceRegexes.external_pattern
   def self.reference_pattern(only_long: false)
     if only_long
-      /(\b[A-Z][A-Z0-9_]*-)(?<issue>\d+\b)/
+      /(\b[A-Z][A-Z0-9_]*-)#{Gitlab::Regex.issue}/
     else
-      /(\b[A-Z][A-Z0-9_]*-|#{Issue.reference_prefix})(?<issue>\d+\b)/
+      /(\b[A-Z][A-Z0-9_]*-|#{Issue.reference_prefix})#{Gitlab::Regex.issue}/
     end
   end
 

app/models/project_services/youtrack_service.rb

@@ -8,7 +8,7 @@ class YoutrackService < IssueTrackerService
     if only_long
       /(?<issue>\b[A-Za-z][A-Za-z0-9_]*-\d+\b)/
     else
-      /(?<issue>\b[A-Za-z][A-Za-z0-9_]*-\d+\b)|(#{Issue.reference_prefix}(?<issue>\d+\b))/
+      /(?<issue>\b[A-Za-z][A-Za-z0-9_]*-\d+\b)|(#{Issue.reference_prefix}#{Gitlab::Regex.issue})/
     end
   end
 

ee/app/models/design_management/design.rb

@@ -165,7 +165,7 @@ module DesignManagement
     def self.link_reference_pattern
       @link_reference_pattern ||= begin
         exts = SAFE_IMAGE_EXT + DANGEROUS_IMAGE_EXT
-        path_segment = %r{issues/(?<issue>\d+)/designs}
+        path_segment = %r{issues/#{Gitlab::Regex.issue}/designs}
         filename_pattern = %r{(?<simple_file_name>[a-z0-9_=-]+\.(#{exts.join('|')}))}i
 
         super(path_segment, filename_pattern)

lib/gitlab/regex.rb

@@ -144,6 +144,10 @@ module Gitlab
     def utc_date_regex
       @utc_date_regex ||= /\A[0-9]{4}-[0-9]{2}-[0-9]{2}\z/.freeze
     end
+
+    def issue
+      @issue ||= /(?<issue>\d+\b)/
+    end
   end
 end