Corrected with rubocop cop 'Lint/AmbiguousRegexpLiteral'.

.rubocop.yml

@@ -264,8 +264,6 @@ Lint/ShadowingOuterLocalVariable:
   Enabled: false
 Style/InfiniteLoop:
   Enabled: false
-Lint/AmbiguousRegexpLiteral:
-  Enabled: false
 Lint/Eval:
   Enabled: false
 Style/ModuleFunction:

app/controllers/tickets_controller.rb

@@ -38,7 +38,7 @@ class TicketsController < ApplicationController
 
     # create tags if given
     if params[:tags] && !params[:tags].empty?
-      tags = params[:tags].split /,/
+      tags = params[:tags].split(/,/)
       tags.each {|tag|
         Tag.tag_add(
           object: 'Ticket',

app/models/ticket/subject.rb

@@ -56,17 +56,17 @@ returns
     ticket_subject_size = Setting.get('ticket_subject_size')
 
     # remove all possible ticket hook formats with []
-    subject = subject.gsub /\[#{ticket_hook}: #{self.number}\](\s+?|)/, ''
-    subject = subject.gsub /\[#{ticket_hook}:#{self.number}\](\s+?|)/, ''
-    subject = subject.gsub /\[#{ticket_hook}#{ticket_hook_divider}#{self.number}\](\s+?|)/, ''
+    subject = subject.gsub(/\[#{ticket_hook}: #{self.number}\](\s+?|)/, '')
+    subject = subject.gsub(/\[#{ticket_hook}:#{self.number}\](\s+?|)/, '')
+    subject = subject.gsub(/\[#{ticket_hook}#{ticket_hook_divider}#{self.number}\](\s+?|)/, '')
 
     # remove all possible ticket hook formats without []
-    subject = subject.gsub /#{ticket_hook}: #{self.number}(\s+?|)/, ''
-    subject = subject.gsub /#{ticket_hook}:#{self.number}(\s+?|)/, ''
-    subject = subject.gsub /#{ticket_hook}#{ticket_hook_divider}#{self.number}(\s+?|)/, ''
+    subject = subject.gsub(/#{ticket_hook}: #{self.number}(\s+?|)/, '')
+    subject = subject.gsub(/#{ticket_hook}:#{self.number}(\s+?|)/, '')
+    subject = subject.gsub(/#{ticket_hook}#{ticket_hook_divider}#{self.number}(\s+?|)/, '')
 
     # remove leading "..:\s" and "..[\d+]:\s" e. g. "Re: " or "Re[5]: "
-    subject = subject.gsub /^(..(\[\d+\])?:\s)+/, ''
+    subject = subject.gsub(/^(..(\[\d+\])?:\s)+/, '')
 
     # resize subject based on config
     if subject.length > ticket_subject_size.to_i